TGF-β and BAFF derived from CD4+CD25+Foxp3+ T cells mediate mouse IgA isotype switching

TGF-β1 is generally accepted as the physiological IgA isotype switch factor. Nevertheless, it is unclear as to which cells in mucus-associated lymphoid tissue provide this cytokine to B cells. Regulatory T cells (Tregs) play immune-suppressive roles by secreting inhibitory cytokines such as TGF-β and IL-10. Thus, it is plausible that Tregs are involved in IgA class switch recombination (CSR) in MALT. We explored, in the present study, the possibility that CD4 + CD25 + T cells facilitate IgA CSR in murine B cells. In cocultures, CD4 + CD25 + Foxp3 + T cells stimulated IgA production by splenic B cells to a greater extent than did CD4 + CD25 − Foxp3 − T cells. This effect was markedly abrogated by the addition of anti-TGF-β1 Ab. Additionally, IgA production was paralleled by an increase in germ line transcript α (GLTα), an indicator of IgA CSR. In contrast, CD4 + CD25 − Foxp3 − T cells were more potent at inducing GLTγ1 and GLTɛ production by cocultured splenic B cells than were CD4 + CD25 + Foxp3 + T cells. Consistent with these results, phenotypic analyses revealed that TGF-β1 and IL-4 were predominantly expressed by CD4 + CD25 + Foxp3 + T cells and CD4 + CD25 − Foxp3 − T cells, respectively. Furthermore, CD4 + CD25 + T cells strongly expressed BAFF, which led to activation-induced deaminase (AID) expression in B cells. Taken together, our results suggest that CD4 + CD25 + Tregs have an important effect on IgA isotype commitment by expressing TGF-β1 and BAFF in MALT.