Humans may lack resistance to HIV in part because a potentially defensive protein is still guarding against a long-extinct virus, scientists reported Friday.
Researchers have known that some primates, such as macaques, can fight off HIV with an antiviral protein called TRIM5-alpha, whereas the human version, though only slightly different, cannot combat HIV.
Scientists at the Fred Hutchinson Cancer Research Center in Seattle wondered just what virus the human protein was able to attack. One candidate was an ancient virus, PtERV1, which left relics of itself in the DNA of modern chimpanzees.
Retroviruses can insert themselves into their hosts’ DNA, sometimes leaving behind their genes as evidence of the infection. Experts estimate that 8% of human DNA is made up of defunct viral genes.
But the human genome has no trace of the PtERV1 virus.
“This implies that either humans were never infected or humans were selected for resistance to the virus,” said virologist Michael Emerman, senior author of the study, published in Science.
The researchers were able to partly reconstruct the virus, though it has been extinct for 3 million to 4 million years. They found that the human version of the TRIM5-alpha protein could bind to the virus, labeling it for the cell to destroy.
Tinkering with the protein, the scientists found that a single mutation made it better at recognizing HIV but worse at attaching to the ancient virus.
This antiviral protein appears to be able to fight one virus or the other, but not both, the authors concluded.