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FRIDAY, June 18 (HealthDayNews) -- A normally harmless virus can become lethal when a major part of a mouse's immune system is disabled, a new study says.

This research may offer insight into how otherwise innocuous viruses can cause severe disease in humans with weakened immune systems, such as AIDS patients, transplant recipients, and people with autoimmune diseases.

There are two major branches in the immune systems of mice and humans. The innate immune system responds rapidly to threats from viruses and other invaders but is fairly inflexible. The adaptive immune system is slower to respond to a threat but is able to adapt in order to continue attacking a mutating invader.

In this study, scientists at Washington University School of Medicine in St. Louis disabled the innate immune branch in mice. A normally harmless virus called cytomegalovirus (CMV) killed these mice within about a week.

The scientists then disabled the adaptive immune branch in mice. They initially survived the CMV infection. But within three to four weeks, the virus mutated and killed the mice.

"The virus exploits whatever little crack you give it in the mice, so we're very interested in finding out whether this also occurs in humans with weakened immune systems," study first author and pediatrics instructor Dr. Anthony R. French said in a statement.

The study appears in the June issue of the journal Immunity.

More information

The U.S. National Institute of Allergy and Infectious Diseases has more about the immune system.

(SOURCE: Washington University School of Medicine news release, June 15, 2004)

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