Species-specific differences in proteasomal processing and tapasin-mediated loading influence peptide presentation by HLA-B27 in murine cells

Expression of HLA- B27 in murine cells has been used to establish animal models for human spondyloarthritis and for antigen presentation studies, but the effects of xenogeneic HLA- B27 expression on peptide presentation are little known. The issue was addressed in this study. HLA- B27- bound peptide repertoires from human and murine cells overlapped by 75 - 85%, indicating that many endogenous HLA- B27 ligands are generated and presented in both species. Of 20 differentially presented peptides that were sequenced, only 40% arose from obvious inter- species protein polymorphism, suggesting that differences in antigen processing- loading accounted for many species- specific ligands. Digestion of synthetic substrates with human and murine 20 S proteasomes revealed cleavage differences that accounted for or correlated with differential expression of particular peptides. One HLA- B27 ligand found only in human cells was similarly generated in vitro by human and murine proteasomes. Differential presentation correlated with significantly decreased amounts of this ligand in human tapasin- deficient cells reconstituted with murine tapasin, indicating that species- specific interactions between HLA- B27, tapasin, and/ or other proteins in the peptide- loading complex influenced presentation of this peptide. Our results indicate that differences in proteasomal specificity and in interactions involving tapasin determine differential processing and presentation of a significant number of HLA- B27 ligands in human and murine cells.