Gastrointestinal Immunology Notes (Lecture 2)

Gastrointestinal Immunology – Lecture 2 Notes

Overview of GI Immunology and the GALT

This lecture focuses on the immune cells found in the gastrointestinal tract, with emphasis on the innate immune system within the gut-associated lymphoid tissue (GALT) and the role of secreted immunoglobulin A (IgA) as an immune barrier and regulator. The gut structure comprises a lumen filled with microbiota and potential pathogens, an epithelial barrier, and the underlying lamina propria where most immune cells reside. A schematic is recalled showing the lumen, mucus, epithelial layer, and lamina propria, with antigens sampled by microfold (M) cells and delivered to antigen-presenting cells (APCs) such as dendritic cells and macrophages. These APCs sample antigens in the lamina propria and transport signals to the adaptive immune system, primarily within Peyer’s patches, which are the GALT lymph nodes rich in B and T cells. The integration of innate sampling and adaptive responses underpins mucosal immunity and homeostasis within the gut.

Anatomical Organization: Lumen, Epithelium, Lamina Propria, and Peyer’s Patches

In the gut, the lumen contains microbiota alongside potential pathogens (bacteria, viruses, parasites). The epithelial barrier lines the lumen, and immediately beneath lies the lamina propria where most immune cells reside. M cells within the epithelium sample luminal antigens and transfer them to dendritic cells in the lamina propria, facilitating antigen presentation. Dendritic cells can directly uptake antigens through their dendritic processes that extend between epithelial cells, and macrophages can also sample antigens in this region. The coordinated sampling, processing, and presentation of antigens occur in the lamina propria and are crucial for activating both innate and adaptive immune responses. Peyer’s patches (GALT lymphoid nodes) have a high density of B and T cells and function as sites where innate-presented antigens interact with adaptive immunity, enabling crosstalk between APCs, B cells, and T cells.

Antigen Sampling and Antigen-Presenting Cells in the Gut

The primary antigen-presenting cells in the gut are dendritic cells, which uptake antigens through M cells and other routes. Dendritic cells can capture bacterial, parasitic, or viral antigens from the lumen and process them for presentation to T cells in Peyer’s patches, as well as to B cells and other gut lymphoid structures. Macrophages also contribute to antigen sampling. The overarching goal of these APCs is to sample antigens, present them to T cells to initiate adaptive immunity, and may also influence B cell responses. The mucosal immune system must balance protective responses against pathogens with tolerance to commensal microbiota to maintain homeostasis. Dendritic cells contribute to both outcomes: they can induce protective immunity against pathogens while promoting regulatory T cell (Treg) responses to limit unnecessary inflammation against commensals. This balance is essential for gut health and prevention of chronic inflammatory states.

IgA Production, Transport, and Function in the Gut

A major arm of the gut’s adaptive immunity is the production of Immunoglobulin A (IgA). B cells within the lamina propria differentiate and secrete IgA, contributing roughly 70\%-80\% of the daily antibody pool in the body. IgA is produced in the lamina propria and then transported across the mucosal epithelium via the polymeric (poly) IgA receptor (pIgR). IgA is bound by this receptor while in the lamina propria and is transcytosed through epithelial cells; at the luminal surface, the receptor’s extracellular domain is cleaved, leaving secreted IgA (sIgA) with a secretory component that protects it from luminal proteases. This process ensures IgA can function in the lumen without degradation.

IgA's primary role is to bind antigens in the lumen. By binding bacteria, IgA helps shape the gut microbiota’s composition and colonization dynamics. It neutralizes rapidly growing bacteria that could otherwise outcompete commensals and can hinder the invasion and establishment of fast-growing invasive pathogens. IgA also helps stabilize slower-growing commensals, contributing to homeostasis within the microbiome. Importantly, IgA restricts microbial translocation by preventing microbes from crossing the epithelial barrier, thereby protecting mucosal integrity. IgA also participates in antigen sampling: luminal antigens that cross via M cells can be captured and presented by dendritic cells, aided by IgA-bound antigens, reinforcing mucosal immune surveillance.

Thus, IgA serves a dual function: a barrier that maintains luminal homeostasis by preventing pathogen access and enabling tolerance to commensal organisms, while participating in antigen sampling that informs local immune responses.

T Cells in Gut Immunity: CD4+ Helper T Cells, IELs, and MAIT Cells

The adaptive arm of gut immunity includes B cells (IgA production) and various T cell populations. Helper T cells (CD4+ T cells) differentiate into several subsets depending on cytokine cues and antigen exposure. In the gut, Th17 cells (CD4+) are key for combating extracellular bacteria and fungi and for supporting mucosal barrier integrity. These cells produce interleukin-17 (IL-17), which is crucial for orchestrating neutrophilic responses against extracellular microbes. Th2 cells (CD4+) contribute to defense against helminths and parasites and support mucus secretion and peristaltic movement, aiding mechanical clearance of pathogens along the gut. Intraepithelial lymphocytes (IELs) are a distinct population residing within the epithelial layer, consisting of CD4+ and CD8+ T cells. They are antigen-experienced and can respond rapidly without requiring priming by dendritic cells, providing immediate protection at the initial entry points of pathogens and enabling the killing of infected epithelial cells. MAIT cells, described here as “mate cells,” are another lymphocyte subset that reside in certain tissues (notably the liver) and provide defense against microbes that may breach mucosal barriers and enter the bloodstream, forming a back-up line of defense if pathogens disseminate beyond the gut mucosa.

In addition to conventional T cells, the mucosa harbors innate-like cells that contribute to early defense, which are discussed under Innate Lymphoid Cells. Together, these T cell compartments coordinate both immediate and longer-term adaptive responses to gut infections while contributing to tissue integrity and barrier function.

Innate Lymphoid Cells (ILCs) in the Gut: ILC2s and ILC3s

Beyond conventional T cells, the gut contains innate lymphoid cells (ILCs) that provide rapid cytokine-mediated signaling to shape downstream adaptive responses. Two major classes of ILCs in the gut are ILC3s and ILC2s. ILC3s produce cytokines such as IL-17 and IL-22. IL-17 supports inflammatory responses necessary to counter microbes, paralleling the action of Th17 cells, while IL-22 collaborates with IL-17 to bolster mucosal defense by stimulating antimicrobial peptide production (defensins) and by enhancing tight junction integrity, thereby strengthening epithelial barrier function and preventing bacterial translocation. ILC2s contribute to type 2–associated responses by producing cytokines like IL-5 and IL-13, which activate eosinophils and promote mucus production and smooth muscle contractions, aiding in the expulsion of parasites and in facilitating mucus-driven clearance. The production of IL-13 is also associated with mucus secretion, with tuft and goblet cells contributing to mucus production via IL-13 signaling in concert with ILCs. IL-5's role in activating eosinophils further supports defense against helminths and larger parasites.

ILCs play a pivotal role in initiating and shaping the early immune response in the gut, creating an environment that supports Th cell differentiation and activity while directly influencing epithelial and mucosal barriers through cytokine signaling and mucus modulation.

Immune Induction, Specialization, and Homeostasis in the GALT

Immune induction largely takes place in Peyer’s patches and mesenteric lymph nodes, which organize the adaptive response to gut-derived antigens. The mucosa contains specialized cells and organized lymphoid structures within mucosal-associated lymphoid tissue, where interactions among dendritic cells, B cells, T cells, and other immune players occur. The lamina propria houses a diverse array of lymphocytes and plasma cells that secrete IgA, reinforcing the mucosal barrier and contributing to immune surveillance. Overall, the GI immune system integrates innate sampling with adaptive responses to balance protection against pathogens and tolerance toward commensal microbiota, maintaining gut homeostasis and preventing excessive inflammatory damage.

The IgA-centered arm exemplifies this balance: IgA not only neutralizes pathogens but also supports commensal stability and helps regulate luminal antigens encountered by M cells and dendritic cells, promoting appropriate downstream responses while protecting epithelial integrity. In summary, the mucosal immune system relies on a network of epithelial barriers, sampling APCs, IgA-mediated antibody defenses, diverse T cell subsets (including Th17, Th2, IELs, and MAIT-like cells), and innate lymphoid cells to sustain gut health and protect against infection.

Summary and Key Takeaways

The GI tract employs a robust mucosal immune system centered on GALT, Peyer’s patches, lamina propria, and the epithelial barrier, with M cells sampling luminal antigens and presenting them to dendritic cells and macrophages.
IgA is the dominant secreted antibody in mucosal tissues, produced by B cells in the lamina propria (roughly 70\%-80\% of daily antibodies). IgA is transported to the lumen via the poly-IgA receptor (pIgR) and is secreted as secreted IgA (sIgA) after receptor cleavage.
IgA binds luminal bacteria to shape colonization, neutralize fast-growing pathogens, stabilize slow-growing commensals, and prevent microbial translocation across the epithelial barrier. IgA also participates in antigen sampling by facilitating uptake of luminal antigens for dendritic cells.
The adaptive immune compartment includes CD4+ T helper cells (Th17 and Th2), intraepithelial lymphocytes (IELs, including CD4+ and CD8+ populations) that provide rapid, pre-primed responses, and MAIT-like cells that defend against blood-borne microbes.
Innate lymphoid cells (ILCs), particularly ILC3s and ILC2s, produce cytokines that regulate inflammation, barrier integrity, and mucus production. ILC3s produce IL-17 and IL-22 (the latter promoting defensins and tight junctions), while ILC2s contribute via IL-5 (activating eosinophils) and mucus-associated pathways (including IL-13-related mucus production by tuft/goblet cells).
Immune induction occurs in Peyer’s patches and mesenteric lymph nodes, with specialized mucosal cells and lymphoid structures coordinating the adaptive response. The overall system aims to provide protective responses against pathogens while maintaining tolerance to commensals, thereby preserving gut homeostasis and barrier function.