Granules secretory proteins: novel autoimmune targets and biomarkers in type 1 diabetes – GRASP

Type 1 diabetes (T1D) results from the destruction of pancreatic islet beta cells by autoimmune CD8+ T cells. We hypothesize that beta cells play a key role in their own autoimmune demise, by increasing their immune visibility and vulnerability. Our analysis of the beta-cell peptidome identified novel antigenic proteins derived from the insulin granules. In particular, secretogranin-5 (SCG5), proconvertase-2 (PCSK2) and urocortin-3 (UCN3) were recognized by human circulating CD8+ T cells and islet-infiltrating CD8+ T cells in the Non-Obese Diabetic (NOD) mouse model of T1D. These T cells are diabetogenic upon in vivo transfer, supporting their pathological role. Two features shared with (pro)insulin may explain this pathogenicity: 1) these granule proteins are released through exocytosis in the bloodstream, thus potentially sensitizing the immune system at distance; 2) they are produced as precursors, and their processing is impaired in T1D, as reflected by increased secretion of unprocessed proinsulin.
Our objectives are to investigate:
#1. Pathogenicity of SCG5, PCSK2 and UCN3. We will address this: 1A) in the NOD mouse, by characterizing CD8+ T cells reactive to these granule proteins and their pathogenicity in T-cell receptor-retrogenic NOD mice; and 1B) in the human, by using CD8+ T-cell clones/transductants recognizing these antigens, to verify their in vitro capacity to be stimulated by dendritic cells exposed to the ?-cell secretome and to destroy ? cells.
#2. SCG5, PCSK2 and UCN3 as novel biomarkers of beta-cell stress and autoimmunity. We will analyze whether these granule proteins: 2A) are increasingly released as pro-proteins (like proinsulin) by beta cells exposed to metabolic or inflammatory stress; and 2B) elicit autoantibody responses in T1D patients.
This project will 1) clarify novel pathogenic mechanisms of T1D, i.e. how beta cells may contribute to their own vulnerability; and 2) provide new biomarkers for T1D diagnosis and staging.