Pioneering work at the California Regional Primate Research Center, University of California, Davis, is bringing a new understanding of the relationship between air pollution, common allergies and asthma.
The research, funded by the National Institutes of Health, shows for the first time that occasional exposure to the air pollutant ozone can change how the lungs of young rhesus monkeys develop, and lead to a disease similar to childhood asthma in humans.
"We have the first real monkey model of human asthma here," says Dallas Hyde, co-investigator and interim director of the CRPRC. "What we are seeing is quite disturbing, as we would not have assumed these levels (of ozone) would change lung development."
Asthma affects nearly 15 million people in the United States, one-third of them children under 18. "Asthma does seem to be increasing," says Dr. Jesse Joad, another member of the team, who runs the pediatric asthma program at the °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis Medical Center in Sacramento. "It is a very common disease, mostly treated in primary care," she says.
Although a direct link between ozone pollution and childhood asthma has yet to be proven, Joad has noticed that when air quality goes down, admissions with severe asthma go up. "We had a really bad week of admissions in mid-September," she says, "when there were high ozone days at the beginning of the week." Children admitted to the clinic are often from inner-city Sacramento, and have severe attacks, sometimes needing artificial ventilation.
Recent data supports the idea that asthma in children and adults might be related to growing up in a polluted environment, according to principal investigator Charles Plopper of the °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis School of Veterinary Medicine, who leads the °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis team. A survey of °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Berkeley freshmen by the °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Berkeley School of Public Health found a strong correlation between lung diseases like asthma and growing up in counties with high ozone levels -- especially if the subjects were exposed to ozone between birth and three years old.
Plopper's team has been studying the effects of ozone pollution and allergies on the lungs of adult and infant rhesus monkeys. "We needed a model that closely replicates the human situation, in a species with lung development similar to humans," says neuroscientist Ed Schelegle. Crucially, in both monkeys and humans, most lung development takes place after birth, he says.
Plopper is clear that these studies could not have been performed in rats, mice or guinea pigs. "The respiratory system of primates (humans and monkeys) is very different to that of rodents," he says. Primates show a completely different response to pollutants than rats, which are relatively resistant to ozone, he adds. In rodents, most lung development is complete at birth or soon after, making them unsuitable for studying childhood lung disease.
In the °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis studies, monkeys of various ages were exposed to air contaminated with ozone for five days, followed by nine days of normal air. This cycle was repeated every two weeks for five months, mimicking the effect of exposure to occasional ozone smog -- for example as it occurs in the Sacramento area.
"The doses of ozone used were about three times the level of a smog alert day in Los Angeles," says Plopper, adding that comparable ozone levels can be found on an average day in Mexico City. In some experiments, the animals were also exposed to house dust mite allergen -- a common cause of allergy and asthma.
Adult monkeys exposed to house dust mite allergen developed lung disease comparable to human asthma, as defined by the NIH, says Schelegle. The disease became worse when the monkeys were also exposed to ozone. This work will be published in a forthcoming article in the American Journal of Pathology.
When young monkeys were exposed to episodic ozone from soon after birth, they showed changes in lung development. The team, including Michelle Fannuchi, Laura van Winkle and Michael Evans, found that the lungs had fewer branches, and there were other structural and cellular changes like those seen in human asthmatics, says Plopper. There were also increases in the response of the airways to stimulation, adds Schelegle. Affected airways closed down more dramatically when stimulated, a condition called airway hyperresponsiveness.
These changes became more pronounced when the monkeys exposed to ozone were also sensitized to house dust mite allergen. When these young monkeys were later exposed to house dust mite in air, they developed rapid, shallow breathing with a drop in oxygen levels in the blood -- symptoms very like the wheezing of childhood asthma.
Exposure of monkeys to dust mite allergen alone, without ozone, causes less severe changes in the lung, says Plopper, speculating that ozone damage might help large allergen particles to enter lung tissue more easily.
A core team of 15 °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis faculty from many disciplines, plus additional researchers, has been assembled for the project, headed by Plopper, Hyde and Joad. "The real unique thing here is the team approach," says Plopper, "we've put together a small village to do this."
Because of the breadth of expertise on the team, from neuroscientists and immunologists to biochemists and physiologists, they are able to extract the most information from every tissue sample. The result is an approach that treats the lung as a complete system.
A major aspect of the work is the role of the immune response in asthma. The °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis team is the first to integrate studies of the immune response with detailed anatomy, biochemistry and other data.
"Studying spatial distribution of the immune response is a unique contribution of this model," says Hyde. The team, including Laurel Gershwin, Lisa Miller and Reen Wu, have already found clear differences in the immune system between normal lungs and those of monkeys exposed to ozone and house dust mite allergen: "I think we're changing the immunology of the lung," says Hyde.
In a surprising finding by the team, toxicologist Alan Buckpitt discovered that exposed infant monkeys lost a key protective chemical, glutathione, from lung fluids and cells. This change has also been found in human asthmatics, but has never before been demonstrated experimentally, according to Plopper.
The researchers have also found changes in the nerve fibers running from the lungs to the brain following episodic ozone treatment. Stimulation of these nerve fibers causes the airways to narrow. Some types of nerve fibers became more sensitive to stimulation, says Joad. At the same time, co-workers Ann Bonham and Chao-Yin Chen found that cells in the brainstem, where these same nerve fibers end, became more sensitive. "These cells do become more excitable," says Joad.
Most worrying is that the studies on infant monkeys suggest that the changes in the lung caused by ozone pollution are long lasting, perhaps permanent, says Joad. "If there are permanent changes that can't be reversed, that has very serious implications," she says. At present, milder cases of childhood asthma are treated only to control symptoms, but if there were long-term consequences, more aggressive treatment with inhaled steroids could be justified in these patients. Experiments with monkeys could help to answer these questions and improve treatment for childhood asthma, she says.
The recent renewal of a grant from the National Institute for Environmental Health Sciences funds about half the cost of the work. Impressed with the results, the National Institute for Allergy and Infectious Diseases is providing supplemental funds for other researchers to make use of the monkey asthma model and facilities established at °ÄÃÅÁùºÏ²Ê×ÊÁÏ¿â Davis.