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Tapasin is critical for efficient loading and surface expression of most HLA class I molecules. The high level surface expression of HLA-B* 2705 on tapasin-deficient 721.220 cells allowed the influence of this chaperone on peptide repertoire to be examined. Comparison of peptides bound to HLA-B* 2705 expressed on tapasin-deficient and-proficient cells by mass spectrometry revealed an overall reduction in the recovery of B* 2705-bound peptides isolated from tapasin-deficient cells despite similar yields of B27 heavy chain and β 2-microglobulin. This indicated that a proportion of suboptimal ligands were associated with B27, and they were lost during the purification process. Notwithstanding this failure to recover these suboptimal peptides, there was substantial overlap in the repertoire and biochemical properties of peptides recovered from B27 complexes derived from tapasin-positive and-negative cells …