Xenotransplantation of the liver

Appropriate donor size for porcine liver xenotransplant

OBJECTIVES

Owing to an imbalance between demand and supply, which is more prominent in pediatric transplant, every year more patients lose their lives on waiting lists. In addition to the use of deceased-donor split and living-donor organs, xenotransplant could provide a solution if associated problems, such as immunologic and physiologic ones, are solved. This study sought to analyze the surgical aspects for liver xenotransplant in a porcine model.

MATERIALS AND METHODS

Landrace pigs (n=22, 23 to 37 kg) underwent a laparotomy under general anesthesia. The hepatic hilum was prepared and the common bile ducts, common hepatic artery, portal vein, supra- and infrahepatic inferior vena cava were identified. The length and diameter of each vessel and bile duct and the weight of the liver were measured.

RESULTS

Pearson tests showed a clear correlation between the increase of the pigs' weight and the livers' weight, and the length of the vessels and the bile ducts. We did not find a clear correlation between the increase of the pigs' liver weight and the diameters of the vessels and the bile duct.

CONCLUSIONS

As the first reporting, this study on xenotransplants from the surgical point of view, we postulate that it could be possible to estimate the size of the liver and the proper length of its vessels and bile duct by weighing only the pigs. It was not feasible to match the diameter of mentioned structures by the livers' weight. However, the weight of pig's liver as well as vascular anatomy of pigs appeared to be suitable alternative for the human liver.

Xenotransplantation of solid organs in the pig-to-primate model

Xenotransplantation using pig organs could solve the significant increasing shortage of donor organs for allotransplantation. In the last two decades, major progress has been made in understanding the xenoimmunobiology of pig-to-nonhuman primate transplantation, and today we are close to clinical trials. The ability to genetically engineer pigs, such as human decay-accelerating factor (hDAF), CD46 (membrane cofactor protein), or alpha1,3-galactosyltransferase gene-knockout (GT-KO), has been a significant step toward the clinical application of xenotransplantation. Using GT-KO pigs and novel immunosuppressant agents, 2 to 6 months' survival of heterotopic heart xenotransplants has been achieved. In life-supporting kidney xenotransplantation, promising survival of close to 3 months has been achieved. However, liver and lung xenotransplantations do not have such encouraging survival as kidney and heart xenotransplantation. Although the introduction of hDAF and GT-KO pigs largely overcame hyperacute rejection, acute humoral xenograft rejection (AHXR) remains a challenge to be overcome if survival is to be increased. In several studies, when classical AHXR was prevented, thrombotic microangiopathy and coagulation dysregulation became more obvious, which make them another hurdle to be overcome. The initiating cause of failure of pig cardiac and renal xenografts may be antibody-mediated injury to the endothelium, leading to the development of microvascular thrombosis. Potential contributing factors toward the development of the thrombotic microangiopathy include: 1) the presence of preformed anti-non-Gal antibodies, 2) the development of very low levels of elicited antibodies to non-Gal antigens, 3) natural killer cell or macrophage activity, and 4) inherent coagulation dysregulation between pigs and primates. The breeding of pigs transgenic for an 'anticoagulant' or 'anti-thrombotic' gene, such as human tissue factor pathway inhibitor, hirudin, or CD39, or lacking the gene for the prothrombinase, fibrinogen-like protein-2, is anticipated to inhibit the change in the endothelium to a procoagulant state that takes place in the pig organ after transplantation. A further limitation for organ xenotransplantation is the potential for cross-species infection. As far as exogenous viruses are concerned, porcine cytomegalovirus has been detected in the tissues of recipient non-human primates, although no invasive disease was reported. Until today, no formal evidence has been presented from in vivo studies in non-human primates or from humans exposed to pig organs, tissues, or cells that porcine endogenous retroviruses infect primate cells. Xenotransplantation is a potential answer to the current organ shortage. Its future depends on; 1) further genetic modification of pigs, 2) the introduction of novel immunosuppressive agents that target the innate immune system and plasma cells, and 3) the development of clinically-applicable methods to induce donor-specific tolerance.

Liver xenografts for the treatment of acute liver failure: clinical and experimental experience and remaining immunologic barriers

A critical element restricting the application of liver transplantation is the shortage of human deceased donor organs. Xenotransplantation using pig organs might be a solution to this shortage. Although the problems that still require resolution include the immunologic barrier, the potential risk of transferring infectious agents with the transplanted organ, and uncertainty about whether the transplanted organ will function satisfactorily in the human environment, recent progress in the genetic manipulation of pigs has led to the prospect that clinical xenografting, at least as a bridge to allotransplantation, may be possible in the foreseeable future. Experience with clinical auxiliary and orthotopic liver xenotransplantation and experimental liver xenotransplantation in nonhuman primate and other large animal models is reviewed, and the remaining immunologic problems are discussed. Evidence suggests that, in patients with hepatic failure, the pig liver may be less susceptible to antibody-mediated injury than other pig organs, such as the heart or kidney. Pig Kupffer cells and other macrophages will recognize and phagocytose primate red blood cells, but this problem should be overcome by pretransplant depletion of macrophages from the organ-source pig. From the evidence currently available, it does not seem unduly optimistic to anticipate that a liver from an alpha1,3-galactosyltransferase gene-knockout pig would survive at least long enough to function as a successful bridge to allotransplantation.

Toward the survival and function of xenogeneic hepatocyte grafts

Xenogeneic hepatocyte transplantation might offer an unobtrusive alternative to whole liver allotransplantation. Having previously found that the immune response to such grafts can be controlled by immunosuppression, we sought approaches to collection and delivery that would optimize survival and function after transplantation. Porcine hepatocytes were isolated by a 2-step collagenase technique and then: 1) used immediately; 2) stored in University of Wisconsin (UW) solution at 4 degrees C; 3) cultured in supplemented Williams E medium; or 4) cryopreserved in UW solution with 10% dimethyl sulfoxide (DMSO). The fate and function of the hepatocytes was determined after they were injected into the spleens of immunodeficient mice. Freshly isolated hepatocytes had better viability (92.2 +/- 1.9%) than hepatocytes cultured for 24 hours (78.4 +/- 6.3%), hypothermically preserved in UW solution for 24 hours (85.8 +/- 3.1%), or cryopreserved (65.0 +/- 2.6%). Freshly isolated hepatocytes secreted more albumin after transplantation than hepatocytes that were cultured, hypothermically stored, or cryopreserved. In conclusion, culture and storage profoundly compromises the function of isolated hepatocytes after transplantation. Freshly isolated hepatocytes are the preferred source for transplantation.

Genetic modification of xenografts

For nearly a century, xenotransplantation has been seen as a potential approach to replacing organs and tissues damaged by disease. Until recently, however, the application of xenotransplantation has seemed only a remote possibility. What has changed this perspective is the advent of genetic engineering of large animals; that is, the ability to add genes to and remove genes from lines of animals that could provide an enduring source of tissues and organs for clinical application. Genetic engineering could address the immunologic, physiologic and infectious barriers to xenotransplantation, and could allow xenotransplantation to provide a source of cells with defined and even controlled expression of exogenous genes. This communication will consider one perspective on the application of genetic engineering in xenotransplantation.

Bridge therapy to liver transplantation in fulminant hepatic failure

The management of patients with fulminant hepatic failure is a major clinical endeavor. Early intensive care at an institution able to perform liver transplantation is essential. It is recognized that therapy focused solely on attempts at preventing/reversing increased intracranial pressure, and the treatment of other failing organs as they occur falls well short of ideal. This review covers the non-biological and biological techniques utilized in efforts to support liver function. The goal is to foster recovery, or to buy enough time for successful liver transplantation. Prospective, controlled trials are beginning to acknowledge subgroups of fulminant hepatic failure and properly randomize therapy. Our understanding of the essential elements of liver support is improving, but no single device has yet proved indispensable.