Crinophagic granules in pancreatic β cells contribute to mouse autoimmune diabetes by diversifying pathogenic epitope repertoire
Hao Hu, Anthony N. Vomund, Orion J. Peterson, Neetu Srivastava, Tiandao Li, Lisa Kain, Wandy L. Beatty, Bo Zhang, Chyi‐Song Hsieh, Luc Teyton, Cheryl F. Lichti, Emil R. Unanue, Xiaoxiao Wan
Abstract
Autoimmune attack toward pancreatic β cells causes permanent loss of glucose homeostasis in type 1 diabetes (T1D). Insulin secretory granules store and secrete insulin but are also thought to be tissue messengers for T1D. Here, we show that the crinophagic granules (crinosome), a minor set of vesicles formed by fusing lysosomes with the conventional insulin dense-core granules (DCG), are pathogenic in T1D development in mouse models. Pharmacological inhibition of crinosome formation in β cells delays T1D progression without affecting the dominant DCGs. Mechanistically, crinophagy inhibition diminishes the epitope repertoire in pancreatic islets, including cryptic, modified and disease-relevant epitopes derived from insulin. These unconventional insulin epitopes are largely undetectable in the MHC-II epitope repertoire of the thymus, where only canonical insulin epitopes are presented. CD4+ T cells targeting unconventional insulin epitopes display autoreactive phenotypes, unlike tolerized T cells recognizing epitopes presented in the thymus. Thus, the crinophagic pathway emerges as a tissue-intrinsic mechanism that transforms insulin from a signature thymic self-protein to a critical autoantigen by creating a peripheral-thymic mismatch in the epitope repertoire. Dense-core granules (DCG) store insulin in pancreatic β cells. Here the authors show that crinosome, formed by fusing lysosome and DCGs, are pathogenic in mouse models of type 1 diabetes by diversifying local insulin epitopes beyond those tolerizing ones expressed in the thymus, thereby inducing autoreactive CD4 T cells for β cell death and insulin deficiency.