CD4+ T cells from type 1 diabetic and healthy subjects exhibit different thresholds of activation to a naturally processed proinsulin epitope

Recent studies suggest that insulin is a primary autoantigen for type 1 diabetes. Several studies have identified preproinsulin (PPI) 76–90 as an immunodominant CD4+ T cell epitope. We developed a class II tetramer reagent using a modified PPI peptide with a lysine to serine substitution at position 88 (PPI 78–9088S) that has high binding affinity to DRA1*0101/DRB1*0401 (DR0401). Using this tetramer, positive responses were observed from both DR0401 healthy and type 1 diabetic subjects when T cells were stimulated with the PPI 78–9088S peptide. Seventy percent of these T cells proliferated in response to both the wild type PPI 76–90 and PPI 78–9088S peptides. However, when T cells were stimulated with wild type peptide and assayed with DR0401/PPI 78–9088S, positive responses were only detected in the diabetic group but not in healthy subjects. When highly purified CD4+CD25−CD45RA+ T cells were stimulated with PPI 78–9088S peptide in the absence of antigen-presenting cells, T cells from the diabetic group were able to respond to peptide stimulation, while T cells from healthy subjects were not. These data suggest that T cells from type 1 diabetic subjects have a lower threshold of activation in response to PPI peptide stimulation as compared to healthy subjects.