Lineage identification of acute leukemias: relevance of immunologic and ultrastructural techniques.

This study assesses the value of immunologic and ultrastructural methods in disclosing the lineage commitment of cells from acute leukemias (ALs). Two hundred and fifty-one ALs were characterized morphologically, cytochemically, and immunologically. Myeloperoxidase (MPO) positivity in > 3% of blasts was regarded as evidence of the myeloid origin of leukemic cells, cytoplasmic CD22 (cCD22) expression was taken as an indication for B-lineage acute lymphoblastic leukemia (ALL), and CD3+ (membrane or cytoplasmic) cases were classified as T-ALL. Diagnosis of minimally differentiated acute myeloid leukemia (AML-M0) was made when blast cells had undifferentiated features by light microscopy, reacted with at least one of the antibodies to myeloid-specific antigens (CD13, CD33, MPO), and lacked CD19, cCD22, and c/mCD3. Megakaryoblastic differentiation was demonstrated by the expression of CD41 and/or CD61. Following these criteria, 209 cases were classified as acute myeloid leukemia (AML) and 39 as ALL. Expression of lymphoid antigens was detected in 45% of AML cases and 30% of ALLs showed myeloid antigens. One case was regarded as a true biphenotypic leukemia because of the combined expression of MPO and CD33 for the myeloid lineage, and cCD3, CD2, and CD5 for the T-cell lineage. Two cases lacked signs of myeloid or lymphoid 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 was considered AML-M0. We conclude that light microscopy and standard immunologic methods can accurately demonstrate the lineage orientation in greater than 99% of ALs. Integration with ultrastructural analysis can define the cell nature of virtually all cases of AL.