Scientists Identify How Key Protein Keeps Chronic Infection in Check

Why is the immune system able to fight off some viruses but not others, leading to chronic, life-threatening infections like HIV and hepatitis C?

A new UCLA AIDS Institute study suggests the answer lies in a protein called interleukin-21 (IL-21), a powerful molecule released by immune cells during chronic infection. Published May 7 in the online edition of Science, the finding could explain how the immune system limits viral replication, restricting a virus's spread through the body.

The researchers looked at two types of T-cells — CD4 T-cells and CD8 T-cells — which are immune cells that play an important role in the body's response to infection. The CD4 T-cells help the immune system by producing IL-21 during chronic infection, bolstering the CD8 T-cells' ability to fight off the virus.

"The CD4 cells are the regulators — the generals, if you will," said principal investigator David Brooks, assistant professor of microbiology, immunology and molecular genetics at the David Geffen School of Medicine at UCLA. "The CD8 cells go out and kill the invaders; they're like the privates in the field."

To shed light on how CD4 T-cells help their CD8 counterparts clear viruses, the researchers infected mice with one of two strains of a virus. They knew that the first strain would generate a short-term infection and the second a chronic infection.