Biologic drug development for treatment and prevention of sexually transmitted infections

SUMMARY

Sexually transmitted infections (STIs) represent a significant global health burden, with over one million new infections occurring daily. In some instances, the prevalence of antibiotic-resistant pathogens is rising, which exacerbates the challenge. STIs cause severe complications, including infertility, ectopic pregnancies, pre-term births, and heightened risks of HIV acquisition. These outcomes underscore the need for innovative therapeutic and prophylactic strategies. In this review, we provide a comprehensive analysis of the current state of biologic drug development targeting key STIs, focusing on Chlamydia trachomatis, Neisseria gonorrhoeae, herpes simplex virus type 2 (HSV-2), and Treponema pallidum. We examine the complexity of host–pathogen interactions that inform biologic drug design, such as multiple mechanisms of infection, immune evasion strategies, and pathogenic latency. We also explore the role of mucosal immunity, highlighting advances in resident memory T cells and cytokine-driven responses that guide therapeutic targeting, concentrating on Chlamydia trachomatis and Neisseria gonorrhoeae, where recent advances in vaccine development appear promising. We conduct a comprehensive survey of platforms, including vaccines, and explore modalities such as monoclonal antibodies and protein therapeutics. Additionally, we examine emerging technologies like nucleic acid-based therapies, microbiome modulation, and phage-based interventions, highlighting their potential against challenging pathogens like HSV-2 and Treponema pallidum. By examining these established and emerging approaches, this review prioritizes critical opportunities for innovation in biologic therapeutics, addressing unmet needs in STI management. It advocates for integrated strategies leveraging antigenic conservation, host immunity modulation, and novel delivery platforms to achieve durable prophylaxis and effective treatment for high-burden infections globally.

INTRODUCTION

According to the World Health Organization, over one million sexually transmitted infections (STIs) are acquired globally each day (1). Even during the coronavirus disease 2019 pandemic, while the estimated number of influenza cases in the United States plummeted from 36 million to approximately 2,000, the estimated number of chlamydia and gonorrhea cases remained mostly flat at around 1.6 million and 850,000, respectively (2). Globally, there are 374 million new infections annually from chlamydia, gonorrhea, syphilis, and trichomoniasis. An estimated 846 million people aged 15–49 (more than one in five of this age group globally) have genital herpes (3). People below the age of 20 and above the age of 55 also carry STI risks, although overall incidence cases in the latter cohort are comparatively lower (Fig. 1). For perspective, malaria affected 247 million people in 2021 worldwide, while tuberculosis caused 10.6 million cases in 2022. In the same year, breast cancer diagnoses reached 2.3 million, and lung cancer cases numbered 2.2 million. By comparison, chlamydia alone accounts for 131 million new cases annually, the highest among bacterial STIs. Trichomonas vaginalis, a protozoal STI, reportedly causes 156 million global cases of trichomoniasis, with nearly 75% of male partners of infected women also becoming infected, demonstrating its high transmissibility. Gonorrhea accounts for over 80 million cases globally, with a penile–vaginal transmission rate of approximately 50% after a single exposure (4, 5). The disease has gained resistance to extended spectrum cephalosporins, the only remaining first-line monotherapy, and treatment failures with ceftriaxone plus azithromycin, the previously recommended dual therapy, are increasingly being reported (6). Pharyngeal infections in gonorrhea are particularly hard to treat with existing antibiotics due to lower drug concentrations in these tissues. This results in a greater potential for drug resistance and reinforces the need for alternate treatments, including those targeting the local mucosa (7). Despite the availability of cheap and effective penicillin treatment, syphilis continues to rise globally with an estimated 70 million cases in 2021. Pre-diagnosis, early-stage syphilis is a risk factor for HIV acquisition in the general population and transmission to the fetus in mothers. Therefore, syphilis prevention using alternate strategies is key to disease elimination. Mycoplasma genitalium, a bacterial pathogen, has emerged as a significant sexually transmitted agent with widespread resistance to multiple antibiotics and with disease pathogenesis that remains poorly understood. Ureaplasma spp., specifically Ureaplasma urealyticum and Ureaplasma parvum, are additional STI pathogens of concern. Although human papilloma virus (HPV) is the most common cause of STI worldwide—with 31% of people infected globally (8) and the prevalence of genital HPV infections estimated to be 10%–20%—highly effective vaccines that prevent HPV infections are available.

Fig 1.

Proportional incidence of chlamydia (gray), gonorrhea (red), and syphilis (blue) in 2022 in females and males under 19, 20–54, and over 55 years of age (US Centers for Disease Control and Prevention [CDC]). Incidence cases are calculated based on 2022 rates reported by the US CDC.

Collectively, STIs cause significant morbidity in men and women. They can lead to urethritis, epididymitis, and prostatitis in men and to genital warts and sores in both men and women. Furthermore, STIs can lead to infertility in both men and women, ectopic pregnancies in women, pre-term births, and an increased risk of complications during pregnancy and childbirth. Taken together, STIs represent a major public health concern globally. This review article critically evaluates the current state of biologic drug development to treat and prevent STIs, highlighting challenges related to pathogens: multiple mechanisms of infection, immune evasion, intracellular habitat, and latency. We focus on describing established biologic drug modalities of vaccines, monoclonal antibodies (mAbs), and protein therapeutics while also exploring emerging platforms of nucleic acid medicines, microbiome, and phage therapeutics. Unlike conventional antimicrobials that typically target bacterial or viral replication pathways, biologics leverage immune modulation, pathogen neutralization, and receptor-blocking mechanisms to achieve durable prophylaxis and therapeutic efficacy. The complexity of host–pathogen interactions, including immune evasion strategies and intracellular persistence, necessitates innovative biologic approaches beyond standard antimicrobial interventions. Among STIs, we examine progress in four leading causes of genital infections: chlamydia, gonorrhea, syphilis, and genital herpes. Focusing on these four specific STIs targets high-prevalence infections that cause significant health burden, have unique treatment challenges, and align well with contemporary public health priorities around STIs. Unlike HPV or Mpox, these STIs do not have the availability of effective vaccines to implement adequate prevention strategies. Thus, our approach ensures research impact by concentrating on infections with high morbidity, potential for severe outcomes, and, in some cases like gonorrhea, increasing antibiotic resistance.

COMPLEX BIOLOGY

STI pathogens often target specific mucosal membranes mostly in the urogenital tract or anal and pharyngeal cavities or ocular mucosa, adapting to the host environment using various cellular and molecular mechanisms (Fig. 2). Some bacteria, like Chlamydia trachomatis, create intracellular inclusions, evading immune detection while hijacking host metabolism for replication and for ongoing chronic survival (9). Others, like N. gonorrhoeae, express surface molecules that mimic host glycoproteins, further camouflaging themselves (10). Some STIs, like herpes simplex virus, can establish latent infections within the host, remaining dormant for extended periods before re-activating and causing symptoms. This makes the achievement of sterile immunity challenging and necessitates lifelong monitoring in some cases (11). The complex microbial communities inhabiting the ano-urogenital tract can influence the susceptibility to and outcome of STIs. For example, certain commensal bacteria as well as phages can act as competitors against pathogenic microbes, while others may modulate the immune response (12). Particularly in females, vaginal microbiota plays a pivotal role in immunity against STI pathogens. Microbiome in a healthy female reproductive tract (FRT) is typically dominated by Lactobacillus spp.—particularly Lactobacillus crispatus, Lactobacillus jensenii, and Lactobacillus gasseri—are associated with lower susceptibility to STI infections (13). Bacterial vaginosis (BV) results from colonization of the FRT with Lactobacillus-deficient organisms and an increase of anaerobic bacteria, such as Gardnerella vaginalis, Atopobium vaginae, Prevotella bivia, and Sneathia amnii, that are linked to local inflammation and elevated STI acquisition risk (14). These bacteria alter the immune landscape in the FRT by diminishing the expression of mucins, which compromises epithelial barrier integrity and induces higher levels of proinflammatory cytokines such as interleukin (IL)-6, IL-8, and tumor necrosis factor alpha (TNF-α). These may in turn promote recruitment of permissive target cells to the area. Therefore, understanding these complex interactions is crucial for developing new prevention and treatment countermeasures. Below, we briefly describe salient biologic features of each of the four STI pathogens that are relevant to the development of biologic treatment and therapies.

Fig 2.

Summary of Neisseria gonorrhoeae infection and pathophysiology. Infection begins with bacterial attachment to mucosal columnar epithelial cells, usually but not exclusively, in the urogenitalia (1). Bacteria use the host microenvironment to attach, to replicate into colonies, and to form biofilms, which in certain cases can lead to invasion and transcytosis across the epithelial barrier (2). A complex array of bacterial components is released at distinct stages of colonization. Lipo-oligosaccharide (LOS) fragments and outer membrane vesicles (OMVs) are released early during attachment, triggering inflammation, while peptidoglycan fragments are shed during bacterial growth and biofilm formation, activating NOD-like receptors and causing epithelial damage. IgA protease is secreted during early colonization to cleave host secretory IgA and evade mucosal immunity, while Opa proteins shed fragments during tight adhesion to modulate host signaling. Porin B (PorB) protein is shed during adhesion and invasion, inducing apoptosis and immune modulation. Lactoferrin- and transferrin-binding proteins are expressed during colonization to scavenge iron from the host, ensuring bacterial survival (3). Released bacterial components interact with specific host receptors, triggering molecular responses that facilitate bacterial colonization, immune evasion, and tissue damage. LOS fragments interact with Toll-like receptor (TLR)4 on macrophages, dendritic cells (DCs), and epithelial cells, activating NF-κB signaling and inducing proinflammatory cytokines like IL-1β, IL-6, and TNF-α. OMVs interact with TLR4 and TLR2 to deliver virulence factors, induce cytokine production, and cause epithelial damage. Peptidoglycan fragments are recognized by cytosolic receptors NOD1 and NOD2, leading to RIPK2-mediated NF-κB activation and neutrophil recruitment (4). Neutrophil-mediated inflammatory response, while aimed at clearing the infection, contributes to tissue damage and bacterial transmission. N. gonorrhoeae spp. evade killing by polymorphonuclear leukocytes and extracellular neutrophil traps through numerous mechanisms, including delayed phagosome–granule fusion, inhibiting reactive oxygen species production, and engaging immunoinhibitory Siglecs. This results in the formation of purulent exudates containing high levels of bacteria, a vehicle for transmission during sexual contact (5).

Chlamydia trachomatis

C. trachomatis is a human-restricted, obligate intracellular bacterial pathogen with a complex life cycle that consists of invasion, replication, propagation, and re-infection (9). C. trachomatis initiates host cell invasion in the form of infectious but metabolically inactive elementary bodies (EBs), which, upon cellular internalization, are enclosed in a membrane-bound protective vacuole called the inclusion. Within the inclusion, EBs transform into metabolically active, non-infectious reticulate bodies (RBs). RBs initiate protein synthesis, DNA replication, and metabolic processes necessary for replication, whereas exposure to certain environmental factors like nutrient deprivation, antibiotics, or temperature changes can induce persistence, a metabolically dormant form of C. trachomatis. Persistent chlamydia can regain infectivity and re-enter the replication cycle under favorable conditions, with release of EBs through either lysis of the host cell or extrusion of the inclusion without cell death (9). What makes C. trachomatis biology challenging is not only that it is an intracellular pathogen but also that it is incapable of replication outside a host cell, residing instead within a specialized intracellular niche and evading host defense. Furthermore, C. trachomatis consists of many serovars with diverse pathologies: serovars A, B, Ba, and C cause chronic infection of the eyes, leading to scarring and eventually blinding trachoma, primarily affecting children in developing countries; serovars D, Da, E, F, G, Ga, H, I, Ia, J, and K cause genitourinary infections and related sequelae, while serovars L1, L2, L2a, and L3 cause lymphogranuloma venereum (15). Even within a specific clinical indication, such variation in C. trachomatis serovars presents a challenge in developing broadly protective treatments and therapies. Extracellularly, the major outer membrane protein (MOMP) contains highly diverse extracellular loops that differ between C. trachomatis serovars (16). Intracellularly, C. trachomatis encodes 50 inclusion body (Inc) proteins, many with unknown function. Among secreted factors, the C. trachomatis chlamydial protease-like activity factor (CPAF) disrupts the host’s programmed cell death machinery, ensuring a prolonged intracellular home for chlamydial replication (17). CPAF degrades cytoskeletal proteins to allow inclusion body expansion and release of EBs. CPAF not only dampens neutrophils’ oxidative burst but also disrupts their chemical activation by cleaving N-formyl peptide receptor 2, a key chemotactic and phagocytic receptor, from their surface, actively paralyzing neutrophils, including their extracellular trap formation, effectively shielding the bacteria from the host’s innate immune response (18).

Neisseria gonorrhoeae

N. gonorrhoeae, the causative agent of gonorrhea, is also a human-restricted gram-negative diplococcus. It infects the mucosal membranes of the urogenital tract, rectum, pharynx, and the eyes, often as a polyclonal infection creating functional redundancy and therapeutic resistance (19). N. gonorrhoeae’s unique abilities depend in part on its outer membrane molecules, namely, outer membrane proteins (OMPs) and lipo-oligosaccharide (LOS), which work in concert to facilitate colonization and evasion of host defenses through molecular mimicry, antigenic variation, complement inhibition, LOS sialylation, and immune suppression. This complex set of evasion pathways introduces multiple potential treatment strategies, including targeted monoclonal antibodies, vaccines, complement disruption, microbiome-based therapies, and bacteriophages, all aimed at enhancing immune clearance. Porin B (PorB), the most abundant outer membrane protein, serves as the main gateway for essential nutrients like glucose and iron. It can also facilitate the invasion of N. gonorrhoeae into host epithelial cells, potentially by providing an entry point or interacting with host cell receptors (10). Additionally, PorB’s surface properties shield N. gonorrhoeae from the complement system by binding to the complement inhibitors C4BP and factor H (FH), thereby hindering complement activation (20, 21) and preventing opsonization, a crucial step for phagocytosis of bacteria. Pilin proteins (type IV pilus) are fibrillar structures that mediate adhesion to cells and exert retractile forces to bring bacteria proximate to cells. Pili are essential for gonococcal colonization of the human genital tract. Recombination events with silent pil “donor” loci result in tremendous antigenic variation in Pil sequences across strains, rendering this essential protein unsuitable as a vaccine candidate. N. gonorrhoeae also possesses a collection of 11 opacity-associated (Opa) proteins, each encoded by a separate gene scattered across its genome. Remarkably, the bacteria can express any combination of these proteins due to phase variation. This allows N. gonorrhoeae to constantly adjust its surface profile, presenting a set of shifting antigens for the host immune system and contributing to N. gonorrhoeae’s adaptability and resilience. N. gonorrhoeae LOS can express carbohydrate structures resembling human glycolipids, acting as a molecular disguise (22). This shields N. gonorrhoeae from immune recognition, preventing opsonization and phagocytosis by neutrophils and macrophages. Additionally, N. gonorrhoeae encodes an IgA1 protease that masks epitopes by cleaving antigen-bound IgA (23). IgA protease also cleaves lysosome-associated membrane protein 1 and facilitates gonococcal intracellular survival and transcytosis (24–26). Furthermore, N. gonorrhoeae can attach sialic acid molecules to its LOS, mimicking host cell surfaces (27). Sialylated N. gonorrhoeae LOS enhances binding of the complement inhibitor factor H (28), which disrupts the complement cascade, preventing formation of membrane attack complexes (MACs) and subsequent bacterial lysis. N. gonorrhoeae LOS can also trigger the release of IL-10 from host cells, suppressing the immune response, including induction of anergy in T cells, favoring bacterial survival and colonization (29). IL-10 released following engagement of carcinoembryonic antigen-related cell adhesion molecules by gonococcal Opa prevents shedding of colonized endocervical cells, thereby promoting gonococcal colonization (30). Therefore, key considerations for biologic drug development include prioritizing conserved antigens and formulating strategies to counteract immune evasion, such as disrupting PorB engagement with C4BP and FH.

Given that STI pathogens infect primarily at the mucosal surface, it is important to design antigens, adjuvants, and platforms for enhanced mucosal delivery. Both C. trachomatis and N. gonorrhoeae cause mucosal infections by attachment to columnar epithelial cells. N. gonorrhoeae expresses several surface molecules such as Opa and LOS that facilitate interactions with host epithelial cells by binding to specific receptors, which aids in colonization. Similarly, C. trachomatis uses heparan sulfate proteoglycan interaction via bacterial OmcB as well as adhesion molecule binding through proteins like MOMP for cell attachment. Thus, surface molecules on C. trachomatis and N. gonorrhoeae that are involved in attachment become key targets for vaccine as well as therapeutic approaches, if molecular redundancies used by the bacteria can be overcome.

Herpes simplex virus type 2

Herpes simplex virus type 2 (HSV-2), a double-stranded DNA virus, encapsulates its genome within an icosahedral capsid, stabilized by 20–23 tegument proteins essential for maintaining viral architecture, facilitating replication, and evading host immune responses. The capsid is enveloped by a lipid bilayer incorporating 12 glycoproteins: glycoprotein B (gB), (gC), gD, gE, gG, gH, gI, gJ, gK, gL, gM, and gN. These viral glycoproteins enable membrane fusion with host cells and help evade the immune system. HSV-2 shares ∼74% nucleotide level sequence identity and close structural resemblance with herpes simplex virus type 1 (HSV-1) but exhibits subtle differences in its cell entry repertoire. Both viruses share the dependence on viral gB for initial attachment to heparan sulfate proteoglycans. However, unlike HSV-1, HSV-2 bypasses the need for gC in this step (31). Once anchored, HSV-2 gD binds either nectin-1/nectin-2 receptors on epithelial and neuronal cells or the immune-specific herpesvirus entry mediator (HVEM) on dendritic and T cells (32). Notably, HVEM interacts with gD based on its orientation (cis or trans), impacting intracellular signaling. Following ligand binding, gD undergoes conformational changes, priming the activation of the gH/gL complex. Gated by gB’s fusion machinery, this complex orchestrates the final membrane fusion event. Additionally, the gH/gL complex exhibits versatility in its action. By binding αvβ3, αvβ6, and αvβ8 integrins, it induces heterodimer dissociation and triggers gH activation for entry through acidic endosomes. This intricate mix of entry routes, coupled with subtle structural variations, underscores the remarkable adaptability and resilience of HSV-2, necessitating a multi-pronged approach to biologic drug design. Importantly, after the initial infection, HSV-2 travels up nerve fibers to sensory ganglia, where it establishes latency, often lasting a lifetime. Traditional antivirals do not affect latent HSV in ganglia, partly because of the blood–brain barrier (BBB). This explains why antivirals cannot cure infection, and re-activations usually occur once therapy is discontinued. To address this, therapeutic approaches must consider mechanisms for overcoming or bypassing the BBB and targeting latent reservoirs. These will be discussed in further detail later in this review.

Treponema pallidum

Treponema pallidum subsp. pallidum, the spirochete that causes syphilis, also utilizes a multi-faceted strategy to evade the immune system and establish chronic infection in humans, its only known natural host. To begin with, T. pallidum infects mucosa in the genitalia and, in addition, other tissues that include but are not limited to, particularly in patients with HIV, ocular and central nervous systems (CNS), where immune privilege prevents clearance of the pathogen. CNS infection predisposes individuals to relapsing infections, which makes treatment challenging. Furthermore, due to its low metabolism, T. pallidum can persist from months to years in a latent phase, making treatment prolonged and difficult. T. pallidum has a striking lack of metabolic capabilities, including lack of a complete pathway for gluconeogenesis (33). As such, the organism depends on the host for alternative carbon sources, enzyme co-factors, nucleotides, amino acids, and fatty acid synthesis, although it does have limited interconversion pathways (34). Furthermore, T. pallidum’s outer membrane is structurally fragile, lacks lipopolysaccharide, and exhibits a small proteome, with few surface-exposed proteins, thus minimizing recognition by antibodies and complement factors. The main basis of immune evasion by T. pallidum is via resistance to antibody binding, which in turn is driven by heterogeneity in surface antigenicity as well as by low abundance of surface-exposed T. pallidum repeat proteins (Tpr) (e.g., only ~200 copies/cell for Tpr C/TprD) (33). Among the outer membrane proteins, TprK antigenic variation is thought to play a major role in immune evasion, although antibodies to TprK have also been shown to be opsonic and protective (35). The TprK protein is predicted to have two hydrophobic transmembrane domains with seven variable (V1–V7) regions. In vivo cloning of T. pallidum using rabbits identified that the antibodies elicited were highly specific to the V domains of the respective infecting clone (34). However, antibody cross-reactivity among strains was rare, resulting in evasion of immune responses in infected individuals and assisting T. pallidum in establishing chronic infection. Analysis of 21 T. pallidum isolates from patients with primary and secondary syphilis using long-read DNA sequencing revealed around 400 full-length TprK sequences characterized by sequence heterogeneity and linked to immune evasion (36). Thus, effective strategies for T. pallidum prevention will need to focus on conserved regions within antigens like TprK and additionally consider other surface-exposed antigens for effective multivalent vaccine design.

CO-INFECTIONS

Co-infections between C. trachomatis, N. gonorrhoeae, HSV-2, and T. pallidum are common due to shared routes of acquisition, presenting a complex interplay of pathogens, host-immune responses, and onset of diseases like pelvic inflammatory disease. A study conducted in the UK revealed significant co-infection of chlamydia and gonorrhea, especially among women (38%) and younger individuals (over 50% in 15- to 19-year-olds) (37). In one US study of patients with syphilis, 27% of the subjects were co-infected with one or more bacterial STIs, of which over 37% also had gonorrhea, nearly 28% had syphilis and chlamydia, and 30% had all three (38). In another study in China involving men who have sex (MSM) with men, 24% of subjects had co-infections, the most (50%) being chlamydia, gonorrhea, and genital herpes co-infections (39). As such, co-infection by multiple pathogens can alter the host immune response, weakening defenses against individual pathogens and creating a more favorable environment for their replication. For example, C. trachomatis-induced inflammation can attract immune cells that become infected by T. pallidum (33). In itself, HSV-2 infection is a potent risk factor for HIV-1, with a threefold increase in susceptibility (40). C. trachomatis and N. gonorrhoeae infections also increase the risk of acquiring HIV (41, 42). While C. trachomatis is an obligatory intracellular pathogen and N. gonorrhoeae is not, both N. gonorrhoeae and C. trachomatis can survive extracellularly as well as within mucosal epithelial cells. Upon infection, these cells, together with resident immune cells, induce cytokines that recruit neutrophils, leading to tissue damage. In addition, C. trachomatis and N. gonorrhoeae induce production of type I interferon, which blocks phagocytosis, and IL-1α, which drives further tissue damage. Type I interferon also lowers levels of major histocompatibility complex class II (MHC-II) on macrophages for robust activation of CD4+ T cells (43). Therefore, it is critical for biologic drug development programs to enhance mucosal immune barriers and activate CD4+ T cells for pathogen clearance at mucosal surfaces, with the hope of cross-protection against opportunistic STIs. Overall, addressing the interplay of immune disruption, inflammation, and pathogen dynamics is essential for developing effective interventions. Clinical trials must account for co-infections to accurately evaluate efficacy.

ROLE OF EXTRAGENITAL INFECTIONS

Extragenital (i.e., rectal and pharyngeal) STI testing remains poorly monitored, with screening rates largely unknown. While the US Centers for Disease Control and Prevention recommends extragenital testing for high-risk groups (e.g., MSM), there is no national requirement for extragenital C. trachomatis or N. gonorrhoeae as part of routine screening at the present time. In the case of N. gonorrhoeae, studies have shown that 13%–33% of infections would be missed if only genital sites were tested (44, 45). A US survey carried out across 42 clinics found that in MSM, despite being asymptomatic and despite testing negative in genital tests, >85% of subjects were positive for C. trachomatis and 70% for N. gonorrhoeae when extragenital testing was conducted (46). A European study found that the most frequent extragenital pathogen detected in an outpatient clinic was N. gonorrhoeae (71.8% of all pharyngeal and 55.7% of anorectal STIs), followed by C. trachomatis (41.0% of all anal infections and 5.6% of pharyngeal infections). Detecting infections at these sites, such as N. gonorrhoeae, can help identify reservoirs that contribute to persistent transmission and antimicrobial resistance, allowing for targeted biologic treatments with improved efficacy and that prevent the development of multi-drug-resistant strains. On the preventative side, extragenital testing highlights the need for vaccines targeting mucosal surfaces in sites like the pharynx and rectum, where pathogens like N. gonorrhoeae can evade immune responses. Different anatomical sites have distinct immunological environments and microbiome compositions, influencing susceptibility to infection, immune responses, and the effectiveness of biologic treatments. Thus, identifying infections at extragenital sites can guide research into microbiome modulation, leading to strategies like probiotics or microbiome stabilizers that enhance mucosal defense as described in more detail later in the review.

NATURAL IMMUNITY

During infection, STI pathogens induce innate and adaptive immunity through both the humoral as well as cellular arms, leading to the induction of a variety of immune factors (Fig. 3). For example, in females, the FRT mucosa contains mucins, epithelial cell secretions, antimicrobial peptides, and immunoglobulins. Surprisingly, unlike other mucosal secretions, those in the cervix are more abundant in IgG than IgA (47, 48). This could be partly because the neonatal Fc receptor responsible for salvaging IgG from catabolism is present in the epithelium of the female genital tract that permits transport of systemic IgG into mucosal secretions (47). At a cellular level, FRT columnar epithelial cells express NOD-like receptors as well as pattern recognition receptors (PRRs), such as toll-like receptors (TLRs) 1–9, leading to neutrophil and monocyte activation, migration, adhesion, cell death pathways, and IL-1 and inflammasome-related genes. NK cells in the FRT release interferon (IFN)-γ early during C. trachomatis infections. Depletion of NK cells results in decreased IFN-γ expression, a shift to Th2 responses, and delayed bacterial clearance. IFN-ε, highly expressed by endometrial epithelial cells, protects against C. trachomatis and HSV-2 from the earliest stages of infection, highlighting IFN-ε’s importance in enhancing protective innate immune responses in the FRT (49).

Fig 3.

Mechanisms of immune modulation and bacterial defense in gonococcal infections by antibody (top panel) and T cell-mediated (bottom panel) mechanisms. (A) Antibodies neutralize toxins and quorum sensing (QS) molecules, reducing bacterial virulence factors as well as inhibiting bacterial–host cell receptor interactions, preventing bacterial adhesion and invasion. (B) Antibodies enhance opsonophagocytosis by macrophages via Fc receptor uptake. (C) Antibodies also enhance complement recruitment through C1q binding to Fc domains for bacterial clearance. (D) Antibodies directly interfere with bacterial biofilm formation by blocking adhesion molecules and by neutralizing biofilm matrix. TGF-β, transforming growth factor beta.

Recent work has shown that mucosal resident memory CD8+ T cells are critical for protective antiviral immunity (50). Studies in mice show a similar correlation for the protective role of CD4⁺ T cells against bacterial pathogens like C. trachomatis when humoral immunity alone is neither sufficient to clear infection nor to prevent re-infection (51). IgA-deficient mice resolve primary and secondary chlamydial genital infections similarly to IgA-sufficient mice, suggesting an incomplete role of mucosal humoral immunity. Reasons behind this inability are complex and remain unclear. Below, we briefly discuss underlying cellular and molecular mechanisms that describe the nature and extent of natural immunity against individual STI pathogens, and we explore how this information could be exploited to develop durable preventative and therapeutic strategies. Of note, most studies of immunological mechanisms are conducted in mice, although the pathogens are human specific. Therefore, caution is required in interpreting the results.

Immunity to C. trachomatis is multi-faceted, involving both innate and adaptive immune responses. While humoral immunity plays a role, particularly in the production of antibodies against C. trachomatis surface proteins, cellular immunity, particularly CD4+ T cells, is crucial for controlling infection and preventing re-infection. MHC-II-restricted, IFN-γ-producing CD4+ T cells are the primary mediators of immunity against both primary and secondary Chlamydia muridarum (Cm) infections in mice (52). Although their role is debated, major histocompatibility complex class I (MHC-I)-restricted CD8+ T cells may also contribute to protection through IFN-γ production. Mucosal B cells and antibodies form a crucial second line of defense. For example, a study conducted in mice demonstrated that a MOMP subunit vaccine, adjuvanted with CpG-1826 and Montanide ISA 720, elicited robust protection via both CD4+ T cells and antibodies, as evidenced by reduced vaginal bacterial shedding, inclusion-forming unit (IFU) counts, and hydrosalpinx incidence (53). Depletion studies confirmed the pivotal role of CD4+ T cells, while B cell-deficient mice receiving immune serum exhibited superior protection compared to animals receiving non-immune serum, highlighting the importance of humoral immunity (52). Further studies have supported this notion, demonstrating inverse correlations between cervical bacterial burden and IgG/IgA levels, but much stronger associations between Th1-polarized cytokine profiles and protection from re-infection/ascension (54). The exact correlation between mouse models and human disease has not been established.

Natural immunity to N. gonorrhoeae is complex and often not fully protective since some N. gonorrhoeae-infected patients are prone to re-infections. In uncomplicated genital infections, the appearance of serum antibodies to PorB and LOS is frequently observed, with levels of serum antibodies rising with the severity of disease. Antibody levels vary widely between individuals. When protective, they tend to be effective against homologous infections involving the same PorB type (55). In addition, induction of antibodies against reduction-modifiable protein (Rmp) increased the risk of infection compared with subjects who lack anti-Rmp antibody, potentially by blocking the protective effect of bactericidal antibodies (56). Thus, the protective effect observed applies specifically to homologous infections with the same PorB type, as antibody levels and protection vary significantly between individuals, and some antibodies, such as those against Rmp, may even increase susceptibility rather than confer protection. An important tool for the study of natural immunity against N. gonorrhoeae, albeit only in males, is the controlled human infection model (CHIM), wherein N. gonorrhoeae is introduced into the urethra of healthy male volunteers (57). CHIM studies have revealed N. gonorrhoeae’s adaptability with pilus antigens and Opa proteins dynamically switching during infection to evade immune defenses. Similarly, LOS undergoes phase variation, with increased sialylation. Interestingly, CHIM studies have also found that the men who resisted N. gonorrhoeae re-infection were significantly more likely to have had a >4-fold increase in anti-LOS IgG following initial infection than those who did not resist re-infection (58).

Like bacterial STI pathogens, HSV-2 also triggers a complex and multi-faceted immune response in humans, involving both innate and adaptive arms. PRRs on epithelial cells and immune cells like macrophages and dendritic cells detect viral components like glycoproteins and viral DNA. PRR activation triggers the release of proinflammatory cytokines like IFN-α and IFN-β, chemokines, and inflammatory mediators. However, at many levels, HSV-2 interferes with innate immunity that includes viral glycoprotein gC binding to complement protein C3b and subsequently blocking C5 binding, thereby attenuating complement activation (59). HSV-2 gD protein also binds to and dampens the effects of NK cells and interferes with dendritic cell function (60). At the humoral level, although HSV-2 infection elicits antibodies against viral proteins, particularly gB and gD, these responses are not protective by themselves. Furthermore, viral protein gE binds to the Fc region of IgG, limiting both complement-mediated virus killing as well as phagocytosis. At the cellular level, HSV-2 inhibits host transporter associated with antigen processing protein, thereby limiting viral peptide on MHC-I for presentation to CD8+ T cells (61). Additionally, various viral proteins reduce T-cell activation and function by decreasing IL-2 secretion, by blocking TNF-associated factor 6 activity and altering linker for activation of T cell (LAT) function, and by promoting Treg cell function with increased IL-10 secretion. Taken together, while HSV-2 infection triggers innate and adaptive immune responses, evasive mechanisms of the virus, combined with its latency, make it impossible to eliminate the virus by natural immunity alone.

T. pallidum infection results in a complex yet ineffective immune response that does not limit bacterial spread or disease progression. In primary syphilis, polymorphonuclear leukocytes infiltrate the infection site, secreting antimicrobial peptides and exerting phagocytosis. Dendritic cells also phagocytose bacteria and present treponemal antigens to T cells, stimulating the production of proinflammatory cytokines like IL-12, IL-6, and IL-1β. These activated T cells accumulate at the infection site, coinciding with peak T. pallidum burden. Notably, Th1 CD4+ T cells dominate, though cytotoxic CD8+ T cells are also present, potentially targeting T. pallidum reservoirs in non-phagocytic cells (62). Macrophages, activated by Th1 cytokines and IFN-γ, rise dramatically and contribute to bacterial clearance through phagocytosis. Nonetheless, T. pallidum employs various strategies to evade the host immune system and establish persistent infections, such as switching on and off the expression of OMPs like Tpr proteins, using mechanisms like polyG repeats (33). T. pallidum can utilize host proteins like albumin and others to form a protective coat, further masking itself from the immune system (63). Furthermore, many OMPs have functional redundancy and high genetic variability that protect against immune-mediated damage. In addition, OMPs have low density and spacing that limit antibody binding avidity. T. pallidum can also modify some OMPs, like Tp0435, through palmitoylation (64). This can alter protein location and function, potentially hindering immune recognition. Finally, T. pallidum can infiltrate tissues like the nervous system, placenta, and eyes, where immune activity is restricted, allowing it to persist undetected. Due to the redundancy and functional variability of OMPs, a vaccine that targets multiple conserved epitopes may enhance the chance of overcoming immune evasion. Furthermore, to counter the pathogen’s infiltration of immune-privileged sites, it may be necessary to develop vaccines that elicit strong immune responses at these locations or monoclonal antibodies capable of crossing such barriers.

BIOLOGIC DRUG DEVELOPMENT

Chlamydia trachomatis

Earliest generation C. trachomatis vaccines comprised whole-cell formulations, employing live or attenuated bacteria (65, 66). These vaccines demonstrated robust protection in mice. However, in human trials, the trachoma vaccines failed primarily due to their inability to provide lasting immunity, with protection diminishing significantly within a year. Vaccination also induced delayed-type hypersensitivity, leading to severe inflammatory responses upon subsequent infections (67, 68). Further development of whole-cell vaccines was therefore discontinued. Subunit vaccines present a safer and more targeted strategy, with key C. trachomatis antigens under investigation at present. MOMP, a surface, outer-membrane β-barrel porin, representing ~60% of total membrane protein and crucial for C. trachomatis attachment, has been a favored vaccine target. Structurally, MOMP is complex, consisting of 16–18 transmembrane β-strands arranged in a cylindrical structure, creating a β-barrel. Additionally, MOMP contains 8–10 cysteine residues, atypically high for membrane proteins. These cysteines form intra- and intermolecular disulfide bonds, stabilizing the protein structure and potentially mediating interactions with other proteins. Four variable domains (VDs) (VD1–VD4) are interspersed within the constant domains, contributing to serovar-specific antigenic properties. Within such structural constraints, a handful of C. trachomatis vaccines are currently being developed. These vaccine candidates are adjuvanted to enhance cell-mediated immunity, particularly CD4+ T cells that produce IFN-γ, which is essential for effective Chlamydia clearance.

CTH522 is the most advanced vaccine candidate against C. trachomatis currently in phase II human clinical trials (69). This vaccine is composed of around 570 amino acids derived from residues 24–259 of MOMP from serovar D fused to residues 260–327 of MOMP from serovars D and E, and residues 261–329 from MOMP of serovars F and G. Compared to the full MOMP, CTH522 is truncated and lacks cysteines that have been converted to serine residues. CTH522 does not have a well-defined secondary structure of the typical β-barrel of MOMP. For example, CTH522 spontaneously forms nanoparticles in solution. These nanoparticles are not random aggregates but have a specific, denaturant-dependent structure held together by hydrophobic interactions (70, 71). CTH522 is purified as a recombinant polypeptide from Escherichia coli and is further formulated with CAF01, a liposomal adjuvant system. Another version of CTH522 is formulated with aluminum hydroxide, a traditional adjuvant commonly used in many licensed vaccines. In a phase I clinical study of 35 women, both CAF01 and aluminum hydroxide-adjuvanted formulations induced 100% seroconversion, with CAF01 generating a stronger systemic and mucosal response. A second phase I trial in 65 adults tested intramuscular, intradermal, and ocular administration with CAF01 and CAF09b adjuvants. The CAF01 formulation containing 85 µg antigen elicited the highest serum IgG and ocular IgA levels, highlighting its potential for both urogenital and ocular Chlamydia protection. Phase II trials of CTH522 are ongoing. Additional Chlamydia vaccination strategies have been undertaken based on engineered antigens, immunological mechanisms, and delivery methodologies, all in pre-clinical studies. Key candidates for C. trachomatis surface antigens using various preparations of the entire MOMP have elicited serovar-specific protection in both mice and non-human primates (72). Naturally expressed native major outer membrane protein offers superior efficacy but faces production and scalability challenges due to its complicated molecular structure as described above (73, 74). Recombinant major outer membrane protein from diverse serovars is likely required for broad coverage but necessitates stable, natural folding of this complex antigen, which has been proven difficult (75). Expression of MOMP in cell-free systems has been attempted with varying degrees of success (76, 77). For example, cell-free co-expression of MOMP from Cm was successfully formed within telodendrimeric nanoparticles as proof of concept and then scaled up (76). A novel approach in addressing structural complexity combines the VDs of MOMP from Cm with transmembrane domains of PorB from Neisseria lactamica with promising results in mice (78). Such PorB/VD chimeric proteins are most easily expressed as stable antigens that are immunogenic and cross-reactive with MOMP, suggesting that if combined in a single vaccine, a broad coverage against the major Chlamydia serovars may be possible. Polymorphic membrane proteins (Pmps), a diverse set of surface proteins that hold the potential for cross-serovar protection, have also demonstrated success in the mouse model using Cm (79, 80). Immunization against CPAF, a protein secreted into the cytosol and released into the extracellular milieu upon host cell lysis, has been shown to shorten the duration of infection, reduce inflammation, and may even offer protection against infertility in pre-clinical studies (81, 82). Plasmid-encoded proteins, such as Pgp3, have demonstrated partial protection in mice using both DNA immunization as well as recombinantly expressed Pgp3 homotrimers (83, 84). Additional candidate antigens, such as OmcB and Tarp, also show promise in mouse studies but require further investigation (85, 86). Delivery methodologies such as bacterial ghosts, halobacterial gas vesicles, and chlamydial exosomes, containing both host and Chlamydia proteins, represent exploratory platforms for identifying novel vaccine targets (87–89).

Recently, GrGA, a chlamydial regulatory protein, has been identified as a promising candidate for vaccine development. GrGA functions as a global regulator, binding to specific DNA sequences and activating or repressing the expression of various chlamydial genes. This control over gene expression is crucial for various aspects of the chlamydial developmental cycle, including early infection. GrGA is not found in humans or other bacteria, making it a highly specific target for vaccine development against C. trachomatis (90). In a detailed study that used a whole C. trachomatis proteome to identify immunoprevalent and immunodominant chlamydial proteins, the vast majority of serum antibodies to specific C. trachomatis proteins predicted protection from neither ascension nor repeated infections, except those toward three C. trachomatis antigens—Pgp3, IncV, and CT_619 (a putative type 3 secretion system effector)—where IgG levels were associated with reduced odds of endometrial ascension of C. trachomatis (91). Vaccine development against these antigens holds promise and should be explored. In early 2025, Sanofi (Paris, France) announced an investigational mRNA vaccine targeting C. trachomatis had received fast-track designation from the US Food and Drug Administration (FDA) (92). The vaccine, engineered to elicit both humoral and cellular immune responses against key chlamydial antigens, entered a phase I/phase II clinical trial evaluating its safety, immunogenicity, and dose response in healthy adults aged 18–29. The trial will assess the vaccine’s ability to induce mucosal immunity, critical for preventing primary genital tract infection and re-infection. No further details were disclosed. Vaccine candidates against C. trachomatis are further described in Table 1.

TABLE 1.

Vaccine candidates against C. trachomatis^a

Antigen/platform	Composition	Adjuvant	Outcome	Stage	Reference
CTH522	Residues 24–259 of C. trachomatis MOMP from serovar D fused to residues 260–327 of MOMP from serovars D and E, and residues 261–329 from MOMP of serovars F and G	CFA01 liposomes and separately alum	Both formulations were safe and well tolerated, with CFA01 formulation being more immunogenic than AlOH.	Phase II	(69–71)
mRNA	Not disclosed	Not disclosed	Not disclosed	Phase I/phase II	(92)
nMOMP	Native mouse pneumonitis major outer membrane protein	MF59, LT-K63, or LT-R72	Vaccine showed limited protection against Cm infection and long-term sequelae.	Pre-clinical	(73, 74)
rMOMP	C. trachomatis serovar D (UW-3/Cx) rMOMP (full length) expressed in E. coli	CpG DNA 1826 and Montanide ISA 720	Vaccinated mice were protected against challenges with serovars D and E but not F, based on fertility and pathology.	Pre-clinical	(75)
MOMP-PorB fusion	Cm MOMP loops VD1, VD2, VD3, or VD4 fused to PorB from N. lactamica, expressed in E. coli	CpG DNA 1826 and Montanide ISA 720	PorB/VD chimeric proteins were immunogenic, antigenic, and cross-reactive with Cm MOMP.	Pre-clinical	(78)
CPAF	Recombinant CPAF from C. trachomatis L2 serovar plus murine IL-12	CpG	Vaccine improved fertility in mice following secondary, but not primary, genital chlamydial challenge.	Pre-clinical	(81, 82)
Pgp3 (DNA)	Plasmid DNA containing gene encoding C. trachomatis serovar D Pgp3	None	DNA vaccination prevented salpinx infection in 56% of mice.	Pre-clinical	(83)
Pgp3 (protein)	C. trachomatis D/UW-3/CX homotrimer protein Pgp3	CFA01	Trimer-immunized mice had shortened lower genital tract chlamydial shedding and decreased upper genital tract pathology.	Pre-clinical	(84)
OmcB	OmcB fused to CT521 and expressed recombinantly as chimeric antigen in E. coli	CpG-IFA	Vaccine reduced bacterial shedding in Ct serovar D and Cm challenge in mice and promoted a CD4(+) T cell-dependent protective response.	Pre-clinical	(85)
Tarp	Cm Tarp recombinantly expressed in E. coli	None	Vaccine induced Th1 response with reduced shedding of Cm from the lower genital tract and reduced pathology in the fallopian tubes.	Pre-clinical	(93)
Bacterial ghosts	Recombinant Vibrio cholerae ghosts containing C. trachomatis serovar D MOMP plus OMP2 (rVCG-M1-2)	None	Adoptive transfer of T cells from immunized to naïve mice led to 80% reduction in vaginal shedding upon challenge with C. trachomatis serovar D.	Pre-clinical	(88)
Halobacterial gas vesicles	MOMP, OmcB, and PompD from C. trachomatis serovar B expressed in Halobacterium salinarium gas vesicles (r-Gv)	None	r-Gv induced TLR4/TLR5 in human foreskin fibroblasts, leading to MyD88 activation and TNF-α, IL-6, and IL-12 production in vitro.	Pre-clinical	(87)
Exosomes	Exosomes from Cm-infected HeLa cells	None	Immunized mice had lower inclusion-forming units in lungs upon intranasal Cm challenge.	Pre-clinical	(89)

^a AIOH, aluminum hydroxide; nMOMP, native major outer membrane protein; r-Gv, recombinant gas vesicle; rMOMP, recombinant major outer membrane protein.

While not yet fully a therapeutic reality for STIs, the potential of mAbs in tackling sexually transmitted diseases is garnering interest. Ibalizumab (Trogarzo, TaiMed Biologics Inc), a humanized mAb against HIV-1, remains the only mAb therapeutic licensed for an STI. Approved by the US FDA in 2018, ibalizumab is used in combination with optimized background regimens (OBRs) for patients with limited treatment options due to extensive resistance to conventional HIV antiretrovirals. Ibalizumab binds to the CD4 receptor on host immune cells, blocking post-binding conformational changes required for HIV entry, thereby inhibiting viral replication without compromising normal immune function. Clinical trials demonstrated that 50% of patients achieved virological suppression at 6 months, with sustained suppression in those completing 12 months of therapy. Adding ibalizumab to OBR increased 5 year survival rates from 38% with OBR alone to 47% (94). Thus, ibalizumab plays a crucial role in providing lifesaving therapy for a small but high-need population (~12,000 people in the United States) with multidrug-resistant HIV, demonstrating the value of targeted biologics in addressing complex infectious diseases where conventional treatments fail.

Early attempts at testing potential mAb therapeutics against C. trachomatis were based on murine antibodies derived from mouse immunizations with EBs derived from Cm (also referred to as Chlamydia pneumonitis biovar, mouse pneumonitis [MoPn]). In one study, murine mAbs B-A2, B-B6, B-B5, and B-D3, all of which recognize surface-exposed MOMP epitopes against C. trachomatis serovar B, protected Swiss Webster mice from lethality when the antibody was pre-incubated with twice the LD₅₀ of C. trachomatis EBs and injected into tail veins (95). These mAbs also protected cynomolgus monkey eyes from infection based on microbiologic and clinical measurements, although their relevance to human disease remains to be established. In a study comparing an IgA mAb (MP-A5d) with an IgG3 mAb (MP-33b), both antibodies consistently reduced shedding and mitigated inflammatory responses in the upper genital tract of BALB/c mice, even with high challenges of Cm (96). In another study, an IgA mAb MoPn4-2 was raised against a conformational epitope in MOMP Cm MOMP (97). MoPn4-2 neutralized C. trachomatis in vitro and reduced the number of Cm-infected mice when EBs that were pre-incubated with MoPn4-2 were placed in the uterine horn of BALB/c mice. Furthermore, >70% of antibody-treated mice were fertile in the 50 µg/mL dose compared to 31% of control mice. BALB/c mice passively immunized with MoPn4-2 and challenged intravaginally with Cm demonstrated significant protection as measured by percentage of mice shedding, mean number of IFUs shed per mouse, and length of time of shedding. Pal et al. (98) investigated the ability of three mAbs targeting different parts of C. trachomatis MOMP to neutralize its infectivity. MoPn-23 bound a non-linear epitope, MoPn-40 targeted a linear stretch, and MoPn-32 attached to the chlamydial lipopolysaccharide. In vitro, MoPn-23 was the most potent neutralizer, requiring only 1% of the concentration needed for MoPn-40 to block half of the Chlamydia infection. MoPn-32 showed no neutralizing activity. Further experiments involved injecting mice with each mAb before exposing them to Cm via intranasal challenge. MoPn-23 provided significant protection, evident in smaller weight changes, lighter lungs, and fewer Chlamydia units in the lungs compared to control mice. MoPn-40 offered less protection, while MoPn-32 failed to protect. Interestingly, MoPn-23 effectively shielded both mice with normal immune systems and those lacking immune cells (SCID mice) against the chlamydia challenge. This suggests that in the mouse model, specific MOMP-targeting antibodies can defend against infection even without the usual cellular immune response. In particular, monoclonal antibodies targeting the homologous epitope on C. trachomatis bound by MoPn-23 hold good promise as therapeutic candidates for further development.

In a study that examined whether C. trachomatis infection could be blocked at the level of host epithelia, special antibodies called diabodies (99) were raised against murine epithelial membrane protein 2 (EMP2), a member of the PMP22/GAS3 transmembrane protein family involved in the trafficking of integrins, MHC class I molecules, immunoglobulin superfamily members (e.g., CD54), and GPI-linked molecules in the genital tract and the eye (100). Topical administration of one diabody KS83 into the mouse vagina reduced Cm infection in the oviduct and uterine horn as compared with the control. KS83 also reduced production of proinflammatory cytokines such as IFN-γ, TNF-α, and granulocyte-macrophage colony-stimulating factor in early Chlamydia infection. Neutrophil influx was reduced, as was host tissue damage judged by histopathology. The study by Shimazaki represents a rare example of addressing host factors in limiting infection by an STI pathogen.

In the studies described above, monoclonal antibodies were administered pre-exposure in pre-clinical animal models, whereas for the most part, if successful, these agents will likely be used post-exposure in real-world settings. However, some data suggest therapeutic benefits in established infections, especially when targeting key antigens like MOMP (98) or host factors like EMP2 (100). Translating these findings to human disease will require clinical trials designed to evaluate mAbs in both prophylactic and therapeutic settings, with emphasis on their ability to reduce inflammation and prevent sequelae post-infection.

Beyond mAbs, protein and peptide therapeutics are an emerging platform in the fight against STIs. Their unique advantages, like targeted action, potency, and rapid response, hold promise for overcoming the limitations of traditional mAbs and preventing the spread of these infections. In an interesting study on C. trachomatis, certain N-benzyloxycarbonyl hydrophobic dipeptide-containing aldehydes or fluoroketones were found to be effective for the inhibition of intracellular growth of C. trachomatis serovar L2 and D. The target enzyme for these dipeptides is intracellular calpain, whose inhibition via small interfering RNA (siRNA) paradoxically had no effect on chlamydial growth (101). Hence, data need to be interpreted with caution. Antimicrobial peptide protegrin-1 completely prevented productive infection of C. trachomatis serovars L2, D, and H1 when pre-incubated in the presence of fetal calf serum with EBs in McCoy cells and also blocked EB formation after successful infection of cells by C. trachomatis (102). Protegrins 1, 2, 3, and 5 were also found to be effective against complement-resistant N. gonorrhoeae strain FA19 (103). In another study, plasmid DNA containing antimicrobial peptide melittin suppressed C. trachomatis infection upon vaginal administration and induction of gene transcription with doxycycline, suggesting a key role played by melittin as an anti-Chlamydia peptide (104).

Recently, ΦCPG1 was discovered as a lytic phage specific to C. trachomatis (105). In this study, ΦCPG1 significantly inhibited Chlamydia caviae growth by inducing abnormally large reticulate bodies, suggesting a potential therapeutic avenue. Previously, a study utilizing M13 bacteriophage explored antigenic fitness that could be achieved by imposing conformational constraints on a 12-residue stretch from the VD1 loop of C. trachomatis serovar C (106). Building upon this finding, Bhattarai et al. utilized M13 phages’ genetic malleability to create nanofibrous scaffolds exhibiting various signaling peptides, mimicking cellular microenvironments, and influencing cell fate through biochemical and physical stimuli (107). Focusing on C. trachomatis, they engineered a novel M13 phage named M13-RGD8-PmpD3 expressing two functional peptides: RGD (integrin-binding) and C. trachomatis-associated protein PmpD, both an autotransporter and a pan-neutralizing antigen. RGD facilitates integrin-mediated endocytosis, while the conserved portion of a C. trachomatis polymorphic membrane protein (PmpD) expressed on the phage surface potentially interferes with C. trachomatis propagation. M13-RGD8-PmpD3 pre-treatment or co-treatment significantly reduced C. trachomatis infection in both HeLa and primary endocervical cells. C. trachomatis inclusion size and number decreased notably in both pre- and co-treatment experiments with M13-RGD8-PmpD3, suggesting disruption of C. trachomatis’s nutrient acquisition from the host cell. Blocking PmpD on M13-RGD8-PmpD3 with specific antibodies significantly reduced its inhibitory effect, confirming the direct role of PmpD in targeting C. trachomatis inclusions. This study demonstrates the promising potential of engineered M13 phage as a novel strategy against C. trachomatis infections, especially given that these phage matrices can deliver large payloads (>1.5 × 10¹³ epitopes per cm²) without compromising phage integrity. Table 2 contains further details on mAbs, proteins, and phage candidates against C. trachomatis.

TABLE 2.

Biologic drug candidates at different stages of development against Chlamydia trachomatis

Modality	Name	Antigen or production method	Remarks	Stage	Reference
mAbs	B-A2, B-B6, B-B5, and B-D3	MOMP	mAbs protected mice from toxic death after i.v. injection of EB serovar B and blocked ocular infection in monkeys.	Pre-clinical	(95)
	MP-A5 and MP-33b	MOMP	Both serovar-specific and serogroup-specific mAbs protected mice from toxic death.	Pre-clinical	(108)
	Mo-33b	MOMP	mAb conferred immunity to re-infection and reduced chlamydial shedding by >100×.	Pre-clinical	(52)
	MoPn 4–2 (IgA)	MOMP	mAb protected mice against infertility and vaginal shedding.	Pre-clinical	(97)
	MoPn-23 and MoPn-40	MOMP	MoPn-23, and to a lesser extent MoPn-40, protected mice against intranasal challenge with Cm.	Pre-clinical	(98)
	KS83	Epithelial membrane protein 2	Pre-treatment with KS83 reduced bacterial load, tissue production of inflammatory cytokines, recruitment of polymorphonuclear leukocytes, and local tissue inflammation.	Pre-clinical	(100)
Proteins/peptides	Protegrin-1	Synthetic protegrin made using solid-phase chemistry	Protegrin rapidly inactivated C. trachomatis serovars L2, D, and H.	Pre-clinical	(102)
	RTD-1, protegrin-1, cryptdin-4, and indolicidin	RTD-1 extracted from rhesus macaque leukocytes by RP-HPLC. Other peptides chemically synthesized.	Protegrin-1 had the strongest antichlamydial activity; susceptibility of L2 > serovar E > MoPn (Cm).	Pre-clinical	(109)
	Melittin	Plasmid expressing melittin under control of tetracycline-responsive promoter of human CMV	Peptide was effective in keeping 50% of mice pathogen-free.	Pre-clinical	(104)
Phage	ΦCPG1	Lytic phage specific to C. trachomatis	ΦCPG1 blocked C. trachomatis entry into HeLa cells in a dose-dependent manner.	Pre-clinical	(105)
	M13-RGD8-PmpD3	Engineered M13 filamentous bacteriophage expressed RGD peptide motif	M13-RGD8-PmpD3 pre-treatment or co-treatment reduced C. trachomatis infection in both HeLa and primary endocervical cells.	Pre-clinical	(107)

In summary, early whole-cell vaccines against C. trachomatis failed due to transient immunity and inflammatory responses, leading to the development of antigen-focused approaches targeting MOMP, Pmps, and CPAF to enhance immune protection. Monoclonal antibodies against MOMP also show pre-clinical promise, while engineered M13 phage nanoparticles and antimicrobial peptides such as LL-37 and melittin are being explored for targeted infection control, though delivery and stability challenges remain. CTH522 remains the most viable vaccine candidate for near-term clinical impact, together with Sanofi’s mRNA vaccine, while monoclonal antibodies offer future potential with further refinement. Despite promising pre-clinical findings for various biologic candidates, significant challenges remain, highlighting the need for optimized formulations and translational strategies to accelerate clinical progress.

Neisseria gonorrhoeae

On the gonorrhea front, there has been good recent progress in gonococcal vaccine development. Vaccines containing outer membrane vesicles (OMVs) have shown promise in protecting against bacteria, like Neisseria meningitidis (Nm). Efforts in New Zealand’s mass vaccination campaign with MeNZB, a local epidemic-causing strain-derived MenB OMV vaccine, revealed a retrospective and serendipitous cross-protective effect against gonorrhea. Vaccinated individuals were 31% less likely to contract gonorrhea compared to those without vaccination (110). Another MenB vaccine, 4CMenB, also offers protection against gonorrhea, while Trumenba that uses factor H-binding protein (FHbp) alone does not (111–113). 4CMenB incorporates an OMV derived from the New Zealand Nm epidemic strain (i.e., MeNZB) alongside three recombinant proteins: neisserial adhesin A, neisserial heparin-binding antigen (NHBA), and FHbp. Notably, NHBA has shown promise in generating antigonococcal antibodies, making 4CMenB potentially relevant for future gonorrhea prevention strategies. In the United States, a large-scale observational study involving over 165,000 subjects reported among 109,000 individuals in two US cities revealed a significant association between 4CMenB vaccination and reduced risk of gonorrhea. Individuals who completed the 4CMenB vaccination series exhibited a 40% lower risk of contracting gonorrhea, while those who received partial vaccination demonstrated a 26% reduction (112). This observed vaccine effectiveness mirrors the 31% protection against gonorrhea noted in the New Zealand MeNZB study (110). Of note, 4CMenB vaccination did not show any protective effect against co-infection with gonorrhea and Chlamydia (112). This discrepancy might be rooted in the complex interplay between chlamydia and the host’s immune response to gonorrhea. C. trachomatis appears to dampen neutrophil activity, crucial for clearing bacterial invaders, and alters the immune landscape at the infection site, creating a more permissive environment for N. gonorrhoeae to thrive. This raises questions about the effectiveness of 4CMenB in the field where co-infections are common. Nonetheless, 4CMenB’s effectiveness against gonorrhea remained consistent throughout the 3 year study period, regardless of assuming 6 or 12 month duration of protection. These findings offer compelling evidence for the potential of 4CMenB vaccination as a valuable tool for curbing gonorrhea rates. Efforts to elucidate the protective antigens in 4CMenB have revealed bactericidal activity of antibodies directed against PorB, LOS, and NHBA (114–116).

Building on the success of 4CMenB, an OMV-based vaccine named NgG that is derived from antigenic components such as OpaA, PorB, and LOS contained within N. gonorrhoeae was advanced into phase II stage (NCT05630859) for efficacy in adults after promising safety results in phase I (FDA Fast Track) by GlaxoSmithKline (GSK, Brentford, UK). NgG is based on OMVs absorbed on aluminum hydroxide, where LOS is genetically detoxified by deleting lpxl1, which results in a pentacylated LOS that does not engage TLR4. These vesicles also lack RmpM (also called protein III) to enhance bactericidal activity because antibodies against N. gonorrhoeae RmpM are often “blocking” in nature (i.e., they block killing by otherwise bactericidal antibodies) (117). In a recent announcement, GlaxoSmithKline terminated further development because the NgG candidate did not meet pre-defined efficacy criteria, representing a setback to the overall gonorrhea vaccine clinical pipeline. No further details were provided.

At the pre-clinical stage, a gonorrhea vaccine, NGoXIM, combines gonococcal outer membrane vesicles with IL-12 microspheres for intranasal delivery, with the aim to drive CD4⁺ Th1-mediated mucosal immunity. Early animal trials showed a strong and durable immune response (118). The most advanced version of this vaccine contains OMVs isolated from the double-mutant (ΔlpxL1 Δrmp) N. gonorrhoeae strain MS11, wherein the gene encoding for the enzyme LpxL1 is deleted, eliminating one of the six acyl chains of lipid A, as well as deletion of the gene encoding for Rmp responsible for inducing subversive antibodies that block complement-mediated killing of N. gonorrhoeae (119). This genetically modified OMV generates antigens that are likely to be more tolerogenic in humans and more effective, as they do not induce subversive antibodies to Rmp. Intranasal immunization with IL-12 microspheres promotes the development of Th1 cells (IFN-γ-producing CD4+ cells) in the iliac lymph nodes, given that resistance to genital gonococcal infections depends on both IFN-γ and B cells. Intranasal immunization of mice with IL-12 microspheres did not increase Th2 (IL-4-producing CD4+ cells) levels on the one hand while increasing levels of Th17 (IL-17A-producing CD4+ cells) on the other hand. Given that the unified mucosal immune system links immune responses across different mucosal sites, intranasal immunization with NGoXIM can activate mucosal-associated lymphoid tissue, inducing antigen-specific IgA and T-cell responses that provide cross-protection in the genital mucosa, offering a strategic approach for combating N. gonorrhoeae infections. These promising results pave the way for NGoXIM to advance toward human safety and efficacy studies. Key issues will be translating animal data to human disease, given that N. gonorrhoeae is human specific.

Gonococcal LOS has emerged as a promising vaccine antigen. While susceptible to phase variation, LOS has surface abundance and accessibility, critical qualities for an effective antigen. Although some LOS components share undesirable similarity to human structures, two distinct epitopes recognized by monoclonal antibodies 2–1-L8 and 2C7 offer unique targeting options. The 2C7 epitope’s broad conservation and immunogenicity during natural infections solidify its appeal as a vaccine target (120, 121). Given the complexities of purifying or synthesizing oligosaccharides, creating peptide mimics (“mimitopes”) with similar structures as the 2C7 epitope offers an attractive alternative (122). Subsequent work by Gulati et al. (123) demonstrated that immunization with this mimitope paired with a Th1-stimulating adjuvant GLA-SE significantly boosts bactericidal IgG production, reduces colonization, and accelerates bacterial clearance in infected mice. Whether peptide mimics of carbohydrate antigens can elicit a durable and broadly protective memory response remains an open question.

Other pre-clinical candidates have also emerged in the gonococcal vaccine arena, such as gonococcal NHBA. This surface-exposed antigen plays a crucial role in N. gonorrhoeae interactions with host cells, making it a prime target for inducing protective immune response. NHBA exhibits remarkable stability across N. gonorrhoeae genomes, with high sequence identity among isolates. Furthermore, recombinant NHBA has proven its ability to effectively stimulate the immune system, generating antibodies that recognize a broad spectrum of N. gonorrhoeae NHBA variants (124). These antibodies not only neutralize the bacteria but also enhance their clearance through complement activation (C3 deposition and opsonophagocytosis). NHBA’s importance extends beyond immune recognition. It contributes significantly to N. gonorrhoeae’s pathogenic arsenal, playing a key role in both microcolony formation and epithelial cell adherence. Targeting NHBA could therefore disrupt these crucial steps in the colonization process. To optimize NHBA for enhanced immune responses and vaccine production, Taha et al. genetically fused the C-terminal fragment of NHBA (cNHBA) with full-length MetQ or its N-terminal (Met1) and C-terminal (Met2) fragments, adjuvanted with aluminum hydroxide (125). All cNHBA and MetQ fragments, as well as fusion antigens, were immunogenic in mice, eliciting predominantly IgG1 antibodies that demonstrated bacterial killing via serum bactericidal activity (SBA), opsonophagocytic activity (OPA), and reduced adherence to cervical and urethral epithelial cells. Among the fusions, MetQ-cNHBA and cNHBA-Met2 induced the highest SBA, OPA, and adherence-blocking titers, making them promising candidates for N. gonorrhoeae vaccine development.

Yet another interesting set of gonococcal vaccine targets is the transferrin-binding proteins, TbpA and TbpB. These antigens are highly conserved, are not subject to antigenic variation, and have been validated in a human male challenge model of infection (126). Intranasal, but not parenteral, immunization of mice with full-length TbpA/TbpB made in E. coli and chemically conjugated to cholera toxin B subunit resulted in bactericidal antibodies (127). Chimeric TbpA/TbpB that is genetically fused to cholera toxin produced 25-fold weaker responses in the same model system (128). Other surface-exposed, highly conserved, and immunogenic gonococcal antigens include NspA (129), MsrA/MsrB (130), MtrE (131), and Acp (132). Of these, MtrE is particularly interesting in that when its 13 amino acid surface-exposed loop 2 antigen is expressed as a fusion protein and combined with a Th1 polarizing adjuvant such as CpG 1826, the resulting vaccine induces both a prophylactic as well as therapeutic (i.e., attenuates colonization when administered during infection) responses in a mouse vaginal challenge model of N. gonorrhoeae (133), making it an attractive gonococcal vaccine candidate. Surprisingly, a pair of intracellular cell division proteins—NGO1549 (FtsN) and NGO0265—identified by an artificial intelligence-driven platform and fused into a single chimeric antigen induced bactericidal antibodies against a wide array of gonococcal isolates and reduced both the duration as well as the burden of gonococcal colonization of mouse vaginas (134). How such intracellular proteins become effective vaccine antigens remains to be understood. Table 3 contains additional details on vaccine candidates against N. gonorrhoeae.

TABLE 3.

Vaccine candidates at different stages of development against Neisseria gonorrhoeae^a

Antigen/platform	Composition	Adjuvant	Outcome	Stage	Reference
MenZB	OMV derived from Neisseria meningitidis NZ98/254	AlOH	Vaccine effectiveness after adjustment for ethnicity, deprivation, geographical area, and sex was 31%.	Retrospective case–control study	(110)
4CMenB	Neisseria meningitidis adhesin A, factor H-binding protein fused with GNA2091, NHBA fused with GNA1030, and OMV NZ98/254 containing PorA1.4	AlOH	Estimated vaccine effectiveness in adolescents and young adults was 32.46%.	Cohort and case–control study	(112, 113, 135)
NgG	OMV from N. gonorrhoeae FA1090 ΔLpxL1/ΔRmpK containing OpA, PorB, and LOS	AlOH	Vaccine was safe and well tolerated in healthy adults.	Phase II (terminated)	https://clinicaltrials.gov/ct2/show/NCT05630859
NgoXIM	OMV from N. gonorrhoeae MS11 ΔLpxL1/ΔRmpK formulated in polylactic acid microspheres containing IL-12	None	Intranasal immunization induced serum IgG, salivary IgA, and vaginal IgG and IgA anti-N. gonorrhoeae antibodies. In addition, vaccinated mice resisted challenge with N. gonorrhoeae.	Pre-clinical	(118, 119)
LOS peptide mimic	Synthetic tetra-multi-antigenic peptide	GLA-SE	Vaccine elicited bactericidal IgG and reduced N. gonorrhoeae colonization while accelerating clearance.	Pre-clinical	(123)
NHBA	Full-length or NHBA-c expressed in E. coli	Freunds or separately AlOH	Vaccination produced predominantly IgG1 antibodies with bactericidal and opsonophagocytic activity, with slightly higher titers seen for NHBA-c.	Pre-clinical	(124)
TbpA/TpbB (full length)	Full-length TbpA and TbpB made in E. coli and chemically conjugated to cholera toxin B subunit	None	Intranasal vaccination resulted in IgA and IgG antibodies, with bactericidal activity from both immunogens.	Pre-clinical	(127)
TbpA/TpbB (chimeras)	Amino acids 2–406 from the N-terminal domain of TbpB (NB) fused to the A2 domain of cholera toxin. Separately, amino acids 198–280 from L2 of TbpA fused between NB and cholera toxin. Both immunogens were expressed in E. coli along with cholera toxin B subunit.	None	Both immunogens induced bactericidal and vaginal growth-inhibiting antibodies when administered intranasally or subcutaneously. Responses were ~25× weaker than full-length immunogens.	Pre-clinical	(128)
NspA	Plasmid encoding NspA and with rNspA protein used as booster	None	Mice generated anti-NspA IgG and IgA in serum and in vaginal wash fluids, with IgG having bactericidal and opsonic activities.	Pre-clinical	(129)
MsrA/MsrB	Recombinant protein made in E. coli	CFA/IFA or AlOH	Antibodies induced by CFA/IFA but not AlOH were bactericidal and opsonophagocytic.	Pre-clinical	(130)
MtrE	Loop 2 fused either as linear or as structural peptide to the N-terminus of the oligomerization domain of chicken C4b-binding protein (IMX315), expressed in E. coli	CpG 1826	Loop2-based antigens induced bactericidal antibodies and provided protection against N. gonorrhoeae challenge in the mouse vaginal tract infection model with accelerated clearance.	Pre-clinical	(133)
Acp	Full-length and mature proteins expressed in E. coli	Liposomes containing MPLA or separately AlOH	Vaccine induced bactericidal antibodies.	Pre-clinical	(132)
MetQ (tagged)	Full-length protein expressed in E. coli	CFA/IFA	Induction of antibodies that were bactericidal and reduced adherence to cervical epithelial cells	Pre-clinical	(136)
MetQ (untagged)	Full-length protein expressed in E. coli as untagged protein	CpG	Immunized mice cleared infection faster and had lower N. gonorrhoeae burden than control.	Pre-clinical	(137)
NGO1549/NGO0265	Artificial intelligence designed chimeric proteins expressed in E. col	GLA-SE	Vaccine induced bactericidal antibodies and reduced duration and burden of gonococcal infection in a mouse model.	Pre-clinical	(134)

^a NHBA, Neisseria heparin-binding antigen; NHBA-c, C-terminal fragment of NHBA.

Regarding mAb therapeutics, a monoclonal antibody specific to gonococcal LOS (2C7) recognizes an epitope that has been shown to be important for immune defense mechanisms and bacterial virulence strategies (121). The parent murine mAb 2C7, as well as a human Fc chimera, was shown to be effective against gonococci in mouse models, both as purified mAb administered to mice or DNA-encoded mAb (11, 138, 139). Complement, but not FcγR, played a vital role in driving the efficacy of this antibody, as no efficacy was observed in C9^−/− mice (138). These findings have significant implications for antibody-based therapeutics against mucosal pathogens. While meningococcal capsular polysaccharide conjugate vaccines rely on antibody-mediated elimination at mucosal surfaces, the mechanisms involved (complement, phagocytes, or both) remain unclear. Studies by Gulati et al. establish complement as a key player in 2C7’s success, highlighting its potential as a valuable correlate of protection for gonococcal vaccine development. The presence of sufficient complement in the genital tract, though lower than that in serum (140), is supported by various sources: plasma exudation, local production by epithelial cells (141), and contribution from neutrophils and macrophages (142). Recent findings revealed that the terminal Gal residue of the lactose extension from HepII on LOS can be sialylated, enhancing complement resistance and boosting gonococcal colonization in the mouse genital tract (121). This explains the widespread presence of the 2C7 epitope and the reduced virulence of mutants lacking HepII lactose, making both the 2C7 targeted epitope as well as the 2C7 antibody relevant in the treatment of gonorrhea. Moreover, lactose extensions from HepII require LOS glycosyltransferase G, whose expression is phase variable. Thus, if lgtG is turned off due to slipped-strand mispairing, resistance to 2C7 would come at a significant fitness cost for the bacteria (121).

Another interesting, potentially pan-bacterial STI mAb (F598) binds the ubiquitously present poly-N-acetylglucosamine (PNAG) (143). A study using antibodies raised against a synthetic PNAG nonasaccharide or a human IgG1 mAb recognizing both native and deacetylated PNAG effectively bound PNAG in vivo, killed diverse microbes and fungi, and protected mice against bacterial, fungal, and even plasmodial infections, making anti-PNAG antibodies a possibility for broad-spectrum immunoprophylaxis (144). The key mechanism underlying PNAG-mediated protection involves Fc-mediated immune clearance or, more commonly, opsonization and complement deposition onto microbial surfaces. Clinical development of PNAG-based immunotherapies is already under way, with mAb F598 successfully completing a phase I clinical trial for safety and pharmacokinetics, showing no significant adverse events. Despite broad expression of PNAG on a diverse set of microbes, pre-clinical tests revealed no binding to human and monkey tissues and acceptable safety profiles in mice (145). Earlier, a small randomized, controlled trial conducted using the N. gonorrhoeae CHIM model (57) showed no protection offered by mAb F598, suggesting that circulating monoclonal bactericidal antibodies might be insufficient for protection against N. gonorrhoeae infection in men. The potential role of F598 equilibration within urogenital tissues or with effector cells to enhance antibacterial responses following antibody infusion remains unexplored.

On the translational side, prophylactic use of mAbs could involve passive immunization via mucosal delivery such as vaginal or rectal gels, dissolvable films, and topical formulations, providing temporary immunity in high-risk populations, while therapeutic applications could involve systemic administration to clear infections, particularly for antibiotic-resistant strains. Commercial feasibility of mAbs depends on production costs, durability of protection, and delivery mechanisms. While systemic mAb therapies are costly, localized or mucosal delivery could reduce dosing requirements and improve cost-effectiveness. Additionally, long-acting mAbs, potentially administered via injections similar to HIV pre-exposure prophylaxis, could enhance market viability. Successful commercialization will require demonstrated clinical efficacy, scalable production, and cost reductions through optimized antibody engineering or alternative bioproduction strategies.

A well-characterized example of a protein therapeutic against gonorrhea is C4BP-IgM, a novel antimicrobial fusion protein (146). The concept behind this therapeutic is the finding that of 190 clinical isolates from Sweden collected over the past decade, 89.7% of PorB1a and 19.3% of PorB1b strains bind C4BP, a strategy used by N. gonorrhoeae to evade the human complement system. C4BP-IgM is designed to counter this evasion by combining only the first two domains of C4BP with the Fc portion of IgM. Lack of the CCP3 domain for C4b binding and the β chain of C4BP for apoptotic cell binding in this construct ensures that complement is not inactivated by the chimeric molecule, and off-target binding and complement activation do not occur, respectively. Thus, C4BP-IgM selectively binds to the bacterial surface and triggers complement activation and bacterial killing through the formation of MACs. In a mouse vaginal colonization model, C4BP-IgM demonstrated efficacy in clearing infections both in terms of infection duration and bacterial load, highlighting the potential of C4BP-IgM as a targeted therapeutic for gonococcal infections. Of note, two fusion proteins—C4BP-IgG and C4BP-IgM—were tested, but only C4BP-IgM effectively competed with C4BP for bacterial binding and was more potent in vivo on a molar basis, suggesting a requirement for the polymeric form of C4BP for stable interaction with PorB.

A second interesting antigonococcal protein candidate is the Fc fusion (FH-Fc) containing the N. gonorrhoeae-binding domains of human FH, a key inhibitor of the alternative pathway of complement. The C-terminus of FH (domains 19–20) binds to sialylated LOS on gonococci; the sialic acid–FH interaction on N. gonorrhoeae also requires N. gonorrhoeae PorB (147). As in C4BP-IgM, several FH-Fc constructs were designed and tested. Initially, a molecule combining gonococcal-binding FH domains 18–20 with a D1119G mutation in the FH domain 19 (FH*/Fc) and fused N-terminally to human IgG1 Fc was found to prevent human RBC lysis while retaining activity against gonococci (148). FH*/Fc showed promise in killing Ng in vitro and reducing bacterial burden in a mouse model. To further optimize the construct, additional variants were developed, of which Fc3/FH*, wherein the FH segment was placed C-terminal to the Fc domain of human IgG3, displayed superior binding to N. gonorrhoeae and complement activation compared to other designs (149). Fc3/FH* effectively killed all sialylated PorB1b N. gonorrhoeae strains tested but only a minority (2 of 15) of PorB1a strains. When administered intravaginally, Fc3/FH* significantly reduced bacterial burden and shortened the duration of colonization in the mouse model. Importantly, Fc3/FH* retained its activity after lyophilization or spray-drying, suggesting potential for formulation to allow topical delivery. In fact, a compelling case can be made for combination therapy with C4BP-IgM for PorB1a and FH*/Fc targeting PorB1b isolates. This combination targets distinct molecules crucial for immune evasion (complement inhibition) and gonococcal viability (PorB), minimizing resistance development. “Escape mutants” arising from treatment that lack the ability to bind FH and/or C4BP would incur significant fitness costs and rapidly be cleared by the host’s immune system.

Reconciling pre-clinical data on protein and peptide therapeutics, particularly for mucosal surfaces, with real-world application will vary by case. For example, vaginal administration of melittin-containing plasmid DNA has been shown to suppress experimental C. trachomatis infection, while topical delivery of Fc fusion proteins like Fc3/FH* has demonstrated reductions in bacterial load in experimental gonococcal infections. However, real-world treatments present timing and efficacy challenges. For instance, protegrin-1, which can block both chlamydial EB formation and productive infection, suggests some potential for post-exposure use, although this is not as extensively validated in models. Therapies like C4BP-IgM or FH*/Fc, designed to counter immune evasion mechanisms of pathogens, offer potential in established infections. Mucosal surfaces themselves introduce challenges, such as high microbial loads, immune responses, and environmental variability (e.g., pH and pathogen adhesion), which can affect the potency of therapies like host defense peptides (HDPs), such as LL-37, discussed in detail later in this review. However, promising therapeutic examples, such as C4BP-IgM fusion proteins and Fc3/FH*, demonstrate both prophylactic and therapeutic efficacies in post-exposure settings by targeting bacterial surfaces and leveraging immune activation. In this case, microbial endpoints for uncomplicated infections would include nucleic acid amplification test 2 weeks after treatment. While these candidates have shown promising microbial endpoint reductions, they have not yet entered early clinical trials, with further development and regulatory work needed to transition these biologics into human studies.

No phages active against N. gonorrhoeae have been found despite the fact that every N. gonorrhoeae isolate sequenced to date contains phage DNA sequences (150) and that filamentous phages produced by N. gonorrhoeae are a mixture of Ngoφ6, Ngoφ7, and Ngoφ8 (Ngofil). Interestingly, meningococcal disease-associated phage (MDAφ) belonging to the Nf1 genogroup of Neisseria meningitidis infects the host via pili, replicates within the cytoplasm, and integrates into the bacterial genome to produce prophage, which is closely related to phage Ngoφ6 found in N. gonorrhoeae. N. gonorrhoeae strain ExNg63 isolated from cerebrospinal fluid of a patient with gonococcal meningitis had 90% similarity to Nf1 prophage MDAφ, suggesting that Nf1 prophage may be more widely distributed among the Neisseria genus, especially N. lactamica and Neisseria cineria (151), and raising the question of whether Nf1 prophages can act as accessories in the colonization of N. gonorrhoeae. In a study wherein rabbits were orally vaccinated with Salmonella Typhimurium strain χ3987 harboring phagemid NgoΦ6 fm, induction of antiphage IgG and IgA antibodies that demonstrated N. gonorrhoeae bactericidal activity was observed (152). In a follow-up study, when Ngoφfil was injected subcutaneously into rabbits, immune sera containing antiphage IgG and IgA antibodies bound to the surface of N. gonorrhoeae (153). Immune sera had bactericidal activity and blocked adhesion of gonococci to tissue culture cells. Collectively, these data demonstrate that N. gonorrhoeae filamentous phage may be potential therapeutic candidates to counter its rising antibiotic resistance.

A novel concept worth discussing, called phage steering, has emerged as a promising therapeutic strategy. In this approach, phages actively steer bacterial colonies toward antibiotic susceptibility rather than simply killing them (154). An example is multi-drug-resistant Pseudomonas aeruginosa. Steering this pathogen toward vulnerability utilizes phage OMKO1 that targets a specific protein (OprM) in P. aeruginosa, acting as its receptor (155). P. aeruginosa develops resistance to OMKO1 by downregulating OprM. However, this adaptation makes them susceptible to certain antimicrobials. Combining OMKO1 with antimicrobials in a phage cocktail proves even more effective than either treatment alone. Beyond OMKO1, Acinetobacter baumannii phages φFG02 and φCO01 have also been shown to steer this pathogen toward susceptibility by causing capsule loss, exposing it to antimicrobials and other phages (156). Thus, phage steering has potential toward STI pathogens like N. gonorrhoeae that have historically shown resistance toward antimicrobial drugs. Table 4 further describes mAb, protein, and phage therapeutics being developed against N. gonorrhoeae.

TABLE 4.

Biologic drug candidates at different stages of development against Neisseria gonorrhoeae^a

Modality	Name	Antigen or production method	Remarks	Stage	Reference
mAbs	2C7 mAb	N. gonorrhoeae LOS	mAb was effective in clearing bacteria, in wild-type, and in (humanized) FH/C4BP dual transgenic mice.	Pre-clinical	(121, 138, 157, 158)
	DNA enclosing 2C7 (DMab)	N. gonorrhoeae LOS	2C7 DMAbs were rapidly produced and persisted for 4 months and facilitated rapid clearance following N. gonorrhoeae challenge.	Pre-clinical	(139)
	mAb F598	PNAG expressed on gram-negative bacteria and other microbes	PNAG mAb-mediated complement-dependent bactericidal killing and protected mice against local/systemic infections.	Pre-clinical	(143, 145)
Protein/peptides	Protegrins 1, 2, 3, and 5	Native as well as synthetic peptide of porcine origin	PG-1 was most potent against clinical isolates of N. gonorrhoeae.	Pre-clinical	(103)
	LL-37	Synthetic peptide	Peptide demonstrated bactericidal activity; efficacy decreases with N. gonorrhoeae LOS sialylation.	Pre-clinical	(159, 160)
	C4BP-IgM	Fusion of human C4BP α-chain domains 1 and 2 with Fc portion of IgM	Hexameric C4BP-IgM attenuated the duration and burden of colonization of N. gonorrhoeae in a mouse vaginal colonization model.	Pre-clinical	(146)
	FH-Fc	Fusion of human FH N. gonorrhoeae-binding domains with Fc portion of IgG	FH-Fc reduced the duration and burden of vaginal colonization of three N. gonorrhoeae strains (including two antimicrobial-resistant strains).	Pre-clinical	(148, 149, 161)
Phage	NGOφ6/X3987	NgoΦ6 delivered by the S. enterica χ3987, serovar S. Typhimurium	Oral immunization of rabbits resulted in bactericidal antibodies.	Pre-clinical	(152)
	Nfil	Purified phage preparations	Parenteral immunization of rabbits elicited sera that had bactericidal activity and blocked adhesion of gonococci to tissue culture cells.	Pre-clinical	(153)

^a PNAG, poly-N-acetylglucosamine.

To summarize, OMV-based vaccines derived from N. meningitidis have shown partial but consistent cross-protection, with 4CMenB vaccination associated with a 40% reduction in gonorrhea risk. They remain the best prophylactic approach available to date to control the spread of gonorrhea. The NGoXIM vaccine, which combines OMVs with IL-12 microspheres for intranasal delivery, has demonstrated strong and durable immune responses in pre-clinical models by eliciting Th1-mediated mucosal immunity, positioning it as a promising candidate for clinical development. Monoclonal antibodies such as 2C7, targeting conserved N. gonorrhoeae lipo-oligosaccharides, have shown strong bactericidal activity in pre-clinical studies and may be viable for passive immunization strategies. Engineered bacteriophage approaches, particularly phage steering, aim to drive antibiotic-resistant strains toward susceptibility rather than direct bacterial clearance, but no gonococcal-specific therapeutic phages have been identified. Thus, OMV-based antigonococcal vaccines and monoclonal antibodies are the most promising biologic candidates, though neither has progressed beyond early-stage human studies, and phage-based strategies remain in pre-clinical development. Future efforts should prioritize broadening vaccine antigen targets and accelerating monoclonal antibody clinical translation to address the threat of antimicrobial-resistant gonorrhea.

Herpes simplex virus type 2

Among the STI pathogens reviewed in this article, HSV-2 has been the subject of most extensive vaccine development and testing over the past decades (162). Several candidates reached late-stage human clinical trials, but none has succeeded yet. A diverse set of platforms has been used to design HSV-2 vaccines, including live-attenuated virus (LAV), adjuvanted protein subunit vaccines, and nucleic acid vaccines (DNA and mRNA). For preventative vaccines, the focus has been on eliciting neutralizing antibody responses, which are mostly directed against glycoprotein D (gD2) and gB2. Of these, the gD2 subunit vaccine adjuvanted with alum and monophosphoryl lipid A (MPL) most recently completed phase III clinical trials, but without success. Two trials were conducted: the first enrolled participants with no prior exposure to either HSV-1 or HSV-2 (double seronegative); the second included participants with any HSV serological status, both studies measuring the vaccine’s effectiveness in preventing genital herpes disease. In the double-seronegative group consisting of men and women, the vaccine reduced disease by 38% compared to the placebo group. However, the vaccine was not efficacious in women who were seropositive to HSV-1, and it was ineffective in men, suggesting interference with vaccine-mediated immune responses against HSV-2 as well as gender-related differences (163). In a follow-up phase III study that monitored herpes genital lesions caused by HSV-1 or HSV-2 in double-seronegative women, the overall vaccine efficacy was 20%. The vaccine was not efficacious against HSV-2 genital disease or HSV-2 infection but reduced the risk of HSV-1 infection and disease (164). More recently, a single-cycle ΔgD-2 vaccine carrying a gD deletion was tested in a mouse model in the presence of alum/MPL adjuvant (165). The ΔgD-2 vaccine provided superior long-term protection against lethal HSV-2 challenge. ΔgD-2 also elicited a durable neutralizing antibody as well as antibody-dependent cellular cytotoxicity (ADCC) response, effectively eliminating infected cells for over a year in the pre-clinical study. In a different study conducted earlier by another sponsor, a randomized, placebo-controlled trial using a vaccine containing two HSV-2 glycoproteins, gB2 and gD2, combined with the adjuvant MF59 showed a promising initial effect during the first 5 months: time to infection was reduced by 50% compared to those who received the placebo. However, this benefit did not last beyond the initial 5 month period, and no significant difference in infection rates was observed between the vaccinated and placebo groups after that point (166).

Past HSV-2 vaccines likely failed due to a combination of immune evasion by the virus, inadequate immune activation, and vaccine design limitations. Pre-existing immunity to HSV-1 may have altered the quality of the immune response, particularly in HSV-1 seropositive individuals, leading to reduced vaccine efficacy. Many past vaccine candidates focused on eliciting antibody responses but failed to induce strong CD4+ and CD8+ T-cell immunity, which is crucial for clearing infection and preventing re-activation. Short-lived immunity was another challenge, as some vaccines initially showed promise but failed to sustain long-term protection, possibly due to repeated subclinical re-activation of HSV-2. An overreliance on gD2 as a target may have contributed to failure, as HSV-2 can downregulate gD expression in vivo, reducing immune recognition. Critically, most vaccine candidates focused on preventing initial infection but did not address the virus’s ability to establish latency in sensory neurons, allowing for re-activation and recurrent disease. Future vaccine strategies must counteract these challenges by incorporating stronger T-cell-based immunity, broadening antigenic targets beyond gD2, optimizing adjuvants, and accounting for prior HSV-1 exposure in clinical trial design.

In addition to the preventative vaccines described above, a number of therapeutic vaccine approaches have also been tested. An LAV ΔgH HSV-2 vaccine with the gH protein deleted was found to be safe in a phase I/phase II human clinical trial but was ineffective in improving symptoms or viral activity among healthy men and women (167). Another recent phase I clinical trial investigated an LAV ΔUL5/ΔUL29 HSV-2 vaccine candidate (HSV529) lacking two crucial genes for viral replication: UL5 and UL29. The study found the vaccine to be safe and well tolerated. However, the immune responses it elicited were lower than the antibody levels observed in people who had previously been infected with HSV (seropositive). In participants who were already infected with HSV-1 or HSV-2, the HSV529 vaccine did not boost their existing levels of neutralizing antibodies, while it did increase CD4+ and CD8+ T-cell responses (168). A general conclusion drawn from these studies is that targeting blockage of viral entry alone is ineffective as a vaccine strategy, especially considering HSV-2’s evasion capabilities. In addition, given the prevalence of HSV-1 worldwide, it is critical that an HSV vaccine protects against HSV-2 in persons who are HSV-1 seropositive. The changing epidemiology of genital herpes, with HSV-1 now being responsible for over 50% of primary cases in the United States, has significant implications for biologic drug development, as many prior vaccine candidates and therapeutics were designed with HSV-2 as the primary target. Given that HSV-1 and HSV-2 share considerable genetic and antigenic similarities but exhibit different re-activation patterns and disease burdens, future biologics must ensure cross-protection against both serotypes. Additionally, since pre-existing HSV-1 immunity can interfere with immune responses to HSV-2-targeted vaccines, new vaccine designs may need to account for this immunological interference. Biologic strategies should also consider the distinct latency and re-activation dynamics of HSV-1 in the genital mucosa. With that in mind, new HSV vaccine candidates are currently in pre-clinical development, some of which have advanced into human clinical trials. A novel DNA vaccine utilizes two plasmids, one expressing full-length gD2 and a second plasmid encoding a shortened version of gD2 fused with ubiquitin, which targets intracellular degradation of gD2 into smaller peptides for MHC class I presentation. Studies in mice showed a well-balanced immune response involving both antibody production against gD2 and the activation of CD8+ T cells (169). In a placebo-controlled, randomized double-blind phase I human trial, this vaccine (COR-1) was safe and well tolerated in HSV-2-positive subjects with a rise in gD2-specific antibodies and cell-mediated immune responses in peripheral blood, validating the rationale for further tests of this innovative vaccine (170). GEN-003 is a novel vaccine containing a transmembrane deletion mutant of gD (gD2ΔTMR) combined with a large fragment of infected cell protein 4 (ICP4.2), an HSV-2 T-cell antigen, together with a matrix-M2 adjuvant (171). In three separate phase II trials to evaluate safety, immunogenicity, and impact on viral shedding, three intramuscular doses 21 days apart of GEN-003 resulted in reduced viral shedding and lesion rates immediately following immunization, while no serious adverse events were attributed to GEN-003 (172). Despite initial reports suggesting a 58% reduction in viral shedding, the difference between the vaccine and placebo was not clinically meaningful in the longer follow-up studies, leading to its abandonment. GSK advanced an adjuvanted protein subunit vaccine GSK3943104 in phase II clinical trials as an investigational treatment for the suppression of recurrent genital herpes in adults aged 18 years and older. Study results indicated that GSK3943104 did not achieve the primary efficacy goal of the study, and as a result, this vaccine candidate will not advance to phase III trials, although no safety concerns were observed during the phase II study. An mRNA vaccine encoding gD2, gC2, and gE2 and formulated with lipid nanoparticles triggered higher levels of serum antibodies in serum and vaginal secretions of mice, including those targeting crucial gD2 epitopes, as well as stronger T-cell responses compared to a vaccine wherein the same three proteins were produced in baculovirus and adjuvanted with CpG and alum, making this nucleoside-modified mRNA-LNP vaccine a promising candidate for further evaluation in human trials (173, 174). ModernaTX (Cambridge, Massachusetts) also has an mRNA therapeutic vaccine—mRNA 1608—in a phase I/phase II, randomized, dose-ranging study in healthy adults 18–55 years of age with recurrent HSV-2 genital herpes. BioNTech (Mainz, Germany) has initiated a phase I clinical trial with their BNT163 mRNA vaccine against HSV-2. Although their antigenic composition has not been publicly disclosed, both vaccines likely focus on envelope proteins for neutralizing antibody generation. mRNA vaccines have the additional advantage that they can promote the development of resident memory T cells in the ganglia, where latency occurs. Additional details on vaccine candidates against HSV-2 are contained in Table 5.

TABLE 5.

Vaccine candidates at different stages of development against herpes simplex virus type 2

Vaccine type	Antigen/platform	Composition	Adjuvant	Outcome	Stage	Reference
Preventative	gD2	Protein subunit vaccine comprising gD2	Alum/MPL	Vaccine showed 20% efficacy against genital herpes.	Phase III (complete)	(163, 164)
	gB2/gD2	Dual protein subunit vaccine with mix of gB2 and gD2	MF59	Vaccine induced high levels of HSV-2-specific neutralizing antibodies, but overall efficacy was 9%.	Phase III (complete)	(166)
	ΔgD-2	Vaccine containing deletion of gD-2 protein	Alum/MPL	Vaccine elected a durable response in a mouse model that consisted of both neutralizing antibodies as well as those inducing ADCC.	Pre-clinical	(165)
Therapeutic	LAV ΔgH	Live-attenuated vaccine with gH deletion	None	No clinical or virological effect observed	Phase I/phase II	(167)
	LAV ΔUL5/ΔUL29 (HSV529)	Live-attenuated vaccine with UL5 and UL29 deletion	None	Vaccine induced neutralizing antibodies and modest CD4+ T-cell responses.	Phase I	(168)
	DNA gD2/gD2-ubiquitin (COR-1)	Plasmid DNA vaccine with full-length gD2 plus second plasmid containing shortened gD2 fused to ubiquitin	None	Immunizations were safe and well tolerated at doses up to 1 mg. It induced T-cell but not antibody responses.	Phase I/phase IIa	(169, 170)
	GSK3943104	Protein subunit	Unknown	Not available	Phase II	https://www.gsk.com/en-gb/innovation/pipeline/?infectious-diseases
	GEN-003-gD (gD2ΔTMR) plus ICP4.2 HSV-2 T-cell antigen mRNA gD2/gC2/gE2	Recombinant proteins made in baculovirus	Matrix-M2	Three doses of 30 and 100 µg GEN-003 given 21 days apart reduced viral shedding and lesion rates by up to 15%.	Phase I/phase II	(171, 172)
	GEN-003-gD (gD2ΔTMR) plus ICP4.2 HSV-2 T-cell antigen mRNA gD2/gC2/gE2	mRNA vaccine enclosing gD2, gC2, and gE2 proteins formulated into lipid nanoparticles	None	mRNA vaccine prevented dorsal root ganglia infection, and vaginal cultures were negative in 98% of mice. Separately, only 20% of guinea pigs shed HSV-2 DNA.	Phase I	(173, 174)
	mRNA-1608	Lipid nanoparticle formulated mRNA vaccine	None	Not available	Phase II	https://trials.modernatx.com/study/?id=mRNA-1608-P101
	BNT163	Lipid nanoparticle formulated mRNA vaccine	None	Not available	Phase I	https://clinicaltrials.biontech.com/trials/BNT163-01

A wealth of pre-clinical evidence supports the crucial role of antibodies in shielding against HSV infection. This aligns with clinical observations where prior maternal infections confer protection against HSV disease. Notably, these studies extend beyond simply preventing illness, demonstrating in animals that maternally derived HSV-specific antibodies can access the nervous system and mitigate neurological symptoms associated with neonatal infection (175). Administration of purified HSV-specific IgG to pregnant mothers further strengthens the argument for passive antibody-mediated protection in newborns (176). Similar compelling evidence emerges in the context of adult infection. A guinea pig model revealed that passive vaccination with HSV-antiserum just 24 hours after infection significantly reduced vaginal lesions, disease re-activation, and latent viral genome copies (177). Antibodies also exhibit the remarkable ability to restrict the spread of HSV within the nervous system via passive immunization, protecting the mouse eye, skin, and even limiting the number of affected sensory ganglia. Taken together, this pre-clinical research paints a clear picture of antibody-mediated protection against HSV infection. Thus, mAbs have emerged as promising contenders with cross-reactivity against HSV-1 and HSV-2. Three are noteworthy: 2c, HSV8, and UB-621/E317. 2c, originally isolated from HSV-1-immunized mice, targets a crucial epitope in the gB protein essential for viral infectivity (178). 2c exhibits neutralizing activity against the virus with or without complement and triggers ADCC (179). In C57/B6 mice, 2c prevents viral shedding at mucous membranes, subsequent neural spread, and even death following intravaginal challenge. Humanized 2c (h2c) further builds on this success, demonstrating remarkable protection against HSV-1 strains in NOD/SCID mice and preventing herpetic stromal keratitis in a corneal model. In ongoing clinical trials in Germany and the United States (NCT04539483 and NCT04165122), h2c is being evaluated for its effectiveness in preventing orolabial and anogenital lesions caused by HSV-1 and HSV-2, respectively (180). HSV8, a human IgG1 mAb targeting gD, has neutralizing activity against both HSV-1 and HSV-2 (181). Topical application of HSV8 shields mice from genital HSV disease, while systemic delivery offers protection against lethal infection in athymic nude mice challenged via corneal or flank models. Notably, HSV8 also exhibits efficacy when topically administered to human cervicovaginal mucus, an effect that has been tested in human clinical trials using the MB66 vaginal film (182). UB-621/E317, another human IgG1 mAb specific for gD, also has neutralizing activity against both HSV-1 and HSV-2 through dual blockade of engagement of nectin-1 and HVEM receptors (183). Systemic administration of E317, the precursor to UB-621, significantly improves survival rates in SCID mice challenged with HSV-1, even when administered after infection. The clinically relevant UB-621 is currently undergoing clinical trials for prevention of orolabial and genital HSV infections (NCT02346760, NCT03595995, NCT04714060, and NCT04979975).

Decoy peptides, mimicking viral or host receptor binding sites, offer a new strategy by competitively inhibiting, particularly viral attachment and entry. For example, gH-derived peptide CB-1 showed a high level of virucidal activity against HSV-2 (184). HSV-2 gD genetically engineered to truncate the transmembrane domain was shown to inhibit HSV-2 plaque production and virus penetration (185). Fujimoto et al. capitalized on the observation that HSV-2 depends on gD interactions with nectin-1 and HVEM for cell entry by constructing a soluble form of human nectin-2 (hNectin-2Ig). Viral growth of HSV-2 strains was significantly inhibited in cells transfected with hNectin-2Ig compared to control cells (186). Further work by this group demonstrated that in a transgenic mouse expressing nectin-1Ig, significant protection was achieved against lethal doses of HSV-2. Even better, transgenic mice expressing HVEMIg showed complete protection against the HSV-2 lethal infection as well as to ocular herpes (187, 188). Table 6 lists further information on mAbs and proteins being developed against HSV-2.

TABLE 6.

Biologic drug candidates at different stages of development against herpes simplex virus type 2

Modality	Name	Antigen	Remarks	Stage	Reference
mAbs	UB-621	gD	mAb reduced mortality in intranasal HSV-2 challenge of neonatal mice.	Phase I	(183)
	h2c	gD	mAb was safe and well tolerated.	Phase I	(178, 180)
	HSV8	gB	Antibody showed high neutralization against low-passage clinical isolates of HSV serotypes	Pre-clinical	(181)
Proteins/peptides	gD-2t	Soluble gD with truncated transmembrane domain	gD-2t efficiently blocked HSV-2 entry and reduced plaque formation in vitro.	Pre-clinical	(185)
	CB-1	gH derived soluble peptide	CB-1 showed high virucidal and antiviral activity against HSV-2 in vitro.	Pre-clinical	(184)
	Nectin-1Ig	Soluble nectin-1 fused to the Fc portion of IgG.	Fusion protein blocked severe symptoms in a mouse model of ocular herpes.	Pre-clinical	(186–188)
	HVEM-Ig	Soluble HVEM fused to the Fc portion of IgG.	Transgenic mouse expressing HVEMIg showed resistance to ocular herpes.	Pre-clinical	(186–188)

Additionally, antisense oligonucleotides (ASOs) targeting the translation initiation site of an HSV-2 large subunit of ribonucleotide reductase have been studied for their ability to inhibit its expression with good success (189). siRNA-based therapies are being explored to silence viral genes necessary for viral replication and persistence for HSV (190). HSV DNA aptamers targeting the viral gB glycoprotein are under investigation as potential topical microbicides to block viral entry (191). Several studies describe the use of genome editing techniques as a strategy to limit HSV‐1 infections, including the use of meganucleases to reduce the HSV viral load, adeno‐associated virus‐delivered homing endonucleases, and the more efficient CRISPR/Cas9 system (192). HSV re-activation rates are proportional to viral loads in latently infected ganglia. Therefore, even partial inactivation of latent genomes can notably reduce disease severity and viral shedding or lead to remission (193). CRISPR has been proven to be remarkably effective in pre-clinical research, and it has now been licensed for treatment of sickle cell disease and transfusion-dependent beta thalassemia (Vertex Pharmaceuticals, Boston, MA). It has also been applied in clinical trials for treating cancer, diabetes, inflammatory diseases, chronic bacterial diseases, and HIV/AIDS (194). Therefore, CRISPR/Cas9 targeting of the latent HSV genome would be an appealing strategy to mutate or clear the virus from latent cells.

HSV-2 vaccine development has been extensive, with multiple candidates that include LAV adjuvanted protein subunit vaccines and nucleic acid vaccines (DNA and mRNA), primarily targeting gD2 and gB2 antigens, but none achieving success. More recently, the ΔgD-2 live-attenuated vaccine, which lacks gD2 expression, has demonstrated long-term protection and robust ADCC responses in mice but has remained at the pre-clinical stage. Beyond prophylactic vaccines, therapeutic approaches such as the LAV ΔgH HSV-2 vaccine and the ΔUL5/ΔUL29 vaccine have been tested in phase I/phase II trials but with limited impact on viral shedding and symptom control. Advances in gene-targeting therapies, including CRISPR/Cas9, ASOs, and siRNA, offer novel therapeutic strategies to silence viral replication genes or disrupt latency, though none have yet entered clinical testing. Live-attenuated vaccine candidates like ΔgD-2 and CRISPR-based latency disruption are some of the most promising biologic approaches being tested. Nonetheless, the need for improved vaccine designs, latency-targeting therapeutics, and strategies that account for prior HSV-1 exposure persists.

Treponema pallidum

Developing a vaccine against T. pallidum has proven to be particularly challenging due to the bacterium’s unique properties. Historically, T. pallidum could not be grown in culture, hindering traditional vaccine development methods. Additionally, T. pallidum evades the immune system through diverse mechanisms, making it difficult to elicit a protective response, although most but not all patients remain serologically positive in the treponema-specific tests. Distinguishing active infection from past infection can be difficult, leading to unnecessary retreatment and potential antibiotic overuse. Nonetheless, over the past years, researchers have explored diverse syphilis vaccine candidates with limited success, including whole-cell inactivated, live-attenuated, and genetically engineered options (195). While the T. pallidum repeat family paralogs, like TprK, initially sparked excitement due to their surface-exposed loops in the NH₂ terminus, their antigenic variability and role in immune evasion presented challenges. Nevertheless, rabbits immunized with a recombinant NH₂-terminal fragment of TprK displayed reduced cutaneous lesion development and progression but with a high degree of strain specificity, as evidenced by the limited cross-reactivity between Nichols and clonal Chicago V-region peptides (35, 196, 197). One promising vaccine candidate utilizes Tp0751, an adhesive lipoprotein that facilitates direct interaction with the host environment by binding to laminin, fibronectin, and fibrinogen. Rabbits immunized with Tp0751 demonstrated reduced lesion development, including lower T. pallidum burden, reduced T. pallidum dissemination, and increased cellular infiltration at lesion sites, markers used to measure efficacy (198). Animals immunized with Tp0751 demonstrated a significantly reduced bacterial load in their organs upon T. pallidum challenge compared to unimmunized animals, although complete sterilization was not achieved. Nonetheless, the introduction of lymph nodes from Tp0751-immunized and T. pallidum-challenged animals into naive animals did not result in infection, thereby confirming sterile protection. Furthermore, Tp0751 amino acid sequences are conserved across T. pallidum strains, suggesting this vaccine can be broadly protective, assuming rabbit data translate to human disease. A subsequent study found that Tp0751 induced weak antibody responses in rabbits. In humans, the antibodies were not opsonophagocytic, and immunization with Tp0751 failed to protect rabbits in a syphilis model. Notably, a different Tp0751 fragment was used, complicating comparisons with Lithgow et al.’s study and emphasizing the need for careful antigen design (199). Separate studies found that immunization of rabbits with adhesion proteins Tp0136 or the outer membrane protein Tp0663 attenuated lesion development and reduced dissemination, with protection observed like that with Tp0751 (200). Furthermore, immunization with Tp0954, a T. pallidum placental adhesin, impeded the development of cutaneous lesions, induced inflammatory cellular infiltration at the primary lesion, and blocked T. pallidum spreading to distal tissues and organs (201). In pursuit of a more comprehensive approach, another team opted for a whole-cell bacterial vaccine by engineering Borrelia burgdorferi to express T. pallidum genes (202). This heterologous system, involving the Tp0435 lipoprotein, offered partial protection in rabbits, though complete immunity remained elusive. Also, plasmid DNA encoding T. pallidum flagellin FlaB3 emerged as a promising candidate, generating significant bacterial burden reduction in immunized rabbits’ blood, liver, spleen, and testicles (203). Ongoing research aims to optimize immunization methods, understand the elicited immune response, and determine the duration of immunity conferred by these diverse vaccine strategies. For example, building on partial protection seen with individual antigens, studies have shown that immunization with a TprC/TprK/Tp0751 tri-antigen cocktail protected rabbits from progressive syphilis lesions and substantially inhibited dissemination of the infection (204). Table 7 contains more information on vaccines against T. pallidum.

TABLE 7.

Vaccine candidates at different stages of development against Treponema pallidum

Antigen/platform	Composition	Adjuvant	Outcome	Stage	Reference
TprK	Amino acids 37–351 of TprK recombinantly expressed in E. coli	RIBI (MPL+TDM+CWS)	Vaccination decreases treponemal burden at the sites of challenge correlating with differences in TprK V regions.	Pre-clinical	(35, 196, 197)
Tp0751	Amino acids 24–237 of Tp0751 recombinantly expressed in E. coli	TiterMax Gold	Immunized animals have reduced bacterial burden on challenge, and introduction of lymph nodes from Tp0751-immunized, Tp-challenged animals to naive animals protects from infection.	Pre-clinical	(198)
Tp0751/TprK/TprC tri-antigen cocktail	Tp0751 produced as mature, full-length soluble protein in E. coli and TprK and TprC purified from refolded peptide-based inclusion bodies.	Custom adjuvant mimicking RIBI natural and RIBI synthetic	Study demonstrated attenuation of lesion progression and reduction of T. pallidum dissemination as well as reproducibility of the results at different locations and using rabbits from different sources.	Pre-clinical	(204)
Tp0136/Tp0663	Proteins expressed recombinantly in E. coli	CFA/IFA	Vaccine reduced lesion development, increased cellular infiltration at lesion sites, and blocked T. pallidum dissemination to distant organs.	Pre-clinical	(200)
Tp0954	Proteins expressed recombinantly in E. coli	CFA/IFA	Vaccine delayed the development of cutaneous lesions, promoted infiltration of inflammatory cells at lesion sites, as well as inhibited T. pallidum spread to distal tissues.	Pre-clinical	(201)
Tp0435	Non-infectious Borrelia burgdorferi strains expressing Tp0435	None	Vaccination led to partial protection, while immunity to Tp0435 was not protective.	Pre-clinical	(202)
FlaB3	Plasmid DNA carrying FlaB3 gene	CFA/IFA	Vaccinated animals had a reduction of bacterial organ burden and improved histopathology.	Pre-clinical	(203)

Proof of concept of antibody treatment for prevention of syphilitic lesions was established decades ago. Pavia et al. (205) immunized guinea pigs with killed or sonicated T. pallidum organisms and passively administered immune sera or purified antibodies into healthy guinea pigs. Immunized guinea pigs showed a significant reduction in testicular lesions compared to non-immunized animals, the degree of protection correlating with antibody levels, particularly those directed against outer membrane antigens of T. pallidum. In a study by Wicher et al. (206), a single intradermal injection of immune serum or purified IgG raised against T. pallidum, from which antibodies directed to non-pathogenic treponemes were depleted, provided healthy guinea pigs protection on the same but not the opposite hind leg, highlighting the role of homocytotropic antibodies in local protection. Blanco et al. (207) isolated a mouse IgM mAb M131 directed against phosphorylcholine on the surface and subsurface of T. pallidum. M131 did not bind red blood cells, which also have phosphorylcholine in their outer membrane. M131 showed complement-dependent killing activity in vitro. When passively administered to rabbits, it conferred immunity, evidenced by lesion disappearance at 29% of challenge sites and an average delay of 8 days in lesion appearance at the others.

In terms of peptide therapeutics, a platform with significant potential for STI treatment is that of HDPs (208), a distinct class of antimicrobials, characterized by their gene-encoded, ribosomal synthesis and limited length of less than 100 amino acids. This definition separates HDPs from non-ribosomally synthesized peptide antibiotics with atypical amino acids, often found in fungi and bacteria. HDPs are a part of the immediate and rapid innate immune response, contrasting with the slower, acquired immunity mediated by B- and T-cell clonal expansion. The importance of this rapid response becomes evident when considering that bacterial generation times are significantly faster than the days or weeks needed for a specific immune response to develop (209). Many HDPs, like human β-defensin (hBD)-2, show increased expression during infection and inflammation. For example, hBD-2 is upregulated in various cell types by bacterial components activating the TLR to the NF-kB pathway. The multi-functional HDP LL-37 possesses microbicidal action against a host of microbes, including STI pathogens N. gonorrhoeae and T. pallidum. Cox et al. (210) demonstrated that the full-length LL-37 is potently active against T. pallidum in vitro. However, LL-37 activity is less than its truncated synthetic variant, WS22-N-amide. Reduced activity of LL-37 is due to both the structure of T. pallidum cell envelope and the loss of an aspartic acid residue within the core peptide, rendering it more negatively charged compared to WS22-N-amide that contains an asparagine (210). Sambri et al. (211) tested five synthetic cathelicidin-derived peptides from various mammalian sources, including LL-37 in vitro: SMAP-29 from sheep, mCRAMP from mice, LL-37 from humans, BMAP-28 from cattle, and PG-1 from pigs. PG-1 was the most potent HDP followed by SMAP-29. Surprisingly, LL-37, the only tested HDP of human origin, was the least potent, a result different from the one obtained by Cox et al. and attributed by the latter to assay conditions (210). Table 8 lists mAbs and protein candidates against T. pallidum.

TABLE 8.

Biologic drug candidates at different stages of development against Treponema pallidum^a

Modality	Name	Antigen	Remarks	Stage	Reference
mAbs	M131	Phosphorylcholine expressed on surface and subsurface of T. pallidum	Passive immunization reduced lesions in 29% of challenge sites and had a mean delay of 8 days to lesion appearance at the remaining sites.	Pre-clinical	(207)
Protein/peptides	LL-37	Peptide was synthesized by solid-phase chemistry and purified using RP-HPLC.	LL-37 effectively killed T. pallidum spirochetes in vitro, especially in low-salt conditions.	Pre-clinical	(210)
	WS22-N-amide (truncated version of LL-37)	Peptide was synthesized by solid-phase chemistry and purified using RP-HPLC.	WS22-N-amide had EC50 30× less than whole peptide and blocked infectivity of T. pallidum in a rabbit model.	Pre-clinical	(210)

^a RP-HPLC, reversed-phase high-performance liquid chromatography.

No microbiome-based treatment is available for STIs to date. A phase IIb clinical trial assessed Lactobacillus crispatus CTV-05 (Lactin-V) for preventing BV recurrence. Women aged 18–45, post-treatment with vaginal metronidazole gel, were randomized in a 2:1 ratio to receive Lactin-V or placebo for 11 weeks, with follow-up until week 24. By week 12, BV recurred in 30% of the Lactin-V group (46 of 152) and 45% of the placebo group (34 of 76) (risk ratio 0.66; 95% confidence interval [CI], 0.44–0.87; P = 0.01). By week 24, the risk ratio was 0.73 (95% CI, 0.54–0.92). L. crispatus CTV-05 was detected in 79% of the Lactin-V group at week 12. The incidence of adverse events was similar between groups, with no significant differences in local or systemic side effects through week 24. In addition, in an exploratory clinical study, the potential of vaginal microbiome transplantation (VMT) as a therapeutic alternative for recurrent, intractable bacterial vaginosis was explored. Among five treated patients, four achieved long-term remission at 5–21 months post-VMT, characterized by symptom improvement and restoration of a Lactobacillus-dominated microbiome. One patient experienced partial remission, and three required repeated VMT for sustained results. Importantly, no adverse effects were reported, highlighting VMT’s safety and potential efficacy. These findings underscore the need for randomized, placebo-controlled trials to validate VMT as a therapeutic approach for recurrent BV (212).

In summary, vaccine efforts have been historically hindered by T. pallidum’s inability to be cultured in vitro and its immune evasion mechanisms, but recent studies have identified Tp0751, TprK, and Tp0136 as promising antigen targets, albeit all at pre-clinical stage. While recombinant Tp0751 vaccination in rabbits reduced bacterial burden and lesion progression, complete sterilization was not achieved, highlighting the need for multi-antigen strategies. Passive immunization studies in animal models have demonstrated that antibodies targeting T. pallidum outer membrane antigens can provide localized protection, with monoclonal IgM M131, specific for phosphorylcholine, showing complement-dependent killing in rabbits. In addition, HDPs such as LL-37 have demonstrated in vitro activity, with synthetic variants like WS22-N-amide exhibiting enhanced microbicidal effects. In our view, multi-antigen vaccines incorporating Tp0751 and TprK, as well as monoclonal antibody-based immunotherapy, are the promising biologic candidates, though none have advanced beyond pre-clinical studies.

DOXYCYCLINE POST-EXPOSURE PROPHYLAXIS

Therapeutic strategies discussed in this review should be framed in the context of recent guidelines issued by health agencies worldwide regarding the use of doxycycline post-exposure prophylaxis (doxy-PEP) for high-risk STI populations. These recommendations provide critical context for understanding the potential role of doxy-PEP in STI prevention strategies. In a 2018 French study on high-risk MSM using pre-exposure prophylaxis (PrEP) for HIV prevention, the use of doxy-PEP following condomless sexual activity was associated with an overall 47% relative reduction in the risk of acquiring new bacterial STI such as gonorrhea, chlamydia, or syphilis (213). In this cohort, doxy-PEP was associated with a 70% and 73% relative reduction in the incidence of chlamydia and syphilis, respectively, but did not affect the rates of developing gonorrhea. Recently, in a US study conducted at HIV clinics and sexual health clinics, MSM and transgender women who were taking PrEP against HIV infection or living with HIV infection and who also had had gonorrhea, chlamydia, or syphilis were randomly assigned to take 200 mg of doxycycline within 72 hours after condomless sex (214). The study found that the combined incidence of gonorrhea, chlamydia, and syphilis was lower by 66% with doxycycline doxy-PEP than with standard care. Unlike the French study where doxy-PEP was ineffective against gonorrhea, the US study showed a 55% reduction in incident gonorrhea, which likely reflected the higher rates of tetracycline resistance in France and the United States (56% vs 20%) at the time the studies were conducted. Doxy-PEP was ineffective in preventing any of the three bacterial STIs in a study of cis-gender women in Kenya (215). Poor adherence to doxycycline prophylaxis likely contributed to lack of efficacy, although differences in the primary sites of infection (cervical vs rectal mucosa) could impact drug efficacy. Based on these findings, various health agencies have recommended the use of doxy-PEP among MSM with recent bacterial STIs (216). Doxy-PEP efficacy has set a benchmark for post-exposure interventions, yet concerns about antimicrobial resistance and impacts on the microbiome remain key challenges. The goal should be integrating doxy-PEP into a broader, sustainable STI prevention strategy while minimizing resistance risks and exploring complementary prophylactic measures.

CONCLUSION

STIs remain a pressing global public health challenge, affecting millions annually with significant consequences for reproductive health, quality of life, and healthcare systems. Despite advancements in antibiotics and antivirals, the rise of resistant pathogens like N. gonorrhoeae underscores the urgency for vaccines and novel therapeutic strategies. This review highlights critical gaps in the prevention and treatment of four important STIs—chlamydia, gonorrhea, genital herpes, and syphilis—while also addressing promising developments in emerging technologies and biologics. These infections were prioritized because they are among the most prevalent STIs globally, causing significant morbidity, including infertility, neonatal complications, and increased HIV susceptibility.

Advances like the CTH522 vaccine against C. trachomatis demonstrate the feasibility of inducing strong serum and mucosal immunity, though challenges in achieving broad serovar coverage remain. Similarly, for N. gonorrhoeae, vaccines like 4CMenB and OMV-based candidates offer modest cross-protection, although recent clinical setbacks highlight the complexity of targeting this pathogen. For viral STIs such as HSV-2, significant efforts in vaccine research have revealed critical insights into immune mechanisms, with candidates like ΔgD-2 showing durable protection in pre-clinical studies. However, challenges persist in addressing HSV latency and immune evasion, which limit the efficacy of conventional approaches. Syphilis caused by T. pallidum remains elusive due to its antigenic variability and limited metabolic capacity, necessitating novel strategies to address latent infections and tissue tropism. Additionally, co-infections between pathogens such as C. trachomatis, N. gonorrhoeae, and HSV-2 exacerbate disease severity and hinder treatment efficacy, emphasizing the need for integrated therapeutic approaches. Beyond vaccines, biologic innovations offer significant promise in combating these diverse pathogens. Monoclonal antibodies targeting conserved epitopes, antimicrobial peptides, and engineered bacteriophages represent next-generation therapies with broad-spectrum potential. Phage therapy has shown promise in steering bacterial populations toward antibiotic susceptibility, a critical advantage in combating multi-drug-resistant strains. Host defense peptides and lysins provide potent antimicrobial activity while complementing traditional therapies. Furthermore, advancements in microbiome modulation and nucleic acid therapeutics such as those based on CRISPR-Cas9 are poised to revolutionize STI prevention and treatment.

Resources should be prioritized against the most promising biologic candidates targeting critical STI pathogens. Product development should focus on innovative biologic solutions that address critical gaps in prevention and treatment. For chlamydia, advancing the CTH522 vaccine, particularly formulations with the CAF01 adjuvant, is essential due to its strong mucosal and systemic immunity. Further work on recombinant MOMP-based vaccines, including MOMP-PorB chimeric constructs, and exploration of novel antigens such as Pmps, Incs, and GrGA should be prioritized to achieve broad serovar coverage as well as thwarting the pathogen’s immune evasion strategies. For gonorrhea, efforts should focus on OMV-based vaccines, including enhancements to 4CMenB and NGoXIM, and LOS-targeted strategies coupled with other neisserial proteins such as PorB and NHBA. Continued research on conserved surface antigens, such as TbpA/B and MtrE, is critical to designing effective gonorrhea vaccines. In addition, mAbs such as 2C7 as well as protein therapeutics like C4BP-Fc and FH-Fc, targeted toward making gonococcal LOS susceptible to complement, should be pursued. HSV-2 vaccine development should prioritize ΔgD-2 for its durable immunity and ADCC, alongside mRNA-based platforms and CRISPR-based therapeutic vaccines to address both viral latency and recurrence. CNS-penetrant mAbs targeting specific viral proteins associated with latent infection, potentially delivered via BBB-crossing strategies like receptor targeting, are one example of such an approach. Bispecific antibodies engineered with dual-binding domains—one targeting a BBB receptor and the other binding a therapeutic target in the CNS—can be used in a Trojan horse strategy to enhance treatment delivery. In addition to mAbs, nanoparticles (liposomes, polymeric nanoparticles, or lipid nanocarriers) can encapsulate antivirals and facilitate their transport across the BBB via mechanisms such as receptor-mediated endocytosis. Next-generation CRISPR-Cas9 or Cas13 technologies can be used to excise HSV-2 latent genomes or disrupt essential latency-maintenance genes like LATs. Such nanoparticle technology can also encapsulate siRNAs or antisense oligonucleotides designed to specifically target viral latency-associated transcripts that could silence the molecular machinery that maintains latency. For T. pallidum, while penicillin continues to be a cheap and effective drug for treatment, newer biologic approaches must be explored for disease prevention. Antigen-based therapies focusing on highly variable TprK proteins and metabolic vulnerabilities, such as reliance on host carbon sources, may limit immune evasion and persistence and should therefore be further pursued. Microbiome modulation with probiotics such as Lactobacillus spp. or the use of VMT could complement these efforts. Leveraging platform technologies, including combination therapies, nanotechnology, and liposomal delivery systems, can enhance immunogenicity and treatment efficacy. Integrated strategies must also consider co-infections, as interactions between pathogens and host immune responses can amplify disease progression, necessitating combination therapies. Finally, increasing the use of extragenital testing and developing interventions targeting asymptomatic rectal and pharyngeal infections are crucial for comprehensive STI management. As such, efforts must be directed toward optimizing these technologies for safety, efficacy, and delivery in resource-constrained settings. This requires collaboration between academia, biotech firms, regulatory bodies, and global health organizations to ensure innovations are not confined to high-income countries but are accessible to populations disproportionately affected by STIs.

Biographies

Davinder S. Gill is a biotechnologist and CEO at StiRx Inc., a Boston, Massachusetts-based biotechnology company focused on developing best-in-class vaccines and biologics for sexually transmitted infections. Gill earned his B.Tech with honors in Chemical Engineering from the Indian Institute of Technology before pursuing a Ph.D. in Chemical Engineering at the University of Houston, Texas. He further specialized in antibody engineering during a postdoctoral fellowship at Massachusetts General Hospital and Harvard Medical School. With over three decades of leadership in the biotech and pharmaceutical industries, Gill has held senior roles, including Vice President and Global Head of Biotherapeutics at Pfizer, where he led research teams across multiple therapeutic areas, advancing numerous drug candidates to clinical trials. As CEO of Hilleman Laboratories, a joint venture between Merck and the Wellcome Trust, he championed affordable and sustainable vaccine platforms for underserved populations, progressing multiple vaccines, including Hillchol™ to commercial launch, and raising funds for several other vaccine initiatives. His tenure at Wyeth and Millennium Pharmaceuticals saw the early establishment of biologics, monoclonal antibodies, and novel protein scaffolds for treatment of human disease. An active advisor, Gill has contributed to global public health through roles with GAVI, ACT-A, the Indian government’s National Biotechnology Council, and other strategic advisory boards. He has been honored with the Cancer Research Institute Fellowship, the Cullen Fellowship and the W.H. Peterson Award by the American Chemical Society. He is co-author on over 100 publications, patents, and abstracts. His work champions a deep commitment to leveraging cutting-edge biomedical innovation to address global health challenges.

Sanjay Ram is an Infectious Diseases physician-scientist and Professor of Medicine at the University of Massachusetts Chan Medical School, with a career dedicated to studying and combating sexually transmitted infections (STIs). A graduate of Seth G. S. Medical College in Mumbai, he completed his residency in Internal Medicine at SUNY Buffalo and a fellowship in Infectious Diseases at Boston University. Known for his research on the interactions between Neisseria gonorrhoeae and the complement system, Ram has pioneered the development of innovative immunotherapies and vaccines targeting antimicrobial-resistant gonorrhea, including Fc fusion proteins, monoclonal antibodies, and sialic acid analogs. As co-founder of StiRx, Inc., he advances the mission to create safe, affordable vaccines for low- and middle-income countries. He has over 100 peer-reviewed publications, contributing to advancements in understanding complement interactions with pathogens like Neisseria meningitidis, which informed FDA-approved meningococcal vaccines. He has been honored with the Maxwell Finland Young Investigator Award and election to the American Academy of Microbiology, Ram is also a Fellow of the Royal College of Physicians and Infectious Diseases Society of America. His ongoing NIH-funded projects span vaccine development, antibody therapeutics, and novel treatments for multidrug-resistant gonorrhea, underscoring his pivotal role in addressing global STI challenges.

Peter A. Rice is Emeritus Professor of Medicine at the UMass Chan Medical School and a leading figure in infectious diseases, with extensive contributions to Neisseria gonorrhoeae vaccine development. A graduate of Yale University and the University of Pennsylvania (now Perelman) School of Medicine, he completed his residency in Internal Medicine at Yale-New Haven Hospital and Peter Bent Brigham (now Brigham and Women’s) Hospital, followed by a fellowship in Infectious Diseases at Boston City Hospital (now Boston Medical Center) and Harvard Medical School's Channing Laboratory. Dr. Rice's groundbreaking work focuses on overcoming immune barriers to develop gonococcal vaccines, particularly through the identification of the 2C7 epitope on gonococcal lipooligosaccharide (LOS). His research has led to the creation of peptide mimics that elicit cross-reactive immune responses, accelerating bacterial clearance in preclinical models, including coinfections with Chlamydia. He holds multiple patents related to vaccine candidates and has co-authored numerous influential studies, advancing multiantigenic vaccine platforms and adjuvant strategies. With a career spanning decades, Dr. Rice has held prominent roles, including Chief of Infectious Diseases at Boston University Medical Center and Director of the Maxwell Finland Laboratory for Infectious Diseases. A former Epidemic Intelligence Officer at the CDC, he has also served as Permanent / Charter Member of several NIH review committees and was a MERIT Awardee from the NIH. He is a Fellow of the American Academy of Microbiology and an active member of numerous professional societies, such as the Infectious Disease Society of America and the American Society of Microbiology. Honored with awards like the Roche Award and election to leadership positions at international/national conferences, Dr. Rice continues to shape the field of vaccine research with his expertise and innovative approaches.