CORECEPTOR (CD4 CD8) ENGAGEMENT ENHANCES CD3-INDUCED APOPTOSIS IN THYMOCYTES IMPLICATIONS FOR NEGATIVE SELECTION/

Negative selection of self-reactive immature T cells is mediated by TC R engagement and is thought to occur via apoptosis (programmed cell de ath). The requirement for the co-receptors CD4 and CD8 in negative sel ection has been demonstrated, but the biochemical mechanisms underlyin g 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 tyro sine phosphorylation of a number of cellular substrates, including the gamma isoform of phospholipase C, and with increased association of t yrosine phosphoproteins, including the protein tyrosine kinase p56(lck ), with the TCR complex. Co-receptor engagement also potentiated CD3-m ediated 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 an d CD8 may participate in negative selection by enhancing TCR/CD3-induc ed tyrosine kinase activation and sustained Ca2+ increases that lead t o endonuclease activation and apoptosis in self-reactive CD4(+)CD8(+) thymocytes.