Characterisation of native human pancreatic mesenchymal stromal cells in type 1 diabetes

Aims/hypothesis: Culture-expanded mesenchymal stromal cells (MSCs) reduce immune cell activation and improve islet functional survival. However, little is known about native human pancreatic MSCs (npMSCs) in health or how they are altered in type 1 diabetes. Here, we determined the number, density and islet-protective phenotype of npMSCs in situ in control individuals and those with type 1 diabetes. Methods: Multiplex immunohistochemistry was used to identify npMSCs (CD90 ⁺ /CD105 ⁺ /CD73 ⁺ /CD31 ^- /CD45 ^- /CD34 ^- ) in human pancreas sections from 38 donors (Network for Pancreatic Organ Donors with Diabetes and Exeter Archival Diabetes Biobank). Donors were categorised as either < 13 years at type 1 diabetes diagnosis (n = 8) or ≥ 13 years at type 1 diabetes diagnosis (n = 11) or were age- and sex-matched individuals without diabetes. Consecutive sections were immunostained with antisera against insulin, glucagon, and the established islet-protective and immunomodulatory factors Annexin A1 (ANXA1) and indoleamine 2,3-Dioxygenase-1. Whole-slide scans were acquired and npMSCs either inside or at the periphery (within 10 μm) of islets were quantified on an individual-islet basis. We identified 53,375 npMSCs and performed an analysis of 26,376 individual islets. Culture-expanded MSCs were exposed to cytokines and viability and proliferation was assessed by flow cytometry. Results: npMSC were identified in situ in the human pancreas where they wrap around the islet periphery in an expected spindle-like morphology. ANXA1 was expressed by 33.2% of npMSCs and was expressed constitutively among individuals with or without diabetes. The density of both intraislet npMSCs and npMSCs within 10 μm of the islet periphery was increased for insulin-containing islets in individuals with type 1 diabetes compared to individuals without diabetes (p < 0.001). npMSC density within 10 μm of the islet periphery was preferentially increased in individuals ≥ 13 years at type 1 diabetes diagnosis compared to individuals < 13 years at type 1 diabetes diagnosis (p < 0.001). npMSC density was reduced around insulin-deficient islets compared to insulin-containing islets in individuals with diabetes (p < 0.001), consistent with an islet-protective role for npMSCs. Exposure of culture-expanded MSCs to an aggressive cytokine combination led to increased cell death and reduced proliferation. Conclusion/interpretation: npMSCs express ANXA1 constitutively suggesting an islet-protective role in health. The density of npMSCs was increased around insulin-containing islets and lost around insulin-deficient islets in individuals with type 1 diabetes which aligns with this hypothesis. npMSC density at the periphery of insulin-containing islets was preferentially higher in individuals with later onset type 1 diabetes, correlating with a less intense immune cell infiltration. The reduced ability of npMSCs to survive in the more intense pro-inflammatory environment around islets in younger onset type 1 diabetes may contribute to the rapid rate of beta cell loss in these individuals.