Asthma triggers: Researchers look to fungal infections as a cause
Volume 8, Issue 2
How does the lining of the human airway shape immune response to disease? This is the question addressed by Mayo Clinic investigators whose research may reduce cases of asthma and have implications for inflammatory diseases of the gut and nervous system.
Hirohito Kita, M.D., is the Walter and Leonore Annenberg Professor of Pulmonary Medicine at Mayo Clinic.
Mayo Clinic allergist Hirohito Kita, M.D., is a medical detective who's long been on the trail of a type of white blood cell called an eosinophil. It's not the culprit in this case, but a sort of cell of interest in the ongoing mystery surrounding asthma.
Initially, Dr. Kita was curious to know whether eosinophils, which trigger asthma, have similar initiation pathways as allergic reactions, which involve mast cells and the IgE antibody that prompts histamine, the nitrogen compound central in localized immune responses.
"We looked very hard at whether eosinophils get stimulated by IgE, but the result was negative," Dr. Kita says. "We needed to look for something else."
The big surprise turned up when Dr. Kita's Allergic Diseases lab found a culprit in the fungus Alternaria. The fungus lives in moist places, such as bathrooms, but is also present among vegetables and is prevalent on Midwestern farms. For most people, the fungus is harmless, although a 1992 New England Journal of Medicine case study by Mayo Clinic researchers described the sudden death of a young adult with asthma after coming in contact with Alternaria.
Dr. Kita's research provided evidence, the direct connection: Enzymes known as proteases, which are produced by the fungus, activate the eosinophils, which produce an allergic reaction. Published in the Journal of Immunology in 2008, the work has raised the potential for developing a drug therapy against Alternaria.
"For the fungus to germinate from a hard spore, it has to digest its shell with proteases," Dr. Kita explains. "Eosinophils also have a receptor for these proteases, called protease activated receptor. They recognize the protease and respond."
The same receptor also recognizes house dust mites and cockroach feces, which contain digestive enzymes and are major asthma triggers. The protease discovery also spawned new thinking about the significant role that infection can play in inciting inflammation.
"The link between fungal infection and inflammatory signaling in a disease like asthma, which traditionally has not been thought of as an infectious disease, has been incredibly important. It's paradigm-shifting science," says Mayo Clinic lung researcher Andrew H. Limper, M.D.
From Japan to Mayo Clinic
Eosinophils (stained green) become bright with activation.
Eosinophils (green) fighting with fungi in a laboratory view.
Looking back, Dr. Kita recalls his first study in the field of asthma in the late 1980s as "a simple experiment." Just after training in pediatrics at Mie University Hospital in Japan, during a research fellowship in allergy and immunology, he became aware that the university hospital was housing 50 children with asthmatic wheezing so severe that they could not attend school or even live at home. He recognized this unique opportunity to learn about the disease and began studying disease-fighting lymphocytes in the children's blood. With flow cytometry, a cell-sorting technique that had just come on the scene, Dr. Kita found that cells carrying a particular allergy-associated antibody decreased during treatment.
He may have regarded the work as simple, but it profoundly affected him and his career. It focused his research interest on asthma and on the mystery of minute biochemical changes that wreak havoc on the body. Dr. Kita also began to appreciate asthma's complexity.
"Wheezing is a phenotype we see in the patient, but behind it is inflammation — many different cell types interacting with each other," Dr. Kita says. "It's a disease with environmental factors. It also gets worse with stress, so it has psychosocial factors, too."
Dr. Kita set about providing answers about the intricate process of inflammation in asthma and its upper-airway counterpart, chronic sinusitis. But the work, for which he's internationally known, has also applied to a range of other diseases, from ulcerative colitis to chronic obstructive pulmonary disease (COPD). Along the way, his prolific lab, which has trained more than 70 postdoctoral researchers and hundreds of students and residents, has spawned numerous collaborations at Mayo Clinic and institutions around the world.
During Dr. Kita's training, medical thinking about asthma focused on the body's lymphocytes. Research was also turning toward the unusual white blood cells known as eosinophils. The tiny, sparse inflammatory cells, which likely evolved to fight parasitic infections, originate in bone marrow and exist in the bloodstream and the gastrointestinal system. They don't turn up in the lungs — except in connection to asthma and allergy. But it wasn't clear how the cells contributed to inflammation.
After his early work in Japan, Dr. Kita traveled to Mayo Clinic in 1988 for a fellowship with immunologist Gerald J. Gleich, M.D., an expert in isolating eosinophils and studying their unique biochemistry.
"The basic questions were, why do we have these cells in the blood and what makes them turn harmful?" Dr. Kita recalls of the work he began with Dr. Gleich, who today runs a lab at the University of Utah.
Dr. Kita first set out to understand what activates eosinophils to release toxic molecules, called eosinophil granule proteins, which damage the airway's epithelial cells. He began by isolating the elusive eosinophils — which make up a mere 3 percent of the body's cells — and stimulating them. He found a variety of triggers, including dust mites and cockroach particles. He returned to Japan, where he intended to continue the studies, but found that the hectic schedule for physician-scientists didn't allow enough time for research. He eventually returned to Mayo Clinic to devote his time to laboratory work.
Broadening the scope of asthma research
Dr. Kita's knowledge of eosinophils has become crucial to studies of several other disease processes.
"He's a world expert in eosinophil and mast cell biology, and he's always open to new ideas and projects, amid a very busy lab," says Mayo Clinic gastroenterologist David A. Katzka, M.D., who studies eosinophilic esophagitis, a severe food allergy in young people that can scar the esophagus when eosinophils disintegrate and release cytokines.
Dr. Kita developed a means to detect the cytokines in the inflamed tissue, which is crucial for diagnosis and treatment of the condition. A project with Mayo Clinic neurologist Claudia F. Lucchinetti, M.D., focuses on the role of eosinophils and other inflammatory cells in demyelinating diseases, such as multiple sclerosis and neuromyelitis optica, which damages the optic nerve and spinal cord.
Dr. Kita's insights about inflammatory processes continue to help other researchers. Many postdocs come to work with him from Japan and continue to collaborate after they join university faculties. At Mayo Clinic, lung researcher Robert Vassallo, M.D., describes Dr. Kita as both collaborator and mentor. Dr. Vassallo works with Dr. Kita investigating mechanisms in cigarette smoke that predispose people to asthma and COPD, which are on the rise worldwide. "He's an extraordinary individual," Dr. Vassallo says, "energetic, positive, with a can-do attitude. And he's down-to-earth and humble, despite the fact that he's very accomplished."
Dr. Kita recently turned his investigative focus to the gate-keeping epithelial cells lining the airways, which, like eosinophils, are part of the innate immune system. When exposed to Alternaria or other inhaled antigens, the epithelial cells produce chemical messengers that regulate lymphocytes, eosinophils and dendritic cells in an immune response. He's interested in specific messengers, for instance the cytokine IL-33 and how it prompts a destructive cascade of events.
One line of investigation is whether inhibitors to those molecules in cells right at the surface of the airways might cure or moderate inflammatory diseases. The process he's studying is relevant to asthma, sinusitis and to gastrointestinal conditions, such as inflammatory bowel disease and ulcerative colitis, a condition in which epithelial cells are in constant surface contact with bacteria in the gut and yet an adverse immune response gets underway inside the cells.
"How important are the cytokines? We don't know yet," Dr. Kita acknowledges. "Our hope is that what we learn will ultimately provide a means to treat many of these diseases."