HLA-DQB1 CODON-57 IS CRITICAL FOR PEPTIDE BINDING AND RECOGNITION

The association of specific HLA-DQ alleles with autoimmunity is correl ated with discrete polymorphisms in the HLA-DQ sequence that are local ized within sites suitable for peptide recognition. The polymorphism a t residue 57 of the DQB1 polypeptide is of particular interest since i t may play a major structural role in the formation of a salt bridge s tructure at one end of the peptide-binding cleft of the DQ molecules. This polymorphism at residue 57 is a recurrent feature of HLA-DQ evolu tion, occurring in multiple distinct allelic families, which implies a functional selection for maintaining variation at this position in th e class II molecule. We directly tested the amino acid polymorphism at this site as a determinant for peptide binding and for antigen-specif ic T cell stimulation. We found that a single Ala-->Asp amino acid 57 substitution in an HLA-DQ3.2 molecule regulated binding of an HSV-2 VP -16-derived peptide. A complementary single-residue substitution in th e peptide abolished its binding to DQ3.2 and converted it to a peptide that can bind to DQ3.1 and DQ3.3 Asp-57-positive MHC molecules. These binding studies were paralleled by specific T cell recognition of the class II-peptide complex, in which the substituted peptide abolished T cell reactivity, which was directed to the DQ3.2-peptide complex, wh ereas the same T cell clone recognized the substituted peptide present ed by DQ3.3, a class II restriction element differing from DQ3.2 only at residue 57. This structural and functional complementarity for resi due 57 and a specific peptide residue identifies this interaction as a key controlling determinant of restricted recognition in HLA-DQ-speci fic immune responses.