Neonatal Porcine Islets Exhibit Natural Resistance to Hypoxia-Induced Apoptosis

Abstract

Despite the success of the Edmonton protocol for human islet transplantation, an alternate source of islet tissue must be developed if beta-cell replacement therapy is to see widespread application. Neonatal porcine islets (NPI) represent one potential source of tissue. When human or rodent islets are transplanted, the majority of cells undergo hypoxia-induce apoptosis soon after the grafts are placed in the recipient. In the present study, we investigated whether NPI were similarly sensitive to hypoxia. NPI were exposed to hypoxia and hypoxia/reoxygenation using an in vitro hypoxic chamber. Afterwards, viability, frequency of apoptosis, and beta-cell function were evaluated. NPI and adult porcine islets were transplanted into chemically diabetic, immunodeficient mice and graft apoptosis was assessed 24 hours and seven days posttransplant. NPI demonstrated a remarkable capacity to resist apoptosis and maintain insulin secretion despite severe stresses such as hypoxia/reoxygenation. One day after transplantation, NPI grafts showed limited apoptosis, confined to rare strongly insulin positive cells. In contrast, adult porcine islet grafts underwent widespread apoptosis. Western blotting revealed that NPI express high levels of at least one potent endogenous antiapoptotic protein (XIAP). The majority of cells within transplanted human islets undergo apoptosis soon after portal infusion. In contrast, NPI have the capacity to resist this early posttransplant apoptosis, with likely reduced antigen release and diminished immune stimulation. NPI appear to contain a population of insulin-low to insulin-negative pre-beta-cells, which are resistant to hypoxia-induced apoptosis and still capable of differentiating into mature beta-cells.