CHIMPANZEES may have survived their own AIDS-like pandemic more than 2 million years ago.
Researchers in the Netherlands and the US have found evidence of a devastating viral pandemic among chimpanzees in the form of a genetic scar it left behind in their genome. If they鈥檙e right, this could explain why modern chimps don鈥檛 suffer from the devastating effects of HIV infection 鈥� they are the descendants of AIDS-resistant animals.
Ronald Bontrop at the Biomedical Primate Research Centre in Rijswijk, Paul Gagneux of the University of California, San Diego, and their team compared immune system genes from captive chimps with those from the human genome. All genes come in variant forms or alleles that differ slightly between individuals. But the group of genes Bontrop and Gagneux鈥檚 team was interested in, known as the major histocompatibility complex class I (MHC I), are unusually diverse.
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These genes encode proteins that sit on the surface of cells. If a cell becomes infected with a virus, the MHC I molecule grabs a bit of viral protein, which attracts killer cells to destroy the infection before it can spread. Because the immune system must deal with so many different types of infection, natural selection pushes it to have a good mix of different MHC I genes, each with a different talent for detecting viruses. But to the researchers鈥� surprise, when they compared genes from 47 captive chimps with those in human genome databases, they discovered that our close cousins have only about half as many MHC I alleles as we do (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.182420799).
By contrast, for most genes chimpanzees have 5 times as much genetic variation as humans. Having compared these genes in humans and different chimp subspecies, the researchers conclude that the chimps lost their variation in MHC I genes between 2 and 3 million years ago. 鈥淚t was a surgical selection, some genes got streamlined, other types of genes weren鈥檛 affected at all,鈥� says Gagneux.
No one knows why, but the most obvious explanation is that ancestral chimps became widely infected with a deadly virus, says Gagneux. In effect, the virus must have ruthlessly selected for chimps with particular MHC I genes that made them resistant, reducing the range of MHC I genes in the population.
Stewart Cooper of the University of California, San Francisco, whose research has also suggested that MHC I genes are less variable in chimps, says the new work is rigorous and provocative. It should now be checked in wild chimp populations, since captive chimps tend to come from just a few areas, he says.
Gagneux agrees. As for the identity of the pathogen that caused the ancient pandemic, an HIV-like virus is an obvious candidate. Bontrop鈥檚 group has already found some evidence that MHC I genes are involved in HIV resistance in chimps. And some people infected with HIV who never develop the symptoms of full-blown AIDS have class I genes associated with resistance. Understanding how MHC I molecules contribute to that resistance in different species could help researchers design more effective vaccines, says Gagneux.