Lung cancer and COPD affect manly the elderly population (31, 32). In the elderly, the propensity for aberrant immune function increases significantly (33). The aging process goes along with changes of the innate, as well as the adaptive immune function (34, 35). Cancer and autoimmune disorders are encountered in elderly patients more frequently, which is partly due to age-related immunological changes, termed immunosenescence (36). Changes of the innate immunity include dendritic cell function, natural killer cells, neutrophil function and macrophages (37). Dendritic cells generally decrease in quantity during the aging process, like for example Langerhans cells in the skin or plasmocytes (38). Aged dendritic cells are characterized by dysfunction of their mitochondria, leading to a reduced ATP turnover and coupling efficiency as well as greater reactive oxygen species (ROS) production (39). Immunosenescene also leads to a dysfunctional maturation and function of natural killer cells (40). Changes in natural killer cell properties that are aging-related comprise a slower response to inflammatory conditions and a general increase of bacterial and fungal infections in the elderly population, which is due to the decline in natural killer cell quantity (41). During aging, spontaneous neutrophil apoptosis, mediated by the increased secretion of proinflammatory cytokines such as INK-1, occurs less frequently (42). Immunosenescence also impairs neutrophil migration to the lung tissue, as demonstrated in a mouse model (43). Insufficient neutrophil migration in elderly patients is most probably due to an increased constitutive PI3K activation, since lower rates of PI3K activation are normally observed in young individuals (44). Macrophages, like other cell types in aging persons, feature telomere shortening, which leads to a decreased GM-CSF-dependent proliferation of these cells, resulting from a decreased phosphorylation of STAT5. In aged mice, it was demonstrated that macrophages became increasingly susceptible to oxidative stress, explaining why DNA damage is caused by ROS to a much greater extent in old people, paving the way for malignant transformation (45). In elderly people, macrophages produce considerably lower amounts of cytokines such as TNF-α and IL-6, and show a reduced expression of the B7 receptor, resulting in a reduced activation of T-cells (46).

The adaptive immune system is also affected by immunosenescence, altering the function of B- and T-lymphocytes. Humoral immunity is impaired in aging patients, because of a relative increase of the memory B cells (IgG(+)IgD(-)CD27(-), double negative, DN) population, leading to a classical low-level chronic inflammatory microenvironment. Naïve/memory B-cell populations in older adults feature a different receptor expression, which has been discussed using the term “inflamm-aging” (47). Interestingly, naïve B-lymphocytes from young persons need very strong signals in order to be activated in vitro, whereas B-cells from elderly patients can be activated when only physiologically stimulated (48). The plasma cell count in the bloodstream of elderly patients was also observed to decline, as compared to young individuals (49). T-lymphocytes feature an altered functionality and distribution in old patients, which is why the susceptibility for infections and other diseases increases with age (50). A decline in naïve T-lymphocyte output is due to an age-related regression of the thymus (51). Another effect of thymus regression in the elderly is a reduced T-lymphocyte diversity (52).

Taken together, the above listed data shows distinct immunological differences in elderly, as compared to young patients. The increased prevalence of lung cancer and COPD in old subjects, smokers and non-smokers alike, is probably a synergistic event of cumulative noxa and immunosenescence.

Mounting evidence suggests that fibrotic lung conditions also predispose individuals to lung carcinogenesis. This was first assumed based on autopsy studies, where coexisting interstitial lung disease and lung cancer have been found (175). Idiopathic pulmonary fibrosis is the most common form of fibrotic lung disease (176). Many patients with pulmonary fibrosis also have a history of smoking, rendering them especially susceptible for lung cancer. Still, non-smokers with fibrotic lung disease seem to have a higher lung cancer risk as well, as idiopathic pulmonary fibrosis increases lung cancer risk by 7-20%, regardless of other risk factors (177). Fibrosis of the lung originates from perpetuated and excessive connective tissue remodeling in response to recurring alveolar microinjuries. Excessive deposition of components of the extracellular matrix leads to an irreversible remodeling of lung tissue (178, 179). Fibroblasts, during excessive and aberrant wound healing, respond by hyperproliferation and change to a pro-fibrotic phenotype resistant to apoptosis (178, 179). Activated fibroblasts are highly responsive to growth factors and cytokines, e.g. TGF-β, connective tissue growth factor, platelet-derived growth factor and IL-6 (179, 180). This chronic low-level inflammatory process leads to an increased risk of lung carcinogenesis in patients with pulmonary fibrosis. T helper cells type 1 and 2 (Th1/Th2) play an important role in the inflammatory phase of pulmonary fibrosis (181). It was demonstrated in a mouse model that depletion of T-lymphocytes by means of an anti-CD3 monoclonal antibody impeded the fibrotic lung remodeling (182). The role of regulatory T-cells in fibrotic lung disease has not been completely clarified. A reduction of regulatory T-cells in the peripheral blood and BAL of pulmonary fibrosis patients was shown in on study, while other data showed an increase of regulatory T-cells (183). A pro-fibrotic impact of regulatory T-cells was shown in one study, increasing TGF-β1 release and collagen deposit (184). However, in late stages of lung fibrosis, regulatory T-cells acted anti-fibrotic (184). It may be that regulatory T-cell dysfunction contributes not only to pulmonary fibrosis but also to lung carcinogenesis. Particularly in the early stages of tumor development regulatory T-cells play a crucial role. A mouse model of mutant Kras-driven lung adenocarcinoma showed that Kras transgenic mice who lacked FoxP3+ regulatory T-cells were 75% less likely to develop lung tumors (185). Tobacco smoke led to an increase in FoxP3+ lymphocytes, according to the mouse study, which poses one among many factors contributing to smoking-related lung cancer. The depletion of regulatory T-cells led to tumor cell death at early stages of lung carcinogenesis according to another mouse study (186), resulting in elevated levels of granzyme A and B, perforin and IFN-γ in infiltrating CD8+ T-cells. Correspondingly, Xiong et al. demonstrated that regulatory T-cell depletion was a protective factor in radiation-induced lung fibrosis (187). Viewing the data on regulatory T-cells and lung fibrosis, it is evident that they can exert pro- as well as anti-fibrotic roles, dependent on the stage of pulmonary fibrosis and the mutual interaction with other T-cells. Th9 and Th22 T-cells, producing IL-9 and IL-22 were also involved in fibrotic remodeling of the lung, exerting pro- and anti-fibrotic properties similarly to regulatory T-cells (188–190). Macrophages, as a source of proinflammatory and pro-fibrotic cytokines, have been linked to lung fibrogenesis as well (191). The role of macrophages in lung carcinogenesis has been widely investigated, showing the important role macrophages play in this context. Macrophages make up the majority of tumor-infiltrating immune cells and are a key link from inflammation to lung cancer (192). Two hypotheses on the effect of macrophages and lung carcinogenesis have emerged: An extrinsic pathway driven by inflammatory conditions increasing the risk for lung cancer, as well as an intrinsic one driven by genetic changes (192). Tumors triggered by chronic inflammation at distinct tissue sites are characterized by infiltrating leukocytes, predominantly macrophages, cytokines, chemokines and growth factors. Tumor-infiltrating macrophages secrete ROS and nitrogen intermediates, inducing DNA damage. In response to tissue damage at sites of chronic inflammation, inflammatory cytokines are released to recruit cells that initiate tissues repair. However, the cytokines endorse tumorigenesis themselves by inhibiting key enzymes (193). Two types of tumor-associated macrophages are known today: canonically activated M1 and alternatively activated M2 macrophages. Both in lung cancer and in lung fibrosis, these two subtypes contribute considerably to disease progression, exerting beneficial as well as harmful functions (194, 195). M2 macrophages have been known to accumulate in fibrotic lungs and have been linked to pro-fibrotic activities (194). Conversely, anti-fibrotic roles have been reported for macrophages as well, mediated by diverse mechanisms such as scavenging of proinflammatory cellular debris, digestion of extracellular matrix components and by secretion of mediators inducing myofibroblast apoptosis (196). The effects of macrophages both in fibrotic lung disease and in lung cancer may explain why lung fibrosis is linked to lung cancer, and why changes in the lung microenvironment associated with lung fibrosis render the patients more susceptible for lung cancer as well.

Neutrophils play a role in chronic inflammation in general, and their role in lung carcinogenesis as well as in lung fibrosis has been commonly acknowledged (197, 198). Levels of IL-8/CXCL8, which is a key chemotactic factor for neutrophil function, are increased in patients with pulmonary fibrosis (199), and the number of neutrophils in the BAL fluid correlated well with G-CSF levels, an important neutrophil growth factor (200). Airway neutrophils are generally activated at a higher level in idiopathic pulmonary fibrosis, compared with healthy individuals, which is demonstrated by increased levels of their main proteolytic product, neutrophil elastase (201). Neutrophil elastase is a protease of neutrophil extracellular traps which are formed as a response to chronic lung inflammation. One mouse study showed that neutrophil elastase proteolytically remodeled laminin in lung tissue, leading to the proliferation of dormant lung cancer cells via the activation of integrin α3β1 signaling (202). Tissue remodeling processes in lung fibrosis, mediated by neutrophils, could therefore facilitate concomitant lung carcinogenesis. Summing up all the above-mentioned data, a great variety of immunological changes can be linked to fibrotic lung disease. Many of these changes are generally associated with chronic inflammation or tissue remodeling, paving the way for malignant transformation of cells. The correlation of lung fibrosis with an increased risk of lung cancer – particularly in never-smokers – can partly be explained by these distinct immunological changes.

Mycotoxins and fungal spores are especially prevalent in damp buildings or at farms, in the malt and wood industry, i.e. all occupations that involve handling moldy materials (203). Different fungi are sources of mycotoxins as secondary metabolites, including trichothecenes synthesized by the genera Fusarium, Myrothecium, Trichoderma, Trichothecium, Cephalosporium, Verticimonosporium, and Stachybotrys (204). Trichothecenes lead to a suppression of lymphocyte-related immune response and stimulate the production of IL-1β by macrophages (205). Additionally, these toxins inhibit protein synthesis by targeting the ribosome, impairment of the mitochondria and activation of MAPKs (206). Trichothecenes also upregulate gene expression related to the inflammatory response, which leads to further ribosome damage through inflammatory cytokines (207, 208). Deoxynivalenol is a trichothecene which is found in contaminated cereal grains. It may exert cytotoxic effects and inflammation synergistically with particulate matter, inducing the inflammatory response (209). Animal studies have demonstrated that upon exposure, mycotoxins not only damage the lung but are distributed to the liver, kidney and spleen (210). Consecutively, alveolar macrophages and neutrophils are recruited, promoting pulmonary hemorrhage, cytokine secretion and ultimately, damage to various organs (211). Notable, even after ingestion of mycotoxins, the lung tissue is injured in response (212).

Ricin is a ribosome-inactivating protein found in beans of the castor plant Ricinus communis. Ricin aerosol does not occur naturally but may be used as a biological weapon because of its high toxicity. Ricin is used as a compound of immunotoxins to target cancer cells (213). Ricin intoxication leads to a strong inflammatory cascade, i.e. the migration of neutrophils to the airways, causing apoptosis and necrosis of lung epithelial cells (214). Conversely to cigarette smoke, ricin also causes apoptosis of alveolar macrophages (214). While severe ricin poisoning leads to interstitial pneumonia, alveolar edema, respiratory failure and death within days, a sublethal ricin dosage leads to lung fibrosis and pulmonary hemorrhage (215). Like cigarette smoke and mycotoxins, ricin increases the expression of proinflammatory genes in airway epithelial cells and pulmonary macrophages (137). Both ricin and mycotoxins are factors predisposing exposed individuals to fibrotic lung disease. The above-listed inflammatory mechanisms contribute not only to fibrosis, but to DNA damage of lung epithelial cells as well, which may lead to lung cancer in the long term. Lastly, also non-biological chemicals can lead to lung inflammation when inhaled. These include volatile organic compounds from household items, office supplies and craft materials, e.g. formaldehyde, benzene and perchloroethylene, and chronic exposure results in damage of the lung epithelium, fibrotic remodeling and malignant transformation (216).

Second-hand smoke counts among the risk factors for COPD in non- and never-smokers (217). Similarly, second-hand smoke, also termed “environmental tobacco smoke”, has been classified as a main pulmonary carcinogen (218). Second-hand smoke is a composition of mainstream- and sidestream smoke, wich are mostly similar in quality and composition (219), yet, sidestream smoke arises at lower burning temperature and the quantities of its chemical compounds in vapor and particular phase (i.e. the quantity of semi-volatile or non-volatile agents) differ from the quantities in mainstream smoke (220). Certain carcinogens, such as aromatic amines, are more abundant in sidestream smoke (220, 221). Second-hand smoke, once released into the air, has the potential to aggregate and mix with other types of air pollutants (222). Smokers are generally exposed to a much larger quantity of smoke-related carcinogens (221). Interestingly, sidestream smoke was associated with a higher potency to induce mouse skin tumors, which gave rise to the assumption that second-hand smoke in non-smoking subjects might even be more carcinogenic than mainstream smoke inhaled by smokers (223).

Since data on second-hand smoke, COPD and lung cancer is gathered epidemiologically, the evidence linking second-hand smoke to these conditions remains weak (224), and a direct biological link is difficult to outline. In a mouse study, it was shown that alveolar macrophage recruitment was increased after exposure to second-hand smoke in a dose-dependent fashion (217). Moreover, an increased expression of alveolar macrophage markers and an increase of the COPD-associated pro-inflammatory markers CCL2 and TNFalpha was also observed in this study (217), showing a direct causative effect of second-hand smoke in COPD.

In lung carcinogenesis, second-hand smoke is an established risk factor (22, 225), however, attempts to epidemiologically confirm the associated risk have only brought forth disputable evidence (226). In COPD and lung cancer, the uncertainties in risk assessment of second-hand smoke exposure have arisen, because exposure assessment is difficult to standardize, given temporal variabilities in source, formulation and concentration of effluent second-hand smoke (227). However, for lung cancer, the risk does significantly increase due to second-hand smoke with many years of latency (228). Novel studies have focused on outlining second-hand smoke biomarkers with the aim to measure exposure over a relevant time period which augments the risk of lung carcinogenesis (221, 229). Human epidemiologic studies on second-hand smoke and lung cancer have been observational (226), correlating the incidence of lung cancer in long-term second-hand smoke exposed individuals, such as family members of smokers, to the level of second-hand smoke exposure (230). Rodent studies on the carcinogenic potential have been performed, allowing for a better standardization, however, animals are usually treated with considerably higher doses of smoke than humans would be exposed to in real life (231). Moreover, animals with smoke exposure often respond with appetite suppression and the consecutive inadequate caloric intake may further contribute to tumor initiation (231). Thus, it is difficult to exactly quantify the carcinogenic potential of passive smoking.

Very few data exists on the immunologic changes induced by second-hand smoke. Presumably this is due to the assumption that the changes are very similar to those caused by active smoking. One mouse study investigated in utero and early-life passive smoke exposure, and the resulting immunologic changes (232). Changes in plasma cytokine concentrations and in immune cell populations, i.e. decreased percentages of B-cells and increased percentages of myeloid cells were observed in the bone marrow of smoke-exposed versus control mice. In the spleen of passive smoke exposed mice, the abundance of B-cells decreased and that of T-cells increased, whereas myeloid cells were significantly lower in the peripheral blood in smoke exposed mice (232). The authors of this study conclude that the immune changes resulting from second-hand smoke in utero and in early life are particularly threatening, contributing to carcinogenesis by lowering host defense mechanisms. In a review article about the immunologic changes of second-hand smoke in humans, it becomes evident that the literature is yet too limited to draw conclusions on how exactly the immunologic changes induced by second-hand smoke differ from that of active smoking (233). Environmental smoke exposure may induce hypersensitivity, leading to asthma and may furthermore suppress immunoregulatory mechanisms. Some studies even failed to detect any immunological changes in passive smoke-exposed individuals (233). Thus, more research is clearly warranted on this topic.

There is evidence for an increased risk of lung cancer due to a history of pulmonary infections caused by Mycobacterium tuberculosis (234). This evidence is supported by large cohort studies, like for instance a Chinese analysis amongst individuals with a history of tuberculosis infection. A positive correlation with lung adenocarcinoma as well as squamous cell carcinoma has been shown (235). According to previous data, the risk of lung cancer is increased by about 50% after a prior diagnosis of tuberculosis (236). The more recently the infection was diagnosed, the greater the risk of lung cancer became (236). Moreover, it is discussed whether a chronic pulmonary colonization with Chlamydia pneumoniae led to an increment in the risk of lung cancer as well (237). As shown by serologic testing, a chronic Chlamydia pneumoniae infection resulted in impaired apoptosis of the infected cells, because IL-10, a prime immunosuppressive cytokine, was activated, resulting in reduced antitumor immune response (238). Moreover, an upregulation of IL-8, mediated by Chlamydia pneumoniae, has been reported to increase the risk of malignancy, endorsing angiogenesis and cell proliferation, as shown in a mouse model (239).

The lung microbiome in general seems to impact the likelihood of lung cancer, in smokers and non-smokers alike (240). Apart from chronic inflammation and bacterial and viral lung infections, even periodontal disease and not only pathogenic but also opportunistic microorganisms can drive lung carcinogenesis in the long term, as shown for Haemophilus influenzae, Enterobacter spp., E. coli, Pneumococcus (241), Legionella (242) and Moraxella genera (137, 243). These pathogens can increase the likelihood of lung cancer, with an association even with certain histological subtypes (240). Klebsiella, Rhodoferax, Acidovorax, Comamonas and Polarmonas genera, for example, are frequently found in the lung microbiome of small-cell lung cancer (SCLC) patients, but not in patients with adenocarcinoma (244). It is proposed, that in some cases not the infection itself increases the risk for subsequent lung cancer, but the significant disruption of the lung microbial landscape. Alpha diversity of the lung microbiome, namely the number of different microbes in one habitat, is usually low in lung cancer patients. Interestingly, the beta diversity, meaning the diversity of microbes between habitats is not different when comparing healthy lungs and those from lung cancer patients (245). In recent years, the importance of the microbiome as an indicator of malignant transformation was recognized increasingly. A study on the lung microbiome, comparing samples from patients with benign and malignant tumors via next generation sequencing, showed the genera Veillonella and Megasphaera to be potential lung cancer biomarkers (243). Moreover, a correlation between Acidovorax genus and SCLC has been shown (244), and Pseudomonas is associated with lung adenocarcinoma (240, 246). Within the last decades, the molecular mechanisms by which microbes endorse malignant transformation, have been investigated in greater detail. The microbial carcinogenic effects comprise bacterial toxins, inflammatory stimuli from immune cells and the direct effects of microbes on epithelial cells. A study on mice with a dysfunctional lung microbiome and impaired function of γδT17 T-cells due to long-term antibiotic treatment, showed an increased risk for Lewis lung carcinoma. The total bacterial count of the lung microbiome in these mice decreased dramatically, showing that the commensal microbiome is indeed crucial for anti-tumor immune cell functions (247). Also bacterial toxins change the immune landscape in a way that may contribute to tumorigenesis. For instance, cytolethal distending toxin (CDT), cytotoxic necrotizing factor 1, and Bacteroides fragilis toxin can damage the DNA repair system, allowing for malignant transformation (248–250). In a murine model of A427 non-small cell lung cancer (NSCLC) it was demonstrated that the toxin of Cyanobacteria decreases CD36 protein levels and increases the concentration of PARP1, an enzyme profoundly linked to tumor development. In the mouse model, these results were proven with bacteria-positive lung cancer (251). Moreover, heat-inactivated E. coli bacteria were found to enhance migration and metastatic dissemination of NSCLC cells in vivo (252). Apart from these direct effects of microorganisms on lung cancer development, it was also shown that a shift in the lung microbiome causes ROS rates to increase. Higher amount of ROS lead to an increase in DNA damage in the long term and predispose to tumor development. Interestingly, TP53 mutated lung tumors are known to be associated with the colonization with Acidovorax genus in the tissue microenvironment (244).

A disruption of the lung microbiome, triggered by lung infections or possibly chronic exposure to carcinogenic dusts or second-hand smoke, may therefore be an important factor predisposing non- and never-smokers for lung cancer.