Heat shock preconditioning impairs revascularization of freely transplanted pancreatic islets

Revascularization of freely transplanted pancreatic islets is essential for appropriate graft function and survival. During the first days after transplantation, however, islet transplants are avascular, and successful engraftment is believed to be markedly hampered by hypoxia-induced tissue injury. Because heat shock has been shown to induce cell resistance against hypoxia, it seems reasonable to stress pancreatic islets by heat before transplantation. In contrast, hypoxia is a major stimulus for angiogenesis, and thus heat shock preconditioning-induced resistance against hypoxia may decrease stimulation of angiogenesis. The authors therefore studied in vivo whether heat shock preconditioning of isolated islets affects angiogenesis and revascularization after free transplantation.

After collagenase isolation, heat shock-preconditioned islets (42 degrees C for 30 min) were transplanted syngeneically into nontreated skinfold chambers of Syrian hamsters. In a second group of animals, nontreated islets were transplanted into heat shock-preconditioned chambers. Nontreated islets transplanted into nontreated chambers served as controls. Islet angiogenesis and revascularization were quantitatively analyzed during 14 days after transplantation using intravital fluorescence microscopy. Expression of heat shock proteins (HSP) was confirmed by immunohistochemistry and Western blotting.

Immunohistochemistry revealed expression of HSP32 (heme oxygenase [HO]-1), HSP72, and also intracellular insulin in isolated and transplanted pancreatic islets. Western blot analysis showed enhanced HSP32 but slightly decreased HSP72 expression in heat shock-preconditioned islets when compared with controls. Intravital microscopy revealed appropriate vascularization of control islets within 14 days after transplantation. Heat shock preconditioning of the host tissue (i.e., the skinfold chambers) did not affect islet vascularization when compared with controls. In contrast, heat shock preconditioning of the isolated islets resulted in a significantly (P < 0.05) impaired take rate, a reduced (P < 0.05) size of the newly formed microvascular network, and thus a smaller area (P < 0.05) of microvascularly perfused endocrine tissue.

These data suggest that heat shock preconditioning of isolated pancreatic islets before transplantation impairs the process of graft angiogenesis and revascularization. Therefore, transient exposure of isolated islets to heat may not be considered a promising tool to improve the outcome of islet transplantation.