In this permissive society, it's still called "the kissing disease."

"Epstein-Barr virus [EBV] is very well established in most people in the U.S. and the world," observed viral immunologist Herbert Virgin. "For transmission, each virus is different. EBV is spread by mucosal secretions, so it causes mononucleosis - kissing disease. It's also common in children. Kaposi's sarcoma-associated herpesvirus [KSHV] can clearly be spread by sexual transmission.

"For EBV," he continued, "the prevalence of herpesvirus in the U.S. depends on socioeconomic groups, but it's generally quoted as greater than 80 percent. KSHV is much less frequent - at most 5 [percent] to 10 percent of the population."

Virgin occupies two professorial chairs at Washington University School of Medicine in St. Louis - one in pathology/immunology and the other in molecular biology. He is senior author of a paper in the August 2002 issue of the Cell press journal Immunity. Its title: "Critical role of complement and viral evasion of complement in acute, persistent, and latent g-herpesvirus infection."

"Our finding in this paper," Virgin told BioWorld Today, "is that this particular class of herpesviruses inhibits the complement pathway via expression of the so-called RCA protein [regulator of complement activation]."

Like a heavily armed military police SWAT unit, complement is the immune system's Special Weapons and Tactics Team. It enlists some 20 dedicated molecules, of which nine killer "C" proteins, led by C3, activate the destruction of invading pathogens. RCA acts like a safety valve that prevents backlash or friendly fire.

"RCA works during acute infection but not during viral latency," Virgin pointed out. "Therefore the host system complement regulates latency but not acute infection. What's novel in our paper is that it's the first demonstration of the physiologic role of this class of proteins in this virus - namely, gamma herpesviruses. The second point," he added, "is that complement, which is normally considered to be of great importance combating acute infection, also turns out to play a role in chronic infection."

The co-authors' in vivo model is a mouse virus called gamma-herpes virus 68 (gHV68). It is similar to EBV and KSHV. The latter causes Kaposi's sarcoma, a cancer that occurs in some immune-deficient people.

Mouse Virus Mimics Human EBV, KSHV

"The mouse virus latently infects B lymphocytes," Virgin explained, "a property it shares with EBV and KSHV. Hanging out latently it also infects macrophages and dendritic cells. While thus holed up for months or years," Virgin volunteered, "whether it affects those cells in any way is an open area of great interest.

"The mouse virus infects mice and other small rodents," he noted. "It was discovered in Czechoslovakia in the early 1980s. And it is the only small-animal gamma herpesvirus. The gamma herpesviruses of humans are EBV and KSHV. And there are primate viruses, including a gamma version called herpesvirus saimiri.

"The mouse virus provides us an opportunity to do mechanistic studies," Virgin said, "because we can mutate the virus, and also its host, the mouse itself. Thus we can study a mouse that's deficient in complement, for example, and a virus that's deficient in the complement regulatory protein. Then we can mix and match those two. So the primary role of the mouse virus is to provide a platform for fundamental research, utilizing genetic approaches that are not available for the human or primate viruses.

"Our in vivo experiments make use of established mice that are deficient in components of the complement system, including C3 or Factor B - C3 being critical for all aspects of complement activation and Factor B for the alternative factor of complement activation. What we did do ourselves was make the virus mutant in the RCA protein and its appropriate control. To do this, we knocked the RCA gene out, then took the mutant virus and rescued the mutation back with wild-type sequences. There is a WT virus, a mutant virus and a marker rescue. The marker rescue was the mutant virus to which we then restored the WT sequences. This we did to avoid problems with secondary mutations elsewhere in the genome that might give us a phenotype we might mistakenly map to a single mutation in our target area."

Viral Virulence Weakened, Then Restored

"In an acute infection experiment," Virgin related, "the viral mutant was about 100-fold less virulent after intracranial inoculation into mice. And that lack of virulence was restored - the mutant virus became virulent again - when we took the C3 complement out of the host. That indicated that the viral protein, during acute infection, was targeting the complement system. When the virus is mutant in a WT mouse it's attenuated, but if we delete the host component C3, the virus isn't attenuated any more. We believe that's because its host component is gone so it doesn't need its own gene that counteracts the host component.

"During persistent infection and latent infection, the viral gene does play a role - that is, ongoing replication of the virus that occurs over weeks to months. However, the gene doesn't play a role in latency; complement does. So what we interpret is that the viral protein has an important role during acute infection by targeting C3 and the complement system in the host, but that during latent infection the viral gene can't counteract the effects of complement. That means the viruses harbor immune-evasion proteins, which are quite specific. They were designed, speaking teleologically, to function in acute infection or perhaps latent infection. But they don't necessarily function in each aspect of the virus' life cycle in vivo."

Virgin made the point: "I believe it is important to consider the function of human complement in our viral system to playing a role in both chronic and acute infections for human KHSV, which causes significant disease in AIDS, and EBV, which causes such ailments as lymphomas. In the field as a whole there's a large number of immunomodulatory proteins, in pox viruses, herpesviruses, even in RNA viruses, which I believe are going to reveal a significant amount about the pathways of their host that controls inflammation and immunity. It's a little bit of a paid political announcement in a certain sense because I think the people in biotechnology who study immune evasion proteins all believe what I just said, and that is," he concluded, "these proteins are good tools for defining novel pathways and inflammation."