Chimp Change: Did an HIV-like virus ravage early chimps?

Two million years ago, chimpanzees experienced a drastic loss of variability in certain genes that help them fight viruses, a new study reports. Widespread infection with an HIV-like virus may have pruned from the species individual chimps with some of these immunity genes, the study’s authors hypothesize. Ironically, the change could have conferred AIDS resistance on modern chimpanzees.

Chimpanzees are more genetically diverse than people (SN: 11/06/99, p. 295). That is, specieswide, they tend to have more polymorphisms–or different versions of a given gene–than humans do. But this trend is unexpectedly reversed for the immunity genes known as class I of the major histocompatibility complex (MHC), Ronald E. Bontrop of the Biomedical Primate Research Center in Rijswijk, the Netherlands, and his colleagues report in the Sept. 3 Proceedings of the National Academy of Sciences.

Many viruses circumvent a host’s immune response by holing up in host cells. An animal’s MHC class I gene products undermine that strategy by enabling the host cell to present on its surface a telltale piece of the virus. This snippet signals circulating killer cells to attack. Different variations of MHC molecules choose different viral pieces to present, and some pieces are more effective than others in alerting a killer cell to fight off disease.

The Dutch scientists compared sequences of a stretch of MHC class I DNA from a group of chimpanzees and from a group of people. They found that chimps have far fewer different versions than people do. Though these species inherited MHC genes from a common ancestor, humans retain many lineages of MHC molecules that chimps appear to have lost about 2 million years ago, the scientists show.

A pandemic viral infection, possibly with an ancestor of HIV, was the likely cause of the selective sweep, says Bontrop. In history, often when a pathogen has wiped out a large percentage of an animal species, a few individuals–usually with some genetic advantage that helped them survive–have started a new population, he says.

HIV-1, the strain responsible for the current human AIDS pandemic, arose from simian immunodeficiency virus (SIV), which infects chimps (SN: 2/06/99, p. 84). However, chimpanzees infected with SIV or HIV-1 don’t develop AIDS, suggesting that modern chimps may be the offspring of AIDS-resistant survivors of an HIV-like pandemic.

What’s most amazing, Bontrop says, is that the chimps’ remaining MHC molecules target viral pieces similar to those selected by the MHC molecules in a small fraction of people–those rare individuals who, if they become infected with HIV, never develop AIDS. Focusing on the pieces of HIV-1 that these people’s immune systems target may help researchers design a vaccine, he says.

A past chimp die-off from infectious disease is probably the culprit for the paucity in chimpanzees’ MHC repertoire, agrees MHC researcher Mary Carrington of the National Cancer Institute in Frederick, Md. “The question is, Which pathogen or pathogens are responsible?” SIV is a reasonable candidate, but multiple disease agents may have been involved, she says.

“There is another lesson here,” Bontrop explains. “HIV, which is a new virus for the human population, is a formidable opponent, and we shouldn’t underestimate it.”