Autologous activated T cells induce proliferation of CLL cells through a CD40L- and IL-21-dependent mechanisms. (A) Purified CLL B cells were stained with DDAO and cultured for 48 hours with autologous T cells, in the absence (resting T cells, Trest) or presence of agonistic antibodies against CD3 and CD28 (Tact), or with the addition of a blocking antibody against CD40L (αCD40L) or a decoy receptor for IL-21 (IL-21R Fc). After 48 hours, proliferation was assessed by Ki-67 staining. Representative histograms of Ki-67 expression in DDAO⁺ gated cells are shown. (B) Results are depicted as the percentage of Ki-67⁺ cells after treatment with the blocking reagents, or an IgG1-Fc control molecule, in relation to the percentage of Ki-67⁺ cells in the cultures with Tact (100%) for 5 to 6 patients analyzed at day 5 (samples 6B, 7, 8, 10, 28, 32).