Angiostatic factors normally restrict islet endothelial cell proliferation and migration: implications for islet transplantation

New blood vessel formation in transplanted islets occurs within 7–14 days post-transplantation through both the expansion of donor islet endothelium and ingrowth of blood vessels from the implantation organ. However, several studies indicate that although the islets attract recipient blood vessels, the formed intra-islet vascular network is insufficient, which affects islet post-transplant function. This study aimed to develop an in vitro model to investigate the migration and proliferation properties of isolated liver and islet endothelium. Rat islet or liver endothelium was purified using Bandeiraea simplicifolia (BS-1)-coated Dynabeads. The liver endothelium displayed an increased migration and proliferation to islet-conditioned medium. These effects were fully prevented by adding a neutralizing vascular endothelial growth factor (VEGF)-antibody. In contrast, islet-produced VEGF failed to induce islet endothelial cell migration and only had marginal effects on islet endothelial cell proliferation. These properties could, however, be activated through blocking the effects of either endostatin, thrombospondin-1 or α₁-antitrypsin. In conclusion, VEGF may attract recipient blood vessels towards intrahepatically transplanted islets, but intra-islet vascular expansion is hampered by angiostatic factors present within the islets and the islet endothelium. Inhibition of angiostatic factors early after transplantation may provide a strategy to restore the islet vascular network and improve islet graft function.