The results of this study showed that Cpn infection was associated with the risk of lung cancer. Patients with serum both Cpn IgG+ and IgA+ had 2.00 times the risk of developing lung cancer. The stratified analysis showed that smokers or drinkers with Cpn IgG+ or IgA+ were more likely to develop lung cancer. Besides, Cpn IgG or IgA had a combined effect on smoking, passive smoking, and family history of cancer.
Our results were consistent with the results of other studies. Several studies (6, 19, 20) supported that Cpn infection is associated with a higher risk for lung cancer. Furthermore, other studies (6-8) showed ORs of 1.2 to 2.8 after adjustment for smoking status indicated that chronic Cpn infection was an independent risk factor for lung cancer. Several case-control studies showed that Cpn infection increased the risk of lung cancer development (10-13, 16), but failed to show a correlation between serum Cpn antibodies and the risk (14, 15, 21).
Although it is unclear how Cpn infection would induce or cause lung cancer, the process may involve chronic inflammation. Chronic Cpn infections may prolong inflammatory mediator stimulation to increase cell necrosis, apoptosis, and mitosis. Thus, the relationship between Cpn infection and lung cancer seems biologically plausible. Furthermore, Cpn proteins have been shown to trigger lung cancer growth potential by altering host cellular replication, transcription, and DNA damage repair (22). During tissue repair, active cellular splitting can result in the occurrence, accumulation, and fixation of mutations, deletions, ectoplasias, and amplifications; these changes increase the risk of malignant transformation at the site of infection (23). Besides, cellular experiments also showed that Cpn infection could transform mesothelial cells, which in turn could increase lung cancer risk (24). Researchers have also established a Cpn infection-induced lung cancer model in rats (25).
Cpn infections are common among specific patient subgroups (e.g. young person (6, 11) , men (12, 13), and smokers (6, 8, 11)). Furthermore, the relationship between Cpn infection and lung cancer risk may vary in environmental factors (e.g. age, gender, smoking history). Our results showed that the OR values of Cpn Ig A or IgG were 2.27 (95% CI = 1.38-3.72) or 1.79 (95% CI = 1.10-2.91) among smokers, which were consistent with those studies (8, 11). The current study also suggested that passive smokers with Cpn infection had a higher risk of lung cancer. The same carcinogen from cigarette smoke of smoking and passive smoking may all induce lung cancer (26-28). Reactive nitrogen and oxygen species (RNOS) produced by smoking can activate NF-κB to promote the expression of inflammatory genes, and directly or indirectly activate the production of inflammatory mediators through regulation of various protein modifications and degradation (29). Therefore, Cpn combines with smoking may promote lung cancer via elevated levels of inflammatory factors that, thereby. Though many studies indicated that smoking might induce lung cancer by aggravating lung inflammation (29, 30), a further research on the underlying mechanism and molecular mechanism of Cpn infection in the pathogenesis of lung cancer are still required.
Moreover, the current study also showed, for the first time, that Cpn IgG (OR=2.45; 95%CI=1.27-4.75) and IgA (OR=2.68; 95%CI=1.40-5.13) were more closely associated with lung cancer among alcohol drinkers. Alcohol exposure reduces airway mucociliary clearance with a progressive desensitization of ciliary response. As a result this important innate primary defense mechanism is weakened, chronic alcohol exposure alters the adaptative immune response to pathogens and leads to an inflammatory response (31). Therefore, Cpn combines with alcohol drinking may also promote lung cancer via elevated levels of inflammatory factors. Furthermore, the combined effects of Cpn IgG+ (OR=2.493, 95% CI = 1.474-4.215) or IgA+ (OR=2.594, 95% CI = 1.409-4.776) and family history of cancer were found on lung cancer. HE et al.(32) proposed that non-small cell lung cancer (NSCLC) patients with family history of cancer, especially family history of lung cancer, might have a significantly higher incidence of epidermal growth factor receptor (EGFR) activating mutation. And EGFR is an important predictive biomarker of EGFR tyrosine kinase inhibitors (TKIs) in NSCLC. Moreover, Cpn proteins have been shown to trigger lung cancer growth potential by DNA damage repair (22). Therefore, Cpn infection might combined with family history of cancer to induce lung cancer by mutation. But further studies are warranted to confirm the results and further explore the role of family history of cancer.
In this study, serum Cpn IgG and IgA were detected by MIF, which is the standard for serologic detection of Chlamydia infection. However, the use of MIF is limited by its subjectivity and reproducibility (33). Therefore, our experiment was conducted by two different people. A skilled technician performed the preliminary experiment, and the other person conducted a blind interpretation of the results. Furthermore, 10% of the samples were randomly selected for retesting. Previously published studies have had variable definitions for "chronic" chlamydial infection. For example, one study (6) used a combination of specific IgA titers (1:16 or higher) and immune complex titers (1:4 or greater), whereas others have used IgA titers of 1:64 or higher (10) or IgG titers of 1:512 or higher (12-14). Still, in several studies (7, 8, 11, 34, 35) IgG antibody titers of 1:16 or more were considered as the evidence of past or present Cpn infection, whereas IgA antibody titers of 1:16 or more likely indicated chronic infection. Thus, IgG and IgA antibody detection was used to explore the relationship between chlamydia and lung cancer in the current study.
The present study was the most extensive retrospective case-control study to evaluate the role of Cpn in lung cancer pathogenesis. Meanwhile, stratification and multivariate analysis were used to identify possible effect modifiers associated with Cpn and lung cancer. However, still several potential limitations in this study should be considered. Firstly, there were some unavoidable selection and recall biases. Secondly, it is hard to explore the causal inference between Cpn infection and lung cancer when the blood was collected after the cancer diagnosis to determine Cpn infection status. Thirdly, our results may underestimate the effect of the association between Cpn infection and lung cancer due to non-disaggregated misclassification bias caused by the pre-selected criteria for determining chlamydial infection. Despite these limitations, our findings are biologically plausible. Studies have suggested that higher infection rates in patients with cancer are often caused by the immunosuppressive effects of cancers (6). However, studies in which serum was collected before lung cancer diagnosis showed that the association between serum Cpn and lung cancer still existed when blood samples obtained 1 to 5 years before diagnosis were excluded, suggesting that Cpn infection pre-dated the cancer diagnosis (8).