Antigen presentation function of type-1 diabetes associated HLA-DQ molecules

Autoreactive CD4+ T cells initiate the autoimmune disease Type-1 diabetes (T1D). Expression of DQ2, DQ8, and DQ2/8 trans-dimers are highly prevalent in T1D. However, the underlying molecular mechanisms are poorly understood. HLA-DM is essential for editing peptides bound to Major Histocompatibility Complex class II (MHCII) on antigen presenting cells, thus influencing the repertoire of peptides mediating selection and activation of CD4+ T cells. Here we explore the structural characteristics of DQ and the role of DM function, as well as the potential molecular mechanism of DM editing, as they may contribute to an understanding of autoreactive CD4+ T cell development in T1D. Cells coexpressing DM with these DQ molecules were observed to express elevated levels of class II-associated invariant chain peptides (CLIP), consistent with HPLC-MS/MS analysis of eluted peptides. Assays with purified recombinant soluble proteins further confirmed that T1D-associated DQ2 and DQ8 are resistant to DM editing. DM sensitivity was enhanced in mutant DQ8 with disruption of hydrogen bonds (H-bonds) that stabilize DQ8 near the DM-binding region. Compared to T1D nonassociated DQ1 and DQ6, the percentages of shared peptides among T1D-associated DQ molecules were significantly higher. Predicted peptide binding motifs of T1D-associated DQ molecules shared charged anchor residues, iv while hydrophobic anchors were present in DQ6 peptides. Competition binding assays and peptide dissociation rate measurements indicated that peptide with high affinity is necessary but not sufficient for DM editing resistance. DM editing dominates the stability of MHCII/peptide complexes in the cellular environment. DM catalyzes peptide exchange in MHCII through a mechanism that has been proposed to involve the disruption of specific conserved H-bond in MHCII/peptide complexes. HLA-DR1 molecules with alanine substitutions at each of the six conserved H-bonding positions were expressed in cells, and susceptibility to DM editing was evaluated by measuring the release of CLIP. Our results support the conclusion that no individual component of the conserved H-bond network plays an essential role in the DM catalytic mechanism. Taken together, our data support that the relative DM resistance in T1Dassociated DQ molecules affects the peptide repertoires and preferential presentation of T1D-associated autoantigenic peptides may contribute to the pathogenesis of T1D.