TCRα genes direct MHC restriction in the potent human T cell response to a class I-bound viral epitope

The underlying generic properties of alpha beta TCRs that control MHC restriction remain largely unresolved. To investigate MHC restriction, we have examined the CTL response to a viral epitope that binds promiscuously to two human leukocyte Ags (HLAs) that differ by a single amino acid at position 156. Individuals expressing either HLA-B*3501 ((156)Leucine) or HLA-B*3508 ((156)Arginine) showed a potent CTL response to the (407)HPVGEADYFEy(417) epitope from EBV. Interestingly, the response was characterized by highly restricted TCR beta-chain usage in both HLA-B*3501(+) and HLA-B*3508(+) individuals; however, this conserved TRBV9(+) beta-chain was associated with distinct TCR alpha-chains depending upon the HLA-B*35 allele expressed by the virus-exposed host. Functional assays confirmed that TCR alpha-chain usage determined the HLA restriction of the CTLs. Structural studies revealed significant differences in the mobility of the peptide when bound to HLA-B*3501 or HLA-B*3508. In HLA-B*3501, the bulged section of the peptide was disordered, whereas in HLA-B*3508 the bulged epitope adopted an ordered conformation. Collectively, these data demonstrate not only that mobile MHC-bound peptides can be highly immunogenic but can also stimulate an extremely biased TCR repertoire. In addition, TCR alpha-chain usage is shown to play a critical role in controlling MHC restriction between closely related allomorphs.