Peritoneal cavity B-1a cells promote peripheral CD4+ T-cell activation.

Innate-like murine B-1a cells are well known for their ability to secrete natural IgM. Their non-Ab mediated functions, including Ag presentation to CD4(+) Tcells, are less well explored. Using combined adoptive transfer experiments with peptide-pulsed peritoneal cavity (PerC)-derived B-1a cells and CFSE-labeled Tcells, we show that B-1a cells present Ag to CD4(+) Tcells from the periphery in vivo. In vitro characterization, using co-cultures in which B-1a or splenic B cells presented whole OVA protein to OVA-specific Tg Tcells, shows that B-1a cells differentially promote intracellular cytokine-expressing Tcells. PerC-derived B-1a cells increase the percentage of IL-10-producing Tcells along with IL-4- and IFN--producing CD4(+) Tcells. These data suggest that B cells in the PerC have the potential to influence peripheral immune responses without the necessity to migrate out of this location. This, to our knowledge previously undescribed, immuno-logical pathway potentially plays a role in the presentation of gut microbiota-derived Ags to peripheral Tcells.