Development and Maintenance of the Gut-Associated Lymphoid Tissue (Galt): the Roles of Enteric Bacteria and Viruses

Journal of Immunology Research, Aug 2018

GALT can be subdivided into several compartments: (a) Peyer's patches (PP); (b) lamina propria (LP); and (c) intraepithelial leukocyte (IEL) spaces. The B-cell follicles of PP are quiescent in neonatal and germ-free (GF) adult mice. Germinal centers (GC), including sIgA

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Development and Maintenance of the Gut-Associated Lymphoid Tissue (Galt): the Roles of Enteric Bacteria and Viruses

Development and Maintenance of the Gut-Associated Lymphoid Tissue (GALT): The Roles of Enteric Bacteria and Viruses JOHN J. CEBRA 0 SANGEETA BHARGAVA PERIWAL 0 GWEN LEE 0 FAN LEE 0 KHUSHROO E. SHROFF 0 0 Department of Biology, University of Pennsylvania , Philadelphia, PA 19104-6018 , USA GALT can be subdivided into several compartments: (a) Peyer's patches (PP); (b) lamina propria (LP); and (c) intraepithelial leukocyte (IEL) spaces. The B-cell follicles of PP are quiescent in neonatal and germ-free (GF) adult mice. Germinal centers (GC), including slgA blasts, appear in the B follicles of formerly GF adult mice about 10-14 days after monoassociation with various gut commensal bacteria. The GC wax and wane over about a 3-week period, although the bacterial colonizers remain in the gut at high density. Neonatal mice, born of conventionally reared (CV), immunocompetent mothers, display GC reactions in PP postweaning, although pups of SCID mothers display precocious GC reactions at about 14 days of life. Normally, gut colonization of neonates with segmented filamentous bacteria (SFB) leads to explosive development of IgA plasmablasts in LP shortly after weaning. Commensal gut bacteria and the immunocompetency of mothers also appears to control the rate of accumulation of primary B cells from "virgin" B cells in neonates. Enteric reovirus infection by the oral route can cause the activation of CD8 T cells in the interfollicular regions of PP and the appearance of virus-specific precursor cytotoxic T lymphocytes (pCTL) in the IEL spaces. Such oral stimulation can also lead to "activation" of both CTL and natural killer (NK) cells in the IEL spaces. More normally, colonization of the gut with SFB also leads to similar activations of NK cells and "constitutively" cytotoxic T cells. Enteric viruses; gut-associated lymphoid tissue (GALT); gut commensal bacteria; IgA responses in gut; intraepithelial leukocytes; Peyer's patches INTRODUCTION GALT includes both organized lymphoid compartments, consisting of PP, regional lymphatics, and mesenteric lymph nodes (MLN), and dispersed lymphoid cells in the IEL spaces and the gut LP (Owen and Jones, 1974; Cerf-Bensussan and Guy-Grand, 1991) . The PP consist of a single-layer cluster of Bcell follicles, divided by T-cell-rich wedges, unevenly distributed in the wall of the small intestine. A specialized follicle-associated epithelium (FAE) overlies these clusters, forming a relatively mucus-free "dome" among the absorptive villi and their interspersed crypts. Among the cells of FAE are specialized M (microfold) cells, which actively pinocytose or endocytose droplets and particles from the gut lumen Owen and Jones, 1974). These M cells serve as "afferent lymphatics" for PP, delivering antigens (Ags) and pathogens to the underlying, organized lymphoid tissue (Wolf et al., 1981; Jones et al., 1994) . The B-cell follicles in PP of CV mammals are not typically quiescent (primary), as in lymph nodes or spleen, but rather display continuous GC reactions and are chronically activated (secondary). Presumably, these GC reactions are constantly driven by the environmental Ags delivered via M cells. The GC in PP of mice typically bind high levels of peanut agglutinin (PNA--a marker for the B blasts) and contain a preponderance of dividing, IgA-expressing B blasts (Lebman et al., 1987; Weinstein et al., 1991) . Surface IgD-bearing B cells (primary B cells) are confined to the recirculating population comprising the surrounding mantle zone. Lymphocyte Recirculation from and to GALT Among the B cells leaving PP via efferent lymphatics are IgA B cells, generated and selected for survival in GC (Craig and Cebra, 1971; Lebman et al., 1987) . These pass through MLN, where some are maturing to IgA plasma blasts (McWilliams et al., 1975). Eventually, these IgA B cells are found in thoracic duct lymph (Pierce and Gowans, 1975) and blood, having developed a homing propensity to exit into and accumulate in mucosal tissues via transit of small venules (Husband, 1982) . Many of the IgA plasmablasts generated in PP eventually accumulate and secrete their IgA Abs in the gut LP. At least one T-cell subset that can contribute to gut immunity (CD8 CTL) can be generated by Agstimulation in the interfollicular regions of PP (London et al., 1987, 1990) . Some of these emigrate to the IEL spaces, where they selectively lodge and can function as one subset of CTL among other NK and CD8 T cells from other sources (Cuff et al., 1993). Roles of IgA Abs and T Lymphocytes in GALT Dimeric IgA Abs, produced locally by IgA plasma cells in gut LP, are actively transported via poly Igreceptors (pig-R) into and through gut epithelial cells, especially crypt cells (Mostov et al., 1980) . During exocytosis into crypt and gut lumen, the pIg-R is cleaved and a portion (secretory component) remains complexed to IgA dimer. This "secretory" IgA can function as a "blocking" or "neutralizing" Ab in the gut l (...truncated)


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John J. Cebra, Sangeeta Bhargava Periwal, Gwen Lee, Fan Lee, Khushroo E. Shroff. Development and Maintenance of the Gut-Associated Lymphoid Tissue (Galt): the Roles of Enteric Bacteria and Viruses, Journal of Immunology Research, 6, DOI: 10.1155/1998/68382