CD 95-independent mechanisms of IL-2 deprivation-induced apoptosis in activated human lymphocytes

Growth factor deprivation-induced apoptosis plays an important role in seve ral cellular systems. However, knowledge of the molecular mechanisms involv ed are restricted to a few murine models or tumor cell lines. Therefore, we aimed studying signaling pathways leading to apoptosis in activated human peripheral T cells after IL-2 withdrawal. Lymphoblasts from patients with C D 95 (Fas/APO-1)-deficiency revealed that functional CD95 was not required to induce apoptosis after IL-2 withdrawal, Moreover, apoptosis induction in response to various cytotoxic stimuli was found to be mediated in the abse nce of functional CD95 but was affirmatorily influenced by IL-2 signaling. Immunoblots showed no downregulation of Bcl-2 or Bcl-x(L) and no upregulati on of Bar, whereas decreased mitochondrial membrane potential was readily m easurable 24 h after cytokine deprivation. Tetrapeptide inhibitors showed l imited efficacy in preventing apoptosis whereas the caspase inhibitor zVAD- FMK potently blocked induction of apoptosis, Cleavage of different fluoroge nic substrates revealed multiple caspase enzyme activities in lymphoblasts, which were not negatively affected by the fas mutation. Starting at 8 h af ter IL-2 withdrawal, upregulation of active caspase-3 but not of caspase-8 could be detected, Taken together, our data argue for molecular mechanisms of cytokine deprivation-induced apoptosis in activated human lymphocytes in dependent of CD95.