B cell proliferation following CD40 stimulation results in the expression and activation of Src protein tyrosine kinase.

Resting normal human B cells express negligible c-src mRNA or Src protein tyrosine kinase; however, upon induction of proliferation, these cells express high levels of both mRNA and protein and show a concomitant increase in tyrosine kinase activity of immunoprecipitated Src. Src expression was most pronounced upon stimulation with CD154, and to a lesser extent CD70, Staphylococcus aureus, Cowan strain I and phorbol ester, and correlated with the activation of the cells. Transfection of cDNA for human wild-type or kinase-dead Src into Raji B cells resulted in an increase and decrease, respectively, of the cell numbers in culture, showing a direct correlation of proliferation to the expression of Src that was corroborated using anti-sense oligodeoxynucleotides and chemical inhibitors. Furthermore, the human B cell lines, Namalwa, Daudi and Raji express low levels of Src but express very high levels of Src after stimulation with CD154 that showed a correlation with increased activation. This is the first report of Src detectable in normal B cells. The finding that Src expression is inducible and correlates with stimulation by CD154 and the proliferation of the B cells suggests that Src may play a specific role in normal and transformed B cell activation/proliferation pathways mediated primarily through CD40 stimulation.