The results of this study showed that Cpn infection was associated with the risk of lung cancer. Patients with both serum 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. Additionally, Cpn IgG and IgA each 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) showed that Cpn infection is associated with a higher risk of lung cancer. Furthermore, other studies (6–8) showed ORs of 1.2 to 2.8 after adjusting for smoking status, indicating that chronic Cpn infection is 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 cancer 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). Furthermore, 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, particularly young people (6, 11), men (12, 13), and smokers (6, 8, 11). Furthermore, the relationship between Cpn infection and lung cancer risk may vary when combined with environmental factors (e.g., age, gender, and smoking history). Among patients aged 60 years and older in our study, the association between Cpn IgA and lung cancer was statistically significant. This may be due to the increase of Cpn infection with age. In addition, a significant association was found between Cpn IgA or IgG and lung cancer among males and smokers. The OR values among smokers in our study were consistent with the results of other studies (8, 11). The current study also suggested that passive smokers with Cpn infection had a higher risk of lung cancer. The same carcinogens in cigarette smoke may induce lung cancer in people with a history of smoking and passive smoking (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 the regulation of various protein modifications and degradation (29). Therefore, Cpn combined with smoking may promote lung cancer via elevated levels of inflammatory factors. Although many studies have indicated that smoking might induce lung cancer by aggravating lung inflammation (29, 30), further research on the underlying mechanisms of Cpn infection in the pathogenesis of lung cancer is still required.
Moreover, the current study also showed, for the first time, that Cpn IgG and IgA were more closely associated with lung cancer among alcohol drinkers. Alcohol exposure reduces airway mucociliary clearance through the progressive desensitization of ciliary response. As a result, this important innate primary defense mechanism is weakened. Chronic alcohol exposure also alters the adaptative immune response to pathogens and leads to an inflammatory response (31). Therefore, Cpn combined with alcohol drinking may also promote lung cancer via elevated levels of inflammatory factors. Furthermore, the combined effects of Cpn IgG+ or IgA+ and family history of cancer on lung cancer were found. He et al. (32) proposed that non-small cell lung cancer (NSCLC) patients with a family history of cancer, especially a family history of lung cancer, might have a significantly higher incidence of epidermal growth factor receptor (EGFR) activating mutation. 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 combine with a family history of cancer to induce lung cancer by mutation. However, further studies are warranted to confirm the results and 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 second person conducted a blind interpretation of the results. Furthermore, 10% of the samples were randomly selected for retesting. Previously published studies have had varying 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). Moreover, in several studies (7, 8, 11, 34, 35) IgG antibody titers of 1:16 or more were considered as evidence of past or present Cpn infection, whereas IgA antibody titers of 1:16 or more were considered to indicate chronic infection. Thus, IgG and IgA antibody detection were used to explore the relationship between Chlamydia and lung cancer in the current study.
The present study is 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, several potential limitations in this study should be considered. First, there were some unavoidable selection and recall biases. Second, it is difficult to explore the causal inference between Cpn infection and lung cancer when the blood for the study is collected after the cancer diagnosis to determine Cpn infection status. Third, 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. Finally, although Cpn IgG+ means patients have at some point had a Cpn infection, Cpn IgA+ only means patients have present or chronic Cpn infection because of the short half-life of Cpn IgA. 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).