| Chronic lymphocytic leukemia (CLL) is characterized by the relentless accumulation of mature-appearing B cells in the blood, marrow and lymphoid tissues. CLL cells are ineffective antigen presenting cells (APC), and are inept at inducing the proliferation of leukemia specific T cells that are required for a productive anti-tumor immune response. Furthermore, a major issue facing the development of immunotherapies for this disease is that CLL cells are resistant to apoptosis, or programmed cell death.; The poor APC capability of CLL cells can be reversed by CD40-activation. Cytotoxic T lymphocytes (CTL) primed with CD40-activated CLL cells effectively induced CTL-mediated apoptosis of CLL cells. Resting and CD40-activated CLL cells were equally sensitive to class-1-restricted CTL-mediated apoptosis. Experiments performed with known inhibitors of CTL killing demonstrated that CTL-mediated apoptosis of CLL cells utilized the granule-exocytosis, but not the Fas-mediated apoptosis pathway. These findings indicate that CD40-activation does not impair the sensitivity of CLL cells to Fas-independent CTL mediated apoptosis.; Autologous CD4 CTL were also found to induce Fas-dependent apoptosis of CD40-activated CLL cells. Further studies with CHO-FasL transfectants showed that after CD40-ligation, CLL cells were initially resistant to Fas-mediated apoptosis despite being induced to express high levels of Fas. However, after 72 hours, increased sensitivity to Fas-ligation was observed; this correlated with a progressive decline in FLIP protein. Downregulation of FLIP with an anti-sense oligonucleotide or pharmacologic agent, however, was not sufficient to render CLL cells sensitive to Fas-mediated killing 24–72 hours after CD40-activation.Instead, the acquired sensitivity to Fas also was associated with increased FADD, and the ability of CLL cells to form an active death inducing signaling complex. Collectively, these studies reveal that CD40 activation of CLL cells initiates a programmed series of events that shifts the balance of anti-apoptotic and pro-apoptotic proteins, thereby changing the sensitivity of CLL cells to Fas-mediated apoptosis.; In separate studies, CD27 activation increased the immunostimulatory nature of Ramos and CLL B cells, as demonstrated by upregulation of immune co-stimulatory molecules and increased T cell proliferation in the mixed lymphocyte reaction. CD27 may play an important role in modulating the immune response in chronic lymphocytic leukemia. |
