and possess a complex array of innate and adaptive mechanisms of immunity. The adaptive immune system of the mucosa-associated lymphoid tissues differs from that of the rest of the peripheral lymphoid system in several respects: the immediate juxtaposition of mucosal epithelium and lymphoid tissue; diffuse lymphoid tissue as well as more organized lymphoid organs; specialized antigen uptake mechanisms and distinctive dendritic cells and macrophages; the predominance of activated/memory lymphocytes and distinctive innate lymphoid cells (ILCs) even in the absence of infection; the production of dimeric secretory IgA as the predominant antibody; and the downregulation of immune responses to innocuous antigens such as food antigens and commensal microorganisms. No systemic immune response can normally be detected to these antigens. In contrast, pathogenic microorganisms induce strong protective responses. The key factor in the decision between tolerance and the development of powerful adaptive immune responses is the context in which antigen is presented to T lymphocytes in the mucosal immune system. When there is no inflammation, presentation of antigen to T cells by dendritic cells induces the differentiation of regulatory T cells. By contrast, pathogenic microorganisms crossing the mucosa induce an inflammatory response in the tissues, which stimulates the maturation of antigen-presenting cells and their expression of co-stimulatory molecules, thus favoring a protective T-cell response. This decision-making process is controlled mostly by the way in which specialized dendritic cells react to their environment before migrating to present antigen to naive T cells. The mutualistic loop formed between the host immune response and the local microbiota plays a central role in maintaining health and in the development of disease.
Questions.
12.1 Multiple Choice: Which of the following is an incorrect statement?
A. Microfold cells have a folded luminal surface and possess a thick layer of mucus that allows the entry of microbes to Peyer’s patches.
B. Microfold cells recognize several bacterial proteins by GP2 and release the material to the extracellular space by a process called transcytosis.
C. Gut‑associated lymphoid tissues attract dendritic cells trough chemokines such as CCL20 and CCL9.
D. Pathogens such as Yersinia pestis and Shigella target microfold cells to gain access to the subepithelial space.
12.2 True or False: Intraepithelial lymphocytes are mostly CD4 T cells, in contrast to the lamina propria, where CD8 T cells predominate.
12.3 Matching: Match each chemokine or chemokine receptor to its tissue homing function.
A. CXCL13 i. Recruitment of lymphocytes to the colon, lactating mammary gland, and salivary glands 12.4 Multiple Choice: Which of the following is a correct statement?
A. CD11b+ dendritic cells stimulate ILC3s and are the main source for IL‑12 in Peyer’s patches.
B. CD11b– dendritic cells require BATF3 for their development.
C. Retinoic acid production by naive T cells is required for dendritic cells for the generation of Treg cells.
D. CCL20 prevents entrance of dendritic cells into the epithelial layer of Peyer’s patches.
12.5 Short Answer: IgA:antigen complexes can be reexported to the gut lumen in order to enhance pathogen excretion from the organism. In contrast, the formation of IgA:antigen complexes can also enhance the uptake of luminal antigen. How can uptake of antigen be beneficial to the organism?