A new study examining how HIV evolves "[makes] clear what a huge challenge making a vaccine is," said co-author Dr. Bruce Walker of the Ragon Institute at Massachusetts General Hospital. Scientists from Oxford University in England, Kumamoto University in Japan, and Royal Perth Hospital and Murdoch University in Australia took part, along with Ragon researchers.

"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," Walker said. HIV is evolving to escape the human immune system, he said, much as bacteria mutate in response to antibiotics.

To see how HIV evolves in response to human leukocyte antigens (HLA) in the immune system, the investigators studied HIV genetic sequences from more than 2,800 HIV patients in the United Kingdom, South Africa, Botswana, Australia, Canada, and Japan. Genes that encode HLA molecules vary among humans, resulting in very different responses to HIV infection and helping explain why some patients progress to AIDS more rapidly than others.

The study's results indicate that HIV mutations are occurring not only in individuals but also on a population level. If a certain genetic immune sequence was common in a population, then the HIV mutation that evolved to overcome it became the most common viral strain, even in persons without the particular HLA 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."

The report, "Adaptation of HIV-1 to Human Leukocyte Antigen Class I," was published online in Nature (2009; doi:10.1038/nature07746