Parameters favouring successful adult pig islet isolations for xenotransplantation in pig-to-primate models

Background: In the near future, adult porcine islets of Langerhans appear as an unlimited source of insulin-producing cells which could play a major role for treating diabetes mellitus. There is, however, an obvious lack of pre-clinical results and data in the pig-to-primate model. One of the main hurdles of this model is certainly related to the difficulty of reproducing regularly successful porcine islet isolation. This experimental work was designed to provide guidelines applicable in pig pancreas procurement and islet isolation for successful islet xenotransplantation into primates. Methods: Pancreases were harvested from adult Belgium Landrace pigs (n = 79) in a single centre. The impact on islet yield of (1) pancreas procurement (blood exsanguination and warm ischaemia time (WIT)), (2) cold storage solutions (classic UW and modified UW (without hydroxyethyl starch and inverse K+/Na+ concentration)), (3) a dynamic or static method of pancreas digestion, and (4) the endotoxin content and enzymatic activity from five different batches of Liberase PI was studied. In addition, pancreatic biopsies (n = 18), performed before isolation, were retrospectively analyzed to study the impact of histomorphometry on porcine islet yield. Finally, two diabetic cynomolgus monkeys were transplanted without immunosuppression with 15 000 pig islet equivalents/kg body weight of recipient to assess in vivo the function of freshly isolated islets. Univariate and multivariate analyses were performed. Results: By multiple linear regression, the most significant variables that significantly improved islet yield were, firstly, the presence of < 30 EU (endotoxin units) of endotoxin in Liberase batches, followed by a WIT under 10 min and the use of blood exsanguination before pancreas harvesting (P < 0.005). In contrast, isolation method (dynamic vs. static) and the solution used for storage (short-term) (UW vs. modified UW) did not significantly influence islet yield. The correlation of retrospective histomorphometry analysis of native pancreas and extemporaneous biopsy before isolation clearly determined a positive relationship between isolated islet number and the number of islets/cm(2) (r = 0.708, P < 0.01) or with the percentage of large islets (r = 0.680, P < 0.01) found in pancreas biopsies. Pig pancreases containing more than 82 islets/cm(2) and more than 42% of large islets (> 100 mu m) thus enabled more than 120 000 islet equivalents to be obtained in 90% of the cases, which is an ideal amount of islets to transplant into a primate of 4 to 5 kg. In vivo, a reduction of blood glucose (< 200 mg/dl), associated with porcine C-peptide production, was observed in two primates after transplantation with adult pig islets. At day 7 post-transplantation, however, loss of islet function was associated with graft destruction and immune reaction. Conclusions: Morphological screening of the pig pancreas before isolation, optimal blood exsanguination, WIT < 10 min, and an endotoxin content < 30 EU/mg in Liberase PI batches determine successful pig islet isolation for xenotransplantation in primates.