The study shows that the antimicrobial peptide LL-37 can talk to a gut immune receptor (FPR2) on special cells that help B‑cells work. This signaling makes the gut produce more signals (CXCL13 and BAFF) that tell B‑cells to grow and make IgA antibodies, which protect the intestine.

Peyer's patches (PPs) are the major mucosal immune-inductive site, and germinal centers (GCs) in PPs determine the quality of the Abs produced. PP GCs are continuously induced by the gut microbiota, and their maintenance contributes to the induction of strong IgA responses to Ags. In this study, we investigated the role of formyl peptide receptor (FPR)-mediated signaling in the maintenance of PP GCs, because FPRs recognize the microbiota and initiate an innate immune response by chemotaxis. We found that follicular dendritic cells (FDCs), a key organizer of B cell follicles and GCs in mucosal immunity, express Fpr2. Additionally, Fpr2-mediated signaling in PP FDCs promoted Cxcl13 and B cell activating factor expression, as well as B cell proliferation and activation. Therefore, we suggest that Fpr2-mediated signaling in FDCs plays a key role in GC maintenance in PPs and results in an Ag-specific IgA response in the gut mucosal immune compartment.