Previous studies indicated the increase of HLA-B39 among HLA-B27 negative patients with spondylarthropathies (SpA). This study was performed to examine whether the natural ligands of HLA-B27 are capable of binding to HLA-B39. Methods. Peptides were synthesized according to the sequences of known natural ligands of HLA-B27 or B39 and were tested for their binding to HLA-B*3901 and B*2705 by quantitative peptide binding assay, using a TAP-deficient RMA-S cell line transfected with human  2 -microglobulin and HLA class I heavy chain genes. Results. Four of the 10 HLA-B27 binding peptides significantly bound to HLA-B*3901. All 4 peptides had hydrophobic/aromatic amino acids (Leu or Phe) at the C-terminus. In contrast, peptides with basic residues (Lys, Arg) or Tyr at the C-terminus did not bind to B*3901. In parallel experiments, 1 of the 2 natural ligands of HLA-B*3901 was found to bind to B*2705.
Conclusion.
A subset of natural HLA-B27 ligands was capable of binding to B*3901. In addition to Arg at position 2 (Arg 2 ), hydrophobic/aromatic C-terminal residues, such as Leu or Phe, seemed to be crucial for the cross-specificity. These results suggested that HLA-B27 and B39 recognize overlapping peptide repertoires, sup-porting the hypothesis that the peptides presented by both of these class I antigens play a role in the pathogenesis of SpA.
Although the crucial role of HLA-B27 in the development of seronegative spondylarthropathies (SpA) has been validated in transgenic animals (1,2) and in human subjects through linkage studies (3), how HLA-B27 causes the disease remains an open question. Hypotheses such as molecular mimicry, presentation of arthritogenic peptides, influence on bacterial invasion or persistence, and B27-mediated modification of intracellular signaling are currently being tested (4).
The molecular feature that most strikingly distinguishes HLA-B27 from other class I antigens lies in the structure of the peptide-binding pocket B (5). This structure is related to the peptide-binding motif of HLA-B27 possessing Arg at position 2 (Arg 2 ), which is rather unique among class I molecules (6). We previously reported an increase of HLA-B39 in HLA-B27 negative Japanese patients with SpA (7). Of interest, HLA-B27 and HLA-B*3901, the predominant subtype of B39, share some of the polymorphic amino acid residues constituting pocket B (Glu 45 , Cys 67 ). In fact, the peptide motif of HLA-B*3901 has also been shown to have Arg 2 as well as His 2 (8). These observations led us to postulate that a subset of the peptides presented by HLA-B27 could also be presented by HLA-B39, and that the peptides of pathogenic significance might be present in such a group of peptides.
This study was performed to experimentally evaluate the hypothesis that a proportion of peptides bound by HLA-B27 is capable of binding to HLA-B*3901.
Methods
Peptides. Peptides were synthesized utilizing an automated multiple peptide synthesizer utilizing the Fmoc strategy