ZAP-70 enhances IgM signaling independent of its kinase activity in chronic lymphocytic leukemia

Abstract

We transduced chronic lymphocytic leukemia (CLL) cells lacking ZAP-70 with vectors encoding ZAP-70 or various mutant forms of ZAP-70 and monitored the response of transduced CLL cells to treatment with F(ab)₂ anti-IgM (anti-μ). CLL cells made to express ZAP-70, a kinase-defective ZAP-70 (ZAP-70-KA³⁶⁹), or a ZAP-70 unable to bind c-Cbl (ZAP-YF²⁹²) experienced greater intracellular calcium flux and had greater increases in the levels of phosphorylated p72^Syk, B-cell linker protein (BLNK), and phospholipase C-γ, and greater activation of the Ig accessory molecule CD79b in response to treatment with anti-μ than did mock-transfected CLL cells lacking ZAP-70. Transfection of CLL cells with vectors encoding truncated forms of ZAP-70 revealed that the SH2 domain, but not the SH1 domain, was necessary to enhance intracellular calcium flux in response to treatment with anti-μ. We conclude that ZAP-70 most likely acts as an adapter protein that facilitates B-cell receptor (BCR) signaling in CLL cells independent of its tyrosine kinase activity or its ability to interact with c-Cbl.