Correction of acute liver cell failure disorders through liver xenoperfusion: experimental study

Experimental hepatocyte xenotransplantation--a comprehensive review of the literature

Hepatocyte transplantation (Tx) is a potential therapy for certain diseases of the liver, including hepatic failure. However, there is a limited supply of human livers as a source of cells and, after isolation, human hepatocytes can be difficult to expand in culture, limiting the number available for Tx. Hepatocytes from other species, for example, the pig, have therefore emerged as a potential alternative source. We searched the literature through the end of 2014 to assess the current status of experimental research into hepatocyte xenoTx. The literature search identified 51 reports of in vivo cross-species Tx of hepatocytes in a variety of experimental models. Most studies investigated the Tx of human (n = 23) or pig (n = 19) hepatocytes. No studies explored hepatocytes from genetically engineered pigs. The spleen was the most common site of Tx (n = 23), followed by the liver (through the portal vein [n = 6]) and peritoneal cavity (n = 19). In 47 studies (92%), there was evidence of hepatocyte engraftment and function across a species barrier. The data provided by this literature search strengthen the hypothesis that xenoTx of hepatocytes is feasible and potentially successful as a clinical therapy for certain liver diseases, including hepatic failure. By excluding vascular structures, hepatocytes isolated from genetically engineered pig livers may address some of the immunological problems of xenoTx.

Hepatic function after genetically engineered pig liver transplantation in baboons

BACKGROUND

If "bridging" to allo-transplantation (Tx) is to be achieved by a pig liver xenograft, adequate hepatic function needs to be assured.

METHODS

We have studied hepatic function in baboons after Tx of livers from alpha1,3-galactosyltransferase gene-knockout (GTKO, n=1) or GTKO pigs transgenic for CD46 (GTKO/CD46, n=5). Monitoring was by liver function tests and coagulation parameters. Pig-specific proteins in the baboon serum/plasma were identified by Western blot. In four baboons, coagulation factors were measured. The results were compared with values from healthy humans, baboons, and pigs.

RESULTS

Recipient baboons died or were euthanized after 4 to 7 days after internal bleeding associated with profound thrombocytopenia. However, parameters of liver function, including coagulation, remained in the near-normal range, except for some cholestasis. Western blot demonstrated that pig proteins (albumin, fibrinogen, haptoglobin, and plasminogen) were produced by the liver from day 1. Production of several pig coagulation factors was confirmed.

CONCLUSIONS

After the Tx of genetically engineered pig livers into baboons (1) many parameters of hepatic function, including coagulation, were normal or near normal; (2) there was evidence for production of pig proteins, including coagulation factors; and (3) these appeared to function adequately in baboons although interspecies compatibility of such proteins remains to be confirmed.

Selected physiologic compatibilities and incompatibilities between human and porcine organ systems

The shortage of donor organs is a major barrier to clinical organ transplantation. Although xenotransplantation is considered one of the alternatives to human organ transplantation, there are immunologic and physiologic incompatibilities between humans and pigs. With the exception of coagulation, the major potential physiologic incompatibilities relating to function of the kidney, heart, liver, lungs, pancreatic islets, and hormones are reviewed. Some of these physiologic differences can be overcome by producing genetically altered pigs to improve compatibility with humans. The possibility of producing such pigs for organ transplantation is considered.

Extracorporeal liver support: waiting for the deciding vote

Because acute liver cell failure is associated with an exceedingly high mortality, liver support has been proposed since the 1950s to improve patient outcome. Early devices, including hemodialysis, hemofiltration, exchange transfusion, plasmapheresis, hemoperfusion, plasma and cross-hemodialysis or cross-circulation, appeared inefficient. Meanwhile, documented results of extracorporeal liver perfusion (ECLP) suggested its superiority over conventional treatment. These devices were abandoned with the development of liver transplantation (LT), which allowed a better outcome and longer survival rate. In the present day, the fact that patients die while waiting for LT because of organ shortage led to a renewed interest in liver support as bridge to LT or regeneration. These devices can be classified according to the presence or lack of hepatocytes, whereas biologic devices refers to the presence of cells or other organic and biochemical component. The absence of individual success of early models led to the development of combined hepatocyte free devices, or artificial liver, which are based upon the hemodiabsorption principle (Biologic-DT) or on the "albumin bound toxin hypothesis" (Molecular Adsorbents Recirculating System) with encouraging results. Meanwhile, hepatocyte based bioartificial liver devices (BLD) were conceived for a global "metabolic support." BLD were developed with the use of human hepatoma cell line (C3A) or primary or cryopreserved porcine hepatocytes. Preliminary experience gave promising results bridging patients to LT. Based upon the same principle of global hepatocyte metabolic support, ECLP regained interest, particularly with the development of transgenic pigs. Several concerns were raised about these devices. Artificial livers lacked any metabolic synthetic activity, the use of human liver for ECLP seems hardly acceptable because of organ shortage, and the accepted use of borderline livers for transplantation is pending trials for the use of xenogenic livers. For BLD, the concerns were the low hepatocyte mass, the absence of accessory liver cells, and the potential risk of seeding tumor cells into patient with the use of human hepatoma cell line. The use of porcine hepatocytes (BLD or ECLP) raised physiologic and immunologic concerns and particularly the fear of a possible transfer of porcine viral material. Although recent studies clearly demonstrate clinical improvement of patients with the use of recently developed liver support devices, most of reported prospective, controlled, or randomized trials had a small number of patients. To give the deciding vote and avoid previous pitfalls, trials need to be developed with a larger number of patients based upon statistically significant models with the following characteristics: 1) comprehensive understanding of the acute liver cell failure mechanisms, 2) world wide classification of conditions that require liver support, and 3) a clear definition of treatment success pending patients to LT or recovery without transplantation. There has not yet been conclusive evidence to support the benefits of extracorporeal liver support. We are still waiting for the deciding vote.