In this study, pretreatment peripheral and/or bone marrow blasts from 12 patients with acute unclassifiable
leukemia (AUL) expressing the myeloid-related
cell-surface antigen (CD 11) were isolated for further analysis. Despite a lack of
myeloperoxidase (MPO) activity, 1 patient's blasts contained cytoplasmic Auer rods. The circulating blasts from another patient expressed MPO while maintaining the same surface phenotype during 20 months of clinical follow-up. In addition, the blasts from 3 cases demonstrated both myelomonocytic and monocyte-specific
surface antigens, whereas the remaining 9 cases completely lacked any monocyte-specific
antigen detectable by
monoclonal antibodies, Mo2, My4 and Leu M3 (CD 14). The first case eventually was diagnosed as
acute myelomonocytic leukemia and the second as
acute myelogenous leukemia by means of immunophenotypic analysis using flow cytometry (FACS IV). In addition, the presence of MPO
protein was identified in the cytoplasm of blast cells from 5 patients with AUL by means of a cytoplasmic immunofluorescence test using a
monoclonal antibody (MA1). Our study indicates that non-T, non-B AUL expressing OKM1 (CD 11)
antigens include acute
leukemias which are unequivocally identifiable as being of either myeloid or myelomonocytic origin. However, further investigations, including immunophenotypic and cytoplasmic analysis, ultrastructural cytochemistry and gene analysis with
molecular probes (tests applicable to normal myeloid cells), are necessary in order to determine the actual origin of blasts and to recognize the differentiation stages of the various types of leukemic cells from patients with undifferentiated forms of
leukemia.