c‐Rel regulation of the cell cycle in primary mouse B lymphocytes

Surface-expressed BCR mediates the proliferation and expansion of antigen-specific B lymphocytes during a humoral immune response. Although several studies extensively characterize BCR proliferative signaling, the mechanisms linking these pathways to the cell cycle remain elusive. Using knockout mice, we show that c-Rel, a proto-oncogenic member of the NF-kB transcription factor family, is essential to BCR-mediated proliferation and cell cycle progression. Splenic B cells obtained from gene-targeted c-Rel knockout mice display a defective proliferation response to antigen receptor cross-linking, resulting in G1 arrest. At the molecular level, we see that BCR stimulation of resting c-Rel‐/‐ B cells fails to induce proper cyclin D3 and cyclin E expression, thereby negatively impacting G1 phase cyclin-dependent kinase (CDK) activity. c-Reldeficient B cells also exhibit incomplete phosphorylation of the Retinoblastoma protein (pRb) and poor expression of E2Fs, thus impeding the G1 to S phase transition. Down-regulation of the pRbrelated p130 protein during the G0 to G1 transition and removal of the CDK inhibitor p27 KIP1 in late G1 parallel that of wild-type cells, suggesting that Rel-deficient B cells can exit the G0 resting state and enter G1 phase normally. Finally, we demonstrate that restoration of proliferation can be achieved partially upon reintroduction of cyclin E using a protein transduction method to reconstitute primary B cells. Collectively, these studies emphasize the importance of c-Rel in lymphocyte proliferation and oncogenesis, and highlight a requirement for c-Rel in establishing an effective humoral immune response.