Lymphocyte surface proteins recognized by an anti-thymocyte-globulin.

Rat lymphocyte surface proteins were labelled by lactoperoxidase-catalysed iodination and immunoprecipitated by an anti-thymocyte globulin (ATG). ATG-Reactive proteins from cortical and medullary thymocytes, peripheral T cells and B cells were examined by gel electrophoresis. Several surface molecules were identified which showed differences in their distribution in the four cell populations. A 105 000-Da component was found in relatively high density on thymocytes and peripheral T cells only. Two high-molecular mass components of 205 000 Da and 190 000 Da were observed which were highly specific for peripheral T cells. These molecules appeared to share antigenic determinants with two other proteins found on the surface of both peripheral T cells and thymocytes. Limited proteolysis with Staphylococcus aureus protease showed these four proteins to have a similar peptide composition indicating the existence of a family of related surface molecules with varying expression on different cells. One of these T cell-specific proteins (P 190) was present at a comparatively high density, making up about 4% of the membrane protein. The main protein recognised by ATG was a component with a molecular mass of 175 000 Da found on all lymphocytes which made up about 7% of membrane protein. The other ATG-reactive proteins were all minor components of the cell surface, constituting less than 1% of membrane protein.