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In vitro and in vivo function of macroencapsulated adult porcine islets is enhanced by changes in the capsular matrix microenvironment

Robert Holdcraft¹, Lawrence Gazda¹, Richard Hall², Barry Smith^3,4,5

¹Xenia Division, The Rogosin Institute, Xenia, OH, United States; ²Bob Evans Farms, Inc., Columbus, OH; ³The Rogosin Institute, New York, NY; ⁴Department of Surgery, Weill Medical College of Cornell University, New York, NY; ⁵NewYork-Presbyterian Hospital, New York, NY; United States

Encapsulation of isolated adult porcine islets in agarose macrobeads (PIM) allows longterm islet culture and assessment of function prior to implantation. Changing the agarose concentration in the matrix supporting the islets from 1.5% to 0.8% yields enhanced in vitro insulin production (76.8±20.8 vs. 89.0±22.2 milliunits/PIM/24 hours, 4 week pre-implant average, n=13 isolates) without affecting the strength of the macrobead (2.63±0.48 vs. 2.51±0.47 lbf, n=30 PIM/group from 10 isolates).

Bromodeoxyuridine (BrdU) staining demonstrated cell proliferation within the islets in both 1.5% and 0.8% agarose, peaking at 4 weeks post-encapsulation (50.3% (n=174) vs. 55.2% (n=169) BrdU-positive islets) and decreasing to 19.8% (n=140) vs. 6.9% (n=180) at 8 weeks post-encapsulation (3 isolates per group). Cell death, as measured by TUNEL staining and regardless of agarose concentration, was ubiquitous in encapsulated islets 24 hours post-isolation but gradually decreased over the first several weeks in culture and was rare in PIM between 4 and 10 weeks post-encapsulation.

When high and low concentration PIM were implanted into streptozotocin-induced diabetic female CD rats, both were highly effective at restoring normoglycemia (90-day post implant averages of 161±92 vs. 132±28 mg/dL, respectively). This result is noteworthy given that blood glucose was more tightly regulated in the 0.8% PIM-implanted animals despite the fact that they received fewer average PIM per animal (19 vs. 17), commensurate with the increased insulin production per PIM observed in vitro.

We conclude that the matrix microenvironment provided by the agarose capsule and its interaction with the islets is critically important to islet survival, proliferation and function, both in vitro and in vivo. Optimization of the microenvironment can produce significant improvements in the ability of isolated islets to control blood glucose in vivo, thus making the most of a limited resource and improving the cost effectiveness of islet transplantation.