The molecular basis underlying T cell specificity towards citrullinated epitopes presented by HLA-DR4

CD4⁺ T cells recognising citrullinated self-epitopes presented by HLA-DRB1 bearing the shared susceptibility epitope (SE) are implicated in rheumatoid arthritis (RA). However, the underlying T cell receptor (TCR) determinants of epitope specificity towards distinct citrullinated peptide antigens, including vimentin-64cit_59-71 and α-enolase-15cit_10-22 remain unclear. Using HLA-DR4-tetramers, we examine the T cell repertoire in HLA-DR4 transgenic mice and observe biased TRAV6 TCR gene usage across these two citrullinated epitopes which matches with TCR bias previously observed towards the fibrinogen β-74cit_69-81 epitope. Moreover, shared TRAV26-1 gene usage is evident in four α-enolase-15cit_10-22 reactive T cells in three human samples. Crystal structures of mouse TRAV6⁺ and human TRAV26-1⁺ TCR-HLA-DR4 complexes presenting vimentin-64cit_59-71 and α-enolase-15cit_10-22, respectively, show three-way interactions between the TCR, SE, citrulline, and the basis for the biased selection of TRAV genes. Position 2 of the citrullinated epitope is a key determinant underpinning TCR specificity. Accordingly, we provide a molecular basis of TCR specificity towards citrullinated epitopes.