αβ T cell receptors (TCRs) are genetically restricted to corecognize peptide antigens bound to self-major histocompatibility complex (pMHC) molecules; however, the basis for this MHC specificity remains unclear. Despite the current dogma, evaluation of the TCR-pMHC-I structural database shows that the nongermline-encoded complementaritydetermining region (CDR)-3 loops often contact the MHC-I, and the germline-encoded CDR1 and -2 loops frequently participate in peptide-mediated interactions. Nevertheless, different TCRs adopt a roughly conserved docking mode over the pMHC-I, in which three MHC-I residues (65, 69, and 155) are invariably contacted by the TCR in one way or another. Nonetheless, the impact of mutations at these three positions, either individually or together, was not uniformly detrimental to TCR recognition of pHLA-B*0801 or pHLA-B*3508. Moreover, when TCR-pMHC-I recognition was impaired, this could be partially restored by expression of the CD8 coreceptor. The structure of a TCR-pMHC-I complex in which these three (65, 69, and 155) MHC-I positions were all mutated resulted in shifting of the TCR footprint relative to the cognate complex and formation of compensatory interactions. Collectively, our findings reveal the inherent adaptability of the TCR in maintaining peptide recognition while accommodating changes to the central docking site on the pMHC-I.MHC restriction | T cell recognition | structural immunology | immunogenetics D uring thymic selection αβ T cell receptors (TCRs) are selected for weak reactivity with one or more self-peptides in complex with self-MHC (major histocompatibility complex), resulting in mature T cells being restricted to recognize processed peptides only when they are presented by self-MHC molecules (1). The exquisite specificity of TCR-pMHC binding must address the highly polymorphic nature of the MHC and variable peptide cargo. The antigen-recognition site of the TCR is made up of six complementarity-determining regions (CDRs), three each from the α and β chains (2). Currently, it is postulated that the CDR1 and 2 loops underpin the specificity or "bias" toward the MHC, whereas the CDR3 loops recognize the diverse range of bound peptides (3-5). Recent studies support the view that the T cell repertoire is intrinsically biased toward the MHC through contacts mediated by conserved TCR Vα and Vβ residues (6). However, the factors governing MHC restriction, a central paradigm of antigen-specific T cell immunity, remain unclear.Presumably as a consequence of the polymorphic pMHC landscape, and the inherent diversity of the responding T cell repertoire, structural studies have shown that the TCR engages the pMHC-I and pMHC-II surfaces in a range of different docking modes (2). Nevertheless, conserved pairwise interactions between closely related Vβ8.2 + TCRs and pMHC-II molecules were identified, leading to the concept that there are defined interaction "motifs" or "codons" between the Vα and/or Vβ domains of a TCR and a given MHC allotype (3,(6)(7)(8). Central to the "int...

Human leukocyte antigen (HLA) gene polymorphism plays a critical role in protective immunity, disease susceptibility, autoimmunity, and drug hypersensitivity, yet the basis of how HLA polymorphism influences T cell receptor (TCR) recognition is unclear. We examined how a natural micropolymorphism in HLA-B44, an important and large HLA allelic family, affected antigen recognition. T cell–mediated immunity to an Epstein-Barr virus determinant (EENLLDFVRF) is enhanced when HLA-B*4405 was the presenting allotype compared with HLA-B*4402 or HLA-B*4403, each of which differ by just one amino acid. The micropolymorphism in these HLA-B44 allotypes altered the mode of binding and dynamics of the bound viral epitope. The structure of the TCR–HLA-B*4405EENLLDFVRF complex revealed that peptide flexibility was a critical parameter in enabling preferential engagement with HLA-B*4405 in comparison to HLA-B*4402/03. Accordingly, major histocompatibility complex (MHC) polymorphism can alter the dynamics of the peptide-MHC landscape, resulting in fine-tuning of T cell responses between closely related allotypes.

In comparison to human leukocyte antigen (HLA) polymorphism, the impact of allelic sequence variation within T cell receptor (TCR) loci is much less understood. Particular TCR loci have been associated with autoimmunity, but the molecular basis for this phenomenon is undefined. We examined the T cell response to an HLA-B*3501–restricted epitope (HPVGEADYFEY) from Epstein-Barr virus (EBV), which is frequently dominated by a TRBV9*01+ public TCR (TK3). However, the common allelic variant TRBV9*02, which differs by a single amino acid near the CDR2β loop (Gln55→His55), was never used in this response. The structure of the TK3 TCR, its allelic variant, and a nonnaturally occurring mutant (Gln55→Ala55) in complex with HLA-B*3501HPVGEADYFEY revealed that the Gln55→His55 polymorphism affected the charge complementarity at the TCR–peptide-MHC interface, resulting in reduced functional recognition of the cognate and naturally occurring variants of this EBV peptide. Thus, polymorphism in the TCR loci may contribute toward variability in immune responses and the outcome of infection.

The underlying generic properties of αβ 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 (156Leucine) or HLA-B*3508 (156Arginine) showed a potent CTL response to the 407HPVGEADYFEY417 epitope from EBV. Interestingly, the response was characterized by highly restricted TCR β-chain usage in both HLA-B*3501+ and HLA-B*3508+ individuals; however, this conserved TRBV9+ β-chain was associated with distinct TCR α-chains depending upon the HLA-B*35 allele expressed by the virus-exposed host. Functional assays confirmed that TCR α-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 α-chain usage is shown to play a critical role in controlling MHC restriction between closely related allomorphs.