AN ESSENTIAL ROLE FOR TYROSINE KINASE IN THE REGULATION OF BRUTONS B-CELL APOPTOSIS

Mutations of the Bruton's tyrosine kinase (btk) gene cause X-linked ag ammaglobulinemia (XLA) in humans and X-linked immune deficiency (Xid) in mice, To establish the BTK role in B-cell activation we examined th e responses of wild-type and Xid B cells to stimulation through surfac e IgM and CD40, the transducers of thymus independent-type 2 and thymu s-dependent activation, respectively, Wild-type BTK was necessary for proliferation induced by soluble anti-IgM (a prototype for thymus inde pendent-type 2 antigen), but not for responses to soluble CD40 ligand (CD40L, the B-cell activating ligand expressed on T-helper cells), In the absence of wild-type BTK, B cells underwent apoptotic death after stimulation with anti-IgM, In the presence of wild-type but not mutate d BTK, anti-IgM stimulation reduced apoptotic cell death, In contrast, CD40L increased viability of both wild-type and Xid B cells, Importan tly, viability after stimulation correlated with the induced expressio n of bcl-X(L). In fresh ex vivo small resting B cells from wild-type m ice there was only barely detectable bcl-X(L) protein, but there was m ore in the larger, low-density (''activated'') splenic B cells and per itoneal B cells. In vitro Bcl-X(L) induction following ligation of sIg M-required BTK, was cyclosporin A (CsA)-sensitive and dependent on ext racellular Ca2+. CD40-mediated induction of bcl-x required neither wil dtype BTK nor extracellular Ca2+ and was insensitive to CsA. These res ults indicate that BTK lies upstream of bcl-X(L) in the sIgM but not t he CD40 activation pathway, bcl-X(L) is the first induced protein to b e placed downstream of BTK.