The study showed how an initial viral infection can block production of critical immune system proteins known as type I interferons, which plays a range of roles including direct antiviral effects, activating innate natural killer cells and adaptive T lymphocytes, which destroy a wide range of infectious invaders.
"It's the first demonstration that a virus causes suppression of the interferon response in vivo," said senior author Michael Oldstone, a Scripps Research professor and a pioneer in immune system studies.
"This model explains how a secondary infection can be caused by a normal virus infection and this provides the guide for what to do and where to look in human diseases, which are of course more difficult," he added.
The research team led by Elina Zuniga, formerly a postdoctoral fellow in the Oldstone lab, worked with mice infected with lymphocytic choriomeningitis virus.
They found that the virus suppressed the mouse immune system by interacting with immune cells known as plamacytoid dendritic cells, which are key producers of one of two critical groups of interferons, known as type I.
When plasmacytoid dendritic cells come in contact with viruses and other foreign invaders, they bind with them via membrane proteins known as toll-like receptors.
Under normal conditions, this binding triggers massive production of type I interferon that then triggers other immune responses.
But the lymphocytic choriomeningits virus, and presumably other immunosupressor viruses like measles and HIV, disables this system.
It also affects the activation of the natural killers that would otherwise destroy the virus-infected cells, as well as other invaders.
The study also showed that while the initial lymphotcytic choriomeningitis virus effectively blocked interferon production, it did not kill the dendritic cells, instead allowing them to function as long-term hosts. This allows such viruses to persist, causing persistent immunosuppression.
"I think the implications are that many of the diseases we don't know the causes for, be they behavioral, mental, cardiovascular, or endocrine, may well be caused by viruses that persist without destroying the differentiated cells they infect, alter their functions, and by this means alter homeostasis and cause disease," said Oldstone.
He said that knowing such basic details about how a virus can suppress the mouse immune system could well aid the development of new treatments for the many immunosuppressive conditions such as HIV and measles that plague humans.
The study appears in the journal Cell Host & Microbe.