No Evidence for Recipient-Derived Hepatocytes in Serial Biopsies of Sex-Mismatched Liver Transplants

Re-evaluation of hepatocyte replacement by recipient-derived cells after allogenic liver transplantation: Discrepancy between clinical observations and a rat model

AIM

Reports suggest that hepatocyte replacement by recipient-derived cells is an active phenomenon after allogenic liver transplantation in rats. However, this phenomenon is rarely observed in humans, and further evaluation is necessary to bridge the gap between clinical practice and animal experiment.

METHODS

Fifty percent volume of the liver from green fluorescent protein (GFP) transgenic Lewis rats were transplanted into wild-type Dark Agouti (DA) rats, in which GFP negative hepatocytes were considered as host (DA rat)-derived cells. The transplanted liver was observed on whole imaging system and fluorescent microscope 7-10 days after transplantation. As a different method from previous reports, hepatocytes isolated from transplanted livers were cultured, and the expression of GFP was examined.

RESULTS

The sliced liver (2 mm) after allogenic transplantation demonstrated decreased intensity of GFP signals compared with the positive control. The hematoxylin-eosin staining of the section revealed abundant infiltration of inflammatory cells, suggesting an immunological rejection reaction. Large polygonal cells with significantly decreased or negative GFP signals were also demonstrated, which was consistent with the results of previous studies. However, cell culturing demonstrated that none of the examined albumin positive large polygonal cells were host-derived cells. The same results were obtained irrespective of reconstruction of hepatic artery.

CONCLUSION

Our result implies that rejection reaction does not promote parenchymal replacement by recipient-derived cells, in contrast to previous reports. If so, the phenomena occurring in rats are consistent with clinical observation of liver transplantation in humans.

Functional Immune Anatomy of the Liver-As an Allograft

The liver is an immunoregulatory organ in which a tolerogenic microenvironment mitigates the relative "strength" of local immune responses. Paradoxically, necro-inflammatory diseases create the need for most liver transplants. Treatment of hepatitis B virus, hepatitis C virus, and acute T cell-mediated rejection have redirected focus on long-term allograft structural integrity. Understanding of insults should enable decades of morbidity-free survival after liver replacement because of these tolerogenic properties. Studies of long-term survivors show low-grade chronic inflammatory, fibrotic, and microvascular lesions, likely related to some combination of environment insults (i.e. abnormal physiology), donor-specific antibodies, and T cell-mediated immunity. The resultant conundrum is familiar in transplantation: adequate immunosuppression produces chronic toxicities, while lightened immunosuppression leads to sensitization, immunological injury, and structural deterioration. The "balance" is more favorable for liver than other solid organ allografts. This occurs because of unique hepatic immune physiology and provides unintended benefits for allografts by modulating various afferent and efferent limbs of allogenic immune responses. This review is intended to provide a better understanding of liver immune microanatomy and physiology and thereby (a) the potential structural consequences of low-level, including allo-antibody-mediated injury; and (b) how liver allografts modulate immune reactions. Special attention is given to the microvasculature and hepatic mononuclear phagocytic system.

The hematopoietic stem cell number in the peripheral blood of pediatric recipients correlates with the outcome after living donor liver transplantation

It has been proposed that circulating HSCs play a role in graft survival after liver transplantation. The aim was to analyze the relationship between the number of HSCs before and after LDLT and liver function, immune biomarkers, and clinical outcomes in pediatric patients. We studied 15 pairs of adult healthy liver donors and pediatric recipients with ESLD. The CD34/CD45+ cell number was measured in the blood via flow cytometry, and plasma levels of immune biomarkers - via ELISA. CD34/CD45+ cell number in the recipients decreased within the first week after LDLT. The cell number before LDLT was negatively correlated with the plasma levels of CRP and the development of graft dysfunction in the early post-transplant period. After LDLT, the CD34/CD45+ cell number was positively correlated with the pretransplant plasma level of sCD40L, a T-cell activation marker. In adult liver donors, the cell number did not change within the first week after liver resection and was lower than in pediatric recipients. The results suggest that in pediatric recipients, the HSC number may be associated with graft function and could be regarded as a potential predictor of the clinical outcome after LDLT.

Implication for bone marrow derived stem cells in hepatocyte regeneration after orthotopic liver transplantation

The liver has the outstanding ability to regenerate itself and restore parenchymal tissue after injury. The most common cell source in liver growth/regeneration is replication of preexisting hepatocytes although liver progenitor cells have been postulated to participate in liver regeneration in cases of massive injury. Bone marrow derived hematopoietic stem cells (BM-HSC) have the formal capacity to act as a source for hepatic regeneration under special circumstances; however, the impact of this process in liver tissue maintenance and regeneration remains controversial. Whether BM-HSC are involved in liver regeneration or not would be of particular interest as the cells have been suggested to be an alternative donor source for the treatment of liver failure. Data from murine models of liver disease show that BM-HSC can repopulate liver tissue and restore liver function; however, data obtained from human liver transplantation show only little evidence for liver regeneration by this mechanism. The cell source for liver regeneration seems to depend on the nature of regeneration process and the extent of injury; however, the precise mechanisms still need to be resolved. Current data suggest, that in human orthotopic liver transplantation, liver regeneration by BM-HSC is a rather rare event and therefore not of clinical relevance.