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The X‐ray Structure of IAd in Complex with a Self Peptide Offers New Insights Into the Basis of Autoimmune Gastritis (AIG)

We developed a mouse model of autoimmune gastritis in which BALB/c mice expressing a transgenic αβ TCR, designated A51, spontaneously develop the disease. The TCR recognizes a 12‐mer peptide from the alpha subunit of gastric parietal cell H/K ATPase residues 889–900 (PLLCVGLRPQWE (H/K ATPase 889–900)), presented in the context of IAd. The disease, a model of human pernicious anemia, is characterized by slow and incomplete penetrance, anti‐gastric mucosal cell antibodies, inflammatory eosinophilic infiltrates, and parietal cell destruction. To investigate the contribution of MHC/peptide interactions to the initiation and maintenance of AIG, we generated a soluble recombinant version of the IAd/H/K ATPase889‐900 complex for biophysical and crystallographic analyses. We report here the 2.5 Å X‐ray structure of the complex. The structure was solved by molecular replacement and refined to Rfree of 28.9 by standard methods. The identification of peptide anchor residues and of solvent‐exposed side chains as T cell epitopic residues readily explains T cell proliferation data and off‐rate measurements of a panel of truncated, chimeric, and mutant peptides. This first structure of IAd in complex with an autoimmunogenic peptide reveals a heretofore unobserved peptide binding motif for IAd.