The constitutive tyrosine phosphorylation of CD3ζresults from TCR-MHC interactions that are independent of thymic selection

The TCR complex, when isolated from thymocytes and peripheral T cells, contains a constitutively tyrosine-phosphorylated CD3 molecule termed p21. Previous investigations have shown that the constitutive phosphorylation of CD3 results from TCR interactions with MHC molecules occurring in both the thymus and the periphery. To determine what contribution the selection environment had on this constitutive phosphorylation, we analyzed CD3 from several distinct class I- and II-restricted TCRtransgenic mice where thymocyte development occurred in either a selecting or a nonselecting MHC environment. Herein, we report that constitutively phosphorylated CD3 zeta (p21) was present in thymocytes that developed under nonselecting peptide-MHC conditions. These findings strongly support the model that the TCR has an inherent avidity for MHC molecules before repertoire selection. Biochemical analyses of the TCR complex before and after TCR stimulation suggested that the constitutively phosphorylated CD3 subunit did not contribute to de novo TCR signals. These findings may have important implications for T cell functions during self-MHC recognition under normal and autoimmune circumstances.