By David N. Leff

Message to teen-agers:

You must remember this: A kiss is still a kiss -- and it's still the entryway for the Epstein-Barr Virus (EBV), which causes infectious mononucleosis.

That's why mono is known as "the kissing disease." In effect, it's a tough-love way of vaccinating amatory youngsters against the viral infection.

Between that saliva-swapping osculation and the onset of mono's fever, malaise, fatigue and swollen tonsils, two or three weeks go by. During that grace period, the EBV replicates in the oral mucosa lining mouth and throat, then honkers down in the immune system's B lymphocytes.

"To find viral antigen being expressed in a very good antigen-presenting cell," observed molecular virologist Fred Wang, of Harvard University Medical School, "is a very alarming finding to the body's immune system. So the system exclaims, 'We've got to control that!' and explodes. Meanwhile, the virus is causing those B cells to proliferate.

"That explosion's goal," Wang went on, "is to kill abnormal, virus-infected cells. The big B cell proliferation and T cell activation cause atypical lymphocytosis. A classic way of diagnosing infectious mononucleosis is to look at a blood smear and see those big, foamy, abnormal lymphocytes."

Over time, cytotoxic T cells get the EBV-infected B cells under control, and the acute disease resolves. "But the system never gets rid of the virus," Wang said. "EBV persists in the B cells for life."

Epstein-Barr virus permanently infects 98 percent of the adult population worldwide. Most of the time, it lies low in the B cells, while replicating in the oral pharynx, biding its opportunity to strike again.

That return engagement rarely repeats the mononucleosis attack, against which those first-time victims are now immune. Next time around, that same infection can trigger a cancer instead.

"A variety of seemingly unrelated malignancies are associated with EBV," Wang pointed out. "The classic one is Burkitt's lymphoma, which occurs mainly in Africa. Then, nasopharyngeal carcinoma, which is endemic to areas of Southeast Asia. EBV," he added, "probably increases one's risk of such cancers, but is not sufficient to cause malignancy. A cofactor is probably necessary."

He continued: "The other lymphoproliferative disorders occur in the setting of immunodeficiency, whether congenital, as in severe combined immunodeficiency, the 'bubble-boy disease', or iatrogenic, as caused by over-immune suppression, incident to organ transplantation, or infectious, as in HIV. EBV-induced B cell lymphomas," he observed, "are a common malignancy in AIDS patients."

Wang is senior author of a paper in this week's Science, dated June 27, 1997. Its title: "An animal model for acute and persistent Epstein-Barr virus infection."

For Lack Of A Model . . .

He told BioWorld Today: "The diseases we're talking about are key features of EBV biology. Those are factors that we cannot reproduce in tissue culture very well."

Explaining the need for an animal model, he went on: "We can study EBV's replication, gene function and all that within tissue culture, but we can't really understand how the virus persists. What is its interaction with the immune system? How does it go from the oral pharynx into the B cells? Humans are okay," he observed. "You can touch pieces of them on the outside -- like the blind men touching the elephant -- but you can't necessarily get the whole picture."

So he and his co-authors took a hard look at primates as candidate animal models.

"We knew that Old World primates, from gorillas and chimpanzees to orangutans and rhesus monkeys, had the simian counterpart of Epstein-Barr viruses, closely related to human EBV," Wang said, "so we thought that the rhesus might make a good model.

"Others had had the same idea in the 1970s and '80s," he recalled. "Those investigators tried to infect rhesus monkeys with human EBV, but it didn't work. Part of the reason may have been that those animals, like humans, are already infected with their own EBV counterpart virus, and there was probably some degree of cross-immunity."

So Wang and his co-authors reasoned: "If you wanted to infect humans with EBV, you'd give the virus to naive individuals, who hadn't ever kissed other humans and caught their EBV infection."

Extrapolating this rationale to rhesus monkeys (Macaca mulatta), the team approached the New England Regional Primate Research Center (NERPRC), in the Boston exurb of Southborough, where Wang is an affiliated investigator.

"If you go to most primate centers," he observed, "they just house their animals in big colonies, so you'd anticipate they'd all share saliva, and get EBV infection very easily."

Happily, the NERPRC has been raising a colony of pathogen-free rhesus monkeys in isolation, starting with hand-reared animals abandoned by their mothers and nursed by human hands. After confirming that they were antibody-free of the simian equivalent of human EBV, Wang recounted, "We tried to infect two of these naive animals by instilling the virus into their throat -- which is the way one gets naturally infected."

Abandoned Monkeys Fill The Bill

Did those two candidate models show signs and symptoms of mono?

"Getting a history from a monkey is tough," Wang replied.

"We saw an atypical lymphocytosis, which is a hallmark of infectious mononucleosis. We saw lymphadenopathy, and in one animal a decent splenomegaly. Also, within the first week, we observed the very early appearance of activated B cells and monocytes, a phase that we cannot access in humans. What we saw in our study probably reflects the entry of the virus into the B cell compartment, probably the trigger of a very vigorous immune response."

These findings, he said, "give us a way of getting at what triggers infectious mononucleosis, which we could never do in humans. We can't tell who's naive, and who's gotten the big inoculum that's going to result in mono two weeks later. We can get at it by choosing the animals to infect, and following their responses."

Awaiting Wang's ready-made human-surrogate monkeys are three new inquiries: scoping the functions of nine latent EBV genes that immortalize B cells; determining how EBV interacts with the oral and pharyngeal epithelium; and seeking ways to intervene therapeutically in EBV infection, by vaccines or drugs. *