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HIV Transmitted During Sex May Hold Clues to Infection

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Joe DeCapua

HIV / GENITAL TRACT -- A new study finds the AIDS virus circulating in the blood may be very different from the virus that’s transmitted during sex.  And this could have implications in the search for an effective vaccine or microbicide.

University of North Carolina School of Medicine researchers at Chapel Hill say HIV may undergo changes in the genital tract of men.  As a result, the virus contained in semen could hold clues about how to block infection.  More research is needed, however.

Why important

“Most of what we know about HIV is based on analysis of virus that’s in the blood.  And much of the transmission that goes on goes from men to women or men to men.  But the male genital tract, male seminal tract, is involved in those transmissions,” says Ron Swanstrom, professor of biochemistry at UNC and the senior author of the study.

“Anything that makes the virus different in semen, which is the site of the transmitted virus, becomes important to understand just to find out if it’s relevant to the transmission event,” he adds.

Do researchers know why HIV can change in the seminal tract?  “That’s the big question for us,” he says, “and no.”

It could be one of two reasons.

Swanstrom says, “There’s a phenomenon known called the founder’s effect where…by chance it’s one thing that gets started and it grows from that….  So it could be just by luck that one or a few viruses get established in the seminal tract and grow out .  In which case presumably there won’t be any differences in terms of the biology of the virus compared to what’s in the blood.”

On the other hand, there’s another, more important possibility.

“There could be some selective pressure.  That the virus that grows in the seminal tract has some other different set of circumstances that it deals with and has to adapt to that environment.  And therefore, it’s a little bit different than what’s in the blood,” he says.

What’s the difference?

Much of the time HIV in the semen and HIV in the blood may not differ at all.  But other times there may be two different scenarios.

Professor Ron Swanstrom
Professor Ron Swanstrom

“One virus or a couple of viruses seem to take off over a very short period of time and represent a significant fraction of the virus in the semen.  We don’t know why that happens, but it’ll be a much less complex population.  The population of the blood is very complex genetically.  The viruses are all kind of different from each other.  Suddenly the semen looks much more homogeneous,” he says.

Those particular viruses, however, disappear fairly quickly.  Another possibility, he says, is that the virus gets established in the seminal tract and eventually becomes separated from the HIV population in the blood.

“That’s when we see this strong compartmentalization where the virus is very different.  But again, we don’t know what that difference is at this time,” he says.

Vaccines, microbicides

At this stage, it’s unclear whether the finding will affect vaccine and microbicide research.  But Swanstrom says, “If this is the pool of viruses that are being transmitted, then it’s really the virus that we want to understand the most about.”

He cautions that while the differences can be seen at the genome level, they may not have any biological difference from the HIV in the blood and therefore won’t behave any differently.

“But if they do confer some special property, then we certainly need to know about that. If we were really lucky, it might help inform us on either a vaccine or a more efficacious microbicide,” he says.

It’s just a different kind of virus

“I think that’s fair to say.  I’ve been a virologist for a long time and probably what’s different about HIV is that it grows constantly in the host.  And this is also true for HCV (Hepatitis C Virus).”

The biochemistry professor says these are special viruses in that “they know how to survive in the face of our immune system.  For HIV it’s always evolving.  The virus is growing every day.”

The HIV lifecycle is only about a day long.  “So 365 times a year it divides.  It grows.  If you think about that in terms of generations and how long it would be to have 365 generations of a person, that gets compressed into every year for HIV.  And whatever the host is able to throw at it, it evolves away from it.  And so there’s this titanic battle that goes on constantly between the virus and the host,” he says.

The next step is to clone the HIV found in the semen and study it further to learn whether its difference will make a difference in the search for better treatment or a cure.

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