Sex hormone regulation of innate immunity in the female reproductive tract: the role of epithelial cells in balancing reproductive potential with protection against sexually transmitted pathogens.

We have previously shown that polarized human uterine epithelial cells secrete antimicrobial molecules that kill or inhibit pathogenic bacteria, fungi, and viruses. 54 , 67 , 134 Apical secretion of these molecules is probably one reason why the uterine lumen has a relatively low number of microorganisms compared to the lower FRT despite flow of fluids throughout the entire tract. Secretion of antimicrobials is both constitutive and induced by TLR agonists and microorganisms and is affected by many factors such as chemokines (some chemokines are themselves antimicrobials), cytokines, and steroid hormones. To demonstrate the effect of estradiol treatment on antimicrobial activity, secretions from polarized epithelial cells were tested directly against bacteria ( Fig. 13 ). Human uterine epithelial cells were treated with or without two concentrations of estradiol for 48 hr. An aliquot of apical conditioned media was then incubated with Staphylococcus aureus for 1 hr, and the bacteria were cultured overnight. Control colony-forming units (CFU) refers to the colonies grown in media in the absence of uterine epithelial cells. The inhibition seen with no estradiol represents bacterial growth in the presence of antimicrobials constitutively produced by epithelial cells. The lower CFU seen with secretions obtained from estradiol treatment indicates that estradiol is inducing the secretion of antimicrobial(s) in addition to that seen constitutively. These data reinforce the premise that estradiol can have a significant effect on antimicrobial secretion by human uterine polarized epithelial cells. In other studies, we have shown that constitutive conditioned media from human uterine epithelial cells, as well as human cervical and ectocervical epithelial cells inhibit CFU of Candida albicans and Neisseria gonorrhoe ae, as well as the infection of TZM-bl cells by HIV-1 (Wira et al. manuscript in preparation). Interestingly, the commensal Lactobacillus crispatus was not affected by any of these conditioned media. This suggests that FRT epithelial cell antimicrobial secretions are selective for potentially pathogenic microorganisms.

Sexually transmitted infections have reached epidemic proportions throughout the world with more than 20 pathogens transmitted through sexual intercourse. The World Health Organization estimates more than 300 million new infections of Trichonomas vaginalis , Chlamydia trachomatis or Neisseria gonorrhea occur annually throughout the world. 1 Some STI can be transmitted vertically to the fetus, resulting in preterm deliveries and/or life-threatening systemic illness in newborn infants. Generally, adolescents and young adults are the demographic age groups most frequently affected with STI, and women are more likely than men to suffer the consequences of these serious infections. Human immunodeficiency virus (HIV) is recognized as a life-threatening sexually transmitted disease that is unique in its rapid spread and the depth of its impact. With 25 million deaths worldwide and an additional 33.2 million (of which 50% are women) infected worldwide, HIV/AIDS is one of the world’s worst pandemics. 2 Since the 1980s, HIV has shifted from a disease spread predominantly through needles and male–male contact to a sexually transmitted disease in which women worldwide are more likely to be infected than men. Presently, women and girls make up almost 57% of all people infected with HIV in Sub-Saharan Africa, where a striking 76% of young people (aged 15–24) living with HIV are female. 2 Within the FRT, the mucosal immune system functions as the first line of defense. 3 – 5 In response to the unique requirements of balancing immune protection with procreation, the immune system in the FRT, which consists of both innate and adaptive immune components, is responsive to and precisely regulated by estradiol and progesterone, both of which are produced in a cyclic fashion by the ovary over the course of the menstrual cycle. In preparing the reproductive tract for fertilization and implantation, estradiol and progesterone simultaneously regulate the immune system in the fallopian tubes, uterus, cervix, and vagina to compliment the reproductive process (see 6 for review). The mucosal immune system in the FRT consists of immune cells that migrate into the uterus, cervix, and vagina as well as resident epithelial cells and supportive stromal cells. 6 Sex hormones influence the migration of macrophages and dendritic cells as well as T and B cells by affecting the expression of adhesion molecules and chemotactic factors. 6 – 9 Among those cells pivotal in conferring immune protection, epithelial cells are recognized as pluripotential in their ability to confer immune protection. Epithelial cells, in addition to providing barrier protection, transport immunoglobulins (IgA and IgG) into FRT secretions and produce antimicrobials that are both bactericidal and viricidal. 7 , 10 Through the production of cytokines and chemokines, these cells signal the recruitment and activation of other cells of the innate and adaptive immune. What is unique to the FRT is that epithelial cells are responsive to both the direct and indirect effects of sex hormones. 7 , 9 In this dynamic balance, epithelial cells throughout the FRT respond directly to estradiol and progesterone, as well as indirectly to the cytokines and growth factors produced by resident (fibroblasts) and migratory cells (immune cells) in the reproductive tract. What is clear is that this responsiveness is part of the bidirectional communication that occurs in which epithelial cells direct both reproductive as well as immune function to maintain an effective level of protection, which distinguishes between pathogens, commensals, allogeneic sperm, and the developing fetus. The pleiotrophic capacity of epithelial cells has led to their recognition as sentinels, the functions of which are only now being recognized. 6 , 11 , 12 This review will focus on current knowledge regarding the sentinel role of epithelial cells in the human female reproductive tract with special emphasis on uterine and vaginal epithelial cells, especially as it pertains to protection against genital tract pathogens. Our goal is to highlight some of the unique responses of these cells to estradiol and progesterone and to point out that, in addition to direct hormonal effects on particular cells, there are the equally important indirect actions of estradiol mediated through growth factors, cytokines, and chemokines.

Epithelial cell interactions with underlying stromal fibroblasts are essential in facilitating steroid hormone-induced growth and development in the endometrium. Cooke et al. used FRT tissues from estrogen receptor (ER) knockout mice to demonstrate that underlying estrogen receptor–positive stromal cells regulate the differentiation of adjacent epithelial cells. 98 These studies indicated that estradiol acts on stromal cells to release one or more paracrine factors that then modulate estradiol effects on FRT epithelial cell for growth and differentiation. Hepatocyte growth factor (HGF), a pleiotrophic agent initially shown to stimulate proliferation of hepatocytes in vivo , 99 is produced primarily by stromal fibroblasts. 100 , 101 HGF has been well characterized in terms of its normal physiological roles of increasing epithelial cell motility and proliferation, wound healing, 102 angiogenesis, 103 , 104 epithelial cell scattering, and embryogenesis 105 (see review 106 ). Using primary human uterine epithelial cells, Sugawara et al. found that HGF stimulates proliferation, migration, and morphological changes in human uterine epithelial cells and concluded that HGF may modulate the cyclic regeneration and development of the endometrial lining of the uterus. 107 We hypothesized that estradiol might have a regulatory effect on HGF secretion by stromal fibroblasts in the human uterus. Hysterectomy tissues from the uterus were dispersed into epithelial and stromal cell fractions by enzymatic digestion and differential filtering as described previously. 36 Stromal fibroblasts were cultured in 24-well culture plates with and without estradiol at a concentration of 10 −8 m . Conditioned media from each well were collected at 48-hr intervals and analysed for HGF by ELISA. As shown in Fig. 9 , treating uterine stromal fibroblasts with estradiol significantly increased HGF secretion; estradiol-induced secretion of HGF increased with the duration of exposure to estradiol. 97 Cells treated with estradiol for 6 days secreted three times more HGF than cells exposed for 2 days. The long-lasting effect of estradiol on HGF secretion by uterine stromal fibroblasts could have implications for cancer or endometriosis where continued HGF secretion might lead to further proliferation of uterine epithelial cells. In related studies using a co-culture system, mouse uterine stromal fibroblasts cultured below uterine polarized epithelial cells in cell inserts produced HGF that significantly increased TER. 108 When epithelial cells and/or stromal cells were incubated with anti-HGF or anti-HGF receptor (HGFR) antibody prior to the addition of HGF, the effect of HGF was blocked. Addition of recombinant HGF to the basolateral compartment of polarized epithelial cells increased TER in a dose-dependent manner. These findings indicate that epithelial cells express the HGFR at their basolateral surfaces and that HGFR mediates the effects of HGF on TER. 109 In contrast, when cells were incubated with TGFβ, TER was markedly but reversibly suppressed. Based on these findings, we conclude that TGFβ and HGF may play regulatory roles in modulating epithelial cell tight junctions. Studies are underway to determine the relative contributions of hormone balance ( Fig. 2 and Table I ), pathogen exposure ( Fig. 3 ), cytokine, and HGF secretion ( Fig. 9 ) to innate and adaptive immune protection against potential pathogens in the FRT. Monocytes circulate through the periphery, mediating immune recognition and pathogen clearance by phagocytosis and indirect stimulation of the immune system through production of key cytokines and chemokines. As shown in Fig. 10 (top), the TLR4 agonist LPS, the key antigenic component of many pathogenic bacteria, stimulates peripheral blood monocyte secretion of IL-1β, thereby causing a proinflammatory response. In the presence of estradiol, this response is further enhanced in a dose-dependent manner ( Fig. 10 , bottom) 110 and may be detrimental to the host especially in the FRT. 81 , 82 Our studies found that increasing estradiol levels in the FRT can down-modulate immune associated genes such as HBD2 and IL-8 (See Fig. 6 ). We have further shown that this response is mediated through a down-regulation of interleukin-1 receptor type I (IL-1RtI) protein expression ( Fig. 10 ). Overall, estradiol has the ability to enhance pathogen-induced IL-1β expression by monocytes but at the same time decrease IL-1β receptor expression by epithelial cells. These studies demonstrate a close link between the endocrine and immune systems in the FRT that may be crucial for dampening proinflammatory responses during the time of ovulation or pregnancy. Expressed by dendritic cells (DC), DC-SIGN is a calcium-dependent carbohydrate-binding protein. 111 , 112 As an adhesion receptor DC-SIGN interacts with ICAM-2 on endothelial cells to both induce migration of DCs from blood to tissue 113 and mediate clustering of DCs with naive T cells through binding of ICAM-3. 111 Of equal importance in the FRT, DC-SIGN functions as a pattern recognition receptor that induces specific immune responses upon interaction with a number of pathogens. 114 This carbohydrate recognition pattern is the basis of its broad specificity for different pathogens and might also be responsible for its distinct signaling properties. While initial target cells for mucosal transmission of HIV-1 in the reproductive tract have not been clearly determined, studies conducted in nonhuman primate models indicate the potential role of DC as initial targets for mucosal infection and systemic dissemination of HIV-1. 115 – 117 DC can capture HIV-1 using surface expressed C-type lectin receptors (CLRs), 118 – 120 among which DC-SIGN is the best characterized. 113 HIV-1 captured by DC can be transferred to other target cells including CD4+ T-lymphocytes via virological synapses, in a process referred to as trans infection. 20 , 121 Understanding the mechanisms by which DC-SIGN and other HIV-1 receptors are regulated on mucosal target cells remains critical to designing strategies to prevent sexual transmission. In previous studies aimed at elucidating the role of cell–cell interactions in the endometrium on the expression of HIV-1 receptors, we determined the effects of soluble factors produced by uterine epithelial cells on expression of DC-SIGN by DC (Ochiel et al., submitted). These studies indicated that in addition to the direct effects of estradiol conditioned medium (CM) from polarized primary uterine epithelial cells and ECC-1 cells decreased the expression of DC-SIGN on DC. As a part of these studies, we explored the effects of estradiol on the expression of DC-SIGN by DC. In these experiments, immature DC were differentiated from human monocytes in the presence of IL-4 and GM-CSF as previously described. 122 To assess the hormonal effects on DC-SIGN expression, DC were differentiated in the presence or absence of physiological concentrations of estradiol (10 −7 m ) for 7 days prior to FACS analyses. In contrast to the results obtained with CM from uterine epithelial cells, estradiol significantly enhanced DC-SIGN expression on DC ( Fig. 11 ). Our results indicating a stimulatory effect of estradiol on DC-SIGN expression suggests that this sex steroid hormone could increase susceptibility of women to HIV-1 acquisition by enhancing DC-mediated virus uptake and dissemination. The impact of endogenous and exogenous sex steroid hormones on the risk of HIV-1 mucosal transmission in women remains to be fully determined. We recently proposed a model for the existence of a window of vulnerability for HIV-1 infection during a woman’s menstrual cycle. 5 During this critical period, 7–10 days of the normal menstrual cycle, critical components of innate and adaptive immune responses are suppressed by estradiol and/or progesterone to facilitate reproductive processes. HIV-1 presumably exploits this time frame, during which antiviral factors are suppressed, to establish and propagate infection in the female genital tract mucosal. 5 The reported findings showing that estradiol increases DC-SIGN expression reveals yet another potential dimension by which sex steroid hormone may modulate susceptibility to HIV-1 infection in women. It is likely that in addition to suppressing antiviral effector mechanisms, the profile of sex steroid hormones present during the window of vulnerability in women may increase HIV-1 acquisition through enhanced expression of HIV-1 capture receptors by DC. More studies are needed to directly test the effects of sex steroid hormones on infection of DC with HIV-1. Natural killer cells are important in innate immunity, not only for their ability to kill certain tumor cells and viral-infected cells without prior immunization or MHC restriction, but also for their secretion of immunoregulatory cytokines that contribute to early host responses against viruses, bacteria, and fungi. 123 NK cells account for a large percentage of leukocytes in the human endometrium and, because their numbers increase as the menstrual cycle progresses, 16 – 18 , 124 Sentman et al. hypothesized that recruitment and/or expansion of uterine NK cells are regulated by sex hormones. 125 As shown in Fig. 12a,b , they tested the effect of various doses of estradiol on human endometrial slices and found that 10 −9 m estradiol significantly enhanced mRNA expression of the chemokines CXCL10 and CXCL11. The expression of five other chemokines was not affected (not shown). Because the estrogen receptor inhibitor ICI182780 blocked the estradiol-induced increase in these two chemokines, the effect was dependent upon estradiol receptor activity (not shown). Progesterone at 10 −8 m also increased the mRNA expression of CXCL10 (8.6-fold) and CXCL11 (12.2-fold) in the endometrial organ culture system. 125 Sentman et al. have suggested that the sex hormones do not act directly on NK cells, but rather act on stromal and epithelial cells to produce chemokines for NK cell recruitment. 125 Because estradiol and progesterone stimulate production of these cytokines in the endometrium, it is likely that these sex hormones contribute to the migration of NK cells needed for immune surveillance, pathogen response, pregnancy, and normal menstrual cycle function. One innate mechanism that regulates NK activity is TGFβ, which is regulated by sex hormones. TGFβ is typically produced in a pro-form that requires activation; 126 the precise mechanisms that lead to activation of TGFβ in tissues are not well understood. It has previously been shown that uterine NK cell responses to monokines and blood cell NK cell responses to TLR agonists were inhibited by endogenous TGFβ. 127 , 128 Uterine NK cells express TLR and can respond to TLR agonists by producing cytokines. 129 Eriksson et al. explored how endogenous TGFβ modulates the production of interferon-γ (IFN-γ) by human uterine NK cells. 130 As shown in Fig. 12c,d , IL-12 and IL-15 in combination and poly (I:C) increase the percent of IFN-γ producing NK cells in the uterus, and antibody to TGFβ enhances the number and fold change of uterine NK cells induced by poly (I:C) that produce IFN-γ. Thus, endogenous TGFβ suppresses poly (I:C)-induced IFN-γ production by uterine NK cells. Because one way that estradiol regulates FRT immunity is by modulating the production and/or activation of TGFβ, it is likely that estradiol regulates NK activity via endogenous TGFβ. At mucosal surfaces, neutrophils are responsible for rapid elimination of potential pathogens through phagocytosis of microbes, release of antimicrobial compounds, and production of toxic oxygen and nitrogen species. 131 In studies to determine whether ovarian hormones modulate neutrophil phenotype, Smith et al. examined blood neutrophils from women at days 7, 14, 21, and 28 of the menstrual cycle for expression of surface receptors, granule proteins, and intracellular cytokines. 19 , 132 Blood neutrophil phenotype varied during the menstrual cycle with decreased expression of CD89 (IgA Fc receptor) and TNFα during the periovulatory period. In other studies, cytokines were analysed for their ability to enhance the innate immune potential of neutrophils by altering receptor expression and cell function. 133 These studies indicated that GM-CSF, known to be produced by FRT epithelial cells, acted synergistically with the chemoattractant IL-8 to promote neutrophil chemotaxis. Use of antibody neutralization and conditioned medium from primary confluent cultures of epithelial cells led to the conclusion that FRT epithelial cells are a potent source of neutrophil chemoattractant activity.

The immune system in the FRT has evolved to be responsive to and precisely regulated by the female sex hormones, estradiol, and progesterone, which are produced in a cyclic fashion by the ovary over the course of the menstrual cycle. In preparing the reproductive tract for fertilization and implantation, estradiol and progesterone simultaneously regulate the immune system in the fallopian tubes, uterus, cervix, and vagina to compliment the reproductive process (See 5 for review). We and others have found that over the course of the menstrual cycle, immune cells are present in substantial numbers and nonuniformly distributed in both the stromal layer and the epithelium of the FRT. 13 – 16 When tissues were dispersed by enzymatic or mechanical means for quantitative flow cytometry or functional analyses, leukocytes were found to be 6–20% of the total number of cells within the FRT. 17 When Natural killer (NK) cells were included in this analysis, the population of immune cells doubled. 18 These studies indicated that T-lymphocytes (CD4+ and/or CD8+) are a major constituent of reproductive tract leukocytes from all tissues. The fallopian tube contained granulocytes as a second major constituent but were significantly less numerous in the other tissues. All tissues contained B-lymphocytes and monocytes as clearly detectable but minor components. Over the course of the menstrual cycle, subtle changes were observed in the migration of macrophages, B cells and neutrophils into the lower tract, 17 , 19 and in dendritic cells entering the squamous epithelium. 20 As seen in Fig. 1 , the distribution of immune cells in the FRT varies with the site examined. Exclusive in the uterus are lymphoid aggregates, which consist of a B-cell core surrounded by T cells and an outer halo of macrophages ( Fig. 1a, f ). The B-cell core (CD19+) was most often seen in large aggregates present in the late proliferative and secretory stages of the cycle. 21 Phenotypic analysis indicated that T cells are almost exclusively CD3+, CD8+, and CD4−). Aggregates containing only cells of the CD3+CD4+ phenotype were occasionally found as were individual CD4+ cells located outside the aggregates in the stroma. Monocytes/macrophages (CD14+ cells) were found as a mantle around the T cells. FRT aggregates are anatomically and functionally distinct from Peyer’s patches in the intestine. 21 The size of lymphoid aggregates was found to vary with the stage of the menstrual cycle, in that aggregates were significantly larger during the secretory (3000–4000 cells) than the proliferative stage (300–400 cells). The distribution and frequency of CD8+ T cells in aggregates using expression of Vb2 or Vb8 as markers of clonality and Ki-67 as markers of dividing cells led to the conclusion that lymphoid aggregates form largely by the trafficking of cells to nucleation sites within the endometrium, rather than by division of precursor cells. 22 The lower reproductive tract, while lacking lymphoid aggregates, contains a full spectrum of immune cells located both within the submucosa and epithelial lining ( Fig. 1g–i ). Coincident with aggregate formation in the uterus, White et al. found that CD8+ cytotoxic T-lymphocyte (CTL) activity, measured in a redirected lyses assay, is suppressed in the uterus and fallopian tubes during the secretory stage of the cycle. 23 This suppression occurs without any drop in CD8+T cell numbers. In the ectocervix and vagina, in contrast to the upper FRT, we found that CTL activity was measurable in tissues from women at the proliferative or secretory stages of the menstrual cycle. 24 Recognized as important molecules in implantation, 25 , 26 we measured the expression of chemokine receptors CXCR4 and CCR5 as well as CD4 on uterine epithelial cells and found that expression varies with the stage of the menstrual cycle ( Fig. 1a–d ). 27 All three were low during the proliferative stage of the cycle, peaked at the time of ovulation and then either plateaued (CXCR4, CD4) or declined (CCR5) during the secretory stage of the cycle. 27 Chemokine receptors on cultured endometrial epithelial cells showed an up-regulation and polarization of CXCR1, CXCR4, and CCR5 receptors when a human blastocyst was present. 25 The distribution and regulation of these receptors in the endometrial epithelium and the human blastocyst suggest that each is essential in the apposition and adhesion phases of human implantation. In addition to expression in the upper FRT, leukocytes and epithelial cells in the lower tract ( Fig. 1g–i ) express CCR5 and GalCer on both. 28 Yeaman et al. showed that basal and parabasal epithelial cells of the ectocervix express CD4, CCR5, and GalCer, unlike the mid-zone and superficial cells lining the lumen. 28 Although changes in protein expression were not as pronounced as those seen in the uterus, histological evidence supported the conclusion that CD4 and CCR5 expression was greater during the proliferative stage than during the secretory stage of the cycle. An unintended consequence of chemokine expression in the upper and lower tract is that HIV-1, as a sexually transmitted pathogen, most likely uses these coreceptors to infect cells in the FRT. The humoral immune system is hormonally controlled and varies with site analysed and stage of the menstrual cycle (for review, see 10 ). In the uterus, levels of pIgR, the epithelial cell receptor responsible for transporting IgA from tissue to lumen varies with the menstrual cycle. 29 When expressed as the percentage of total protein, luminal uterine SC levels were highest during the secretory phase, significantly reduced during the proliferative phase and lowest during menstruation. Total SC was also greatest during the secretory phase, averaging approximately 2-fold higher than SC in proliferative and menstrual samples. In other studies, IgG levels in secretions from the uterine mucosa were highest during the peri-ovulatory phase, whereas levels in the fallopian tube were lowest at that time. 30 This study reached the conclusion that each organ (fallopian tubes, uterus, cervix, and vagina) and even different sites within each organ can respond independently from each other to changes in hormone levels, producing different types and amounts of secretory proteins. In the lower FRT, IgA, IgG, and lactoferrin levels in cervical mucus were depressed by 10- to 100-fold at mid-cycle relative to that seen early in the proliferative phase, only to rise toward the end of the menstrual cycle. 31 When women were placed on oral contraceptives, immunoglobulins and lactoferrin levels were suppressed for the duration of hormone exposure. In other studies, in which cervical mucus was evaluated from 5 days before to 3 days after ovulation, IgA and IgG had a biphasic pattern with a peak before ovulation followed by a small increase after ovulation. 32 More recently, Nardelli-Haefliger et al. demonstrated that titers of antihuman papillomavirus 16 virus-like particle (VLP) IgG in cervical secretions dropped approximately 9-fold at mid-cycle during ovulatory cycles suggesting increased vulnerability to pathogens at mid-cycle. 33 Chemokines and cytokines are central to the progressive tissue growth and remodeling that occur during each menstrual cycle, while antimicrobials protect against potential pathogens. 6 While relatively little has been done to measure changes in upper FRT secretions during the menstrual cycle, analysis of cervical–vaginal secretions demonstrate that chemokines and cytokines (IL-6 and IL-8) as well as endogenously produced antimicrobials [secretory leukocyte protease inhibitor (SLPI), HBD2, HNP1-3, and lactoferrin] dropped significantly at mid-cycle (day 13) and remained depressed for 7–10 days, returning to proliferative stage levels just before menstruation. 31 , 34 In contrast, total protein and transforming growth factor-beta (TGFβ) levels remained unchanged throughout the menstrual cycle. In other studies, human intestinal defensin-5 was highest in cervico-vaginal lavage during the secretory stage of the menstrual cycle. 35 Overall, these studies support the hypothesis that hormonal changes during the menstrual cycle regulate the immune system throughout the FRT in a way that is synchronized with reproductive function which optimizes FRT conditions for successful sperm migration, fertilization, implantation, and pregnancy. The extent to which changes in the immune system are directly or indirectly mediated by estradiol is addressed in the following sections.

In conclusion, analysis of the FRT indicates that the innate and adaptive immune systems are present and functional throughout the menstrual cycle. Not widely appreciated is a growing body of data indicating that the immune system in the fallopian tubes, uterus, cervix, and vagina function independently from each other to changes in hormone levels during the menstrual cycle. The net result is coordinated immune protection that compliments the reproductive demands of each organ. As the first line of this protection, epithelial cells, macrophages, dendritic cells, NK cells, and neutrophils function to meet the challenges of STI, while at the same time supporting an immunologically distinct fetal placental unit. The studies presented demonstrate overwhelmingly that estradiol secreted by the ovary during the menstrual cycle act both directly and indirectly on epithelial cells and other immune cells in the FRT to regulate a spectrum of immune functions specific to each site in the FRT. At the center of immune protection, epithelial cells that line the FRT provide a level of protection not previously recognized. Acting as a physical barrier, these cells are sentinels that function as an integral part of the innate and adaptive immune systems by recognizing and protecting against bacterial and viral challenges as well as signaling the recruitment and activation of underlying immune protection when pathogenic challenge exceeds their protective capacity. Along with other immune cells, epithelial cell responses are uniquely programmed to respond to hormone changes so that immune coverage occurs in a way that confers continuous protection. This review indicates that epithelial cell and immune cell functions are complicated by the recognition that sex hormones alter epithelial permeability, microbicide activity, and cytokine/chemokine secretion in the FRT to ensure maternal and fetal protection. As discussed previously, 5 aspects of the innate, humoral, and cell-mediated immune systems are suppressed by sex hormones to optimize conditions for procreation. Suppression occurs in the upper (fallopian tubes, uterus, endocervix) and lower (ectocervix and vagina) FRT, and coincides with the recruitment of potentially infectable cells and upregulation of coreceptors involved in pathogen uptake. The studies presented suggest that by understanding the ways in which sex hormones regulate epithelial cell and immune cell function in the FRT, new avenues may be identified both to protect against potential pathogens and to enhance the quality of women’s reproductive health.