Pediatric HIV: the potential of immune therapeutics to achieve viral remission and functional cure

Abstract

Purpose of Review

In the absence of antiretroviral therapy (ART), more than 50% of perinatally HIV infected children die by two years of age. Early ART from infancy is therefore a global recommendation and significantly improves immune health, child survival and disease outcome. However, even early treatment does not prevent or eradicate the latent reservoir necessitating life-long ART. Adherence to life-long ART is challenging for children and longstanding ART during chronic HIV infection led to higher risks of non-AIDS co-morbidities and virologic failure in infected children. Thus, HIV-infected children are an important population for consideration for immune-based interventions to achieve ART-free remission and functional cure. This review summarizes how the uniqueness of the early life immune system can be harnessed for the development of ART-free remission and functional cure, which means complete virus control in absence of ART. In addition, recent advances in therapeutics in the HIV cure field and their potential for the treatment of pediatric HIV infections are discussed.

Recent Findings

Preclinical studies and clinical trials demonstrated that immune-based interventions target HIV replication, limit size of virus reservoir, maintain virus suppression, and delay time to virus rebound. However, these studies have been performed so far only in carefully selected HIV-infected adults, highlighting the need to evaluate the efficacy of immune-based therapeutics in HIV-infected children and to design interventions tailored to the early life maturing immune system.

Summary

Immune-based therapeutics alone or in combination with ART should be actively explored as potential strategies to achieve viral remission and functional cure in HIV-infected pediatric populations.

Introduction

With the worldwide implementation of HIV screening during pregnancy (1) and option B plus practices that include triple drug anti-retroviral therapy (ART) for all pregnant women irrespective of their CD4 counts and plasma viral loads (2), the risk of perinatal (in-utero and intrapartum) and postnatal (during breastfeeding) mother-to-child transmission (MTCT) of HIV has dropped to <5% (3). Yet, 160,000 infants were newly infected with HIV in 2018 (4). ART treatment of HIV-infected pregnant women will not eliminate MTCT of HIV due to multiple challenges such as lack of access to therapy (5), late presentation for prenatal care (6), and non-adherence to therapy during pregnancy and breastfeeding (7), which resulted in higher risk of primary HIV infection associated with pregnancy and breastfeeding. Thus, even with the effectiveness of maternal ART to prevent in utero and intrapartum transmissions, the breastfeeding route now accounts for >50% of pediatric infections (8). Because of the inherent limitations of ART-based prophylactic interventions, 1.7 million children live with HIV worldwide (4). The WHO guidelines recommend immediate initiation of ART in all infected children, committing them to a lifelong treatment. Prolonged ART exposure in children is associated with long-term metabolic consequences (9) and children are at a higher risk than adults for developing virologic failure (10). Thus, this population will highly benefit from interventions that afford viral remission and functional cure.

Early initiation of ART in HIV-infected infants is associated with significant benefits in HIV disease outcomes including reduction in mortality and morbidity, immune recovery, viral suppression, improved growth and development as well as prevention of end organ effects (11, 12). Although, ART initiation within hours of birth did not prevent establishment of the HIV reservoir of a newborn who was positive at birth and thus infected in utero (13), there could be a therapeutic window within hours of birth to prevent or limit the reservoir in peripartum-exposed infants. Globally, only approximately 50% of the infants who are exposed to HIV in utero, peripartum, and breastfeeding are tested before 8 weeks of age (3). With breastfeeding, it is particularly challenging to identify acute HIV infection in infants or to develop HIV testing algorithms to feasibly detect acute infection in this population. Delayed diagnosis and ART initiation in these infected infants result in high viral loads and potential to increase the pool of latently infected CD4⁺ T cells that comprise the latent viral reservoir. Thus, reducing the size of the latent viral reservoir through very early treatment in order to attain and sustain viral remission after ART discontinuation has been the focus of the global effort aimed at finding a functional cure for HIV in perinatal infection. In this review, we briefly summarize the progress made in the development of immune therapeutics in HIV cure preclinical and clinical studies. Additionally, we discuss the potential use of these therapeutics for the treatment of HIV-infected children and highlight data supporting their potential to achieve long-term HIV remission and functional HIV cure in the context of the early life maturing immune system.

ART for treatment of pediatric HIV infection

The introduction of combination ART regimens for children has significantly improved the prognosis of pediatric HIV infection. However, the current selection of antiretroviral (ARV) drugs that can be used for initial therapy in the pediatric HIV populations is very limited. The three-drug ARV regimens typically prescribed for HIV therapy to newborns with documented HIV-infection consists of two nucleoside reverse transcriptase inhibitors (NRTIs) and either an integrase strand transfer inhibitor (INSTI) or a non-nucleoside reverse transcriptase inhibitor (NNRTI) or a protease inhibitor (PI) (14). The timing of ART initiation post-infection is also a critical factor for ART efficacy. ART initiation in infants between 6 to 12 weeks of age immediately upon diagnosis (defined by a plasma HIV RNA level of >1000 copies/ml and a CD4⁺ T cell percentage of 25% or more) has been associated with reduced infant mortality rate (76% decrease) and slower HIV disease progression (75% decrease) as demonstrated in the Children with HIV Early Antiretroviral Therapy (CHER) trial based in South Africa (15). Based on these data, clinics have now implemented the practice of treating every infant with ART within 1–2 weeks of HIV diagnosis, regardless of their clinical status, CD4⁺ T cell percentages, or plasma viral load (14).

Several ongoing trials are investigating whether very early ART initiation (within days to a week of life) in infants limits the viral reservoir size and subsequently results in viral remission (Table 1). The Early Infant HIV Treatment in Botswana trial (EIT) (NCT02369406) will evaluate the virologic and immunologic outcomes of very early ART on the size and composition of viral reservoir (16). Similarly, the Latency and Early Neonatal Provision of Antiretroviral Drug Clinical Trial (LEOPARD) (NCT02431975) (17) will evaluate the impact of very early ART on establishment and maintenance of the viral reservoir in 60 HIV-infected infants identified within 48 hours of birth. Meanwhile, the Very Early Intensive Treatment of HIV-Infected Infants to Achieve HIV Remission (IMPAACT P1115) (NCT02140255) will monitor if the initiation of ART within 48hr of birth is capable of achieving HIV remission (18). Findings from these studies will provide novel insights into timing of ART, HIV reservoir dynamics and potential for ART-free remission that will inform the development of strategies to achieve long term viral remission in HIV-infected children. In evaluating the outcomes of these studies, it will be important to distinguish which treatments are most effective for the different types of exposure as these could differ significantly.

Table 1.

Current pediatric HIV interventions in clinical development.

Intervention strategy	Phase (Status)	Clinical trial number	Target population
Combination ART
Early Infant HIV Treatment in Botswana (EIT): Nevirapine (NVP), Kaletra (LPV/r), Lamivudine (3TC), Zidovudine (ZDV)	Phase II/III (Recruiting)	NCT02369406	HIV-infected
Latency and Early Neonatal Provision of Antiretroviral Drugs Clinical Trial (LEOPARD): NVP, ZDV, 3TC, LPV/r	Phase IV (Active)	NCT02431975	HIV-infected
Very Early Intensive Treatment of HIV-Infected Infants to Achieve HIV Remission: NRTIs, NVP, LPV/r, RAL, VRC01	Phase I/II (Recruiting)	NCT02140255 (IMPAACT P1115)	HIV-infected
Passive immunization (bNAbs)
VRC01 (CD4BS targeting) + combination with ART	Phase I (Recruiting)	NCT03208231 (IMPAACT 2008)	HIV-infected
VRC01LS (CD4BS targeting)	Phase I (Recruiting)	NCT03707977	HIV-infected
10–1074 (V3-glycan)
ART
VRC01 (CD4BS targeting)	Phase I (Recruiting)	NCT02256631	HIV-exposed infant
VRC01LS (CD4BS targeting)
VRC07–523LS (CD4BS targeting)

Unique aspects of the pediatric immune system that could be harnessed for HIV cure

The immune system is exquisitely adapted to the immunological challenges at different developmental stages of life. Notably, early life innate immune cells display distinct cytokine profiles when compared to adults, in which they produce lower levels of proinflammatory cytokines (IFN-γ, TNF, and IL-12) (19, 20) and secrete higher levels of immunoregulatory and anti-inflammatory cytokines. These cytokines promote a tolerogenic milieu with a distinct Th17 (IL-6, IL-1β, and IL-23) (19), Th2 (IL-4, IL-5, IL-9, and IL-13) (21, 22), and regulatory T cell (IL-10 and TGF-β) bias (23). Thus, immune ontogeny in utero and in early life establishes a highly tolerogenic and broadly anti-inflammatory immune environment that may influence the efficacy of immune interventions towards a functional HIV cure in pediatric populations. Some of the unique aspects of the early life immune system that may favor HIV cure include: (1) the tolerogenic immune environment, which promotes low immune activation (19, 20) that contributes to the low expression of the HIV co-receptor CCR5 in memory CD4⁺ T cells, thus could limit the establishment of latent virus reservoir (24); and (2) the unbiased CD8⁺ T cell repertoires that generate de novo autologous variant-specific anti-HIV cytotoxic T lymphocytes (CTL) responses early in HIV-infected children (25). The immune system in utero and in early life favors low levels of immune activation, at least initially, following HIV infection but is then skewed towards increased immune activation associated with increased viral reservoir size, viral replication, microbial translocation, and mucosal inflammation as the disease progresses. The overall balance between these opposing influences may depend crucially on the timing of ART initiation. Early ART initiation will minimize the size of the latent viral reservoir as well as chronic immune activation and provide an opportunity for introduction of immune therapeutic interventions. Despite lack of HIV-specific immunity in infants who are treated early with ART, they appear to maintain normal immune system development and more active thymus (26), suggesting the potential for therapeutic vaccines that control latent or persistent HIV reservoir in the absence of ART.

Differences in pathogenesis between HIV infected adults and children may also enhance the potential for a cure in HIV-infected neonates. The latent virus reservoir in HIV-infected infants is more homogeneous than in adults as it has not been selected by CTL and other immunologic pressures (27). Importantly, the rate of cell-associated virus decay following very early ART initiation is faster in infants than in early-treated children and adults, and the decrease in virus reservoir size continues for a longer time period of time in children as compared to adults (28–31). Recent studies have reported that HIV-infected infants develop bNAb responses earlier and more frequently than untreated HIV-infected adults, suggesting that this immune response could be harnessed in immune strategies towards a cure (32, 33). Importantly, although infants showed a delay in the development of antibodies capable of mediating antibody-dependent cellular cytotoxicity (ADCC), these effector functions are associated with a better disease outcome in HIV-infected infants, suggesting the ability to mediate clearance of HIV-infected cells (34, 35). Altogether, these observations provide a strong rationale for the development of strategies that harness the uniqueness of the early life immune system for the remission and/or eradication of HIV in children.

Immunomodulatory agents as adjunctive HIV treatment

HIV infection, in the absence of ART, results in profound immunological abnormalities with declines in CD4⁺ T cell counts, chronic immune activation, and cytokine deregulation [reviewed in(26, 36)]. The immune system can partially be restored after sustained ART-mediated viral suppression, but complete immune reconstitution might not be possible in chronically infected individuals (37). While several completed and ongoing clinical trials have evaluated the potential of immunomodulatory agents in reducing the size of the viral reservoir and in delaying rebound in HIV-infected adults, in this review, we focus on the immunoreconstitutive, anti-inflammatory, and anti-proliferative regimens that have shown promising results in adults and therefore could be beneficial in the pediatric settings.

a. Immuno-reconstitutive HIV therapy

A paradigm of HIV therapeutics involves the use of immune stimulants in conjunction with ART to expand the immune repertoire and enhance HIV-specific immunity while the virus is suppressed. Interleukin-2 (IL-2) is a secretory cytokine derived from T and NK cells which can induce T cell proliferation and regulate differentiation and survival of T cells (38). Early clinical studies in HIV-infected adults indicated that incorporating IL-2 in the ART regimen resulted in significantly increased in CD4⁺ T cell counts compared to ART by itself (39). Based on these results, two large multicenter phase III randomized open-label clinical trials, the Subcutaneous Recombinant, Human Interleukin-2 in HIV-Infected Patients with Low CD4⁺ Counts under Active Antiretroviral Therapy (SILCAAT) and the Evaluation of Subcutaneous Proleukin in a Randomized International Trial (ESPRIT) were conducted (40). These trials demonstrated that while IL-2 therapy of HIV-infected adult patients on ART resulted in sustained increase in CD4⁺ T cell counts, this therapeutic strategy did not provide clinical benefit in terms of the risk of AIDS-related opportunistic diseases and death or HIV RNA loads. IL-2 has also been recently hypothesized to purge the HIV latent reservoir by activating T cells for recognition by autologous HIV-specific CD8⁺ T-cells, thereby reducing the size of viral reservoirs and a pilot study was initiated to investigate the impact of IL-2 therapy on the size of HIV reservoir in HIV-infected ART suppressed patients (NCT03308786) (41). IL-2 was also shown to be safe and was associated with immune recovery in children and resulted in increased CD4⁺ T cell counts (42).

Interleukin-15 (IL-15) is a cytokine that is important for NK cell functions and CD8⁺ T cell homeostasis (43). IL-15 super agonist, a combination of IL-15 and IL-15R, enhanced CD8⁺ T cells and NK cell activity in pre-clinical models (44, 45). Additionally, IL-15 super agonist, ALT-803, inhibited HIV infection in a humanized mouse model and simian immunodeficiency virus (SIV) rhesus macaque model (44, 46). Based on these results, ALT-803 is currently evaluated in a phase I clinical trial for its potential towards clearing latent HIV infection (NCT02191098) (47–49). Similar to IL-2 and IL-15, interleukin-7 (IL-7) is another immunomodulatory therapy that has been demonstrated to expand naïve and memory CD4⁺ T cells in phase I/II trials (50, 51) and might be a potent candidate for adjunct HIV therapy. Conversely, proliferative cytokines such as IL-15 and IL-7 may also increase the size of the virus reservoir due to enhanced lymphocyte activation and proliferation. However, these proliferative effects were shown to be negligible compared to their ability to perturb the immune system (49).

b. Anti-inflammatory and anti-proliferative HIV therapy

Highly proliferating and activated CD4⁺ T cells are preferred targets for HIV replication, thereby resulting in disease progression. Furthermore, cellular proliferation has been associated with viral persistence during latency (52–55). Using mathematical models, Reeves et. al. have demonstrated that efficient reductions in the rate of CD4⁺ T cell proliferation will exhibit reduction in latent reservoir size (56), and the use of anti-inflammatory and anti-proliferative therapeutic agents has been proposed to limit activation-induced viral replication and viral reservoir establishment (57). One of the anti-inflammatory agents that has been investigated for targeting HIV replication is Cyclosporine A (CsA), a therapeutic agent that inhibits IL-2 transcription and hence IL-2-mediated activation and proliferation (58). While a combination therapy of CsA and ART has been demonstrated to improve levels of CD4⁺ T cells and HIV-specific immune responses in acutely HIV-infected adults (59), no notable impact was observed on HIV viral loads (60).

On the other hand, Mycophenolate mofetil (MMF), a morpholinoethyl ester of the active compound mycophenolic acid (MPA) has been hypothesized to inhibit HIV replication by a dual mechanism, which involves (1) an antiviral mechanism that depletes the substrate for reverse transcriptase, and (2) an immunologic mechanism that targets the proliferation of lymphocytes (61). The effect of MMF on cellular proliferation and HIV replication has been extensively studied both in vitro and in vivo (62). The potential of MMF as adjunct therapeutic along with ART has been investigated in pilot clinical studies. In one study involving chronically HIV-infected patients, MMF was found to reduce the size of dividing CD4⁺ T cells and the reduction was found only to be correlated with viral replication in patients who maintained the capacity to inhibit lymphocyte proliferation (61). In another placebo-controlled pilot study in HIV-infected adults, MMF was associated with decreased T cell activation; however, the impact on viral load was not clearly understood (63). More importantly, in both studies, MMF appeared to be safe with no adverse side effects. A phase I clinical trial is currently ongoing to evaluate the impact of MMF on HIV reservoir size (NCT03262441) (64).

Sirolimus or Rapamycin is an mTOR inhibitor (65) that interferes with IL-2 signaling, thereby inhibiting activation of T cells (66). It has been demonstrated to dampen HIV replication in vitro (67), in murine preclinical models of HIV infection (68) and in HIV-1 infected patients undergoing liver transplantation (69). Currently, Sirolimus is being investigated in a phase I/II trial for its potency to reduce the HIV reservoir size in individuals on suppressive ART (NCT02440789) (70). Additionally, newer anti-inflammatory anti-proliferative approaches are under evaluation for their potency to reduce viral reservoir size and delay time to HIV rebound. Heat shock protein 90 (Hsp90) inhibitors have shown promising results in reducing HIV reservoir and delaying viral rebound in humanized mouse models (71) and appear promising to be studied in non-human primate models of HIV cure. Recently, an inhibitor of the Wnt/β catenin pathway, PRI-724, was demonstrated to decrease proliferation of central memory (CM) and T memory stem cells (SCM) and induce modification of the SCM and CM CD4⁺ T-cell transcriptome towards a more differentiated memory T-cell profile in SIV-infected ART-suppressed rhesus macaques. However, short-term treatment of ART-suppressed macaques with PRI-724 was not associated with reduced viral reservoir size (72).

In children infected with HIV early in life who remain on lifelong suppressive ART, persistent immune dysregulation can predispose them to non-AIDS related morbidity and mortality (73). Furthermore, a combination therapy of immunomodulatory agent and ART offers the unique opportunity of targeting the virus in a multi-pronged manner, by inhibiting cell-to-cell viral spread using immunomodulatory components in addition to the antiviral effect of ART (Figure 1). This approach might be particularly relevant in rapidly controlling viral replication in infants who are infected early in life, thereby reducing their HIV reservoir size and hence delaying rebound post-treatment interruption (Figure 1). While no major adverse effects have been reported in trials with these immunomodulatory agents in HIV-infected adults, the unique differences in early life immune system versus in adulthood pose potential safety concerns that warrant caution for their use in the pediatric populations. The long-term effects of these immunomodulatory agents on a developing immune system in infants/children are less known and therefore necessitate extensive area of investigations in order to fully capitalize on the full benefit of immunomodulatory agents for eradication of pediatric HIV.

Fig 1.

Timing for immune therapeutic interventions in HIV-infected children. In utero infection and breast feeding period provide a unique early intervention opportunity using combination of ART and passive adminstration of bNAbs to supress viremia and limit establishment of latent virus resevoir as well as virus diversity. During the main window of opportunity (from 2 to 9 years of age), children’s immune ontogeny favors the developemnt of cell-mediated immunity that is likely to play a major role in eradication of HIV infection. Late immune therapeutic interventions provide further potentials to achive virus control, remission, and functional cure prior to sexual debut as well as ART interruption. Created with BioRender.

Broadly neutralizing antibodies (bNAbs) for HIV treatment

Daily burden and unwanted clinical toxicities associated with ART emphasize the need for alternative strategies to induce long-term HIV remission upon ART cessation. The dual functionality of bNAbs via their antigen binding variable (Fab) and constant (Fc) domains make them attractive both as prophylactic agents and as immunotherapeutic adjunctives or alternatives to ART (74). Unlike ART, bNAbs could potentially neutralize virus via the Fab domain (75), clear infected cells via the Fc domain engagement with host immune cell receptors (76, 77), and stimulate the host immune cells effector functions via the formation of antigen-antibody immune complexes that enhance antigen presentation (76, 78). Previous reviews have summarized preclinical and clinical studies assessing the role of bNAbs for HIV prevention (79–82). In this study, we therefore focus on preclinical and clinical studies assessing bNAbs as HIV therapeutics (81–83).

a. Preclinical evidence of bNAbs efficacy for HIV therapy

Early studies demonstrating the therapeutic effects of passively infused bNAbs were conducted in humanized mice models of HIV infection. Passive infusion of humanized mice infected with the HIV YU2 strain with a single bNAb or a combination of three or five bNAbs, effectively controlled HIV-infection, suppressing viral load levels to below detection and delaying the time to virus rebound after ART cessation (84). Importantly, rebound viruses following tri- and penta-mix bNAb therapy cessation remained sensitive to neutralization by combination bNAbs (84). In a separate study, combination therapy with 3BNC117, PG16, and 10–1074 in HIV YU2-infected humanized mice starting 2-weeks post-infection resulted in reduction of both plasma viral RNA and cell-associated viral DNA, suggesting a potential sterilizing effect of combination bNAbs (85). Triple bNAb therapy (3BNC117, PG16, and 10–1074) was also demonstrated to interfere with the establishment of latent viral reservoir, and when combined with latency reversing agents (LRAs), resulted in delayed time to viral rebound in 50% of HIV-infected humanized mice (86).

Subsequent studies in non-human primate models also supported the potential use of bNAbs in HIV therapy (87, 88). Intravenous administration of 3BNC117 and 10–1074 to adult rhesus macaques chronically infected with simian-human immunodeficiency virus (SHIV) SHIV AD8EO resulted in a rapid decrease of plasma viral RNA, although no consistent change was observed in cell-associated viral DNA (87). When compared to monotherapy with a single bNAb (3BNC117 or 10–1074 alone), the administration of combination bNAb therapy (3BNC117+10–1074) increased the time of virus suppression from 4–7 days to 18–36 days in SHIV chronically-infected macaques (87). Early administration of monotherapy with a single bNAb (3BNC117 or 10–1074 alone) resulted in lower levels of persistent viremia, which led to establishment of robust CD8⁺ T cell responses in SHIV-infected adult macaques, suggesting that early treatment with bNAb can control or eliminate infection (89). In another study, dual bNAb therapy with 3BNC117 and PGT121 resulted not only in rapid control of plasma viral RNA in SHIV SF162P3-infected adult macaques, but also reduced cell-associated viral DNA in gut and lymph node tissues (88). In infant macaques, the effectiveness of combination bNAbs passive infusion in controlling peak viremia during acute infection was demonstrated using an oral challenge SHIV model (90). Dual bNAb therapy (PGT121+VRC07–523) protected all (10/10) infant macaques when given subcutaneously 24-hour after exposure to SHIV SF162P3, with absence of viremia, lack of antiviral immunity, and evidence of virus clearance in the blood and tissues at necropsies (90). This bNAb cocktail given subcutaneously at 30-hours after exposure protected all (6/6) infant macaques, but when given at 48-hours only protected half (3/6) infant macaques (91). On another hand, anti-HIV-specific bNAbs including PGT121, PG16, PG9, VRC01, VRC03, and 10E8, tested individually or in combination of two antibodies were demonstrated to effectively block HIV viral replication induced in latently infected CD4⁺ T cells isolated from ART-suppressed individuals in an in vitro model (75). Altogether, these results underscore the importance of combination bNAb cocktails and they suggest that more durable effects will be achieved with early intervention as opposed to chronic infection. The success of this therapy in non-human primates, including infants, in impacting the viral reservoir and improving antiviral immunity support the assessment of bNAb combinations in clinical trials.

b. Clinical trials of bNAbs for HIV therapy

Over the years, several clinical trials have evaluated the safety and tolerability of bNAbs in HIV-uninfected individuals and further evaluated the antiviral efficacy of the bNAbs in the absence of ART in HIV chronically infected subjects. Most of these studies demonstrated that bNAbs were safe and well tolerated in patients (92–96) apart from a phase I trial evaluating the safety of subcutaneous administration of 10E8VLS that was halted due to development of local reactogenicity as well as other clinical symptoms including fever and malaise (NCT03565315) (97). Detailed findings from these studies have been previously reviewed (80, 98, 99).

A recently completed phase I clinical trial in HIV-infected adults (NCT02825797) (96, 100), used a dual combination of bNAbs (3BNC117+10–1074) that target non-overlapping sites of HIV envelope (CD4bs- and V3-specific regions, respectively). The bNAbs were safe and well tolerated (96), with reduced risks of viral escapes to one bNAb and no observed viral escapes to both bNAbs in patients with preexisting sensitivities to both 3BNC117 and 10–1074 (95). Importantly, the dual bNAb therapy resulted in increased time to virus rebound following treatment interruption in individuals that harbor preexisting sensitive viruses and led to a reduction in the size of the latent viral reservoir, suggesting that bNAbs are able to mediate clearance of HIV-infected cells and potentially activate host immune cells (95). This study indicated that combination of bNAbs with distinct HIV envelope specificities can effectively maintain viral suppression in individuals harboring antibody sensitive viruses whilst restricting development of resistant variants and escape pathways conferred to given epitopes when present at sufficient plasma antibody levels. Additionally, the bNAb combination may have mediated clearance of virus-infected cells by engaging Fc effector functions of host immune cells.

Phase I clinical trials evaluating the safety, tolerability, and efficacy of engineered bNAbs designed to increase half-life, potency, and breadth are currently underway in both HIV-infected and HIV-uninfected adult populations. Notably, several trials are evaluating bNAbs with the LS mutation that significantly increases antibody half-life: VRC01-LS and VRC07–523LS (NCT02840474) (101), VRC07–523LS (NCT03387150) (102), 3BNC117-LS (NCT03254277) (103), and 10–1074-LS alone or in combination with 3BNC117-LS (NCT03554408) (104). Engineered bNAb constructs with specificities against multiple epitopes on HIV envelope (CD4bs, V1V2, MPER) and conserved cellular co-receptors (CD3, CD4, CD16) found on host immune receptors for increased breadth and Fc effectors functions are also evaluated for both safety and antiviral activities in chronically infected HIV positive adults including: bispecific antibody, 10E8.4/iMAb (NCT03875209) (105) and trispecific antibody, SAR441236 (NCT03705169) (106). Antibody modifications for enhanced Fc effectors functions such as the MGD014, a dual affinity re-targeting protein (DART) (NCT03570918) (107) and the GS-9722, a bNAb with modified Fc domain are currently evaluated for therapeutic and preventive efficacy in combination with ART or other bNAbs in the clinics after promising preclinical results (98).

Only a few clinical trials have been conducted in pediatric populations to date (Table 1). A recent clinical trial with passive administration of the bNAbs, VRC01 and VRC01-LS, at birth in infants born to HIV-infected mothers in USA, South Africa, and Zimbabwe demonstrated the safety and tolerability of these bNAbs in the pediatric setting (NCT02256631/IMPAACT P1112) (108). Ongoing studies are evaluating the impact of VRC01 in combination with ART on the clearance of HIV-1-infected cells in infants (NCT03208231/IMPAACT 2008) (109), and the effect of dual bNAb therapy of with VRC01-LS and 10–1074, in combination with ART in maintaining virus suppression in pediatric HIV populations (NCT03707977) (110). Effective bNAb-based strategies for long term viral remission in pediatric settings will likely require combination of bNAbs with the following characteristics: (1) targeting multiple HIV Env epitopes for prevention of viral escape; (2) with high potency and breadth for lower administered concentrations and larger coverage of target populations; (3) longer half-life to extend intervals between administration; and (4) capable of mediating robust Fc effector functions for clearance of HIV-1-infected cells.

Prospects for other immune therapeutics interventions in pediatric HIV cure

Preclinical studies in non-human primates have demonstrated that therapeutic vaccines can modulate immune responses in SIV-infected animals. Administration of a DNA vaccine that expressed the SIV capsid p27 Gag homolog of the highly conserved elements of the HIV proteome generated a robust anti-SIV-Gag cytotoxic T lymphocytes (CTL) response and resulted in controlled virus infection and lower plasma viral loads in SHIV-infected macaques (111). Several studies have demonstrated Toll-like receptor (TLR) agonists potency as adjuvants for pediatric vaccines (112–114), suggesting a high sensitivity of pediatric immune cells to these compounds. In one study, administration of Ad26/MVA (recombinant adenovirus 26 serotype (Ad26) prime/modified vaccinia Ankara (MVA) boost) with a TLR7 agonist resulted in a delayed viral rebound and lower plasma viral loads (a 2-log reduction) post treatment interruption in SIV-infected rhesus macaques treated with ART 7 days after infection (115). Notably, the breadth of the immune responses in these SIV-infected animals was directly correlated with the time to virus rebound albeit inverse correlated with plasma viral loads at set point. Importantly, recent studies have demonstrated that children are capable of generating high magnitude HIV-specific antibody responses and in some circumstances infant responses are more potent than adults (33, 116–118). The PEDVAC trial (Pediatric HIV Vaccine Trial) and the follow up EPIICAL (Early-treated Perinatally HIV-infected Individuals) trial aim to evaluate the proof-of-concept for combined DNA/MVA therapeutic vaccine administered with TLR4 agonist in inducing cellular and humoral responses in pediatric HIV populations (119, 120). Moreover, because of its broader unmodified naive T and B cell repertoires that could be effectively primed by vaccines, the maturating immune system in infants could provide an immunological advantage for therapeutic vaccination.

Identification of a novel class of drugs known as latency reversing agents (LRAs) present a novel strategy to expose and eliminate (“shock” and “kill”) latently infected cell in HIV infection. Different classes of LRAs and their mechanisms of action towards eradication of latent HIV provirus reservoirs including cytokines/receptor agonists, epigenetic modifiers, intracellular signaling modulators, and transcriptional elongation regulators have been reviewed in detail by Sadowski and Hashemi (121). The ability of receptor agonist such as TLR agonists to activate HIV expression in latently infected cells derived from ART-suppressed HIV-infected individuals (122, 123) and in ART-suppressed SIV-infected macaques (124), supports that TLR agonist can stimulate diverse immune effector functions and thus, mediate clearance of HIV-infected cells. The efficacy of TLR7 agonist co-administered with the bNAb PGT121 in stimulating immune system and maintained virus control after discontinuation of ART by targeting the virus reservoir in preclinical animal model demonstrated the potential combination strategy of targeting latent virus reservoirs during ART (124, 125). However, a recent clinical trial using TLR3 agonist, Poly-ICLC in HIV-infected adults treated with ART did not observe similar effect in reversing HIV latency or decreased size of latent virus reservoirs despite transient innate immune activation (126), suggesting that TLR agonist may act instead as a global immune activator. More recently, a study in SIV-infected ART-treated rhesus macaques demonstrated that while administration of TLR7 agonist resulted in transient increased in immune cell activation, plasma cytokines, and interferon stimulated genes (ISGs), it did not result in increase in plasma viremia or viral RNA-to-viral DNA ratio nor decrease in viral DNA in PBMCs or tissues (127). Consequently, the inconsistencies in these findings underscore the necessity for more preclinical studies to further our understanding of TLR agonists activities in order to explore their potentials in combination with other “shock” and “kill” interventions in the clinics towards pediatric HIV cure. The potential advantages of chimeric antigen receptor (CAR) T cells to target residually active HIV-infected cells has been discussed in detail (128). Recently, a universal CAR-T cell platform based on CTLs engineered to bind a variety of broadly neutralizing anti-HIV antibodies, known as convertibleCAR-T cells (cCAR-T) were developed and demonstrated to effectively kill HIV-infected CD4⁺ T cells from blood and tissues but not uninfected CD4⁺ T cells when tested in vitro (129). The main advantage of cCAR-T is that the platform is readily multiplexed with different anti-HIV antibodies targeting distinct epitopes. Thus, the promising in vitro efficacy of cCAR-T in reducing reactivated latent viral reservoir size from HIV-infected individuals warrants further exploration in preclinical animal models of HIV infection to determine both efficacy and safety of this novel approach in the pediatric population.

Conclusion

Global implementation of maternal ART to prevent MTCT and early infant ART to limit the size of latent reservoirs are vital elements in achieving the goal to eliminate pediatric HIV. However, continued progress towards eradication of pediatric HIV infections potentially requires combination of ART with other immune-based interventions. The early life immune system presents a unique opportunity for HIV cure potentials due to several reasons: (1) limited size of initial viral reservoirs, (2) immunotolerant immune environment that favors low levels of activation, (3) highly dynamic and adaptable immune ontogeny, and (4) optimal timing for both passive and active immunizations as part of childhood vaccination program. Importantly, distinct immune interventions may be required to achieve functional cure in pediatric populations depending on their age, duration of infection, and the timing of ART initiation (Figure 1). Early ART initiation could be administered with passively infused bNAbs at birth to suppress viremia and limit establishment of latent virus reservoirs as well as viral diversity in early pediatric HIV infections. As cell-mediated immunity has been shown to play a key role in eradication of HIV infection (130), immune interventions such as therapeutic vaccines that target the endogenous T and B cell can potentially augment the cellular and humoral immunity for eradication of early and established/chronic stage of pediatric HIV infection (Figure 1). Moreover, immune ontogeny between 2 to 9 years of age presents an optimal window of opportunity for therapeutic cure interventions in HIV-infected children due to increase immune activation (131). To fully harness the therapeutic potentials of immune interventions in eliminating pediatric HIV, there is an urgency to evaluate the optimal timing for their administration in both early and late ART-treated children.