Detection of terminal deoxynucleotidyl transferase.

The recently published study of peripheral blood lymphocyte subpopulations in patients with acute immunodeficiency syndrome, by Murray et al,’ may have contributed additional insight into normal T cell ontogenesis. However, the data on terminal transferase (TdT) must be interpreted with caution. It is stated that TdT levels are especially elevated in acute lymphocytic leukemia (ALL) of T cell origin. The concentration of TdT is actually lower in T cell than in null cell ALL.2’3 Furthermore, the enzyme is not detectable in mitogen-activated normal lymphocytes by biochemical assay4 or by Western blotting5; the report of putative TdT expression in such cells, as determined by indirect immunofluorescence (IF),6 may be a false-positive identification of TdT antigen.’ Murray et al purport to measure ‘cytoplasmic TdT’ with a kit supplied by Bethesda Research Laboratories (BRL). It should be noted that, at least in fresh human lymphocytes (whether normal8 or malignant9), TdT is predominantly a nuclear enzyme, as measured by immunoassay. Moreover, problems attendant on the use of the BRL kit have been described.’0