Mucosal Immunity in Female Reproductive Organs


Introduction

  • Presentation by Dr. Narjes Saheb, Assistant Professor of Immunology.

  • Date: January 11, 2026.

  • Email contact: nsharifaskari@sharjah.ac.ae

References

  1. Bagri P, Yu E, Kaushic C. 2021. Mucosal immunology of the female reproductive tract and its regulation by female sex hormones. In: Mor G, ed. Reproductive Immunology. Academic Press: 253-276.

  2. Wira CR, Rodriguez-Garcia M, Patel MV. 2015. The role of sex hormones in immune protection of the female reproductive tract. Nat Rev Immunol. 15(4):217-30. doi: 10.1038/nri3819.

  3. Swaims-Kohlmeier A, Sheth AN, Brody J, et al. 2021. Proinflammatory oscillations over the menstrual cycle drive bystander CD4 T cell recruitment and SHIV susceptibility from vaginal challenge. eBioMedicine. 69:103472.

  4. Monin L, Whettlock EM, Male V. 2020. Immune responses in the human female reproductive tract. Immunology. 160(2):106-115.

Learning Objectives

  • Assess the role of the mucosal immune system in the female reproductive tract (FRT), focusing on the immune-endocrine system interactions.

  • Define site-specific mucosal immune responses in the upper and lower FRT, analyzing cell contributions to immune protection and reproductive potential.

  • Identify hormonal changes during the menstrual cycle and their regulation of immune cell phenotype and function, focusing on estradiol (OE₂) and progesterone (P₄).

  • Discuss the concept of a 'window of vulnerability' in the menstrual cycle that allows for optimal conditions for fertilization but increases STI susceptibility.

Immunologically Privileged Sites

  • Definition: Locations in the body where immune responses are restricted to prevent damage to delicate tissues or structures.

  • Examples:

    • Brain

    • Eye

    • Testis

    • Uterus (fetus)

The Nature of the Body's First Line of Defense

  • The mucosal immune system serves as a critical barrier, protecting the internal body from external pathogens.

  • Components include:

    • Lachrymal gland

    • Salivary gland

    • Gastrointestinal tract, including stomach, pancreas, and intestines

    • Urogenital tract: uterus, bladder, vagina

    • Respiratory tract: trachea and lungs

    • Mammary glands

A Multi-layered Defense Strategy

  1. Anatomic Barriers: Composed of epithelial layers and mucus. Acts as a physical barrier to pathogens.

  2. Antimicrobial Proteins: Include defensins and components of the complement system.

    • Notable proteins: C3C_3, defensins

  3. Innate Immune Cells: Include macrophages and natural killer (NK) cells.

  4. Adaptive Immunity: Involves B cells (producing antibodies) and T cells.

  • Mucosal tissues: Found in gastrointestinal, respiratory, and urogenital systems, oral cavity, mammary glands, and eyes.

The Function of the Mucosal Immune System in the FRT

  • The mucosal immune system is essential for defending against a wide array of pathogens (viral, bacterial, fungal, parasitic).

  • The FRT has adapted to accommodate a semi-allogenic fetus while defending against pathogens.

Mucosal Membrane and Barrier Mechanism

  • Aqueous protection: Rich in immunoglobulins and antimicrobial peptides; crucial for immunity.

  • Pathogen Transmission: pH maintenance through commensal bacteria that produce lactic acid and hydrogen peroxide, enhancing antimicrobial activity.

  • Physical Barrier: Mucus includes mucins that form a thick gel, offering protection against pathogens like HIV.

  • Dynamic Structure: Epithelial cells, mucus, lactic acid from bacteria, and proteins work to prevent infections in the upper tract.

Transformation Zone

  • Intersection of columnar and squamous epithelium in upper and lower FRT, rich in immune cells.

  • Different compartments have distinct reproductive duties coinciding with phases of the menstrual cycle.

Structure of the Female Reproductive Tract

  • Lower FRT: Vagina and ectocervix, featuring stratified squamous epithelium (superficial, intermediate, and basal cells).

  • Upper FRT: Endocervix, endometrium, Fallopian tubes, lined by columnar epithelium. Contains lymphoid aggregates with B cells and T cells.

Hormonal Influence on Immune Responses

Hormonal Phases of the Menstrual Cycle

  1. Menstrual Phase

  2. Proliferative Phase

  3. Mid-cycle (Ovulation)

  4. Secretory Phase

  • Estradiol (OE₂): Dominates the proliferative phase, promoting epithelial growth and modulating inflammation.

  • Progesterone (P₄): Predominates in the secretory phase, preparing the endometrium for implantation and generally acting in an anti-inflammatory capacity.

Immune Cell Dynamics During the Cycle

  • The immune landscape is regulated by sex hormones, affecting cell distribution and function crucial for adapting immune protection aligned to reproductive needs.

  • Unique immune cell phenotypes differ from peripheral blood and other mucosal sites.

Regulatory Actions of Hormones on Immune Cell Function

  • Dendritic Cells (DCs): Differentiation and frequency influenced by OE₂ and P₄.

  • NK Cells: Uterine NK (uNK) cells are distinct; their peak frequency occurs in the secretory phase, preparing the uterus for remodeling during implantation. P₄ inhibits uNK cytotoxicity.

  • Neutrophils and Macrophages: Neutrophil numbers are stable, while macrophages fluctuate with hormonal levels in the endometrium.

T Cell Distribution and Activity

  • Distribution: CD4+ and CD8+ T cells in lower FRT vs high CD8+ in the upper FRT.

  • Lymphoid aggregates peak around ovulation, and CTL activity decreases to minimize rejection of sperm and fetus in the context of immune tolerance.

Antibody Functions in FRT

Immunoglobulins

  • IgG (Lower FRT):

    • Mechanism: Transported via the neonatal IgG receptor (FcRn) on epithelial cells.

    • Primary Function: Phagocyte responses and complement activation.

  • IgA (Upper FRT):

    • Mechanism: Transferred via the polymeric immunoglobulin receptor (plgR).

    • Primary Function: Mucosal immunity, blocking pathogen adherence.

  • Fluctuations in antibody levels (IgG and IgA) occur, influencing fertility and protection.

Hormonal Regulation Throughout Menstrual Cycle

  • Immunological responses must balance infection defense and reproductive needs, predominated by OE₂ and P₄.

  • Their secretion modifies epithelial, fibroblast, and immune cell functioning, affecting STI susceptibility.

The Molecular Language of Hormones

  • The signaling roles of OE₂ and P₄ include direct modulation of cytokine and chemokine release, shaping immune responses favorably for reproductive processes.

The Price of Preparation: The 'Window of Vulnerability'

  • After ovulation, increased P₄ creates conditions amenable for fertilization and implantation but also risks infection susceptibility.

    • Characteristics: Reduced CTL activity, inhibited uNK functions, and overall immune dampening.

Case Study: HIV Exploiting Vulnerability

  • HIV infection of CD4+ T cells correlates with hormonal changes affecting immune responses in FRT, heightening susceptibility to infection when the hormonal balance is conducive.

The Cycle of STIs and HIV Susceptibility

  • Pre-existing STIs enhance susceptibility to HIV, leading to increased inflammatory responses and greater recruitment of HIV target cells.

  • High-risk phases are influenced by hormonal activity preceding menstruation.

Conclusions: Key Takeaways on FRT Immunity

  1. Dual Mandate: Robust defense vs. immunological tolerance required for reproduction.

  2. Hormonal Effectors: OE₂ and P₄ are central to balancing the immune system's response.

  3. Site-Specific Defense: Immune strategies vary between upper and lower FRT.

  4. Window of Vulnerability: Immune regulation decreases during secrets phase, risking STIs, highlighting a biological compromise inherent to reproductive functionality.