We report here a novel mouse model that supports efficient engraftment of both human immune cells and human liver cells. The Balb/C Rag2-γC-null mouse with the AFC8 transgene enabled us to inducibly deplete murine hepatocytes. In addition to human immune cells in lymphoid and liver organs, AFC8-hu HSC/Hep mice were also efficiently repopulated with human albumin+ liver cells. AFC8-hu HSC/Hep mice supported HCV infection, which induced HCV-specific human immune response, liver infiltration, hepatitis and fibrosis.
In the absence of a functional immune system, uPA/SCID or Fah-Rag-γC-null mice transplanted with only human hepatocytes supported HCV infection but no significant liver fibrosis17, 21
. Immuno-deficient mice expressing the uPA transgene in the liver or carrying the Fah mutation allow human adult hepatocytes to efficiently re-populate the liver17-20
. These mice have poor health including neonatal death and, most importantly, the lack a human immune system. The severe liver injury in these mice may impair the development and function of human liver and immune cells. To overcome these deficiencies, the AFC8 mouse enabled us to inducibly deplete murine hepatocytes through a programmed cell death mechanism without bystander cell killing. In addition, the Rag2-γC-null mouse in Balb/C background permits efficient engraftment of human immune cells. Therefore, AFC8-hu HSC/Hep mice provide the first humanized mouse model with both human immune and liver cells.
We inoculated AFC8-hu HSC/Hep mice with HCV genotype 1a clinical isolates. We were able to detect HCV genomic RNA in the liver of AFC8-hu HSC/Hep mice ( and ), but we did not detect significant HCV viremia in the blood of HCV-inoculated AFC8-hu HSC/Hep mice. This may be due to the relative low level of human hepatocyte engraftment (~15%). To detect HCV viremia in the blood, the adult hepatocyte-engrafted uPA or Fah models support >50% engraftment of human hepatocytes17, 19-21
. In the future we will improve the AFC8-hu HSC/Hep model by optimizing the death induction conditions with different doses and times of dimer injections. It is also reported that the NOD-Scid-γC-/-
(NSG) mouse is more permissive in accepting human cells30
. It is likely that the introduction of the AFC8 transgene into NSG mice will create a better host for engrafting human immune and liver cells. To get different sources of human HSC and liver progenitor cells, the AFC8-hu mouse model will be useful to study functional differentiation of human embryonic stem (hES) or induced plural-potent (iPS) cells to the human liver lineages. The construction of AFC8-hu HSC/Hep mouse with HSC and liver progenitor cells derived from ES or iPS cells will be an exciting future direction.
HCV infection appeared to induce human immune responses in AFC8-hu SHC/Hep mice. We observed significant infiltration into the liver of HCV-infected AFC8-hu HSC/Hep mice, including human T cells, macrophages, DC and NK cells. Not only did we observe a preferential expansion of human T cells from HCV-infected AFC8-hu HSC/Hep mice with HCV-derived peptides, the expanded T cells also expressed IL2, TNFα and IFNγ in response to HCV peptide re-stimulation (). Therefore, HCV infection induced HCV-specific human T cell responses in AFC8-hu HSC/Hep mice. It will be interesting to test if depletion of human T cells (CD4 and/or CD8) will affect HCV infection and pathogenesis in AFC-hu HSC/Hep mice. It will also be of interest to characterize and study human T cell subsets such as regulatory T cells and NKT cells in future experiments. Consistent with poor B cell response has been reported in all humanized mouse models, we failed to detect HCV-specific human antibodies in HCV-infected AFC8-hu HSC/Hep mice (data not shown).
In human patients, immune responses against HCV are implicated as mediators of liver diseases31, 32
. Remarkably, HCV-infected AFC8-hu mice developed human leukocyte infiltration, hepatitis and liver fibrosis throughout the liver parenchyma with bridging septa (). Human CD3+ T cells and CD68+ macrophages, as well as human albumin+ hepatocytes and αSMA+ activated stellate cells, were detected in and near the fibrotic region (supplementary Figure 7
), suggesting contribution of human leukocytes to liver fibrosis. We also showed that HCV infection led to increased level of activated human stellate cells in fibrotic livers. Human extracellular matrix protein Col1A1 and inhibitor of matrix degradation TIMP1 were induced in HCV-infected AFC8-hu HSC/Hep mouse livers. It is of interest that the corresponding mouse fibrosis-associated genes were not induced by HCV infection. The lack of liver fibrosis in chronically infected chimpanzees also suggests a species-specific nature of HCV-induced liver fibrosis33
. The AFC8-hu HSC/Hep mouse will provide an excellent model to elucidate the mechanisms of HCV-induced human liver fibrosis.
Since HIV co-infection occurs in nearly 25% of HCV patients and often leads to accelerated end-stage liver disease32, 34, 35
, there is a significant need for a model system to study HCV/HIV co-infection. It will be of interest to test how HIV-1 infection will affect HCV infection and pathogenesis in the AFC8-hu HSC/Hep mouse. In addition to HCV infection and immuno-pathogenesis, AFC8-hu HSC/Hep mice will also be useful to study other liver-tropic pathogens such as HBV and malaria.