With existing treatments only partially effective with major adverse effects and no vaccine currently available, hepatitis C virus (HCV) infection is a major health problem worldwide. An estimated 120 million people are chronically infected around the world and, therefore, at increased risk of liver damage (fibrosis and cirrhosis) and liver cancer. Research into potential new vaccines and therapies for HCV has been severely hampered by the lack of a small animal mouse model, often a crucial research tool to investigate disease progression and to test new drugs. Now, US scientists have for the first time made a genetically humanized mouse model for hepatitis C, which could prove vital in HCV infection research.
HCV is spread via blood-to-blood contact; anything from blood transfusions, sharing contaminated needles in injection drug use, and, as Pamela Anderson found out, contaminated tattoo needles. Diagnosis can be problematic and disease progression can be unpredictable, infected individuals range from being asymptomatic, to clearing the virus naturally or suffering progressive liver damage that, ultimately, leads to liver failure and need for transplantation.
Mice are normally resistant to HCV infection, only humans and chimpanzees are naturally permissive to HCV, and at least four human factors are critical for HCV entry, claudin 1, occuldin, CD81 and scavenger receptor type B class I (SCARBI). In their paper published this week in Nature, Marcus Dorner and colleagues built on existing knowledge that in vitro rodent cells only need to express occludin and CD81 to enable HCV entry. They reasoned that expressing these key human genes (CD81 and occludin) in mice could make living animals susceptible to HCV infection.
The scientists made mice that expressed human SCARB1, claudin 1, occludin and CD81 using an adenovirus as a vector to deliver the human genes into the mouse liver. Although mouse liver cells expressed these human genes (5% of cells expressed all four genes, whilst 18–25% expressed both CD81 and occludin), infecting these mice with HCV and proving they were infected was the major stumbling block as HCV infection in murine cells in vitro and in vivo is inefficient. Even though mice were infected with bioluminescent HCV (tagged with firefly luciferase), which can be easily detected if they replicated (the cells would ‘glow’), bioluminescent signals were not above background levels making it difficult to detect the virus. As an alternative approach, the mice were engineered to express the luciferase reporter whilst the HCV genome was engineered to express a protein that activates the bioluminesce reporter gene, such that delivery and replication of HCV in the liver leads to a bioluminescent signal. In this way, the researchers showed that all mice expressing at least human occludin and CD81 could indeed be infected with HCV. They then went on to validate their new model and demonstrated the in vivo role of SCARB1 in viral entry and uptake into host cells. Furthermore, the study authors managed to block HCV entry using passive immunisation (transfer of readymade anti-HCV antibodies) in the humanized mice. A promising HCV vaccine candidate (a recombinant vaccine virus vector expressing HCV proteins that has been shown to work in chimps) was also tested in the model mice and was shown to induce immunity and partial protection against HCV infection.
“To our knowledge, this is the first time that any step in the viral life cycle has been recapitulated in a rodent simply by the expression of human genes,” write the study authors. This new mouse model should enable scientists to closely study hepatitis C disease progression in a small animal model that is more amenable to lab research. Hopefully, new improved strategies (both drugs and vaccines) against HCV can be developed and used to guide any future clinical trials.
Dorner, M., Horwitz, J., Robbins, J., Barry, W., Feng, Q., Mu, K., Jones, C., Schoggins, J., Catanese, M., Burton, D., Law, M., Rice, C., & Ploss, A. (2011). A genetically humanized mouse model for hepatitis C virus infection Nature, 474 (7350), 208-211 DOI: 10.1038/nature10168