Survival of autoreactive T lymphocytes by microRNA-mediated regulation of apoptosis through TRAIL and Fas in type 1 diabetes

Autoreactive CD8⁺ T cells recognizing autoantigens expressed by pancreatic islets lead to the destruction of insulin-producing beta cells in type 1 diabetes (T1D), but these T cells also occur in healthy subjects. We tested the hypothesis that uncontrolled expansion of diabetogenic T cells in patients occurs, resulting from failure to activate apoptosis. We compared function, transcriptome and epigenetic regulation thereof in relation with fate upon repeated exposure to islet-autoantigen of islet autoreactive T cells from healthy and type 1 diabetic donors with identical islet epitope specificity and HLA-A2 restriction. Patient's T cells proliferated exponentially, whereas those of non-diabetic origin succumbed to cell death. Transcriptome analysis revealed reduced expression of TRAIL, TRAIL-R2, FAS and FASLG (members of the extrinsic apoptosis pathway) in patient-derived compared with healthy donor-derived T cells. This was mirrored by increased expression of microRNAs predicted to regulate these particular genes, namely miR-98, miR-23b and miR-590-5p. Gene-specific targeting by these microRNAs was confirmed using dual-luciferase reporter assays. Finally, transfection of these microRNAs into primary T cells reduced FAS and TRAIL mRNA underscoring their functional relevance. We propose that repression of pro-apoptotic pathways by microRNAs contributes to unrestricted expansion of diabetogenic cytotoxic T cells, implicating microRNA-mediated gene silencing in islet autoimmunity in T1D.