Leu-M1 antigen as a marker of acute nonlymphoid leukemia

The monoclonal antibody Leu-M1 was originally described as a marker of myeloid/monocytic cells and their precursors. Its usefulness as a myeloid marker in bone marrow specimens was further supported by a limited number of studies using flow cytometry and cell suspensions. Recent reports on its utility in paraffin sections for the diagnosis of acute leukemia have not shown it to be an effective marker of myeloid differentiation. To further evaluate the diagnostic usefulness of Leu-M1 in acute leukemia, we investigated the immunoreactivity of Leu-M1 antigen in a series of 100 plastic-embedded bone marrow biopsies from patients with acute leukemia and compared these results to those obtained in a series of 30 paraffin-embedded specimens. We also investigated the usefulness of neuraminidase pretreatment of sections to enhance the sensitivity of Leu-M1. In plastic sections, we were able to demonstrate positive Leu-M1 staining in 48 of 50 (96%) cases of acute nonlymphoid leukemia (ANL) (FAB classification M1-M5) and 0 of 50 cases of acute lymphocytic leukemia (ALL). Comparatively, Leu-M1 stained positively in 10 of 20 (50%) cases of ANL and 0 of 10 ALL embedded in paraffin. In both plastic and paraffin sections, pretreatment with neuraminidase enhanced the sensitivity of Leu-M1 reactivity, but markedly decreased its specificity. Our study suggests Leu-M1 is both a reliable marker of myelomonocytic differentiation and a useful diagnostic tool in the differentiation of ANL from ALL in plastic sections, and confirms its limited usefulness in paraffin-embedded material.