Epithelial cells form a first-line defense to combat any pathogenic intrusion. Epithelial cells have a role in bi-directional communication with immune cells to maintain a baseline effective level of protection that helps to differentiate the “self” and “non-self” antigens. Moreover, these cells have a pleiotropic capacity as sentinel sites. The barrier is composed mainly of non-keratinized stratified squamous cells that line vagina and ectocervix and columnar cells in uterus, with the presence of tight junctions between basal layers. These tight junctions are composed mainly of transmembrane proteins, occluding, claudin and junctional adhesion molecules (Grant and Wira, 2003; Wira and Fahey, 2004; Fahey et al., 2005). These serve as directional conduit; help maintain the integrity of FRT monolayers and aids epithelial cells to functionally polarize response to different stimuli. However, these are absent in vagina that permits the penetration of molecules to deeper layers, where “adherans junctions” are present to restrict the passage of escaped molecules. These epithelial cells on disruption by pathogenic attack lead to perturbation of cytoskeleton directly and release of cytokine-chemokines indirectly. Transforming growth factor (TGF-β) and tumor necrosis factor (TNF-α) are secreted by basolateral and apical surface, respectively. The secretion is mostly unwavering toward the luminal or apical compartment, leading to generation of a potent gradient for drawing immune cells toward the surface. An electrochemical gradient “transepithelial resistance” has also been prominently observed across the uterine monolayer, owing to the presence of tight junctions and barrier integrity. This transepithelial resistance is hormone-dependent, being the least in secretory phase (Simons and Wandinger-Ness, 1990; Tsukita et al., 2001; Grant and Wira, 2003; Shin et al., 2006). The same has been confirmed by administering selective estrogen receptor modulators (SERMs) that led to a decrease in viral infections by increasing the TER (Brotman et al., 2014). Another family of glycosylated proteins “murins” is also present in cervico-vaginal mucus. The pores of these proteins are large enough for the viruses to pass easily; however, they restrict the diffusion of penetrated particles by 800 folds. (Lai et al., 2010) Epithelial cells are also known to secrete a plethora of cytokines and chemokines, namely, HBP 1-14, HD5, elafin, SLP1, MIP-1 α, RANTES, SPF-1 α, and trappin-2, via upregulation of IL-1 β, IL-6, IL-8, and CCL-2 (Cocchi et al., 1995; Bleul et al., 1996; Quayle et al., 1998; Valore et al., 1998; King et al., 2003; Schaefer et al., 2005; Jv et al., 2008; Ghosh et al., 2009; Ghosh et al., 2010).

Fibroblasts are the active structural component that facilitate steroid hormone, playing a pivotal role in growth and development in epithelial membrane. They secrete hepatocyte growth factors, IL-18, RANTES, and MIP-3 α, which have a role to play in epithelial cell motility, proliferation, wound healing and embryogenesis (Kankuri et al., 2005; Skibinski et al., 2007; Shen et al., 2019).

Natural Antimicrobial Peptides (NAPs) are whey acidic protein motifs containing peptides and are abundantly present at the epithelial surfaces and are activated once the membrane is disrupted by attack of pathogens. TLR 3, 7, 8, and 9 present at the mucosal surfaces recognize the PAMPs, in turn generating intracellular signals and cytokine-chemokine storm via activation of NF-kb pathway. NAPs are present throughout FRT and counter-act enzymes, preventing the host tissue damage. Secretory leucocyte protease inhibitor (SLPI) is one of the most potent NAPs, which inhibit mononuclear elastase, trypsin, and cathepsins. Elafin is another NAP and inhibits elastase and proteinase. NAPs are activated after pathogen attack (Helmig et al., 1995; Pfundt et al., 1996), during menstruation, secretory phase (King et al., 2003) and pregnancy (Hein et al., 2002; Fahey et al., 2005).

Defensins are secreted by WBCs and epithelial cells after microbial attack. The most common defensins produced include β-defensins 1–4 and α-defensins. Defensins though present throughout the epithelium, have a unique temporal expression profile. Defensins 1–3 are more commonly present in uterine epithelium, 2 in cervico-vaginal epithelium, and 5 in fallopian tubes, vagina and cervix. Defensins 1, 3, and 5 are more abundantly produced in secretory phase, 2 in menstruation, and 4 in proliferative phase. (Quayle et al., 1998; Valore et al., 1998; King et al., 2003)

Toll-like receptors (TLRs) are the most abundant group of PRRs present in FRT that play an indispensable role in recognition of conserved motifs of pathogenic antigens. FRT has all the 10 TLRs with a temporal profile. Upper FRT is rich in TLR-4, while TLR-2 is more common in fallopian tubes and cervix. Vaginal epithelial cells are lined with TLR 1, 2, 3, 5, and 6; however, endocervix is rich in TLR 1, 2, 3, and 6. TLR 1–9 is abundantly encountered in endometrial epithelial cell lines and TLR 2 and 4 in fallopian tubes and endometrium. (Krikun et al., 2007) Moreover, this TLR expression is cycle-dependent as well, but many studies have delineated contradictory results. Few studies have shown higher expression of TLR 2 and 4 in peri-menstrual phase, while secretory phase favors TLR 2-6, 9, and 10. Similar selective expression has been seen in pregnancy as well that favors the expression of TLR 2, 4, and NOD 1, 2 in trophoblasts. (Costello et al., 2007; Krikun et al., 2007)

Monocytes and macrophages circulate throughout FRT and mediate immune recognition as well as microbial elimination via phagocytosis. These cells also induce secretion of cytokines and chemokines, via down-regulation of HBP2, IL-8, and IL-1. Macrophages form 10% of all white blood cells (WBCs) of FRT. These are present in highest numbers in endometrial stroma and connective tissues, however, is cycle-dependent. On the contrary, vaginal macrophages remain constant in number, irrespective of the hormonal changes. These are distinct from gastrointestinal tract as these express more of CCR5 and CXCR4, making the sites more susceptible to infections.

Dendritic cells are the primary armor chunks of antigen presentation. These can be either myeloid or plasmacytoid, based on embryonic origin. DCs are present in subepithelial stromal layer in endometrium an in epithelial layer in vagina. Of these, pDCs form the pillars of innate immune system of FRT owing to the sensitivity to TLR 7 and 9, subsequently, leading to secretion of interferons III and I. These professional APCS are pivotal in generation of adaptive immune response as well. DC-SIGN, a calcium-dependent carbohydrate binding protein, interacts with ICAM-2 on endothelial cells, inducing migration of DCs and triggering the immune response. (Schaefer et al., 2005)

NK cells account for 10% of systemic WBCs and 70% of mucosal WBCs, exhibiting phenotypic differences from other immune cells with presence of CD9, CD69, and CD94 markers. These are abundantly present in endocervix and endometrium, being absent from ectocervix. Upon attack by viruses and parasites, a pro-inflammatory cascade is triggered via generation of GM-CSF, IL-10, IL-8, and IFN-Υ. NK cells also generate angiogenic growth factors and leukemia inhibitory factors. (Wira and Fahey, 2004; Wira et al., 2005; Wira et al., 2013; Wira et al., 2014)

Polymorphonuclear cells (PMNs) are present throughout FRT, with maximum population in fallopian tubes and progressively decrease toward the vagina. The PMN influx is preceded by IL-1 secretion. PMNs play a role in immune activation via secretion of elastase and activation of metalloproteases. Moreover, innate immune defense is activated more via disruption of epithelial barrier. The phagocytic and pathogen clearance is mediated by secretion of oxidative compounds, trappins, α-defensins, phospholipases, and cytokines (Wira et al., 2013; Wira et al., 2014).

Interferons are the anti-viral compounds that modulate immune activity via autocrine and paracrine action. Three classes of IFNs have been noted, I, II and III. Type I interferons include 13, II include Υ and III include λ 1–3. Type I interferons play a role in innate and adaptive immune system, II in adaptive system, and III in innate system. A ubiquitous expression of IFNs has been reported throughout the FRT to induce an extensive antiviral state, creating a hostile environment for pathogen survival (Schaefer et al., 2005; Fahey et al., 2006; Brotman et al., 2014).

Humoral immune system is distinct in secretion of immunoglobulins against the invading pathogens, which acts either via neutralization, opsonization or complement activation. Neutralization involves binding of antibody to free or bound antigens, preventing the cell entry. Opsonization, on the other hand, refers to coating of pathogen surface to facilitate the phagocytosis. Complement activation activates the complement pathway to induce an inflammatory response. Viral invasion leads to activation of B-cells, which in-turn interacts with CD4+T cells, culminating into differentiation of B-cells. These differentiated B cells play a role in generation of long-lived plasma memory cells (Charles A Janeway et al., 2001; Wira et al., 2005; Wira et al., 2013; Wira et al., 2014).

Immunoglobulins are the major components of humoral immune system. A temporal shift has been noted, with IgG being more common in cervico-vaginal site, lower FRT. However, endocervical region is also quite rich in IgA. It has been revealed that IgG in FRT is plasma-derived, but IgA is locally generated. IgA produced is distinct in being resistant to IgA1 proteases and 70% of cervical IgA is locally produced. Moreover, transport of IgA is against the concentration gradient. An influence of hormones and contraceptives has also been demonstrated in FRT (Kutteh et al., 1996).

In addition to humoral immunity, cell-mediated immune (CMI) also plays a substantial role in FRT. T cells have been noted to be present in stroma of uterus, cervix and vagina. Moreover, distinctive intraepithelial lymphocytic clusters are also noted to be present within epithelial cells. Both the classes of T cells, CD4+ and CD8+ T cells are known to be present in FRT. Due to absence of mucosa-associated lymphoid tissue (MALT) in lower reproductive tract, the primary priming of CD4+ T cells occurs in draining lymphoid organs. However, upper tract has few traces of lymphoid associated tissues, which play a role in antigen presentation. CD4+T cells have a pivotal role in both humoral, as well as CMI immune arms, by helping CD8+T cells in their effective differentiation, mobilization, and concurrently, stimulating B cells to produce immunoglobulins. CD4+T cells might also take the role of cytotoxic cells to inhibit viral replication via secretion of IFN-Υ (Schaefer et al., 2005; Wira et al., 2013).

Cytotoxic T-cells (CTLs) are the main lytic cells of immune armor that exhibit the pathogen clearance via secretion of perforins and granzymes. Owing to the abundance of TGF-β and IL-16, pre-requisite for Th-17 differentiation, in genital tract, CD3+T cell-mediated cytolytic action is noted throughout the reproductive tract. The main inflammatory mediator cells in this regard are Th-17 cells, which trigger the immune response. Th-17 cells act as a double-edged sword, being of utmost significance in immune activation but simultaneously protecting the pathogen by inhibition of Th1 and Th2 response.

Another important subset of T cells, intraepithelial γδ cells are also present in genital tract. γδ T-cells rapidly initiate cytokine secretion and enhance the cytolytic activity. An important role has been reported in HSV infection. Moreover, these cells have a role to play in immunoregulation during pregnancy (Wira et al., 2013).

Memory cells are present as a focus of cells within the submucosa and in intraepithelial sites. These are present as lymphoid aggregates, with a central B core, surrounded by CD4+T and CD8+T-cells, which is in turn encapsulated by macrophage shell. These cells can either be of CD4+, CD8+, or B-cell lineage and are unique to FRT, distinct from Peyer’s patches in intestines. The most substantial role is in rapid progressive response to secondary infections (Yeaman et al., 2001; Zhou et al., 2018).