Innate Immunity and Cell Surface Receptors in the Pathogenesis of COPD: Insights from Mouse Smoking Models

Abstract

Chronic obstructive pulmonary disease (COPD) is mainly associated with smoking habit. Inflammation is the major initiating process whereby neutrophils and monocytes are attracted into the lung microenvironment by external stimuli present in tobacco leaves and in cigarette smoke, which promote chemotaxis, adhesion, phagocytosis, release of superoxide anions and enzyme granule contents. A minority of smokers develops COPD and different molecular factors, which contribute to the onset of the disease, have been put forward. After many years of research, the pathogenesis of COPD is still an object of debate. In vivo models of cigarette smoke-induced COPD may help to unravel cellular and molecular mechanisms underlying the pathogenesis of COPD. The mouse represents the most favored animal choice with regard to the study of immune mechanisms due to its genetic and physiological similarities to humans, the availability of a large variability of inbred strains, the presence in the species of several genetic disorders analogous to those in man, and finally on the possibility to create models "made-to-measure" by genetic manipulation. The review outlines the different response of mouse strains to cigarette smoke used in COPD studies while retaining a strong focus on their relatability to human patients. These studies reveal the importance of innate immunity and cell surface receptors in the pathogenesis of pulmonary injury induced by cigarette smoking. They further advance the way in which we use wild type or genetically manipulated strains to improve our overall understanding of a multifaceted disease such as COPD. The structural and functional features, which have been found in the different strains of mice after chronic exposure to cigarette smoke, can be used in preclinical studies to develop effective new therapeutic agents for the different phenotypes in human COPD.

Keywords: airway remodelling; cigarette smoking; emphysema; persistent inflammation; smoking cessation; vascular remodelling.

Figure 1

Emphysema-associated pulmonary lesions which can be observed in different mouse strains after chronic exposure of CS. Representative histologic sections from lungs of C57 Bl/6J mice at 6 months after air- (A) or CS-exposure (B). Mice practically do not have goblet cells in their bronchi and bronchioles. Note the appearance of clusters of goblet cells in airways after chronic exposure to CS. (C and D) DBA/2 mice at 6 months after CS exposure show evident areas of fibrosis (sea-green stain in the black square) associated with disseminated foci of pulmonary emphysema. This histological picture firstly described in smoking DBA/2 mice was subsequently described also in man as “Combined Emphysema –Fibrosis Syndrome” (CPFE) In (D), high magnification of the lung parenchyma present in the black square of (C). (E) Histologic section from distal airways from FVB^PAR−2-TgN mice showing muscularisation of small (≤80 mm) intrapulmonary vessels that precedes the development of PH (~45% increase) and right ventricular hypertrophy. (F) Note in the excessive thickening of a-SMA-positive layers in small intrapulmonary vessels. (in insets: higher magnification of lung parenchyma present in black squares). (G and H) Lung sections from an air-exposed (G) and a CS-exposed (H) C57 Bl/6J mouse at 10 months from the start of the exposure. Distal airways of the air-exposed mice show a normal appearance. Peribronchiolar region from a mouse at 10 months after CS exposure is thickened by an evident fibrotic reaction (sea-green stain) (arrowheads). (A and B): PAS staining; (E): haematoxylin and eosin staining; (C, D, G and H): Masson’s Trichrome staining; (F): Immunostaining with anti-a-SMA antibodies. Scale bars = 40 μm. These images are property of the authors.

Figure 2

Role of formylated peptides and FPRs in the initiation and progression of COPD in smokers. Formyl peptides are present in tobacco leaves. They are active components of mainstream and side stream cigarette smoke. Formyl peptides are also actively secreted by pathogens or passively released from dying host cells after tissue injury. The major component of formyl peptides, N-Formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP), can promote by itself the recruitment of inflammatory cells. FMLP engagement of its high-affinity receptor FPR1 can lead neutrophil degranulation and release of superoxide anion, as well as macrophage activation and polarization. These inflammatory cells cause an overload of oxidants and proteases, which lead to epithelial cell death and DAMPs release (ie endogenous ATP, AGE, mitochondrial formyl peptides, HMGB1, MyD88, etc.) in the microenvironment. This may result in the activation of other PPRs, which amplify the inflammatory response. The reactivation of desensitized FPRs by P2Y2 ligation is also reported in the figure. This image is the property of the authors.

Figure 3

Studies on cell surface receptors and COPD were carried out in genetically modified mice. Genetic ablation of the formyl-peptide receptor-1 (fpr-1) gene in mice, or treatment with specific antagonists of FPRs prevents recruitment of inflammatory cells in the lung leading to a complete protection from smoking-induced lung emphysema and airway remodelling (84). CS-induced neutrophilic inflammation and emphysema can be also statistically lowered upon inhibition of RAGE, or purinergic receptor subtypes (such as P2Y2R or P2X7R) as demonstrated in RAGE (60), or P2Y2R and P2X7 knockout mice (44, 45). After chronic CS-exposure, par2 gene overexpression in FVB mice leads to emphysematous changes associated with PH and RVH (26). The absence of proteinase-activated receptor-1 signaling in C57 Bl/6 mice confers protection form FMLP-induced goblet cell metaplasia (51). This image is the property of the authors.