epatitis C virus (HCV) is an RNA virus that causes a persistent infection in more than 170 million people worldwide. It is associated with cirrhosis and forms of liver cancer and is a growing threat to public health. Unfortunately, current therapy regimes are not very effective in most patients and there is no vaccine. One reason is that hitherto it has been impossible to grow HCV in cell culture, so that the impact of various potential drugs and possible vaccines has been nearly impossible to study.

The two papers at #5 and #6 are set to change all that, for both describe the complete replication of HCV particles in cell culture. At #5 is a group led by Charles Rice at the Rockefeller University in New York while at #6 is a group led by Francis Chisari at the Scripps Research Institute in La Jolla, California. The similarities between the two papers are much greater than the differences.

There have been previous efforts to culture at least part of the HCV genome with limited success. Those methods required a series of mutations in the host cells. Takaji Wakita, a member of the Chisari group, then discovered that a version of HCV called JFH-1 would replicate in human cell lines from liver and non-liver, but very inefficiently, and the resulting particles could not be propagated further. Chisari’s group started with a liver cell line called Huh-7.5 (produced by Rice’s group) that allows various forms of HCV to replicate. These Huh-7.5 cells contain a form of HCV integrated into their genome. The group "cured" the cells of this infection by treating them with a form of interferon. This new cell line—Huh-7.5.1—was then infected with JFH-1 and monitored closely. To begin with, the amount of HCV RNA that could be detected declined, but after seven days it began to rise and two weeks after infection the cells were producing large amounts of fully infective virus particles.

Rice’s group took a different tack. They too started with JFH-1 (a gift from Wakita). From it they used only the non-structural genes, which manage the virus’s replication inside the liver cell. To the JFH-1 genes they spliced structural genes, which code for the protein coat that envelops and protects the viral genome, from other isolates. The resultant chimera replicated efficiently in ordinary Huh-7.5 cells.

From this point on, both groups followed almost identical paths to assess the success of their efforts. First, were the virus particles being made by the infected cells themselves infective? Yes. The medium in which the cells were growing routinely contained large numbers of "focus-forming units"—infective virus particles. Furthermore, those virus particles were themselves fully competent. They could be serially passaged through two or more rounds of cells without losing either infectivity or the ability to replicate.

Was it actually HCV? Yes. An antibody to one of the HCV coat proteins effectively blocked the infectivity of the particles, the extent of the block being related to the amount of antibody. Another antibody, this time to the cell-surface protein to which HCV particles must bind before they can enter the cell, also blocked infectivity in a dose-dependent manner.

That said, there are also differences between the two papers. Rice’s group, using chimeric viral genomes, found that replication was much more rapid, with a peak of infectivity just 48 hours after infection, compared to the 21 days of Chisari’s group. The significance of this is not clear, but may be related to the host liver cells in which the particles were multiplying.

There is also an interesting discrepancy in the physical nature of the particles produced by the two groups. Both centrifuged the supernatant on a density gradient and measured the amount of HCV RNA and the infectivity of the various fractions. For Chisari, infectivity and RNA were closely linked and peaked at a density of 1.105 g/ml. Rice’s group, by contrast, found a greater spread of both RNA and infectivity at several densities, with no clear peak at any given density. Moreover, the fractions that contained the most HCV RNA had very low infectivity.

These differences will doubtless be resolved. What matters is that researchers now have two robust methods for multiplying HCV, a prerequisite for developing vaccines and more effective therapies. And the methods emerged from a race characterized by Science as "a friendly but fierce competition."