The CT antigenic determinants have previously been shown to be present on the T200 glycoproteins and other proteins of murine cytotoxic T cell clones but not of T helper clones or nonactivated lymphocytes (1, 2). Two determinants recognized by mAbs CT1 and CT2 are also expressed on thymocytes in a developmentally regulated fashion during fetal thymus ontogeny and are found in a subset of Lyt-2+ intraepithelial lymphocytes in the intestinal mucosa (3-5). Previous studies of the biosynthesis of CT+ proteins suggested that these determinants were composed of carbohydrate (8). We now demonstrate that the anti-CT mAbs react with a carbohydrate determinant at the nonreducing terminus of O-linked oligosaccharides that has the configuration GalNAc beta 1,4[SA alpha 2,3]-galactose. The CT antibodies detected this determinant not only on CTL clones but also in the human blood group antigens Cad and Sda+. Variant CTL lines, non-Cad erythrocytes, and Sda- glycoproteins that lacked the GalNAc residue did not bind the CT mAb. Sialic acid was essential for CT antigen expression since neuraminidase or mild periodate treatment abrogated CT antibody binding. In addition, other carbohydrate structures with terminal GalNAc residues such as the A or Tn blood group antigens were not recognized. The CT antibodies thus define GalNAc and sialic acid containing carbohydrate antigens that are expressed on discrete subsets of T lymphocytes and may also be useful reagents for the detection of Cad and Sda+ blood group antigens.

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