The modulated expression of Mo5, a human myelomonocytic plasma membrane antigen

Mo5 is a 94-kd protein antigen expressed by human peripheral blood monocytes, neutrophils, and by all bone marrow myeloperoxidase-positive myeloid precursors (promyelocytes, myelocytes, metamyelocytes, and bands). Mo5 is borne by the malignant cells of 74% of patients (N = 27) with acute monocytic leukemia (French-American-British [FAB] group M4, M5), and 50% of patients (N = 38) with acute granulocytic leukemia (FAB M1, M2, and M3). Nonmyeloid cells in peripheral blood and bone marrow are Mo5-negative. The surface expression of Mo5 by myeloid cells is modulated by several experimental conditions: Exposure of neutrophils to calcium ionophore (1 mumol/L, 37 degrees C, ten minutes) under conditions resulting in degranulation of specific granules produces a three- to fourfold increase in the plasma membrane density of Mo5 antigen. This suggests that, in neutrophils, there is an intracellular pool of Mo5 antigen, which may be associated with specific granules, and that granule-associated Mo5 is translocated to the plasma membrane upon degranulation. Conversely, incubation of monocytes, neutrophils, U-937, and Mo5-positive leukemia cells in medium containing anti-Mo5 monoclonal antibody results in a significant decrease in surface Mo5 expression. This loss of surface Mo5 is a rapid, temperature-dependent process (occurring within 30 minutes at 37 degrees C) that is produced by divalent anti-Mo5 immunoglobulin [F(ab')2 but not F(ab)]. After down-modulation, Mo5 is reexpressed by monocytes within 48 hours. Mo5 is therefore a human myelomonocytic differentiation antigen whose expression is modulated up or down depending on the nature of extracellular stimuli.