BALTIMORE, Oct 18 (Reuters Health) -- Rhinovirus infections cause exacerbations of asthma by promoting preexisting allergic inflammation, according to Dr. William Busse from the University of Wisconsin in Madison.
"The common cold virus is the most frequent cause of asthma exacerbations, even though the rhinovirus is really not a (cell-damaging) organism in terms of (the) airway," Busse said in his presentation at the Tenth Annual Symposium of the Johns Hopkins Asthma and Allergy Center in Baltimore, Maryland.
According to research cited by Busse, 61% of asthma exacerbations in children and 44% of asthma exacerbations in adults are related to infection with rhinoviruses (RV). Unfortunately, he said, little is know about the immune response to RV.
Busse explained that RV infection in asthma patients causes increased airway responsiveness and allergic reactions that can persist for weeks after RV exposure.
Busse suggested that rather than causing direct damage, "RV changes the inflammatory environment..., upregulating the intensity of the... response should exposure occur."
In his model, the RV directly stimulates special cells called macrophages to release substances such as interferon that wield antiviral activity. Macrophages also stimulate lymphocytes, a type of white blood cell important in the immune response. Both antiviral and inflammatory effects result. The balance between these effects determines the clinical course of the rhinovirus infection.
Among patients examined by Busse and his team, a greater quantity of virus was found in those with the least degree of interferon-gamma production, indicating the antiviral effects of T-cell lymphocyte activation. Patients who had lower levels of interferon-gamma experienced more severe restrictions in respiratory capacity and more wheezing, suggesting to Busse that their "diminished capacity to generate interferon-gamma allowed the persistent effects of the virus and increased the likelihood of an asthma exacerbation occurring."
"As far as we can tell," Busse said, "these airway changes do not occur in patients who do not have asthma."
"Individuals who experience a drop in lung function at the time of their respiratory infection may be individuals in whom there's an alteration in the relativity of their immune response, enabling the rhinovirus to persist and sustain an ongoing inflammatory reaction," Busse said. "People with allergic diseases may have very subtle alterations in their ability to respond to these viruses in an appropriate.... fashion, such that the complications and consequences of infection are more persistent."
