CD40-MEDIATED LYMPHOTOXIN-ALPHA EXPRESSION IN HUMAN B-CELLS IS TYROSINE KINASE-DEPENDENT

The cytokine lymphotoxin (LT)alpha is known to play a role in B cell a ctivation. As the engagement of the B cell antigen CD40 is known to le ad to B cell proliferation and differentiation, we studied LT alpha ex pression in human B cells after CD40 ligation. We demonstrate that ant i-CD40 monoclonal antibody (mAb) induces strong LT alpha mRNA and surf ace expression in human tonsil B cells. Induction of LT alpha mRNA and surface expression by CD40 ligation is inhibited by the protein tyros ine kinase (PTK) inhibitors herbimycin and genistein in a dose-depende nt manner. The protein kinase C (PKC)-specific inhibitors sphingosine and bis-indolylmaleimide caused negligible inhibition of anti-CD40-ind uced LT alpha mRNA and surface expression. No inhibition is observed w ith the protein kinase (PKA) inhibitors H89 and HA1004. Cross-linking of the transmembrane phosphatase CD45 to CD40 by using goat-anti-mouse F(ab')(2) fragments strongly inhibits CD40-mediated LT alpha expressi on in human B cells, confirming the role of PTK activation in CD40-med iated induction of LT alpha expression. Inhibitors of the serine/threo nine protein phosphatases PP1 and PP2A, okadaic acid and calyculin ind uce LT alpha mRNA expression. In contrast, cyclosporin A, an inhibitor of the serine/threonine phosphatase calcineurin has no effect on anti -CD40-induced LT alpha expression. These results suggest that inductio n of LT alpha expression in B cells following engagement of CD40 invol ves activation of protein tyrosine kinases.