Peyer’s patches may also support some IgA production through a TI mechanism [[78]]. In addition to IgA-inducing FDCs, Peyer’s patches include TipDCs, a TNF-inducible nitric oxide synthase (iNOS)-producing DC subset that usually occupies the intestinal lamina propria [[79]]. These TipDCs elicit IgA production learn more by increasing the expression of the TGF-β receptor on B cells via nitric oxide, thereby rendering B cells more responsive to IgA-inducing signals provided by TGF-β [[79]]. Of note, recent findings

show that IgA-secreting plasma cells acquire TipDC-like phenotypic features in the intestinal microenvironment, including expression of the antimicrobial mediators, TNF and iNOS [[80]]. Thus, some of the functions previously ascribed to intestinal TipDCs also involve IgA-secreting plasma cells. Follicular B cells from Peyer’s patches and mesenteric lymph nodes further undergo IgA CSR and production in response to TI signals from plasmacytoid X-396 ic50 DCs (pDCs), which release large amounts of BAFF and APRIL upon being “primed” by type I interferon from intestinal stromal cells [[81]]. Together with Peyer’s patches and mesenteric lymph nodes, isolated lymphoid follicles represent another intestinal site for IgA induction. Isolated lymphoid follicles contain lymphoid tissue-inducer cells that

release the TNF family member lymphotoxin-β upon exposure to TLR signals from commensals [[42]]. The interaction of lymphotoxin-β with its cognate receptor stimulates local stromal cells to release TNF and DC-attracting chemokines

such as CCL19 and CCL21 [[42]]. By inducing DC production of matrix metalloproteases 9 and 13, TNF stimulates DCs to process active TGF-β from a latent precursor protein [[42]]. In the presence of TLR signals, DCs further release BAFF and APRIL, which activate Tau-protein kinase a TI pathway for IgA production by cooperating with TGF-β [[42]]. In addition to isolated lymphoid follicles, the intestinal lamina propria contains a diffuse lymphoid tissue comprised of scattered B cells that can undergo IgA class switching and production, although less efficiently and at a lower frequency than follicular B cells (reviewed in [[82, 83]]). This IgA production is supported by multiple subsets of lamina propria DCs that can activate B cells in a TI manner. When exposed to microbial TLR signals, lamina propria TipDCs release nitric oxide, which in turn enhances the production of BAFF and APRIL [[79]]. Another lamina propria DC subset with IgA-licensing function is represented by DCs constitutively expressing the flagellin receptor TLR5 [[84]]. These DCs express little or no TLR4 and induce TI IgA class switching and production by releasing retinoic acid and IL-6 upon sensing flagellin from commensal bacteria [[84]]. Also, epithelial cells deliver IgA-inducing signals to lamina propria B cells by releasing BAFF and APRIL after recognizing bacteria via multiple TLRs [[38, 85]].