Demonstration of terminal deoxynucleotidyl transferase in single cells by indirect immunofluorescence — II. An examination of specificity

Advantages in the use of indirect immunofluorescence for the identification of terminal transferase (TdT) in single cells may be offset by lack of specificity, as compared to the biochemical assay of the enzyme, especially in analyses of lymphocyte populations. False positive results were obtained in 15/15 tonsillectomy samples and in 9/27 specimens from children with acute lymphoblastic leukemia in remission, perhaps due to antihuman lymphocyte activity in the rabbit heteroantisera which are used in the indirect immunofluorescence technique. This phenomenon may be more pronounced in 'activated' normal lymphocytes. Such reactions are not due to antibodies directed against adenovirus, papovavirus or EB virus antigens, although these are common constitutents of human tonsillar cells. Additional problems with TdT heteroantisera may result from immunization with non-TdT determinants in calf thymus extracts, as was manifest in human non-lymphoid (KB) cells cultured in fetal bovine serum. These difficulties will be overcome only by production of a monoclonal antibody using human TdT as the antigen.