Smoker's macrophages

The uptake of tar from cigarettes accumulates in alveolar macrophages and causes autofluorescence.[4] The intensity of fluorescence, however, is independent of cigarette exposure. This indicates a maximum capacity of tar uptake; excess tar cannot be retained by macrophages. Another pigment in smoker’s macrophage is hemosiderin which is involved in iron homoeostasis. Hemosiderin-laden pigmented macrophages are yellowish brown and found in the bronchiole and peribronchiolar alveolar space.[9] The presence of these dirty macrophages has been a characteristic of many smoke-related lung diseases.[9][10]

A macrophage can be polarized into the classic M1 or M2 phenotype, and this phenomenon can be seen in cigarette consumption.[2] In this polarization scheme, lower M1 markers and higher M2 markers have been observed.[4] The reprogram of macrophage implies a dysregulated inflammation that can damage healthy lung cells.

Macrophage polarization is mediated by three major signaling pathways: NF-κB, MAPK, and JAK/STAT.[2] Each signaling cascade can lead to different results depending on the length of smoke history. It is therefore important to evaluate the characteristics of research participants and specify the experimental conditions when examining smoker’s macrophages. It is anticipated that cigarette smoking inhibits signal transduction which alters gene expression and cytokine profile with increasing M2-like phenotype. This trait is involved in anti-inflammation and tissue repair, but this can also be pro-fibrotic.[11] However, some studies found variation in the conventional polarization and found dual polarization in multiple diseases, yet the direction and extent of polarization are also different across diseases.[4] Despite the contradiction, treatments targeting the polarization process have promising results.

In long-term smoking or established diseases, not only does CSE decrease the production of pro-inflammatory cytokines, but also impairs TLR2 and TLR4 signaling.[2] Its inhibitory effect on NF-κB also induces apoptosis of alveolar macrophage. Prolonged exposure to CSE hence leads to M2 polarization. Meanwhile, NF-κB pathways will be activated with low concentration of CSE or in previously unexposed individuals. The increased activity of NF-κB upregulates the production of pro-inflammatory cytokines TNF-α, IL-1β, and IL-8. The short-term exposure attracts macrophages and neutrophils to the lung with a 4-fold increase in cellularity.[4][2] Short duration also biases polarization towards M1 phenotype. The number of immune cells however will be normalized in 6 months, demonstrating the shift in signaling direction.

Similar time and dose dependent effects of CSE are exerted on macrophage polarization through the MAPK signaling pathway which involves JNK and ERK as intermediate signaling molecules.[2] In diseased conditions due to long-term smoking, the inactivation of JNK reduces the levels of reactive nitrogen species and pro-inflammatory cytokines with more M2-like phenotype. However, brief exposure to CSE triggers the activation of ERK that increases MUC1, TNF-a, and IL-8 levels to produce inflammatory effects.

Cigarette contents also modulate multiple STAT proteins activities.[2] In response to the smoking, STAT3 and STAT6 signaling are stimulated to potentiate M2-like phenotype with elevated IL-6, IL-10, IL-12, and TGF-b. In the meantime, the toxic nitrogenous chemicals and oxidative stress would be reduced. In post-smoke situations, reduction in STAT1 is associated with M1-like phenotype and the downregulation of IFN-γ signaling.

Nicotine in cigarettes modulates the above signaling pathways by binding to α-7 nicotinic receptors on macrophage or neurons, hence activating the cholinergic anti-inflammatory pathway.[12] Changes can thus be directly mediated by binding of nicotine to macrophage or indirectly via the Vagus nerve. Upon binding, the inhibition of the NF-κB and activation of JAK2/STAT3 pathways lead to over-inhibition of pro-inflammatory cytokines and thus an imbalance toward anti-inflammatory cytokines. The result may be lethal if inflammation is not controlled.

Diagram of iron homeostasis. A: The role of hepcidin; B:The effects of environmental factors; C: Genetic variation in iron homeostasis. Exposure to cigarette contents leads to abnormal ferritin metabolism and increased TFR1 activity

Cigarettes contain a small amount of iron, but cumulatively a larger quantity in daily smoking.[13] The increasing iron exposure in the lung and airway affects both respiratory and systemic iron homeostasis by modifying cellular response. Although direct etiologic link has not been established, there is a 4-fold increase in intracellular iron level and a concomitant iron release observed in smoker’s macrophage.[4][13] While iron-loading affects macrophage activation and functions, excessive extracellular iron favors bacterial growth. Normally, activated alveolar macrophage secretes lipocalin-2 which traps bacterial siderophores and prevents bacterial iron uptake. Iron imbalance locally in the lung thus results in higher risk of infection.

Hemosiderin is the iron storage in smoker's macrophage rather than ferritin. It is formed during hemorrhage or abnormal metabolism of ferritin.[14][15] Indeed, buildup of iron causes oxidative stress resulting in lung damage and mitochondrial dysfunction.[13] The level of hemosiderin-laden macrophage is also associated with pulmonary hemodynamics parameters which is used to evaluate pulmonary hypertension in the early stage of diseases.[16]

Iron homeostasis has been associated with macrophage polarization and reprogramming despite unclear causality in cigarette iron.[13] M1 macrophage demonstrates high TF, HAMP, and FTH1 gene activities that mediates iron uptake. M2 macrophage on the other hand expresses FPN1 which causes iron release. Supplementing iron to mice predisposes macrophage to M2 phenotype and inhibits M1-mediated inflammation.

The immune functions in smoker’s macrophages are compromised, so the airway pathogens are more likely to accumulate and cause infection.[17] Smoker’s macrophages have reduced expression of HLA-DR antigens, causing immunosuppression.[18][19] In addition, nicotine impairs the phagocytosis of M. tuberculosis and also induces immunosuppression via the activation of alpha-7 nicotinic receptors.[4][17] Meanwhile, due to the impaired TLR2 and TLR4 signaling, macrophages fail to recognize pathogens, so there is a decrease in pathogen clearance.[17] Therefore, smokers are prone to acute respiratory tract infection and community acquired pneumonia.[20]

A patient's lung diagnosed with lung cancer

Asthma has been proven to have a causal relationship with smoking due to the modified inflammation reaction.[21] Alveolar macrophages will be excessively recruited onto the airway wall, leading to a narrower airway for oxygen to pass through. Some patients may also be affected by airway remodeling. Smoker's macrophages affect the elastic fibers in the mucus layer of the airways, tightening the lumen and causing asthma. Symptoms of asthma include wheezing, coughing, and chest discomfort. To ameliorate the situation, drugs that either suppress the inflammatory response or relaxes the airway will be administrated, so air can pass through.[22]

Smoking is found to be the most important causative factor leading to COPD.[23] Because of the altered inflammatory response of the macrophages, smoking induces inflammation across the entire airway, which in turns obstructs the airflow. Symptoms of COPD include persistent coughing, wheezing, chest infections and breathlessness. Treatments for COPD usually focus on the source of the problem, which is smoking, thus the general treatment is going through smoking rehabilitation which including nicotine replacement therapy, mental therapy for advice, and support to quit smoking. In certain urgent cases, direct constriction also occurs, in which bronchodilators allow the airway to dilate.

Tobacco smoking has been associated with cancer mainly along the respiratory tract, but may also lead to cancer in the bladder and renal pelvis.[24] Upon smoking, carcinogenic chemicals are inhaled, affecting the inflammation response. As inflammation plays an important role in inducing cancer, with smoking affecting the inflammatory response of macrophages in the lungs, the dysregulated inflammatory response poses a higher risk in developing cancer along the airway. Symptoms of cancer mediated by cancer include lumps on the body, sudden weight loss, persistent coughing and tough swallowing. Treatments for cancer are generally surgery, chemotherapy and radiation therapy. These treatments directly target the cancer cells to kill the cancer prior to smoking rehabilitation programs.