Adherent reformed islets: a long-term primary cell-based platform for exploring mouse and human islet biology.

Pancreatic islets are 3-dimensional micro-organs that maintain β-cell functionality via cell-cell and cell-matrix communication. Isolated primary islets are the gold standard for in vitro models. However, native islets present experimental challenges for long-term mechanistic studies owing to their short culture life (approximately 1 week). We developed a novel long-term protocol to study the function of primary islets. The protocol employed reformed islets following dispersion and a fine-tuned culture environment. Reformed islets are highly similar to their primary counterparts across various physiological characteristics. Long-term culture of reformed islets enables high-resolution imaging, repeated functional assessment, and the study of cell-cell communication. Unlike other platforms such as stem cell-derived organoids, reformed islets retain their resident immune populations, making them ideal for studying both resident and infiltrating immune cells and their interactions with hormone-producing islet cells. Qualitative and quantitative analyses revealed that the composition and cytoarchitecture of the reformed islets mimicked those found in primary islets, including the presence of macrophages and CD4+ and CD8+ T cells, which are the key resident immune cell types. Reformed islets secrete insulin and are glucose-responsive, and their β-cells can be stimulated to proliferate using GLP-1 receptor agonism. Furthermore, a comparison of the transcriptomic landscape of isolated human islets and reformed islets generated from the same donor demonstrated a high degree of similarity. Our reformed islets provide an ideal platform to study diabetes pathology. We recapitulated both the T1DM and T2DM disease milieu and validated our model for studying islet immune trafficking and invasion using activated macrophages and T cells. Our data illustrates that reformed islets are an anatomical and functional alternative to native human and mouse islets. Moreover, reformed islets have an advantage over mouse and human β-cell lines, including MIN6 and EndoC-βH1cells, that lack the signalling input of non-β-endocrine cells and immune cell crosstalk. In this study, we showed that reformed islets are a durable paradigm (cell-based model) for islet-based exploration and a means of target discovery/validation for diabetes research. ### Competing Interest Statement The authors have declared no competing interest.