Two HLA-B27 subtypes, B*2702 and B*2705, both associated with ankylosing spondylitis, were tested for binding affinity with a panel of polyalanine model nonapeptides carrying Arg at position 2 (P2) and a series of different amino acids at position 9 (P9). The alpha chains were isolated from BTB(B*2705), C1R/B*2702 (a B*2702 transfectant cell line) and from the NW(B*2702) cell line that has a peculiar peptide presentation behavior. Peptide binding was measured by the HLA alpha chain refolding assay. The results obtained show that: 1) Peptides with basic residues (Arg and Lys) and also aliphatic (Leu) and aromatic (Phe and Tyr) peptides at P9 have a similar high affinity in the binding to B*2705; 2) B*2702 binds well to P9 aliphatic and aromatic peptides but only very weakly to P9 basic peptides. Since both B*2702 and B*2705 are associated with AS the presumed arthritogenic peptide is hypothesized to have an aromatic or aliphatic residue at position 9. Peptides with basic residues in this position would be excluded as candidates because of their low binding affinity with B*2702.
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