Transplanted hepatocytes: Wiped out or washed out?
Authors: Malcolm R. Alison, Fabio Marongiu, Ezio Laconi
Journal: Journal of Hepatology (2011)

Hope for the long-term effectiveness of hepatocyte transplantation (HTx) for metabolic liver disease in pediatric patients is based on the premise that hepatocytes in the main constitute a static population, quite unlike for example, the small bowel where there is constant turnover. However, several recent studies on the dynamics of hepatocyte turnover raise important questions about the long-term success of HTx. For example, a recent lineage tracing study in the mouse has demonstrated that, within a year, almost the total parenchymal population is replaced by new hepatocytes differentiating from Sox9-expressing interlobular bile ducts [1Furuyama K. Kawaguchi Y. Akiyama H. Horiguchi M. Kodama S. Kuhara T. et al.Continuous cell supply from a Sox9-expressing progenitor zone in adult liver, exocrine pancreas and intestine.Nat Genet. 2011; 43: 34-41Crossref PubMed Scopus (655) Google Scholar, 2Alison M.R. Lin W.R. Hepatocyte turnover and regeneration: virtually a virtuoso performance.Hepatology. 2011; 53: 1393-1396Crossref PubMed Scopus (17) Google Scholar], suggesting that transplanted hepatocytes will inevitably be ‘washed out’ of the liver within the turnover time – in about 1 year in the mouse. Other murine studies have pointed to a small sub-population of biliary duct cells, also being Sox9-positive, as being bipotential, driving progenitor cell-mediated liver regeneration [[3]Dorrell C. Erker L. Schug J. Kopp J.L. Canaday P.S. Fox A.J. et al.Prospective isolation of a bipotential clonogenic liver progenitor cell in adult mice.Genes Dev. 2011; 25: 1193-1203Crossref PubMed Scopus (194) Google Scholar], seemingly giving rise to Foxl1-expressing progenitor cells, also with bipotentiality, but with limited proliferation potential [[4]Shin S. Walton G. Aoki R. Brondell K. Schug J. Fox A. et al.Foxl1-Cre-marked adult hepatic progenitors have clonogenic and bilineage differentiation potential.Genes Dev. 2011; 25: 1185-1192Crossref PubMed Scopus (127) Google Scholar]. A further murine study has shown that 0.076% of all albumin-expressing hepatocytes are born from albumin-naïve cells every 4 days in normal healthy mice, again highlighting the dynamic cell state of the normal liver [[5]Iverson S.V. Comstock K.M. Kundert J.A. Schmidt E.E. Contributions of new hepatocyte lineages to liver growth, maintenance, and regeneration in mice.Hepatology. 2011; 54: 655-663Crossref PubMed Scopus (33) Google Scholar]. These murine studies have re-awakened the discussion as to whether hepatocytes ‘stream’ from portal tracts to hepatic veins, first described for the rat liver [[6]Zajicek G. Oren R. Weinreb Jr., M. The streaming liver.Liver. 1985; 5: 293-300Crossref PubMed Scopus (224) Google Scholar]; subsequently with the liver being formally proposed as a ‘stem cell and lineage system’, hierarchically organized into stem, amplifying and differentiating compartments [[7]Sigal S.H. Brill S. Fiorino A.S. Reid L.M. The liver as a stem cell and lineage system.Am J Physiol. 1992; 263: G139-G148PubMed Google Scholar]. Very recently, these cell maturational stages have been characterized in great detail in terms of the likes of morphology, ploidy, proliferation, biochemistry and gene expression [[8]Turner R. Lozoya O. Wang Y. Cardinale V. Gaudio E. Alpini G. et al.Human hepatic stem cell and maturational liver lineage biology.Hepatology. 2011; 53: 1035-1045Crossref PubMed Scopus (239) Google Scholar]. Moreover, our own recent studies in the human liver, demonstrating a portal tract to hepatic vein orientation of groups of clonally-derived hepatocytes, are in line with the concept of a dynamic lineage system [[9]Fellous T.G. Islam S. Tadrous P.J. Elia G. Kocher H.M. Bhattacharya S. et al.Locating the stem cell niche and tracing hepatocyte lineages in human liver.Hepatology. 2009; 49: 1655-1663Crossref PubMed Scopus (127) Google Scholar]. On the other hand, not all studies have concurred with this concept, for example Bralet and colleagues [[10]Bralet M.P. Branchereau S. Brechot C. Ferry N. Cell lineage study in the liver using retroviral mediated gene transfer. Evidence against the streaming of hepatocytes in normal liver.Am J Pathol. 1994; 144: 896-905PubMed Google Scholar] genetically labeled rat hepatocytes in vivo at 24 h after partial hepatectomy, but failed to observe any change in their location (periportal and mid-zonal) over the proceeding 15 months – observations not consistent with a ‘streaming’ liver. There are recent examples of the fairly long-term beneficial effects of HTx and survival of engrafted cells, for example, a Crigler-Najjar patient survived well for 4 years after HTx before an OLT and he still had conjugated bilirubin in his blood at 3.5 years [[11]Fisher R.A. Strom S.C. Human hepatocyte transplantation: worldwide results.Transplantation. 2006; 82: 441-449Crossref PubMed Scopus (372) Google Scholar]. In 2006, Sokal’s group performed a HTx for the correction of argininosuccinate lyase deficiency, and cell tracking confirmed their durable presence (12.5%) in the liver at 7 months after the last infusion [[12]Stephenne X. Najimi M. Sibille C. Nassogne M.-C. Smets F. Sokal E.M. Sustained engraftment and tissue enzyme activity after liver cell transplantation for argininosuccinate lyase deficiency.Gastroenterology. 2006; 130: 1317-1323Abstract Full Text Full Text PDF PubMed Scopus (172) Google Scholar]. More recent information on that same case confirmed that the patient was still doing well at up to 18 months when she received an OLT [[13]Fitzpatrick E. Mitry R.R. Dhawan A. Human hepatocyte transplantation: state of the art.J Intern Med. 2009; 266: 339-357Crossref PubMed Scopus (128) Google Scholar]. On the other hand, the majority of children undergoing HTx for urea cycle disorders have only been monitored for a relatively short time before OLT, though one 3-year-old patient with citrullinemia was still doing well 30 months after HTx [[14]Meyburg J. Hoffmann G.F. Liver, liver cell and stem cell transplantation for the treatment of urea cycle defects.Mol Genet Metab. 2010; 100: S77-S83Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar]. Many other cases of HTx have also only provided short-term benefit including glycogen storage disease type I [[15]Muraca M. Burlina A.B. Liver and liver cell transplantation for glycogen storage disease type IA.Acta Gastroenterol Belg. 2005; 68: 469-472PubMed Google Scholar], and factor VII deficiency [[16]Dhawan A. Mitry R.R. Hughes R.D. Lehec S. Terry C. Bansal S. et al.Hepatocyte transplantation for inherited factor VII deficiency.Transplantation. 2004; 78: 1812-1814Crossref PubMed Scopus (223) Google Scholar]. In most studies, the absence of a sustained benefit of HTx in the medium to long-term has been ascribed to rejection (‘wipe-out’) or other causes not directly related to HTx itself (e.g. infections), but we would like to suggest that hepatocyte egress (‘wash-out’) could be an alternative, but non-exclusive explanation. We believe it is beholden upon hepatologists to once and for all establish the cell replacement dynamics of the liver, preferably in a large animal model. If the ‘streaming liver’ hypothesis wins the day then attempts at the correction of metabolic liver disease should be directed towards targeting cholangiocytes and/or other hepatocyte progenitors or only transplanting hepatocytes into extrahepatic sites. The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.