Within liver transplantation, the decisions surrounding a patient with fulminant hepatic failure are among the most difficult faced by the transplant team. The challenge is the prognostic uncertainty of a patient’s future course. Underutilization of liver transplantation can result in devastating outcomes, whereas overly aggressive use commits patients to a lifetime of immunosuppression and deprives other individuals in chronic liver failure who have no hope of spontaneous recovery. Despite the fact that patients with fulminant hepatic failure are given priority on organ allocation lists, the paucity of available organs can delay transplantation resulting in avoidable mortality.

In an attempt to determine which patients will require a liver transplant, predictive criteria have been analyzed and algorithms developed [9]. Prognosis for recovery from fulminant liver failure depends upon several factors. Etiology is important with spontaneous recovery rates from hepatitis A and acetaminophen toxicity being high [12,13], whereas recovery rates from other types of viral etiologies and drug reactions are lower [14]. A retrospective review of more than 200 patients with acute liver failure who underwent transplantation showed a 5-year survival of 66.9% with a 56.6% 5-year graft survival. Analysis showed that for graft survival, donor age, ethnicity and race, time from onset of jaundice to encephalopathy, need for veno-venous bypass, and intracranial pressure monitoring were important prognostic indicators. For patient survival, patients who were intubated had elevations in preoperative creatinine, bilirubin, and INR, and the time to onset of encephalopathy after jaundice was found to be associated with diminished survival. In general, after transplantation, patients who were sickest before surgical therapy fared the worst, suggesting the benefit of early intervention [9].

Transplantation for acute liver failure is also made more difficult by the potential for a more complicated postoperative course. In patients with acute liver failure, primary nonfunction occurs in up to 16% of transplantations as compared with those cases where transplantation is performed for other indications, thought to be closer to 5% [9]. Therapies that may aid the recovery of an acutely failing liver and improve outcomes or those that could successfully avoid the need for transplantation would be valuable. Stem cells have shown some potential in assisting the regenerating liver in recovery, and further understanding of the mechanism of action may prove useful in designing therapeutics.

The regenerative capacity of the liver is significant and occurs after surgical insult or other inflammatory causes. After surgical resection, the regenerative response in humans is proportional to the amount of liver removed, with liver mass being precisely regulated by physiologic stimuli [15]. After partial hepatectomy, proliferation of the existing mature cellular population, including hepatocytes, biliary epithelial cells, fenestrated epithelial cells, Kupffer cells, and cells of Ito, rebuilds the lost hepatic tissue. In contrast to other regenerating tissues, liver regeneration is not dependent on a small group of progenitor or stem cells [15]. Proliferation begins at periportal areas of the hepatic lobule and proceeds pericentrally within 36 to 48 hours [16]. Gradually, typical hepatic histology is restored [17]. The regenerative capacity of the hepatocyte itself is almost unlimited, with injected hepatocytes able to restore liver mass multiple times over [18].

Similar to other organs, the cellular lineage of the liver consists of stem cells, precursor cells, and mature hepatocytes. Mature cells respond to partial hepatectomy and centrilobular injury for example, as induced by carbon tetrachloride (CCl₄). Ductular progenitor cells, fewer in number, respond to centrilobular injury when the proliferation of hepatocytes is inhibited. In rodents, these cells, termed oval cells, have been thought to exist within the terminal bile ductules, the canals of Hering, and have been termed bipolar because of their ability to differentiate into hepatocytes or ductular epithelium in vitro [19–21]. Examination of the livers of human fetuses has identified a population of CD34⁺ cells, termed side population cells, from which epithelial and hematopoietic cells arise and that can potentially contribute to hepatocyte generation. Side population cells in developing human liver may share a temporal relationship with oval/progenitor cells, responsible for liver regeneration after massive or chronic hepatic injury [22]. Finally, there are also rare cells of exogenous origin, hematopoietic stem cells originating in the bone marrow, supported by data showing that hepatocytes may express genetic markers of donor hematopoietic cells after bone marrow transplantation [2,4].

Much attention has been given to the triggers of regeneration. Hepatocyte growth factor (HGF) and its receptor c-Met are key factors and play an important role in liver growth and function [23]. HGF levels have been shown to increase substantially after a decrease in hepatic mass in humans, leading to changes in gene expression, termed immediate-early genes, within the hepatocyte [24].

In addition, the relationship of the hepatocyte to its surrounding matrix and the action of urokinase on the matrix and HGF are important determinants of regeneration. Other factors, including tumor necrosis factor-alpha and interleukin (IL)-6, are components of the early signaling pathways leading to regeneration, whereas growth factors, including epidermal growth factor, transforming growth factor, fibroblast growth factor, vascular endothelial growth factor, as well as norepinephrine and insulin, all have important effects, with the roles of some, like HGF, being essential, whereas others are facultative [19].

Cells with stem cell properties may appear in large numbers when mature hepatocytes are inhibited from proliferation [15]. Bone marrow-derived hematopoietic stem cells (HSCs) participate in liver injury recovery under strong positive selection pressure when normal mechanisms of regeneration are either blocked or are inadequate [2]. After bone marrow transplantation from male rats into lethally irradiated syngeneic female rats, Petersen and colleagues [2] demonstrated the presence of the sry region of the donor male Y chromosome within the female recipients’ livers 13 days after injury. Similarly, after bone marrow transplant from dipeptidyl peptidase-IV-positive (DDPIV-IV+) male rats into females with a deletion of this enzyme, DDPIV-IV+ hepatocytes were found in the female recipients. An extrahepatic source for the repopulating liver cells was demonstrated when, after whole-liver transplantation of L21-6 antigen negative livers into rats expressing L21-6 antigen, cells expressing the antigen were detected in the ductal structures of the donor liver after injury. These results have been repeated by other investigators. However, in these experiments only a small amount of cells from the recipient populate the liver, with 2% of mouse hepatocytes of recipient origin and 4% to 40% of hepatocytes and cholangioles described as a recipient phenotype in human recipients of male bone marrow [5]. Therefore, repopulation of the regenerating liver with hematopoietic stem cells does occur, but only on a limited basis. It is unclear why the physiologic response to liver injury does not more completely use recruitment of endogenous marrow-derived cells.

The mechanisms by which stem cells exert their beneficial effects on the injured liver are uncertain. Hypotheses about the mechanism by which HSCs contribute to liver regeneration have included direct contribution of the bone marrow-derived stem cells to the recovering hepatocyte population through transdifferentiation into hepatocytes, cell fusion creating hepatocyte cell hybrids, and finally, paracrine effects promoting endogenous processes, in particular by enhancing angiogenesis [3,25,26]. Enhanced angiogenesis is thought to occur when hematopoietic stem cells commit to sinusoidal endothelial cells and consequently play a central role in coordinating angiogenesis [27].

In attempting to elucidate the role of the heterogeneous population of stem cells, multiple cell surface markers have been studied as candidates to uniquely identify the specific subset of stem cells responsible for influencing hepatic regeneration. Classic HSC surface antigens include CD34 and CD133 [28]. However, these cell surface markers represent a heterogeneous population of immature hematopoietic and endothelial cells with a continually and reversibly changing phenotype depending upon the state of activation. CD34 is a glycoprotein involved in cell-cell adhesion interactions that is also expressed on cells in the umbilical cord, mesenchymal stem cells, endothelial progenitor cells, and on mature endothelial cells [29]. Reports that levels of CD34⁺ cells increase in patients after hepatic resection exist [30], although other studies report data that conflicts with this and suggest differences between resection for benign versus malignant disease [31]. Other investigators have supported the hypothesis that mobilization of hematopoietic stem cells occurs after partial hepatectomy or the ischemic insult of orthotopic liver transplantation [32,33]. A highly heterogeneous population of stem cells was mobilized in living liver donors at 12 hours after resection. These cells were found to be rich in CD133 and coexpressed CD45 and CD 14. In addition, a small subset of mobilized cells expressed CD34. CD34⁺ HSCs were also found to be mobilized in patients after liver transplant by Lemoli and colleagues [33], however, they demonstrated that only ischemia/reperfusion injury associated with liver transplant resulted in the mobilization of bone marrow (BM) stem/progenitor cells.

Mobilization of hematopoietic stem cells from the stem cell niche is already currently in use in certain clinical applications. HSCs reside in the marrow within a highly organized microenvironment consisting of marrow stromal cells, osteoblasts, osteoclasts, and their associated extracellular matrix proteins [34]. The in vivo regulatory microenvironment of the HSC has not been extensively explored.

Pharmacologic mobilization of HSCs from the stem cell niche has emerged as the standard of care for patients with hematologic disorders, including a variety of malignant and nonmalignant conditions, of which the most common are multiple myeloma [34,35]