CELL-CYCLE SPECIFIC EFFECTS OF TUMOR-NECROSIS-FACTOR-ALPHA IN MONOCYTE MEDIATED LEUKEMIC-CELL DEATH AND THE ROLE OF BETA-2-INTEGRINS

Human monocytes are involved in host defense against neoplastic cells. In view of cellular immunotherapy with cytotoxic monocytes in minimal residual disease of acute myeloid leukemia we have studied the role o f monocytes in cell cycle dependent leukemic cell death of U937, THP-1 , and HL-60 cells in vitro. Leukemic cells separated in G1 of the cell cycle by countercurrent centrifugal elutriation were highly susceptib le to monocyte mediated cytotoxicity, whereas cells in S and G2-M Were less sensitive or completely resistant as compared to unfractionated control cells. HL-60 cells resistant to cytotoxic monocytes became sen sitive to monocyte mediated cytotoxicity upon differentiation inductio n with 1,25-dihydroxyvitamin D3 which paralleled an accumulation of ce lls in G1 of the cell cycle. The differences in susceptibility of cell phase separated populations to monocyte mediated cytotoxicity paralle led differences in sensitivity to the cytotoxic effects of tumor necro sis factor alpha, as secreted by gamma-interferon activated monocytes. Furthermore, monocyte mediated cytotoxicity was markedly inhibited in the presence of anti-CD11/CD18 monoclonal antibodies recognizing the alpha and beta chains of the beta2-integrin adhesion proteins. By fluo rescence activated cell sorter immunofluorescence a marked increase in mean fluorescence density of the beta2-integrins could be demonstrate d on cells in G1 of the cell cycle as compared to unseparated leukemic cells. A decrease in mean fluorescence density was shown for cells in G2-M. By blocking experiments with anti-CD11/CD18 monoclonal antibodi es, the differences in mean fluorescence density were functionally rel evant since cells in G1 were shown to be the most sensitive cells to b eta2-integrin dependent monocyte mediated cytotoxicity. In conclusion these data show that differences in sensitivity to tumor necrosis fact or and in the expression of beta2-integrins may play a central role in cell cycle dependent monocyte mediated antileukemic activity.