By David N. Leff

About 4 million Americans - and 170 million people worldwide - have been dragooned into a game of Russian roulette. The pathogenic revolver holds four chambers - three loaded, one blank.

That sinister weapon is the hepatitis C virus (HCV). The cartridges in its four cylinders reflect the fates that pulling the viral trigger will inflict on its hapless players - all infected by HCV:

¿ Bullet one is the blank. An infected person may never even know he or she was transiently infected.

¿ Bullet two causes mild symptoms; recovery (about 15 percent of cases) is quick and uneventful.

¿ Bullet three brings on lifelong, chronic hepatitis in 80 percent to 85 percent of individuals infected with HCV. It causes, among other ills, 30 percent of all liver transplants in the U.S.

¿ Bullet four signals fulminant hepatitis C, the rare, lightning-fast killer.

In 1987, virologists at Chiron Corp. in Emeryville, Calif., identified and named HCV as a separate liver-targeting virus, after A and B. It infected patients who received contaminated blood transfusions, decades before its existence was known, and before donor blood was tested for viral impurities. Nowadays, it's transmitted by dirty needles in the hands of drug addicts.

The human body's immune system is locked in a take-no-prisoners war with the viral envelope proteins on HCV. These immunogenic targets keep several jumps ahead of the antibodies and killer T cells deployed by the immune defenses. They outwit and outplay these defender molecules by constantly changing the gene sequences, and resulting amino acid proteins, of their viral envelopes.

What the immune system "sees" in this never-ending, quick-change act looks like swarms of brand-new viruses, for which it's not prepared. Virologists call these imposters "quasispecies."

"Quasispecies," explained NIH virologist Harvey Alter, "are the simultaneous presence of multiple immunologically distinct viral variants, narrowly separated from each other. These quasispecies' changes are small - 1, 2 or 3 percent alterations in the viral structure.

"There are patients who have 20 or 30 different hepatitis C viruses at any one time," Alter observed. "These quasispecies are expressed particularly in the hypervariable region [HVR] of the viral genome. That's the part that mutates rapidly. HVR is in the envelope - the part of the virus that's on its surface. Therefore, it's the part that is presumably the site of immune attack."

Does Immune System Make Virus Do It?

"The whole concept," Alter continued, "is that this viral variation is driven at the same point in time that the immune system is developing antibodies against the HVR, and the presumption is that the immune attack is what forces the variation in the virus."

Alter is chief of the Infectious Diseases Section, and associate director for research, in the Department of Transfusion Medicine at NIH. He is senior author of a paper in the current issue of Science, dated April 14, 2000, titled: "The outcome of acute hepatitis C predicted by the evolution of the viral quasispecies."

"It reports a literally unique experiment," Alter said, "carried out on 12 patients infected with what is now known as hepatitis C virus, who underwent open-heart surgery for coronary bypass or valve replacement. They came from prospective studies of post-transfusion hepatitis that we've been conducting since the 1970s," Alter recounted. "So these patients, all at the NIH clinical center, were followed every week or two after open-heart surgery. They had varied outcomes - recovery, chronic, fulminant. And it was this unique availability of serum samples from this very early period of infection that made this study different from any other. These people were infected at the time of their transfusion, when there was no test for the virus, but we could go back and show that they were infected with HCV."

The paper's lead author, virologist Patrizia Farci - who splits her time between NIH and Italy's University of Caglieri - "took the serum samples," Alter recounted, "sampled four time points in each patient, and at each point isolated the viral RNA, cloned it and sequenced 10 clones. Then we looked to see how different these clones were from each other - the viral diversity - and from the prototype hepatitis C viral sequence. What we found," Alter narrated, "was that in the patients who were going to become chronically infected, the number of variants kept increasing, whereas in those who were going to recover, they kept decreasing. So in the recovery situation, it looked as if the immune system was able to contain and limit the amount of viral variation, and eventually get rid of the virus.

"Whereas, in the more usual chronic situation - maybe 80 percent to 85 percent of people with HCV - the virus keeps escaping. So even if the immune system can neutralize one variant, the next variant is sufficiently different that it requires yet a different antibody or a different T cell. So eventually the number of variants is more than the immune system can contain."

More Research Now, Clinical Uses Later

"What this means," he pointed out, "is that in the natural history of the infection, the majority of people will become chronic, and 15 percent to 20 percent will recover. In other words, very early in the infection, the battle takes place between the immune system and the virus, and if you could do these technically sophisticated studies on a routine basis, you could determine who's winning."

Alter sees no immediate clinical utility in these findings. Rather he made the point: "I think in order to conquer a virus you have to understand how the virus works, and how the body attempts to fight it. When you have that understanding - which is our goal - then treatment approaches come out of that.

"One of the key things with hepatitis C," he added, "is that people have a natural mechanism to recover. It only happens in a minority, but it's different from HIV, where there is no natural recovery. "What we want to do is figure out how some individuals recover from HCV. What is it that these people have that's different and then tap into that - try to reproduce or create it. It looks," he concluded, "as though this will involve tweaking the immune system."