Investigating the Biological Mechanisms underlying the Initiation of Autoimmunity against Beta Cell Antigens in Type 1 Diabetes

Type 1 diabetes (T1D) is an autoimmune disease characterized by circulating autoantibodies, lymphocytic infiltration of pancreatic islets of Langerhans, and cell-specific destruction of beta cells, leading to insulin deficiency and symptomatic hyperglycemia. Some of the major target autoantigens in T1D are intracellular proteins, such as GAD65, IA-2 and proinsulin, whose initial encounter with the immune system is poorly understood. Here we describe a method for culturing monolayers of primary pancreatic islet cells in vitro, which enables detailed observation of proteins and sub-cellular processes in primary human and rat beta cells by super-resolution and live-cell light microscopy. This technical advance is broadly applicable to obtaining novel biological insights as demonstrated by our identification of a mechanism to control proliferation in primary beta cells through growth and disassembly of primary cilia. This protocol also allowed us to identify two novel interrelated mechanisms for how beta cell proteins, such as the antigen GAD65, can become target of autoimmunity and initiate T1D. A triggering factor that these two pathways have in common is represented by endoplasmic reticulum (ER) stress, to which pancreatic islet beta cells are particularly susceptible, and can be induced by inflammatory factors such as Th1 cytokines. In the first mechanism, we show that ER stress in beta cells perturbs the palmitoylation cycle controlling GAD65 endomembrane distribution, resulting in aberrant accumulation of the palmitoylated form in Golgi membranes. The palmitoylated form has heightened immunogenicity, exhibiting increased uptake by antigen presenting cells (APCs) and T cell stimulation compared to the non-palmitoylated form. Similar accumulation of GAD65 in Golgi membranes is observed in human beta cells in pancreatic sections from GAD65 autoantibody positive individuals, who have not yet progressed to clinical onset of T1D, and T1D patients with residual beta cell mass and ongoing T cell infiltration of islets. Thus, aberrant accumulation of immunogenic GAD65 in Golgi membranes facilitates inappropriate presentation to the immune system following release from stressed and/or damaged beta cells, triggering autoimmunity. In the second mechanism, we present that, following a pathway originating from the Golgi, both rat and human pancreatic islets release the intracellular beta cell autoantigens GAD65, IA-2 and proinsulin in a subtype of extracellular vesicles called exosomes. Islet-exosomes are then taken up by and can activate APCs including dendritic cells. Accordingly, anchoring of GAD65 to exosome-mimetic liposomes strongly boosts antigen presentation and T cell activation in the context of the human T1D susceptibility haplotype HLA-DR4. Furthermore, cytokine-induced ER stress enhances exosome secretion by beta cells, induces exosomal release of the immunostimulatory chaperones calreticulin, Gp96 and ORP150 and increases exosomal stimulation of APCs. Therefore, stress-induced exosomal release of intracellular autoantigens and immunostimulatory chaperones may play a role in initiation of autoimmune responses in T1D.