Tapasin facilitation of MHC-I separates closely related allomorphs, is strongly influenced by peptide length and depends on stability

Only a small fraction of the peptides inside a cell are eventually presented by HLA-I on the cell surface. The presented peptides have HLA-I allomorph-specific motifs and length restrictions. Tapasin influences HLA-I antigen presentation both qualitatively and quantitatively to different degrees depending on both peptide sequence and HLA-I allomorph. The tapasin-dependence in cellular context has been shown to correspond to the facilitation of peptide- HLA-I complex formation by the first 87 amino acids of tapasin (Tpn1- 87) (i.e., tapasin-facilitation = Bmax Tpn1-87/Bmax Ctrl) in a biochemical assay. Both peptide length and tapasin-facilitation are important for HLA-I antigen presentation and we here set out to study if these two parameters relate to each other. We used a luminescent oxygen channeling assay and seven different peptide libraries (X7- X13) to study 16 HLA-A and -B allomorphs and the results show a broad spectrum of tapasin-facilitation of HLA-I allomorphs and that HLA-A allomorphs were generally less restricted than -B allomorphs
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to peptides of the classical lengths of 8-10 amino acids. Since both stability and tapasin-facilitation have been suggested as discriminators of immunogenic peptides we used a scintillation proximity based assay to study the stability of peptide-HLA-I complexes formed with peptides of different lengths. The results demonstrate an inverse correlation between tapasin-facilitation and stability valid for different peptide mixes of specific lengths but also on the level of HLA-I allomorphs, suggesting that molecules of poor stability are either not in a conformation that allows tapasin to interact or have a conformation where association has no effect.