Immunological definition of acute minimally differentiated myeloid leukemia (MO) and acute undifferentiated leukemia (AUL).

Immunophenotyping has become an important tool in the diagnosis of acute leukemia for several reasons. Indeed the use of a standardized panel of monoclonal antibodies (MoAb) to B and T cells, and myeloid cells, as well as non lineage restricted antigens, permits allocation of more than 98% of acute leukemia to their respective lineage.(1-3) In ALL, immunophenotyping has established a basis for precise and biologically oriented classification of the disease which may be of prognostic importance.(2) In AML immunological markers are particularly important for identification of acute leukemia with minimal myeloid, erythroblastic or megakaryoblastic differentiation. Immunological markers also allow the identification of acute leukemias with minimal or aberrant marker expression, acute biphenotypic leukemia in which single cells coexpress different lineage associated markers and acute bilineage leukemia where there are two separate blast cell populations (usually lymphoid and myeloid).(3) There is sometimes confusion in the literature about the definition of acute unclassifiable and acute undifferentiated leukemia.(4-6) This is mainly due to misinterpretation of phenotypic data or to the lack of relevant lineage specific markers in these studies, especially for the detection of cytoplasmic antigens. Indeed, it is important to stress that in hematopoietic precursors, antigens detected by monoclonal antibodies first appear in the cytoplasm during early differentiation and are only expressed on the membrane later. This has been demonstrated not only for the T lineage (CyCD3), the B lineage (CyCD22) but also for the myeloid lineage (CyCD13).(7,8) For all these reasons, it seems reasonable to consider that acute leukemia unclassifiable by morphocytochemical evaluation and standard membrane markers immunotyping includes at least: acute truly undifferentiated leukemia, acute minimally differentiated myeloid leukemia and acute true biphenotypic leukemia.(5) We will discuss here, the immunological definition of the MO AML and AUL.