Assessment of pancreatic islet transplants using in vivo bioluminescence imaging

Abstract

Pancreatic islet transplantation is a promising treatment for type 1 diabetes. However, current efforts to study islet transplantations are hampered by the lack of a non-invasive method of imaging islets and quantifying islet mass post transplantation. Transplanted pancreatic islets can be imaged and quantified non-invasively using in vivo bioluminescence imaging (BLI). Pancreatic islets transfected with the firefly reporter gene, luciferase, emit light that can be quantified using photon-counting measurements. Pancreatic islet number is linearly related to light emission both in vitro and in vivo. Application of bioluminescence imaging for this application can be greatly enhanced by relating light emission to the number of islets surviving post-transplantation. Determining this relationship requires detailed knowledge of the factors that influence photon-counting measurements. Bioluminescence was modeled using constant light emitting phosphorescent beads implanted at the two common sites of islet transplantation: the renal capsule and liver. This model was used to quantify light attenuation by tissues overlying the islet transplantations. The ratio of implanted light emission to in vitro light emission was found to be .2394+/- 0.0261 for renal implantation and 0.0645 +/- 0.0140 for hepatic implants. Mathematical modeling of light propagation using Monte Carlo simulation is in excellent agreement with these experimental results. Monte Carlo modeling yields an in vivo to in vitro luminescence ratio for renal and hepatic sources to be 0.2860 and 0.0495, respectively. Surgical artifacts were found to influence bioluminescence measurements. Surgical scar tissue leads to lower light emission the week immediately post-op, but this attenuation is negligible two weeks after surgery. The orientation of the subject also influences quantification of bioluminescence. Rotation of 50 degrees from flat can lead to a 73% decrease in light transmission for renal implants and 52% decrease for hepatic implants. The rate of luminescence decrease with increasing angle depends on the surface light is projected upon. Flatter surfaces lead to a slower decrease in luminescence while higher curvature leads to more rapid decrease in luminescence. Spot size of bioluminescence was found to increase with increasing tissue depth. The spot size of hepatic implants was found to be 17% larger than renal implants, as measured by full width at half maximum measurements. Constant light emission modeling of transplanted islet bioluminescence permits quantification of actual islet number from photon counting measurements and insight into factors which influence these measurements.