Activation of apoptosis pathways in peripheral blood lymphocytes by in vivo chemotherapy

In addition to myelosuppression, anticancer drugs cause rapid and persisten t depletion of lymphocytes, possibly by direct apoptosis induction in matur e T and B cells. Induction of apoptosis regulators was analyzed in peripher al blood lymphocytes from pediatric patients undergoing first-cycle chemoth erapy for solid tumors. In vivo chemotherapy induced a significant increase in lymphocyte apoptosis ex vivo. The activation of initiator caspase-8 and effector caspase-3 and the cleavage of caspase substrates was detected 12 to 48 hours after the onset of therapy. Caspase inhibition by Z-VAD-fmk did not reduce ex vivo lymphocyte apoptosis in all patients, indicating the ad ditional involvement of caspase-independent cell death. No evidence for the involvement of activation-induced cell death was found in the acute phase of lymphocyte depletion as analyzed by activation marker expression and sen sitivity for CD95 signaling. Lymphocyte apoptosis in vivo appeared to be pr edominantly mediated by the mitochondrial pathway because a marked decrease of mitochondrial membrane potential (Delta Psi (M)) was detected after 24 to 72 hours of treatment, preceded by the increased expression of Bax. Inte restingly, despite the use of DNA-damaging agents, p53 remained completely undetectable throughout treatment. In contrast, in vitro treatment with cyt arabine and etoposide induced p53 protein, CD95 receptor expression, CD95 s ensitivity, and CD95 receptor-ligand interaction in stimulated cycling lymp hocytes, but no such induction was seen in resting cells. These data sugges t that chemotherapy-induced lymphocyte depletion involves distinct mechanis ms of apoptosis induction, such as direct mitochondrial and caspase-depende nt pathways in resting cells and p53-dependent pathways in cycling lymphocy tes. (C) 2001 by The American Society of Hematology.