Protein linked to immune system response identified

LONDON, Jan 18 (Reuters) Scientists have identified a protein that could help them understand why symptoms of the common cold such as sniffles and congestion last for a limited time.

The previously unknown protein called carabin cranks up the immune system response during an infection and then shuts it off when it has done its job so that it doesn't harm healthy cells.

''We found an important missing brake within white blood cells that keeps the system in check so it doesn't override itself to cause problems during an virus infection or in the common cold,'' Professor Jun O Liu, of Johns Hopkins University in Baltimore, Maryland, said in an interview.

Liu and his team were searching for immune system proteins that were active during infections when they homed in on carabin.

The protein is usually present in low concentrations in white blood cells but during an immune system reaction, levels increase.

As they rise, the system gradually shuts down.

''It allows the immune system a window of time to respond and hopefully that is enough to wipe out the viral infection,'' said Liu, whose findings are reported online by the journal Nature.

White blood cells in the immune system attack cells that have been infected with a virus. They also switch on genes to fight the infection.

When the scientists added the protein to white blood cells already primed to fight infection, the more carabin they placed in the cells, the less active they became.

''This is an important brake, but most likely, it is not the only brake. We think it plays a very prominent role,'' said Liu, adding that it controlled the duration and the extent to which the immune system is activated.

''It basically sheds new light on how the immune system is controlled. The deregulation of the immune response is responsible for many, many human diseases,'' he added.

If further studies confirm the importance of carabin as a major inhibitor of immune response, Liu believes it could have important implications for understanding and treating auto-immune diseases.

It could also play a role in preventing the rejection of donor organs in transplant patients.

''We now have a better grasp of how the (immune) system works, a more complete picture of how the system is laid out,'' Liu added.

Reuters SP GC0929