CaMKII promotes BCR-induced apoptosis through Bcl-xL downregulation in immature B cells

B cell antigen receptor (BCR) signaling plays a critical role in regulating B cell fate, including activation, tolerance, and apoptosis. In immature B cells, strong BCR engagement induces apoptosis, which contributes to the elimination of autoreactive clones during negative selection. However, the molecular mechanisms linking BCR signaling to apoptotic pathways remain incompletely understood. In this study, we investigated the role of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in BCR-induced apoptosis using the immature B cell line WEHI-231. Stimulation of the BCR with anti-IgM antibody induced a rapid increase in intracellular Ca2+ levels and promoted apoptosis in WEHI-231 cells. BCR engagement also induced phosphorylation of CaMKII, indicating activation of this kinase downstream of Ca2+ signaling. Pharmacological inhibition of CaMKII with KN-93, a CaMKII inhibitor, attenuated BCR-induced apoptosis, whereas overexpression of CaMKII enhanced cell death. We further found that BCR stimulation resulted in downregulation of the anti-apoptotic protein Bcl-xL, and inhibition of CaMKII prevented this reduction. Conversely, CaMKII overexpression further enhanced Bcl-xL downregulation following BCR stimulation. Importantly, restoration of Bcl-xL expression significantly rescued CaMKII-mediated apoptosis. These findings identify a Ca2+-CaMKII-Bcl-xL signaling axis that promotes BCR-induced apoptosis in immature B cells.