The peptide specificity of HLA-B*1403, an allotype associated with ankylosing spondylitis (Lopez-Larrea, C., Mijiyawa, M., Gonzalez, S., Fernandez-Morera, J. L., Blanco-Gelaz, M. A., Martinez-Borra, J., and Lopez-Vazquez, A. (2002) Arthritis Rheum. 46, 2968-2971) was compared with those of the non-associated B*1402 and the prototypic disease-associated B*2705 allotypes. Although differing by a single residue (L156R), B*1402 and B*1403 shared only 32-35% of their peptide repertoires. Subtype-related differences observed in multiple peptide positions, including P3 and P7, were largely explained by a direct effect of the L156R change on peptide specificity. The HLA-B14 subtypes shared only approximately 3% of their peptide repertoires with B*2705. This was due to distinct residue usage at most positions, as revealed by statistical comparison of B*1402, B*1403, and B*2705-bound nonamers. Nevertheless, shared ligands between B*2705 and B*1403 were formally identified, although ligands common to B*2705 and B*1403, but absent from B*1402, were not found. Alloreactive T-cells were used as a tool to analyze epitope sharing among B*1402, B*1403, and B*2705. The percentage of cross-reactive T-cell clones closely paralleled peptide overlap, suggesting that shared ligands tend to maintain their antigenic features when bound to the different allotypes. Our results indicate that B*1403 and B*2705 can present common peptides. However, both the disparity of their peptide repertoires and the lack of binding features shared by these two allotypes, but not B*1402, argue against, although do not exclude, a mechanism of spondyloarthritis mediated by specific ligands of B*2705 and B*1403.