Cure for killer flu 'discovered'

The immune system reacts strongly to flu

Scientists believe they may have found a way to beat even the most powerful strains of flu and maybe even Sars.

Researchers at Imperial College London say they are able to control the immune system's response to flu.

Previous studies have suggested the immune system may sometimes do more harm than good when it comes to flu.

This is because it responds too strongly to an attack, preventing recovery and in extreme cases attacking the body and causing death.

A study published in The Lancet late last year suggested this was why flu can turn from a nuisance virus into a killer virus.

Dr Tracy Hussell and colleagues at Imperial carried out tests on mice, which had been infected with influenza A.

Killer strain

The "A" strain is the most powerful strain. It was responsible for the 1919 flu pandemic, which claimed the lives of 20m people around the world.

Many experts believe it is only a matter of time before another major outbreak occurs. If it does happen, it is likely to be down to a mutated "A" strain.

If the clinical symptoms of Sars are caused by an excessive immune responses it too could be effectively treated Dr Tracy Hussell, Imperial College London

When flu infects the body it triggers a two-fold response from the immune system: T cells, which patrol the body keeping a look out for invaders, seek out flu cells and try to destroy them; and B cells follow close behind and produce antibodies to prevent the body from allowing the virus in again.

The T cells produce high levels of cytokines, which enable the body to kill harmful cells. However, this can lead to problems.

"During flu infection, the immune system has an 'all hands on deck' attitude to the viral assault," said Dr Hussell.

"But it's this that causes most of the damage. The exaggerated immune response produces inflammatory molecules that lead to what's known as a 'cytokine storm'.

"Essentially too many cells clog up the airways and prevent efficient transfer of oxygen into the bloodstream."

Scientists have been trying to tackle this problem by stopping T cells in the first place. However, this leaves the patient unable to clear the virus and susceptible to other infections.

Dr Hussell's team has developed a way of controlling these T cells. They have discovered that blocking a molecule called OX40 causes these cells to move on more quickly. As a result, they are not clogging up airways.

"OX40 sends out a survival signal instructing activated T cells to remain in the lungs for longer to help fight the infection," said Ian Humphreys, who was also involved in this study.

"However, because new cells are arriving all the time this prolonged presence is not needed," he said.

"Inhibiting this signal therefore allows T cells to vacate the lungs earlier whilst leaving behind a sufficient immune presence."

This molecule can be blocked by using a drug called OX40:Ig.

'Reverses symptoms'

The tests on mice showed that those that were treated with this drug no longer had flu. The drug was effective both in the early stages of infection and also 20 days into the attack.

Dr Hussell said the findings suggested the drug could be used to treat any disease that is caused by an excessive T cell inflammatory response. This could range from asthma and bronchitis to pneumonia or even Sars.

"If the clinical symptoms of Sars are caused by an excessive immune responses it too could be effectively treated," Dr Hussell said.

Further research is needed before this technique can be tested in humans. Scientists will also have to ensure that the body is still able to fight off infections.

"The cytokine response is what causes you to feel ill when you have flu," Professor Alan Hay, a flu expert from the National Institute for Medical Research in London, told BBC News Online.

"If you can reduce that you can affect the symptoms. But it is also important that you control the virus infection."

The study is published in the Journal of Experimental Medicine.