Peyer’s patches contain abundant isotype-switched B cells with activated phenotypes and are inductive sites for T-independent anti-DNA IgA

Cecal Patches Generate Abundant IgG2b-Bearing B Cells That Are Reactive to Commensal Microbiota

Gut-associated lymphoid tissue (GALT), such as Peyer's patches (PPs), are key inductive sites that generate IgA⁺ B cells, mainly through germinal center (GC) responses. The generation of IgA⁺ B cells is promoted by the presence of gut microbiota and dietary antigens. However, the function of GALT in the large intestine, such as cecal patches (CePs) and colonic patches (CoPs), and their regulatory mechanisms remain largely unknown. In this study, we demonstrate that the CePs possess more IgG2b⁺ B cells and have fewer IgA⁺ B cells than those in PPs from BALB/c mice with normal gut microbiota. Gene expression analysis of postswitched transcripts supported the differential expression of dominant antibody isotypes in B cells in GALT. Germ-free (GF) mice showed diminished GC B cells and had few IgA⁺ or IgG2b⁺ switched B cells in both the small and large intestinal GALT. In contrast, myeloid differentiation factor 88- (MyD88-) deficient mice exhibited decreased GC B cells and presented with reduced numbers of IgG2b⁺ B cells in CePs but not in PPs. Using ex vivo cell culture, we showed that CePs have a greater capacity to produce total and microbiota-reactive IgG2b, in addition to microbiota-reactive IgA, than the PPs. In line with the frequency of GC B cells and IgG2b⁺ B cells in CePs, there was a decrease in the levels of microbiota-reactive IgG2b and IgA in the serum of GF and MyD88-deficient mice. These data suggest that CePs have a different antibody production profile compared to PPs. Furthermore, the innate immune signals derived from gut microbiota are crucial for generating the IgG2b antibodies in CePs.

Crosstalk between sIgA-Coated Bacteria in Infant Gut and Early-Life Health

Gut microbiota transmission from mother to offspring has attracted much interest in recent years. The gut microbiota in the infant plays a potentially significant role in modulating and maintaining the development of infant immunity. Secretory immunoglobulin A (sIgA), the major immunoglobulin in the intestine, can target polysaccharides and flagellin on the bacterial surface, resulting in sIgA-coated bacteria. The presentation of specific bacteria coated with sIgA may be a signal of disease and provide novel insights into the relationship between infant microbiota and disease. Here, we review the composition of sIgA-coated bacteria in the adult intestine, human milk, and the infant intestine, as well as the factors that influence the development of gut microbiota in early life. Then, we highlight the diseases that are related to variations in sIgA-coated bacteria in the infant and adult intestine. Furthermore, we discuss the possibility that sIgA-coated bacteria could play a role in mediating both innate and adaptive immune responses. Finally, we propose directions for future research to promote our understanding within this field.

IL-21 and IL-5 coordinately induce surface IgA+ cells

Intestinal IgA is induced by microbes and food antigens. Peyer's patches (PPs) are known as one of the inductive sites for intestinal IgA production. However, the precise mechanism of IgA induction is as yet unknown. IgA secretion was induced from IgD⁺ B cells in vitro by stimulus with lipopolysaccharide in the presence of only retinoic acid (RA) and low doses of TGF-β1. Surface IgA⁺ cells were effectively induced from IgD⁺ B cells in vitro by the mixture of RA and the cytokines TGF-β1, APRIL, IL-5 and IL-21. rIL-21 upregulated surface IgA⁺ but impaired the proliferation of stimulated B cells in the presence of rTGF-β1, RA and rAPRIL, in vitro. The addition of rIL-5 restored the impaired proliferation by rIL-21, resulting in the expansion of IgA⁺ cells. rIL-21 induced the expression of Aicda and Prdm1, and impaired Rel in IgD⁺ B cells. Blockade of IL-21R signaling by a neutralizing mAb in vivo led to lower frequencies of IgA⁺ and IgG2b⁺ cells and lower germinal center B cells in PPs in a homeostatic condition. Although amounts of small intestinal IgA and titers of anti-dsDNA, the major target of intestinal IgA, in these mice were not altered, anti-OVA IgA titers induced by OVA drinking in OVA-specific T-cell receptor (TCR) transgenic mice were decreased. PP-deficient TCR transgenic mice showed diminished anti-OVA IgA induction. Blockade of IL-5R signaling in vivo led to similar results with relatively weaker effects than that of IL-21R mAb administration. These results suggest that IL-21 and IL-5 play cooperative roles in surface expression of IgA in PPs.