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IMMUNOLOGY Putting the Squeeze On Asthma InnatelyDiscovery of Instigating Agent Could Lead to Drug Target Just the act of breathing—usually so effortless and assured—can turn into a terrifying ordeal with an attack of asthma. Legions of immune cells rush into the airways of the lungs, causing the delicate tubules to become swollen and inflamed. The mucous coating that normally cleanses the tubes, the bronchi and bronchioles, becomes so thick that it clogs the pathways rather than clearing them. Making matters worse, the muscles surrounding the bronchial tubes constrict, causing the airways to be strangled from outside, as well.
"Whenever you get a basic discovery like this, it has bigger implications,"says Craig Gerard, second from left. Coworkers on the project are, from left, Bao Lu, assistant professor of pediatrics; Norma Gerard; and Alison Humbles. For years, researchers have struggled to understand what triggers this suffocating siege. Fifteen years ago, they identified a group of inflammatory agents, the leukotrienes, which appear to recruit immune cells to the lungs and incite the mucus-secreting cells to overproduction. Blocking these inflammatory agents, they found, could halt small muscle attacks and mucus secretion, but only in a minority of patients. Researchers suspected that something else must be goading the attack, but so far they have not been able to finger another inflammatory agent. In the Aug. 31 Nature, Craig Gerard and his colleagues, led by Alison Humbles, HMS research fellow in pediatrics at Children's Hospital, and Norma Gerard, HMS professor of medicine at Beth Israel Deaconess, report that they have flushed out a candidate, and from the most unlikely of ranks. The immune system is divided into two main arms, the innate and the acquired. Asthma was thought to be due to an overzealous acquired immunity, but the newly identified target, the C3a receptor, is a key player in the innate immune system. An Unusual SuspectIn contrast to the acquired system, the innate system does not need to be primed by a previous foreign invader, which suggests it could react immediately to asthma-inducing allergens."My suspicion is that a member of the innate immune system is acting way downstream as part of the dirty work in this disease," said Craig Gerard, an associate professor of pediatrics at Children's Hospital. He and his colleagues uncovered the agent by knocking out the receptor for C3a in mice, thereby depriving the animals of their ability to heed the C3a inflammatory call. The mutants had fewer problems breathing and displayed less sensitivity when challenged with an allergen. In a follow-up study, they found that the lungs of human asthmatics that had been challenged with asthma-inducing allergens produced more C3a than unchallenged lungs, suggesting the inflammatory molecule is produced in greater amounts in human asthma. If so, depriving humans of the use of their C3a receptors, and thus of their ability to heed C3a's call, could produce the same salutary effects in humans as in mice. "Antagonizing the C3a receptor could be a next generation treatment for asthma," Gerard said. Future generations may have a burgeoning need for such treatments. Asthma is becoming more prevalent worldwide, especially in inner cities, where dirt, poor hygiene, pollution, and other allergen-provoking forces are triggering new cases of asthma at unprecedented rates. "Asthma may be on the rise because the innate immune system is getting kicked up by the environment," Gerard said. What Goes Around...The irony, of course, is that the immune system, including C3a, was originally designed to protect the respiratory tract from allergens. Foreign particles—dust mites, pathogens, bits of physical and chemical debris—flood the lung's passages with each and every breath. If it were not for the action of immune cells and mucus, which protect and cleanse, the life-supporting flow of air could turn into a deadly armada.While they are designed to protect the lungs, immune cells—and the eosinophils, in particular—become perversely triggered during an asthma attack. In an effort to find out what triggers the eosinophils, Gerard and his colleagues knocked out the mouse receptor for an inflammatory agent known to activate eosinophils, CCR3. "We thought we'd screw up eosinophil traffic," Gerard said. While that turned out to be true in some parts of the mouse body, the eosinophils continued to home to the lungs in large numbers. Even more perplexing, when naive mice were exposed to the respiratory allergen methacholine for the very first time, the mutants displayed the hyperresponsiveness, or twitchiness, that characterizes the bronchial muscles of human asthmatics. A Model CureReasoning that some other agent might be instigating the twitchiness, the researchers knocked out in a second set of mice the receptor for C3a, which is present on both eosinophils and the bronchial smooth muscle cells. Aiming this time to produce the mouse equivalent of human disease, they injected newborns with ovalbumin, a substance that stimulates the production of a particular class of acquired immune cells and antibodies found in human asthmatics. The mice were reexposed to the allergen in aerosolized form two and a half to three weeks later, and their lung function was measured. The mice displayed little of the twitchiness seen in the first set of mutants."The animals didn't wheeze as much with the C3a receptor absent," said Gerard. "But you always have to appreciate that while the mouse leads you in a direction, people aren't mice. It's a model. That's why our human study was so important." Drawing upon experiments conducted by associate professor of medicine Elliot Israel and assistant professor of medicine Craig Lilly—both of the Asthma Center at Brigham and Women's Hospital—Gerard and his colleagues analyzed the lung contents of asthmatics. In the experiments, asthmatic subjects had salt water squirted into one lung and whatever they were allergic to into the other. The lungs were then washed out and the liquid retrieved and analyzed for inflammatory mediators and immune cells. Gerard and his colleagues found C3a levels were higher in the challenged lungs—where asthma symptoms had been induced—than in the saline-filled ones. "All of this taken together suggests that the discovery of the role of the C3a receptor in the mouse could have a direct translational bearing on humans," Gerard said, "because the mystery since the leukotriene story has been what else is causing the mucus secretion and the small muscle contractions seen in asthma, and it now looks like C3a could be playing a role," he said. Sorting Out the PlayersBut C3a may be only a partial solution to the mystery. Researchers at Johns Hopkins report in the September Nature Medicine that mice lacking a C3a relative, C5a, another innate immune system agent, exhibit symptoms opposite those of mice lacking the C3a receptor: they display much greater levels of bronchospasm when exposed to allergens. "It looks like there is a yin-yang effect between C3a and C5a that no one expected or understands," he said.Nor is it clear yet whether the genes for the C3a and C5a receptors are defective in human asthmatics. Gerard thinks the genetic underpinnings of asthma will turn out to be more subtle. "I don't want to think of the C3a receptor gene as an asthma gene," he said. "But it is possible that there may be single nucleotide polymorphisms, or SNPs, in the human C3a and C5a gene pool that make you 10 times more susceptible to asthma." He believes the C3a and C5a receptor genes are probably two of many polymorphic genes that play a role in asthma. It is also possible that different genes come into play in different asthma patients—essentially creating different types of asthma—a prospect that could be explored once the genes are known. "What's cool about the next phase of the genomics revolution is that having a target like the C3a receptor, you can then go back and look for polymorphisms within a defined patient population," he said. Even more exciting is the prospect of tailoring treatment to a patient's particular brand of asthma. "Going SNP hunting could lead to better drug targets," said Gerard. —Misia Landau |
