Human islet microtissues as an in vitro and in vivo model system for diabetes Joan Mir-Coll1, Tilo Moede2, Meike Paschen2, Aparna Neelakandhan1, Ismael Valladolid-Acebes2, Barbara Leibiger2, Adelinn Biernath1, Carina Ämmälä3, Ingo B. Leibiger2, Burcak Yesildag1 and Per-Olof Berggren2 1 InSphero AG, Wagistrasse 27a, 8952 Schlieren, Switzerland; email@example.com; firstname.lastname@example.org; email@example.com; firstname.lastname@example.org. 2 The Rolf Luft Research Center for Diabetes and Endocrinology, Karolinska Institutet, Karolinska Sjukhuset L1:03, 17176 Stockholm, Sweden; email@example.com; firstname.lastname@example.org; email@example.com; firstname.lastname@example.org; email@example.com; firstname.lastname@example.org. 3 Bioscience, Research and Early Development, Cardiovascular, Renal and Metabolism, BioPharmaceuticals R&D, AstraZeneca, 431 38 Mölndal, Gothenburg, Sweden; email@example.com. Loss of pancreatic β-cell function is a critical event in the pathophysiology of type 2 diabetes. However, studies of its underlying mechanisms as well as the discovery of novel targets and therapies have been hindered due to limitations in available experimental models. In this study we exploited the stable viability and function of standardized human islet microtissues to develop a disease-relevant, scalable, and reproducible model of β-cell dysfunction by exposing them to long-term glucotoxicity and glucolipotoxicity. Moreover, by establishing a method for highly-efficient and homogeneous viral transduction, we were able to monitor the loss of functional β-cell mass in vivo by transplanting reporter human islet microtissues into the anterior chamber of the eye of immune-deficient mice exposed to a diabetogenic diet for 12 weeks. This newly developed in vitro model as well as the described in vivo methodology represent a new set of tools that will facilitate the study of β-cell failure in type 2 diabetes and could accelerate the discovery of novel therapeutic agents.