(A) Fas/FasL interaction mediated cytotoxicity. Jurkat cell apoptosis by leukemic cells from P2, P10, P11, P12, P22 at an E/T ratio of 10:1 was inhibited by the antagonist Fas:Fc. Results are expressed as the percentage inhibition of apoptosis induced by leukemic cells in the presence of the blocking MoAb. Leukemic cells were incubated with 15 μg/mL Fas:Fc for 30 minutes and than used as effector cells in the JAM test. The experiment was repeated 3 times with each patient. (B) Effect of Fas-Fc concentration on apoptosis. Results are expressed as the percentage of Jurkat apoptosis induced by leukemic cells from patient P2, in the presence or absence of the blocking MoAb and shows 1 representative experiment out of 2.