In vitro mutagenesis of HLA-B27. Substitution of an unpaired cysteine residue in the α₁ domain causes loss of antibody-defined epitopes

The HLA class I molecules identified serologically as HLA-B27 are highly associated with ankylosing spondylitis and related human disorders. All known HLA-B27 amino acid sequences contain a cysteine residue at position 67; no other published HLA class I sequence contains a cysteine within the hypervariable region of the α₁ domain, which extends from amino acid residues 63-84. To investigate the role of this cysteine residue in the antigenic structure of HLA-B27, we isolated a genomic clone encoding a molecule of the HLA-B27.1 subtype and performed oligonucleotide-directed mutagenesis to convert the cysteine at position 67 to a tyrosine. When transfected into mouse L cells, both the wild-type and Cys₆₇ → Tyr₆₇ mutant B27 genes directed the synthesis and surface expression of molecules reactive with the monomorphic anti-HLA class I antibody W6/32. However, only the L cells transfected with the wild-type B27 gene reacted with the anti-B27 antibody ME1; L cells transfected with the mutant B27 were completely unreactive with this antibody. Experiments with hybrid exons created from the HLA-B27 and HLA-A2 genes yielded results consistent with the mapping of the ME1 epitope to the B27 α₁ domain. A second anti-B27 antibody, GS145.2, also showed markedly reduced binding to the Cys₆₇ → Tyr₆₇ mutant. These studies document the importance of the unique Cys₆₇ residue in the antigenic structure of HLA-B27.