Updated publication reference for PubMed record(s): 29769354. Birdshot chorioretinopathy (BSCR) is a rare inflammatory eye disease very strongly associated with HLA-A*29:02, with >95% of patients carrying this allele. BSCR is also associated with endoplasmic reticulum aminopeptidase (ERAP)2, an enzyme involved in processing HLA class I ligands. We analyzed the relationship between both risk factors, to address the basis for their joint association with BSCR. Label-free quantitative mass spectrometry was used to characterize the effects of ERAP2 on the A*29:02-bound peptidome. An ERAP2-negative cell line was transduced with lentiviral constructs containing GFP or GFP-ERAP2, and the A*29:02 peptidomes from mock- and ERAP2-transduced cells were compared. A similar analysis was performed with two A*29:02-positive, ERAP1-concordant, cell lines differing in their expression or not of ERAP2. In both comparisons the presence of ERAP2 affected the following features of the A*29:02 peptidome: 1) Length, with increased amounts of peptides >9-mers, and 2) N-terminal residues, with less ERAP2-susceptible and more hydrophobic ones. The paradoxical effects on peptide length suggest that unproductive binding to ERAP2 might protect some peptides from over-trimming. The influence on N-terminal residues is consistent with a double effect of ERAP2: a direct one on peptide trimming and improved processing in concert with ERAP1. The alterations in the A*29:02 peptidome suggest that the association of ERAP2 with BSCR is through its effects on peptide processing. These differ from those on the ankylosing spondylitis-associated HLA-B*27. Thus, ERAP2 alters the peptidome of distinct HLA molecules as a function of their specific binding preferences, influencing different pathological outcomes in an allele-dependent way.