CD40-Activation of Chronic Lymphocytic Leukemia Cells Induces Latent Sensitivity to Fas/TRAIL-Mediated Apoptosis Via a P53-Independent, C-Abl-Dependent Pathway.

Patients with chronic lymphocytic leukemia (CLL) experienced rapid reductions in leukemia cell count and lymph node size following an intravenous infusion of autologous CLL cells that were transduced to express a recombinant ligand for CD40 (CD154), including patients with leukemia cells that had inactivating mutations in P53 . Subsequent studies demonstrated that ligation of CD40 on CLL cells induced latent sensitivity to Fas (CD95)-dependent apoptosis and that this might account, in part, for the early reductions in leukemia cell counts observed in patients treated with CD154 gene therapy (Proc Natl Acad Sci U S A 100:3854–9, 2002). We found that ligation of CD40 induces CLL cells to express P53, as well as P53-dependent genes encoding CD95, DR5, and p21. Similar effects were observed following treatment of CLL cells with γ-radiation. In contrast to the effects of γ-radiation, however, CD40-ligation also induced CLL cells to express high-levels of Bid, a pro-apoptotic protein that mediates cross talk between extrinsic death receptors and mitochondrial-dependent apoptotic pathways. CLL cells that had inactivating mutations in P53 also were induced to express CD95, DR5, p21, and Bid following CD40-ligation, but not following γ-radiation, demonstrating that the gene expression response to CD40-ligation is dependent on a factor(s) other than p53. A good candidate for this is P73, a member of the P53 gene family that has been implicated in lymphocyte activation-induced cell death. Consistent with this notion, we found that ligation of CD40 on CLL cells induced expression of the α isoform of P73. The induction of P73 and latent sensitivity to CD95 (and TRAIL)-mediated apoptosis could be inhibited with the c-abl kinase inhibitor, imatinib mesylate, when added at the time of CD40-ligation. However, transduction of leukemia cells with a mutant gene encoding a c-abl kinase resistant to imatinib mesylate resulted in CLL cells that could be induced by CD40-ligation to express P73 and Bid even in the presence of imatinib mesylate. We conclude that sensitization to CD95-mediated apoptosis following CD40-activation is via a p53-independent, c-abl-kinase dependent process that is associated with activation of p73. Unlike p53, p73 mutations are extremely rare in human cancer. As such, CD154-based immune gene therapy may have activity even in patients who have CLL cells with dysfunctional P53 that are resistant to many of the anti-leukemia drugs currently used in the treatment of this disease.