Microbiota metabolite short-chain fatty acid acetate promotes intestinal IgA response to microbiota which is mediated by GPR43.

Intestinal IgA, which is regulated by gut microbiota, has a crucial role in maintenance of intestinal homeostasis and in protecting the intestines from inflammation. However, the means by which microbiota promotes intestinal IgA responses remain unclear. Emerging evidence suggests that the host can sense gut bacterial metabolites in addition to pathogen-associated molecular patterns and that recognition of these small molecules influences host immune response in the intestines and beyond. We reported here that microbiota metabolite short-chain fatty acid acetate promoted intestinal IgA responses, which was mediated by "metabolite-sensing" GPR43. GPR43-/- mice demonstrated lower levels of intestinal IgA and IgA+ gut bacteria compared with those in wild type (WT) mice. Feeding WT but not GPR43-/- mice acetate but not butyrate promoted intestinal IgA response independent of T cells. Acetate promoted B-cell IgA class switching and IgA production in vitro in the presence of WT but not GPR43-/- dendritic cells (DCs). Mechanistically, acetate-induced DC expression of Aldh1a2, which converts Vitamin A into its metabolite retinoic acid (RA). Moreover, blockade of RA signaling inhibited the acetate induction of B-cell IgA production. Our studies thus identified a new pathway by which microbiota promotes intestinal IgA response through its metabolites.

Interleukin (IL)-21 promotes intestinal IgA response to microbiota.

Commensal microbiota-specific T helper type 17 (Th17) cells are enriched in the intestines, which can convert into T follicular helper (Tfh) in Peyer's patches, and are crucial for production of intestinal immunoglobulin A (IgA) against microbiota; however, the role of Th17 and Tfh cytokines in regulating the mucosal IgA response to enteric microbiota is still not completely known. In this study, we found that intestinal IgA was impaired in mice deficient in interleukin (IL)-17 or IL-21 signaling. IL-21, but not IL-17, is able to augment B-cell differentiation to IgA(+) cells as mediated by transforming growth factor ß1 (TGFß1) and accelerate IgA class switch recombination (CSR). IL-21 and retinoic acid (RA) induce IgA(+) B-cell development and IgA production and drives autocrine TGFß1 production to initiate IgA CSR. Repletion of T-cell-deficient TCRßxδ(-/-) mice with Th17 cells specific for commensal bacterial antigen increased the levels of IgA(+) B cells and IgA production in the intestine, which was blocked by neutralizing IL-21. Thus IL-21 functions to strongly augment IgA production under intestinal environment. Furthermore, IL-21 promotes intestinal B-cell homing through α4ß7 expression, alone or with TGFß and RA. Together, IL-21 from microbiota-specific Th17 and/or Tfh cells contributes to robust intestinal IgA levels by enhancing IgA(+) CSR, IgA production and B-cell trafficking into the intestine.

Txp40, a ubiquitous insecticidal toxin protein from Xenorhabdus and Photorhabdus bacteria.

Xenorhabdus and Photorhabdus are gram-negative bacteria that produce a range of proteins that are toxic to insects. We recently identified a novel 42-kDa protein from Xenorhabdus nematophila that was lethal to the larvae of insects such as Galleria mellonella and Helicoverpa armigera when it was injected at doses of 30 to 40 ng/g larvae. In the present work, the toxin gene txp40 was identified in another 59 strains of Xenorhabdus and Photorhabdus, indicating that it is both highly conserved and widespread among these bacteria. Recombinant toxin protein was shown to be active against a variety of insect species by direct injection into the larvae of the lepidopteran species G. mellonella, H. armigera, and Plodia interpunctella and the dipteran species Lucilia cuprina. The protein exhibited significant cytotoxicity against two dipteran cell lines and two lepidopteran cell lines but not against a mammalian cell line. Histological data from H. armigera larvae into which the toxin was injected suggested that the primary site of action of the toxin is the midgut, although some damage to the fat body was also observed.