Bacterial Infections May Modulate Allergic Response

Researchers seeking to explain why incidences of allergy and asthma are increasing, especially in the developed countries, used a mouse model for allergic asthma to show that infection with Mycoplasma pneumoniae prior to exposure to an allergen caused the mice to experience milder asthma attacks after challenge with the allergen. Their findings were published in the March 2003 issue of Infection and Immunity.

Investigators at National Jewish Medical and Research Center (Denver, CO, USA; www.njc.org) injected mice with either M pneumoniae or with a saline solution. Three days later the mice were sensitized against the egg protein ovalbumin. Two weeks later, the mice were again injected with ovalbumin and their allergic response was evaluated. In another experiment, mice were first injected with ovalbumin and then two days later with M pneumoniae. Two weeks later they were injected with ovalbumin and their allergic response was evaluated.

When mycoplasma infection occurred three days before ovalbumin sensitization and challenge, the infection reduced bronchial hyper-responsiveness (BHR) and inflammatory-cell influx into the lung. This was accompanied by a significant induction of Th1 responses (increased gamma-interferon and decreased interleukin-4 production). Conversely, when mycoplasma infection occurred two days after allergen sensitization and challenge, the infection initially caused a temporary reduction of BHR and then increased BHR, lung inflammation, and IL-4 levels.

"For the first time, we have shown that a bacterial infection can modify the allergic response,” said senior author Dr. Richard Martin, vice chair of the department of medicine at National Jewish. "Timing is everything, however. Our results suggest that M pneumoniae, or a related pathogen, could help prevent asthma and other allergic diseases, but only if the infection occurs before a person is sensitized to an allergen.”

The authors believe that their results give weight to the hygiene hypothesis, which contends that compared with the past, children living in the developed countries today are exposed to fewer infectious organisms, which are necessary to properly train their developing immune systems. As a result, their immune systems overreact to relatively harmless irritants, leading to allergies and asthma.