The role of sex hormones in immune protection of the female reproductive tract

Abstract

Within the human female reproductive tract (FRT), the challenge of protection against sexually transmitted infections (STIs) is coupled with the need to enable successful reproduction. Oestradiol and progesterone, which are secreted during the menstrual cycle, affect epithelial cells, fibroblasts and immune cells in the FRT to modify their functions and hence the individual's susceptibility to STIs in ways that are unique to specific sites in the FRT. The innate and adaptive immune systems are under hormonal control, and immune protection in the FRT varies with the phase of the menstrual cycle. Immune protection is dampened during the secretory phase of the cycle to optimize conditions for fertilization and pregnancy, which creates a 'window of vulnerability' during which potential pathogens can enter and infect the FRT.

Figure 1. Anatomy and histology of the FRT

The female reproductive tract (FRT) is composed of distinct anatomical regions that undergo morphological changes during the menstrual cycle. The lower FRT consists of the vagina and ectocervix and is protected by a stratified squamous epithelium, which is composed of superficial, intermediate and basal epithelial cells. The thickness of the squamous epithelium remains fairly constant in humans during the menstrual cycle. By contrast, the upper FRT, which consists of the endocervix, endometrium and Fallopian tubes, is covered by a single-layer columnar epithelium. In the endometrium, the columnar epithelial cells proliferate during the menstrual cycle and form glands in the secretory phase. The transformation zone is where the columnar epithelium of the upper FRT meets the squamous epithelium of the lower FRT. Overlying the epithelial surface in the lower FRT and endocervix is mucus, the consistency of which changes across the cycle, becoming thick and viscous in the secretory phase. Also present is a dynamic population of bacteria, primarily composed of lactobacilli in most women, that acidify the lumen of the lower FRT. Underlying the epithelium is a dense layer of fibroblasts, interspersed with immune cells (T cells, macrophages, B cells, neutrophils, natural killer (NK) cells and dendritic cells (DCs)). The transformation zone contains a particularly high number of immune cells compared with the rest of the FRT. In the endometrium, immune cells form lymphoid aggregates that reach peak size around ovulation and during the secretory phase of the cycle.

Figure 2. The menstrual cycle

The 28-day menstrual (ovarian) cycle is divided into four stages — menstrual phase, proliferative phase, mid-cycle (ovulation) and secretory phase — that are characterized by cyclic changes in hormone levels. Day 0 is defined by the onset of menstrual bleeding, which lasts for 3–5 days in most women. Menses is followed by the proliferative phase, during which the endometrial lining is reconstituted. Follicle-stimulating hormone (FSH) produced by the anterior pituitary gland induces oestradiol (OE₂) production by the ovary. OE₂ levels increase during the proliferative phase and peak before mid-cycle (ovulation), followed by a rapid drop in concentration. Rising OE₂ levels stimulate luteinizing hormone (LH) production by the anterior pituitary, the levels of which surge in the late-proliferative phase within 24–36 hours of the OE₂ peak, leading to ovulation and increasing progesterone (P4) synthesis. At the same time, FSH levels increase by a smaller amount. Both LH and FSH levels rapidly drop in the early secretory phase. After ovulation, the concentrations of P4, and to a lesser extent OE₂, which are both produced by the corpus luteum in response to LH, steadily increase before peaking at mid-secretory phase. Both FSH and LH levels remain low throughout the secretory phase. In the absence of fertilization, OE₂ and P4 levels drop, which leads to endometrial shedding and the onset of menses. Immune changes in the FRT that occur as a result of cyclic changes in hormone levels create an optimal environment for successful fertilization and implantation during the secretory phase. This environment of regulated immune responses creates a ‘window of vulnerability’ during this phase, with permissive conditions for the entry and survival of pathogens.

Figure 3. Fluctuations in immune cell populations in the FRT during the menstrual cycle

Bars represent the mean ± standard error of the mean (SEM) from different studies analysing immune cell subsets in the female reproductive tract (FRT; endometrium^,,,,, endocervix and ectocervix^,,,,). Studies included in the figure are limited to those that used flow cytometry, as microscopic analysis of tissues does not enable immune cell frequency to be accurately obtained. Statistical analyses are not possible because of the limited number of flow cytometry studies that have been carried out. NK, natural killer.

Figure 4. Oestradiol-mediated control of interactions between epithelial cells, fibroblasts and immune cells in the FRT

Oestradiol (OE₂) functions directly through receptor expression on multiple cell types of the female reproductive tract (FRT) or indirectly through intermediary molecules to regulate gene transcription and protein expression, and to alter the number, distribution and phenotype of cells in the FRT. OE₂ induces the expression of multiple cytokines, chemokines, growth factors and antimicrobial proteins. For example, OE₂-mediated stimulation of epithelial cells increases the luminal secretion of antimicrobial proteins (such as secretory leukocyte protease inhibitor (SLPI), elafin and human β-defensin 2 (HBD2)) in the uterus but decreases the secretion of elafin and HBD2 in the vagina, possibly leading to differences in antiviral and antibacterial activity in the FRT lumen depending on anatomical location. Transforming growth factor-β (TGFβ), granulocyte–macrophage colony-stimulating factor (GM-CSF), macrophage colony-stimulating factor (M-CSF) and CC-chemokine ligand 20 (CCL20) are secreted by epithelial cells into the tissue environment under the influence of OE₂, where they modulate immune cell chemotaxis and function — for example, leading to changes in dendritic cell (DC) responses to Toll-like receptor (TLR) ligands. In contrast to its direct effects on epithelial cells, OE₂ can indirectly alter the proliferation and the barrier function of uterine epithelial cells by stimulating the secretion of hepatocyte growth factor (HGF) by uterine fibroblasts, which in turn modulates tight junction expression and cell replication. OE₂ also directly affects uterine fibroblasts to increase their secretion of CCL2 and interleukin-8 (IL-8), which leads to increased chemotaxis of neutrophils, monocytes and DCs. Less clear is the role of OE₂ in regulating the contributions of immune cells to the mucosal environment in the FRT; these cells secrete CCL5, tumour necrosis factor (TNF) and fibroblast growth factor 2 (FGF2). TNF, which is a pro-inflammatory cytokine, activates fibroblasts and degrades tight junction integrity (and thus the barrier function) of epithelial cells. Similarly, FGF2 stimulates growth of uterine epithelial cells and fibroblasts, and also alters epithelial structure and integrity (not shown). NK, natural killer.

Figure 5. Influence of sex hormones on mucosal immunity in the lower and upper FRT during the window of vulnerability

This figure depicts the key immunological mechanisms present in the female reproductive tract (FRT) that are essential for successful reproduction and that directly or indirectly affect pathogens that enter the FRT and threaten reproductive health. These immune mechanisms are under hormonal control. During the ‘window of vulnerability’, oestradiol (OE₂) and progesterone (P4), selectively stimulate and/or suppress aspects of the innate and adaptive immune systems as shown, in ways that vary according to the FRT site. For example, in the lower FRT, innate components (such as human β-defensin 2 (HBD2)) in the lumen are suppressed at a time when CD8⁺ cytotoxic T lymphocyte (CTL) activity and natural killer (NK) cell cytotoxic activity are maintained. By contrast, CD8⁺ CTL and NK cell activities are suppressed in the uterus at a time when luminal innate components are enhanced. These uterine changes are consistent with increased luminal pathogen killing and/or inactivation at a time when semi-allogeneic blastocyst rejection might otherwise occur. The resulting alterations in immune protection optimize conditions for successful implantation but also lead to an increased risk of acquiring sexually transmitted infections (STIs). In the table, double-headed arrows indicate that there is no change. CCL, CC-chemokine ligand; CXCL12, CXC-chemokine ligand 12; CX₃CL1, CX₃C-chemokine ligand 1; DC, dendritic cell; FcRn, neonatal Fc receptor; HGF, hepatocyte growth factor; IFNγ, interferon-γ; IL-8, interleukin-8; pIgR, polymeric IgA receptor; SLPI, secretory leukocyte protease inhibitor; TGFβ, transforming growth factor-β.