The immunomodulatory role of seminal plasma in endometrial receptivity and embryo implantation

Abstract

Successful implantation and pregnancy rely on complex interactions between the embryo and the maternal reproductive tract. Seminal plasma components, including proteins, cytokines, and growth factors, are pivotal in enhancing endometrial receptivity and inducing maternal immune tolerance to the developing conceptus. Exposure to seminal plasma facilitates pathogen clearance, supports embryo development, and modulates immune responses by altering the endometrial transcriptome and promoting regulatory T cell (Treg) expansion. Proteomic studies have identified seminal plasma factors involved in these processes. Changes in the immunomodulatory components of seminal plasma can diminish its positive effects on the endometrium, potentially resulting in reduced fertility and increased risk of adverse pregnancy outcomes. This review explores how seminal plasma influences maternal immune responses and highlights the clinical implications, particularly its potential to improve outcomes in assisted reproductive technologies (ART) like in vitro fertilization (IVF). Understanding the molecular dialogue between seminal plasma and the endometrium may lead to new strategies for enhancing fertility and promoting healthy pregnancy.

Introduction

The successful establishment of pregnancy is a complex and delicate process that requires a harmonious interplay between the embryo and the maternal immune responses in the uterine environment [1]. The old concept that pregnancy is associated with immune suppression has created a myth of pregnancy as a state of immunological weakness and, therefore, of increased susceptibility to infectious diseases [2]. However, recent evidence underscores that the immune system during pregnancy is highly active, balancing the dual roles of fostering a supportive environment for fetal development and protecting both mother and fetus from pathogens.

A critical component of this dynamic interplay is the role of seminal plasma in modulating endometrial receptivity and inducing maternal tolerance to the semi-allogenic conceptus. Seminal plasma is a complex mixture of biomolecules such as proteins, lipids, carbohydrates, and signaling molecules. Seminal plasma serves as a protective and nourishing environment for sperm while also playing an essential role in facilitating communication between the male and female reproductive tissues. Seminal plasma is instrumental in preparing the female reproductive tract for pregnancy [3,4]. Upon deposition in the female reproductive tract, seminal plasma triggers immune responses that help pathogen clearance, enhance tissue remodeling, and establish maternal tolerance. This immune tolerance induction by seminal plasma has implications for improving outcomes in assisted reproductive technology (ART) and managing pregnancy complications.

This review discusses the immunological components of seminal plasma and their interactions with the female reproductive tract, emphasizing their role in shaping maternal immune tolerance and reproductive success.

Seminal plasma

Seminal plasma is a complex biological fluid that contains secretions from the testis, epididymis, prostate, seminal vesicles, and Cowper’s glands. It comprises various biochemical components, including exosomes, microRNAs (miRNAs), immunomodulatory agents (cytokines and chemokines), peptides, and proteins (Figure 1). Seminal plasma is traditionally considered a medium for sperm transport and protection within the female reproductive tract [5]. Beyond its transport function, seminal plasma facilitates maternal immune adaptation and endometrial receptivity, critical for successful implantation and pregnancy maintenance [6-8]. Here is a summary of seminal plasma components and their roles in modulating female reproductive immune responses.

Figure 1.

Anatomy of the male reproductive system and composition of seminal plasma. Left: Key anatomical structures involved in seminal plasma production, including the seminal vesicle, prostate gland, bulbourethral gland, epididymis, and testis. Right: Major constituents of seminal plasma: (A) immune factors (cytokines, chemokines); (B) exosomes; (C) proteins and enzymes; (D) sugars; (E) lipids; (F) minerals (Zn, Cu, Fe, Cr).

Immunomodulatory agents

The seminal plasma contains various signaling molecules with immunomodulatory effects that regulate immune responses in the female reproductive tract [9-13]. These molecules influence endometrial gene expression, immune cell recruitment, and local tolerance. Key agents include TGF-β, interleukin (IL)-10, IL-1, IL-6, IL-8, tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), granulocyte-macrophage colony-stimulating factor (GM-CSF), Activin A, prostaglandin E2 (PGE2), and soluble human leukocyte antigen G (sHLA-G) [14,15]. Their collective actions promote Treg induction, leukocyte activation, and tissue remodeling, which are essential for successful implantation (Table 1).

Table 1.

Key immunomodulatory agents in seminal plasma and their functions

Agent	Type	Primary Functions in Female Reproductive Tract
TGF-β (β1, β2, β3)	Anti-inflammatory cytokine	Induces Tregs, inhibits Th1 responses, enhances GM-CSF, IL-6, LIF expression; promotes immune tolerance [16]
IL-10	Anti-inflammatory cytokine	Suppresses inflammatory responses; promotes immune tolerance [17,18]
IL-1, IL-6, IL-8, IL-18	Pro-inflammatory cytokines	Stimulate leukocyte recruitment, endometrial gene expression (e.g., LIF), and embryo implantation readiness [19,20]
TNF-α, IFN-γ	Pro-inflammatory cytokines	Enhance early inflammatory responses; may influence uterine receptivity and local immunity [13,21]
GM-CSF	Growth factor/cytokine	Promotes endometrial remodeling, leukocyte activation, and embryotrophic support [13,22,23]
Activin A, Follistatin	TGF-β superfamily members	Induce cervical inflammation post-coitus; modulate immune response [19,20]
GDF15	Divergent TGF-β family	Possibly attenuates immune response during coitus [24]
PGE2	Lipid mediator	Promotes Treg development; induces tolerogenic DCs; enhances PTGS2 expression and immune modulation [25]
sHLA-G	Immune checkpoint molecule	Contributes to maternal-fetal tolerance; inhibits NK and T-cell responses [26]
VEGF, EGF, FGF, G-CSF	Growth factors	Support angiogenesis, embryo growth, endometrial receptivity [27]

TGF-β, Transforming Growth Factor Beta; IL, Interleukin; TNF-α, Tumor Necrosis Factor Alpha; IFN-γ, Interferon Gamma; GM-CSF, Granulocyte-Macrophage Colony-Stimulating Factor; GDF15, Growth Differentiation Factor 15; PGE2, Prostaglandin E2; sHLA-G, Soluble Human Leukocyte Antigen-G; VEGF, Vascular Endothelial Growth Facto; EGF, Epidermal Growth Factor; FGF, Fibroblast Growth Factor; G-CSF, Granulocyte Colony-Stimulating Factor; PTGS2, prostaglandin-endoperoxide synthase 2; LIF, leukemia inhibitory factor.

Exosomes

Seminal plasma is a rich source of exosomes and extracellular vesicles that carry proteins, lipids, and nucleic acids, including miRNAs. Examining proteins in seminal exosomes has found a diverse collection of proteins that play a crucial role in the interaction with the female reproductive system. Dysregulation of exosome-associated proteins has been linked to male infertility, particularly in conditions like varicocele. Profiling the exosomal proteome can provide insights into the underlying causes of infertility. Some important proteins can be found in seminal exosomes: RAB27A, KIF5B, CRISP1, SPAG11B, DEFB126, Transferrin, SEMG1, and protease [28]. The result of a study showed that exosome-associated proteins in seminal plasma actively signal to cells of the endometrium and can promote decidualization of endometrial stromal fibroblasts (eSFs) in women with and without inflammatory disorders through induction of IL-11 [24].

Researchers have also observed exosomes interact with the female reproductive tract epithelium, influencing gene expression and promoting an immune-tolerant environment [29]. The conditioned medium obtained from human endometrial cells exposed to seminal plasma exosomes stimulates the production of pro-inflammatory cytokines such as IL-1α and IL-6 while reducing the levels of the anti-inflammatory cytokine IL-10 [30]. Another study revealed that seminal exosomes are internalized by human endometrial stromal cells and subsequently induce them to produce IL-8 and IL-6, which are involved in embryo implantation [31]. It suggests seminal exosomes promote a pro-inflammatory uterine environment for implantation [5]. Seminal exosomes also interact with DCs and induce immature, tolerogenic DCs to promote a tolerogenic immune environment. The proteins constituted by seminal exosomes play an essential role in Tregs expansion, which recognizes paternal antigens and contributes to maternal immune tolerance [32].

Proteins

Seminal plasma is rich in a diverse array of proteins, which can be categorized into two main groups: those derived from blood plasma, such as albumin, prealbumin, and globulin, and those synthesized and secreted by the male reproductive organs, including clusterin, plasmin, lactoferrin, pro cathepsin D, and cholesterol transfer protein. These proteins play vital roles in regulating osmotic pressure, maintaining pH balance, and facilitating the transport of ions, lipids, and hormones within the seminal plasma [33]. The composition of seminal plasma proteins is both complex and species-specific. Ongoing research is focused on identifying the active factors within these proteins and understanding their mechanisms of action, particularly in humans.

Seminal plasma roles in pregnancy

Seminal plasma is crucial to pregnancy success by initiating maternal immune responses, facilitating embryo implantation, and modulating gene expression in the endometrium (Figure 2).

Figure 2.

Key roles of seminal plasma in reproductive processes. Seminal plasma influences multiple aspects of female reproductive physiology, including immune activation, sperm maturation and survival, maternal immune tolerance, embryo attachment, and modulation of gene and miRNA expression. These mechanisms collectively support implantation and early pregnancy.

Induction of maternal inflammatory responses

Following deposition in the female reproductive tract, seminal plasma triggers an immune response in females by inducing an influx of inflammatory cells. This intricate process involves the production of GM-CSF, IL-6, IL-1, IL-8, and a diverse array of chemokines [4]. These pro-inflammatory factors stimulate the extravasation and infiltration of macrophages, DCs, and granulocytes into the subepithelial stroma (Figure 3).

Figure 3.

Immune response to seminal plasma in the female reproductive tract. Seminal plasma components such as TGF-β, PGE2, PGI2, and proteins stimulate epithelial cells to produce pro-inflammatory cytokines and chemokines, recruiting immune cells, including neutrophils, macrophages, DCs, and uNK cells. Dendritic cells exposed to TGF-β acquire a tolerogenic phenotype and present antigens to naïve T cells in draining lymph nodes, promoting Treg expansion. These Tregs migrate to the implantation site to support maternal immune tolerance. TGF-β, transforming growth factor beta; PGE2, prostaglandin E2; PGI2, prostaglandin I2; DC, dendritic cell; uNK, uterine natural killer cell; Treg, regulatory T cell.

The maternal inflammatory responses by seminal plasma are diminished when a barrier method such as a condom is used. Bromfield et al. evaluated in mice the consequences for offspring of ablating the plasma fraction of seminal fluid by surgical excision of the seminal vesicle gland [15]. They found that the absence of seminal plasma was associated with down-regulation of the embryotrophic factors LIF, CSF-2, IL-6, and EGF and up-regulation of the apoptosis-inducing factor Trail in the oviduct [15].

The post-mating inflammatory response created by seminal plasma has been extensively studied in humans and animals. In a study conducted on pigs, researchers investigated the effects of seminal plasma exposure on the uterine environment [34,35]. The findings revealed that seminal plasma elicited a distinct immune response within the uterus, characterized by the upregulation of specific cytokines and chemokines. Notably, the expression of GM-CSF, IL-6, and monocyte chemoattractant protein (MCP)-1 was induced in the uterine tissue. Human studies have also demonstrated that sexual intercourse elicits a distinct immune response within the cervical tissues. Sharkey et al. showed an increase in CSF-2, IL-6, IL-8, and IL-1A expression in the human cervix after unprotected intercourse by seminal plasma exposure [36]. This inflammation leads to the robust recruitment of macrophages and dendritic cells to cervical tissue, including the epithelial layers and the underlying stromal compartment [36]. The influx of leukocytes requires direct interaction between seminal plasma and the female reproductive tract tissues, and using a condom during intercourse prevents the development of an inflammatory response [36]. Gutsche et al. conducted an in vitro study to examine the effects of seminal plasma exposure on the mRNA expression of cytokines in human endometrial epithelial and stromal cells [17]. Incubation of epithelial cells with 0.1, 1, and 10% seminal plasma resulted in concentration-dependent stimulation of IL-1β, IL-6, and LIF mRNA expression [17]. Cytokine mRNA expression increased 2-fold for IL-1β, 2.5-fold for IL-6, and 2.2-fold for LIF following stimulation with 10% seminal plasma [17]. Ochsenkühn et al. also found that seminal plasma can increase the expression of IL-1β and TGF-β in human endometrial epithelial cell cultures [37].

Research shows that sperm regulates female reproductive immune responses through seminal plasma components attached to sperm, like sialic acid residues [38]. This mechanism is believed to have an essential role in the dispersion of seminal components by sperm throughout the uterine environment [39]. A contribution of sperm to the regulation of female tract immune responses is indicated in several species. A study conducted in vivo on cows examined the local and systemic immune responses triggered by sperm following artificial insemination [40]. The results indicated that most sperm were quickly transported to the uterus within one hour, leading to pro-inflammatory responses in neutrophils and peripheral blood mononuclear cells (PBMCs). The upregulation of TNF-α, IL-8, IL-1β, and PGE characterized this response. In humans and mice, evidence showed that sperm signal in conjunction with the seminal plasma fraction elicits a maximal effect. In humans, artificial sperm insemination into the cervix results in an influx of neutrophils into surrounding tissues, potentially through complement activation [4].

Facilitation of embryo attachment

Seminal plasma facilitates embryo implantation and pregnancy success by modulating the maternal uterine environment. A wide range of molecules, such as GM-CSF, LIF, IL-6, TGF-β, TNF-α, TGF-α, insulin, insulin growth factor (IGF)-I and II, EGF, and heparin-binding-EGF (HB-EGF) have been found to contribute to embryo attachment potentially. These molecules are partially regulated by exposure to seminal plasma in the oviductal epithelium [24,41-43]. Seminal plasma contains a heterogeneous population of extracellular vesicles (EVs) involved in several reproductive physiological processes. EVs carry proteins, lipids, and microRNAs and can modulate endometrial gene expression. The major types of EVs found in seminal plasma are prostasomes (originating from the prostate), epididymosomes (from the epididymis), and Sertoli cell-derived EVs (from testicular cells like Sertoli cells) [44]. These EVs interact with the female reproductive tract epithelium and can modulate gene expression to create an environment conducive to embryo implantation [45]. Specific miRNAs like miR-21-5p encapsulated in seminal EVs can regulate endometrial receptivity and embryo-maternal communication [46]. Specific proteins like spermadhesins and enzymes like paraoxonase type 1 (PON-1) in seminal plasma may also play roles in embryo-endometrial interactions [47].

Induction of endometrial gene expression

Seminal plasma has been shown to modulate gene expression in the female reproductive tract, particularly in the uterus, which is essential for successful pregnancy. Various seminal plasma components, including PGs and TGF-β, mediate these effects.

Seminal plasma components like IL-8 and TGF-β induce LIF and LIF receptor (LIFR) expression [48]. Their expression decreases in individuals with infertility and those suffering from recurrent early abortions, suggesting their involvement in the process of implantation and early pregnancy [49]. Seminal plasma upregulates the expression of these genes, thus enhancing the receptivity of the endometrium for embryo implantation [42,49-51]. Homeobox A-10 (HOXA-10) is a member of the GATA family of transcription factors and plays a key role in several signaling events during implantation [52]. Seminal plasma has been shown to regulate its expression, which is critical for successful implantation [3,53]. The human endometrium activates HOXA-10 during the implantation window. Its levels significantly increase during the middle secretory phase of the menstrual cycle [54]. Animal studies have shown that mice lacking the HOXA-10 gene are remarkably infertile and exhibit structural abnormalities in their reproductive systems. Seminal plasma also increases the expression of Mucin-1 (MUC-1), which helps create a tolerogenic environment for the embryo [3,55-57]. This gene is involved in the innate immune response and prevents microbes from entering the uterus. Investigation showed that the expression level of MUC-1 was increased in the human endometrial epithelial cells treated with the semen of healthy men compared to oligoasthenoteratozoospermia ones [58].

Exposure of endometrial epithelial cells to seminal plasma increased the expression of VEGF, EGF, FGF-1, and FGF-2. These genes are involved in endometrial angiogenesis and tissue proliferation, leading to successful implantation [17,42,59,60]. An increased expression of VEGF and FGF-1 was observed in the oviduct of seminal plasma-treated gilts [61]. A study using HeLa cervical adenocarcinoma cells investigated the role of seminal plasma in regulating neoplastic cervical epithelial cell growth and tumorigenesis [62]. They showed that seminal plasma-induced cytokine production, VEGF-A expression, and cell proliferation by induction of the inflammatory PTGS pathway [62]. HB-EGF belongs to the EGF family of growth factors. This gene plays a role in preparing the uterus for endometrial receptivity. Seminal plasma enhances HB-EGF upregulation, preparing the uterus for implantation [3]. The expression of this growth factor associated with VEGF, FGF-2, EGF, LIF, LIF-R, HOXA10, MUC1, and CSF increased in mice after seminal plasma exposure compared to vasectomized mice [3]. GM-CSF is continuously released by the epithelial cells that line the uterine endometrial stroma and make up the endometrial glands, with the highest production occurring during ovulation [63,64]. Seminal plasma exposure significantly enhances GM-CSF expression in the female reproductive tract, particularly in the uterus [16,35,65,66]. This enhancement is critical for a successful pregnancy and the prevention of sexually transmitted diseases. The TGF-β components of seminal plasma are proposed as a potent stimulator for GM-CSF expression in uterine epithelial cells. Comparable amounts of recombinant TGF-β1 stimulated GM-CSF release in cultures of uterine epithelial cells from estrous mice and, when instilled into the uterine lumen, caused an increase in GM-CSF content and an infiltration of leukocytes into the endometrium similar to the post-mating response [67].

IL-1β, IL-6, IL-17A, IL-8, and TGF-β1 are inflammatory cytokines important for embryo implantation during pregnancy. These cytokines are expressed by uterine epithelial cells after seminal plasma exposure [68-71]. A dose- and time-dependent induction by seminal plasma of IL8, IL6, CSF-2 and CCL-2 mRNA expression in ectocervical epithelial (Ect1) cells was verified by quantitative RT-PCR [36]. Seminal plasma components, especially PGE-2, are considered the primary stimulator of IL-8 release from human cervical explant [72].

Modulation of miRNA expression

MicroRNAs are small, non-coding RNAs with significant regulatory functions in biological processes, such as modulating the immunological environment. After fertilization, miRNAs perform essential roles in pregnancy, with numerous miRNAs being associated with endometrial receptivity (miR-30 family, miR-494, and miR-923) [73], implantation (miR-101 and miR-199a) [74], placental function (miR-17-92, miR-371-) [75], and labor (miR-223, miR-3, and iR-200) [76]. The exposure of the female reproductive tract to different seminal plasma fractions resulted in the differential expression of several miRNAs, with the most significant changes observed in the media of uterine explants incubated with seminal plasma from the post-sperm-rich fraction. Seminal plasma exposure primarily affected the miRNAs miR-34b, miR-205, miR-4776-3p, and miR-574-5p [77]. Seminal plasma also causes an increase of various immune-regulatory miRNAs in the female reproductive tract in patterns that correlate with the activation of tolerogenic DCs (tDCs) and Tregs. Interaction with seminal plasma induces two crucial miRNAs associated with immunological tolerance, including miR-223 and miR-146a [78]. Bioinformatics analysis identified that predicted target genes of dysregulated miRNAs, mainly miR-34b, miR-205, miR-4776-3p, and miR-574-5p, were involved in several immune-related pathways, such as Th1 and Th2 cell differentiation, cytokine-cytokine receptor interaction, T cell receptor signaling pathway, TGF-β signaling pathway and pathways involved in cellular processes, such as PI3K-Akt signaling pathway, focal adhesion, cell adhesion molecules, MAPK signaling pathway, and Wnt signaling pathway [77].

Enhancement of maternal immune tolerance

Seminal plasma plays a central role in establishing maternal-fetal immune tolerance by modulating populations of uterine natural killer (uNK) cells, Tregs, dendritic cells, macrophages, and neutrophils (Figure 3).

Uterine natural killer cells

uNK cells are an immune subset located in the uterus. uNK cells have distinct tissue-specific characteristics compared to their peripheral blood and lymphoid organ counterparts. These cells are abundant in the secretory endometrium and decidua, especially during early pregnancy, accounting for 70% of leukocytes. uNK cells protect the host from pathogen invasion and contribute to a series of physiological processes that affect successful pregnancy, including uterine spiral artery remodeling, fetal development, and immunity tolerance [79]. Seminal plasma significantly influences uNK cells, which facilitate immunological tolerance during pregnancy. Some studies have documented the impact of seminal plasma exposure on the populations of uNK cells in the human reproductive tract [36,80]. The number of CD56⁺ uNK cells in the uterine endometrium of women is higher [80], as are the CD57⁺ NK cells in the ectocervix after unprotected sexual intercourse [36]. uNK cells are less cytotoxic than peripheral ones, and their cytotoxic activity is downregulated during preimplantation. Some in vitro studies have shown the suppressive effects of seminal plasma exposure on the cytotoxicity mediated by NK cells [81]. PGE and polyamines in seminal plasma are responsible for such an inhibitory effect on the lytic activity of NK cells [82,83].

T cells

A key component of the female reaction to seminal plasma is the production of an adequate T-cell reaction to paternal antigens. To have a successful pregnancy, the T cell response must promote the expansion of immune-suppressive Tregs, identified by the transcription factor Foxp3. Different Th cells exert various effects in the context of pregnancy. Th1 is needed to induce an inflammatory response that facilitates embryo implantation, while Th2 is necessary for pregnancy maintenance. Besides these, Tregs play essential roles in inducing and mediating a tolerogenic state during pregnancy [84,85]. The modulation of this T cell response by seminal plasma during fertilization has been thoroughly investigated in human and animal models [86-88].

Seminal plasma is rich in immune-regulatory substances that contribute to its capacity to stimulate the production of Tregs, including TGF-β and PGE-related molecules, specifically 19OH-PGE1 and 19OH-PGE2 [89,90]. They have been linked to the generation of Tregs by enhancing the development of naïve CD4⁺ CD25^- T cells into suppressor T cells that express Foxp3 [91]. Remarkably, the concentration of TGFβ in seminal plasma is one of the highest recorded in biological fluids, reaching around 500 μg/mL in human semen. Male accessory glands, such as the seminal vesicle and prostate, mainly produce it [90]. Investigations have shown that the administration of TGFβ from an external source increases the number of Tregs in the vagina and decreases the occurrence of fetal loss in the abortion-prone CBA/J × DBA/2J mouse model [92]. Robertson et al. examined male seminal fluid’s role in female tolerance induction to fetus using paternal tumor cell grafts and by delayed-type hypersensitivity (DTH) challenge on Day 3.5 postpartum [7]. They found that exposure to seminal fluid inhibited rejection of paternal tumor cells through decreased type 1 immunity. The efficacy of this effect is particular to antigens, as seminal plasma from males with different MHC is less effective at establishing tolerance. They also showed that mating with intact males suppressed the DTH response to paternal alloantigens in an MHC-specific fashion. At the same time, excision of the seminal vesicle glands diminished the tolerance-inducing activity of seminal fluid. In addition, they detected an increase in CD4⁺CD25⁺ cells expressing Foxp3 in the para-aortic lymph nodes, draining the uterus of mice after mating with intact males. The increase in CD4⁺CD25⁺ cells was abrogated when males were vasectomized, or seminal vesicles were excised. Collectively, these data provide evidence that exposure to seminal fluid at mating promotes a state of functional tolerance to paternal alloantigens that may facilitate maternal acceptance of the conceptus at implantation, and the effects of seminal fluid are likely to be mediated by expansion of the Tregs pool and modulation of type 1 immunity [7].

Further studies have demonstrated that Treg cells’ activation and expansion directly result from seminal plasma exposure during mating (Figure 3). Initially, it was discovered that mating results in the expansion of T cells in the para-aortic lymph nodes that receive drainage from the uterus. Subsequently, it was shown that many reactive lymphocytes that responded were Treg cells. By the 3.5th day after sexual intercourse, the number of Tregs increases by approximately two times compared to the estrus cycle in mice. This increase is due to the enhancement of their recruitment from the circulation into the implantation site by the effect of CCL19 [7,93].

The development of Tregs following mating necessitates DCs transporting antigens to the lymph nodes, which drain the uterus. DCs prompt naïve T cells to proliferate and acquire a Treg phenotype [94]. Tolerogenic DCs form paternal antigen-specific Tregs during pregnancy [95,96].

T cells with a regulatory phenotype can emerge due to interactions with other immune cells. Neutrophils can activate pro-angiogenic Tregs by transporting pro-apoptotic proteins [97]. In addition, Tregs interact with mast cells to generate a pro-angiogenic phenotype [98]. This phenomenon is clearly shown in the CBA/J × DBA/2J model, where the transfer of Tregs increases mast cells. These mast cells promote T cell-mediated tolerance by producing IL-9 and TGF-β [99].

In vitro studies have also shown that seminal plasma can directly influence the differentiation of DCs into a tolerogenic phenotype characterized by increased production of IL-10 and TGF-β [72,100,101] and the differential activation of Tregs [86,101]. Our previous research has demonstrated that exposure to seminal plasma can stimulate the expression of IL-10 and TGF-β, as well as the expansion of Tregs, which collectively contribute to improving IVF outcomes in couples with unexplained infertility [102-104].

Dendritic cells

DCs are renowned for their ability to shape the adaptive immune response by presenting antigens and guiding T cells to adopt specific immune functions. Additionally, they can interact with non-immune cells and play a vital role in forming the uterine lining and preparing the uterus for embryo implantation. These cells are present in the non-pregnant endometrium of both humans and rodents and accumulate in the uterus before implantation, persisting throughout the entire pregnancy [105,106]. Rodents, pigs, and human seminal plasma are essential in recruiting DCs [35,66,107,108]. DCs in the uterus take paternal alloantigens in seminal plasma and transport them to lymph nodes via the uterine lymphatic system. Once in the lymph nodes, the cells present the antigen and stimulate T cells that mediate immunological tolerance towards the paternal alloantigens expressed by the implanting embryo [94]. Human monocytes exposed to highly diluted seminal plasma differentiated to DCs that did not express CD1a but displayed higher levels of CD14, known as tolerogenic DCs [107]. The inhibitory effect of seminal plasma on DCs is presumably induced by specific prostaglandins, including PGE-l, 19-OH-PGE-l, and PGE-2, found in seminal plasma at high concentrations [107]. It is impossible for them to fully develop following exposure to lipopolysaccharides (LPS), TNF-α, CD40L, Pam2CSK4 (TLR2/6 agonist), or Pam3CSK4 (TLR1/2 agonist). Upon activation, they produced low levels of the inflammatory cytokines IL-12p70, IL-1β, TNF-α, and IL-6 but showed a significant capacity for producing IL-10 and TGF-β [107,109]. Seminal plasma proteins, such as clusterin, stimulate DCs’ ability to induce CD25⁺FoxpF3⁺CD4⁺ T lymphocyte expansion via DC-SIGN (a C-type lectin receptor selectively expressed on DCs) [110]. Additionally, the immunomodulatory properties of seminal plasma may assist spermatozoa to circumvent the attack of DCs in the female reproductive tract, thereby promoting successful fertilization [111].

Macrophages

Macrophages at the maternal-fetal interface play a critical role in fetal tolerance, priming of cervical tissues, parturition, postpartum tissue repair, and displaying self-renewal capacity [112]. Macrophages efficiently capture and transport antigens from seminal plasma to draining lymph nodes. It leads to the activation of immune responses against paternal MHC and other antigens found in semen. Macrophages release enzymes and signaling molecules that alter the luminal epithelial glycocalyx and stromal extracellular matrix composition. This modification supports embryo attachment and promotes trophoblast invasion during the initial stages of placental development [14]. Seminal plasma increases the expression of pro-inflammatory cytokines, such as IL-8, IL-1β, MCP-1, and GM-CSF, in cervical epithelial cells, which can promote the recruitment and activation of macrophages in the female genital tract [60,113]. Exposure to seminal plasma in mice increases the number of macrophages in the corpora lutea. These cells are crucial in remodeling activities to maintain steroidogenic function [114]. Studies have demonstrated a clear correlation between administering seminal plasma in pigs during ovulation and increased numbers of corpus luteum macrophages, enhanced steroid production, and heightened levels of progesterone in the bloodstream [115]. Researchers gave gilts a mixture of seminal plasma through a transcervical catheter. They found that when seminal plasma interacts with uterine cells, it raises GM-CSF, IL-6, and MCP-1 levels and helps white blood cells move around [116]. The inflammatory cells that invade the endometrium seem to remain and undergo differentiation in the tissue for several days, while the luminal neutrophil response is resolved within 24 hours. This process increases locally activated macrophage and dendritic cell populations throughout preimplantation [116].

Neutrophils

After mating, the initial influx of neutrophils in the female reproductive tract plays a crucial role in regulating the immune response. These neutrophils, abundant during the early post-mating inflammatory response, exhibit multifaceted immune functions. Their primary capabilities include effectively eliminating pathogens and tissue debris, which enables them to clear excess sperm and seminal plasma, thereby preventing the spread of sexually transmitted infections [4]. The constituents of seminal plasma actively contribute to the transit and viability of spermatozoa in the female reproductive tract [117]. In vitro studies have reported that seminal plasma can reduce neutrophil function, which may benefit sperm survival and fertility [118].

The interactions between seminal plasma and neutrophils may enhance the adaptive immune response to antigens in seminal plasma. Neutrophils, which can function as antigen-presenting cells, can influence the antigen-presenting milieu by promoting the growth of Tregs, thereby modifying the immune response [97,119].

Song and his colleagues discovered that after insemination in mice, neutrophils move and gather around the uterine epithelium, accompanied by a rise in IL-17A levels [70]. Inhibiting IL-17A decreased the quantity of neutrophils in the uterus by reducing the chemokines CXCL1, CXCL2, and CXCL5. They discovered that seminal plasma can stimulate γδ T cells to produce IL-17A.

Assisted reproductive technologies

Seminal plasma is vital in ART because it strengthens the reproductive environment’s quality and increases the likelihood of successful implantation and conception. Studies have shown that seminal plasma exposure can improve the chances of successful pregnancy in women undergoing IVF and intracytoplasmic sperm injection (ICSI). A meta-analysis of seven randomized controlled studies found that using seminal plasma as an auxiliary measure during IVF significantly increased clinical pregnancy rates (CPR) [120]. An analysis combining data from eight randomized controlled studies found that intracervical seminal plasma application during oocyte collection significantly affected CRP. These findings indicate that localized seminal plasma administration at the cervix can improve IVF effectiveness by creating a more favorable environment for embryo implantation [121]. Only a few randomized controlled studies provide the available data on birth outcomes. These trials showed a slight rise in live birth rates, but this increase was not statistically significant in all meta-analyses. This highlights the need for more comprehensive and well-designed research to understand better the influence of seminal plasma on IVF success rates and birth outcomes [120,121]. Researchers have found that seminal plasma components like proteins and cytokines most effectively improve IVF success. Key proteins linked to successful IVF outcomes include A2LD1, ATP1B3, FBXO2, PTGDS, clusterin, and NPC2 [122-124]. These proteins contribute to sperm maturation, capacitation, antioxidative defense, and immune modulation (Table 2).

Table 2.

Seminal plasma proteins associated with IVF outcomes, their functions, and expression patterns

Protein	Function/Role in Reproduction	Expression Level in IVF Outcome
A2LD1	γ-glutamyl amine acyltransferase; may degrade crosslinked proteins in oocyte/granulosa cells	↑ Upregulated in successful IVF [122]
ATP1B3	Maintains ion gradients critical for sperm motility and function	↑ Upregulated in successful IVF [122]
FBXO2	E3 ubiquitin ligase; regulates protein turnover and impacts sperm quality	↑ Upregulated in successful IVF [122,125]
PTGDS	Converts PGH2 to PGD2; linked to sperm maturation and motility	↑ Positively correlated with better IVF outcomes [124]
DJ-1	Antioxidant; protects sperm from oxidative damage, maintains motility	↓ Downregulated in asthenozoospermia [126]
Clusterin	Chaperone; involved in sperm maturation, motility, and capacitation	↑ Higher in successful IVF [123]
NPC2	Cholesterol transport; maintains sperm membrane integrity and protects against oxidative stress	↑ Higher in successful IVF [123]
PSA	Facilitates semen liquefaction; correlated with improved sperm quality	↑ Higher in successful IVF [127]

A2LD1: γ-glutamyl amine acyltransferase; ATP1B3: ATPase Na⁺/K⁺ transporting subunit beta 3; FBXO2: F-box only protein 2; PTGDS: Prostaglandin D2 Synthase; NPC2: Niemann-Pick C2 protein; PSA: Prostate-Specific Antigen.

Seminal plasma contains cytokines such as IL-1β, IL-6, IL-8, LIF, IL-18, TGF-β1, and IL-11. These are released by various immunocompetent cell subsets and are thought to affect sperm cell function and the reproductive process [127,128]. Some studies assessed the relationship between seminal plasma cytokines and fertilization rates in men attending an IVF program. IL-11 is part of an exclusive group of genes that are essential for implantations and was found in significant levels in the seminal plasma of men with successful IVF [129]. Elevated levels of proinflammatory cytokines IL-1β, IL-6, IL-8, and TNF-α in seminal plasma may impair implantation and pregnancy establishment in couples who underwent IVF treatment [21]. A study indicates that an excess of seminal IL-18 negatively affects IVF and ICSI outcomes, which could potentially be mitigated by the TGF-β1 content found in seminal plasma [130].

Variability in seminal plasma composition driven by age, metabolic disorders, infections, or lifestyle habits (e.g., smoking, diet, physical activity) can significantly impact its immunological and molecular profile. Studies have shown that aging and obesity, for example, may alter the levels of key cytokines, prostaglandins, and antioxidant proteins, potentially impairing sperm function and endometrial immune responses [4,131-133]. These alterations could serve as biomarkers for male reproductive health or predictors of ART outcomes [28]. Furthermore, therapeutic strategies targeting seminal plasma composition, such as dietary interventions, antioxidant therapy, or modulation of exosomal content, warrant further investigation as adjunctive approaches to improve fertility outcomes.

Pregnancy disorders

Studies have demonstrated that seminal plasma exposure during the preconception period, in addition to its immunomodulating properties, reduces the risk of certain pregnancy disorders, such as preeclampsia. According to the results of some studies, limited exposure to the conceiving partner’s seminal plasma increases the risk of developing preeclampsia [134,135]. Women diagnosed with preeclampsia exhibit a decreased number of Tregs [136]. The protective effect of seminal plasma exposure is partner-specific. Multiparous women conceiving with a new partner (different from previous pregnancies) have an increased risk of preeclampsia compared to conceiving with the same partner, a phenomenon called primipaternity [137,138]. In assisted reproduction using donor sperm or surgically obtained sperm (no seminal plasma exposure), the incidence of preeclampsia is higher compared to the use of the partner’s ejaculated sperm containing seminal plasma [139,140]. Therefore, it appears that repeated preconception exposure to the conceiving partner’s seminal plasma is crucial for priming maternal immune tolerance to paternal antigens and reducing the risk of preeclampsia, a pregnancy disorder associated with impaired maternal-fetal tolerance.

The impaired immunomodulatory effects of seminal plasma may play a role in recurrent spontaneous abortion (RSA) or unexplained recurrent pregnancy loss (URPL) [101]. These problems may contribute to the underlying pathophysiology and higher risk of recurrent pregnancy losses (RPL). Decreased female Treg function was shown after stimulation with the seminal plasma of RPL males compared to control males [101]. Metabolomic analysis revealed differences in seminal plasma and sperm cell metabolites and pathways related to oxidative stress, nucleic acid synthesis, and hormone metabolism in men from URPL couples compared to fertile controls [141]. Cluster analysis showed that men with RPL had a less favorable expression pattern for pro-inflammatory cytokines (IL-1α, IL-1β, IL-6, IL-8, IL-12, IL-18, and TNF-α) [142].

Future directions

Advancing our understanding of seminal plasma’s immunological and molecular functions opens new avenues for clinical translation. Identification of key seminal plasma components such as exosomal miRNAs, immunoregulatory cytokines (e.g., TGF-β, GM-CSF), and fertility-associated proteins may support the development of novel biomarkers to predict ART success or diagnose subfertility. Additionally, therapeutic modulation of the female immune response through seminal plasma supplementation, exosome-based therapy, or cytokine mimetics could improve endometrial receptivity and implantation rates. Future research should explore these possibilities in well-designed clinical trials to integrate seminal plasma-based tools into personalized reproductive medicine protocols.

Conclusion

The growing body of evidence reviewed here highlights the crucial role of seminal plasma in priming the female reproductive tract and optimizing the endometrial environment for successful embryo implantation and healthy pregnancy. Seminal plasma benefits through a multifaceted molecular interaction with the endometrium. Seminal plasma upregulates key genes that make the endometrium more receptive. These genes include LIF, LIFR, MUC1, VEGF, EGF, and FGF2. These genes enhance endometrial-embryo communication, fostering a supportive environment for implantation. Seminal plasma controls the female reproductive tract’s immune responses and inflammatory pathways. It helps the mother accept the semi-allogenic embryo and mitigates immune rejection by increasing Tregs. These findings suggest that exposure to seminal plasma during embryo transfer or oocyte retrieval may improve ART success rates, such as IVF. However, it is crucial to note that abnormal seminal plasma composition is associated with some pregnancy disorders. Further research is warranted to elucidate the complex molecular mechanisms that control the interaction between seminal plasma and the endometrium and to find the best ways to use seminal plasma or its parts in assisted reproductive settings. Nonetheless, the current understanding of this crucial dialogue highlights the importance of maintaining a healthy seminal plasma composition for optimal reproductive success.

Disclosure of conflict of interest

None.