Cbl-b Positively Regulates Btk-mediated Activation of Phospholipase C-γ2 in B Cells

Genetic studies have revealed that Cbl-b plays a negative role in the antigen receptor–mediated proliferation of lymphocytes. However, we show that Cbl-b–deficient DT40 B cells display reduced phospholipase C (PLC)-γ2 activation and Ca(2+) mobilization upon B cell receptor (BCR) stimulation. In addition, the overexpression of Cbl-b in WEHI-231 mouse B cells resulted in the augmentation of BCR-induced Ca(2+) mobilization. Cbl-b interacted with PLC-γ2 and helped the association of PLC-γ2 with Bruton's tyrosine kinase (Btk), as well as B cell linker protein (BLNK). Cbl-b was indispensable for Btk-dependent sustained increase in intracellular Ca(2+). Both NH(2)-terminal tyrosine kinase-binding domain and COOH-terminal half region of Cbl-b were essential for its association with PLC-γ2 and the regulation of Ca(2+) mobilization. These results demonstrate that Cbl-b positively regulates BCR-mediated Ca(2+) signaling, most likely by influencing the Btk/BLNK/PLC-γ2 complex formation.