Selection of in vivo retrovirally transduced hepatocytes leads to efficient and predictable mouse liver repopulation

In vivo selection with lentiviral expression of Bcl2T69A/S70A/S87A mutant in hematopoietic stem cell-transplanted mice

Current in vivo selections for hematopoietic stem cell (HSC)-based gene therapy are drug dependent and not without risk of cytotoxicity or tumorigenesis. We developed a new in vivo selection system with the non-phosphorylatable Bcl2 mutant Bcl2^{T69A/S70A/S87A} (Bcl2^AAA), which makes in vivo selection drug independent and without risk of cytotoxicity or tumorigenesis. We demonstrated in HSC-transplanted mice that Bcl2^AAA facilitated efficient in vivo selection in the absence of any exogenously applied drugs under both myeloablative and non-myeloablative conditioning. In mice transplanted with retrovirally transduced sca-1-positive bone marrow cells, the marked cell level increased from 26.38% of input transduced cells to 92.61 ± 0.95% of peripheral blood cells for myeloablative transplantation or to 37.82 ± 6.35% for non-myeloablative transplantation 6 months after transplantation. Bcl2^AAA did not induce tumorigenesis and does not influence hematopoiesis and the function of the reconstituted blood system. However, the high-level constitutive expression of Bcl2^AAA mediated by retroviral vector induced exhaustion of the marked cells after tertiary transplantation. Fortunately, low-level constitutive expression of Bcl2^AAA driven by an internal promoter in lentiviral vector could both maintain the marked cell level (24.13 ± 5.27%, 27.17 ± 5.51%, 24.33 ± 5.08%, and 22.07 ± 4.44% for primary, secondary, tertiary, and quaternary recipients) and avoid the exhaustion of the marked cells even in quaternary recipients. Importantly, the low-level constitutive expression of Bcl2^AAA did not induce tumorigenesis. Thus, the in vivo selection employing the low-level constitutive expression of Bcl2^AAA provides a general platform which is relevant for widespread applications of gene therapy.

Implication of hepatic stem cells in functional liver repopulation

The liver has an enormous potential to restore the parenchymal tissue loss due to injury. This is accomplished by the proliferation of either the hepatocytes or liver progenitor cells in cases where massive damage prohibits hepatocytes from entering the proliferative response. Under debate is still whether hepatic stem cells are involved in liver tissue maintenance and regeneration or even whether they exist at all. The definition of an adult tissue-resident stem cell comprises basic functional stem cell criteria like the potential of self-renewal, multipotent, i.e. at least bipotent differentiation capacity and serial transplantability featuring the ability of functional tissue repopulation. The relationship between a progenitor and its progeny should exemplify the lineage commitment from the putative stem cell to the differentiated cell. This is mainly assessed by lineage tracing and immunohistochemical identification of markers specific to progenitors and their descendants. Flow cytometry approaches revealed that the liver stem cell population in animals is likely to be heterogeneous giving rise to progeny with different molecular signatures, depending on the stimulus to activate the putative stem cell compartment. The stem cell criteria are met by a variety of cells identified in the fetal and adult liver both under normal and injury conditions. It is the purpose of this review to verify hepatic stem cell candidates in the light of the stem cell definition criteria mentioned. Also from this point of view adult stem cells from non-hepatic tissues such as bone marrow, umbilical cord blood or adipose tissue, have the potential to differentiate into cells featuring functional hepatocyte characteristics. This has great impact because it opens the possibility of generating hepatocyte-like cells from adult stem cells in a sufficient amount and quality for their therapeutical application to treat end-stage liver diseases by stem cell-based hepatocytes in place of whole organ transplantation.

The nude mouse as model for liver deficiency study and treatment and xenotransplantation

We aimed at reviewing the various uses of Nude mouse for the development of liver deficiency models and evaluation of efficacy of hepatic cell xenotransplantation. The first part records the large range of liver deficiency models that can be developed in Nude mice: surgical partial hepatectomy, acute toxic liver deficiency, chronic cirrhosis, and transgenic liver injury. The second part tackles the outcome of rat hepatocyte as well as human cell transplantation, both mature hepatocyte and hepatic progenitor, into Nude mouse submitted to liver injury. Results are discussed and compared to other available immunodeficient mouse models. The issue of humanized liver creation is also addressed. Altogether, these results show that Nude mouse appears to be a suitable small animal model to expand our insight into liver cell engraftment and regeneration.

Non-viral S/MAR vectors replicate episomally in vivo when provided with a selective advantage

The ideal gene therapy vector should enable persistent expression without the limitations of safety and reproducibility. We previously reported that a prototype plasmid vector, containing a scaffold matrix attachment region (S/MAR) domain and the luciferase reporter gene, showed transgene expression for at least 6 months following a single administration to MF1 mice. Following partial hepatectomy of the animals, however, we found no detectable vector replication and subsequent propagation in vivo. To overcome this drawback, we have now developed an in vivo liver selection strategy by which liver cells transfected with an S/MAR plasmid are provided with a survival advantage over non-transfected cells. This allows an enrichment of vectors that are capable of replicating and establishing themselves as extra-chromosomal entities in the liver. Accordingly, a novel S/MAR plasmid encoding the Bcl-2 gene was constructed; Bcl-2 expression confers resistance against apoptosis-mediated challenges by the Fas-activating antibody Jo2. Following hydrodynamic delivery to the livers of mice and frequent Jo2 administrations, we demonstrate that this Bcl-luciferase S/MAR plasmid is indeed capable of providing sustained luciferase reporter gene expression for over 3 months and that this plasmid replicates as an episomal entity in vivo. These results provide proof-of-principle that S/MAR vectors are capable of preventing transgene silencing, are resistant to integration and are able to confer mitotic stability in vivo when provided with a selective advantage.

Rodent models of liver repopulation

The liver has an extraordinary faculty to regenerate. Hepatocytes are highly differentiated cells that, despite a resting G0 state in the normal quiescent liver, can re-enter the cell cycle to reconstitute the organ after an injury. However, the first cell therapy approaches trying to harness this specific characteristic of the hepatocytes came up against the competition with resident hepatocytes in the ability to proliferate. This review will describe the different rodent models that have been developed in the last 15 years to demonstrate the concept of liver repopulation with transplanted cells harbouring a selective advantage over resident hepatocytes. Examples will then be given to show how these models demonstrated the therapeutic efficiency of cell transplantation in specific disorders. The transplantation of human hepatocytes into some of these mouse models led to the creation of humanized livers. These humanized mice provide a powerful tool to study the physiopathology of human hepatotropic pathogens and to develop drugs against them. Finally, emphasis will be placed on the role of these rodent models in the demonstration of the hepatocytic potential of stem cells.

Transplanted hepatocytes over-expressing FoxM1B efficiently repopulate chronically injured mouse liver independent of donor age

Orthotopic liver transplantation is limited by the shortage of liver donors, leading to elderly patients being enrolled as donors with increasing frequency. Alternative strategies such as cell therapy are therefore needed. Because transplanted hepatocytes do not proliferate into a recipient liver, repopulation strategies have been developed. We have previously published a proof of concept that hepatocytes harboring a survival selective advantage can efficiently repopulate a mouse liver. We develop here an alternative approach by conferring a selective proliferative advantage on transplanted hepatocytes over resident ones. FoxM1B is a transcription factor that, when over-expressed into hepatocytes, accelerates the cell cycle and maintains the hepatocyte in vivo proliferative capacity of aged livers. We now demonstrate that transplanted hepatocytes over-expressing FoxM1B repopulate the liver of mice subjected to continuous injury far more efficiently than control hepatocytes. We show that old hepatocytes that over-express FoxM1B retain their cell division capacity and repopulate liver as well as young ones, in contrast with old non-modified hepatocytes, which lose their proliferative capacity. In conclusion, our results point to the potential use of FoxM1B expression in hepatocyte-based therapy protocols in diseases where host hepatocytes are chronically injured, especially if donor hepatocytes come from old livers.

Liver repopulation by transplanted hepatocytes and risk of hepatocellular carcinoma

BACKGROUND

Transplantation of isolated hepatocytes in rats treated with retrorsine (RS) results in massive repopulation of the host liver. In this study, the long-term fate of hepatocytes transplanted into RS-treated recipients was followed for up to two years.

METHODS

Dipeptidyl-peptidase type IV-deficient (DPPIV) Fischer 344 rats were given two injections of RS (30 mg/kg), followed by transplantation of 2 million hepatocytes, isolated from a syngenic, DPPIV donor.

RESULTS

Extensive (91+/-7%) liver replacement by transplanted hepatocytes was observed in animals sacrificed 18 months posttransplantation. Similar levels of repopulation persisted at two years (87+/-5%). No evidence of preneoplastic and/or neoplastic evolution of the transplanted cell population was present in the RS-treated and repopulated livers at any time point considered. Furthermore, serum parameters related to hepatocyte function and integrity were in the normal range. In control groups given cell transplantation in the absence of prior treatment with RS, only small clusters of donor-derived, DPPIV hepatocytes were discerned.

CONCLUSIONS

These results indicate that liver repopulation in this model is largely stable, persisting for up to two years and allowing for a normal liver function. In addition, no increased risk of neoplastic transformation appears to be associated with the process of liver repopulation for as long as over two thirds of the life span of the recipient animal.

N/A

N/A

Repopulation of rat liver by fetal hepatoblasts and adult hepatocytes transduced ex vivo with lentiviral vectors

Recent studies have shown that nondividing primary cells, such as hepatocytes, can be efficiently transduced in vitro by human immunodeficiency virus-based lentivirus vectors. Other studies have reported that, under certain conditions, the liver can be repopulated with transplanted hepatocytes. In the present study, we combined these procedures to develop a model system for ex vivo gene therapy by repopulating rat livers with hepatocytes and hepatoblasts transduced with a lentivirus vector expressing a reporter gene, green fluorescent protein (GFP). Long-term GFP expression in vivo (up to 4 months) was achieved when the transgene was driven by the liver-specific albumin enhancer/promoter but was silenced when the cytomegalovirus (CMV) enhancer/promoter was used. Transplanted cells were massively amplified ( approximately 10 cell doublings) under the influence of retrorsine/partial hepatectomy, and both repopulation and continued transgene expression in individual cells were documented by dual expression of a cell transplantation marker, dipeptidyl peptidase IV (DPPIV), and GFP. In this system, maintenance or expansion of the transplanted cells did not depend on expression of the transgene, establishing that positive selection is not required to maintain transgene expression following multiple divisions of transplanted, lentivirus-transduced hepatic cells. In conclusion, fetal hepatoblasts (liver stem/progenitor cells) can serve as efficient vehicles for ex vivo gene therapy and suggest that liver-based genetic disorders that do not shorten hepatocyte longevity or cause liver damage, such as phenylketonuria, hyperbilirubinemias, familial hypercholesterolemia, primary oxalosis, and factor IX deficiency, among others, might be amenable to treatment by this approach.

Principles of hepatocyte repopulation

Hepatocyte transplantation (HTx) is technically feasible and can be clinically beneficial. Current research focuses on optimizing parameters which relate to the outcome of HTx, including site of transplantation, cell number and, most notably, the preferred cell type to be transplanted (differentiated adult vs. fetal hepatocytes vs. putative progenitor or precursor cells). However, the single major impediment towards the clinical effectiveness of HTx is the limited expansion of donor cells in the recipient liver. To this end, a relative growth advantage must be present or is to be imposed on transplanted hepatocytes versus resident cells. Possible strategies are presented and discussed.

Bone marrow transplantation in mice leads to a minor population of hepatocytes that can be selectively amplified in vivo

Cell-based therapy may some day be a therapeutic alternative to liver transplantation. Recent observations indicating that hematopoietic stem cells can differentiate into hepatocytes have opened new therapeutic prospects. However, the clinical relevance of this phenomenon is unknown. We have previously developed a strategy based on the protective effect of Bcl-2 against Fas-mediated apoptosis to selectively amplify a small number of hepatocytes in vivo. We now show that this approach can be used to amplify a minor population of bone marrow-derived hepatocytes. Normal mice were transplanted with unfractionated bone marrow cells from transgenic animals expressing Bcl-2 under the control of a liver-specific promoter. Recipients were then submitted to weekly injections of the anti-Fas antibody, Jo2. Upon sacrifice, the liver of the recipients showed bone marrow-derived clusters of mature hepatocytes expressing Bcl-2, which showed that the hepatocyte progeny of a genetically modified bone marrow can be selectively expanded in vivo. In contrast, no Bcl-2 expression could be detected without the selective pressure of Jo2, suggesting that differentiation of bone marrow cells into mature hepatocytes is very inefficient under physiologic conditions. We conclude that a selection strategy will be required to achieve a therapeutic level of liver repopulation with bone marrow-derived hepatocytes.

Liver repopulation by Bcl-x(L) transgenic hepatocytes

Liver repopulation could constitute a potential therapeutic alternative to liver transplantation in the future. Therefore, the development of liver repopulation strategies is of major interest. We have previously reported that Bcl-2-expressing hepatocytes are resistant to Fas-mediated apoptosis and that these hepatocytes, when transplanted into the spleen, are able to repopulate the liver of normal mice submitted to Fas-mediated apoptosis. We now show that Bcl-x(L)-overexpressing hepatocytes are able to repopulate up to 10% of a normal mouse liver treated with successive injections of anti-Fas antibody. We show that a twofold overexpression of Bcl-x(L) is sufficient to confer a selective advantage to hepatocytes submitted to anti-Fas antibody. Moreover, repopulation percentages obtained here were comparable to those obtained when Bcl-2 hepatocytes were transplanted, suggesting that both proteins are equivalent in conferring a selective advantage to hepatocytes submitted to anti-Fas antibody.