Cross‐linking of the T cell antigen receptor interferes with the generation of CD4⁺8⁺ thymocytes from their immediate CD4⁻8⁺ precursors

The majority of thymocytes are immature cells co-expressing the surface markers CD4 and CD8. About two thirds of these cells also express the T cell antigen receptor (TcR), though at a level distinctly lower than found on mature T cells. The direct precursors of these "double-positive" thymocytes are cycling cortical blast cells of the CD4-8+ phenotype. Using a new monoclonal antibody to a constant determinant of the rat TcR alpha/beta, it is shown here that (a) about 50% of these CD8 "single-positive" committed precursor cells already express the TcR alpha/beta, though at very low levels, (b) during short-term suspension culture in medium supplemented only with fetal calf serum they not only acquire CD4 but also TcR alpha/beta levels characteristic of CD4,8 "double-positive" thymocytes, and (c) cross-linking of the TcR during culture inhibits the acquisition of the CD4 antigen in the majority of these cells.