Isolated islets of Langerhans trigger an instant blood mediated inflammatory reaction : a finding with implications for intraportal islet transplantation

Abstract

Islet transplantation offers a logical means to treat insulin-dependent diabetes. However, for reasons poorly understood, the clinical results with islet transplantation have been vastly inferior to those obtained with whole organ pancreas transplantation. If islet transplantation is to be widely applied, the supply of human islets will not suffice. Pig islets could then serve as an alternative. The conventional technique for transplanting isolated islets is by intraportal injection, with the islets being trapped in the sinusoids of the liver. The aim of this thesis was to investigate whether isolated islets of Langerhans when exposed to blood in the portal vein, could become damaged, and if this is the case, to elucidate the cause of this damage.

The studies revealed that isolated human islets when exposed to human blood in vitro (allogenic system), trigged and "instant blood mediated inflammatory reaction", IBMIR, characterised by platelet consumption, and activation of the coagulation and complement systems. The islets became surrounded by clots and infiltrated with leukocytes, and there was evidence of islet injury as reflected in insulin dumping. When heparin and a complement inhibitor (sCR1), was added to the system, IBM IR was suppressed and islet damage was reduced. When pig islets were injected into the portal vein of pigs (allogenic system) it was found that the islets became surrounded by clots, this corroborating the in vitro findings. When porcine islets were exposed to human blood in vitro (xenogeneic system), IBMIR was also induced and there was severe islet injury reflected in massive dumping of porcine insulin. Histological examination revealed disrupted islet integrity. The addition of sCR 1 to the system, significantly reduced the islet injury. Porcine islets were then injected intraportally into Cynoinolgus monkeys (xenogeneic system) and again IBM IR, and severe islet injury was induced. Pre-treatment of the monkeys with sCR I caused a significant reduction in islet injury. IBM IR shares several characteristics with hyperacute vascular xenorcjection. However, IBM IR seems to be initiated by activated platelets, while HAR is initiated by an antibody -antigen reaction.

Using flow cytometry and immunohistochemical staining, it was found that both human and porcine islet cells expressed complement regulatory proteins. However, porcine islet cells were readily lysed when exposed to fresh human sera, this confirming that the complement regulatory proteins are species restricted. Moreover, islet cells obtained from hDAF transgenic pigs were less susceptible to human complement mediated cell lysis albeit the hDAF expression was low. Studies of adult and fetal porcine islets revealed that adult porcine islets express little or no Gal[alpha](1,3)Gal while fetal islets do express the epitope. Despite this difference both adult and fetal islet cells were susceptible to lysis by fresh human sera. Thus, IBMIR inflicts a significant damage to both human and porcine islets exposed to human blood and IBMIR will also, most likely, enhance a subsequent specific, cell mediated, rejection. Platelet and complement activation seem to be most important factors in the pathogenesis of IBMIR. Our findings indicate that reducing IBMIR will be of major importance for the outcome of intraportal islet transplantation.