Detection of terminal transferase in acute myeloid leukemia by flow cytometry

The purpose of this study was twofold: (1) to develop an optimized, reliable method for the flow cytometric analysis of the intranuclear DNA polymerase, terminal deoxynucleotidyl transferase (TdT) in acute myeloid leukemia, and (2) to establish the usefulness of a novel, fluorescein-isothiocyanate conjugated monoclonal anti-TdT antibody (HT-6) in double-fluorescence staining for surface antigens in the characterization of leukemic cells. Inclusion of an aldehyde blocking buffer in the staining protocol reduced background fluorescence sufficiently to allow for the detection of the low-level fluorescent TdT+ myeloblasts. When admixed to normal peripheral blood mononuclear cells, 0.4-0.5% of HLA-DR+ or myeloid surface antigen+, TdT+ double-stained myeloblasts could be reliably detected above background levels. Flow cytometric TdT measurements using the HT-6 antibody in 55 patients with TdT+ acute lymphocytic or myelocytic leukemia or blast crisis of chronic myelogenous leukemia were equal or superior to the results obtained with a mixture of monoclonal anti-TdT antibodies (anti-HTDT-Mix) and comparable to those obtained by the conventional slide method employing polyclonal rabbit anti-human TdT antiserum. This flow cytometric TdT determination in combination with surface antigen staining using a novel anti-TdT monoclonal antibody (HT-6) allows for the recognition of minimal leukemic blast cells during clinical remission in acute myeloid leukemia.