Primate Virus Shows How
HIV Escapes Early
Immune Response
LONDON (Reuters) - Scientists have figured out how a primate virus similar to HIV evades the immune system in the first weeks of infection, a discovery they say could pave the way for the development of a new AIDS vaccine. The HIV virus that causes AIDS is particularly cunning because it replicates and mutates so quickly that it can outsmart the body"s normal defense mechanisms.
SIV, a monkey virus, is very similar to HIV but it infects monkeys much more quickly than its human equivalent. Scientists at the University of Wisconsin and the Wisconsin Regional Primate Research Center in Madison have shown that SIV in monkeys evades a massive immune response early in the infection by replicating and mutating into a new variant. The new strain is not recognized by the killer T cells sent by the body"s immune system to destroy the initial virus.
"What we have discovered is that in the course of a normal infection there is a hitherto unknown and unexpected massive killer cell response in the acute phase of the infection," David Watkins, one of the researchers, said in a telephone interview. "These results show that infected individuals do make immune responses that the virus cannot tolerate. The challenge will be to mimic these responses in an HIV vaccine," he said. Although drugs have prolonged the lives of AIDS patients, experts believe that the only way to fight the pandemic that has killed 19 million people is with a vaccine. More than 70 AIDS vaccines are in various stages of development and testing around the world.
The scientists, whose research was published in the science journal Nature Wednesday, discovered that the change in the virus occurs in the region of a viral protein called TAT, which could be an important target for a new AIDS vaccine. "This is the first real direct evidence that the important immune response that is controlling the virus is this killer cell response," Watkins explained. The scientists are convinced the immune response is there because several weeks after the macaque monkeys had been injected with SIV they could not find any trace of the original virus, only a mutated form.
"We put in virus A but at four to eight weeks, after the infection, the only replicating virus is virus B, implying that the killer cells had actually got rid of the first virus," Watkins explained. The researchers believe that the TAT-specific killer T-cells can get rid of the virus in the monkeys and that a vaccine incorporating TAT could induce a similar response in humans. "If vaccines can induce these killer T cell responses before infection occurs, the opportunity for the virus to subsequently escape from these immune responses would be greatly reduced," Watkins said.
The researchers have started to vaccinate monkeys with regions of the TAT protein that they have shown are recognized by T cells. "This should induce a massive killer T cell response at the onset of infection," Watkins said. If the monkey trials are successful, the scientists hope to begin human trials in the United States and Canada by the end of the year.

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