CD40 ENGAGEMENT TRIGGERS SWITCHING TO IGA1 AND IGA2 IN HUMAN B-CELLS THROUGH INDUCTION OF ENDOGENOUS TGF-BETA - EVIDENCE FOR TGF-BETA BUT NOT IL-10-DEPENDENT DIRECT S-MU-]S-ALPHA AND SEQUENTIAL S-MU-]S-GAMMA, S-GAMMA-]S-ALPHA DNA RECOMBINATION

IgA are major effecters of antimicrobial defense in the respiratory an d digestive tracts. We have analyzed the requirements for and the moda lities of switching to IgA using our recently identified monoclonal mo del of human germinal center differentiation, CL-01 B cells. CL-01 cel ls bear surface IgM (sIgM) and sIgD and switch to all seven downstream isotypes in response to physiologic stimuli, In these cells, CD40 eng agement by CD40 ligand induces production of endogenous TGF-beta and I L-10, expression of germline I alpha 1-C alpha 1 and I alpha 2-C alpha 2 transcripts, mature V(H)DJ(H)-C alpha 1 and V(H)DJ(H)-C alpha 2 tra nscripts, and IgA secretion. These events are associated with not only direct S mu-->S alpha, but also sequential S mu-->S gamma, S gamma--> S alpha DNA recombination, and are ablated by neutralizing anti TGF-be ta but not IL-10 Ab, and indicating that TGF-beta, not IL-10, is a cru cial mediator of the transcriptional activation and recombination of h uman C alpha 1 and C alpha 2 genes. Our findings in CL-01 cells were r eproduced in freshly isolated naive sIgM(+) sIgD(+) B lymphocytes. Thu s, engagement of CD40, in the absence of other (known) stimuli, is suf ficient to effectively induce switching to IgA in human B cells. This is effected by direct and sequential DNA recombination events, which a re both dependent upon endogenous TGF-beta secreted by the CD40L-induc ed B cells.