TdT⁺/nonlymphoid antigen⁺ acute leukemias: Immunologic and karyotypic monitoring during therapy and at relapse suggests the transformation of a bipotential stem cell

Immunophenotype and karyotype were monitored in 19 adult acute leukemia patients with blast cell populations expressing terminal transferase (TdT) and nonlymphoid antigens either at presentation or at relapse. Three patterns of immunophenotypic course were observed when following the patients through at least one, sometimes two (six patients), or three relapses (one patient). Induction chemotherapy induced predominantly TdT⁺ leukemias with a minor monoblastic component to become TdT-negative, purely monoblastic without clinical response or change in karyotype in five patients (group 1). In group 2, relapse was associated with the disappearance (four patients) or the appearance of TdT⁺/nonlymphoid antigen⁺ features (four patients). In two instances, new nonrandom cytogenetic abnormalities, in one case, evolution of an initial abnormal cytogenetic clone, were found at relapse. Six patients (group 3) presented and relapsed with identical TdT⁺ myeloblastic, promyeloblastic, or monoblastic immunophenotype and karyotype. In general, FAB classification did not reflect expression of TdT in nonlymphocytic leukemias or the presence of nonlymphoid blast features in lymphocytic leukemias. Lymphoid-specific antigens in addition to TdT were not detected in any of the cases at the time of nonlymphoid antigen expression. In 11 of the 19 patients, simultaneous expression of TdT and myeloid or monocytic antigens could be demonstrated at the single cell level using double-fluorescence staining. These follow-up data are best consistent with a drug-induced maturation drive of a TdT⁺/monocytic (majority of cases) or TdT⁺/myelocytic leukemic stem cell with its differentiation commitment being influenced by chemotherapy or by other as yet undefined conditions predisposing to proliferation of the leukemic cell at relapse.