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The Distribution of Terminal Deoxynucleotidyl Transferase (TdT) among Subsets of Thymocytes in the Rat¹

From the Department of Pathology, University of Connecticut Health Center, Farmington, Connecticut 06032 and the Department of Biochemistry, University of Kentucky, Medical Center, Lexington, Kentucky 40506

Abstract

Terminal deoxynucleotidyl transferase (TdT), a unique DNA-polymerizing enzyme, has been shown to be present in a moderately dense subpopulation of rat thymocytes separated on discontinuous Ficoll density gradients. This subpopulation has been characterized by using antigenic and functional markers to identify directly and quantify cortical and medullary thymocytes. The TdT-positive thymocytes are depleted by cortisone administration, lack responsiveness to phytohemagglutinin, concanavalin-A, and histocompatibility alloantigens, bear surface antigens characteristic of cortical thymocytes (bone marrow lymphocyte antigen) and lack surface antigens characteristic of medullary thymocytes (rat-masked thymocyte antigen and histocompatibility antigens). The results indicate that TdT is present exclusively (or in markedly higher concentrations) in a subset of cells which comprises about 65% of cortical thymocytes. Two other major subsets of cortical thymocytes were identified which appeared to be TdT-negative. A minor subset of very low density cortical thymocytes was also defined. These observations have provided insight into the possible pathways of thymocyte ontogeny.

Footnotes

¹ This work is supported by the Connecticut Research Foundation and by Grant AI-09649 from the National Institute of Allergy and Infectious Diseases, and CA-08487 from the National Cancer Institute.

² Randall Barton is supported by National Institutes of Health Training Grant AI-00438.