A murine monoclonal antibody specific for a cell-surface antigen expressed by a subgroup of human myeloid leukaemias.

Murine monoclonal antibody YB5.B8 was raised against leukaemic blasts from a patient with M1-type acute non-lymphocytic leukaemia (ANLL). The antibody, which is of IgG1 class, bound to the majority of leukaemic blasts in the immunizing population, but not to cells of an autologous EBV-transformed B cell line. The antigen was not detected on normal blood or bone marrow cells, or on any of the eleven haemopoietic cell lines tested. It was present on some cells in peripheral blood specimens from 7/37 patients with ANLL and 1/5 patients with chronic myelomonocytic leukaemia and one patient with myelofibrosis with blastic change. In contrast, the antigen was not detected on cells in any of the 32 lymphoid leukaemic specimens tested, or on cells from four patients with chronic granulocytic leukaemia in accelerated chronic phase. In the ANLL group, expression of the antigen usually occurred on cells from types M1 or M2 according to the F.A.B. classification, and appeared to be associated with an unfavourable response to chemotherapy. The antigen was removed from the cell surface by digestion with pronase, and was re-expressed after 24 h in culture. Re-expression was prevented by the protein synthesis inhibitor, cycloheximide, but not by tunicamycin which inhibits glycosylation. Therefore, it seems likely that YB5.B8 binds to a peptide antigenic determinant.