Current Situation

Rejection: Immunology of Organ Xenografting

2. There are four immunological barriers that must be overcome for achieving successful organ xenotransplantation from pig to primate (human and non-human). First, hyperacute rejection, which is caused by xenoreactive natural antibodies and complement of the recipient acting against endothelial cells of the source animal organ. Second, acute vascular rejection caused by the combined effect of elicited xenoreactive antibodies and activated host natural killer cells and monocytes. In combination these stimuli (the anti-graft antibodies and the activated host cells) result in activation of the endothelial cells of the source organ. Endothelial cell activation leads to general inflammation with resultant thrombosis (platelet aggregation and activation of the coagulation cascade) resulting in organ rejection. Third, the xenograft counterpart of classical T cell mediated rejection of allografts (transplantation between individuals of the same species) will almost certainly occur. Finally, xenografts may also be subject to chronic rejection in a manner analogous to allografts.

Hyperacute Rejection. Recipient xenoreactive natural antibodies and complement are the two major factors that result in hyperacute rejection of an immediately-vascularized organ. Pre-existing xenoreactive natural antibodies bind with vascular endothelial cells of the pig organ.(¹³) These antibodies are directed primarily towards a sugar moiety, the Gal-a (1,3)-Gal-ß (1,4)-GlcNac antigens of the pig, also known as "a-gal".(¹⁴) The bound antibodies fix and activate complement, with the combination of antibodies and activated complement leading to endothelial activation which result in thrombosis, rapid graft ischemia and rejection. Elimination of xenoreactive natural antibodies provides one method to overcome hyperacute rejection.(¹⁵) Hyperacute rejection has also been overcome by methods that inhibit complement.(¹⁶)

Among the different approaches for achieving inhibition of complement, the one that has proven most effective is based on in vitro experiments in which a human protein that inhibits human complement activation is introduced into the membrane of pig endothelial cells. The molecule first tested was human Decay Accelerating Factor, or hDAF. The presence of hDAF in the pig endothelial cells prevented lysis of those cells and would thus, presumably, prevent the activation of the cells.(¹⁷) These findings suggested that the production of transgenic pigs expressing hDAF might provide an approach for overcoming hyperacute rejection of pig organs transplanted into primates.

Certain research groups have produced such transgenic pigs and have demonstrated that organs from these pigs usually do not undergo hyperacute rejection.(¹⁸) Based on these results with transgenic hDAF-expressing pigs, it appears that hyperacute rejection can be overcome, which is the first major triumph of gene therapy in the field of organ transplantation.

Another possible solution to hyperacute rejection is to eliminate, or greatly reduce the expression of "a-gal" from pigs by knocking out the 1,3 galactosyl transferase gene, which is needed for the expression of "a-gal".(¹⁹) This has not yet been accomplished in pigs, although present-day cloning technology could make this possible.

Acute Vascular Rejection. Acute Vascular Rejection is precipitated by elicited xenoreactive antibodies and by the possible infiltration of host inflammatory cells, monocytes and natural killer cells, that invade the xenograft.(²⁰) Endothelial cells are activated resulting in thrombosis, compromised blood flow and rejection.(²¹) Acute vascular rejection now represents the principle immunological barrier to successful xenotransplantation. Studies of acute vascular rejection in animals has shown that the use of immunosuppression leads to organ survival for a far greater length of time than is seen in untreated cases.(²²) An alternative approach for overcoming acute vascular rejection is further genetic engineering animals/organs.(²³) A number of genes that may suppress the inflammatory response that appears to cause acute vascular rejection are now being studied.

T Cell Response. If acute vascular rejection can be overcome, it is expected that there will be a xenograft counterpart of the allogeneic T cell rejection response.(²⁴) There are disagreements whether the xenogenic T cell response will be more difficult to overcome than the allogeneic one, which today is easily controlled. In addition to the use of immunosuppression, there is the possibility that in pig-to-primate transplants we might achieve tolerance (non-reactivity of the immune system of the recipient to pig antigens without immunosuppression).(²⁵) Such tolerance is the hope of transplantation in general and may be aided in the xenogenic arena by further genetic engineering of the source animal.

Chronic Xenograft Rejection. There is evidence that - as with allotransplants, - even when a transplant survives all the above rejection phases, there is the possibility that it will be rejected months or years later.(²⁶) This is referred to as "chronic" rejection. The main pathology of chronic graft failure involves smooth muscle cell proliferation and obliteration of the lumens of blood vessels.