Phenotypic conversion of TdT+ adult AML to CALLA+ ALL

A case of acute leukaemia is described in which the blast cells showed Sudan Black and terminal deoxynucleotidyl transferase (TdT) positivity at presentation and did not react with the monoclonal antibodies (McAb) My9 (anti-myeloid) and J5 (anti-CALL antigen). The myeloid lineage of these cells was confirmed by the myeloperoxidase (MPO) reaction at electron microscopic level. The patient entered complete remission but relapsed one year later with blasts showing negative Sudan Black and MPO reactions. These cells were still TdT+ but J5 was now positive. This case points to the existence of a clonogenic leukaemic cell with potential for evolution in both myeloid and lymphoid lineages. This is also suggested by the reactivity of both types of blast cells with the McAb 3C5 which recognises an antigen present both on early myeloid and lymphoid precursors. It is possible that patients with such leukaemias may benefit from combined therapy directed against cells of both myeloid and lymphoid lineages.