T-cell acute lymphoblastic leukemia in Israel: clinical and laboratory features.

T-cell acute lymphoblastic leukemia (ALL) comprises a third of the cases of childhood ALL in Israel. This high proportion of T-ALL is most probably due to a deficiency in pre-B/common ALL. The T-ALL patients had significantly worse 4-yr survival compared to standard risk or non-T high risk patients. In view of these special epidemiologic and clinical features a study of the immunophenotype of all consecutive cases of T-ALL and T-non Hodgkin's lymphoma (NHL) observed in our medical center was performed. Twenty-eight ALL and 3 NHL patients were studied and their cells characterized using a panel of monoclonal antibodies, TdT reactivity and E-rosette formation. Assays of the activities of adenosine deaminase (ADA) and purine nucleoside phosphorylase (NP) were also performed. Based on the surface antigen expression, the tumor cells could be classified into one of the three known developmental stages of T cells. It was found that the immunophenotype of the T-ALL cases in Israel was similar to that observed in other countries. Considerable heterogeneity of surface antigen expression was found and in a number of cases the phenotype analysis was not easily reconciled with models of T-cell ontogeny. The activities of ADA and NP were correlated with the developmental stage, as defined by the surface antigenic expression. Contrary to observations on normal T-cells, where ADA activity decreases and NP activity increases as T-cells mature and differentiate, this was not found in the malignant T cells. These findings as well as the existence of atypical immunophenotypes suggest that the leukemic T cell has an abnormal gene expression.