Structural insights into the CD4+ T cell response to the α1-Gliadin peptide in celiac disease

Celiac disease (CD) is a common T cell mediated disease in which the body develops an inappropriate immune response to dietary gluten. This results in malabsorption, with symptoms such as bloating, weight loss, and fatigue. At present, the only available treatment for CD is a strict adherence to a lifelong gluten free diet. CD is specifically associated with Major Histocompatibility Complex (MHC) class II human leukocyte antigens (HLA) DQ2 and/or DQ8, both of which have been shown to be critical for the development of an inflammatory CD4+ T cell response towards gluten. The structural nature of the CD-associated T cell receptors (TCRs) that recognise gluten-bound MHCs has yet to be explored. To date, no structure of a TCR specific for a HLA-DQ2/DQ8-bound gliadin peptide have been reported. This thesis presents the first ternary crystal structure of a gluten specific TCR (SP34) in complex with gluten peptide SGEGSFQPSQENP (α1-gliadin)-bound HLA-DQ8. This is also the first ternary complex structure of HLA-DQ8. In addition, the crystal structure of the SP34 TCR in its unliganded state is shown. Comparison of the unliganded and liganded structures of both the α1-gliadin-bound HLA-DQ8 and the SP34 TCR allowed analysis of conformational changes that have occurred upon binding. Examination of the ternary TCR-pMHC-II complex revealed important contacts that the SP34 TCR makes with the presented gluten peptide and HLA-DQ8. Furthermore, gluten peptide screening of T cell clones raised from a Celiac donor show that the SP34 TCR binds to a number of peptides from gluten, most of which share a recurring motif that appears to be central to recognition by this TCR. Thus TCR promiscuity towards a number of HLA DQ8-restricted gluten epitopes may contribute to the pathogenesis of disease. The identification of the interactions that the SP34 TCR made with the gluten peptide, as described in this thesis, may aid in the development of new treatments for CD patients in the future. These treatments could include the development of peptide vaccines or the production of non-toxic wheat varieties in which these immunogenic gluten epitopes have been mutated. In order to study the binding affinities of the numerous gluten peptides that are presented by the MHC-II molecules HLA-DQ2 and HLA-DQ8, this thesis also presents the development of a method to purify recombinant HLA-DQ2/DQ8 molecules without a peptide attached to the construct. The creation of recombinant peptide-empty constructs has the potential to be a highly useful tool in the development of specific assays that will more directly measure the TCR affinity for the large number of diverse gluten peptides that can elicit a CD4+ T cell immune response in CD. Also, HLA-DQ2/DQ8 molecules could be loaded with gluten peptides without the need to clone each construct, allowing more rapid crystallization and structure solution of bound immunodominant epitopes. This identification and structure determination of strongly binding, highly immunogenic gluten peptides will greatly aid the design of peptide-based vaccines and provide further insight into MHC-II-peptide binding.