Immunodiagnosis of acute leukemia displaying ectopic antigens: proposal for a classification of promiscuous phenotypes.

The nature of the blast cells in 163 cases of acute leukemia was investigated by immunophenotyping, with particular emphasis on the expression of "ectopic" surface membrane structures. Although no antigen included in our panel except CD3 revealed absolute lineage restriction, immunological typing allowed a definite characterization of blast cells in more than 90% of cases. Four groups of patients were identified (A, B, C, D) with different degrees of antigen ectopic expression. We classified as group A leukemias (74%) those expressing conventional antigenic patterns, in absence of cross-lineage markers. Samples classified as group B (18%) showed a single ectopic membrane specificity, apparently discordant with the overall composite phenotype; such a "low-grade deviation" did not prevent a definite immunodiagnosis. Pattern C specimens (5%) revealed a promiscuous coexpression of markers related to different lineages (biphenotypic leukemias), whereas group D included unclassifiable phenotypes, characterized by no antigen or DR-only expression. Our findings suggest the possibility of interpreting complex phenotypic constellations of membrane markers in a consistent and logical manner.