Hepatitis C relies on specific 'host entry factors' to infect cells - making it impossible to study the disease in lab mice until now. Credit: Elie Dolgin

Researchers have produced the first mouse model with a functional immune system that is susceptible to infection by the hepatitis C virus — by getting it to express two human genes.

The advance paves the way for testing potential vaccines and drug targets. With few therapies available — the first targeted ones were approved by the US Food and Drug Administration just last month — the hepatitis C virus (HCV) puts 170 million people around the world at a heightened risk of cirrhosis, liver failure and cancer.

HCV infects only humans and chimpanzees, using specialized molecules found in these organisms to establish infection. It is this specificity that has made it so difficult to develop treatments, because mice and other common laboratory animals are resistant to the virus.

Previously, researchers had humanized mice by engineering them to support human liver cells1. In order for this to work, however, the mice had to be immunodeficient, which led to other problems. "Mice with human livers require much technical expertise, take several months to produce and do not have an adaptive immune system," says Karl-Dimiter Bissig, a stem-cell biologist at Baylor College of Medicine in Houston, Texas, who was not involved with the new work.

So Alexander Ploss, a virologist at the Rockefeller University in New York City, and colleagues decided that the key to a robust mouse model of HCV would be to provide the mouse cells with the human components necessary to take up the virus.

Entry granted

Over the past 13 years, several molecules, dubbed 'host entry factors', have been identified as essential for efficient viral uptake. To trick mouse cells into granting HCV entry, the team delivered the genes for up to four human entry factors into the cells using modified adenoviruses — common cold viruses.

The expression of just two of these human genes proved sufficient to make mouse cells susceptible to HCV. The results are published today in Nature2.

"This new model is elegant in its simplicity," says Bissig. "Simply by overexpressing two proteins, they've created the first situation in which hepatitis C encounters a mouse immune system."

The research community has been eagerly anticipating this advance. "Although this step was expected given the vast amount of previous work that has been done to define entry factors, it's still an impressive step forward," says Michael Gale, an immunologist at the University of Washington in Seattle.

And the mice are not the only recent development in this area: the host factors required for HCV entry have already opened the door to new antiviral strategies. In April, Thomas Baumert, a virologist at the University of Strasbourg in France, and his colleagues reported the identification of two further factors that facilitate HCV entry in humans3. By blocking the molecules pharmacologically in a chimaeric mouse model his group inhibited infection.

Illuminating approach

Despite the excitement, the new mouse model does present one problem, says Gale — it is not yet very robust. "At this current stage it is not amenable to biochemical analyses of virus–host interactions because viral replication is so low," he says.

In working to overcome this expected limitation in mice with functioning immune systems, the authors created a sensitive detection system. As soon as the virus enters a cell — before it starts to replicate — it triggers the expression of a reporter gene that can be detected by luminescence. Ploss says that, without the reporter system, the team couldn't have made the progress they did.

Not surprisingly, Ploss has his sights set on achieving full viral replication in this mouse model to strengthen its ability to assess immune responses to potential vaccines.

His team is pursuing two possibilities. First, they are trying to identify any additional human factors that may be required to establish replication in mouse cells. And they are testing whether the viral entry signalling pathways somehow limit virus replication in mouse cells — to see whether they can disrupt target genes to create conditions more conducive to replication. Gale says these are important steps towards realizing what he expects will become a robust mouse model for HCV infection.

And the approach may find uses in other research areas. "This work does raise the hope that genetically humanized mouse models could become available for other viral infections, such as hepatitis B or HIV," says Baumert.