A B-1a cell subset induces Foxp3(-) T cells with regulatory activity through an IL-10-independent pathway

Regulatory T (Treg) cells play a critical role in the maintenance of tolerance. B-1a cells belong to a specific and functionally important B-cell subset that exerts its regulatory role through the production of IL-10. While IL-10 has been correlated with the induction of type 1 Treg (Tr1) cells or Tr1-like cells, whether IL-10-producing B-1a cells are able to induce Treg cells, especially the Tr1 lineage, is poorly understood. We have demonstrated that, similar to the reported B-2 cells, B-1a cells are able to convert naïve CD4(+)CD25(-) T cells into a subset of T cells with suppressive function, which we called 'Treg-of-B1a' cells. Treg-of-B1a cells do not express Foxp3, but upregulate the Treg markers OX40, programmed death 1 (PD-1), inducible costimulator (ICOS) and IL-10R. Moreover, Treg-of-B1a cells do not express Foxp3 and produce high levels of IFN-γ and IL-10, but minimal amounts of IL-4; therefore, they resemble Tr1 cells. However, utilizing IL-10(-/-) mice, we showed that IL-10 was not involved in the induction of Treg-of-B1a cells. On the contrary, CD86-mediated costimulation was essential for B-1a cells to drive the induction of Treg-of-B1a cells. Finally, we demonstrated that, in contrast to the Treg cells generated by B-2 cells that mediate contact-dependent suppression, Treg-of-B1a cells suppress through secreting soluble factors. While Tr1 cells mediate suppression mainly through IL-10 or TGF-β secretion, Treg-of-B1a cells mediate suppression through an IL-10- and TGF-β-independent pathway. Together, these findings suggest that B-1a cells induce a functionally and phenotypically distinct Treg population that is dissimilar to the reported Foxp3(+) Treg or Tr1 cells.