LINEAGE IDENTIFICATION OF ACUTE LEUKEMIAS - RELEVANCE OF IMMUNOLOGICAL AND ULTRASTRUCTURAL TECHNIQUES

This study assesses the value of immunologic and ultrastructural metho ds in disclosing the lineage commitment of cells from acute leukemias (ALs). Two hundred and fifty-one ALs were characterized morphologicall y, cytochemically, and immunologically. Myeloperoxidase (MPO) positivi ty in >3% of blasts was regarded as evidence of the myeloid origin of leukemic cells, cytoplasmic CD22 (cCD22) expression was taken as an in dication for B-lineage acute lymphoblastic leukemia (ALL), and CD3+ (m embrane or cytoplasmic) cases were classified as T-ALL. Diagnosis of m inimally differentiated acute myeloid leukemia (AML-MO) was made when blast cells had undifferentiated features by light microscopy, reacted with at least one of the antibodies to myeloid-specific antigens (CD1 3,CD33, MPO), and lacked CD19, cCD22, and c/mCD3. Megakaryoblastic dif ferentiation was demonstrated by the expression of CD41 and/or CD61. F ollowing these criteria, 209 cases were classified as acute myeloid le ukemia (AML) and 39 as ALL. Expression of lymphoid antigens was detect ed in 45% of AML cases and 30% of ALLs showed myeloid antigens. One ca se was regarded as a true biphenotypic leukemia because of the combine d expression of MPO and CD33 for the myeloid lineage, and cCD3, CD2, a nd CD5 for the T-cell lineage. Two cases lacked signs of myeloid or ly mphoid differentiation and were studied by electron microscopy methods . One displayed platelet peroxidase (PPO) activity and was classified as a megakaryoblastic variant, one other reacted with anti-CD33 and wa s considered AML-MO. We conclude that light microscopy and standard im munologic methods can accurately demonstrate the lineage orientation i n greater than 99% of ALs. Integration with ultrastructural analysis c an define the cell nature of virtually all cases of AL.