HIV may have “accidentally” evolved to become deadly, rather than benign, like similar viruses in monkeys, say researchers at a presentation at the General Microbiology’s 162nd Meeting today in Edinburgh, Scotland. These new data may help explain why HIV damages the immune system in most humans, whereas its close cousin, simian immunodeficiency virus (SIV), doesn’t harm most monkeys.

While most people would intuitively assume that an active immune system response to the virus is a good thing, researchers have actually shown that an overactive response in people with HIV causes great harm to the immune system. What remained unknown, however, was why human immune systems react so powerfully to HIV, when monkey immune systems barely respond to SIV.

Building on previous studies, researchers today presented data showing that differences in the viruses’ NEF proteins may explain this discrepancy. The NEF protein in both SIV and HIV helps the viruses evade detection by the immune system. SIV’s NEF protein, however, also removes a receptor called CD3 from the surface of T cells, which allows them to become active. HIV’s NEF protein does not remove this molecule, thus resulting in the hyperstimulation of the immune system and, eventually, its destruction.

The researchers stated that their next step is to genetically alter SIV in such a way that it makes NEF proteins that, like HIV, don’t remove CD3 receptors and see if this will cause SIV to become deadly in monkeys. Should those experiments confirm that NEF plays such a key role, it may be possible to develop treatments for HIV that play off the unique immune-dampening quality of SIV’s version of NEF.