AIDS Virus Traced to Chimp Subspecies

CHICAGO--Most AIDS researchers have long believed that HIV-1, the main form of the AIDS virus, jumped from chimpanzees into humans. But data supporting this theory has been hard to come by. Now Beatrice Hahn from the University of Alabama at Birmingham and co-workers have pieced together what is being hailed as the best case yet for the chimp connection.

Hahn's genetic detective work--which she described in the keynote speech here at the opening of the Sixth Conference on Retroviruses and Opportunistic Infections--indicates that one particular chimp subspecies, found in a region that includes Gabon, Cameroon, and Equatorial Guinea, is the original source of human HIV-1 infections. She speculates that butchering chimpanzees--a practice, she notes, that is common in parts of west equatorial Africa--may have provided the route for transmission. A paper on the findings will be published in Thursday's Nature.

Scientists had previously found HIV-like viruses in only three chimps: two from Gabon and a third from what was then Zaire. (The viruses do not appear to cause disease in the animals.) An analysis of the genetic sequences of these viruses, named SIVcpz, had revealed that the two Gabonese strains are closely related to HIV-1 strains found in humans, but that the Zairian strain is quite different, leading some to doubt that chimps were the original reservoir. Hahn has now isolated another SIVcpz strain from a tissue sample from a chimp named Marilyn that died in a research colony in the United States in 1985; she found that it is similar to the Gabonese strains.

Hahn and her colleagues analyzed DNA of mitochondria from all four chimps and discovered that the Gabonese animals and Marilyn all belonged to one subspecies, Pan troglodytes troglodytes. The Zairian animal belonged to a different subspecies, P.t. schweinfurthii. Hahn believes that SIVcpz may have been in chimps for hundreds of thousands of years, and that viral strains have evolved to be specific to particular chimp subspecies, which are isolated from each other by rivers. Some subspecies may not harbor the virus, she says, while others may be infected with a strain that is less likely to spread in humans.

Vanessa Hirsch, a primate researcher at the National Institute of Allergy and Infectious Diseases, says she is impressed by the data. "Everyone has kind of been pussy-footing around the question of whether chimps were the origin," says Hirsch. "As more isolates are studied, it becomes more believable."