Conformation of MHC class II I-Ag7 is sensitive to the P9 anchor amino acid in bound peptide (original) (raw)

Type I diabetes is a chronic autoimmune disease resulting in the destruction of insulin-producing b cells in the pancreas. In humans, disease incidence is linked to expression of specific MHC class II alleles and in mice type I diabetes is associated with the class II allele I-A g7. I-A g7 contains a polymorphism that is shared by human class II alleles associated with the disease, at position 57 in the b chain, in which aspartic acid is changed to a serine. The P9 pocket in the peptide-binding groove is in part shaped by b57, and therefore the structure of this pocket is modified in I-A g7. Using mAbs, we have previously determined that alternative conformations of I-A g7 form in response to peptide binding. In this study, we have extended these findings by examining how peptides induce I-A g7 molecules to adopt different conformations. By mutating the amino acid in the P9 position of either class II-associated invariant chain peptide (CLIP) or glutamic acid decarboxylase (GAD) 65 (207-220), we have determined that the chemical nature of the P9 anchor amino acid, either acidic or small hydrophobic, affects the overall conformation of the I-A g7 class II molecule. T cell hybridomas specific for GAD 65 (207-220) in the context of I-A g7 were also examined for recognition of I-A g7 bound to GAD 65 (207-220), in which Glu 217 in the P9 position was changed to alanine. We found that although some TCRs were able to recognize both peptides in the context of I-A g7 , and thus both class II conformations, approximately one-third of the T cells tested were not able to recognize the alternate class II conformation formed with the mutated peptide. These results indicate that the I-A g7 conformations may affect functional activation of T cells, and thus may play a role in autoimmunity.