Systemic Inflammation in COPD: Is Genetic Susceptibility a Key Factor?

Abstract

COPD is a multicomponent disease characterized by abnormal inflammatory response of the lungs to noxious particles that is accompanied by systemic effects like weight loss, muscle wasting, reduced functional capacity and impaired health status. A persistent low-grade systemic inflammatory response reflected by enhanced levels of acute phase proteins like C-reactive protein (CRP) and pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α, is present in part of the COPD population. The production of inflammatory proteins is partly genetically determined. Several studies have shown that polymorphisms within genes coding for these inflammatory mediators may modulate systemic inflammatory responses. Among all of these genes, the TNF family (TNF-α, lymphotoxin (LT)-α and their receptors TNF-R55 and TNF-R75), interleukin (IL)-6 and CRP gene polymorphisms are the most prominent candidates. However, large carefully designed studies in well-characterized COPD cohorts are required to unravel the exact role of genetic background in the systemic component of this disease.

Key words:

• COPD
• Smoking
• Systemic inflammation
• Gene polymorphisms.

INTRODUCTION

Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and mortality worldwide and further increases in its prevalence are expected in the coming decades [[1]]. The definition given in the statement of the Global Initiative for Chronic Obstructive Lung Disease (GOLD) says that COPD is “a disease state characterized by airflow limitation that is not fully reversible. The airflow limitation is usually both progressive and associated with an abnormal inflammatory response of the lungs to noxious particles and gases” [[2]]. The chronic inflammatory response is characterized by increased numbers of macrophages, neutrophils, and T lymphocytes [[3], [4], [5]]. Most recently, Hogg et al. [[6]] indicate that progression of COPD was strongly associated with an increase in percentage of airways that contained neutrophils, macrophages, CD4+ cells, CD8+ cells and B cells [[6]]. In turn, all types of activated inflammatory cells may be an important source of inflammatory mediators and proteases in COPD. Indeed, analysis of induced sputum and bronchial biopsies revealed that levels of interleukin (IL)-8 and tumor necrosis factor α (TNF-α), which are generally considered to be important mediators in neutrophil recruitment, are elevated in patients with COPD. Moreover, the degree of airway obstruction is inversely correlated with numbers of CD8+ T lymphocytes in lung biopsies and with levels of neutrophils and IL-8 in sputum derived from COPD patients [[7], [8], [9]].

Therefore, the critical role of the local inflammatory process in the pathogenesis of COPD is recognized and generally accepted today.

Less is known about so called low-grade systemic inflammation present in COPD as well as in other chronic inflammatory diseases [[10], [11]]. Many studies have reported changes in levels of oxidative status, levels of inflammatory cells and mediators not only in the lungs but also in the circulation of COPD patients [[5], [12], [13], [14], [15], [16], [17]]. Gan et al. [[18]] have performed a meta-analysis of published articles concerning systemic inflammatory markers and summarized that several inflammatory mediators, like TNF-α, some interleukins (IL-6 and IL-8), acute phase proteins (C-reactive protein (CRP), fibrinogen), and leukocytes, are significantly increased in the circulation of stable COPD patients. In turn, this systemic inflammation is related to several systemic effects of COPD: involuntary weight loss [[12], [19]], muscle wasting [[20], [21]], reduced functional capacity and health status [[22]], increased cardiovascular morbidity and mortality [[23]] in COPD. These findings have put forward importance to take into account not only local but also systemic inflammatory effects of disease.

Despite the growing general consensus regarding the presence of systemic inflammation in COPD, remarkably little is known about the underlying mechanisms. Several pathways may be involved. So far, it is not clear whether they are partly related or independent. In the present article, current knowledge regarding the origin of systemic inflammation in COPD is reviewed with special attention to the genetic hypothesis. Common polymorphisms in genes coding for inflammatory mediators and their relationships with COPD characteristics are discussed.

Origin of systemic inflammation

The currently accepted hypothesis concerning the nature of the abnormal inflammatory response seen in lungs of COPD patients is based on theory of amplification of so-called “physiological response” to environmental inflammatory stimuli, particularly, cigarette smoke [[24]]. Noxious components contained in cigarette smoke induce endothelial cell injury, cause activation of macrophages and neutrophils, shift oxidant-antioxidant stability and impair antiprotease screen of lungs [[25], [26], [27]]. Normally, ‘physiological’ repair processes restore the broken balance [[28]]. Due to unknown reasons, this damage/repair equilibrium in some individuals switches towards aberrant, suggesting amplified inflammation. Indeed, the inflammatory changes that occur in COPD are also seen in lungs of cigarette smokers without COPD, but to a lesser extent [[29], [30], [31]]. Moreover, it has been shown that airway inflammation, marked by a significant increase in the percentage of neutrophils in the sputum and cells containing IL-8, persists in COPD patients at least 1 year after smoking cessation [[30], [32]]. These data suggest that the inflammatory response in COPD is likely to be a self-perpetuating mechanism.

As have been found by many studies and discussed above, for today we have sufficient data to believe of low-grade systemic inflammation present in COPD patients as well as respiratory impairment. The most evident question is the source of this systemic inflammation?

The most believed hypothesis of systemic inflammation development is spill over of the intense inflammatory process in the airways, parenchyma and pulmonary vasculature into the systemic circulation, promoting a generalized inflammatory reaction [[33], [34]]. Lung-derived inflammatory mediators and reactive oxygen species may release directly from airways to the general circulation through the pulmonary vascular system and also are likely to be involved in priming (preactivation) of peripheral blood leukocytes [[35], [36]]. Most recently new evidence supporting this hypothesis came from a study performed by Oudijk et al. [[36]] showing that expression of genes coding for IL-1β, MIP-1β, IL-1 receptor 2 and CD83 by peripheral blood neutrophils of patients with stable disease after stimulation by TNF-α, correlated with severity of COPD, as measured by FEV1. These findings suggest that progression of COPD is likely associated with the activation of neutrophils in the systemic compartment. Other evidence of possible contribution of smoking and reduced FEV1 to systemic inflammatory process comes from a recent large NHANES III population-based study from the United States. Gan et al. [[37]] have shown that as smoking as well as reduced lung function by itself are associated with increased odds of elevated CRP, fibrinogen and blood leukocytes, but having both risk factors suggest an additive effect contributing to higher level of systemic inflammation.

However, another study performed to evaluate the relationship between local and systemic inflammation has revealed that the systemic inflammatory response in mild-to-moderate COPD does not fit with the overflow concept. Vernooy et al. [[5]] compared levels of soluble TNF-receptors (TNF-R) and IL-8 in sputum and plasma and did not reveal direct correlations, suggesting that the inflammatory processes in the airways and the systemic circulation are independent processes. Further evidence for this hypothesis can be derived from a recent study conducted by van der Vaart et al. [[38]]. They have attempted to find beneficial effects of anti-TNF treatment by infliximab on the level of pulmonary inflammation but have not succeeded, although the same management was beneficial in patients with other systemic inflammatory diseases like rheumatoid arthritis, improving both the local as well the systemic component of disease [[39]]. A recent study revealed that anti-TNF systemic treatment reduces general joint pain intensity, plasma levels of IL-6 and CRP and increases anti-inflammatory cytokines and receptors in synovial fluid and plasma of rheumatoid arthritis patients [[40]]. However, it is important to note that the COPD patients studied by van der Vaart et al. suffer from mild to moderate disease, who will probably have no pronounced systemic inflammation, in view of the reported negative association between FEV1 and CRP levels in plasma of COPD patients [[22]].

Alternatively, increased levels of inflammatory mediators in the blood of COPD patients may originate from extrapulmonary cells (circulating leukocytes, endothelium or muscle cells). A particular problem in COPD patients with marked alveolar wall destruction is intermittent or continuous hypoxia. Significant inverse correlations between Pa_O2 and circulating TNF-α and soluble TNF-R levels in patients with COPD were reported [[41]]. Similarly, a significant relationship was found between the reduced oxygen delivery and the TNF-α levels in the peripheral circulation, stressing the role of tissue hypoxia [[42]].

Recently, Agusti et al. [[43]] have formulated an autoimmune hypothesis for COPD suggesting that the adaptive immune response to newly created or altered epitopes is an essential component in the COPD pathogenesis. The adaptive response is driven by proliferation of B and T cells after antigen presentation by specialized cells such as dendritic cells and macrophages [[44]]. Tobacco smoking and chronic bacterial colonization have been named as a major source of new antigens and epitopes. At least one study gives support to this proposed hypothesis: it has been shown that not only cigarette smoke condensate (CSC) but anti-idiotypic antibody directed against CSC also prime purified human neutrophils, i.e., had tobacco-like activity [[45]]. Generation of tobacco anti-idiotypic antibodies would require an initial immune response to tobacco antigens [[46]]. Components of tobacco smoke may help sensitize the immune system by stimulating release of IL-1α, IL-1ß, IL-6, from macrophages [[47]]. These data suggest that these antibodies may drive the inflammatory process contributing to an ongoing inflammation process in ex-smokers, assuming an unrecognized autoimmune component of COPD [[45]].

Genetic hypothesis

One can also hypothesize that the systemic inflammation could be primary, and that some inflammatory mediators and circulating abnormalities combined with risk factors like smoking to cause pulmonary injuries. It is reasonable to speculate that circulating cells from predisposed individuals are more susceptible to the effects of smoking or occupational exposure and generate an enhanced response to the same rank of stimulation leading to hyper-production of inflammatory mediators premising a higher than normal degree of systemic inflammation. Indeed, several lines of evidence suggest that the systemic inflammatory response is modulated by genetic background [[48], [49]]. Moreover, since it is known that levels of inflammatory markers vary between individuals and have low intra-individual variability over time, as shown for patients with atherosclerosis and coronary syndromes [[50], [51]], systemic inflammation seems to be a stable factor with high systemic impact. Therefore, all genes encoding proteins potentially involved in systemic inflammatory responses like acute-phase proteins (CRP, fibrinogen, etc.) and closely related cytokines-inductors (members of TNF family, IL-6, IL-1) are candidate genes to determine the genetic background that is responsible for inter-individual differences in systemic inflammatory responses.

However, it has to be noted that genetic factors may independently influence the development of inflammatory response in lungs as well. It is known that not all smokers develop abnormal airway inflammation despite a comparable level of smoke exposure [[52], [53]]. Indeed, a variety of genetic association studies have compared the distribution of polymorphisms in candidate genes hypothesised to be involved in the development of progressive and irreversible airway obstruction in COPD patients and control subjects [[54]]. In particular, studies of genes, coding for enzymes metabolizing products of cigarette smoke; proteins of the proteolysis-antiproteolysis system; mediators involved in the efficiency of mucociliary clearance in the lungs; and, genes encoding for different inflammatory cytokines have been performed. Given facts suggest the idea that genetic factors may predispose individuals to pulmonary inflammation.

A diversity of variations can occur in DNA. The simplest change is the substitution of one nucleotide for another, which is known as a single nucleotide polymorphism (SNP), the most common variant. A number of other variations occur, including deletion or insertion of one or more nucleotides, and insertion of multiple repeating sequences. Different states of the same gene that arise due to polymorphism are referred to as alleles.

Some of SNPs lead to a change in function or production of gene products-proteins. Genetic polymorphisms within genes coding for pro-inflammatory cytokines and acute-phase proteins can significantly influence their plasma levels as well as SNPs could be responsible for defects in endogenous anti-inflammatory proteins. For example, changes that occur in a promoter region may alter the binding of a transcription activating or suppressing factor, altering the rate of transcription of the gene [[55]]. The co-inheritance of multiple ‘impaired’ SNPs in multiple proteins, each leading to small changes in production or function of inflammatory molecules, is likely contributes to serious deleterious effects like systemic inflammation [[56]]. The rationale for studying gene SNPs in COPD as well in any other disease is to identify potential markers of susceptibility, severity, and clinical outcome (high-risk or protective genotypes); seeks to identify potential markers for responders and non-responders in clinical trials, and seeks to identify targets for therapeutic intervention [[57]].

In this review we discuss the association between inflammatory mediators, gene polymorphisms and various aspects of COPD. Further the relationship between most pronounced circulating mediators, their polymorphisms and, where possible, systemic manifestations of COPD are described.

Table 1 Summary of candidate genes and polymorphisms potentially implicated in the systemic inflammation of COPD

Candidate gene	SNP and location	Functionality	Prevalence of risk allele in Caucasians	Reference
CRP	1059G/C Exon 2	Plasma CRP reduced in GC or CC carriers	0.09	[[69], [70]]
	−390C/T/A Promoter	High CRP values in individuals with −390TT genotype	A-0.033 T-0.354	[[73]]
	GT-repeat intron	Two-fold lower serum CRP in carriers of genotypesGT16/GT16, GT21/GT21, GT16/GT21	N/A	[[71]]
	1846G/A 3′UTR	Associated with low basal CRP levels	0.66	[[69]]
	+1444C/T 3′UTR	Higher basal and stimulated CRP level	0.304	[[72]]
TNF	−238G/A Promoter	High TNF-α production associated with −238G allele	0.05; 0.084	[[105]]
	−308G/A Promoter	Conflicting results (increased transcription or no effect)	0.15; 0.2	[[101], [102]] [[104], [105]]
	+489G/A	Not found	0.08; 0.10	[[103]]
LTA	252A/G intron 1	1.5-fold enhanced transcription	0.30	[[117], [119]]
	804C/A (Asp60Thr) Exon 3	2-fold increase in induction of several cell-adhesion molecules	0.41	[[117]]
TNFR1	36A/G Exon 1	Unknown	0.46	[[127]]
TNFR2	587T/G (Met196Sarg) exon 6	Lower soluble TNF-R75 plasma levels with TT genotype	0.23	[[131]]
IL6	−174G/C Promoter	Conflicting results (influences IL6transcription or no effect)	0.23; 0.50	(143) (144)
	(A)n(T)n repeat region Promoter	Complete lack of transcriptional induction of IL-6 by IL-1β in combination with −174C	N/A	[[129]]

*Frequencies of alleles derived from SNP online databases (dbSNP NCBI) or relevant papers. If more than one frequency shown, it means that were extracted from different sources.

CRP: C-reactive protein, IL6: interleukin-6, LTA: lymphotoxin-α, N/A: not available, TNF: tumor necrosis factor-α, TNFR1: tumor necrosis factor receptor 55, TNFR2: tumor necrosis factor receptor 75, 3′UTR: 3′-untranslated region.

C-reactive protein

One of the markers of systemic inflammation that is repeatedly shown increased in COPD patients compared to healthy controls is C-reactive protein (CRP), one of the major acute-phase proteins. CRP specifically binds to specific microbial polysaccharides (phosphocholine moieties), which gives this substance a host defensive role. Upon binding to these structures, CRP activates the classical complement pathway and opsonises ligands for phagocytosis. The formation of acute-phase reactants is strongly induced by cytokines such as IL-6 or TNF-α. It has been shown that CRP is a predictor for acute exacerbations of COPD [[58]] and appears a major determinant of hospitalization and death risk in patients with end-stage respiratory failure [[59]]. Most recently, an increased CRP was also shown to be directly associated with impaired muscle function and exercise capacity, independently of muscle mass [[22]]. Furthermore, a decreased health status, often seen in COPD, as assessed by the St. George's Respiratory Questionnaire was associated with a higher CRP in a large well-designed cohort of patients with COPD GOLD II-III [[60]]. CRP is widely used as a marker of cardiovascular morbidity and mortality, even in COPD population [[23], [61]]. CRP may have direct effects on the pathogenesis of atherosclerosis and endothelial dysfunction [[62]]. CRP stimulate IL-6 and endothelin-1 production and upregulates adhesion molecules, promoting a cascade of events that can lead to clot formation [[63]] and even promote atherosclerosis in apolipoprotein E-deficient mice [[64]]. However, the exact role of CRP in development of cardiovascular diseases is still under discussion [[65], [66]].

Both the population-based NHLBI Family Heart Study and twin studies indicate that heritability of serum CRP levels is substantial and approximates 40–50% [[67], [68]]. Several studies have been performed and reported that polymorphisms within the CRP gene are associated with plasma CRP levels. In fact, it was found that a silent SNP in the coding region of gene (not causing amino-acid change) 1059G/C in exon 2 of the CRP gene is associated with decrease in baseline CRP levels [[69], [70]]. In addition, Russel et al. [[69]] have reported that 1846G/A polymorphism in the 3′untranslated region of the CRP gene influences basal levels of CRP and is associated with system lupus erythrematosis. Previously, Szalai et al. [[71]] showed that the variation in baseline CRP amongst individuals is associated with a polymorphic dinucleotide repeat in the CRP intron. Besides, 1444C/T polymorphism in the 3′untranslated region of the CRP gene is associated with higher baseline and stimulated CRP levels [[72]]. However, none of these reported polymorphisms in the CRP gene were shown to have functional capacity. Most recently, Szalai et al. [[73]] has found that the −390C/T/A promoter polymorphism is functional: transcription factor within (E-box 2) binds only when the −390 T allele is present. The tendency was toward high CRP values in individuals with −390TT genotype. SNPs within CRP gene have never been reported in COPD patients. Currently, studies on the influence of CRP polymorphisms on CRP circulating levels and disease severity in COPD patients are in progress.

TNF family

TNF-α, also known as cachectin, and lymphotoxin-α (LT-α) also known as tumor necrosis factor β (TNF-β), are two closely related pro-inflammatory cytokines (about 34% amino acid residue homology) that bind to the same cell surface receptors (TNF-R55 and TNF-R75) and produce a vast range of similar, but not identical, effects. TNF-α was found to be the prototype of a family of molecules that are involved in immune regulation and inflammation [[74], [75]]. The receptors for TNF also constitute a superfamily of related proteins [[76]]. The TNF gene cluster is located within the class III region of the highly polymorphic major histocompatibility complex (MHC) on human chromosome 6p21.3. The genes (TNF and LTA) that encode TNF-α and LT-α, are structurally comparable.

Tumor necrosis factor alpha

TNF-α is one of the major pro-inflammatory cytokines synthesized as a membrane protein, which is cleaved by TNF-alpha-converting enzyme to produce its soluble form. Soluble TNF-α exerts a range of inflammatory and immunomodulatory activities that are important in host defense. TNF-α activates nuclear factor NF-κ B, which switches on the transcription of inflammatory genes, including cytokines, chemokines, and proteases, in epithelial cells and macrophages. It similarly activates p38 mitogen-activated protein (MAP) kinase, which in turn may activate a similar array of genes and may interact with the NF-κ B pathway.

Elevated levels of TNF-α have been found in the lung tissues by bronchial biopsies, bronchoalveolar lavage and induced sputum reflecting local inflammation in COPD patients [[29], [77], [78]]. Moreover, increased circulating levels of TNF-α have been reported in COPD [[79], [80], [81]]. Several studies have provided clear evidence for involvement of TNF-α in the pathogenesis of tissue depletion in patients with COPD. Increased plasma levels of TNF-α were found in patients with COPD suffering from weight loss [[12]]. De Godoy et al. [[19]] reported higher lipopolysaccharide (LPS)-induced TNF-α levels in monocytes from weight-losing patients compared to weight-stable patients and healthy controls. Therefore, due to these multiple effects TNF-α is considered a major biological mediator of systemic inflammation in COPD.

A number of polymorphisms have been described within the TNF gene. In the promoter, these are at positions (relative to the transcription start site) −1031T/C, −863C/A, −857C/A, −851C/T, −419G/C, −376G/A, −308G/A, −238G/A, −162G/A, and −49G/A, although those at positions −419, −163, −49 are rare in Caucasians. In addition, there is an insertion of a cytosine at position +70 in the first exon, a G/A substitution at position +489 in the first intron, and a deletion of a guanine at position +691 in the first intron of the TNF gene [[82]].

The −308G/A SNP has been the most studied polymorphism. Several studies have reported higher TNF-α production by cells from G/A donors than by G/G cells [[83], [84]] but other studies could not confirm this effect [[85], [86], [87], [88]]. Carriage of the A allele of TNF-α -308 has been associated with an increased risk for many diseases, including septic shock [[89]] and severe cerebral malaria [[90]], but not with rheumatoid arthritis [[91]]. Contradictory results are reported on the relationship between TNF-α -308G/A polymorphism and asthma [[92], [93]].

The association with COPD found for Asian populations [[94], [95]] was not replicated in studies of European COPD subjects [[96], [97], [98]]. However, Keatings et al. [[99]] showed that homozygosity for the A allele predisposes to a worse prognosis for COPD in Irish. Most recently, Hersh et al. [[100]] have found significant associations with quantitative and qualitative COPD-related phenotypes for the TNF –308G/A promoter polymorphism in a family-based group which was not replicated in case-control group among American Caucasians. The possible reason of inconsistent results could be different definitions for study subjects between reports. Studies of TNF polymorphism have defined cases based on airflow obstruction, emphysema, decline in lung function, or chronic bronchitis. It is possible that a given genetic variant may confer susceptibility to a specific COPD-related phenotype.

Similar conflicting results have been demonstrated for effect of the −238G/A polymorphism on TNF gene transcription. One study showed the −238G allele to be associated with high TNF-α production [[88]], but this was not confirmed in another study [[86]]. No differences in distribution of −238G/A polymorphism have been found between COPD patients and population controls [[98]]. However, polymorphism at position −238 were associated with multiple sclerosis [[88]], alcoholic liver disease [[101]] and death from community-acquired pneumonia [[102]].

In fact, linkage disequilibrium is strong in this area and it may be difficult to study the role of a SNP in isolation. A strong allelic association between −238G and −308A has been observed, which has found to encode high TNF α production in vitro [[82]].

Kaijzel et al. [[103]] investigated the functional consequence of the +489G/A polymorphism. No difference could be detected between the different alleles in TNF-α pre-mRNA yield upon in vitro and physiological stimulation conditions. Nevertheless, a number of studies have examined the TNF +489G/A intron polymorphism, showing an association with prostate cancer, and with a subgroup of common variable immune deficiency patients with granuloma [[104], [105]]. Minor allele of +489G/A polymorphism was associated with an enhanced risk of development of COPD. Moreover, a subgroup of COPD patients without radiological emphysema was associated with TNF +489G/A gene polymorphism [[98]].

Circulating TNF-α levels do not seem to correspond with the −238G/A and −308G/A TNF promoter polymorphisms in patients with multiple sclerosis and heart failure [[88], [106]]. This relationship is not yet studied in COPD. However, it might be assumed that circulating TNF-α levels are under a multifactorial regulatory process.

Lymphotoxin-α

LT-α is a soluble protein mostly secreted by activated lymphocytes and presumed to act as a modulator in the immune response.

Although most studies of susceptibility genes for immune disease and COPD in particular have focused on the TNF gene, there have been a few reports of functional polymorphism of the LTA gene. Recently, Ozaki et al. [[107]] reported a significant association between the LTA gene polymorphism and myocardial infarction. Of three SNPs studied, two differed significantly between affected individuals and control subjects: the intron 1, 252A/G and exon 3, 804C/A. Functional studies indicated that the substitution in intron 1 enhanced the transcription of LTA and some unknown nuclear factor is binding more tightly to the G allele [[107], [108]]. The 804C/A SNP caused a threonine-to-asparagine substitution at codon 26 (Thr26Asn) and a twofold increase in induction of several cell-adhesion molecules, including vascular cell adhesion molecule-1 in vascular smooth muscle cells [[107]]. Most recently, +80C/A polymorphism of the LTA gene was identified, as a main predictor of LT-α protein production by human B cells [[109]].

Interestingly, it has been suggested that higher levels of TNF-α production are associated with 252A/G polymorphism of the LTA gene [[110]]. Recently, further evidence supporting this association was reported [[111]]. It was found that patients homozygous for the 252G allele might develop an enhanced systemic inflammatory response with larger peak concentrations of TNF-α and IL-6 and manifested an increased risk of cardiopulmonary morbidity after cardiac surgery. Furthermore, LTA 252G allele was found to be associated with high CRP levels.

Previously, LTA 252G allele has been related to clinical asthma and atopy [[93], [112]]. Several studies were conducted to investigate 252A/G polymorphism in COPD patients. Two studies could not link it to COPD associated with emphysema in Italians [[96], [113]]. It seems that this lack of association can be related to the small sample sizes of studied groups in both cases. However, Sandford et al. (2001) also found no association of the TNF gene cluster haplotypes (−308G/A TNF and 252A/G LTA) and the rate of decline of lung function in large Lung Health Study [[97]]. Further studies have to be performed to elucidate the role of genetic polymorphisms within LTA gene in COPD.

Tumor necrosis factor receptors I and II

Two distinct receptor types have been identified that specifically bind TNF-α and LT-α. Virtually all cell types studied show the presence of one or both of these receptor types. TNF-R75 and TNF-R55 are transmembrane glycoproteins with an apparent molecular weight of 75 kDa and 55 kDa, respectively [[76]]. Two receptor types employ different signal transduction pathways: both cytokines induce apoptosis in cells upon binding to TNF-R55, whereas they induce inflammatory responses by activating NFκ B nuclear protein upon binding to TNF-R75 [[114]].

Soluble forms (sTNF-R55 and sTNF-R75) of both types of receptors have been found in human serum [[115]]. These soluble receptors are capable of neutralizing the biological activities of both TNF-α and LT-α. But, at the same time, they stabilize the TNF-α molecule and prevent its degradation. Small but significant increases in circulating levels of both sTNF-R55 and sTNF-R75 have been demonstrated in COPD [[5], [13], [15], [16]]. Because inflammatory stimuli such as TNF-α will induce shedding of membrane-bound TNF-R75, the enhanced levels of sTNF-R may reflect the enhanced inflammatory status of patients with COPD.

Two different genes encode TNF receptors. The TNFR1 gene is located on chromosome 12p13.2 and coding for TNF-R55. The polymorphism in 36A/G exon 1 represents a silent mutation, namely Pro12Pro. An association between 36A/G polymorphism and Crohn's disease is found in Belgium [[116]]. Additionally, the same study has shown that response to infliximab (anti-TNF treatment) was lower in patients carrying TNFR1 36G allele. Interestingly, an association between rheumatoid arthritis and a TNFR1 protective genotype AA was found restricted to the familial form of the disease. Since the functional consequences of this polymorphism have not been studied, the nature of observed associations is remaining unknown.

TNFR2 gene coding for TNF-R75 was mapped to the chromosome 1p36. There are several gene polymorphisms reported in the coding region of this gene, namely in exon 4 (143C/G), exon 5 (365A/C), exon 6 (587T/G). The latter 587T/G polymorphism is of particular interest since it causes amino acid substitution Met196Arg. Till et al. [[117]] have found that 587T/G polymorphism may influence inflammatory signaling pathways resulting in a significantly lower capability to induce TNF-R75-mediated NF-κ B activation. Furthermore, this SNP was associated with altered plasma levels of TNF-R75.

An association between TNFR2 587T/G polymorphism and chronic inflammatory disorders such as rheumatoid arthritis and systemic lupus erythematosus has been described [[118], [119]]. Association between this polymorphism and the response to anti-TNF-α therapy in rheumatoid arthritis patients has also been reported [[120]]. Ferrarotti et al. [[96]] performed the only reported study on TNF-Rs polymorphisms in small group of patients with severe COPD. They have not found evidence of TNFR1 (36A/G) or TNFR2 (587T/G) influence on COPD risk.

Interleukin 6

IL-6, a multifunctional cytokine with a central role in host defense, has diverse functions including stimulation of the hepatic acute phase proteins as CRP in response to infection and injury, differentiation and/or activation of macrophages and T cells as well as growth and terminal differentiation of B cells (reviewed in 121, 122). IL-6 is not constitutively expressed but is highly inducible and is produced in response to a number of inflammatory stimuli such as IL-1β, TNF-α, bacterial products such as endotoxin, and viral infection.

Several studies have shown that IL-6 concentrations are increased in induced sputum, bronchoalveolar lavage, and exhaled breath condensate of COPD patients, particularly during exacerbations [[123], [124]]. Monocytes from COPD patients release more IL-6 in response to stimulation with LPS than cells from normal subjects [[125]]. Furthermore, elevated circulating levels of IL-6 were reported in COPD patients [[80], [81], [126]]. Moreover, during periods of exacerbation, serum levels of IL-6 increase significantly. In addition, IL-6 has been implicated in the pathogenesis of atherosclerosis [[127], [128]].

Circulating levels of IL-6 are largely regulated at the level of expression. The transcription of this molecule is tightly regulated by the transcription factors NF-IL6, NF-κ B, Fos/Jun, cellular retinol-binding protein (CRBP), and the glucocorticoid receptor. Several functional promoter SNPs (−174G/C, −572G/C, −597G/A) gene were described within IL6 gene. The IL6 promoter also contains an (A)n(T)n repeat region running from −373 to approximately −392 that may contribute to gene transcription rates in vitro [[129]]. It has been shown that a polymorphism in the 5′ flanking region of the IL6 gene at position −174G/C appears to affect IL6 transcription and associated with decreased plasma IL-6 levels [[130]]. However, Endler et al. [[131]] did not confirm this data. The −174G/C polymorphism has been found to influence energy expenditure and insulin sensitivity in healthy normoglycemic subjects [[132]]. In addition, presence of the −174C allele was associated with higher baseline CRP levels [[133]].

IL6 gene haplotype analysis has characterized combined synergized effects of several SNPs on expression levels of IL-6. Terry et a. (2000) reported that −174C allele in combination with A₈/T₁₂ allele of −373 (A)n(T)n polymorphism resulted in a complete lack of transcriptional induction of IL-6 by IL-1β [[129]]. However, Rivera-Chavez et al. [[134]] could not confirm these data. In another study evaluating LPS-stimulated IL-6 production by leucocytes it was demonstrated that leucocytes from the homozygous carrier of the GGG-haplotype (−597GG, −572GG and −174GG) produced the highest amount of IL-6 [[135]].

Taking into account that COPD patients have a weight loss as the result of a negative energy balance, partly associated to the level of systemic inflammation [[21]] it would be interesting to relate IL6 SNPs with levels of systemic inflammatory proteins and energy expenditure characteristics.

CONCLUSIONS

The studies discussed in present review suggest the concept that genetic predisposition has to be taken into account when looking for sources of systemic inflammation in COPD. Genetic polymorphisms of most of the discussed inflammatory markers have been intensively studied and related to different chronic inflammatory states. However, despite all these studies the relationship is still unclear. Highlighting the fact that systemic inflammation could be a risk factor for systemic effects of COPD, new studies, elucidating genetic background of systemic inflammation should be explored in adequate phenotyped COPD patients. In future it possibly will help to find new cause-directed management.

ACKNOWLEDGMENTS

We acknowledge the support of the European Respiratory Society (fellowship number 161) and GSK.