April 7 (HealthDay News) -- Not only does smoking help cause chronic obstructive pulmonary disease (COPD), it may cripple the body's immune response to bacteria that can worsen the disease, Canadian researchers report.

The finding, detected in a study involving mice, may influence how smokers with COPD are treated, the experts said.

"It is well established that smoking is the main risk factor for COPD. But our research also suggests that cigarette smoke substantially changes the immune response to bacteria, which means that patients with COPD who smoke are weakening their body's ability to deal effectively with bacterial invaders. This may cause even further progression of the disease," principal investigator Martin Stampfli, an associate professor at McMaster University in Hamilton, Ontario, said in an American Thoracic Society news release.

In the study, one group of mice was exposed to cigarette smoke twice a day for five days a week for eight weeks, while another group of mice was exposed to cigarette smoke for four days. After the cigarette smoke exposure, both groups of mice received a dose of nontypeable Haemophilus influenzae (NTHI) bacteria, which causes many COPD exacerbations.

Mice in a control group weren't exposed to cigarette smoke but were inoculated with NTHI.

Both groups of mice exposed to cigarette smoke showed distinct changes in their immune response, specifically an increase in inflammation in the lungs, after being inoculated with NTHI. They also had increased weight loss in response to bacterial infection and a shift in the expression of inflammatory markers.

The study was published in the second April issue of the American Journal of Respiratory and Critical Care Medicine.

If further research confirms these findings, "they would indicate that treatment targets for smokers with COPD may be markedly different than in non-smokers. Smoking may change the underlying inflammatory pathways elicited after bacterial infection," Stampfli said.

SOURCE: American Thoracic Society, news release, April 7, 2009