EXPERIMENTAL AUTOIMMUNE INSULITIS INDUCTION BY T-LYMPHOCYTES SPECIFICFOR A PEPTIDE OF PROINSULIN

Type I diabetes, an autoimmune disease that occurs in humans and anima ls, is characterized by the destruction of insulin-secreting islet bet a-cells of the pancreas. Antibodies directed toward multiple islet pro teins can be detected before diagnosis of type I diabetes; however the identity of the inciting autoantigen(s) that targets beta-cells for d estruction has not been defined, Autorecognition of many self-proteins by CD4(+) T lymphocytes is restricted by the products of class II imm une response genes of the major histocompatibility complex (MNC), and ht human type I diabetes such a MHC association has been described The present study uses a rat MWC class II (RT1.B-l) peptide binding motif to predict potentially autoreactive CD4(+) T cell epitopes in two key islet beta-cell constituents: the enzyme glutamic acid decarboxylase (GAD) and the insulin precursor hormone proinsulin (PI). Seventeen-ami no-acid-long peptide fragments of GAD and PI containing the binding mo tif were synthesized and used to generate peptide-specific, MHC class II-restricted, CD4(+) T cell lines, Once established, the T cell fines specific for rat islet GAD and PI were adoptively transferred to naiv e, MHC-compatible rats, At 10 days after transfer, insulitis had devel oped in mts receiving PI-specific T cells, whereas no insulitis was ob served in pancreata of mts receiving GAD-specific T cells. Of particul ar interest is the finding that the pathogenic T cell epitope identifi ed in PI spans the endogenous cleavage site between the B-chain and C- peptide of insulin Moreover, the PI-specific T cells were able to reac t specifically with material produced in vitro by a rat insulinoma cel l line. These results demonstrate that pathogenic T cell epitopes can be located in portions of molecules that are subsequently degraded dur ing normal enzymatic processing. As PI is found in highest concentrati ons It the beta-cells of pancreatic islets, it is possible that this m olecule and not its individual degradation products (ie, insulin and C -peptide) might serve as an autoantigen in the pathogenesis of type I diabetes.