The female reproductive tract includes the fallopian tubes, uterus, cervix and vagina. The mucosa of the female reproductive tract varies between the upper and lower reproductive channels. The upper reproductive tract includes the fallopian tube, uterus, and inner cervix, and is covered by a monolayer of columnar epithelium. The lower reproductive tract includes the cervix and vagina; these are covered by a stratified squamous epithelium that forms a more protective barrier than the columnar epithelium. This is a unique system that can balance mucosal immunity to microorganisms and immune tolerance to the sperm, embryo, and fetus (14).

The mucosal immune system is the first line of defense against viral, bacterial, fungal, and parasitic pathogens (15). In the vagina, the main mucosal cells are the epithelial cells, stromal fibroblasts, and leukocytes; these line the surface of the vaginal mucosa and provide a barrier that controls epithelial cell barrier function ( Figure 1 ). Estradiol (E2) increases the proliferation of vaginal epithelium cells, and high levels of progesterone (P4) are associated with vaginal epithelial thinning in animal models, although this has not been observed in humans (16).

Epithelial cells are connected by tight junctions that regulate the movement of molecules through the epithelium. Tight junctions predominate between the basal epithelial cells of the stratified squamous epithelium of the lower female reproductive tract. In contrast, the columnar epithelium in the upper female reproductive tract has a more tightly connected and powerful network (17). A recent study demonstrated that destruction of the epithelium in the female reproductive tract increases the risk of human immunodeficiency virus (HIV) infection by interfering with barrier protection and by promoting the recruitment of HIV target cells. It was shown that the vaginal use of tenofovir (TFV) and tenofovir alafamide (TAF) (a modified TFV prodrug) in HIV prophylaxis trials caused a significant delay in wound closure in the endometrium (EM), endocervix (CX) and ectocervix (ECX). Reconstitution of the tight junctions in epithelial cells of the EM and CX is compromised even after wound closure (18).

A recent study of the mucosal barrier in the reproductive tract showed that the treatment of bovine endometrial epithelial cell lines with astaxanthin (AST, a natural antioxidant carotenoid) reduced the production of lipopolysaccharide-induced interleukin-6 and tumor necrosis factor, increased the activity of cell superoxide dismutase and catalase, and promoted the production of insulin-like growth factor and epithelial growth factor. Furthermore, AST significantly increased the expression of claudin, a tight junction protein that may play an important role in maintaining the host endometrial defense barrier against pathogenic infection. Collectively, these results suggest that AST is a promising agent for endometritis (19).

Human natural killer (NK) cells are a class of innate immune cells that play an important role against pathogenic immunity; this is due to their ability to recognize and lyse infected cells. NK cells are also the dominant form of immune cells at the maternal and fetal interface (20). During the proliferative phase of the endometrium, only a few NK cells are scattered in the matrix of the functional layer. However, during the proliferative phase of the menstrual cycle, only a few NK cells are scattered throughout the stroma of the functional layer. In contrast, there is a dramatic increase in the number of NK after ovulation ( Figure 2 ). During the late secretory phase, the number of NK cells surges up (by up to 30-40% of cells) in the stromal compartment and the number of endometrial leukocytes increases up to 70% (17, 23) but numbers of uterine NK cells are thought to reduce in the second half of pregnancy but the mechanism for this reduction is unclear (24).

Three major subpopulations of NK cells have been identified in the decidua based on mRNA expression profiles, cell surface antigens, and metabolic behavior; these subpopulations are referred to as dNK1, dNK2, and dNK3 (25). Recently, Lamond et al. reported that NK cells protect the placenta to avoid invasion by Listeria, an intracellular bacterial pathogen (26). NK cells can also prevent infection-induced abortion via the injection of granulysin into the placental trophoblast; this removes intracellular pathogens without damaging placental cells, thus reflecting a mechanism that maintains the tolerance of the maternal-fetal interface to external abnormal factors (20).

Macrophage are the second most abundant subset of immune cells in the endometrium after uterine NK cells (24). The population of macrophages increases significantly during the secretion phase of the menstrual cycle and accounts for 10–20% of the population of decidual leukocytes (24). Macrophages bind to molecules that are specific to the cell wall of pathogens by specific pattern recognition receptors that participate in the recognition, phagocytosis, and degradation of microbial cells or ‘self’ cells (27). The phage function of macrophages in the female reproductive tract is controlled by dendritic cell-specific regulators that are locally synthesized by cells (e.g., uterine epithelial cells) and regulated by estrogen (28). Recent research suggest that dysregulation of the functions and estrogen responsiveness of female reproductive tract macrophages may be associated with infertility, estrogen- and macrophage-dependent gynecological diseases (29).

Dendritic cells are a heterogeneous and dynamic population of leukocytes. These are the most potent antigen capture cells (immature dendritic cells) and antigen presentation cells (mature dendritic cells) (30). Recent research demonstrated that uterine dendritic cells exhibit a tolerant phenotype and both uterine dendritic cells and uterine macrophages produce IL-10, TGF-β, and indoleamine 2,3 -dioxygenase, thus helping to maintain a steady state in the microenvironment (31). Dendritic cells in the cervical mucosa can effectively promote the replication of human immunodeficiency virus 1 (HIV-1) and systemic viral dissemination in the cervical mucosa through siglec-1 antibodies (32). A previous study demonstrated that decidualization is a process that involves phenotypic and functional changes of the endometrial stromal cells to sustain immune homeostasis.

Adaptive immune cells in the reproductive tract include both B and T lymphocytes. Although B cells are relatively rare in the female reproductive tract, a recent study showed that circulating memory B cells clustered together in a chemokine receptor 3-dependent manner in the vaginal mucosa after secondary infection with herpes simplex virus and then secreted virus-specific IgG2b, IgG2c and IgA into the lumen. This data indicated that circulating memory B cells act as a rapid induction source of mucosal antibodies in the female genital tract (33).

Tissue-resident memory T cells (TRM cells) are composed of both CD4 and CD8 T cell subsets. We found that the distribution of TRM subsets was uneven in the female genital tract with significantly higher levels of CD69⁺CD103⁺CD4 TRM in the vaginal tissue than in cervical tissue (34). Compared to B cells, T cells are always present in the vagina and uterus. Tissue-resident memory T cells in the mucosa of the reproductive tract respond rapidly to reproductive pathogens via the innate and adaptive immune systems. In cervical tissues, CD103⁺CD8TRM cells are preferentially localized to the cervical epithelial cells, whereas CD69⁺CD8TRM cells are evenly distributed in the epithelial cells and stroma (35). The production of TRM cells in the vaginal mucosa can provide advanced levels of defense against pathogens. Therefore, T cell-induced vaccines can persistently prevent infections in the mucosa of the reproductive tract, such as HIV (36). Hormone levels are also involved in the regulation of tissue T cells in the female reproductive tract. For example, estradiol treatment for herpes simplex virus 2 in mice led to increased levels of Th1 and Th17 TRM cells in the vagina (37). Estradiol has also been shown to prevent Herpes simplex virus type 2 (38), although the underlying mechanisms remain unknown. Recent studies show that compared with pre-menopausal women, the intra-and extra-cervical CD8⁺ T cells can increase cytotoxic activity in post-menopausal women (39).

Bacterial vaginosis is characterized by significant reduction of normal Lactobacillus-dominated microbiota and the overgrowth of anaerobic organisms (including Gardnerella, Prevotella, and vaginal fungi); collectively, these changes lead to an increase in vaginal pH and foul-smelling secretions. The diversity of microbial communities dominated by bacteria associated with bacterial vaginosis leads to an increase in the levels of cytokines, such as IL-8, IL-1a, IL1b, interferon and tumor necrosis factor (103). In a previous study, antimicrobial peptides of human beta-defensin-2, C-C (cervical cancer) chemokine ligand 20 (CCL20), and secretory leucocyte peptidase inhibitor, were found to be upregulated in cases of bacterial vaginosis, although there was no significant change in CCL20 expression following colonization with Lactobacillus species (63). The production of CCL20 is known to be regulated by tumor necrosis factor-α and IL-1β (104). Notably, both CCL20 and HBD-2 encode antimicrobial peptides and ligands for C-C chemokine receptor 6 (CCR6); this is a receptor that is specifically expressed on CD4⁺ T cells, leukocytes, and dendritic cell populations, and regulates the migration of these cells during inflammation. A recent study, based on a three-dimensional human cervical epithelial cell model, found that the expression levels of IL36G were significantly increased in bacterial vaginosis-positive cervical epithelial cells, thus proving that IL-36G is a key regulator of mucosal inflammation, neutrophil transport, and low immunity in the female reproductive tract (105).

Vulvar vaginal candidiasis is the second most common cause of vaginal inflammation and is predominantly causes by Candida albicans as the main pathogen. When Candida albicans invades the vaginal mucosa, it activates the host innate immune system; this induces the upregulation of light chain 3, lysosome-associated membrane protein 1, and cytokines (tumor necrosis factor-αand IL-1), thus leading to the activation of cellular autophagy (106). The specific bacterial species that were found in highly diverse and Lactobacillus-deficient cervicovaginal bacteria communities, were not only associated with sharply elevated levels of genital pro-inflammatory cytokines but also associated with increased genital antigen presentation cell activation through the lipopolysaccharide sensing pathway. Alternatively, antigen presentation cells have also been shown to produce chemokine (C-X-C motif) ligand 10 (CXCL10) which can lead to an increase in the number of activated CD4⁺ T cells (91). Therefore, a small fraction of the vaginal microbiota regulates the local immune system and inflammatory response, thus affecting the susceptibility of infection.

Disorders of the vaginal microbiota are risk factors for the development of cervical intraepithelial neoplasia. Human papillomavirus infection plays an important role in the etiology and pathogenesis of cervical cancer lesions. Vaginal Lactobacillus maintains a low pH environment and produces bacteriocin, thereby maintaining the barrier function of the cervical epithelium to inhibit human papilloma virus (HPV) from entering the basal cells (42). When pathogenic bacteria colonize the epithelium of the reproductive tract, they produce enzymes and metabolites that may impair the barrier and promote the entry of HPV. Recent studies have shown that the risk of developing cervical intraepithelial neoplasia in patients with an abnormal genital microbiota was twice than that in the healthy population (107). The immune response to acute HPV infection was shown to be initially mediated by mucosal NK cells and the production of epithelial antiviral antimicrobial peptides (108). Antivirus-specific immune responses are essential for the eradication of HPV infection; this requires cooperation between CD4 ⁺ Th cells (TH) and cytotoxic CD8⁺ T cells (109). It has also been demonstrated in persistent high-risk human papillomavirus infection, that there are numerous CD4⁺ Th cells, CD25⁺ regulatory T cells, and activated TH-1 and TH-2 cells (110). The high levels of interferon-γsecreted by Th-1 cells are known to mediate cytotoxic T cells and directly block viral cytotoxic activity (111). A previous clinical study confirmed the strong correlation between the Th-1 pattern and the clearance of high risk human papilloma virus (HR-HPV) (112). In contrast, IL-17 has been shown to inhibit immune response effectors in HPV-related diseases (113). The high-risk human papillomavirus has evolved different mechanisms to evade host adaptive responses, including reduced protein secretion or the manipulation of antigen processing machinery (114). The clearance of infection is not a rare event and is often associated with the specific composition of the vaginal microbiota (7).

Microbiota with a reduced content of Lactobacillus may contribute to HPV persistence. For example, the prevalence of bacterial vaginosis in women with persistent HR-HPV was reported to be 11%, while the ratio of bacterial vaginosis in women clearing HR-HPV was only 5% (103). Persistent infection with HR-HPV is the leading cause of cervical cancer worldwide (115). Further studies found that HPV infection and/or subsequent clearance was not associated with inflammation or alterations in the subpopulation of cervical T cells but was associated with an increased number of Langerhans cells (116).