Co-receptor (CD4/CD8) engagement enhances CD3-induced apoptosis in thymocytes: Implications for negative selection

Negative selection of self-reactive immature T cells is mediated by TCR engagement and is thought to occur via apoptosis (programmed cell death). The requirement for the co-receptors CD4 and CD8 in negative selection has been demonstrated, but the biochemical mechanisms underlying their involvement in this process remain undefined. Here we present evidence that co-receptor engagement dramatically enhances CD3-induced endonuclease activation and cell death characteristic of apoptosis in immature thymocytes. The responses are associated with increased tyrosine phosphorylation of a number of cellular substrates, including the γ isoform of phospholipase C, and with increased association of tyrosine phosphoproteins, including the protein tyrosine kinase p56(lck), with the TCR complex. Co-receptor engagement also potentiated CD3-mediated Ca²⁺ increases via a mechanism dependent upon tyrosine kinase activation. Sustained Ca²⁺ availability was found to be necessary for endonuclease activation and apoptosis to occur. We suggest that CD4 and CD8 may participate in negative selection by enhancing TCR/CD3- induced tyrosine kinase activation and sustained Ca²⁺ increases that lead to endonuclease activation and apoptosis in self-reactive CD4⁺CD8⁺ thymocytes.