Quantitative and qualitative influences of tapasin on the class I peptide repertoire

Tapasin is critical for efficient loading and surface expression of most HL A class I molecules. The high level surface expression of HLA-B*2705 on tap asin-deficient 721.220 cells allowed the influence of this chaperone on pep tide repertoire to be examined. Comparison of peptides bound to HLA-B*2705 expressed on tapasin-deficient and -proficient cells by mass spectrometry r evealed an overall reduction in the recovery of B*2705-bound peptides isola ted from tapasin-deficient cells despite similar yields of B27 heavy chain and beta (2)-microglobulin. This indicated that a proportion of suboptimal ligands were associated with B27, and they were lost during the purificatio n process. Notwithstanding this failure to recover these suboptimal peptide s, there was substantial overlap in the repertoire and biochemical properti es of peptides recovered from B27 complexes derived from tapasin-positive a nd -negative cells. Although many peptides were preferentially or uniquely isolated from B*2705 in tapasin-positive cells, a number of species were pr eferentially recovered in the absence of tapasin, and some of these peptide ligands have been sequenced. In general, these ligands did not exhibit exc eptional binding affinity, and we invoke an argument based on lumenal avail ability and affinity to explain their tapasin independence. The differentia l display of peptides in tapasin-negative and -positive cells was also appa rent in the reactivity of peptide-sensitive alloreactive CTL raised against tapasin-positive and -negative targets, demonstrating the functional relev ance of the biochemical observation of changes in peptide repertoire in the tapasin-deficient APC. Overall, the data reveal that tapasin quantitativel y and qualitatively influences ligand selection by class I molecules.