Probes for INSULIN

The canonical notion that type 1 diabetes (T1D) results following a complete destruction of β cells has recently been questioned as small amounts of C-peptide are detectable in patients with long-standing disease. We analyzed protein and gene expression levels for proinsulin, insulin, C-peptide, and islet amyloid polypeptide within pancreatic tissues from T1D, autoantibody positive (Ab+), and control organs. Insulin and C-peptide levels were low to undetectable in extracts from the T1D cohort; however, proinsulin and INS mRNA were detected in the majority of T1D pancreata. Interestingly, heterogeneous nuclear RNA (hnRNA) for insulin and INS-IGF2, both originating from the INS promoter, were essentially undetectable in T1D pancreata, arguing for a silent INS promoter. Expression of PCSK1, a convertase responsible for proinsulin processing, was reduced in T1D pancreata, supportive of persistent proinsulin. These data implicate the existence of β cells enriched for inefficient insulin/C-peptide production in T1D patients, potentially less susceptible to autoimmune destruction.

Acquisition of T cell central tolerance involves distinct pathways of self-antigen presentation to thymocytes. One pathway termed indirect presentation requires a self-antigen transfer step from thymic epithelial cells (TECs) to bone marrow (BM)-derived cells before the self-antigen is presented to thymocytes. The role of indirect presentation in central tolerance is context-dependent, potentially due to variation in self-antigen expression, processing and presentation in the thymus. Here, we report experiments in mice in which TECs expressed a membrane-bound transgenic self-antigen, hen egg lysozyme (HEL), from either the insulin (insHEL) or thyroglobulin (thyroHEL) promoter. Intrathymic HEL expression was less abundant and more confined to the medulla in insHEL mice compared to thyroHEL mice. When indirect presentation was impaired by generating mice lacking MHCII expression in BM-APCs, insHEL-mediated thymocyte deletion was abolished, whereas thyroHEL-mediated deletion occurred at a later stage of thymocyte development and Foxp3+ T-regulatory cell differentiation increased. Indirect presentation increased the strength of TCR signalling that both self-antigens induced in thymocytes, as assessed by Helios expression. Thus, indirect presentation limits the differentiation of naïve and regulatory T cells by promoting deletion of self-reactive thymocytes.