Background: Islet transplantation of encapsulated cells within an immunoprotective device is a promising approach to cure Type 1 diabetes without the need for lifelong immunosuppression. Unfortunately, in order to use a device of a reasonable size, the large number of cells required for diabetes reversal dictates high-cell packing density, which results in oxygen limitations affecting the viability and functionality of the encapsulated cells. Oxygen at a high concentration delivered to encapsulated cells can help reduce the hypoxia induced reduction of encapsulated cell viability and function. However, hyperoxia can also be detrimental to cells, so the toxicity of high oxygen exposure needs to be investigated so as to establish safe levels of oxygen exposure. In this study, we investigated rat and human islets exposed to different concentrations of oxygen and evaluated the viability and function of the cells at various time points.
Methods: Human and rat islets were exposed to normoxic and hyperoxic conditions (40, 60, and 95%) for up to 7 days. Islet viability was assessed by oxygen consumption rate (OCR) and membrane integrity staining with fluorescein diacetate and propidium iodide. Islet function was assessed by dynamic glucose stimulated insulin secretion (GSIS) via perifusion (Biorep). Results: In hyperoxic 40% culture, neither OCR nor GSIS were changed in human or rat islets for as long as 7 days of exposure relative to normoxic control. In hypoxic 60% culture, neither OCR nor GSIS were changed in rat islets relative to normoxic control; however, OCR was reduced by ~15% after 1 day of exposure and by ~30% after 7 days of exposure in human islets. GSIS was also significantly reduced as early as 1 day of exposure. Hyperoxic 95% culture significantly reduced both OCR and GSIS in both rat and human islets after 1 day of exposure and with a very small fraction of surviving cells after 7 days of exposure. Conclusions: Supplemental oxygen, particularly in the peri-transplant period, will be essential to support enough insulin secreting cells within a reasonably sized encapsulation device for the treatment of Type 1 diabetes in a research animal model or human patients. Our data indicates that up to 40% oxygen can be supplied to rat and human islets and it is not detrimental to viability or function. However, hyperoxia of >60% can be toxic to human islets with as little as 24 hours of exposure. Future work will be aimed at further defining the “safe” oxygen exposure limits at which islet viability and function are maintained for primary and stem cell derived islets.