An HLA-DR negative acute leukaemia which stimulates MLC and CMC responses.

We describe an acute myelomonocytic leukaemia (E72) devoid of cell-surface HLA-DR antigens, but capable of inducing cellular responses. Leukaemia E72 induced proliferation of normal lymphocytes in primary mixed lymphocyte culture (MLC), which was only weakly inhibited by anti-DR sera. Depletion of a small percentage (congruent to 4%) of DR+ cells on a cell-sorter failed to abrogate the capacity of E72 to stimulate MLC and cell-mediated cytotoxicity (CMC) responses. We found that normal lymphocytes primed with E72 responded in secondary MLC to lymphocytes and leukaemias, suggesting that the lymphocyte-activating determinant (LAD) on E72 is not leukaemia-specific. In addition, E72 induced CMC responses to both leukaemias and lymphocytes. We suggest that E72 may express a novel HLA or non-HLA LAD.