The activities of two forms (8.2 and 6.2 S) of DNA ligase have been studied in chicken thymocytes at different stages of development (16 and 18 days of incubation, hatching and one month after birth). Thymocytes have been sorted out on the basis of size and T antigenicity (immunofluorescence) with the activated cell sorter. Three kinds of cells can be distinguished: (a) large, antigenically negative cells of 16 days of incubation, expressing only the "heavy" (8.2 S) form of enzyme. The activity of this form decreases during development and is very low after birth; (b) large, antigenically positive cells without DNA ligase activity and (c) small, antigenically positive cells, expressing the "light" (6.2 S) enzyme. This form of enzyme appears only from 18 days of incubation onwards. These results allow us to consider the 6.2 S DNA ligase activity as an additional marker (with cell reduction and T antigenicity) for thymocyte maturation.
Sera from rabbits injected with quail thymocytes were absorbed on quail bursal and liver cells. The absorbed sera reacted with avian T but not B lymphocytes in an immunofluorescence assay. Material precipitated by the anti-T antisera from lysates of radioiodinated chicken or quail thymocytes was analyzed by one- and two-dimensional electrophoresis. Of two anti-T cell antisera studied in detail, one reacted mainly with a protein of an apparent molecular weight of 45 000 - 55 000, and the other with a protein of an apparent molecular weight 65 000 - 70 000. These proteins may be homologues of previously described mammalian T lymphocytes-specific antigens.
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