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The neutrophil and systemic inflammation
Joint Bone Spine 79 (2012) 209–211
Available online at
www.sciencedirect.com
Editorial
The neutrophil and systemic inflammation
a r t i c l e
i n f o
Keywords: Neutrophil Inflammatory diseases
1. Introduction Neutrophils play a pivotal role in innate immunity in humans. Thus, neutrophils constitute a potent defense system against pathogens, most notably bacteria and fungi, that have breached the skin-mucosa barrier; as well as against structures identified by the immune system as non-self, such as altered cells and endogenous molecules [1,2]. When infection or inflammation develops, the neutrophils are the first cells that migrate to the disease site. Mature neutrophils are compartmentalized cells that contain granules filled with “ready-to-use” substances produced during granulopoiesis [2]. Exposure of neutrophils to an appropriate stimulus causes decompartmentalization, leading to the rapid onset of effector functions that underlies the efficiency of neutrophils in innate immune responses. However, neutrophils are also equipped for de novo synthesis of mediators that relay the early effector functions [3,4]. The microbicidal and cytotoxic effects of neutrophils are dependent on a variety of mechanisms, which are interlinked, including the release of proteolytic enzymes and antimicrobial proteins and the rapid production of reactive oxygen species (ROSs) [1,5]. Although phagocytic “killer” activity was long believed to be the only function of neutrophils, a far broader array of effects has now been documented. Thus, neutrophils contribute to trigger and to regulate innate and adaptive immune responses, as well as tissue homeostasis [3,4]. The functions exerted by neutrophils illustrate the complex interconnections among factors contributing to the innate and adaptive immune responses involved in combatting pathogens and in the pathophysiology of various diseases. 2. Physiology of neutrophils 2.1. Destruction of pathogens Neutrophils normally protect the body against pathogens and eliminate altered self-structures via a process that involves the following steps: granulopoiesis, transendothelial migration of neutrophils from the bloodstream to tissues via postcapillary venules in response to danger signals induced or released by the pathogen, recognition by receptors involved in innate responses (e.g., toll-like receptors or TLRs) of molecular patterns that are either conserved
during the evolution of pathogenic agents (pathogen-associated molecular patterns or PAMPs) or found in altered cells or tissues (damage-associated molecular patterns or DAMPs), adhesion to the target, phagocytosis of the target, and destruction and digestion of the target via the release of proteases and antimicrobial peptides within the phagocytosis vacuoles, together with massive production of ROSs by NADPH oxidase [1,2,5,6]. NADPH oxidase activity is regulated by the inflammatory environment and, more specifically, by the balance between proinflammatory cytokines such as TNF-␣ and antiinflammatory cytokines such as IL-10. Several proinflammatory cytokines are capable of priming NADPH oxidase: although they do not directly induce ROS production, they cause NADPH oxidase overactivity with overproduction of ROSs upon subsequent stimulation by appropriate factors [7]. 2.2. Immune system regulation Neutrophils release compounds involved in regulating many immune-system participants, factors involved in neutrophil function, and tissue remodeling processes. For instance, ROSs modulate intracellular signaling, thereby regulating the functions of neighboring cells such as apoptosis, proliferative responses, and cytokine production [8]. Furthermore, the release of proteolytic enzymes regulates the tissue environment and the conformation of various mediators, thereby causing their activation or deactivation. Importantly, neutrophils produce many mediators of inflammation (e.g., platelet-activating factor (PAF) and leukotriene B4), as well as proinflammatory and antiinflammatory cytokines. IL-8 is the most abundant cytokine produced by neutrophils. IL-8, together with other chemoattractants produced by the neutrophil, plays a major role in promoting neutrophil migration to sites of inflammation. Furthermore, neutrophils produce chemokines for most immune cell types including dendritic cells and, consequently, contribute to trigger adaptive immune responses. In addition, neutrophils interact directly with dendritic cells, transferring the antigen to them and regulating their maturation [3,4]. An original feature of neutrophils is the ability to rapidly release mediators and cytokines by exocytosis upon arrival at sites of inflammation, with a subsequent relay via de novo synthesis. Although cytokine production by neutrophils is less abundant than by monocytes, neutrophils are the first cells to massively infiltrate the site of inflammation, and their cytokine production may therefore be crucially important at this early stage of the immune response [4]. Neutrophils have some ability to present antigens to T cells, in particular via crosspresentation [9]. Furthermore, neutrophils migrate to lymph nodes, where they can interact with immune cells. Neutrophil functions are regulated by mediators such as cytokines produced by cells
1297-319X/$ – see front matter © 2012 Société franc¸aise de rhumatologie. Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.jbspin.2011.12.008
210
Editorial / Joint Bone Spine 79 (2012) 209–211
at the site of inflammation and depend on the balance between various lymphocyte subsets, most notably Treg versus TH17; in addition, direct interactions, for instance with CD4+ CD25+ regulator T cells, also contribute to regulate neutrophil functions [4]. 2.3. Death of neutrophils In the absence of inflammation, neutrophils live only 2 to 3 days, after which they undergo apoptosis in the tissues. Neutrophil phagocytosis by macrophages prevents the release of toxic molecules in the environment and regulates granulopoiesis. The death of neutrophils by necrosis or apoptosis, regulated by the inflammatory environment, contributes to determine whether the site of inflammation will recover a state of tissue homeostasis. A unique feature of neutrophils is that death can occur via a third mechanism in which all the cell structures are destroyed and a network of extracellular fibers (neutrophil extracellular traps or NETs) is formed [10,11]. The fibers are unfolded chromatin filaments composed of DNA and histones and coated with numerous antimicrobial molecules from the neutrophil granules (such as myeloperoxidase, elastase, cathepsin G, alpha-defensins, cathelicidin LL37, lactoferrin, and gelatinase). NET formation occurs when neutrophils are activated by various stimuli (e.g., bacterial endotoxins and some cytokines) and depends on ROS production by NADPH oxidase. The networks entrap not only gram-positive and gram-negative bacteria, but also fungi, and keep them in close contact with high concentrations of antimicrobial proteins. NETs are degraded by DNaseI, an enzyme found in the serum of healthy individuals. NET formation may increase both the spatial and the temporal antimicrobial efficiency of neutrophils, by allowing neutrophils to act on neighboring but non-entrapped microorganisms, thus continuing their antimicrobial effects even after their death. However, conceivably, this defense mechanism involving prolonged exposure of host DNA associated with microorganisms might eventually disrupt self-tolerance and lead to autoimmune disorders. 2.4. Resolution of the inflammatory process Once the pathogenic agent is eliminated, the inflammatory response subsides and the tissue repair process begins. This resolution phase is crucial to protect the tissues and to allow their return to a state of homeostasis [12]. For neutrophils, it involves a decrease in neutrophil numbers at the site of inflammation, suppression of neutrophil activation, the production of antiinflammatory mediators, the induction of neutrophil apoptosis, and the clearance of apoptotic neutrophils by macrophages. Phagocytosis of apoptotic neutrophils by macrophages and macrophage reprogramming to an antiinflammatory state play a central role in the resolution of the inflammatory response. Among antiinflammatory mediators, lipid derivates are crucial. In brief, during their response, neutrophils switch from producing proinflammatory mediators such as prostaglandins and leukotrienes to producing lipoxins. Lipoxins promote the phagocytosis of apoptotic neutrophils by macrophages and induce macrophages to synthesize antiinflammatory cytokines (such as TGF-) and lipid mediators (resolvins, protectins, and maresins) that play a key role in inhibiting neutrophil recruitment and activation and in clearing neutrophils from sites of inflammation [12]. 3. Neutrophils and inflammatory diseases Thus, neutrophils usually play a beneficial role by eliminating agents identified as non-self. This beneficial role is illustrated by the occurrence of serious and/or recurrent infections in patients with neutropenia or with qualitative neutrophil deficiencies
such as primary disorders of various neutrophil functions [13]. However, the neutrophil is a two-edged sword. The inflammatory environment plays a key role in regulating each of the steps of the neutrophil response, thus ideally ensuring that the response is appropriate for the inciting stimulus. Any dysregulation of these finely orchestrated mechanisms can lead to persistent tissue infiltration by neutrophils, prolonged neutrophil activation, or neutrophil activation at inappropriate sites, thereby inducing tissue lesions and contributing to the pathophysiology of various inflammatory diseases. We will not discuss the role for neutrophils in infections and their systemic complications, and we will instead focus on the part played by neutrophils in some of the systemic autoimmune diseases. 3.1. Neutrophils and systemic lupus erythematosus (SLE) Few studies have investigated the role for neutrophils in systemic lupus erythematosus (SLE). Increased counts of apoptotic neutrophils have been found in blood samples from SLE patients, as well as basal activation of neutrophils with overexpression of adhesion proteins and overproduction of ROSs compared to healthy individuals. Other abnormalities reported in SLE include an increase in the number of immature neutrophils with overexpression of genes encoding antimicrobial peptides produced by neutrophils, although these findings do not constitute proof that the neutrophil is involved in the pathophysiology of SLE [14]. A role for NETs with incomplete degradation by serum DNaseI was evidenced in about 36% of SLE patients. Incomplete NET degradation correlated with high antinuclear antibody titers, the presence of autoantibodies against NET, and an increased rate of glomerulonephritis. The underlying reason was the presence of either DNaseI inhibitors or of NET-protective antibodies. Defective NET clearance may lead to the release of autoantigens and DAMPs, which may stimulate the inflammatory response and exacerbate the disease [15]. More specifically, studies by Lande et al. and Garcia-Romo et al. [16,17] have established that neutrophils and NETs exert a pivotal effect in the activation of plasmacytoid dendritic cells (pDCs), whose high production of interferon-␣ (IFN-␣) plays a central role in the pathophysiology of SLE. These studies indicate that serum from patients with SLE contains immune complexes composed of autoantibodies against host DNA, ribonucleoproteins, and antimicrobial peptides released by activated neutrophils (LL37, ␣-defensins). Importantly, DNA, ribonucleoproteins, and antimicrobial peptides are all found in NETs. Autoantibodies to antimicrobial peptides block the degradation of DNA in NETs and promote its capture by pDCs via Fc␥RII, which is followed by interaction of the DNA with TLR9 within the endosomes, leading to IFN-␣ production by pDCs. IFN-␣ promotes the release of antimicrobial peptides at the surface of neutrophils, and autoantibodies to these peptides induce neutrophil death via NET formation with a further increase in IFN-␣ production. NETs may also interact with self-reactive B cells, thereby increasing the production of antibodies against DNA and LL37. In addition, IFN-␣ can be produced by neutrophils under the influence of specific stimuli in patients with SLE [18]. Nucleosomes (composed of double-stranded DNA and histones) in the bloodstream of SLE patients induce neutrophil activation with increased production of proinflammatory cytokines and modulation of neutrophil adhesion proteins. This activation response is independent from TLR9, whose role varies across cells and models [19]. The consequences of neutrophil activation in SLE may play a major role in the complex pathophysiology of this disease. 3.2. Neutrophils and autoimmune vasculitides The presence of antineutrophil cytoplasmic antibodies (ANCAs) plays a vital role in the pathophysiology of small-vessel vasculitides
Editorial / Joint Bone Spine 79 (2012) 209–211
such as Wegener’s granulomatosis, microscopic polyangiitis, and Churg-Strauss syndrome. In these diseases, the neutrophils are primed by proinflammatory cytokines. The surface of primed neutrophils carries proteases released from the granules, such as myeloperoxidase or proteinase 3. These primed neutrophils also express adhesion molecules that bind to those found in blood vessels. ANCAs bind to their target antigens and interact with Fc␥Rs, thereby inducing massive production of ROSs and complete degranulation of lytic enzymes near the blood vessels, causing vessel damage [20]. NETs produced by ANCA-stimulated neutrophils have been visualized in the renal glomerules and interstitial tissue of patients with small-vessel vasculitides [21]. In addition, proteinase 3 and myeloperoxidase are found in NETs, expose autoantigens, and perpetuate the autoimmune response. 3.3. Neutrophils in rheumatoid arthritis Rheumatoid arthritis (RA) is a systemic inflammatory disease whose clinical expression usually targets the joints. Immune response dysregulation results in inflammatory synovitis [22]. Neutrophils are not normally resident cells in joints. A major event in the pathogenesis of RA is the influx into the synovial membrane of neutrophils that have destructive capabilities. These neutrophils massively infiltrate the joint fluid, accumulate at the pannus/cartilage junction, and contribute to destroy the cartilage via protease release and ROS production. Among the various chemoattractants that induce neutrophil migration to joints, leukotriene B4 and its receptor have been shown to play a key role in mice. A cascade of events induces cytokine production by the neutrophils and by the joint cells [23]. Several chemokines such as IL-8 are found in high concentrations in the human rheumatoid joint. These chemokines are associated with cytokines, such as TNF␣, which prime NADPH oxidase, inducing overactivation of this enzyme in response to other stimuli such as leukotriene B4, the complement fraction C5a, or formyl peptides. NADPH oxidase results in ROS overproduction, which contributes to explain the joint damage [7,24]. These examples show that, in a dysregulated inflammatory environment, neutrophils are not only potent effectors of tissue lesions, but can also contribute to dysregulate the various partners involved in immune responses, thereby perpetuating the inflammatory and autoimmune responses. Disclosure of interest The author declares that she has no conflicts of interest concerning this article. Acknowledgements I thank the Inserm, the Arthritis Fondation Courtin, and the Agence nationale de la recherche for supporting our research on the role for neutrophils in rheumatoid arthritis.
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References [1] Witko-Sarsat V, Rieu P, Descamps-Latscha B, et al. Neutrophils: molecules, functions and pathophysiological aspects. Lab Invest 2000;80:617–53. [2] Borregaard N. Neutrophils, from marrow to microbes. Immunity 2010;33: 657–70. [3] Nathan C. Neutrophils and immunity: challenges and opportunities. Nat Rev Immunol 2006;6:173–82. [4] Mantovani A, Cassatella MA, Costantini C, et al. Neutrophils in the activation and regulation of innate and adaptive immunity. Nat Rev Immunol 2011;11:519–31. [5] Vignais PV. The superoxide-generating NADPH oxidase: structural aspects and activation mechanism. Cell Mol Life Sci 2002;59:1428–59. [6] Takeuchi O, Akirai S. Pattern recognition receptors and inflammation. Cell 2010;140:805–20. [7] El-Benna J, Dang PM, Gougerot-Pocidalo MA. Priming of the neutrophil NADPH oxidase activation: role of p47phox phosphorylation and NOX2 mobilization to the plasma membrane. Semin Immunopathol 2008;3:279–89. [8] Dröge W. Free radicals in the physiological control of cell function. Physiol Rev 2002;82:47–95. [9] Beauvillain C, Delneste Y, Scotet M, et al. Neutrophils efficiently cross-prime naive T cells in vivo. Blood 2007;110:2965–73. [10] Brinkmann V, Reichard U, Goosmann C, et al. Neutrophil extracellular traps kill bacteria. Science 2004;303:1532–5. [11] Fuchs TA, Abed U, Goosmann C, et al. Novel cell death program leads to neutrophil extracellular traps. J Cell Biol 2007;176:231–41. [12] Serhan CN, Chiang N, Van Dyke TE. Resolving inflammation: dual anti-inflammatory and pro-resolution lipid mediators. Nat Rev Immunol 2008;8:349–61. [13] Andrews T, Sullivan KE. Infections in patients with inherited defects in phagocytic function. Clin Microbiol Rev 2003;16:597–621. [14] Bennett L, Palucka AK, Arce E, et al. Interferon and granulopoiesis signatures in systemic lupus erythematosus blood. J Exp Med 2003;197:711–23. [15] Hakkim A, Fürrohr BG, Amann K, et al. Impairment of neutrophil extracellular trap degradation is associated with lupus nephritis. PNAS 2010;107:9813–8. [16] Lande R, Ganguly D, Facchinetti V, et al. Neutrophils activate plasmacytoid dendritic cells by releasing self-DNA-peptide complexes in systemic lupus erythematosus. Sci Transl Med 2011;3:73ra19. [17] Garcia-Romo GS, Caielli S, Vega B, et al. Netting neutrophils are major inducers of type I IFN production in pediatric systemic lupus erythematosus. Sci Transl Med 2011;3:73ra20. [18] Decker P. Neutrophils and interferon-␣-producing cells: who produces interferon in lupus? Arthritis Res Ther 2011;13:118–20. [19] Lindau D, Rönnefarth V, Erbacher A, et al. Nucleosome-induced neutrophil activation occurs independently of TLR9 and endosmal acidification: implications for systemic lupus erythematosous. Eur J Immunol 2011;41:669–81. [20] Kallenberg CG. Pathogenesis of ANCA-associated vasculitides. Ann Rheum Dis 2011;70:i59–63. [21] Kessenbrock K, Krumbholz M, Schönermarck U, et al. Netting neutrophils in autoimmune small-vessel vasculitis. Nat Med 2009;15:623–5. [22] Boissier MC. Cell and cytokine imbalances in rheumatoid synovitis. Joint Bone Spine 2011;78:230–4. [23] Chou RC, Kim ND, Sadik CD, et al. Lipid-cytokine-chemokine cascade drives neutrophil recruitment in a murine model of inflammatory arthritis. Immunity 2010;33:266–78. [24] Dang PMC, Stensballe A, Boussetta T, et al. A specific p47 phox-serine phosphorylated by convergent MAPKs mediates neutrophil NADPH oxidase priming at inflammatory sites. J Clin Invest 2006;116:2033–43.
Marie-Anne Gougerot-Pocidalo Inserm U773, UF dysfonctionnements immunitaires, faculté de médecine, université Paris-7 Denis-Diderot, CHU Xavier-Bichat, AP–HP, site Bichat, 16, rue Henri-Huchard, 75018 Paris, France E-mail address: [email protected] 15 December 2011 Available online 5 February 2012
Available online at
www.sciencedirect.com
Editorial
The neutrophil and systemic inflammation
a r t i c l e
i n f o
Keywords: Neutrophil Inflammatory diseases
1. Introduction Neutrophils play a pivotal role in innate immunity in humans. Thus, neutrophils constitute a potent defense system against pathogens, most notably bacteria and fungi, that have breached the skin-mucosa barrier; as well as against structures identified by the immune system as non-self, such as altered cells and endogenous molecules [1,2]. When infection or inflammation develops, the neutrophils are the first cells that migrate to the disease site. Mature neutrophils are compartmentalized cells that contain granules filled with “ready-to-use” substances produced during granulopoiesis [2]. Exposure of neutrophils to an appropriate stimulus causes decompartmentalization, leading to the rapid onset of effector functions that underlies the efficiency of neutrophils in innate immune responses. However, neutrophils are also equipped for de novo synthesis of mediators that relay the early effector functions [3,4]. The microbicidal and cytotoxic effects of neutrophils are dependent on a variety of mechanisms, which are interlinked, including the release of proteolytic enzymes and antimicrobial proteins and the rapid production of reactive oxygen species (ROSs) [1,5]. Although phagocytic “killer” activity was long believed to be the only function of neutrophils, a far broader array of effects has now been documented. Thus, neutrophils contribute to trigger and to regulate innate and adaptive immune responses, as well as tissue homeostasis [3,4]. The functions exerted by neutrophils illustrate the complex interconnections among factors contributing to the innate and adaptive immune responses involved in combatting pathogens and in the pathophysiology of various diseases. 2. Physiology of neutrophils 2.1. Destruction of pathogens Neutrophils normally protect the body against pathogens and eliminate altered self-structures via a process that involves the following steps: granulopoiesis, transendothelial migration of neutrophils from the bloodstream to tissues via postcapillary venules in response to danger signals induced or released by the pathogen, recognition by receptors involved in innate responses (e.g., toll-like receptors or TLRs) of molecular patterns that are either conserved
during the evolution of pathogenic agents (pathogen-associated molecular patterns or PAMPs) or found in altered cells or tissues (damage-associated molecular patterns or DAMPs), adhesion to the target, phagocytosis of the target, and destruction and digestion of the target via the release of proteases and antimicrobial peptides within the phagocytosis vacuoles, together with massive production of ROSs by NADPH oxidase [1,2,5,6]. NADPH oxidase activity is regulated by the inflammatory environment and, more specifically, by the balance between proinflammatory cytokines such as TNF-␣ and antiinflammatory cytokines such as IL-10. Several proinflammatory cytokines are capable of priming NADPH oxidase: although they do not directly induce ROS production, they cause NADPH oxidase overactivity with overproduction of ROSs upon subsequent stimulation by appropriate factors [7]. 2.2. Immune system regulation Neutrophils release compounds involved in regulating many immune-system participants, factors involved in neutrophil function, and tissue remodeling processes. For instance, ROSs modulate intracellular signaling, thereby regulating the functions of neighboring cells such as apoptosis, proliferative responses, and cytokine production [8]. Furthermore, the release of proteolytic enzymes regulates the tissue environment and the conformation of various mediators, thereby causing their activation or deactivation. Importantly, neutrophils produce many mediators of inflammation (e.g., platelet-activating factor (PAF) and leukotriene B4), as well as proinflammatory and antiinflammatory cytokines. IL-8 is the most abundant cytokine produced by neutrophils. IL-8, together with other chemoattractants produced by the neutrophil, plays a major role in promoting neutrophil migration to sites of inflammation. Furthermore, neutrophils produce chemokines for most immune cell types including dendritic cells and, consequently, contribute to trigger adaptive immune responses. In addition, neutrophils interact directly with dendritic cells, transferring the antigen to them and regulating their maturation [3,4]. An original feature of neutrophils is the ability to rapidly release mediators and cytokines by exocytosis upon arrival at sites of inflammation, with a subsequent relay via de novo synthesis. Although cytokine production by neutrophils is less abundant than by monocytes, neutrophils are the first cells to massively infiltrate the site of inflammation, and their cytokine production may therefore be crucially important at this early stage of the immune response [4]. Neutrophils have some ability to present antigens to T cells, in particular via crosspresentation [9]. Furthermore, neutrophils migrate to lymph nodes, where they can interact with immune cells. Neutrophil functions are regulated by mediators such as cytokines produced by cells
1297-319X/$ – see front matter © 2012 Société franc¸aise de rhumatologie. Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.jbspin.2011.12.008
210
Editorial / Joint Bone Spine 79 (2012) 209–211
at the site of inflammation and depend on the balance between various lymphocyte subsets, most notably Treg versus TH17; in addition, direct interactions, for instance with CD4+ CD25+ regulator T cells, also contribute to regulate neutrophil functions [4]. 2.3. Death of neutrophils In the absence of inflammation, neutrophils live only 2 to 3 days, after which they undergo apoptosis in the tissues. Neutrophil phagocytosis by macrophages prevents the release of toxic molecules in the environment and regulates granulopoiesis. The death of neutrophils by necrosis or apoptosis, regulated by the inflammatory environment, contributes to determine whether the site of inflammation will recover a state of tissue homeostasis. A unique feature of neutrophils is that death can occur via a third mechanism in which all the cell structures are destroyed and a network of extracellular fibers (neutrophil extracellular traps or NETs) is formed [10,11]. The fibers are unfolded chromatin filaments composed of DNA and histones and coated with numerous antimicrobial molecules from the neutrophil granules (such as myeloperoxidase, elastase, cathepsin G, alpha-defensins, cathelicidin LL37, lactoferrin, and gelatinase). NET formation occurs when neutrophils are activated by various stimuli (e.g., bacterial endotoxins and some cytokines) and depends on ROS production by NADPH oxidase. The networks entrap not only gram-positive and gram-negative bacteria, but also fungi, and keep them in close contact with high concentrations of antimicrobial proteins. NETs are degraded by DNaseI, an enzyme found in the serum of healthy individuals. NET formation may increase both the spatial and the temporal antimicrobial efficiency of neutrophils, by allowing neutrophils to act on neighboring but non-entrapped microorganisms, thus continuing their antimicrobial effects even after their death. However, conceivably, this defense mechanism involving prolonged exposure of host DNA associated with microorganisms might eventually disrupt self-tolerance and lead to autoimmune disorders. 2.4. Resolution of the inflammatory process Once the pathogenic agent is eliminated, the inflammatory response subsides and the tissue repair process begins. This resolution phase is crucial to protect the tissues and to allow their return to a state of homeostasis [12]. For neutrophils, it involves a decrease in neutrophil numbers at the site of inflammation, suppression of neutrophil activation, the production of antiinflammatory mediators, the induction of neutrophil apoptosis, and the clearance of apoptotic neutrophils by macrophages. Phagocytosis of apoptotic neutrophils by macrophages and macrophage reprogramming to an antiinflammatory state play a central role in the resolution of the inflammatory response. Among antiinflammatory mediators, lipid derivates are crucial. In brief, during their response, neutrophils switch from producing proinflammatory mediators such as prostaglandins and leukotrienes to producing lipoxins. Lipoxins promote the phagocytosis of apoptotic neutrophils by macrophages and induce macrophages to synthesize antiinflammatory cytokines (such as TGF-) and lipid mediators (resolvins, protectins, and maresins) that play a key role in inhibiting neutrophil recruitment and activation and in clearing neutrophils from sites of inflammation [12]. 3. Neutrophils and inflammatory diseases Thus, neutrophils usually play a beneficial role by eliminating agents identified as non-self. This beneficial role is illustrated by the occurrence of serious and/or recurrent infections in patients with neutropenia or with qualitative neutrophil deficiencies
such as primary disorders of various neutrophil functions [13]. However, the neutrophil is a two-edged sword. The inflammatory environment plays a key role in regulating each of the steps of the neutrophil response, thus ideally ensuring that the response is appropriate for the inciting stimulus. Any dysregulation of these finely orchestrated mechanisms can lead to persistent tissue infiltration by neutrophils, prolonged neutrophil activation, or neutrophil activation at inappropriate sites, thereby inducing tissue lesions and contributing to the pathophysiology of various inflammatory diseases. We will not discuss the role for neutrophils in infections and their systemic complications, and we will instead focus on the part played by neutrophils in some of the systemic autoimmune diseases. 3.1. Neutrophils and systemic lupus erythematosus (SLE) Few studies have investigated the role for neutrophils in systemic lupus erythematosus (SLE). Increased counts of apoptotic neutrophils have been found in blood samples from SLE patients, as well as basal activation of neutrophils with overexpression of adhesion proteins and overproduction of ROSs compared to healthy individuals. Other abnormalities reported in SLE include an increase in the number of immature neutrophils with overexpression of genes encoding antimicrobial peptides produced by neutrophils, although these findings do not constitute proof that the neutrophil is involved in the pathophysiology of SLE [14]. A role for NETs with incomplete degradation by serum DNaseI was evidenced in about 36% of SLE patients. Incomplete NET degradation correlated with high antinuclear antibody titers, the presence of autoantibodies against NET, and an increased rate of glomerulonephritis. The underlying reason was the presence of either DNaseI inhibitors or of NET-protective antibodies. Defective NET clearance may lead to the release of autoantigens and DAMPs, which may stimulate the inflammatory response and exacerbate the disease [15]. More specifically, studies by Lande et al. and Garcia-Romo et al. [16,17] have established that neutrophils and NETs exert a pivotal effect in the activation of plasmacytoid dendritic cells (pDCs), whose high production of interferon-␣ (IFN-␣) plays a central role in the pathophysiology of SLE. These studies indicate that serum from patients with SLE contains immune complexes composed of autoantibodies against host DNA, ribonucleoproteins, and antimicrobial peptides released by activated neutrophils (LL37, ␣-defensins). Importantly, DNA, ribonucleoproteins, and antimicrobial peptides are all found in NETs. Autoantibodies to antimicrobial peptides block the degradation of DNA in NETs and promote its capture by pDCs via Fc␥RII, which is followed by interaction of the DNA with TLR9 within the endosomes, leading to IFN-␣ production by pDCs. IFN-␣ promotes the release of antimicrobial peptides at the surface of neutrophils, and autoantibodies to these peptides induce neutrophil death via NET formation with a further increase in IFN-␣ production. NETs may also interact with self-reactive B cells, thereby increasing the production of antibodies against DNA and LL37. In addition, IFN-␣ can be produced by neutrophils under the influence of specific stimuli in patients with SLE [18]. Nucleosomes (composed of double-stranded DNA and histones) in the bloodstream of SLE patients induce neutrophil activation with increased production of proinflammatory cytokines and modulation of neutrophil adhesion proteins. This activation response is independent from TLR9, whose role varies across cells and models [19]. The consequences of neutrophil activation in SLE may play a major role in the complex pathophysiology of this disease. 3.2. Neutrophils and autoimmune vasculitides The presence of antineutrophil cytoplasmic antibodies (ANCAs) plays a vital role in the pathophysiology of small-vessel vasculitides
Editorial / Joint Bone Spine 79 (2012) 209–211
such as Wegener’s granulomatosis, microscopic polyangiitis, and Churg-Strauss syndrome. In these diseases, the neutrophils are primed by proinflammatory cytokines. The surface of primed neutrophils carries proteases released from the granules, such as myeloperoxidase or proteinase 3. These primed neutrophils also express adhesion molecules that bind to those found in blood vessels. ANCAs bind to their target antigens and interact with Fc␥Rs, thereby inducing massive production of ROSs and complete degranulation of lytic enzymes near the blood vessels, causing vessel damage [20]. NETs produced by ANCA-stimulated neutrophils have been visualized in the renal glomerules and interstitial tissue of patients with small-vessel vasculitides [21]. In addition, proteinase 3 and myeloperoxidase are found in NETs, expose autoantigens, and perpetuate the autoimmune response. 3.3. Neutrophils in rheumatoid arthritis Rheumatoid arthritis (RA) is a systemic inflammatory disease whose clinical expression usually targets the joints. Immune response dysregulation results in inflammatory synovitis [22]. Neutrophils are not normally resident cells in joints. A major event in the pathogenesis of RA is the influx into the synovial membrane of neutrophils that have destructive capabilities. These neutrophils massively infiltrate the joint fluid, accumulate at the pannus/cartilage junction, and contribute to destroy the cartilage via protease release and ROS production. Among the various chemoattractants that induce neutrophil migration to joints, leukotriene B4 and its receptor have been shown to play a key role in mice. A cascade of events induces cytokine production by the neutrophils and by the joint cells [23]. Several chemokines such as IL-8 are found in high concentrations in the human rheumatoid joint. These chemokines are associated with cytokines, such as TNF␣, which prime NADPH oxidase, inducing overactivation of this enzyme in response to other stimuli such as leukotriene B4, the complement fraction C5a, or formyl peptides. NADPH oxidase results in ROS overproduction, which contributes to explain the joint damage [7,24]. These examples show that, in a dysregulated inflammatory environment, neutrophils are not only potent effectors of tissue lesions, but can also contribute to dysregulate the various partners involved in immune responses, thereby perpetuating the inflammatory and autoimmune responses. Disclosure of interest The author declares that she has no conflicts of interest concerning this article. Acknowledgements I thank the Inserm, the Arthritis Fondation Courtin, and the Agence nationale de la recherche for supporting our research on the role for neutrophils in rheumatoid arthritis.
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Marie-Anne Gougerot-Pocidalo Inserm U773, UF dysfonctionnements immunitaires, faculté de médecine, université Paris-7 Denis-Diderot, CHU Xavier-Bichat, AP–HP, site Bichat, 16, rue Henri-Huchard, 75018 Paris, France E-mail address: [email protected] 15 December 2011 Available online 5 February 2012