The Adaptor Protein BLNK Is Required for B Cell Antigen Receptor-induced Activation of Nuclear Factor-κB and Cell Cycle Entry and Survival of B Lymphocytes

Abstract

B lymphocytes lacking the adaptor protein B cell linker (BLNK) do not proliferate in response to B cell antigen receptor (BCR) engagement. We demonstrate here that BCR-activated BLNK-/- B cells fail to enter the cell cycle, and this is due to their inability to induce the expression of the cell cycle regulatory proteins such as cyclin D2 and cyclin-dependent kinase 4. BCR-stimulated BLNK-/- B cells also do not up-regulate the cell survival protein Bcl-xL, which may be necessary for the cells to complete the cell cycle. In addition, BLNK-/- B cells exhibit a high rate of spontaneous apoptosis in culture. Examination of the various BCR-activated signaling pathways in mouse BLNK-/- B cells reveals the intact activation of Akt and mitogen-activated protein kinases but the impaired activation of nuclear factor (NF)-KB that is known to regulate genes involved in cell proliferation and survival. The inability to activate NF-κB in BCR-stimulated BLNK-/- B cells is due to a failure to induce the degradation of the inhibitory κB protein. In all these aspects, BLNK-/- B cells resemble xid B cells that have a mutation in Bruton's tyrosine kinase (Btk). Recently, phospholipase C (PLC)-γ2 has also been demonstrated to be essential for NF-KB activation. Since BLNK has been shown separately to interact with both Btk and PLC-γ2, our finding of normal Btk but impaired PLC-γ2 activation in BCR-stimulated BLNK -/- B cells strongly suggests that BLNK orchestrates the formation of a Btk-PLC-γ2 signaling axis that regulates NF-κB activation. Taken together, the NF-κB activation defect may be sufficient to explain the similar defects in BCR-induced B cell proliferation and T cell-independent immune responses in BLNK-/-, Btk-/-, and PLC-γ2 -/- mice.