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Characterisation of human pancreatic mesenchymal stromal cells in type 1 diabetes - Diabetologia Aims/hypothesis Culture-expanded mesenchymal stromal cells (MSCs) reduce immune cell activation and improve islet functional survival. However, little is known about human pancreatic MSCs (pMSCs) in health or how they are altered in type 1 diabetes. Here, we determined the number, density and islet-protective phenotype of pMSCs in situ in individuals with and without type 1 diabetes. Methods Multiplex immunohistochemistry was used to identify pMSCs (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 sex-matched individuals of similar age 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 pMSCs either inside or at the periphery (within 10 µm) of islets were quantified on an individual-islet basis. We identified 53,375 pMSCs and performed an analysis of 26,376 individual islets. Culture-expanded MSCs were exposed to cytokines and viability and proliferation were assessed by flow cytometry. Results pMSCs 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 pMSCs and was expressed constitutively among individuals with or without diabetes. The density of both intraislet pMSCs and pMSCs within 10 µm of the islet periphery was increased for insulin-containing islets in individuals with type 1 diabetes compared with individuals without diabetes (p<0.001). pMSC density within 10 µm of the islet periphery was preferentially increased in individuals ≥13 years at type 1 diabetes diagnosis compared with individuals <13 years at type 1 diabetes diagnosis (p<0.001). pMSC density was reduced around insulin-deficient islets compared with insulin-containing islets in individuals with diabetes (p<0.001), consistent with an islet-protective role for pMSCs. Exposure of culture-expanded MSCs to an aggressive cytokine combination led to increased cell death and reduced proliferation. Conclusions/interpretation pMSCs express ANXA1 constitutively, suggesting an islet-protective role in health. The density of pMSCs was increased around insulin-containing islets and lost around insulin-deficient islets in individuals with type 1 diabetes which aligns with this hypothesis. pMSC 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 pMSCs to survive in the more intense proinflammatory environment around islets in younger-onset type 1 diabetes may contribute to the rapid rate of beta cell loss in these individuals. Graphical Abstract https://link.springer.com/article/10.1007/s00125-025-06634-w