Fas/APO-1 (CD95)-mediated cytotoxicity is responsible for the apoptotic cell death of leukaemic cells induced by interleukin-2-activated T cells.

Apoptotic cell death is induced by the cross-linking of Fas/APO-1 receptor (CD95) in acute myelogenous leukaemia (AML) cells. Since CD95 ligand (CD95L) is expressed on interleukin-2 (IL-2)-activated T cells, we investigated the involvement of CD95-CD95L pathway in T cell-mediated cytotoxicity against AML cells. Activated CD8+ T cells could efficiently kill a parental CD95-sensitive AML cell line, MML-1 and, to a lesser extent, a CD95-resistant clone, MML-1R. Neither MML-1 nor MML-1R cells were killed by activated CD4+ T cells. The blocking monoclonal antibody (MoAb) against CD95, ZB4, caused a significant inhibition of T-cell-mediated cytotoxicity against MML-1 cells but not against MML-1R cells. Interestingly, MML-1 cells underwent the classic nuclear morphologic changes and DNA fragmentation characteristic of apoptosis when cultured with activated T cells. Enumeration of apoptotic and necrotic nuclei showed that both apoptosis and necrosis were induced in MML-1 cells, whereas necrosis was exclusively observed in MML-1R cells. Apoptosis of MML-1 cells was completely blocked in the presence of ZB4 MoAb. Similarly, blocking by ZB4 MoAb significantly inhibited T-cell-mediated lysis of fresh AML cells in a CD95-sensitive group, but not in a CD95-refractory group. In addition CD8+ T cells expressed CD95L mRNA more abundantly than CD4+ T cells upon activation by IL-2. These findings suggest that T-cell-mediated cytotoxicity against AML cells requires participation of CD95-CD95L pathway for cytotoxic signal transduction leading to target apoptosis.