An immune signaling protein called interleukin-21 (IL-21) plays a critical role in the long-term control of chronic viral infections such as HIV disease, according to a study published online in Science and reported by ScienceDaily.

If CD4 cells are the directors of the body’s immune response, then “killer” CD8 cells do the dirty work of actually finding and destroying pathogens. Researchers have been certain that the CD4 and CD8 cells are able to communicate with one another, but until now, they haven’t known for sure what chemical proteins were necessary for that communication to take place.

To determine the role played by IL-21, Heidi Elsaesser, PhD, at the University of California in Los Angeles, bred mice whose immune cells are unable to make receptors for IL-21. They then infected the specially bred mice and normal mice with one of two strains of a virus. The first virus causes only a short-term infection, and the other causes a long-term chronic infection. Both types of mice managed to control the short-term infection, but the mice without IL-21 receptors couldn’t make enough CD8 cells to control the chronic infection.

“After the immune system loses CD8 T-cells, it’s unable to clear the virus,” said study coauthor, David Brooks, PhD. “This tells us that IL-21 is a critical player in the body’s fight against chronic infection.” These findings could help researchers understand how and why the immune system ultimately fails to control HIV in most people.

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