Gastrointestinal Mucosal Immune System

4. Gastrointestinal Mucosal Immune System (Figure 5)

The gastrointestinal tract contains a dense population of immune cells with multiple characteristics and functions. Its main task in healthy animals is to achieve tolerance against dietary and endogenous bacterial antigens. On the other hand, the gastrointestinal immune system must be conditionally reactive against pathogenic bacteria or harmful environmental antigens.

Figure 5. The intestinal immune system.

The gut-associated immune system has anatomically defined and diffuse structures. These may act specifically as inductive or effective sites, providing the ability for an adequate immune response. The intestinal mucosa harbors a high density of immune cells that are often organized in cell clusters, either visible as lymph follicles or more prominently as Peyer’s patches. Given the antibody secreting cells, IgA+ plasma cells predominate in the small intestine, and IgM+ plasma cells are found in higher concentrations than IgG+ plasma cells (Waly et al., 2001). Plasma cells are found in all regions of the small intestine with greater numbers in the lamina propria and Peyer's patches compared to the epithelium (Howard et al., 2005).

Cats have elevated numbers of intraepithelial lymphocytes a proportion of which express surface IgM, but the significance of this finding is still uncertain. T-cells (CD3+) and T-cell subsets (CD4+ and CD8+) follow a specific distribution pattern with greater numbers in the villous lamina propria than in the lamina adjacent to the crypts. Intra-epithelial lymphocytes are mainly CD8+ T lymphocytes; CD4+ T-cells dominate in the lamina propria. Antigen presenting macrophages and dendritic cells in the lamina propria express L1 and major histocompatibility complex (MHC) class II. B-cells predominate in Peyer's patches with 40% B-cells, 28% CD4+ T-cells and 20% CD8+ T-cells.

Diseases that are associated with infections or allergic reactions in the gastrointestinal tract involve the local or general immune system (Day, 2005; Stokes & Waly, 2006). IgA is the dominant immunoglobulin in intestinal secretions of cats, as in other species. Normally, oral tolerance is induced for short periods after introduction of novel antigens into the diet. In cases of dysregulated immune response, cats may become hypersensitive to the newly introduced dietary antigen prior to the establishment of tolerance.

MHC class II expression by leukocytes with dendritic cell or macrophage morphology in the lamina propria was significantly greater in cats with inflammatory bowel disease compared to healthy cats. MHC class II expression by enterocytes was also more pronounced in diseased cats (Waly et al., 2004).

Cytokine expression seems to be important in determining the reaction of the gastrointestinal immune system to antigen challenges. Cats with intestinal inflammation had significantly more transcription of pro-inflammatory and immunoregulatory genes encoding IL-6, IL-10, IL-12, p40, TNF-α and TGF-β than cats with normal histology (Cave, 2003; Nguyen Van et al., 2006).