Autoimmunity to glucokinase and P4Hb alters insulin secretion in autoimmune diabetes

Background: Inflammation and oxidative stress in the pancreas amplifies various post-translational modifications (PTMs) on self-proteins. The loss of immune tolerance to PTMs within the stressed islets subsequently impacts the autoreactive T cell epitope repertoire and contributes to the destruction of insulin-producing beta cells in type 1 diabetes (T1D). In this study, we examined the effects of the novelPTM autoantigens on immunogenicity and beta cell functions. Moreover, we also examined autoantibodies arising in diabetes elicited by checkpoint inhibitor therapy (CPI) in cancer patients.

Methods: Mass spectrometry was performed to identify proteins and map specific PTM sites. Autoantibody and T cell responses to modified islet proteins were evaluated by ELISA and HLA tetramers, respectively. Moreover, proinsulin and insulin secretion upon glucose stimulation was examined in human islets in the context of beta cell PTMs. Finally, the autoantibodies profile was screen from CPIs-treated patients serum.

Results:We identified a number of PTM beta cell proteins. In particular, we found carbonyl residues within P4Hb (an insulin folding protein) and nineteen citrullination modifications within glucokinase. Carbonylated-P4Hb is amplified in stressed islets coincident with decreased glucose-stimulated insulin secretion and altered proinsulin to insulin ratios. Autoantibodies against both P4Hb and glucokinase arise in human T1D patients. Likewise,CD4+ T cells specific for citrullinated glucokinase are present in the circulation of T1D patients. Interestingly,anti-glucokinase antibodies correlated with anti-ZnT8 immunity. InCPI-treated cancer patients, we observed autoantibodies to both established T1D biomarkers (insulin, GAD, IA2, ICA and ZnT8) and also to novel T1D autoantibodies (P4Hb and glucokinase) as well as antinuclear antibodies (ANAs) typical of lupus autoimmunity. Taken together, our studies implicate these crucial enzymes as biomarkers, providing new insights into creating autoantigens and define the impact of PTMs on the biological function of beta cells in T1D and CPI-associated diabetes.