Determinants of gliadin-specific T cell selection in celiac disease

In HLA-DQ8-associated celiac disease (CD), the pathogenic T cell response is directed toward an immunodominant alpha-gliadin-derived peptide (DQ8-glia-alpha1). However, our knowledge of TCR gene usage within the primary intestinal tissue of HLA-DQ8+ CD patients is limited. We identified two populations of HLA-DQ8-glia-alpha1 tetramer+ CD4+ T cells that were essentially undetectable in biopsy samples from patients on a gluten-free diet but expanded rapidly and specifically after antigenic stimulation. Distinguished by expression of TRBV9, both T cell populations displayed biased clonotypic repertoires and reacted similarly against HLA-DQ8-glia-alpha1. In particular, TRBV9 paired most often with TRAV26-2, whereas the majority of TRBV9- TCRs used TRBV6-1 with no clear TRAV gene preference. Strikingly, both tetramer+/TRBV9+ and tetramer+/TRBV9- T cells possessed a non-germline-encoded arginine residue in their CDR3alpha and CDR3beta loops, respectively. Comparison of the crystal structures of three TRBV9+ TCRs and a TRBV9- TCR revealed that, as a result of distinct TCR docking modes, the HLA-DQ8-glia-alpha1 contacts mediated by the CDR3-encoded arginine were almost identical between TRBV9+ and TRBV9- TCRs. In all cases, this interaction centered on two hydrogen bonds with a specific serine residue in the bound peptide. Replacement of serine with alanine at this position abrogated TRBV9+ and TRBV9- clonal T cell proliferation in response to HLA-DQ8-glia-alpha1. Gluten-specific memory CD4+ T cells with structurally and functionally conserved TCRs therefore predominate in the disease-affected tissue of patients with HLA-DQ8-mediated CD.