Monoclonal antibody M195: a diagnostic marker for acute myelogenous leukemia.

Monoclonal antibody (mAb) M195 is a mouse IgG2a reactive with a myelomonocytic differentiation antigen found on early myeloid cells and monocytes. The reactivity of M195 with fresh hematopoietic neoplasms in the blood or bone marrow from 227 patients at Memorial Hospital was determined by flow cytometry. M195 was positive on 67% of 61 myeloblastic leukemias. Seventy percent of Tdt-negative ANLL and 30% of Tdt-positive ANLL were positive; 100% of CMMOL and 100% of CML in myeloblastic crisis or accelerated phase were positive. In contrast, M195 was positive on only 8% of 51 lymphoblastic leukemias and 1% of 70 other nonmyeloid samples. M195 binding did not correlate well with FAB classification of ANLL. The pattern of reactivity of M195 was similar but not identical to that of MY9 (CD33) on the same cases (83% concordance). Cross-blocking of M195 binding by MY9 and L4F3 (CD33) was demonstrated. M195 may bind to a different epitope on the same protein antigen. The presence of both MY9 and M195 positivity on a leukemia sample had a 98% specificity of diagnosing ANLL, which was greater than MY9 alone (88%) or M195 alone (92%). Assays of granulocytic-monocytic and erythroid colony-forming units showed M195 to be present on these hematopoietic progenitors. This pattern of reactivity of M195, together with its lack of reactivity with mature granulocytic elements or with adult tissues, make it a candidate for therapy of ANLL in vivo.