Antibody Mediated Rejection in Pig-To-Nonhuman Primate Xenotransplantation Models

Antibody-mediated mechanisms are central to the rejection that occurs when pig organs are transplanted into primates. In this article, the histopathological features of the humoral rejection process in these species combinations, namely hyperacute rejection and acute humoral xenograft rejection, will be illustrated. The profile of the natural and elicited antibodies involved will also be discussed. It has now been demonstrated that the natural immune response to a porcine xenograft is primarily directed to Galα1-3Gal (αGal) specificities, whilst the elicited immune response is directed to both αGal and non-αGal antigens. The principal characteristics of anti-αGal, anti-non-aGal and polyreactive antibodies will be described, together with the identification of the molecules recognised by natural and elicited xenoreactive antibodies. The role of the humoral immune response in the rejection of porcine islets in the primate is still uncertain and the current views on the subject will be discussed. Finally, a concise but comprehensive review of the different strategies that have been attempted to prevent the onset of antibody-mediated rejection is presented. These strategies encompass approaches aimed at interfering with the binding of xenoreactive antibodies with their targets, the use of conventional or novel immunosuppressants and splenectomy. It is undeniable that significant progress has been recently achieved in understanding the humoral rejection process of pig organs transplanted into primates. It is expected that a more comprehensive elucidation of the mechanisms underlying accommodation and tolerance may, in the not too distant future, further extend survival of pig organs transplanted into primates.