Alternative splicing and differential expression of the islet autoantigen IGRP between pancreas and thymus contributes to immunogenicity of pancreatic islets but not diabetogenicity in humans

Thymic expression of self-antigens during T-lymphocyte development is believed to be crucial for preventing autoimmunity. It has been suggested that G6PC2, the gene encoding islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP), is differentially spliced between pancreatic beta cells and the thymus. This may contribute to incomplete elimination of IGRP-specific T lymphocytes in the thymus, predisposing individuals to type 1 diabetes. We tested whether specific splice variation in islets vs thymus correlates with loss of tolerance to IGRP in type 1 diabetes. Expression of G6PC2 splice variants was compared among thymus, purified medullary thymic epithelial cells and pancreatic islets by RT-PCR. Differential immunogenicity of IGRP splice variants was tested in patients and healthy individuals for autoantibodies and specific cytotoxic T lymphocytes using radiobinding assays and HLA class I multimers, respectively. Previously reported G6PC2 splice variants, including full-length G6PC2, were confirmed, albeit that they occurred in both pancreas and thymus, rather than islets alone. Yet, their expression levels were profoundly greater in islets than in thymus. Moreover, three novel G6PC2 variants were discovered that occur in islets only, leading to protein truncations, frame shifts and neo-sequences prone to immunogenicity. However, autoantibodies to novel or known IGRP splice variants did not differ between patients and healthy individuals, and similar frequencies of IGRP-specific cytotoxic T lymphocytes could be detected in both patients with type 1 diabetes and healthy individuals. We propose that post-transcriptional variation of tissue-specific self-proteins may affect negative thymic selection, although this need not necessarily lead to disease