People with cystic fibrosis may produce too little mucus in their airways, according to US scientists.
The surprise finding may lead to new treatment for CF
The team at Wake Forest Baptist Medical Center found people with CF had abnormally low levels of two mucus-making proteins in their spit.
This suggests boosting mucus production might be a new way to treat this fatal genetic disease, they say.
Their findings are published in the American Journal of Respiratory Cell and Molecular Biology.
One in 25 people in the UK are carriers of the faulty gene that causes CF.
Scientists know people with CF are prone to lung infections and when they get an infection their body can produce thick fluid secretions that clog the lung.
Dr Bruce Rubin and colleagues wanted to investigate whether excess mucus was to blame.
They collected sputum from 12 patients with CF and 11 healthy patients without any lung disease.
The sputum from the CF patients had much lower levels of two proteins that make mucus than the sputum from healthy people.
"This showed unequivocally there is much less mucus in the CF airway," said Dr Rubin, a paediatric pulmonologist at Wake Forest Baptist's Brenner Children's Hospital.
The substance that was blocking the CF patients' lungs turned out to be pus - collections of lung cells and bacteria.
"They had loads of phlegm, but in fact that phlegm was pus. We always thought there would be loads of mucus also but there isn't," he said.
Dr Rubin believes that increasing mucus levels could help.
"Mucus is good for you. It traps particles that you inhale and helps them get swept out of the lung. Healthy people produce mucus when they get an infection as a defence.
"Perhaps mucus is critical for preventing infection in the CF airway. In its absence the bacteria get there and find a welcome playground," he said.
To test their theory the researchers plan to do a study in animals to see if mucus can effectively soak up the bacteria they believe is reproducing in the airway of CF patients.
"If it turns out that mucus is protective against bacteria, we may have a treatment for CF," said Dr Rubin.
"There are ways to increase mucus production in normal airways, we just need to show that they are also effective in CF airways," he said.
The scientists also hope to find out why people with CF produce less mucus.
From their study they know that the mucus is not produced rather than produced and then broken down.
"Perhaps the mucus is in the cells and not getting out. It may be that the cells are not producing the mucus. We need to do more work," Dr Rubin said.
Dr Jim Littlewood, chairman of the Cystic Fibrosis Trust said: "I have never heard that it is an underproduction of mucus by these proteins. It's a new theory and there may be some substance in it.
"We have known for a long time that the main problem is the lungs eventually become chronically infected in the majority of patients and the reason for this is still unknown.
"We are interested in any new development which may improve the treatment of people with CF and will look forward with interest to the results of the proposed animal experiments looking into the antibacterial role of mucus," he said.