Pathophysiological roles of interleukin-8/CXCL8 in pulmonary diseases

Naofumi Mukaida

Abstract

Fifteen years have passed since the first description of interleukin (IL)-8/CXCL8 as a potent neutrophil chemotactic factor. Accumulating evidence has demonstrated that various types of cells can produce a large amount of IL-8/CXCL8 in response to a wide variety of stimuli, including proinflammatory cytokines, microbes and their products, and environmental changes such as hypoxia, reperfusion, and hyperoxia. Numerous observations have established IL-8/CXCL8 as a key mediator in neutrophil-mediated acute inflammation due to its potent actions on neutrophils. However, several lines of evidence indicate that IL-8/CXCL8 has a wide range of actions on various types of cells, including lymphocytes, monocytes, endothelial cells, and fibroblasts, besides neutrophils. The discovery of these biological functions suggests that IL-8/CXCL8 has crucial roles in various pathological conditions such as chronic inflammation and cancer. Here, an overview of its protein structure, mechanisms of production, and receptor system will be discussed as well as the pathophysiological roles of IL-8/CXCL8 in various types of lung pathologies.

  • inflammation
  • angiogenesis
  • chemokine
  • neutrophil

interleukin (IL)-8 was first purified and molecularly cloned as a neutrophil chemotactic factor from lipopolysaccharide-stimulated human mononuclear cell supernatants (109, 166). Since then, a family of structurally related cytokines has been identified. Because most of them exhibit chemotactic activity for a limited spectrum of leukocytes, they are now called chemokines (chemotactic cytokines) (12, 103). Chemokines are low-molecular-weight proteins with cysteines at well-conserved positions, exhibiting a basic charge and an affinity for heparin. It is, therefore, believed that chemokines efficiently bind to proteoglycans on vascular endothelium cells and to extracellular matrix proteins in the tissues (12). Chemokine receptors comprise a large branch of the rhodopsin family of cell-surface G protein-coupled receptors with seven-transmembrane domains (122). High-affinity binding to target cells and subsequent signaling and functional effects of chemokines are mediated by these receptors.

Chemokines are divided into four subgroups, CXC, CC, CX3C with four to six cysteines, and C chemokines with only two, corresponding to the second and fourth cysteines in the other groups. CXC and CX3C chemokines are distinguished by the presence of one (CXC) and three (CX3C) intervening amino acids, respectively, whereas the first two cysteines are adjacent in CC chemokines. A novel nomenclature system has been proposed for chemokines and their receptors (122, 169). Systematic chemokine names are based on their cysteine subclass roots, followed by “L” for “ligand”. The numbers correspond generally to the same number used in the corresponding gene nomenclature. Because most chemokine receptors are restricted to a single chemokine subclass, the nomenclature system of chemokine receptors is rooted by the chemokine subclass specificity, followed by “R” for “receptor” and the number. According to this nomenclature system, IL-8 is now called CXCL8.

CXC chemokines can be further subclassified into Glu-Leu-Arg (ELR)+ and ELR CXC chemokines, based on the presence or absence of tripeptide motif ELR of the NH2 terminus before the first cysteine. This classification correlates with the functional differences. ELR+ CXC chemokines bind CXCR1 and/or CXCR2 with a high affinity and have a potent chemotactic effect, particularly on neutrophils (4,121), and exhibit potent angiogenic activity (148). It is presumed that growth-related proteins (GRO; CXCL1, 2, and 3) substitute for the functions of IL-8 in mice and rats, both of which lack the ortholog of human IL-8/CXCL8 and CXCR1. Thus although we will mainly discuss IL-8/CXCL8, we will refer to other ELR+ CXC chemokines, particularly GROs.

MOLECULAR AND CELLULAR BIOLOGY OF IL-8/CXCL8

Protein structure.

The CXCL8 cDNA encodes a 99-amino acid precursor protein with a signal sequence, which is cleaved to yield mainly 77- or 72-residue mature protein (109). CXCL8 is further processed at the NH2 terminus yielding different truncation analogs (77-, 72-, 71-, 70-, 69-amino acid forms). The truncation is caused by proteases that are released from CXCL8-secreting cells or by accessory cells (65, 124, 129, 167), and the occurrence of the NH2-terminal forms depends on the producer cells and culture conditions (153-155). The two major forms are 77- and 72-amino acid proteins with a minor 69-amino acid protein. In vitro, fibroblasts and endothelial cells predominantly produce the 77-amino acid form, whereas leukocytes mainly secrete 72- or 69-amino acid forms. These three forms exhibit neutrophil chemotactic activities with distinct potencies: 69- > 72- >77-amino acid form. In vivo, the 77-amino acid form is rapidly cleaved to yield a 72-amino acid form (86).

Nuclear magnetic resonance spectroscopy and X-ray crystallography have revealed that CXCL8, in a concentrated solution and crystallized state, occurs as a homodimer consisting of two identical subunits (14). The monomer contains a disordered NH2terminus, followed by a loop region, three antiparallel β-strands, and an α-helix extending from amino acids 57 to the COOH terminus. At concentrations >100 μM, CXCL8 exists as a dimer stabilized by six hydrogen bonds between the first β-strands of the partner molecules (residues 23–29) and by other side-chain interactions. The dimer consists of two antiparallel α-helices lying on top of a six-stranded antiparallel β-sheet. However, at nanomolar concentrations, which represent physiologically relevant levels and induce maximal biological activity, most CXCL8 occurs in its monomeric form (27,130). Moreover, a chemically synthesized CXCL8 analog, with a methylated Leu25, which prevents dimerization, exhibits neutrophil activation and receptor binding equivalent to that of natural CXCL8 (134). These observations suggest that the monomer is a functional form.

Mechanisms of production.

CXCL8 can be produced by leukocytic cells (monocytes, T cells, neutrophils, and natural killer cells) and nonleukocytic somatic cells (endothelial cells, fibroblasts, and epithelial cells) (12, 115,116, 128). CXCL8 production is not constitutive but inducible by proinflammatory cytokines such as IL-1 and tumor necrosis factor (TNF)-α (109). Moreover, CXCL8 production can be induced by bacteria (e.g., Helicobacter pylori, Pseudomonas aeruginosa) (5, 44), bacterial products [e.g., lipopolysaccharide (LPS)] (12, 128), viruses (e.g., adenovirus, respiratory syncytial virus, cytomegalovirus, rhinovirus) (6, 33, 75, 119), and viral products (e.g., X protein of human hepatitis virus B, Tax protein of human T cell-leukemia type I virus) (105, 113). This may result in elevated CXCL8 concentrations in body fluids with microbial infection.

Environmental factors can induce CXCL8 production in several types of cells. Hypoxic conditions induce several types of tumor cells to produce high levels of CXCL8 through the cooperative activation of NF-κB and activator protein-1 (AP-1) (93, 162). Because CXCL8 has potent angiogenic activities (18, 87,148), hypoxic conditions may induce neovascularization by inducing the production of CXCL8. Reactive oxygen intermediates (ROI) can activate NF-κB, an essential transcription factor for CXCL8 gene, and eventually induce CXCL8 gene transcription (73).

CXCL8 production is regulated at the level of gene transcription and mRNA stability. In most cases, CXCL8 gene transcription occurs through cooperative activation of two distinct types of transcription factors, NF-κB and AP-1 (115). However, in hepatoma cell lines, the constitutive activation of PEA3 and AP-1, but not NF-κB, eventually results in constitutive CXCL8 gene transcription (71).

Receptor system.

CXCL8 binds to two distinct receptors, CXCR1 and CXCR2, with a similar high affinity (68, 123). CXCR1 and CXCR2 consist of 350 and 360 amino acids, respectively. They are membrane-bound molecules composed of seven-transmembrane domains and coupled to G proteins at the COOH-terminal portion and possibly the third intracellular loop (120, 122). CXCR1 and CXCR2 possess high-sequence homology exceeding 80% at the amino acid level, except in their NH2-terminal portions, and can bind some ELR+ CXC chemokines. Because of the differences in their NH2-terminal portion, their binding specificities differ; CXCR2 binds CXCL1, CXCL2, CXCL3, CXCL5, CXCL6, and CXCL7 with high affinity, whereas only CXCL6 also binds CXCR1, but with a lower affinity than CXCL8 (122).

After ligand binding, human CXCL8 receptors are internalized and subsequently recycled and reappear on the cell surface rapidly within 60 min (140). The inhibition of recycling reduced CXCL8-mediated chemotaxis. Moreover, ligand binding activates pertussis toxin-sensitive and receptor-coupled G proteins, particularly Gαi proteins (120). G proteins, on conversion to the GTP-bound form, dissociate into Gα- and Gβγ-subunits. Generated Gβγ remain near the receptor-ligand complexes, recruiting and activating phosphatidylinositol 3-kinase-γ (PI3K-γ), which in turn generates phosphatidylinositol 3,4,5-trisphosphate (PIP3) (144). PIP3 activates protein kinase B (Akt) as well as GTPases, resulting in directed cell migration (Fig.1).

Fig. 1.

Presumed signaling mechanisms of IL-8/CXCL8. PLC, phospholipase C; PIP-K, phosphatidylinositol 4-phosphate kinase; IP3, inositol 1,4,5-trisphosphate; DAG, diacylglycerol; PIP2, phosphatidylinositol 4,5-bisphosphate; PIP3, phosphatidylinositol 3,4,5-trisphosphate; RGS, regulator of G protein signaling; PI3K, phosphatidylinositol 3-kinase; MAPK, mitogen-activated protein kinase; MAPKK, mitogen-activated protein kinase kinase; PH, plekitin homology; PKC, protein kinase C; Akt, protein kinase B; [Ca2+]i, intracellular calcium concentration.

Deficiency in phospholipase Cβ2+3, a key enzyme to generate inositol 1,4,5-trisphosphate and diacylglycerol, abrogates CXCL8-induced increases in intracellular calcium concentrations, protein kinase C phosphorylation, p47 phox translocation, and superoxide anion generation but enhances chemokine-mediated chemotaxis (100). PI3K-γ-deficient mice exhibit a reduction in CXCL8-mediated neutrophil migration, PIP3 generation, Akt phosphorylation, and respiratory burst with a robust increase in intracellular calcium concentration in response to CXCL8 (67). These results suggest that chemokine-induced chemotaxis is mediated through both PI3K-γ-independent and -dependent pathways.

CXCL8 activates Ras and eventually mitogen-activated protein kinases (MAPK) and extracellular signal-related kinases (ERK)1/2 in neutrophils (85). G protein-coupled receptor-mediated signals can be downregulated by RGS (regulator of G protein signaling) proteins. RGS proteins appear to enhance the endogenous GTPase activities and thus decrease the half-life of the active GTP-bound state. Among RGS proteins, RGS1, RGS3, and RGS4 reduce CXCL8-mediated migration and adherence (47). Moreover, expression of RGS proteins also reduces CXCL8-induced MAPK activation (21).

In leukocytes, CXCR1 and CXCR2 are coordinately expressed (36,114). However, CXCR1 exhibits a slightly lower affinity for CXCL8 in vitro and requires more CXCL8 for internalization and recycling than CXCR2 (35). Thus CXCR1 may transduce CXCL8 signals at a higher concentration of CXCL8 than does CXCR2. Moreover, CXCL8-induced activation of NADPH oxidase and phospholipase D is mediated in vitro mainly by CXCR1 (76). CXCR1- but not CXCR2-mediated signals attenuate the response to other types of chemoattractants, such as C5a andN-formyl-methionyl-leucyl-phenylalanine (FMLP), without any effects on their receptor expression (137). In contrast, C5a and FMLP attenuate CXCR2- but not CXCR1-mediated signals (136). Thus there may be a cross talk between CXCL8 and other chemotactic factors such as C5a and FMLP.

Biological activities.

Accumulating evidence indicates that CXCL8 is involved in the whole process of leukocyte transmigration into tissues. CXCL8 is internalized by endothelial cells abluminally, transcytosed to luminal surface, and presented to the neutrophils (111). There, CXCL8 induces the shedding of l-selectin, regulates the expression of β2-integrins (CD11b/CD18 and CD11c/CD18) and complement receptor type 1 (CR1/CD35) on neutrophils, and alters the avidity of the constitutively expressed integrin molecules (32, 43,70). CXCL8 promotes adhesion of neutrophils to plastic, extracellular matrix proteins, and unstimulated as well as cytokine-stimulated endothelial monolayers through interaction with CD11b/CD18 (32). CXCL8 stimulates neutrophil migration across endothelium (70), pulmonary epithelium (117), and fibroblasts (26) (see Fig.2).

Fig. 2.

Major biological activities of IL-8/CXCL8. LTB4, leukotriene B4; MMP-9, matrix metalloproteinase-9.

Moreover, CXCL8 can activate various functions of neutrophils, including degranulation and respiratory burst (115, 128). CXCL8 activates 5-lipoxygenase with release of leukotriene B4 and 5-hydroxyeicosatetraenoic acid in the presence of exogenous arachidonic acid (141) and induces the synthesis of platelet-activating factor in neutrophils (28).

CXCL8 exhibits activities against other types of leukocytes. CXCL8 induces basophils to exhibit chemotaxis (58) and to adhere to endothelial cells (11). CXCL8 induces a release of histamine and leukotrienes from IL-3-pretreated basophils at higher concentrations (38) and inhibits histamine release induced by IL-3 at lower concentrations (20, 92). Although CXCL8 does not exhibit any chemotactic activity for normal eosinophils (142), it induces chemotaxis of eosinophils obtained from atopic patients' peripheral blood (24, 159) or from those pretreated with IL-3 or granulocyte/macrophage colony-stimulating factor (158). In the presence of shear stress, monocyte adherence to endothelium cells is markedly increased in response to CXCL8 (61), also suggesting a potential role of CXCL8 in in vivo macrophage migration.

CXCL8 exhibits chemotactic activity against freshly isolated T lymphocytes, whereas incubation of purified T lymphocytes resulted in a progressive decrease in CXCL8-binding sites on T lymphocytes in association with reduced chemotactic response (98). In addition to its direct effects on T cells, CXCL8 indirectly causes T cell chemotaxis by inducing neutrophils to release from their granules several factors such as α-defensin-1, α-defensin-2, and CAP37/azurocidin with chemotactic activities for T lymphocytes (34). Thus a selective depletion of neutrophils inhibits the CXCL8-induced in vivo migration of CD4+ but not CD8+ lymphocyte in rats (90). Human CXCL8 and CXCL1 are chemotactic for B cells (74), and CXCL8 selectively inhibits IL-4-induced immunoglobulin E (IgE) production (81, 82).

Angiogenic activities have been documented for all ELR+ CXC chemokines, including CXCL1, 2, 5, 6, 7, and 8 (87, 147,148). These chemokines can induce migration of endothelial cells, which express CXCR1 and CXCR2. Moreover, CXCL8 induces a loss of focal adhesion in fibroblasts and promotes chemotaxis and chemokinesis of these cells (49).

ROLES OF IL-8 IN PULMONARY DISEASES

Bacterial infection.

CXCL8 is frequently detected at sites of infections. This may be because various types of microbes and their products can induce CXCL8 protein production in a variety of cell lines in vitro (115). Moreover, several lines of evidence suggest that neutrophils are recruited by endogenously produced CXCL8 to infection sites to eradicate inciting microbes. CXCL8 can enhance the in vitro intracellular killing of Mycobacterium fortuitum(126) and Candida albicans by neutrophils (45). Lung-specific transgenic expression of mouse CXCL1 induces neutrophil accumulation in the bronchi and terminal bronchioles (101) but enhances resistance to Klebsiella pneumoniae infection (152). Moreover, CXCR2-deficient mice show an enhanced susceptibility to mucosal and systemic C. albicans infection with reduced neutrophil infiltration into the infected tissues (15). In an acute pyelonephritis model caused by Escherichia coli infection, neutrophils cannot transverse the mucosal barrier and eventually accumulate under the epithelium in CXCR2-deficient mice (54, 63). Subepithelial neutrophil entrapment finally results in renal scarring. InListeria monocytogenes infection, CXCR2-deficient mice often develop chronic infection, compared with wild-type mice (37). These observations suggest that CXCL8 and its related molecules (CXCL1, CXCL2, and CXCL3) play a crucial role in regulating the processes of eradication of invasive bacteria, which are restricted to local sites.

Intrapleural injection of LPS into rabbits induces a massive infiltration of neutrophils into pleural cavity (23). The administration of an anti-CXCL8 antibody reduces neutrophil infiltration in this model, implicating CXCL8 as a key mediator in endotoxin-induced pleurisy (Table 1). Intravenous injection of LPS into a normal human volunteer causes a rapid increase in plasma CXCL8 levels, peaking at 2 h and returning to baseline levels within 5 h after LPS injection (106). Moreover, an elevation in plasma CXCL8 levels precedes neutrophil accumulation and activation, as shown by an elevated neutrophil-derived elastase level (135). Similar results are also observed in patients with sepsis (56,64). In lethal and sublethal sepsis induced by infusing primates with live E. coli and LPS, respectively, CXCL8 is detected in the circulation. The CXCL8 levels are higher in animals with lethal bacteremia than in those with sublethal endotoxemia (156). However, the administration of an anti-CXCL8 antibody only marginally improved survival in endotoxin-induced acute lethality inPropionibacterium acnes-primed rabbits (72). Thus it remains elusive on the pathogenic roles of CXCL8 in endotoxemia.

View this table:
Table 1.

Effects of blocking antibodies to IL-8/CXCL8 or related molecules on pathophysiology of animal pulmonary disease models

Viral infection.

Various types of viruses and viral products can induce CXCL8 production in a wide variety of cell types (33, 105, 107, 113, 119,132) at the transcriptional level. In most cases, viruses and viral products can activate both NF-κB and AP-1, which results in rapid CXCL8 gene transcription. However, the involvement of additional transcription factors is suggested in respiratory syncytial virus-induced IL-8 gene transcription (33).

A vicious cycle may exist between CXCL8 and infection with several viruses. CXCL8 inhibits the in vitro antiviral activities of interferon-α or -β (80). This may account for the inverse correlation between serum CXCL8 levels and the sensitivity to interferon treatment among patients infected with hepatitis C virus (133). Adenovirus type 7, a frequent causative agent for neutrophilic pneumonia, induces CXCL8 production by pulmonary epithelial cells through the activation of ERK1/2 (6). Because CXCL8 can activate ERK1/2 (85), adenovirus and CXCL8 may synergistically induce CXCL8 production and eventually cause neutrophilic pneumonia. Human cytomegalovirus causes pneumonitis in patients with immunodeficiency and/or on immunosuppressive treatments. Human cytomegalovirus can induce IL-8 gene transcription in various types of cells by activating both NF-κB and AP-1, which can synergize (119). CXCL8 can enhance human cytomegalovirus replication in fibroblasts (118), suggesting the presence of an amplifying loop between CXCL8 and cytomegalovirus infection.

Accumulating evidence suggests the involvement of virus-encoded chemokine- or chemokine receptor-like molecules in the pathogenesis of virus infection. The UL-146 and UL-147 of human cytomegalovirus encodes CXC chemokine-like molecules, vCXC-1 and vCXC-2, respectively (131). vCXC-1 binds the human CXCR2 and induces calcium mobilization, chemotaxis, and degranulation in human neutrophils, but its pathophysiological relevance remains elusive. Human herpesvirus 8 (Kaposi's sarcoma herpes virus) open reading frame (ORF) 74 encodes a chemokine receptor-like protein, which resembles human CXCR2 (25). CXCL8 and CXCL1 activate ORF74-mediated signaling pathway (60), whereas CXCL10 inhibits it (59). Moreover, ORF74 constitutively activates protein kinase C, c-jun kinase, and p38 MAPK pathways, and eventually induces malignant transformation and angiogenesis (10,13). Furthermore, transgenic expression of ORF74 induces an angioproliferative disease resembling Kaposi's sarcoma (164). Thus ORF74 may be involved in the pathogenesis of angiogenesis, a characteristic pathological feature of Kaposi's sarcoma.

Acute respiratory distress syndrome.

Acute respiratory distress syndrome (ARDS) is identified as an acute respiratory failure associated with a nonhydrostatic pulmonary edema (19). The involvement of neutrophils in ARDS has been documented by several groups (146), and elevated bronchioalveolar lavage fluid (BALF) concentrations of CXCL8 and granulocyte colony-stimulating factor correlated with increased neutrophil numbers in BALF (3). Moreover, CXCL8 levels in BALF were increased in ARDS patients and correlated with the development of ARDS in at-risk patient groups (46). Kurdowska and colleagues (95) claimed that CXCL8 in BALF was associated with anti-CXCL8 autoantibodies and that there was a significant correlation between the concentrations of CXCL8:anti-CXCL8 complex and the onset of ARDS. These clinical observations implicate CXCL8 in the pathogenesis of ARDS.

This hypothesis is supported by results on several animal models. In acid aspiration- and endotoxemia-induced ARDS in rabbits, CXCL8 is produced in the lungs (53, 165). In both models, the abrogation of CXCL8 activity reduces neutrophil infiltration as well as tissue damage. These effects may be explained by the assumption that locally produced CXCL8 can suppress neutrophil apoptosis (48) and induce neutrophil migration into the lungs and damages to lung tissues, including alveolar epithelial barrier function (112).

Reperfusion injury and transplantation.

Reperfusion injury occurring after a transient ischemic episode frequently causes greater injury than the ischemia itself (55, 161) and is frequently observed in myocardial infarction, cerebral infarction, and transplanted organs. Reperfusion of blood flow results in reoxygenation of ischemic tissue, leading to the generation of ROI such as hydrochloric acid and hydrogen oxide (110). These generated ROI damage every component in tissue, including nucleic acids, membrane lipids, enzymes, and receptors. Reperfusion injury is characterized by the adherence and emigration of neutrophils into postcapillary venules (51). Depletion of neutrophils or the prevention of neutrophil adherence with antibodies to leukocyte adhesion molecules abolished reperfusion-induced vascular dysfunction (69, 89). These results imply that neutrophil infiltration has a crucial role in the establishment of reperfusion injury.

ROI can activate NF-κB (73), an essential transcription factor for CXCL8 gene transcription. High levels of CXCL8 are produced massively by infiltrating cells and tissue resident cells when blood flow is reperfused into ischemic lungs (143), myocardium (91), and brain (108). Moreover, administration of an anti-CXCL8 antibody prevents both neutrophil infiltration and tissue damage. Consistent with these observations, the degree of CXCL8 release during early reperfusion can predict later graft function in human lung transplantation (41).

Lung injuries due to other physical conditions.

Reexpansion of collapsed lung induces increased microvascular permeability and neutrophil infiltration, leading to reexpansion pulmonary edema. CXCL8 is produced by alveolar macrophages and epithelial cells in the reexpanded lung in rabbits (125). Moreover, pretreatment with a neutralizing antibody to CXCL8 reduced microvascular permeability and neutrophil infiltration, indicating the crucial roles of CXCL8 in the establishment of reexpansion lung injury. Smoke inhalation also causes lung endothelial injury and formation of pulmonary edema, due to an increase in alveolar epithelial permeability to protein and reduction in the fluid transport capacity of alveolar epithelium. The pretreatment with a neutralizing anti-CXCL8 antibody significantly reduced the smoke-induced increase in bidirectional transport of protein across the alveolar epithelium and restored alveolar liquid clearance to a normal level (96). Thus CXCL8 has a crucial role in edema formation due to these injuries.

High oxygen concentration contributes to lung disease and concomitant neutrophil infiltration in the newborn. High oxygen concentration synergistically increases TNF-α-induced CXCL8 gene expression in vitro in human lung type II epithelial cells (7). Moreover, exposure of newborn rabbits to hyperoxia induces CXCL8 expression in neutrophils and macrophages in BALF, and CXCL8 levels correlated with neutrophil number in BALF (39). CXCL8 levels in tracheal aspirates are increased in preterm infants with acute lung injury and correlates with subsequent progression to bronchopulmonary dysplasia (127). These results suggest a pivotal involvement of CXCL8 and/or related chemokines in hyperoxia-induced lung injury in neonates. This notion is corroborated by the observation that anti-CXCL1 and/or anti-CXCL2 treatment prevents hyperoxia-induced alveolar septal thickening and neutrophil infiltration in newborn rats (40).

Allergic inflammation and asthma.

CXCR2-deficient mice exhibit increased serum IgE levels when they are kept under specific pathogen-free conditions (29). IgE production in allergen-induced pulmonary inflammation is also enhanced in CXCR2-deficient mice compared with wild-type mice (42). These phenotypes may be explained by the observation that CXCL8 selectively inhibits IL-4-induced IgE production (81, 82). Because the treatment of human lungs with IgE results in release of CXCL8 (52), CXCL8 may constitute a negative feedback for IgE production.

Analysis of induced sputum in persistent asthma identifies two different inflammatory patterns (62). The most common pattern is noneosinophilic and is associated with neutrophil influx into the lungs and increased local CXCL8 production. Increases in sputum CXCL8 as well as CCL2 and CCL4 levels precede a late exacerbation of acute asthmatic attack (94). Furthermore, in noninfectious status asthmaticus, neutrophil number and CXCL8 levels in BALF are markedly increased (97). Although the mechanism of CXCL8 production in asthma remains elusive, it is of interest that pulmonary epithelial cells can produce high levels of CXCL8 in the presence of diesel exhaust particles (151), which can contribute to the pathogenesis of asthma (138). Because CXCL8 inhalation has been shown to directly provoke bronchoconstriction in guinea pigs (57), it presumably contributes to the establishment of asthma at various stages directly and indirectly by inducing neutrophil infiltration and activation.

Idiopathic pulmonary fibrosis and other diffuse lung diseases.

Increased CXCL8 expression by alveolar macrophage is observed in idiopathic pulmonary fibrosis (IPF) (31). CXCL8 levels in both serum and BALF are increased significantly, and serum CXCL8 levels are indicative of the disease activity of IPF (30, 168). In mice, the administration with anti-mouse CXCL2 antibodies attenuates bleomycin-induced pulmonary fibrosis by reducing angiogenesis but not neutrophil infiltration (79). In contrast, CXCL10 also attenuates bleomycin-induced pulmonary fibrosis by inhibiting angiogenesis (78). Because the level of CXCL10 is decreased in IPF tissues (77), the imbalance between CXCL8 and CXCL10 may be responsible for the angiogenesis observed in IPF.

In chronic obstructive pulmonary diseases (COPD), both neutrophils and eosinophils are activated in the airway (16). Sputum CXCL8 levels correlated well with levels of neutrophil activation markers such as neutrophil myeloperoxidase and elastase (66, 163). Moreover, sputum CXCL8 levels were inversely correlated with forced expiratory volume. Because COPD is frequently associated with bacterial respiratory infections, invading bacteria may induce CXCL8 production and eventually neutrophil migration and activation.

CXCL8 levels in BALF are increased markedly in diffuse panbronchiolitis (DPB), which is characterized by chronic inflammation of the respiratory bronchioles with leukocyte infiltration (88,139). Macrolide antibiotics, such as erythromycin and clarithromycin, are effective for the treatment of DPB (139), and a regimen with these macrolide antibiotics is associated with inhibition of CXCL8 production at the transcriptional level (1), suggesting the involvement of CXCL8 in the pathogenesis of DPB. Moreover, microsatellite polymorphism of the human CXCL8 gene is reported to be associated with DPB (50). Thus aberrant CXCL8 production may predispose the patient to the development of DPB.

Chloride concentrations in the airway surface fluid overlying respiratory epithelia are elevated in cystic fibrosis when compared with normals. Of interest is that elevated chloride concentration in vitro increased CXCL8 synthesis by neutrophils but decreased their capacity to kill P. aeruginosa (149). This may result in neutrophil-mediated damages to lungs and enhanced susceptibility to bacterial infection in cystic fibrosis.

Cancer.

The roles of leukocyte infiltration in cancer still remain unclear despite a long history of intensive investigation. There are at least two hypotheses to document the role of tumor-associated leukocytes. On one hand, tumor-associated leukocytes may reflect the host's ineffective attempt to reject the tumors. CXCL8 induces the accumulation of neutrophils, which can directly kill tumor cells (99). On the other hand, tumor-associated leukocytes, particularly macrophages, may be a potential source of growth factors for tumor cells and endothelial cells. Thus chemokines with chemotactic activities for monocytes/macrophages, particularly CCL2, are presumed to be involved in tumor progression by recruiting and activating macrophages to produce growth factors (157). In bronchoalveolar carcinoma, tumor cells are a main source of CXCL8, and neutrophils are located mainly in the alveolar lumen, whereas lymphocytes are exclusively in the alveolar wall. Moreover, the presence of increased numbers of neutrophils in BALF is correlated with CXCL8 levels in BALF and associated with poor outcome (17). Thus proangiogenic CXCL8 may promote tumor progression in bronchiolar carcinoma.

Endothelial cells exhibit chemotaxis in response to CXCL8 as well as other ELR+ CXC chemokines, such as CXCL1, CXCL2, CXCL5, CXCL6, and CXCL7 (87, 147, 148). Moreover, these chemokines are angiogenic in the rat corneal vascularization assay. CXCL8 expression is directly correlated with the degree of neovascularization in some tumor tissues, such as nonsmall cell lung cancer and gastric cancer tissues (84, 145). In contrast, ELR CXC chemokines, including CXCL4, CXCL9, and CXCL10, can inhibit the angiogenic effects of ELR+ CXC chemokines and basic fibroblast growth factor (2, 8, 104, 160). Thus the balance of ELR+ vs. ELR CXC chemokines produced in tumor tissues may dictate the degree of angiogenesis and eventually tumor aggressiveness (147).

CXCR1 or CXCR2 is expressed by several types of cancer cell lines, such as melanoma, pancreatic carcinoma, colon carcinoma, and gastric cancer (22, 83, 102, 150). In vitro studies demonstrate that either CXCL8 or CXCL1 can stimulate the proliferation of melanoma, pancreatic carcinoma, and colon carcinoma cell lines, whereas CXCL8 induces the expression of metastasis-related genes, such as matrix metalloproteinases, by a gastric carcinoma cell line. However, it remains to be established whether these CXC chemokines directly modulate tumor growth in vivo.

FUTURE PERSPECTIVES

In 1987, IL-8/CXCL8 was discovered as a neutrophil chemotactic factor (166). Subsequent studies have revealed that CXCL8 exerts a wide variety of actions on various types of cells, including neutrophils, monocytes, lymphocytes, endothelial cells, and fibroblasts. Studies of models of acute inflammation have established CXCL8 as a key mediator in neutrophil-mediated acute inflammation. However, the absence of CXCL8 and CXCR1 orthologs in mice and rats hinders the clarification on the pathophysiological roles of CXCL8 in other pathological conditions, particularly chronic inflammation and cancer. The elucidation of these issues will be made possible by further careful extrapolation of observations on these pathological conditions in mice and rats. This will undoubtedly lead to the development of novel therapeutics and/or preventive modalities by targeting CXCL8.

Acknowledgments

We greatly appreciate Drs. Joost J. Oppenheim, Carole Galligan, and Ying Ying Le (National Cancer Institute–Frederick) for thoughtful comments on the manuscript.

Footnotes

  • This work is supported in part by grants from the Ministry of Education, Science, and Technology, and Osaka Cancer Research Foundation.

  • Address for reprint requests and other correspondence: N. Mukaida, Division of Molecular Bioregulation, Cancer Research Institute, Kanazawa Univ., 13-1 Takara-machi, Kanazawa 920-0934, Japan (E-mail:naofumim{at}kenroku.kanazawa-u.ac.jp).

  • 10.1152/ajplung.00233.2002

REFERENCES

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