Terminal deoxynucleotidyl transferase expression in acute non-lymphoid leukaemia: an analysis by immunofluorescence

Indirect immunofluorescence for terminal transferase enzyme (TdT) was used to study the blasts of 64 patients with acute non-lymphoid leukaemia (ANLL). In 32 patients no TdT positive cells were seen. In 19 cases a small subpopulation of cells expressing TdT was detected; these constituted up to 5% of total nucleated cells, and it was not clear whether these TdT positive cells were part of the leukaemic process or represented residual normal bone marrow lymphoid cells. The remaining 13 patients had TdT positive cells accounting for 7-90% of the total. In two of these cases TdT was expressed on blasts with myeloid features, representing an aberrant expression of TdT by myeloid cells; in contrast, in three cases mixed populations of TdT positive lymphoid blasts and TdT negative myeloid blasts were observed. In the remaining cases it was not possible to determine whether the TdT positive cells had definite lymphoid or myeloid features. Cytogenetic analysis showed no evidence of the Philadelphia chromosome. Response to treatment was assessed in 11 of the 13 patients. Only one patient remitted with the initial choice of therapy (DAT); four failed to respond to initial regimes of vincristine and prednisone (V & P) while the other five patients did not respond to myelotoxic combinations (DAT). Only one patient subsequently entered complete remission on second line therapy (V & P). This group of patients with TdT+ ANLL had a particularly bad prognosis, and appeared to differ from cases of TdT positive acute undifferentiated leukaemia, which often respond to V & P therapy.