THE STRUCTURE OF THE T CELL ANTIGEN RECEPTOR

Recent crystallographic studies of T cell antigen receptor (TCR) fragments from the α and β chains have now confirmed the expected structural similarity to corresponding immunoglobulin domains. Although the three-dimensional structure of a complete TCR αβ heterodimer has not yet been determined, these results support the view that the extracellular region should resemble an immunoglobulin Fab fragment with the antigen-binding site formed from peptide loops homologous to immunoglobulin complementarity-determining regions (CDR). These preliminary results suggest that CDR1 and CDR2 may be less variable in structure than their immunoglobulin counterparts, consistent with the idea that they may interact preferentially with the less polymorphic regions of the molecules of the major histocompatibility complex. The region on the variable β domain responsible for superantigen recognition is analyzed in detail. The implications for T cell activation from the interactions observed between domains of the α and β chains are also discussed in terms of possible dimerization and allosteric mechanisms.