Immunological characterization of the leukemic megakaryocytic line at light and electron microscopic levels

Twenty cases of leukemia involving platelet precursors have been identified by a panel of monoclonal and polyclonal antiplatelet antibodies and by the ultrastructural demonstration of platelet peroxidase (PPO). The two techniques were in close agreement both for identification and for the quantitation of the blast cells except in three cases where PPO was present in the absence of the immunological markers. The immunological appearance of the leukemic megakaryocytic precursors was identical to that of their normal counterparts; the cells were positive with J 15 (anti GP IIb-IIIa complex), C 17 (anti GP IIIa), J 2 (anti GP 26,000) AN 51 (anti GP Ib). A diffuse cytoplasmic labelling was observed with anti factor VIII vwF and anti platelet factor 4 (PF 4). In addition, the leukemic maturation was quite similar to normal megakaryocyte differentiation since in micromegakaryocytes the expression of Gp Ib was strong and an intense granular pattern of labelling with anti factor VIII vwF and anti PF 4 was observed. In no case was the leukemic megakaryocytic series labelled by anti-erythroid antibodies, anti myeloid antibodies or J 5, B 1, OKT 11 antibodies. Using ultrastructural immunoferritin with J 15 it was possible to demonstrate that labelling with this antibody only occurred on PPO-positive cells. Immunogold or peroxidase labelling with AN 51 at the EM level in cases of mixed leukemia showed that Gp Ib was absent from proerythroblasts and myeloblasts. Therefore, in no case were specific platelet markers expressed in the leukemias of other cell lineages.