Progressor Chimpanzees Could Reveal Mechanism For Resisting AIDS

The chimpanzees were originally inoculated with multiple strains of HIV some 15 years ago. All of the animals remain clinically healthy, but show evidence of abnormal immune function typical of progressive HIV infection.

Although chimpanzees are susceptible to HIV infection, they normally do not develop AIDS. In 1996, Dr. Francis Novembre and other scientists at the Yerkes Research Center described the first case of AIDS ever reported in a chimpanzee. That animal (C499) was housed with other HIV positive chimpanzees, including two of the three chimpanzees that are currently progressing to AIDS. Yerkes scientists are conducting genetic analyses to determine whether C499 transmitted his particular subtype of HIV to his companions.

The progressor chimps are among a group of ten HIV-positive chimpanzees that have been regularly monitored since the mid-1980s. Six of the chimps have displayed stable chronic HIV infection without signs of progressing to AIDS. C499 was euthanized in 1996 after developing complications from AIDS.

The Yerkes team, led by Dr. Shawn O'Neil, has found that virologic and immunologic changes among the progressor chimpanzees closely resemble those seen in human progressors, most notably high viral loads and profound reductions in CD4+ T lymphocytes. Likewise, the progressor chimpanzees have displayed evidence of chronic immune activation, another key marker of HIV progression in people.

"The conspicuous absence of immune activation in most HIV-infected chimpanzees has been proposed to explain the relative resistance of this species to the development of AIDS," said O'Neil. "These data suggest that chronic immune activation is important for HIV pathogenesis in chimpanzees, as it is in people."

Scientists believe that chimpanzees serve as the natural reservoir for HIV, and that they have adapted to HIV infection and developed resistance to the virus through natural selection. The HIV pandemic is thought to have originated through cross-species transmission of HIV from chimpanzees to people in West Africa. According to O'Neil, the discovery of the three progressor chimps presents an unique opportunity to identify the evolutionary mechanisms that normally protect chimpanzees from AIDS a mystery that continues to elude scientists.

"Aside from humans, chimpanzees are the only animals that are consistently susceptible to HIV infection," explained O'Neil. "But in most studies, chimpanzees have not developed AI