HIV, the virus that causes AIDS, is one of the fastest evolving entities known. That's why no one has yet been able to come up with a vaccine: The virus mutates so rapidly that what works today in one person may not work tomorrow or in others.
A study published today in the journal Nature confirms that dizzying pace of evolution on a global scale.
"It's very clear there's a battle going on between humans and this virus, and the virus is evolving to become unrecognized by the immune system," said Dr. Bruce Walker, director of the Ragon Institute at Massachusetts General Hospital in Boston and one of the researchers. "It does make clear what a huge challenge making a vaccine is."
HIV evolves to escape the immune system, much in the same way that bacteria mutate under pressure by antibiotics, Walker said. Researchers looked at HIV genetic sequences in the United Kingdom, South Africa, Botswana, Australia, Canada and Japan to see how they evolved in response to a key set of molecules in the human immune system called human leukocyte antigens. These key molecules direct the immune system to recognize and kill HIV and other infectious diseases.
Genes encoding human leukocyte antigens vary among humans, and even small differences can dramatically affect a person's response to HIV infection. For example, an adult infected with HIV will survive on average about 10 years without anti-HIV drugs before developing AIDS. But some people will progress to AIDS within a year, and others can survive without treatment for 20 years.
The study published online today found mutations occurred not just in individuals but on a population level. That is, if a particular genetic immune sequence was common in a population, the HIV mutation that evolved to escape it became the most common strain of HIV even in those without that particular human leukocyte antigen gene.
"What this study does is give an explanation for why there are different HIV strains in different parts of the world," Walker said. "The genetic makeup of people in different regions is influencing the virus in specific ways."
This would appear to be bad news for the director of the newly opened Ragon Institute, which was founded to develop vaccines for HIV and other infectious diseases.
But Walker saw the results as hopeful. Mutations often come at a cost to the virus, he said. They can actually make the virus less fit -- that is, less able to replicate as quickly or do as much damage. His challenge is to find what kind of pressure results in this kind of mutation.
In addition to the Ragon Institute, the study involved researchers from Oxford University in England, Kumamoto University in Japan and the Royal Perth Hospital and Murdoch University in Australia, who analyzed the genetic sequences of HIV and human leukocyte antigen genes in 2,800 people.
-- Mary Engel
Photo credit: Ricardo DeAratanha / Los Angeles Times