Graduate Research Assistant
Department of Radiology
University of Minnesota
Continuous oxygen delivery improves oxygenation of tissue-engineered islet grafts in vivo as measured with fluorine-19 magnetic resonance spectroscopy
Bradley P Weegman1, Samuel A Einstein1, Leah V Steyn2, Thomas M Suszynski3, Meri T Firpo4, Melanie L Graham5, Jody Janacek5, Lynn E Eberly6, Michael Garwood1, Klearchos K Papas2.
1Department of Radiology - Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States; 2Department of Surgery, University of Arizona , Tucson, AZ, United States; 3Medical Center, University of Texas Southwestern, Dallas, TX, United States; 4Department of Medicine - Stem Cell Institute, University of Minnesota, Minneapolis, MN, United States; 5Department of Surgery, University of Minnesota, Minneapolis, MN, United States; 6Division of Biostatistics, University of Minnesota, Minneapolis, MN, United States
Introduction: Pancreatic islet transplantation (ITx) is a potential cure for type 1 diabetes, but favorable long-term outcomes are inconsistent, and the liver may not be the optimal site. Macro-encapsulation of islets within a tissue-engineered graft (TEG) could offer many advantages, however oxygenation is a critical limitation for the development of large-scale and high-cell density implanted TEGs. Islets are especially sensitive to prolonged exposure to hypoxia, which has been shown to decrease graft viability and function. This study describes a 19F magnetic resonance spectroscopy (19F-MRS) method for non-invasive evaluation of oxygen levels (pO2) within implanted TEGs, and a method for delivery of supplemental oxygen (DSO) to increase pO2 in vivo.
Methods: TEGs were constructed by injecting an emulsion of perfluoro-15-crown-5-ether (PFCE) and cross-linked porcine plasma into 40 μl immunoisolation devices (TheraCyte, Inc. Laguna Hills, CA). Grafts were prepared that contained only the control emulsion (no cells, N=6) or 10,000 porcine islet equivalents (PIE) (N=6). A second set of TEGs modified for in vivo DSO were prepared in the same way containing the control emulsion (N=6) or 20,000 PIE (N=6). TEGs were implanted into a dorsal subcutaneous pocket in non-diabetic Lewis rats (Charles River, Wilmington, MA) and pO2 was monitored for 29 days using 19F-MRS. Humidified, oxygen-rich gas (pO2 = 650 mmHg) was continuously delivered to an internal compartment of DSO-modified implants through a transcutaneous cannula using a harness and tether apparatus. Oxygen levels in vivo were determined using a 16.4 T horizontal bore MRS system (Agilent Technologies Santa Clara, CA) to measure the 19F spin-lattice relaxation rate constant (R1) of the PFCE within the graft, and applying a previously established calibration between R1 and pO2. All grafts were explanted after 29 days and fixed in 10% buffered formalin solution for histological assessment.
Results: TEGs without DSO had an average internal pO2 of 39±9 mmHg for controls and 27±6 mmHg for PIE-loaded TEGs one day following implantation. The pO2 decreased to and remained ≤10 mmHg for 29 days in both conditions (Figure 1). Implants with continuous DSO achieved elevated oxygen concentrations with an internal pO2 of 556±18 and 370±49 mmHg the first day after implantation for control implants and PIE-loaded implants, respectively. The elevated pO2 levels persisted for DSO implants with average internal pO2 > 100 mmHg for up to 29 days in both conditions. Oxygen levels in PIE-loaded TEGs remained significantly lower than control TEGs at each time-point studied. Explant histology showed similar vascular formation in surrounding tissues for all implanted TEGs, but islet tissue was only found after 29 days in vivo in grafts that received DSO.
Conclusions: Non-invasive 19F-MRS methods are an accurate and valuable tool for the assessment of oxygenation within implanted tissue engineered grafts. Delivery of supplemental oxygen can successfully increase internal oxygen concentrations of implants with and without islets and may support prolonged islet survival within macro-encapsulated immunoisolation devices.
Figure 1: 19F-MRS oxygen measurements within TEGs implanted for 29 days with and without DSO. Implants without DSO remained at hypoxic levels (pO2 < 10 mmHg) while implants with DSO successfully achieved elevated oxygen concentrations that were maintained for the duration of the study.
Schott Foundation. Minnesota Lions Diabetes Foundation. Juvenile Diabetes Research Foundation (JDRF 5-2013-141). Schulze Diabetes Institute. NIH (P41 EB015894, and S10 RR025031). Giner Inc..
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11:00 - 12:30
|Pancreas and Islet Transplantation: Imaging, Biopsies, and Biomarkers||Continuous oxygen delivery improves oxygenation of tissue-engineered islet grafts in vivo as measured with fluorine-19 magnetic resonance spectroscopy||Room 110|
17:30 - 18:30
|Joint Poster Session||Non-invasive assessment of tissue-engineered graft viability with oxygen-17 magnetic resonance spectroscopy||Main Foyer 1 & 2|