Selective regulation of Lyn tyrosine kinase by CD45 in immature B cells

It has been well established that protein-tyrosine phosphatase CD45 is critically involved in the regulation of initial tyrosine phosphorylation and effector functions of T and B cells. However, the signaling pathway governed by CD45 is not completely understood. In B cells, it has not been unequivocally resolved as to which protein-tyrosine kinases (PTKs) associated with B cell antigen receptor are regulated by CD45 in intact cells. As a first step toward the elucidation of CD45-initiated signaling events, we have tried to identify physiological substrates for CD45 by analyzing PTK activity in CD45-deficient clones recently generated from the immature B cell line WEHI-231. The results clearly demonstrated that among PTKs examined (Lyn, Lck, and Syk), only Lyn kinase is dysregulated in the absence of CD45 such that without B cell antigen receptor ligation, Lyn is hyperphosphorylated and activated in CD45-negative clones. Thus, Lyn seems to be a selective in vivo substrate for CD45 in immature B cells.