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⁺ T cells, are less well explored. Using combined adoptive transfer experiments with peptide‐pulsed peritoneal cavity (PerC)‐derived B‐1a cells and CFSE‐labeled T cells, we show that B‐1a cells present Ag to CD4⁺ T cells 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 T cells, shows that B‐1a cells differentially promote intracellular cytokine‐expressing T cells. PerC‐derived B‐1a cells increase the percentage of IL‐10‐producing T cells along with IL‐4‐ and IFN‐γ‐producing CD4⁺ T cells. 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 T cells.