Quality criteria for porcine islet cells suitable for xenotransplantation

Abstract

Introduction: Intact functional pancreatic islets can be successfully isolated from various domestic pig breeds, including some minipig strains. Nevertheless, the technically demanding pig islet isolation procedure needs further improvements. Few efforts have been made to determine the underlying mechanisms for the greatly varying islet yields in many domestic pig breeds. This problem also relates to Landrace Pigs that are often used as donor animals to insert different human genes into the pig genome in order to overcome the various immunological reactions in response to transplantation of a porcine xenograft.Methods: To obtain a clearer picture on the efficiency of islet yields, we evaluated the number of islets, their morphology and intensity of insulin staining in pancreata of various brain‐dead pigs of crossbred and pure bred strains. We performed histochemistry with frozen tissue sections and anti‐pig‐insulin antibody and applied an evaluation procedure, based upon >1.600 microscopically screened pancreata, >800 isolations, and sub‐sequent functional in vivo and in vitro assays.Results: In first series 40 pancreata (tail end of the splenic lobe) of both sexes of the following donor animals were investigated: Piétrain [PI] × German Landrace Pig [GL] (n = 5), German Pure Bred Pig [GPB] x GL (n = 3), PI x GL/GPB (n = 9), GL (n = 12), GPB (n = 3), PI (n = 7), and Duroc (n = 1). Pigs were 6‐7 months old, and were fed and kept under identical conditions. Only three of 40 pancreata (7.5%) displayed islets of sufficient quality for subsequent isolation [data not shown], i.e. sufficient numbers of mostly rounded islets of ≥200 μm in diameter, displaying intensive and evenly distributed insulin staining. Each of the 3 “good” donor organs belonged to a different pig breed. This test was repeated a second time, using 50 pancreata from brain‐dead pigs (both sexes) collected in a Bavarian pig breeding institution: PI × GL (n = 16), PI × GL/GPB (n = 16), GL × GPB (n = 5), GL (n = 12), GPB (n = 2), PI (n = 1). Age, keeping and feeding conditions were identical with conditions of the first trial. Overall, four of 50 pancreata (8%) were determined to be suitable for subsequent islet isolation [not shown here], according to our microscopic evaluation criteria. Again, those four “good” pancreata belonged to four different pig breeds, i.e., a correlation between a particular pig breed and “good” islets could not be detected. In both tests, pancreata that displayed islets with large insulin‐free areas, pancreata with insufficient islet numbers, and islets that displayed very weak insulin staining, as compared to positive control islets, were determined as being unsuitable for islet isolation.Conclusions: Only 7.5–8% of domestic pig pancreata (n = 90) appear to be suitable for islet isolation; in previous years this number was 35–50%. The reasons for this continuous decline in suitable donor organs are presently unknown. None of the crossbreeds and pure breeds tested here displayed a genetic background that correlated with either “good” or “bad“ islets; sufficient animal numbers from pure bred pig strains could clarify this important point. A high versus low caloric diet could help to elucidate the possible influence of nutrients on islet development. From preliminary findings, we hypothesise that a certain amount of carbohydrate feeding may be beneficial in stimulating islet morphology and also islet function.