RA is produced mainly in the small intestine, whereas SCFAs are mainly produced in the colon.25,27 Therefore, this pathway is likely to be effective mainly in MLN which drains metabolites from both small and large intestines rather than in effector sites Dimethyl phthalate of the intestines where SCFAs and RA are produced in different locations. SCFAs activate GPCRs such as GPR41, GPR43 and GP109A. of pathogenic microbes and their products to the intestinal epithelial surface, thus decreasing their pathogenicity, as evidenced by IgA binding-mediated inhibition of type 3-secretion (T3S) system.7 IgA also facilitates the engulfment of pathogens by Peyer’s patch M cells and phagocytes, such as neutrophils, dendritic cells and macrophages, to mount effective local immune responses to pathogens.8-10 Gut microbiota induce the development of gut-associated lymphoid tissues (GALT), such as isolated lymphoid follicle (ILF) and Peyer’s patches (PPs), which are major inductive sites for IgA-producing plasma B cells.11 Germ-free (GF) animals have reduced numbers of IgA-producing plasma cells, and the gut microbiota are required for normal levels of class switch recombination (CSR) from IgM to IgA. B cells, particularly marginal zone (MZ) B cells, are negatively affected in certain restricted flora (RF) mice which have altered commensal Bp50 microbiota.12 The impaired plasma B cell responses in GF or RF animals were restored by conventionalization of the mice. Also, somatic hypermutation and IgA repertoire diversification were greatly suppressed in GF mice.13 It has been shown that certain microbiota, especially and and via metabolic regulation.23 Thus, microbial products support B cell-regulating T cells. Indirect mechanisms of B Dimethyl phthalate cell regulation by microbiota: Functions of myeloid cell populations DCs produce cytokines and present antigens to T cells, and this function is also required to generate Tfh and T follicular regulatory (Tfr) cells. DCs express several TLRs and are activated by TLR ligands. DCs sense microbial products not only within tissues but also in the gut lumen using their membrane extensions across the epithelial barrier. DCs also directly activate B cells with cytokines and cell-surface ligands (e.g., BAFF and APRIL). Therefore, the effect of microbial products on DCs can indirectly regulate B cell activation and differentiation. For example, MyD88 is required for DCs to enhance antibody responses by enhancing the production of cytokines (IL-6, IL-10 and TGF1) and other B cell-activating molecules.35,36 TLR activation also enhances follicular dendritic cells (FDCs) to activate B cells by secreting BAFF, APRIL, and TGF-137 Moreover, SCFAs up-regulate RALDH2 in DCs to increase Dimethyl phthalate RA production. Thus, it is possible that RA produced by SCFA-activated DCs can promote IgA-producing plasma B cells. RA is usually produced mainly in the small intestine, whereas SCFAs are mainly produced in the colon.25,27 Therefore, this pathway is likely to be effective mainly in MLN which drains metabolites from both small and large intestines rather than in effector sites of the intestines where SCFAs and RA are produced in different locations. SCFAs activate GPCRs such as GPR41, GPR43 and GP109A. Myeloid cells, such as neutrophils, macrophages and DCs, variably express GPR43 and GP109A. Therefore, SCFAs have the potential to indirectly regulate B cells through myeloid cells. For example, SCFAs increase IL-10 production by macrophages and DCs.38,39 via either SCFA-receptor signaling or HDAC inhibition (Fig.?1). RA and IL-10, produced from SCFA-regulated myeloid cells, can promote antibody production, particularly IgA. In contrast, T and B cells do not significantly express SCFA receptors. Eosinophils, abundant in the intestinal lamina propria, sense microbial signals and regulate B cells. Eosinophils, when activated by commensal bacterial products, produce numerous B cell-activating molecules, such as BAFF, APRIL, IL-6 and matrix metalloproteinase 9 (MMP9), which can promote the differentiation and survival of IgA+ plasma cells. Eosinophil-deficient mice (dblGATA-1 and PHIL mice) have reduced numbers of Dimethyl phthalate IgA+ plasma cells but increased numbers of IgG1+ cells in PPs. Eosinophil-deficient mice also have decreased numbers of CD103+ DC and Tregs but increased production of Th2 Dimethyl phthalate cytokines (IL-4 and IL-5) by Tfh cells in PPs40 The gut microbiota activate epithelial cells for production of IL-25, which in turn activates eosinophils. For example, Tritrichomonas muris, a symbiotic protozoon in mice, induces IL-25 production by specialized epithelial cells called Tuft cells. Tuft cell-derived IL-25 mounts a type 2 innate lymphoid cell (ILC2) response and produce IL-5 and IL-13,41 which can increase eosinophil responses and impact B cell responses in the gut. Antibodies are produced by several.