Surface phenotypes of leukemic cells from six patients with adult T cell leukemia (ATL) were analyzed by the use of monoclonal antibodies, both at the time of initial diagnosis and at either relapse or exacerbation phase after chemotherapy. Changes of cell surface antigens were observed in four of the six cases. The majority of the leukemic cells of these patients were reactive with anti-Leu-1 and anti-Leu-3a, but unreactive with anti-Leu-2a and MAS 036c monoclonal antibodies at the time of initial diagnosis, indicating that ATL cells are of peripheral inducer/helper T cell origin. In three cases, the Leu-1 antigen disappeared at relapse or at exacerbation phase, and in one of these cases, a small percentage of ATL cells became reactive with MAS 036c, which identifies cortical thymocyte antigen. ATL cells of one other case did not have Leu-1 antigen from the start, but gained Leu-2a antigen at exacerbation phase and became double-labeled cells (Leu-2a+, 3a+), which is known to be a feature of thymocytes. Thus, it appeared that ATL cells sometimes change their surface phenotype to that of an earlier stage of T cell differentiation at relapse or at exacerbation phase. Chronic myelocytic leukemia (CML) cells also usually change to immature cells at blastic crisis involving morphological change. However, this morphological change was not so prominent in the ATL cases studied, except one, in which typical ATL cells with nuclear indentation changed to large immature cells with basophilic cytoplasm at relapse.

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