Studies on the differentiation of T lymphocytes in sheep. III. Preliminary characterization of an antigen recognized by two anti-pan T-cell monoclonal antibodies.

ST-1a and ST-1b are monoclonal antibodies that selectively react with cells in the T-cell lineage of the sheep. Preliminary structural studies using radioimmunoprecipitation and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) indicated that both ST-1a and ST-1b recognize an antigen of apparent molecular weight 67,000 and 60,000-65,000, under either reducing or nonreducing conditions. Differential expression of these 67,000 and 60,000-65,000 MW proteins was observed in T cells obtained from various anatomical compartments. Cortical thymocytes and efferent lymph lymphocytes expressed predominantly the 67,000 molecule, whereas a thymus cell fraction enriched for medullary thymocytes expressed mainly the 60,000-65,000 MW molecule. Both proteins were found in equivalent amounts in immunoprecipitates obtained from peripheral blood lymphocytes and lymph node lymphocytes. Peptide mapping studies indicated that 67,000 proteins of both ST-1a and ST-1b immunoprecipitates have significant homology in peptide composition. Sequential immunoprecipitation experiments clearly demonstrated that the antigens recognized by ST-1a and ST-1b antibodies are the same. On the basis of the size, tissue distribution and trypsin sensitivity of the antigen, together with cytofluorometry data, we suggest that the target antigen of ST-1a and ST-1b monoclonal antibodies is the ovine homologue of mouse Lyt-1 and human Leu-1.