Rather than the viruses causing infection directly, it is the proteins they make that seem to be the culprit, triggering an immune response that led to arthritis in mice.
Meanwhile, blood tests from a small group of people with arthritis suggest they are more likely than healthy people to carry antibodies to a particular mimivirus protein that is of the type that comes under attack in the condition.
Though bigger studies in people are now needed, the findings suggest mimiviruses could trigger some human cases of rheumatoid arthritis, which is caused by the body's immune system attacking itself.
Discovered more than a decade ago in a cooling tower in Bradford, UK, mimiviruses get their name because their size mimics that of bacteria rather than other viruses. They are known to infect and kill amoebas, but no link to disease in humans had previously been found.
While trawling through publically available gene libraries, Thierry Hennet of the
University of Zurich, Switzerland, noticed that mimiviruses make proteins called collagens that are very similar to those made by humans.
Hennet's main research focus is how arthritis is affected by collagens. These are the basic structural proteins in animals, which break down in rheumatoid arthritis – and have never been seen in viruses before.
He wondered whether people coming into contact with mimiviruses, through exposure to contaminated sea or lake water for example, might make antibodies against the mimivirus collagen. These antibodies might then go on to attack human collagen because it's so similar, eventually leading to arthritis. To investigate, his team injected mice with a mixture of 60 proteins that had been extracted from mimiviruses bred in the lab, including several collagens.
They also injected a second group of mice with just one of these collagens,
L71, and a third group with just cow collagen, already known to trigger arthritis in mice.
The group that received the cow collagen developed the most severe