# Lecture 21: Mucosal Immunity
## Immunology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the organization of the mucosal immune system (MALT, GALT, BALT, NALT)
2. Explain the production, transport, and function of secretory IgA
3. Describe the role of M cells in antigen sampling and Peyer's patches in mucosal immune induction
4. Explain how the mucosal immune system maintains tolerance to commensal organisms and food antigens
5. Discuss the concept of the common mucosal immune system and its relevance to mucosal vaccination
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## Lecture Content
### I. Overview of Mucosal Surfaces
Mucosal surfaces represent the largest interface between the body and the external environment, spanning a total surface area of approximately 400 square meters -- vastly exceeding the approximately 2 square meters of skin. The major mucosal sites include the gastrointestinal tract, respiratory tract, urogenital tract, conjunctiva, middle ear, and salivary gland ducts. Because mucosal surfaces are the primary portal of entry for most pathogens, they are equipped with a dedicated immune system that must strike a delicate balance between **defense** against pathogens and **tolerance** to the approximately 38 trillion commensal bacteria in the gut and to the food antigens encountered daily. The mucosal immune system contains more lymphocytes than all other immune compartments combined, reflecting the enormous immunological challenge it faces.
### II. Organization of Mucosal-Associated Lymphoid Tissue (MALT)
The mucosal immune system is organized into inductive sites, where immune responses are initiated, and effector sites, where those responses are carried out. **Organized MALT** constitutes the inductive sites. In the gut, **GALT (gut-associated lymphoid tissue)** includes **Peyer's patches** (30 to 40 organized lymphoid follicles in the ileum), isolated lymphoid follicles (ILFs) scattered throughout the small and large intestine, mesenteric lymph nodes (MLNs) that drain the gut, and the appendix and cecal patches. **BALT (bronchus-associated lymphoid tissue)** consists of lymphoid aggregates in the bronchial mucosa that are inducible in humans. **NALT (nasopharynx-associated lymphoid tissue)** includes the adenoids and tonsils forming Waldeyer's ring. These organized MALT structures share features with conventional lymph nodes, including B cell follicles, T cell zones, and germinal centers.
**Diffuse MALT** constitutes the effector sites. The **lamina propria** -- loose connective tissue beneath the epithelium -- harbors plasma cells, T cells, macrophages, DCs, mast cells, and eosinophils. **Intraepithelial lymphocytes (IELs)**, mostly CD8+ T cells including many gamma-delta T cells, are interspersed between epithelial cells as a first line of T cell defense. The **epithelial cells** themselves contribute by producing antimicrobial peptides (defensins, RegIII-gamma), cytokines, and expressing innate immune receptors.
### III. Antigen Sampling at Mucosal Surfaces
Several specialized mechanisms allow the mucosal immune system to sample luminal antigens. **M cells (microfold cells)** are specialized epithelial cells that overlie Peyer's patches and other organized MALT. They lack microvilli, have a thin glycocalyx, and possess a basolateral "pocket" containing DCs and lymphocytes. M cells transcytose intact antigens -- bacteria, viruses, and particles -- from the lumen to the underlying immune cells without processing them, delivering them directly to DCs and macrophages for immune activation. Some pathogens, including Salmonella, Shigella, poliovirus, and HIV, exploit M cells as a portal of entry.
Lamina propria DCs can also sample luminal contents directly by extending transepithelial dendrites between epithelial cells into the lumen, capturing bacteria and antigens without disrupting the epithelial barrier. CX3CR1+ macrophages are particularly adept at this process in the gut. Additionally, **goblet cell-associated antigen passages (GAPs)** allow goblet cells to deliver small soluble antigens from the lumen to DCs in the lamina propria, a mechanism that is particularly important for inducing tolerance to dietary antigens.
A cross-sectional diagram of the gut mucosal immune system. The intestinal lumen is at the top, containing commensal bacteria and food antigens. The epithelial layer shows: absorptive enterocytes with microvilli, goblet cells secreting mucus (inner dense mucus layer and outer loose mucus layer), Paneth cells at the crypt base secreting defensins and lysozyme, and M cells overlying a Peyer's patch. The M cell has a thin apical surface and a basolateral pocket containing DCs. Arrows show M cell transcytosis of a bacterium to a DC below. A DC extends a transepithelial dendrite between epithelial cells to sample a bacterium in the lumen. Intraepithelial lymphocytes (IELs, mostly CD8+ and gammadelta T cells) are shown between epithelial cells. Below the epithelium, the Peyer's patch contains B cell follicles with germinal centers, a T cell zone (interfollicular region), and a subepithelial dome rich in DCs. The lamina propria shows scattered plasma cells (secreting dimeric IgA), T cells, macrophages, and DCs. Secretory IgA is shown being transported across the epithelium via the polymeric Ig receptor (pIgR) and released into the lumen.
### IV. Secretory IgA: The Dominant Mucosal Antibody
IgA is the **most abundantly produced immunoglobulin in the body**, with approximately 3 to 5 grams synthesized per day, mostly at mucosal surfaces. While serum IgA is predominantly monomeric IgA1, **secretory IgA (sIgA)** is a dimeric molecule consisting of two IgA monomers joined by a J chain, wrapped in a secretory component derived from the polymeric immunoglobulin receptor.
IgA production begins with B cell activation in Peyer's patches and mesenteric lymph nodes, where TGF-beta, BAFF, APRIL, and retinoic acid promote IgA class switching. IgA-positive plasmablasts then upregulate gut-homing receptors -- alpha-4-beta-7 integrin (which binds MAdCAM-1 on gut endothelium) and CCR9 (which responds to CCL25 from intestinal epithelium) -- and migrate through the blood to the intestinal lamina propria, where they differentiate into IgA-secreting plasma cells.
The transport of IgA across the epithelium follows a defined pathway. Dimeric IgA produced in the lamina propria binds the **polymeric immunoglobulin receptor (pIgR)** on the basolateral surface of epithelial cells. The pIgR-IgA complex is endocytosed and transcytosed to the apical (luminal) surface. There, pIgR is proteolytically cleaved, and the extracellular portion remains attached to the IgA dimer as the **secretory component (SC)**, which protects sIgA from proteolytic degradation in the harsh luminal environment.
Secretory IgA performs several critical functions. Through **immune exclusion**, it binds pathogens and toxins in the lumen, preventing their attachment to the epithelium. During transcytosis, it can perform **intracellular neutralization** by intercepting viruses within epithelial cells. Through **antigen excretion**, IgA bound to antigens in the lamina propria can transport them back to the lumen. sIgA promotes entrapment of pathogens in mucus and, crucially, is non-inflammatory -- it does not activate complement or trigger destructive inflammation, unlike IgG. It also maintains commensal homeostasis by coating bacteria and preventing their invasive growth.
### V. Mucosal T Cell Responses
The mucosal T cell compartment includes several distinct populations. **Intraepithelial lymphocytes (IELs)**, positioned between epithelial cells, constitute the first T cell line of defense. Two major populations exist: **conventional IELs** (alpha-beta TCR, CD8-alpha-beta+), which are antigen-experienced cells that traffic from the periphery, and **unconventional IELs** (gamma-delta TCR or alpha-beta TCR with CD8-alpha-alpha homodimer), which develop extrathymically and recognize stress-induced ligands such as MIC-A and MIC-B. Both populations perform cytotoxicity against infected epithelial cells, produce cytokines, and contribute to epithelial repair.
In the lamina propria, CD4+ T cells predominate, including Th1, Th17, and abundant Tregs. **Th17 cells** produce IL-17 and IL-22, which stimulate epithelial antimicrobial peptide production, enhance barrier integrity, and recruit neutrophils. **Regulatory T cells** are particularly abundant in the gut lamina propria and are essential for tolerance to food antigens and commensals. Peripherally induced Tregs (pTregs) are generated from naive CD4+ T cells by DCs that produce TGF-beta and retinoic acid. These Tregs express FOXP3 and produce IL-10 and TGF-beta to suppress inflammatory responses.
### VI. Mucosal Tolerance
The mucosal immune system must tolerate enormous quantities of harmless antigens, including food proteins and commensal bacteria. **Oral tolerance** is the systemic immunological unresponsiveness induced by oral administration of antigen. The mechanism depends on antigen dose: low doses induce antigen-specific Tregs that actively suppress responses, while high doses can cause anergy or clonal deletion of reactive T cells. DCs in the mesenteric lymph nodes and lamina propria -- particularly CD103+ DCs that produce retinoic acid and TGF-beta -- are key mediators, promoting Treg differentiation.
When mucosal tolerance fails, pathological conditions result. **Food allergies** arise from IgE-mediated responses to food proteins. **Celiac disease** involves T cell-mediated responses to gluten-derived peptides. **Inflammatory bowel disease (IBD)**, including Crohn's disease and ulcerative colitis, reflects dysregulated immune responses to commensal bacteria.
A diagram illustrating the secretory IgA transport pathway and immune exclusion. Left panel: In the lamina propria, an IgA-secreting plasma cell produces dimeric IgA (two IgA monomers linked by a J chain). The dIgA binds the polymeric immunoglobulin receptor (pIgR) on the basolateral surface of an intestinal epithelial cell. The pIgR-dIgA complex is shown being internalized into a vesicle, transcytosed through the epithelial cell, and arriving at the apical surface. The pIgR is cleaved, releasing secretory IgA (dIgA + secretory component) into the intestinal lumen. Right panel: In the lumen, sIgA is shown performing immune exclusion -- binding to bacteria and toxins, trapping them in the mucus layer, and preventing them from contacting the epithelial surface. Coated bacteria are shown being swept away by peristalsis. An inset shows intracellular neutralization: during transcytosis, IgA in an endosome encounters and neutralizes a virus that has entered the epithelial cell.
### VII. The Common Mucosal Immune System
Immune responses initiated at one mucosal site can generate effector cells -- especially IgA-positive plasmablasts -- that home to distant mucosal sites, a concept known as the common mucosal immune system. Tissue tropism is dictated by homing receptors: alpha-4-beta-7 integrin and CCR9 direct cells to the gut, alpha-4-beta-1 integrin and CCR10 direct cells to the respiratory and urogenital mucosa, and CCR10 also guides cells to the mammary gland and salivary glands.
This interconnection has important implications for **mucosal vaccines**. Oral vaccines (such as oral polio and rotavirus vaccines) generate gut mucosal IgA, and intranasal vaccines can generate respiratory mucosal immunity. Parenteral (injected) vaccines generally induce systemic IgG but elicit poor mucosal IgA responses. For pathogens that infect via mucosal surfaces, mucosal vaccines are therefore preferred. However, mucosal vaccine development faces the challenge that tolerance mechanisms at mucosal surfaces can impair vaccine immunogenicity, necessitating effective mucosal adjuvants such as cholera toxin B subunit and heat-labile enterotoxin.
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