IMMUNOLOGICAL DETECTION OF BLAST CELL SUBPOPULATIONS IN ACUTE MYELOBLASTIC-LEUKEMIA AT DIAGNOSIS - IMPLICATIONS FOR MINIMAL RESIDUAL DISEASE STUDIES

The aim of the present study was to analyze the incidence of AML cases displaying more than one blast cell subpopulation by immunophenotype at diagnosis, since, any of them, although minimal, can be responsible for the relapse. For this purpose we have prospectively investigated the immunophenotype of blast cells from 40 de novo AML patients at dia gnosis with a large panel of monoclonal antibodies in double and tripl e staining combinations analyzed at flow cytometry. The discrimination between the different cell populations was based on: (1) the existenc e of aberrant phenotypes; (2) differences in light-scatter characteris tics; and (3) the expression of differen tiation-associated antigens ( CD34, CD117, HLADR, CD33, CD15, CD14, CD11b and CD4). More than one bl ast cell subpopulation was identified in 34 patients (85%), two subpop ulations in 12 patients (30%), three in three cases (7.7%), four in 13 patients (32.5%) and five populations in six cases (15%). The most co mmon criteria for discrimination of blast cell subpopulations was base d on the expression of maturation-associated antigens and, interesting ly, the blast subpopulations defined by higher reactivity for myeloid differentiation associated markers had a more mature FSC/SSC pattern. In 53% of the patients at least one of the subpopulations identified w as minimal (<10% of the total leukemic cells). Regarding the existence of aberrant phenotypes three situations were observed: (1) none of th e subpopulations had antigenic aberrations (10 cases); (2) coexistence of normal and aberrant subpopulations (five cases); and (3) all the s ubpopulations displayed aberrant phenotypes (19 cases). In 17 of the 2 3 patients (74%) who had two or more blast cell subpopulations with ph enotypic aberrations, at least one aberrant criteria was common to all the subpopulations; this criteria by itself would permit the simultan eous identification of all subpopulations in minimal residual disease (MRD) studies. In the remaining cases the investigation of MRD should be based on the phenotypic characteristics of each subpopulation.