Highly Divergent T-cell Receptor Binding Modes Underlie Specific Recognition of a Bulged Viral Peptide bound to a Human Leukocyte Antigen Class I Molecule

Human leukocyte antigen (HLA)-I molecules can present long peptides, yet the mechanisms by which T-cell receptors (TCRs) recognize featured pHLA-I landscapes are unclear. We compared the binding modes of three distinct human TCRs, CA5, SB27, and SB47, complexed with a "super-bulged" viral peptide (LPEPLPQGQLTAY) restricted by HLA-B*35:08. The CA5and SB27 TCRs engaged HLA-B*35:08LPEP similarly, straddling the central region of the peptide but making limited contacts with HLA-B* 35: 08. Remarkably, the the CA5TCR did not contact the alpha 1-helix of HLA-B* 35: 08. Differences in the CDR3 beta loop between the CA5 and SB27 TCRs caused altered fine specificities. Surprisingly, the SB47 TCR engaged HLA-B* 35: 08LPEP using a completely distinct binding mechanism, namely "bypassing" the bulged peptide and making extensive contacts with the extreme N-terminal end of HLA-B* 35: 08. This docking footprint included HLA-I residues not observed previously as TCR contact sites. The three TCRs exhibited differing patterns of alloreactivity toward closely related or distinct HLA-I allo-types. Thus, the human T-cell repertoire comprises a range of TCRs that can interact with "bulged" pHLA-I epitopes using unpredictable strategies, including the adoption of atypical footprints on the MHC-I.