Human Hepatocyte Transplantation: Gene Therapy and More?

This time is a very good one if we but know what to do with it. —Ralph Waldo EmersonLiver transplantation is an irreversible and complicated intervention that is presently the only effective treatment for acute liver decompensation and life-threatening liver-based metabolic disease. Acute liver failure, however, is potentially reversible and inborn errors of metabolism often occur in the face of an otherwise normal liver. In addition, the expense and limited supply of donor organs is likely to exclude liver transplantation as an option for most potential patients in the foreseeable future. Since 1969, when a method for isolating primary hepatocytes by collagenase perfusion was developed,1numerous studies have documented the efficacy of hepatocyte transplantation in the laboratory for the treatment of these disorders. Although animal studies have set the stage for clinical application of hepatocyte transplantation, evidence for long-term survival and function of transplanted human hepatocytes has been tantalizingly slow to come.In the last few years, a number of investigators have performed hepatocyte transplantation for life-threatening liver failure. Unfortunately, patients with fulminant liver failure have an extremely variable course and can recover spontaneously, so that improvement in survival or successful bridging to whole organ liver transplantation are endpoints that are too ambiguous to convincingly prove the function of transplanted liver cells. In fact, Mito, a leading advocate for hepatocyte transplantation, has concluded that transplanted liver cells are probably not responsible for most of the clinical improvement after transplantation.2 It was with this background that our research group began their examination of hepatocyte transplantation for the treatment of inherited metabolic disease, where reconstitution of the defective metabolic function could be determined unequivocally. The recent report of successful human hepatocyte transplantation with partial correction of hyperbilirubinemia in a patient with Crigler-Najjar syndrome (bilirubin-uridine diphosphoglucuronate glucuronosyltransferase deficiency) we believe opensthe door a little wider for the use of cellular and molecular based therapies for liver disease.3Our patient, a 10-year old girl, required 10 to 12 hours of phototherapy daily to maintain serum unconjugated bilirubin levels at 24 to 27 mg/dL (400–460 μmol/L). Hepatocytes were infused into her portal vein through a percutaneously placed catheter. The child was awake during the cell infusion and was discharged 20 hours after it was completed. After the transplant procedure, the serum bilirubin level decreased to 10.6 to 14 mg/dL (180–240 μmol/L) and has remained at that level for more than 1 year even though she receives less phototherapy. Hepatic bilirubin-uridine diphosphoglucuronate glucuronosyltransferase activity increased from barely detectable (0.4%) to 5.5% of mean normal enzyme activity and >30% of the child's bile pigments are now bilirubin glucuronides.This experience is encouraging and confirms that the theoretical advantages of cell transplantation appear to apply in humans. Hepatocyte transplantation was much less invasive, technically less demanding, and less expensive than whole organ transplantation. Hepatocytes, approximating 5% of the liver mass, were transplanted over a relatively short period of time and produced only a mild and transient increase in portal pressure. The resources necessary to perform the transplant procedure were not excessive because the cells were isolated at a distant site and transported for infusion. The engrafted hepatocytes survived and functioned for more than 1 year and are likely to survive indefinitely. In addition, once engrafted, the consequences of graft loss would have been minimal, the procedure would not have interfered with subsequent liver transplantation should that have become necessary and liver-directed gene therapy is still possible for this patient. The level of immunosuppression was similar to that used for whole organ transplantation; it may be possible to reduce this significantly in future patients because the first patient developed no evidence of graft rejection. Although the serum bilirubin was reduced to safe levels, our patient continues to need phototherapy. It is likely, however, that transplantation of additional hepatocytes will reduce bilirubin levels further and eliminate the need for phototherapy.Evidence that transplanted hepatocytes can engraft and function normally in patients with acute liver failure and/or decompensated cirrhosis is still lacking. We have recently shown that intrasplenic hepatoctye transplantation can improve liver function and prolong the survival of rodents with decompensated end-stage cirrhosis, however, the effect was transient and it is possible that liver failure may produce substances that inhibit normal hepatocyte engraftment and function.4There are many obstacles to overcome before hepatocyte transplantation can be performed more generally. Although the Crigler-Najjar patient was transplanted safely, a number of patients have had significant cardiovascular and pulmonary complications during hepatocyte infusion, indicating that there are still technical issues that need to be addressed. Although the ease of administration is appealing, managing transplanted children will require careful monitoring of engraftment and immunosuppression. If hepatocyte transplantation proves to be effective therapy for patients with liver damage and liver-based metabolic diseases, a shortage of human hepatocytes for transplantation can be anticipated. However, techniques not applicable to whole organs may help resolve the problem of hepatocyte scarcity. Hepatocytes have a tremendous capacity to divide and methods that can induce hepatocyte proliferation in culture or after transplantation are being developed rapidly.5,6 In addition, whole organs must be used immediately or wasted whereas hepatocytes can be cryopreserved7 for storage and transportation, to be used as and when required.We are inching forward in human hepatocyte transplantation. Only a handful of patients can expect to receive direct benefit from the experience of this one patient with Crigler-Najjar syndrome, but an important step has been taken. Definitive cures for metabolic diseases await effective gene therapy, but this may be some way off. With further experience and a better knowledge of hepatocyte antigenicity, we believe that hepatocyte transplantation offers more immediate hope to children with debilitating metabolic disease. It is still too early to know what percentage of the millions of patients with acute and chronic liver disease could ultimately benefit from this form of therapy, but, laboratory studies are likely to help solve any hepatocyte donor shortage should it occur.