Adenosine deaminase (ADA) and terminal deoxynucleotidyl transferase (TdT) activities were determined on 97 biopsic specimens obtained from patients with nonneoplastic diseases (12 cases), Hodgkin (30 cases), and non-Hodgkin lymphomas (55 cases). Thirty additional cases were tested only for TdT. TdT was positive in 10 out of 13 lymphoblastic lymphomas (LL) examined and negative in all the other specimens, including the ten cases of the immunoblastic type. Levels of ADA above 350 U/mg of protein were found in 10 out of 12 LL tested, but not in any other specimen. The 3 TdT-LL had high contents of ADA. Therefore, all LL can be detected using both ADA and TdT markers. The 3 TdT- LL had a heterogeneous phenotype and their possible origin is discussed in view of the possibility that they constitute a rare entity distinct from the more common TdT+ LL. Very low levels of ADA (below 100 U/mg of protein) were found in chronic lymphocytic leukemia and immunocytoma, and in Burkitt's lymphoma. In other B-cell non-Hodgkin lymphomas, intermediate values between 100 and 350 U were often found, and this finding could be relevant to the different cellular origin of the various B-cell neoplasias. We conclude that ADA distribution in solid lymphoid tumors reflects the cellular origin of these neoplasias. Adenosine deaminase alone and in combination with TdT can be useful in the diagnosis and classification of childhood lymphomas in which the immature hystotypes predominate.
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