C-peptide of proinsulin is an autoantigen in human type 1 diabetes

Background - The antigens and epitopes recognized by CD4+ T cells in people with type 1 diabetes (T1D) remain poorly defined. We, and others, have found that several epitopes derived from C-peptide are recognized by human islet-infiltrating CD4+ T cells implicating C-peptide as an autoantigen in human T1D. Here we quantified and analyzed CD4+ T cells that responded to full-length C-peptide (PI33-63) in peripheral blood of subjects with and without T1D.

Methods - The CFSE dye-based proliferation assay was used to detect C-peptide-responsive CD4+ T cells in PBMC (1). CD4+ T cells that responded to C-peptide were sorted into individual wells, cloned and characterised.

Results - CD4+ T-cell responses to full-length C-peptide were detected in: >60% (14 of 23) of recent-onset (>100 days of diagnosis) T1D subjects, 13% (2 of 15) of long-standing (>100 days from diagnosis) T1D subjects and 8% (1 of 13) of HLA-matched healthy subjects. A panel of 22 C-peptide-specific CD4+ T-cell clones were generated from 6 individuals with recent-onset T1D. These clones recognized epitopes across the entire 31 amino acids of C-peptide, although most epitopes were towards the C-terminus. Eighty-six percent (19 of 22) C-peptide-specific clones were restricted by high-risk alleles HLA-DQ8, -DQ2, -DQ8trans or -DQ2trans. TCR sequencing revealed that these clones used a wide variety of TCR genes. Finally, titration experiments showed that full-length C-peptide was a much more potent agonist of some CD4+ T-cell clones than an 18mer peptide encompassing their cognate epitope.

Conclusion  - Our findings support the notion that full-length C-peptide is an important target of pathogenic CD4+ T cells in people with DQ8 and/or DQ2 who develop T1D. Consequently, full-length C-peptide may be useful in T-cell assays and antigen-specific therapy protocols.

1. S. I. Mannering, et al. A sensitive method for detecting proliferation of rare autoantigen-specific human T cells. J Immunol Methods 283, 173-183 (2003).