Inhibition of T Cell Receptor Expression and Function in Immature CD4$^+$ CD8$^+$ Cells by CD4

Most immature CD4$^+$CD8$^+$ thymocytes express only a small number of T cell receptor (TCR) molecules on their surface, and the TCR molecules they do express are only marginally capable of transducing intracellular signals. TCR expression and function was not intrinsically low in immature CD4$^+$CD8$^+$ thymocytes, but was found to be actively inhibited by CD4-mediated signals. Indeed, release of CD4$^+$CD8$^+$ thymocytes from CD4-mediated signals resulted in significant increases in both TCR expression and signaling function. These results suggest that, in CD4$^+$CD8$^+$ cells developing in the thymus, increased TCR expression and function requires release from CD4-mediated inhibition.