Immune System Cells May Be Cause of Asthma

New finding could spur development of better treatments, researchers say

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By Serena Gordon
HealthDay Reporter

WEDNESDAY, March 15, 2006 (HealthDay News) -- As medical technologies improve, researchers are rooting out more information about possible causes of common diseases, such as asthma.

One new finding, reported in the March 16 issue of the New England Journal of Medicine, is that immune system cells long thought to cause asthma may not be the primary culprit behind the disease.

"We found that asthma is caused not by T-helper 2 cells as has been previously thought, but by a novel class of cells called natural killer T cells," said one of the study's authors, Dr. Dale Umetsu, a professor of pediatrics at Harvard Medical School and Children's Hospital Boston, and a visiting professor at Stanford University in California.

"The majority of T cells in people with asthma aren't what we thought they were," he added.

According to Umetsu, natural killer T cells were only recently discovered because the technology to differentiate these cells from others hasn't been around long.

T cells are a part of the body's immune defenses and normally help rid the body of foreign invaders, such as viruses or bacteria. In asthma, however, the immune cells don't work as they should and instead produce inflammation in the lungs.

More than 20 million Americans have been diagnosed with asthma, according to the U.S. Centers for Disease Control and Prevention, and about 4,200 people die from the disease each year.

Asthma symptoms include wheezing, coughing, breathing difficulties and a feeling of tightness in the chest. While the exact cause of the disease is unknown, doctors do know that asthma can be exacerbated by exposure to certain triggers, such as dust mites, pollen, pets and even exercise or cold air. There is no cure for asthma, only treatments aimed at managing it.

Because studies in mice uncovered the presence of natural killer T cells only in rodents with asthma, the researchers behind the new study compared samples from 14 people with asthma to samples from six healthy "controls" and five people with another inflammatory lung disorder called sarcoidosis, which is unrelated to asthma.

About 60 percent of the T cells in the asthma group were natural killer T cells, not the expected helper T cells. No natural killer T cells were evident in samples from the healthy control group or the people with sarcoidosis.

"These were very surprising findings -- a turn of events that no one suspected in the past," said Umetsu. "Part of the reason they escaped notice is they have many features that are similar to T helper 2 cells. Now, we need to know more about the biology of natural killer T cells to develop more specific therapies for asthma."

"None of the current [asthma] therapies are focused on targeting natural killer T cells. Perhaps as we develop therapies that can eliminate them from the lungs, we could have more effective and possibly curative therapies for asthma," he said.

The first step, however, is to confirm these findings in a larger group of people, and in a more diverse population of people with asthma, because there are different types of asthma. Some people have asthma that's triggered by allergens, while for others exercise or cold air can induce airway spasms.

Also, Umetsu said that researchers have to learn more about how these cells work and what causes them to go to the lungs initially. Natural killer T cells appear to respond to different things than helper T cells.

Any potential therapy would have to specifically target the lungs because natural killer T cells do have some protective effects in the rest of the body, he added.

Dr. Jonathan Field, director of the allergy and asthma clinic at New York University Medical Center/Bellevue in New York City, said, "This may be a new paradigm of how people develop asthma."

But, he cautioned that more needs to be learned about these cells, such as whether they are the actual cause of disease or if they simply appear in response to the disease.

"You have to wonder which [immune cells] actually are causing the most pathology," Field said. "Are natural killer T cells causing the changes? Which cell is the conductor and which is the actual locomotive?"

More information

The National Library of Medicine has more about asthma.

SOURCES: Dale Umetsu, M.D., Ph.D., the Prince Turki Al-Saud Professor of Pediatrics, Harvard Medical School and Children's Hospital Boston, and visiting professor at Stanford University, Palo Alto, Calif.; Jonathan Field, M.D., director of the allergy and asthma clinic, New York University Medical Center/Bellevue, New York City; March 16, 2006, New England Journal of Medicine

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