Expression of Differentiation Antigens in Subpopulations of Mouse Thymocytes: Regulation at the Level of de Novo Synthesis

Abstract

Mouse thymocytes can be divided into two broad classes by a variety of criteria, including steroid sensitivity, buoyant density, and availability of binding sites for peanut agglutinin. The major class is poorly reactive in immune functional assays, whereas the minor population does contain cells that are immunologically competent. Correlated with these functional differences, the populations differ in their displays of specific surface antigens and in their steady-state terminal deoxynucleotidyl transferase (TdT) activity. To investigate the basis for these phenotypic differences, the two populations were enriched by density gradient fractionation or by differential PNA agglutination; alternatively, the minor population was freed of the major one by in vivo cortisone treatment. These partially purified cells were then examined for their rates of de novo synthesis of four differentiation marker proteins. Incorporation of ^(35)S-methionine into H-2 (K and/or D), TL, TdT, and a Qa-region product (probably Qa-1) was monitored by immune precipitation with specific antisera followed by gel electrophoresis. The metabolic labeling interval was restricted to 30–40 min, to reflect as closely as possible instantaneous rates of synthesis. The two thymocyte populations were found to differ significantly in their production of each of these molecules. Relative to its total protein synthesis, the major population synthesized 10–20 fold less H-2, 5–10 fold more TdT, and 10–20 fold more TL than the minor population. The Qa product was synthesized in both populations, but at a slightly higher rate in the minor population. The biosynthetic rates are consistent with the reported steady-state phenotypes of the cells, and imply that changes in differentiation antigen expression during T lymphocyte development are controlled, at least in part, at a translational or pretranslational level.

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