Promiscuous binding of proinsulin peptides to Type 1 diabetes-permissive and -protective HLA class II molecules

Aims/hypothesis. Presentation of peptide epitopes derived from beta-cell autoantigens, such as insulin and its precursor molecules, by MHC class II molecules to autoreactive T-cells is believed to play a role in the development of Type I diabetes. However, little is known about the interaction between peptides of (prepro)insulin and MHC class II molecules permissive and protective for Type I diabetes. In this study therefore, peptides spanning the human preproinsulin sequence were assessed for their binding characteristics to Type 1 diabetes-protective and -permissive HLA molecules. Methods. HLA-DR2, -DQ6.2 (Type 1 diabetes-protective) and HLA-DR4, -DQ8 (Type I diabetes permissive) molecule binding affinity for overlapping synthetic 20mer peptides spanning human preproinsulin was measured in a direct competition binding assay against a biotinylated indicator peptide. Results. All HLA molecules tested showed similarity in their binding characteristics across the preproinsulin molecule, with regions of the insulin A-chain showing the highest affinity and C-peptide regions the lowest affinity for all HLA molecules tested. Furthermore, an insulin peptide implicated as a major CD4+ T-cell target in disease pathogenesis (B9-23) had high affinity binding to both protective and permissive HLA molecules but did not represent the highest affinity region of (prepro)insulin identified in either case. Conclusion/interpretation. The results suggest that peptide binding affinity alone is unlikely to be the major determinant of disease susceptibility in relation to interactions between (prepro)insulin epitopes and HLA molecules. The identification of epitopes derived from beta-cell autoantigens that bind promiscuously to diabetes-permissive HLA molecules could be important in the design of peptide-based immunotherapeutic strategies for the prevention of Type 1 diabetes.