The effect of mutant β₂-microglobulins on the conformation of HLA-B27 detected by antibody and by CTL

The arthritis-predisposing HLA-B27 consists of a heavy chain, a small peptide, and the monomorphic β₂-microglobulin (β₂-m). CTLs and a mAb, Ye- 2, which recognize the complex with specificities both for the heavy chain and for the peptide, are available. The β₂-m is in noncovalent association with the heavy chain at multiple points and is exchangeable with free β₂-m outside of the complex. The purpose of our experiments was to test whether mutant β₂-m capable of modulating HLA-B27 activity could be created. Eighteen recombinant mutants of the human β₂-m were experimentally generated. In 14 of these, mutations were at or near residues that are either contact residues or interface residues with the heavy chain. Relative to the parent β₂-m, two-thirds of the mutants showed reduced ability to exchange into HLA-B27 complexes. However, at least four of them induced more than 80% decrease in Ye-2 Ab reactivity. Two mutants were able to induce a minor decrease in susceptibility to lysis by four CTL clones. One of the CTL clones was autoreactive. Two of the CTL clones were specific for HLA-B27 cells experimentally infected with arthritis-causing Yersinia enterocolitica. These results indicate that certain β₂-m residues play an indirect role in peptide presentation, although they are not directly associated with the peptide residues.