Association of Chlamydia pneumoniae immunoglobulin A seropositivity and risk of lung cancer.

Chlamydia pneumoniae is a common respiratory pathogen that has also been associated with risk for chronic diseases, including atherosclerotic cardiovascular disease. Two recent studies have reported an association between serological evidence of past infection with the organism and lung cancer. To further evaluate this association, we conducted a case-control study among a subgroup of white male smokers identified for a population-based case-control study of lung cancer in western Washington between 1993 and 1995. Serum specimens obtained at study enrollment from 143 cases and 147 controls were tested for C. pneumoniae IgG, IgM, and IgA antibodies. In multivariate analysis controlling for smoking variables and educational status, IgA antibody titer 216 was independently associated with risk of lung cancer among subjects <60 years of age [odds ratio (OR), 2.67; 95% confidence interval (CI), 1.21-5.89] but not among older subjects (OR, 0.69; 95% CI, 0.34-1.43). Among subjects <60 years of age, there was suggestive evidence of a stronger association among current smokers (OR 4.31; 95% CI, 1.36-13.68) than former smokers (OR 1.50; 95% CI, 0.48-4.75; P for interaction term, 0.26). Additional studies, including prospective serological evaluations, are needed to further assess the possible significance of this association.