Liver and Hepatocyte Xenotransplantation

A severe shortage of human livers for allotransplantation has sparked interest over the years in the potential use of animals in lieu of humans as a source of livers—that is, xenotransplantation. Besides offering a plentiful supply of livers, xenotransplantation might offer a transplant inured to hepatitis C. Xenogeneic livers can also be used for extracorporeal “xenoperfusion,” for devices containing xenogeneic hepatocytes, auxiliary liver transplants, bridge liver transplants, and hepatocyte transplants. The hurdles to xenotransplantation of the liver include the immune response of the recipient against the graft, incompatibility of the graft with complex physiological and biochemical systems of the recipient, and the possibility of transferring infectious agents from the graft to the recipient. Recent progress in characterizing and overcoming these hurdles has encouraged some optimism regarding the ultimate application of xenotransplantation for the treatment of human disease.

Allogeneic organ transplantation is the preferred therapy for patients with irreversible functional failure of such organs as the liver, kidneys, heart, and lungs. However, the supply of human organs and tissues is quite limited, and many die while waiting for an organ to become available. In 2009, 1723 (6.6%) of the 25,912 individuals listed as waiting for a liver transplant died; these numbers include 12% of those with Model for End-Stage Liver Disease (MELD) scores of 21 to 30 and 24% of those with MELD scores over 30.

Those at particular risk for death are young children and adults with acute liver failure who require immediate transplantation. Although the need for liver transplantation will decrease with improvements in the care of patients with liver disease, particularly hepatitis C, a more comprehensive solution to the problem will require the development of artificial organs or the development of another source of organs for transplantation. The use of animals as a source of organs and tissues for transplantation, called xenotransplantation , is thus a way that transplantation can realize its full potential as an approach to human disease. In this chapter we review the various ways that xenogeneic livers and hepatocytes might be applied for the treatment of hepatic disease.

Rationale for Xenotransplantation

The primary rationale for xenotransplantation is to bypass the shortage of human livers and hepatocytes for transplantation. However, xenotransplantation also offers a potential way to avoid recurrence of hepatitis B or hepatitis C in a transplant, because these viruses evidently do not infect animal livers.

Xenogeneic livers might also be used as a source of hepatocytes for treatment of metabolic defects, such as Crigler-Najjar syndrome type 1 and urea cycle disorders. These defects are now treated by orthotopic liver allotransplantation, even though the structure and most functions of the native liver are intact. Instead, these defects might be treated by hepatocyte transplantation, leaving the native liver in place to provide all other functions. Such an approach, using human hepatocytes, has been reported. Because human livers are in short supply, some have advocated use of xenogeneic hepatocytes for this purpose.

Xenotransplantation also offers a means of gene delivery. In animals, such as pigs, but not in humans, genetic material can be introduced into the nucleus of the fertilized egg and become incorporated into the genome. The transgenic animal thus produced might express the gene product, which can be delivered by transplanting xenogeneic cells (or organs). As an extension of this method of gene delivery, targeted changes can be made in the genomic DNA of cultured cells—such changes might include knocking genes out or substituting genes—and the nucleus of cultured cells can be transferred by various means to an enucleated egg. Such a transfer, called cloning , can be used to produce a line of animals with known changes in genetic material, including expression of transgenes. Those cells or organs might then be transplanted as a means of gaining expression of the gene. These technologies are discussed later.

Source of Organs

On first consideration one might think that nonhuman primates such as chimpanzees and baboons would provide the most suitable source of xenografts, because of the genetic proximity to humans. In the early 1960s Reemtsma et al transplanted a series of chimpanzee kidneys into patients with renal failure, and in the early 1990s Starzl et al transplanted baboon livers into two patients with liver failure. The renal and liver xenografts maintained the survival of patients for up to 9 months. However, nonhuman primates have not been used recently as a source of organs for transplantation because they are available in small number, their size is small, and they could transmit lethal infectious agents. Also, current technology for genetic manipulation would be difficult to undertake in primates and might provoke social opposition.

Because of these problems, most investigators today focus on using lower animals, particularly pigs, as a source of organs and tissues for xenotransplantation. Pigs are available in sufficient quantities, and their anatomy and physiology are similar to those of humans. Pigs can also be modified genetically for xenotransplantation. Although the tissue of pigs might transmit some infectious agents, none of these agents poses risks comparable to agents of nonhuman primates.

Applications of Xenotransplantation of the Liver

Xenogeneic livers might be used in various ways to treat diseases of the liver. The potential applications of xenogeneic livers are discussed briefly in the following sections.

Orthotopic Liver Transplantation

Orthotopic allotransplantation of the liver is the most effective treatment for hepatic failure. The indications for orthotopic liver transplantation include acute and chronic liver failure, metabolic liver diseases, and unresectable malignant tumors. Liver allotransplantation is occasionally performed for a variety of metabolic disorders, including Crigler-Najjar syndrome, ornithine transcarbamylase deficiency, and maple syrup urine disease. The indications for orthotopic liver xenotransplantation may be similar to that for allotransplantation. However, additional indications might include temporary transplantation as a bridge for certain patients awaiting a human liver allograft and as a transplant that would avert reinfection by hepatitis viruses.

Five orthotopic liver xenografts have been performed since 1969 ( Table 103-1 ). The chimpanzee-to-human xenografts functioned for only 1 to 9 days, probably limited mainly by the medical and immunosuppressive therapeutics of that era rather than by the biology of the graft. Two patients with hepatic failure caused by hepatitis B, who received liver xenografts from baboons, functioned for 26 days and 70 days without evidence of reinfection by hepatitis virus.

TABLE 103-1

Attempts of Clinical Liver Xenotransplantation

Modified from Taniguchi S, Cooper DKC. Clinical xenotransplantation: a brief review of the world experience. In: Cooper DKC, Kemp E, Platt JL, White DJG, eds. Xenotransplantation: The Transplantation of Organs Between Species . Berlin: Springer; 1997:779.

Year	Donor	Type	Survival (Days)
1966	Chimpanzee	Heterotopic	<1
1966	Chimpanzee	Orthotopic	<2
1969	Chimpanzee	Orthotopic	9
1969	Baboon	Heterotopic	<1
1970	Baboon	Heterotopic	3
1970	Baboon	Heterotopic	<1
1971	Baboon	Heterotopic	<1
1971	Baboon	Heterotopic	3
1974	Chimpanzee	Orthotopic	14
1992	Baboon	Orthotopic	70
1993	Baboon	Orthotopic	26
1993	Pig	Heterotopic	<2

Orthotopic transplantation of swine livers into baboons has been used as a model for pig-to-human xenotransplantation. Ekser et al transplanted 10 porcine livers into baboons treated with a clinically applicable immunosuppressive regimen. Six baboons survived for 4 to 7 days. The porcine livers produced albumin, and the porcine coagulation factors rendered coagulation assays at normal values. However, all of the baboons died from uncontrolled bleeding caused by consumptive coagulopathy.

Auxiliary (or Heterotopic) Liver Transplantation

Auxiliary, or heterotopic, liver transplants are accomplished by inserting the graft beside, or in continuity with, the recipient’s own liver. This operation is usually performed for patients with fulminant or subfulminant hepatic failure but is not routinely performed for metabolic liver diseases because auxiliary liver transplantation in this setting requires banding of the portal vein to prevent shunting of portal blood away from the graft. Auxiliary transplantation leaves the native liver intact in case of graft failure or spontaneous recovery or for future gene therapy. Auxiliary allografts are not often performed in part because human livers are scarce. Xenogeneic livers might be used instead for this reason. Seven auxiliary liver xenotransplants have been performed since 1969 (see Table 103-1 ). The first were performed by Starzl et al using chimpanzee livers. The first auxiliary liver xenograft was performed in a child with hepatic coma. The graft functioned for 24 hours, improving clearance of bilirubin and alkaline phosphatase. However, the child died of gastrointestinal hemorrhage and liver failure. Makowka et al implanted a porcine liver as an auxiliary graft into a patient with fulminant decompensation of autoimmune hepatitis. Although the porcine xenograft functioned, the patient died from neurological complications 34 hours later. These clinical experiences demonstrate that a pig liver can potentially support patients who suffer from acute liver failure. Auxiliary transplantation of the xenogeneic livers might provide an early step in evaluating the feasibility of clinical xenotransplantation.

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