Evidence that a peptide spanning the B-C junction of proinsulin is an early autoantigen epitope in the pathogenesis of type 1 diabetes

The expression of pro(insulin) in the thymus may lead to the negative selec tion of pro(insulin) autoreactive T cells and peripheral tolerance to this autoantigen in type 1 diabetes (T1D). We investigated whether proinsulin is expressed in the thymus of young nonobese diabetic (NOD) mice, whether T c ells from naive NOD female mice at weaning are reactive to mouse proinsulin , and the role of proinsulin as a pathogenic autoantigen in T1D. Proinsulin II mRNA transcripts were detected in the thymus of 2-wk-old NOD mice at si milar levels to other control strains. Despite this expression, proinsulin autoreactive T cells were detected in the periphery of 2- to 3-wk-old naive NOD mice. Peripheral T cells reactive to the insulin, glutamic acid decarb oxylase 65 (GAD65), GAD67, and islet cell Ag p69 autoantigens were also det ected in these mice, indicating that NOD mice are not tolerant to any of th ese islet autoantigens at this young age. T cell reactivities to proinsulin and islet cell Ag p69 exceeded those to GAD67, and T cell reactivity to pr oinsulin in the spleen and pancreatic lymph nodes was directed mainly again st a p24-33 epitope that spans the B chain/C peptide junction. Intraperiton eal immunization with proinsulin perinatally beginning at 18 days of age de layed the onset and reduced the incidence of T1D. However, s.c. immunizatio n with proinsulin initiated at 5 wk of age accelerated diabetes in female N OD mice. Our findings support the notion that proinsulin p24-33 may be a pr imary autoantigen epitope in the pathogenesis of T1D in NOD mice.