Effects of CD40 activation of CLL cells on susceptibility to apoptosis induction. CD40 stimulation results in increased expression of adhesion and costimulatory molecules, thereby activating autologous T cells, which results in increased expression of death receptor ligands and IL-21. Next, CD40-mediated expression of death receptors and IL-21 receptor results in increased susceptibility to nonspecific immune effector–mediated apoptosis, possibly by cleavage of the proapoptotic protein Bid. Bid expression on CD40 activation is mediated by c-Abl–dependent expression of p73. CD40-activated CLL cells are more sensitive to chemotherapy, which could be mediated by p73.