Immune System 'Killer' T-Cells May Not Be Key to Asthma

WEDNESDAY, April 4, 2007 (HealthDay News) -- While previous research suggested that a certain type of immune system cell -- called invariant natural killer T-cells -- might play a role in the development of asthma, a new study concludes that's just not the case.

In stark contrast to a previous study that found that these cells made up about 60 percent of the T-cells in an asthmatic's lungs, the current study found that fewer than 2 percent were invariant natural killer T-cells.

"We not only failed to find any evidence of raised counts of invariant natural killer T-cells in mild asthma and chronic obstructive pulmonary disorder but also failed to reproduce the observations [from past research] in moderate to severe disease," said one of the study's authors, Dr. Ratko Djukanovic, professor of respiratory medicine and director of Allergy and Inflammation Research at Southampton School of Medicine in the United Kingdom.

Djukanovic said he's not sure why this study found such low numbers of killer T-cells, but that improvements in the technology may have allowed his team to identify invariant natural killer T-cells with a higher degree of specificity.

Results of the study are published in the April 5 issue of the New England Journal of Medicine.

Asthma is a chronic lung disorder that causes symptoms such as wheezing, coughing and shortness of breath. About 20 million Americans have the lung disease and more than 4,000 people die of asthma complications each year, according to the U.S. Centers for Disease Control and Prevention.

Despite the disease's prevalence, the exact cause remains unknown. Health experts know that asthma can be triggered by a reaction to certain substances, such as dust mites, pollen or even cold air. It was commonly believed that exposure to allergens would cause the body to produce T-cells known as CD4+ helper T-cells, which would then release substances called cytokines that cause airway inflammation.

However, research published last year suggested that rather than the helper T-cells, it was actually killer T-cells at the root of this process in people with severe asthma.

Intrigued by this possibility, Djukanovic and his colleagues sought to reproduce these findings and to see if natural killer T-cells were also present in large numbers in people with milder forms of asthma.

The researchers examined airway cells from 24 people with asthma, 10 with chronic obstructive pulmonary disease (a combination of emphysema and chronic bronchitis) and 12 healthy controls.

After examining the cells, the researchers found that fewer than two percent of the T-cells were invariant natural killer T-cells.

"Invariant natural killer T-cells represent a minority population within the airways of both healthy lungs and lungs from patients with asthma or COPD," said Djukanovic, who added that this doesn't necessarily mean that killer T-cells don't play any role in asthma.

"It is still unclear what their role is within the highly complex human immune system, and how changes in invariant natural killer T-cell numbers or function could cause disease," he said.

"Whether or not natural killer T-cells expand in numbers to the extent reported [in past research], studies in animals suggest that these cells are likely to have a role in the pathogenesis of asthma," said Dr. Ling-Pei Ho, of Oxford University in the United Kingdom, in an accompanying editorial.

"This research is still in the exploratory phase, and this study highlights the fact that it depends on how you look for these cells" said Dr. Ricardo Vinuya, an asthma specialist at Providence Hospital and Medical Center in Southfield, Mich. The exciting possibility raised by killer T-cells, he said, is that they would give researchers a new, more specific target for developing new medications.

More information

To learn more about the causes and triggers of asthma, visit the U.S. National Heart, Lung and Blood Institute.