TdT expression in acute myeloid leukaemia. Haemopoietic immaturity or maturational asynchrony?

A total of 412 cases of acute leukaemia were examined for the presence of nuclear terminal deoxynucleotidyl transferase (TdT) by indirect immunofluorescence. Of the 129 cases of acute myeloblastic leukaemia (AML FAB groups M1/M2) examined, 18% (n = 23) had significant proportions (greater than 10%) of TdT-positive blasts. Although most of these AML cases (n = 18) were of poorly differentiated (M1) type; 5 cases of AML showing features of granulocytic differentiation (M2) were also found to be TdT-positive. Even though TdT was generally more strongly expressed in the M1 group and associated with other markers of myeloid immaturity (Ia positive and lack of chloroacetate esterase), there was no inverse relationship with Sudan black or myeloperoxidase activity. In addition, although the proportion of AML-M1 cases with increased TdT-positive cells was slightly higher (18/95, 19%) than for the AML-M2 group (5/34, 15%) the results suggest that the presence of nuclear TdT in leukaemic myeloblasts may not only reflect cellular immaturity but may also be due to maturational asynchrony in otherwise well-differentiated blasts.