By David N. Leff

Ebola virus has played starring roles in movies, books, and the news media as a swift, sudden killer. Yet the body count of its victims runs only into the hundreds.

There's a less famous viral scourge, hepatitis C virus (HCV), that infects 3 percent of the world's human population — 170 million people, including 4 million Americans — and condemns 8,000 to 10,000 a year to death in the U.S. alone.

Hepatitis C is known as the "silent disease" because its symptoms aren't noticeable until the viral infection is well-entrenched in the liver, its favorite target organ. HCV is the No. 1 perpetrator of liver disease, and the main reason for liver transplantation.

Once ensconced in the liver, the virus starts replicating, which may leave the hepatic organ scarred with life-threatening cirrhosis.

Clinical symptoms, when they do finally appear, include fatigue, appetite loss, abdominal pain, nausea and often jaundice.

Like HIV, HCV penetrates its victim's body by way of the bloodstream. Infection usually occurs thorugh swapping of contaminated drug needles and syringes, or such other self-inflicted ports of entry as tattooing, ear and body piercing, and possibly, like HIV, unsafe sex.

Unlike HIV, with its life-preserving triple-drug cocktail, medical science has relatively little to offer a chronic hepatitis C sufferer in the way of pharmacotherapy, and nothing at all — so far — in the way of a vaccine. Current treatment consists of antivirals, such as interferon and ribavirin, which benefit fewer than half the patients who get them.

Chiron First Characterized Hepatitis C In '87

It's just over a decade since virologist Michael Houghton and his colleagues at Chiron Corp., in Emeryville, Calif., characterized hepatitis C virus in 1987. Before then, this mysterious pathogen bore the frustrated name "non-A, non-B hepatitis virus." Since then, the alphabetical rap sheet has reached the letter G, and still counting. (See BioWorld Today, Jan. 28, 1998, p. 1.)

Besides being silent, HCV is sneaky, and research virologists and immunologists have helplessly spent the last decade trying to crack its cover.

"So far," observed Chiron immunologist Sergio Abrignani, "no one has been able to reliably culture that virus in vivo in the lab." This was the major problem, he said, in finding a receptor for HCV. Abrignani heads the immunology department at IRIS-Chiron, the company's research outpost in Siena, Italy.

"What we do know for sure," he said, "is that there are infectious blood plasma, sera, which can trap hepatitis C virus in humans or chimpanzees." (Homo sapiens and Pan troglodiyes, the chimp, are the only life forms that HCV can infect.)

"Although we cannot see the virus," the Sennease scientist added, "we know that there is a nucleic acid, an RNA, in that plasma, which can transfer hepatitis infection."

The invisible virus's two envelope proteins, E1 and E2, are also known, since Chiron's Houghton sequenced the virion in 1989.

To get around HCV's black-box lifestyle, Abrignani and his colleagues resorted to what he terms "alternative methods" of blowing its cover. Today's issue of Science, dated Oct. 30, 1998, reveals the prybars by which they forced that box open. Their paper's title is "Binding of hepatitis C virus to CD81."

Abrignani explained: "CD81 is a cell-surface protein, widely expressed on human cell types, particularly hepatic cells, and on B lymphocytes of the immune system, where it causes crygolublinemia. That's the only disease, besides the more reknowned hepatitis C, caused by the hep C virus."

He noted that "All viruses have receptors, a cell-surface molecule to which they bind as the first step in attaching to, then entering into, human cells. For example, HIV has the CD4 receptor; Epstein-Barr virus, CD21. We found that the molecule binding HCV is CD81."

Abrignani said that "CD81 is a known molecule, discovered about a decade ago by people working in B lymphocyte activation."

It doesn't just perch on top of an HCV target cell, but weaves back and forth through the cell's membrane four times, leaving two loops of its amino acid sequence outside. It's in the larger loop that HCV makes its move, binding its major viral envelope protein, E2, to it.

"The way we started thinking of identifying the receptor for HCV," said Abrignani, "was because when we expressed the recombinant E2 envelope protein, we found that we could measure a specific binding of the human cells. So we thought, 'If the envelope binds in a specific way, the entire virus must bind too.'

"And the way we started looking at the binding of the recombinant E2 was in all lymphoma cell lines. It was from one of these that we isolated a cDNA library. This we then screened to see what part of it was responsible for binding E2, and to what. It turned out to be CD81."

So, he said, "we devised a magnetic bead to which we attached the major loop of CD81. We reasoned that since this loop was binding to the recombinant envelope that we produced, it should also bind the entire virus. If this was the case, we should have been able to measure the attachment of nucleic acid to the bead. And this was indeed the case."

Building a Transgenic Mouse To Pinch-Hit

Now, Abrignani and his co-authors are trying to create a small-animal model of the hepatitis C virus, to stand in for humans and chimps, the only HCV-infectable species. This fact, he said, "poses a lot of problems in developing new vaccines and screening new therapeutics. So what we're doing right now is making mice transgenic for human CD81. We'll see whether they're infectable or not by HCV.

"We think that in less than one year we'll have an answer," he said.

Meanwhile, back in the U.S., Chiron vaccinologists have a Phase I clinical trial in progress testing the immunogenicity of a vaccine based on HCV's E2 antigen, to protect people against hep C infection. It has enrolled 48 volunteers at the University of Maryland's vaccine research center, in Baltimore.

Serum and cells from these volunteers are being sent to Siena, where Abrignani and his lab will assay their immune response.

"We know for sure that it's safe," he said, "but we don't have a complete picture of its immunogenicity, because the trial is not complete at this point. It should be in a very few months." *

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