50-OR

Abstract

Aim Assembly of HLA class I complex is assisted by the peptide loading complex and its determinant Tapasin (TPN). The crucial role of TPN in selecting peptides makes it an ideal target for viral immune escape mechanisms. For instance the immune evasion strategy of HCMV prevents the presentation of viral antigens by targeting TPN. Our recent investigations highlighted the importance of residue 156 within the PBR in determining TPN dependence of assembly and peptide binding specificities for HLA-B ∗ 44 allotypes. We extended this work further to HLA-A ∗ 24 allelic variants which are known to process and present the IE-1 peptides following HCMV infection. Methods We compared the TPN dependency and peptide repertoire of A ∗ 24:02 Gln156 , A ∗ 24:06 Trp156 and A ∗ 24:13 Leu156 . Class I negative and class I/TPN negative cells were lentivirally transduced with HLA-A ∗ 24 full length constructs, mRNA transcripts verified by real time PCR and surface expression of complexes assessed by flow cytometry. Peptides eluted from soluble HLA-A ∗ 24 molecules loaded in the presence or absence of TPN were sequenced by LC-ESI-MSMS. Results Both A ∗ 24:06 and A ∗ 24:13 showed high levels of surface expression (∼75%) in the absence of TPN, while A ∗ 24:02 exhibited partial independence. Their peptides clearly showed different patterns and features. Structurally, HLA-A ∗ 24:02 contains the residue triad Met97/His114/Gln156, while A ∗ 24:06 and A ∗ 24:13 contain a Trp and Leu polymorphism respectively at 156 that provides TPN independence by stabilizing the triad residues, thus generating an energetically stable and a more peptide receptive environment. Conclusions TPN independent alleles might help to combat infections. However, presentation of unusual ligands by these alleles could be a risk factor during stem cell transplantation and needs to be considered during donor selection. The future of HSCT relies on our understanding of how successful clinical outcomes can be achieved despite patient-donor allelic mismatches.