CD40 engagement triggers switching to IgA1 and IgA2 in human B cells through induction of endogenous TGF-β: Evidence for TGF-β but not IL-10- dependent direct sμ→sα and sequential Sμ→Sγ, Sγ→Sα DNA recombination

IgA are major effectors of antimicrobial defense in the respiratory and digestive tracts. We have analyzed the requirements for and the modalities of switching to IgA using our recently identified monoclonal model of human germinal center differentiation, CL-01 B cells. CL-01 cells bear surface IgM (sIgM) and sIgD and switch to all seven downstream isotypes in response to physiologic stimuli. In these cells, CD40 engagement by CD40 ligand induces production of endogenous TGF-β and IL-10, expression of germline Iα1-Cα1 and Iα2-Cα2 transcripts, mature V(H)DJ(H)-Cα1 and V(H)DJ(H)-Cα2 transcripts, and IgA secretion. These events are associated with not only direct Sμ→Sα, but also sequential Sμ→Sγ, Sγ→Sα DNA recombination, and are ablated by neutralizing anti-TGF-β but not IL-10 Ab, and indicating that TGF-β, not IL-10, is a crucial mediator of the transcriptional activation and recombination of human Cα1 and Cα2 genes. Our findings in CL-01 cells were reproduced in freshly isolated naive sIgM⁺ sIgD⁺ B lymphocytes. Thus, engagement of CD40, in the absence of other (known) stimuli, is sufficient to effectively induce switching to IgA in human B cells. This is effected by direct and sequential DNA recombination events, which are both dependent upon endogenous TGF-β secreted by the CD40L- induced B cells.