Under the supervision of Raphaël Scharfmann, team Functional pancreatic beta cell mass in rodent and human
Type 1 diabetes is a disease characterized by the destruction of insulin-producing cells (pancreatic beta cells) following the invasion of islets by cytokines-producing lymphocytes. One of the key cytokines in the pathology is the interferon gamma (IFNg). Increased IFNg levels are reported among diabetic patients, both at systemic level and in infiltrating lymphocytes, furthermore its messenger RNA expression correlates with the disease development in diabetes rodent models. Experimental procedure inducing an overexpression of IFNg increases the incidence of diabetes, while its blockade slows it down. Pancreatic beta cells are not the sole cell type in the endocrine pancreas. They cluster with alpha and delta cells, secreting glucagon and somatostatin respectively, to form the islets of Langerhans, the endocrine units of the pancreas. The auto-immune attack executed by lymphocytes is directed toward antigens presented at beta cells surface, yet the cytokines they secrete induce a global, untargeted inflammation. Our hypothesis was that the non-beta endocrine cells are altered during type 1 diabetes. Using our recently described FACS-based strategy to sort the major endocrine populations from mouse islets, we performed a transcriptomic analysis of alpha, beta and delta cells at a pre-diabetic stage using NOD mice (Non-obese diabetic mouse), a mouse model of type 1 diabetes. Those experiments shed the light on a common pathway activated in the three cells types : the IFNg signalling. We thus wanted to study more globally their response to IFNg. For that we treated healthy islets (from C57BL/6 mice) in vitro with IFNg. RNA-seq analysis revealed that alpha and delta cells display a transcriptomic profile very similar to that of beta cells, including the increase of diabetesrelated genes such as the chemokine Cxcl10 (C-X-C motif chemokine 10) or Cd274, encoding the checkpoint protein PD-L1 (Programmed cell death 1 ligand 1). Interestingly, at low IFNg concentration, we observed two beta cell populations (responders and non-responders) based on PD-L1 protein expression. Our islets deconstruction-reconstruction experiments showed that this phenotype was transient, and required an intact islet structure. By immunostaining we demonstrated that IFNg responsive cells were located in the periphery of the islets, suggesting a gradual diffusion of the cytokine into the islet. We corroborated those findings by the in vivo analysis of pancreatic islets from NOD mice, showing a more intense PD-L1 staining on endocrine cells close to immune infiltrate. Our xi results suggest that we should re-consider the importance of non-beta cells in type 1 diabetes, both as victims but also players of the disease, and gives new insight into the propagation of inflammation.