Modelling of the MHC II allele I-A g7 of NOD mouse: pH-dependent changes in specificity at pockets 9 and 6 explain several of the unique properties of this molecule

Abstract

The non-obese diabetic (NOD) mouse is an animal model of Type I (insulin-dependent) diabetes mellitus [1,2]. Extensive work on this model has shown that the chief genetic determinant of diabetes is the unique MHC class II allele of this mouse, termed I-A g7 [2,3]. The MHC class II molecules exist on professional antigen-presenting cells (APCs, i. e. dendritic cells, B lymphocytes and macrophages). They capture processed antigen fragments, forming complexes that are presented to CD4 + helper T-cells. Because of a deletion in their H-2/I-Ea gene, NOD mice do not express an I-E molecule [4]. Sequencing of the I-A g7 molecule showed a number of remarkable features in its beta chain; a histidine in position 56 and a serine in position 57 [5]. The histidine is unique to I-A g7 of all known mouse, rat, porcine, bovine and human MHC class II alleles [6,7]. The b57Ser is unique to the I-A g7 of all mouse MHC class II alleles but is Diabetologia (2000) 43: 609±624Modelling of the MHC II allele I-A g7 of NOD mouse: pH-dependent changes in specificity at pockets 9 and 6 explain several of the unique properties of this molecule Abstract Aims/hypothesis. We modelled the three-dimensional structure of I-A g7 , the chief genetic component of diabetes in non-obese diabetic mice, to understand the unusual properties of this molecule. Methods. Modelling was done, in complex with established antigenic peptides, based on the structure of I-A k .Results. The selectivity of the I-A g7 molecule changes greatly at pockets 9 and 6 but hardly at all at pockets 1, 4 and 7, between endosomal pH (5.0) and extracellular pH (7.0), in agreement with previous results. This selectivity is attributed to the unique combination of b9His, b56His and b57Ser. The positive charges in and around pocket 9 at pH 5, favour binding by negatively charged residues. At pH 7 however, the uncharged a68, b9 and b56 histidines favour the accommodation of the bulky residues lysine, arginine, phenylalanine and tyrosine at pocket 9. The combination of b9His and a66Glu is responsible for the pHdependent selectivity at pocket 6. Furthermore, the lack of repulsion between b56His and a76Arg at pH 7 leads to a more stable ternary complex. Conclusion/interpretation. These results reconcile previous conflicts over the peptide binding ability of I-A g7 and its motif. They furthermore provide possible explanations for the short lifetime of cell-surface I-A g7 complexes in vivo, the higher threshold of thymic negative selection and inherent self-reactivity shown by immunocytes in these mice and the protection from diabetes afforded to them by several transgenically expressed mouse class II alleles. This contributes to an understanding of the pathogenesis of Type I (insulin-dependent) diabetes mellitus in this animal. [Diabetologia (2000) 43: 609±624] Keywords Autoimmunity, I-A g7 molecule, MHC class II structure, protein modelling, NOD mice, Type I diabetes mellitus.Received: 9 August 1999 and in revised form: 27 January 2000Corresponding author: G. K. Papadopoulos, PhD, Labo...

Disabling an integral CTL epitope allows suppression of autoimmune diabetes by intranasal proinsulin peptide

“…When homology modeling was performed, no crystal structures of K d or I-A g7 were available. The two crystal structures of I-A g7 published subsequently (12,13) agree in nearly all respects with the modeled structure used here (14). Modeling of I-A g7 was based on the coordinates of the I-A k /HEL 50-62 peptide complex (15) (17).…”

“…p13D (C36), have the capacity to greatly enhance binding by forming salt bridges with I-A g7 α chain 76R (C33, C35) and 75K (C36). There is also an interaction between R (C32) and E (C33), probably accommodated because of the wider opening of the p9 pocket in I-A g7 compared with other MHC class II alleles (12)(13)(14).…”

Functional consequences of HLA-DQ8 homozygosity versus heterozygosity for islet autoimmunity in type 1 diabetes

“…24,35 The recently determined crystal structures of DQ2cis (DQA1*0501-DQB1*0201) 29 and DQ8cis (DQA1*0301-DQB1*0302) 25 were used as base molecules. The program Quanta (Accelrys, San Diego, CA, USA) was used to obtain a complete structure by providing the missing amino-acid residues from the crystal structure (for example, b105-112), as well as missing atoms from certain residues.…”

“…A similar procedure for the crystal structure of the DQ8cis-InsB13-21 complex did not show any serious deviations from the determined crystal structure. 24,25 All structures were examined for atomic clashes via the Discover program. The two generated HLA-DQ trans-molecules were examined in detail to have a physicochemical characterization of the possible anchor residues in their respective pockets.…”

“…To do so, we simulated the structures of HLA-DQ molecules in both cis-and trans-configuration based on the known crystal structures of the two HLA-DQ2 and HLA-DQ8 molecules in the cis-configuration. 23,24 From these structures, the respective peptide-binding motifs for the HLA-DQ trans-dimers were deduced and the possible complexes of these four HLA-DQ cis-and trans-molecules with peptides from the InsB6-22 region were determined. Second, to verify the peptide-binding motifs, we tested binding of insulin peptides to T1D susceptibility conferring HLA-DQ2, HLA-DQ8 and HLA-DQ2/8 molecules and the dominantly protective HLA-DQ6 molecule, by comparing HLA-DQ homozygous cell lines with heterozygous cell lines (B lymphoblastoid cell lines (BLCL)), based on the assumption that the distribution of the different HLA-DQ2 and HLA-DQ8 cis-and transmolecules on the heterozygous cell surface would be equal.…”