SYK, BUT NOT LYN, RECRUITMENT TO B-CELL ANTIGEN RECEPTOR AND ACTIVATION FOLLOWING STIMULATION OF CD45(-) B-CELLS

B cell Ag receptor (BCR) signaling occurs via tyrosine phosphorylation of CD79a and CD79b ITAMs, leading to recruitment and activation of Ly n and Syk tyrosine kinases and subsequent downstream events. CD45 expr ession is required for BCR triggering of certain of these downstream e vents, such as calcium mobilization and p21(ras) activation. However, the site in the BCR signaling cascade at which CD45 impinges is poorly defined. To address this question, we have studied CD45 function in t he CD45-deficient (CD45(-)) and CD45-reconstituted (CD45(+)) J558L mu m3 plasmacytoma. In both CD45(+) and CD45(-) cells, Ag stimulation led to CD79a and CD79b tyrosine phosphorylation as well as Syk tyrosine p hosphorylation, recruitment to the receptors, and activation. In contr ast to CD45(+) cells, Lyn exhibited high basal tyrosine phosphorylatio n in the CD45(-) cells and was not further phosphorylated upon Ag stim ulation. Mapping studies indicated that the observed constitutive phos phorylation of Lyn reflects phosphorylation of its C-terminal tyrosine , Y508, at high stoichiometry. Constitutively Y508-phosphorylated Lyn was neither recruited to the BCR nor activated upon Ag stimulation. Mo reover, CD79a-ITAM phosphopeptides failed to bind Lyn from the CD45(-) cells. Thus, Y508 phosphorylation of Lyn occurs in the absence of cel lular CD45 expression and appears to render the kinase unable to assoc iate with the phosphorylated receptor complex via its Src homology 2 d omain and to participate in signal propagation. Surprisingly, in view of previous findings implicating Src family kinases in ITAM phosphoryl ation, the data indicate that Ag-induced CD79a and CD79b tyrosine phos phorylation and Syk recruitment and activation can occur in the absenc e of CD45 expression and, hence, Src-family kinase activation.