Combined and interaction effect of chlamydia pneumoniae infection and smoking on lung cancer: a case-control study in Southeast China

BACKGROUND: This case-control study investigated the role of Chlamydia pneumoniae (Cpn) infection in the pathogenesis of lung cancer and the combined and interaction effect of Cpn infection, smoking, and various environmental factors. METHODS: The study comprised 449 lung cancer patients and 512 age- and sex-matched healthy controls. All participants provided a 5 ml fasting peripheral venous blood sample for testing Cpn-specific IgG and IgA by using micro-immunofluorescence. Besides analyzing the associations between Cpn and lung cancer, combined effect analysis, logistic regression, and the Excel table made by Andersson were used to analyze the combined and interaction effects of Cpn and environmental factors on lung cancer. RESULTS: Compared to those with no evidence of serum Cpn IgA or Cpn IgG, those with both Cpn IgG+ and IgA+ had 2.00 times the risk (95% CI: 1.34–3.00) of developing lung cancer. Cpn IgG+ or IgA+ was associated with a significantly increased risk of lung cancer among smokers; the adjusted odds ratio (OR) was 1.79 (95% CI: 1.10–2.91) and 2.27 (95% CI: 1.38–3.72), respectively. Those exposed to passive smoking with Cpn IgG+ or IgA+ also showed an increased risk of lung cancer; the adjusted OR was 1.82 (95% CI: 1.20–2.77) or 1.87 (95% CI: 1.22–2.87), respectively. Similar results were also observed among alcohol drinkers. Multiplicative and additive interactions were not observed between Cpn infection and environmental factors. The combined effects of Cpn IgG+ or IgA+ with smoking, passive smoking, and family history of cancer on lung cancer were determined. CONCLUSION: Cpn infection is potentially associated with primary lung cancer in the Chinese Han population and has combined effects with smoking, passive smoking, and family history of cancer.