Cholera toxin requires microbiota metabolites short-chain fatty acids for its mucosal adjuvant activity

Abstract

Abstract Cholera toxin (CT) is a potent mucosal adjuvant, however, the mechanisms by which CT induces both mucosal and systemic immune responses are still not completely understood. It has been shown recently that gut microbiota is critical in inducing immune response by CT, however, the mechanisms involved remain largely unknown. We demonstrated in this study that gut microbiota metabolites short-chain fatty acids (SCFA), which are solely metabolic products of gut microbiota from dietary fiber, are required for CT in inducing antigen-specific IgG and IgA responses after oral immunization. Depleting of gut bacteria by antibiotics significantly decreased mucosal and systemic IgG and IgA responses in mice orally immunized with ovalbumin (OVA) and CT. Feeding SCFA acetate and butyrate rescued antibody responses in antibiotics-treated WT mice. Furthermore, deficiency of GPR43, a SCFA binding receptor, impaired CT’s adjuvant activity. Acetate and butyrate did not directly act on B cells to promote IgG production, but promoted B cell IgG production and plasma B cell differentiation related genes in the presence of WT but not GPR43−/− DC. Furthermore, SCFA increased IgG production by both WT and GPR43−/− B cells when cultured with WT DC, indicating a crucial role for DC expression of GPR43 in SCFA induction of B cell IgG production. Mechanistically, SCFA promoted DC expressions of BAFF, a B cell activating factor, and Aldh1a2, an enzyme promoting production of retinoic acid (RA) from Vitamin A. Blockade either RA or BAFF signaling significantly inhibited SCFA induced B-cell IgG production. Therefore, our study indicated a critical role of gut microbiota and their metabolites SCFA in promoting the mucosal adjuvant activity of CT through partially GPR43.