An Insertion Mutant in DQA1*0501 Restores Susceptibility to HLA-DM: Implications for Disease Associations

Abstract

HLA-DM (DM) catalyzes CLIP release, stabilizes MHC class II molecules and edits the peptide repertoire presented by class II. Impaired DM function may have profound effects on antigen presentation events in the thymus and periphery that are critical for maintenance of self-tolerance. The associations of the HLA-DQ2 (DQ2) allele with celiac disease and type 1 diabetes mellitus have been appreciated for a long time. The explanation for these associations, however, remains unknown. We previously found that DQ2 is a poor substrate for DM. Here, to further characterize DQ2-DM interaction, we introduced point mutations into DQ2 on the proposed DQ2/DM interface in order to restore the sensitivity of DQ2 to DM. The effects of mutations were investigated by measuring the peptide dissociation and exchange rate in vitro, CLIP and DQ2 expression on the cell surface and the presentation of α-II-gliadin epitope (residues 62-70) to murine, DQ2-restricted T cell hybridomas. We found that the three α chain mutations (α+53G, α+53R or αY22F) decreased the intrinsic stability of peptide-class II complex. More interestingly, the α+53G mutant restored DQ2 sensitivity to DM, likely due to improved interaction with DM. Our data also suggest that α-II-gliadin 62-70 is a DM-suppressed epitope. The DQ2 resistance to DM changes the fate of this peptide from a cryptic to an immunodominant epitope. Our findings elucidate the structural basis for reduced DQ2-DM interaction and have implications for mechanisms underlying disease associations of DQ2.

A TCRα framework–centered codon shapes a biased T cell repertoire through direct MHC and CDR3β interactions

“…A prerequisite may be a small TRBV footprint on DQA1 as well as ignorance to CLIP2 as seen for the prototypic TCR studied here. Since HLA-DQ2.5 is largely inert to HLA-DM-mediated peptide exchange, a high proportion of HLA-DQ2.5 molecules present CLIP peptides (24,(33)(34)(35). The consequence likely is a narrow peptide repertoire available for T cell interactions during thymic selection.…”

Susceptibility to HLA-DM Protein Is Determined by a Dynamic Conformation of Major Histocompatibility Complex Class II Molecule Bound with Peptide

“…Notably, mutations in pocket 4 or pocket 6 of HLA-DR3 increase affinity for CLIP, make the mutant DR3-CLIP complexes resistant to DM and SDS-stable, and reduce their binding to DM (66). An HLA-DQ variant associated with autoimmunity (DQA1*0501) that is a poor substrate for DM (67) has residue ␣53 deleted with consequential structural alterations in the extended strand and 3 10 helical region (62,68); DM susceptibility is restored by introduction of ␣Gly-53 into the extended strand (69). All these data suggest that interactions along the entire peptide binding groove influence DM susceptibility by regulating the ability to adopt conformation(s) in which the 3 10 helix and extended strand regions can move into position for optimal interaction with DM.…”

On the perils of poor editing: regulation of peptide loading by HLA-DQ and H2-A molecules associated with celiac disease and type 1 diabetes

“…CLIP complexes with DQ1 (DQA1*01:01, DQB1*05:01) exhibit a normal degree of DM susceptibility. The deletion of residue α53 in DQ2.5 (as in all DQ alleles with DQA1*04 and *05 chains) is critical, because insertion of Gly, the residue at α53 in DQ1, restores sensitivity to DM (80). Interestingly, the insertion of Arg at this position (found in DQA1*03, and thus in DQ8) fails to restore sensitivity to DM, suggesting that DQ8 also interacts poorly with DM.…”

“…However, DQ2.5 allows sufficient CLIP release for gliadin peptide presentation. Interestingly, recent studies show that DQ2.5 resistance to DM editing actually promotes the presentation of certain gliadin peptides (80). Model APCs expressing wild-type DQ2.5 molecules present an immunodominant αII-gliadin (residues 62-70) peptide to T cells, with or without DM present.…”