Inductive Gut Associated Lymphoid Tissue Structures

Intestinal immune cells are located in three compartments: GALT, the mucosal lamina propria, and the surface epithelium (Fig. 3.2). GALT comprises Peyer's patches, the appendix, and numerous ILFs [2, 6, 30] . These organized lymphoid structures represent inductive sites for intestinal immune responses, while the lamina propria and epithelial compartment principally constitute effector sites but may nevertheless contribute to retention, proliferation, and differentiation of immune cells [31].

Talus Anatomy

Fig. 3.2 Depiction of the intestinal mucosal immune system. Inductive sites for T and B cells are constituted by gut-associated lymphoid tissue (GALT) such as Peyer's patches with B-cell follicles and M cell (M)-containing follicle-associated epithelium through which exogenous antigens (Ag) are transported to reach antigen-presenting cells (APC), including dendritic cells, macrophages (Mf), and follicular dendritic cells (FDC). After being primed, naïve T and B cells become memory/effector cells and migrate from GALT to mesenteric lymph nodes via lymph and then via the thoracic duct to peripheral blood for subsequent extravasation at mucosal effector sites. This process is directed by the profile of adhesion molecules and chemokines expressed on the local microvasculature - the endothelial cells thus exerting a "gatekeeper function" for mucosal immunity (see Figs. 3.3 and 3.4). The lamina propria (effector site) is illustrated with its various immune cells, including B cells (B), the approximate proportions of various Ig-producing plasma cells, and CD4+ T cells. The distribution of intraepithelial T lymphocytes (mainly CD8+ with a/p T-cell receptor; some y/5) is also depicted. Additional features are the generation of secretory IgA (SIgA) and secretory IgM (SIgM) via pIgR/membrane secretory component (mSC)-mediated epithelial export. The combined effect of oral tolerance mechanisms, mainly the action of regulatory T cells (not shown), provides a suppressive tone in the gut, normally keeping inflammation driven by IgG and IgE antibodies as well as cell-mediated (CD4+ T cells and Mf ) delayed-type hypersensitivity (DTH) under control

Fig. 3.2 Depiction of the intestinal mucosal immune system. Inductive sites for T and B cells are constituted by gut-associated lymphoid tissue (GALT) such as Peyer's patches with B-cell follicles and M cell (M)-containing follicle-associated epithelium through which exogenous antigens (Ag) are transported to reach antigen-presenting cells (APC), including dendritic cells, macrophages (Mf), and follicular dendritic cells (FDC). After being primed, naïve T and B cells become memory/effector cells and migrate from GALT to mesenteric lymph nodes via lymph and then via the thoracic duct to peripheral blood for subsequent extravasation at mucosal effector sites. This process is directed by the profile of adhesion molecules and chemokines expressed on the local microvasculature - the endothelial cells thus exerting a "gatekeeper function" for mucosal immunity (see Figs. 3.3 and 3.4). The lamina propria (effector site) is illustrated with its various immune cells, including B cells (B), the approximate proportions of various Ig-producing plasma cells, and CD4+ T cells. The distribution of intraepithelial T lymphocytes (mainly CD8+ with a/p T-cell receptor; some y/5) is also depicted. Additional features are the generation of secretory IgA (SIgA) and secretory IgM (SIgM) via pIgR/membrane secretory component (mSC)-mediated epithelial export. The combined effect of oral tolerance mechanisms, mainly the action of regulatory T cells (not shown), provides a suppressive tone in the gut, normally keeping inflammation driven by IgG and IgE antibodies as well as cell-mediated (CD4+ T cells and Mf ) delayed-type hypersensitivity (DTH) under control

Although Peyer's patches are present at birth, it takes some time before they become activated as signified by germinal centers, and the induction of ILF organogenesis depends on postnatal exogenous stimuli, at least in mice [6, 32] . The B-cell follicles of GALT are covered by a specialized epithelium containing very thin "microfold" or membrane (M) cells which, together with intraepithelial dendritic cells (DCs), transport antigens from the gut lumen into the lymphoid tissue [33].

Peyer's patches resemble lymph nodes with interfollicular T-cell zones and a variety of antigen-presenting cells (APCs) such as DCs and macrophages, whereas ILFs have a more sparse T-cell zone. All GALT structures are devoid of encapsulation and contain no afferent lymphatics; [6] their supply of antigens depends exclusively on sampling directly from the mucosal surface through the specialized follicle-associated epithelium. Induction and regulation of mucosal immunity hence takes place primarily in GALT and draining mesenteric lymph nodes (MLNs), while terminal differentiation of B cells to PCs occurs in the lamina propria (Fig. 3.2) where secondary T-cell signals are generated when antigens are presented by local DCs and macrophages [31]. However, animal experiments have shown that oral tolerance can be induced in the absence of GALT, thus being dependent on antigen transport to MLNs from gut mucosa through lymph by specialized DCs, as discussed later [34, 35],

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