Learning To Ignore Your Viruses

Carl Zimmer in his blog, The Loom:

A couple weeks ago I wrote about the 98,000 viruses that have permanently pasted their genes into our genome over the past 60 million years. What makes these viruses doubly fascinating is that scientists are making new discoveries about them all the time. Over at the open-access journal PLOS Pathogens, two new papers add some pieces to the puzzle of how these viruses get into our genomes, and how they affect our health along the way…

HIV is also a retrovirus, meaning that it inserts its genes into our own. But it is not a live-in virus. It primarily infects one class of white blood cells, and then spreads to other people through shared needles, sex, and other forms of contact. HIV leads to the collapse of the immune system, otherwise known as AIDS. Growing evidence suggests that it does so not by killing cells directly, as once thought, but by chronically overactivating the immune system. As the immune cells divide madly, they eventually start malfunctioning and even committing suicide.


In an opinion piece in PLOS Pathogens, Viktor Muller and Rob J. De Boer point out that most of HIV’s cousins, which infect other primates, don’t do anything of the sort. I’ve reproduced a tree they put together, showing the relationship of HIV-like viruses in apes and monkeys. (Go here for a closer view.) HIV, marked in red, is not a single lineage of viruses. One form, HIV-2, jumped from sooty mangabey monkeys into people several times. The more common form, HIV-1, descends from chimpanzee viruses, which have moved into humans many more times. As the tree shows, lots of primates get infected by their own HIV relatives, and this appears to have been going on for millions of years. But if you look at sooty mangabeys or some other monkey, you generally find abundant amounts of the virus without any sign of an overactive immune system. It’s not that the virus carried by sooty mangabeys is weak. Scientists have injected it into other monkeys, and it has triggered a strong immune response. The blue arrows on the tree mark the rise of new virus strains in macaques that came from sooty mangabeys. This shift appears to have happened at primate research centers in the past few decades. In their new hosts, these viruses cause lots of nasty symptoms.

More here.