Conformation of MHC class II I-Ag7 is sensitive to the P9 anchor amino acid in bound peptide
1 Department of Biology, American University, 4400 Massachusetts Avenue NW Washington, DC 20016, USA
2 David H. Smith Center for Vaccine Biology and Immunology, University of Rochester, 601 Elmwood Avenue, Box 609, Rochester, NY 14642-8609, USA
Correspondence to: L. S. Arneson; E-mail: larneso{at}american.edu
Type I diabetes is a chronic autoimmune disease resulting in the destruction of insulin-producing ß 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-Ag7. I-Ag7 contains a polymorphism that is shared by human class II alleles associated with the disease, at position 57 in the ß chain, in which aspartic acid is changed to a serine. The P9 pocket in the peptide-binding groove is in part shaped by ß57, and therefore the structure of this pocket is modified in I-Ag7. Using mAbs, we have previously determined that alternative conformations of I-Ag7 form in response to peptide binding. In this study, we have extended these findings by examining how peptides induce I-Ag7 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-Ag7 class II molecule. T cell hybridomas specific for GAD 65 (207–220) in the context of I-Ag7 were also examined for recognition of I-Ag7 bound to GAD 65 (207–220), in which Glu217 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-Ag7, 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-Ag7 conformations may affect functional activation of T cells, and thus may play a role in autoimmunity.
Keywords: autoimmunity, MHC, T cells
Transmitting editor: R. A. Flavell
Received 22 August 2006, accepted 19 June 2007.