A structural model for TCR recognition of the HLA class II shared epitope sequence implicated in susceptibility to rheumatoid arthritis.

HLA molecules associated with rheumatoid arthritis (RA) contain a discrete structural element known as the shared epitope, a set of conserved amino acid residues located on the alpha helical portion of the class II beta chain. Each of the different HLA molecules associated with RA contain the same shared epitope sequence, although they may vary markedly in other regions of the class II structure, which also determine peptide-class II interactions. Previous mutagenesis studies and structural modelling indicate that key polymorphic amino acid side chains within the shared epitope sequence are in locations likely to contact the T cell receptor (TCR) during the trimolecular activation reaction between the HLA-peptide complex and TCR. We have evaluated the potential structural basis for such shared epitope recognition by analysing detailed molecular models of the arthritis-associated DRB1*0404 molecule and a T cell receptor from T cell clone EM025, specific for HLA-DR4 molecules which carry the shared epitope. A likely orientation for the trimolecular complex was deduced in which the EM025 alpha chain interacts with the DR alpha chain and the EM025 beta chain interacts with the DR beta chain; residues Q70 and R71 within the DR beta chain shared epitope region are positioned for hydrogen bond interactions directly with Q97 of the TCR beta CDR3 region, D30 of the TCR beta CDR1 region, and possibly N51 of the TCR beta CDR2 region, indicating a degree of specific selection and interaction which encompasses multiple TCR contacts. These findings suggest a structural basis for the genetic associations with the HLA shared epitope and the potential contribution of this region to oligoclonal T cell selection and expansion in RA.