The GALT consists of secondary lymphoid organs [Peyer's patches (PPs) and the mesenteric lymph nodes (MLNs)], tertiary organized lymphoid tissues
[CPs and isolated lymphoid follicles (ILFs)], dispersed cells [intraepithelial T lymphocytes (IELs) and LP T and B lymphocytes], and an organized network of specialized DCs. In addition, Paneth cells, specialized epithelial cells at the base of the intestinal crypts, secrete antimicrobial peptides that exert their action in the lumen of the intestine. Together, these diverse cells and organized lymphoid structures are thought to regulate the microflora while also defending the host from breaches in epithelial integrity and preventing inflammatory responses against harmless antigens. How these different functions are accomplished remains poorly understood, but it is clear that there must be extensive communication between the immune system components within the intestinal mucosa. In addition, the epithelial cell compartment is also regulated by interactions with components of the mucosal immune system. Recent studies with genetically modified mice have advanced our understanding of the organization and relationship between GALT components. However, the mechanisms and mediators involved in the cross talk between these components have yet to be elucidated.
In general, the PPs, ILFs, and CPs have been ascribed different functions based on their developmental and structural differences. However, several studies, as outlined below, have observed redundant or overlapping functions between PPs and ILFs. CP function remains undetermined, although our recent studies suggest that these cells are similar to the LTi cells that, in the fetus, are required for development of all lymph nodes and PPs. The CP cells, like fetal LTi cells, are characterized by the expression of the orphan nuclear hormone receptor RORyt, which is necessary for the development of all three types of the organized GALT within the LP, as well as for development of lymph nodes [5,6].
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