New insights into the resolution of inflammation.

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Review

New insights into the resolution of inflammation Derek Gilroy ∗ , Roel De Maeyer Centre for Clinical Pharmacology and Therapeutics, Division of Medicine, 5 University Street, University College London, London WC1E 6JJ, United Kingdom

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Article history: Received 24 March 2015 Received in revised form 6 May 2015 Accepted 13 May 2015 Keywords: Adaptive immunity Eicosanoids Chronic inflammation Macrophages

a b s t r a c t The goal of treating chronic inflammatory diseases must be to inhibit persistent inflammation and restore tissue function. To achieve this we need to improve our understanding of the pathways that drive inflammation as well as those that bring about its resolution. In particular, resolution of inflammation is driven by a complex set of mediators that regulate cellular events required to clear inflammatory cells from sites of injury or infection and restore homeostasis. Indeed, it may be argued that dysfunctional resolution may underpin the aetiology of some chronic inflammatory disease and that a novel goal in treating such diseases is to develop drugs based on the mode of endogenous pro-resolution factors in order to drive on-going inflammation down a pro-resolution pathway. And while we are improving our understanding of the resolution of acute and chronic inflammation, much remains to be discovered. Here we will discuss the key endogenous checkpoints necessary for mounting an effective yet limited inflammatory response and the crucial biochemical pathways necessary to prevent its persistence and trigger its resolution. In doing so, we will provide an update on what is known about resolution of acute inflammation, in particular its link with adaptive immunity. © 2015 Published by Elsevier Ltd.

1. Introdution Inflammation is a key pathophysiological component of a wide range of diseases, including rheumatoid arthritis, asthma, inflammatory bowel disease atherosclerosis and cancer. Over the past 50 years knowledge concerning the mediators that cause inflammation in various preclinical models and clinical settings has developed to the point that the current mainstay for treating many inflammation-mediated diseases is based on inhibiting the synthesis or activities of these mediators. While non-steroidal anti-inflammatory drugs (NSAIDs), steroids and monoclonal antibodies are effective at treating disease symptoms in specific clinical settings, none of these approaches are curative, and many of them have undesirable side effects. Rather than just focusing on the initiation of the inflammatory response, recent years have seen increasing attention on the other end of the inflammatory spectrum – the resolution of the inflammatory response. Inflammatory resolution is increasingly viewed as an active process involving a number of key mediators, with dysregulation of this process possibly predisposing individuals to the development of chronic inflammatory diseases [1]. While understanding resolution biology will provide important insights into the aetiology of

∗ Corresponding author. Tel.: +44 020 7679 6933; fax: +44 020 7679 6351. E-mail address: [email protected] (D. Gilroy).

chronic inflammatory diseases, it will also provide new targets for the development of novel anti-inflammatory drugs based on triggering and driving pro-resolution pathways. Such approaches provide the exciting potential to reverse the underlying disease processes of a wide range of inflammatory conditions that have high and unmet medical needs. In this review we will attempt to define “inflammatory resolution” and place it in the context of the broader inflammatory response. This will provide a solid basis for understanding the potential clinical utility of compounds, which can modulate inflammatory resolution processes. 2. What is ‘resolution of inflammation’? It is important to distinguish between inflammatory resolution and inflammatory onset. At onset, local release/activation of soluble mediators (e.g. complement, vasoactive amines, cytokines, lipids) from histiocytes and stromal cells and up-regulation of cell adhesion molecules on the microvascular endothelium collectively facilitate extravascular leucocyte accumulation manifesting in Celsus’ cardinal signs of inflammation – heat, redness, swelling and pain (Rudolph Virchoff added loss of function in the 19th century) [2]. This well-characterised phase of the inflammatory response is routinely targeted using drugs including NSAIDS and anti-TNF␣ that inhibit or antagonise the action of these inflammatory drivers forming the mainstay for treating chronic inflammatory disease. Resolution, however, switches inflammation off. In-as-much as

http://dx.doi.org/10.1016/j.smim.2015.05.003 1044-5323/© 2015 Published by Elsevier Ltd.

Please cite this article in press as: D. Gilroy, R. De Maeyer, New insights into the resolution of inflammation, Semin Immunol (2015), http://dx.doi.org/10.1016/j.smim.2015.05.003

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onset is orchestrated by a host of sequentially released mediators, resolution is an active process that is no longer considered a passive event where the response was hitherto thought to simply fizzle out [3,4]. For instance, a critical requirement for the inflammatory response to switch off is the elimination of the injurious agents that initiated it in the first place. Failure to achieve this first step will lead to chronic inflammation as exemplified by chronic granulomatous disease, which results from a failure of the phagocytic NADPH oxidase enzyme system to produce superoxide and kill invading infections leading to a predisposition to recurrent bacterial and fungal infections and the development of inflammatory granulomas [5]. Successfully dispensing with the inciting stimulus will signal a cessation of pro-inflammatory mediator synthesis and lead to their catabolism. This will halt further leucocyte recruitment and oedema formation. These are probably the very earliest determinants for the resolution of acute inflammation, the outcome of which signals the next stage of cell clearance. The clearance phase of resolution, be it polymorphonuclear leucocyte (PMN)- or eosinophil-driven or adaptive (lymphocyte mediated) in nature, also has a number of mutually dependent steps. The clearance routes available to inflammatory leukocytes include systemic recirculation or local death by apoptosis/necrosis of influxed PMNs, eosinophils or lymphocytes followed by their phagocytosis or efferocytosis by recruited monocyte-derived macrophages. Once phagocytosis is complete, macrophages can leave the inflamed site by lymphatic drainage with evidence that a small population may die locally by apoptosis [6]. Dispensing with the injurious agent leads to the next phase of pro-inflammatory mediator catabolism where levels of cytokines, chemokines, eicosanoids, cell adhesion molecules, etc. must revert back to that expressed during the pre-inflamed state. For prostaglandins (PGs), for instance, the first step in their catabolism (reviewed in detail in Ref. [7]) is the oxidation of the 15(S)-hydroxyl group by a 15-hydroxyPG dehydrogenase, which metabolises E-series PGs as well as other eicosanoids including Lxs, 15-HETE, 5,15-diHETE, and 8,15-diHETE to the corresponding 15-keto compounds. The second step involves the reduction of the 13 double bond by an NADPH/NADH dependent 13–15ketoPG reductase. Further catabolism of PGs some HETES and Lxs occurs by the beta-oxidation pathway common to fatty acids in general, i.e. via the carboxyl end of the molecule leading to the formation of short-chain metabolites, which are excreted in the urine. Some of these eicosanoids are also excreted following glucuronidation. 5-HETE and LTs undergo beta-oxidation from the omega-terminus following an initial omega-hydroxylation. In terms of chemokines, the atypical chemokine receptors such as D6 possess the inability to initiate classical signalling pathways after ligand binding thereby acting as a type of scavenging system for pro-inflammatory signals such that in TPA-induced skin inflammation D6-deficient mice exhibit an excess concentration of chemokines resulting in a notable inflammatory pathology with similarities to human psoriasis, for review see [8]. In addition, the work of Ariel et al. showed that CCL3 and CCL5 were increased in peritoneal exudates of Ccr5−/− mice during the resolution of acute peritonitis. Transfer of apoptotic PMNs resulted in CCR5-dependent scavenging of CCL3, CCL4 and CCL5. It transpires that CCR5 surface expression on apoptotic PMNs was reduced by pro-inflammatory cytokines and was increased by pro-resolution lipid mediators including lipoxin (Lx)A4 [9]. Thus, endogenous systems exist to facilitate pro-inflammatory mediator clearance and whose function, when it becomes dysregulated, may lead to chronic inflammation. If all of these pathways of stimulus removal, inhibition of granulocyte trafficking, pro-inflammatory mediator catabolism, appropriate cell death/efferocytosis (phagocytosis of apoptotic cells), etc. are followed then acute inflammation will resolve without causing excessive tissue damage and give little

opportunity for the development of chronic, non-resolving inflammation. Each stage of the resolution cascade represents an opportunity to be harnessed to drive ongoing inflammatory diseases down a pro-resolution pathway. Yet, we caution that this will not be a panacea for all diseases driven by ongoing inflammation. We suspect that resolution processes may vary from tissue to tissue and be dependent of the nature of the injurious stimulus. Thus, designing pro-resolution drugs will have to be organ and disease specific. With that comes the need for more appropriate animal models of ongoing inflammation that best reflect the intended human condition. In addition more studies must be focused on examining resolution pathways in healthy and diseased humans.

3. Inflammatory onset Inflammation is a reaction of the microcirculation; it is a protective response initiated after infection or injury. While both local and systemic responses can be activated, inflammation is an essential biological process with the objective of eliminating the inciting stimulus, promoting tissue repair/wound healing and in the case of infection, establishing memory such that the host mounts a faster and more specific response upon a future encounter. The acute inflammatory response is a complex yet highly coordinated sequence of events involving a large number of molecular, cellular and physiological changes. It begins with the production of soluble mediators (complement, chemokines, cytokines, eicosanoids [including PGs], free radicals, vasoactive amines, etc.) by resident cells in the injured/infected tissue (i.e. tissue macrophages, dendritic cells, lymphocytes, endothelial cells, fibroblasts and mast cells) concomitant with the up-regulation of cell adhesion molecules on both leukocytes and endothelial cells that promote the exudation of proteins and influx of granulocytes from blood [10]. Upon arrival these leukocytes, typically PMNs in the case of non-specific inflammation or eosinophils in response to allergens, primarily function to phagocytose and eliminate foreign microorganisms via distinct intracellular (superoxide, myeloperoxidase, proteases, lactoferrins) and/or extracellular (neutrophil extracellular traps) killing mechanisms [11]. It is likely that the magnitude of the infectious load and its eventual neutralisation signal the next phase of active anti-inflammatory and pro-resolution [12]. In terms of signalling pathways that regulate this very early response to injury/infection, most of our earlier knowledge was garnered from work done on studying members of interleukin 1 (IL-1) and tumour necrosis factor (TNF) receptor families and the Toll-like microbial pattern recognition receptors (TLRs) which in fact belong to the IL-1R family. IL-1 and TNF␣ represent the archetypal pro-inflammatory cytokines, which are rapidly released upon tissue injury or infection. TLRs recognise microbial molecular patterns, hence the term pattern recognition receptor (PRR) and therefore TLRs represent a germline encoded non-self recognition system that is hard-wired to trigger inflammation. However, there is some suggestion that endogenous ligands may trigger TLRs during tissue injury and certain disease states which may act to promote inflammation in the absence of infection [13]. Although structurally different, these receptors use similar signal transduction mechanisms. Receptor engagement results in recruitment of adaptor proteins, that possess either Toll-IL-1 receptor (TIR) domains in the case of TLRs and IL-1R or death domains (DD) in the case of the TNFR family, linked to the regulation of cell survival [14]. Once recruited these adaptors recruit further signalling proteins that belong to the TRAF family [15,16] and various protein kinases, including IRAK1 and 4 in the case of TIR signalling [17] and RIP kinases in the case of TNFR signalling [18,19]. These


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molecules activate several effector pathways, the most important of which lead to activation of Mitogen Activated Protein Kinases (MAPK) [20,21], including JNK [22] and p38 MAPK [23], as well as I␬B kinases (IKK) [24]. The MAPKs lead to direct and indirect phosphorylation and activation of various transcription factors, especially those that belong to the bZIP family: AP-1 [25] and CREB [26], which bind to the promoters of pro-inflammatory genes. MAPKs also regulate pro-inflammatory gene expression through post-transcriptional mechanisms such as mRNA turnover, mRNA transport and translation [27–29]. The IKKs, which form a complex composed of two catalytic subunits-IKK␣ and IKK␤ and a regulatory subunit IKK␥/NEMO, are responsible for activation of the NF-␬B transcription factor [24], which has emerged as a central regulator of inflammatory and immune responses [30,31]. Target genes for the IKK and MAPK pathways include IL-1 and TNF␣, generating a feed-forward mechanism to amplify the inflammatory response. The pro-inflammatory cytokines IL-6, IL-12 and type I interferons (IFNs), which are also target genes for IKK and MAPK regulation, signal via receptor associated tyrosine kinases (RTKs) that belong to the JAK group, whose activation results in phosphorylation and nuclear translocation of STAT transcription factors [32]. Engagement of cytokine receptors as well as TLRs, can also lead to activation of phosphoinositide-3-kinases (PI3K), which in turn activate other proteins kinases such as AKT [33]. Collectively, these proteins kinases coordinate the expression of a large number of proinflammatory mediators to initiate and maintain the inflammatory response.

4. Dampening pro-inflammatory signals Once activated, it is reasonable to expect that the various families of receptors and their signalling pathways that triggered the release of pro-inflammatory mediators be turned off in order to prevent an over-exuberant pro-inflammatory state. To facilitate this, a number of regulatory mechanisms have evolved [34]. For example, Leucine Rich Repeat Containing 33 (LRRC33) is a transmembrane molecule that contains an extracellular domain with 17 LRRs, which negatively regulates TLR signalling and subsequent NF-␬B activation resulting in a dampening of TLR-driven proinflammatory events [35]. LRRC33 is similar to RP105, an inhibitor of TLR4 signalling, but it lacks a TIR domain. Like LRRC33, dendritic cells lacking RP105 exhibit increased NF-␬B activation and pro-inflammatory signalling in response to LPS challenge [36]. In addition to dampening TLR signalling, posttranscriptional regulatory mechanisms including restricting mRNA stability and/or translation have also evolved. Many of these mRNAs possess adenine/uridine-rich elements (AREs) that recruit destabilising factors and translational silencers to their 3 untranslated region (for detailed review see [37]). Also, microRNAS (miRNAs) have emerged as key regulators of TLR pro-inflammatory signalling pathways. miRNA is a small non-coding RNA molecule containing about 21–22 nucleotides, which function by base-pairing with complementary sequences within mRNA molecules. As a result, these mRNA molecules are silenced by cleavage of mRNA, destabilisation of the mRNA through shortening of its poly(A) tail and/or less efficient translation of the mRNA into proteins by ribosomes. In terms of specific pro-inflammatory signalling pathways, PI3K signalling is inhibited by the PTEN phosphatase that belongs to the Protein Tyrosine Phosphatase (PTP) family, some of its other members, for instance SHIP, SHP1/2 and CD45, are responsible for negative regulation of tyrosine kinase signalling [38]. MAPK kinase phosphatases (MKPs), which also belong to the PTP family control the duration of MAPK activation as recently shown for TNF␣-mediated JNK activation [39]. Inducible suppressors of cytokine signalling (SOCS), which function as ubiquitin ligases, are

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responsible for the negative feedback control of JAK-STAT signalling [40]. A20 is another inducible ubiquitin ligase, which functions as a negative feedback regulator of TLR and TNFR signalling to IKK and NF-␬B, A20 is also a direct target gene for the NF-␬B pathway constituting a negative feedback loop for NF-␬B activation [41]. Recently a new pathway for negative regulation of IKK/NF-␬B was described from observations made in mice that harbour a variant of IKK␣, IKK␣AA that cannot be activated by upstream regulators. Although IKK␣AA mice do not develop “spontaneous” inflammation, they develop an exaggerated inflammatory response when challenged with bacteria, fungal cell wall particles, or even immune complexes [42]. These studies established that while IKK␤ catalytic activity is important for the activation of NF-␬B through phosphorylation of endogenous inhibitory (I␬B) proteins [39], IKK␣ is required for termination of NF-␬B activation through phosphorylation of the transcription factors RelA (p65) and c-Rel [42]. IKK-mediated phosphorylation results in polyubiquitination of the target protein, leading to its accelerated degradation via the 26S proteasome. However, while I␬B degradation is essential for NF-␬B activation and nuclear translocation, the accelerated degradation of nuclear Rel proteins via IKK␣-mediated phosphorylation, is important for controlling the duration of NF-␬B activation. The evolution of two catalytic sub-units in the IKK complex with opposing, yet complimentary, activity therefore ensures rapid and transient activation of NF-␬B. Eliminating the injurious agent and dampening inflammatory signalling pathways leads to the next phase of pro-inflammatory mediator catabolism where levels of cytokines, chemokines, eicosanoids, cell adhesion molecules, etc. must revert back to that expressed during the pre-inflamed state. For PGs, for instance, the first step in their catabolism is the oxidation of the 15(S)-hydroxyl group by a 15-hydroxyPG dehydrogenase, which metabolises Eseries PGs, Lxs, 15-HETE, 5,15-diHETE, and 8,15-diHETE to the corresponding 15-keto compounds. The second step involves the reduction of the 13 double bond by an NADPH/NADH dependent 13–15-ketoPG reductase. Further catabolism of PGs some HETES and Lxs occurs by the beta-oxidation pathway common to fatty acids in general, i.e. via the carboxyl end of the molecule, leading to the formation of short-chain metabolites, which are excreted in the urine. Some of these eicosanoids are also excreted following glucuronidation. 5-HETE and LTs undergo beta-oxidation from the omega-terminus following an initial omega-hydroxylation. In terms of controlling levels of chemokines, the atypical chemokine receptors such as D6 is unable to initiate classical signalling pathways after ligand binding and thereby acts as a scavenging system for CC (␤)-chemokines, for review see [8]. In addition, the work of Ariel et al. showed that CCL3 and CCL5 are scavenged form the exudates of resolving inflammation by sequestration on the surface of CCR5-expressing apoptotic PMNs [9]. Thus, endogenous systems exist to facilitate pro-inflammatory mediator clearance and whose function, when it becomes dysregulated, may lead to chronic inflammation.

5. From onset to resolution At this juncture it must be emphasised that inflammation leading to resolution is not a sequence of separate events that occur in isolation, but is a dynamic continuum of over-lapping events where pro- and anti-inflammation blend seamlessly into pro-resolution. For instance, pro-inflammatory signals are activated in an immediate and early manner concomitant with anti-inflammatory signals that serve to temper the magnitude of the early onset phase of acute inflammation with PMN influx being a good barometer of inflammation severity. Over the course hours and only after tissues have sensed that the injurious agent has been neutralised,




is it safe to catabolise pro-inflammatory soluble mediators and switch off pro-inflammatory signalling pathways. Alongside this is the synthesis of factors that terminate further PMN trafficking and prepare the injured tissue for resolution. One well-described event in the transition towards resolution is the replacement of PMNs or eosinophils by monocytes and phagocytosing macrophages. However, until recently our understanding of the signals that control this cell profile switch was unclear. Studies addressing this issue of leucocyte infiltration in peritoneal inflammation have suggested that the interaction between interleukin (IL)-6 and its soluble receptor, sIL-6R, forms one of the major determinants of this switch from PMNs to monocytes [43,44]. It was shown that sIL6R, produced by the infiltrating PMNs forms a complex with IL-6, which, in turn, directly modulates CC and CXC chemokine expression. Thus, CXC chemokine synthesis, induced by IL-1 and TNF␣ was suppressed whereas the CC chemokine CCL2 (MCP-1) was promoted. This chemokine shift suppresses further PMN recruitment in favour of sustained mononuclear cell influx. In addition to chemokines, eicosanoids also orchestrate the early transition to resolution in acute inflammation. Transcellular metabolism of arachidonic acid by lipoxygenase/lipoxygenase interaction pathways gives rise to the specialised pro-resolution mediators (SPMs) [45]. LXs and resolvins, for instance, display differential actions on leukocytes including the inhibition of PMN chemotaxis [46] as well as their adhesion to and transmigration through endothelial cells [47] while at the same time trigger monocyte chemoattraction. It is unclear at this point whether there is any crosstalk between SPMs and IL-6/sIL-6R complex signalling in the control of leucocyte profile switching. Nonetheless, it seems that when acute inflammation needs to resolve the IL-6/sIL-6R, chemokines and LXs represent some of the earliest signals that control the switch from very early PMNs to monocyte/macrophage.

6. Macrophages in inflammatory resolution In humans, monocytes were initially defined on the basis of morphology as well as intracellular monocyte-specific esterase, for instance. Nowadays, flow cytometry using anti-CD14 (directed to the LPS-binding domain) and CD16 (Fc-binding domain) antibodies has given three classifications namely, CD14++ /CD16− (classical), CD14++ /CD16+ (intermediate) and CD14+ /CD16++ (non-classical) [48]. In contrast, in mice, blood monocytes are also sub-divided into three categories including Ly6c++ /CD43+ (classical), Ly6c++ /CD43++ (intermediate) and Ly6c+ /CD43++ (non-classical). In addition, models that use transgenic mice have been highly informative for defining monocyte subpopulations such as mice with differential expression of CX3CR1-promoter-driven marker genes [49–51]. Such studies on chemokine receptor expression have shown that classical monocytes in the mouse are Ly6chi /CCR2high and CX3CR1low , whereas non-classical monocytes are ly6clow /CCR2low and CX3CR1hi . CD115 (M-CSF receptor) has also been used as it discriminates monocytes from granulocytes. One of the primary roles for macrophages during resolution is the clearance of effete cells. Indeed, while clearance of dead and dying cells is essential to prevent further tissue damage and the development of autoimmunity, the manner by which granulocytes die and the impact of this process on macrophage programming is a crucial determinant for efficient resolution. PMNs and eosinophils, for instance, can be eliminated by a number of routes including retro-transendothelial migration back into systemic circulation, lymphatic drainage – in order to participate in adaptive immune responses – or they may meet their demise within the inflamed tissue. Local cell death occurs in many ways including autophagy, excitotoxicity, pyroptosis, necrosis, necroptosis and NET-osis and caspase-mediated apoptosis [52,53]. In particular,

granulocytes undergo apoptosis during inflammation. During apoptosis cells maintain an intact membrane and, therefore, do not release their potentially histotoxic agents. Necrosis of inflammatory leukocytes, on the other hand, involves a loss of membrane integrity leading to the release of potentially toxic intracellular contents [54]. Moreover, apoptotic cells express a repertoire of surface molecules that allow their recognition and phagocytosis by macrophages [55]. Recognition of these apoptotic cells by macrophages does not liberate pro-inflammatory agents from the macrophages themselves but can release anti-inflammatory signals such as IL-10 and transforming growth factor␤ [56]. Thus, as mentioned above, not only is apoptosis a non-inflammatory way of disposing of cells, but this method has the added advantage of conferring upon macrophages an anti-inflammatory phenotype conducive to resolution. It is important to note that if not recognised and disposed of apoptotic cells will eventually undergo secondary necrosis releasing damaging intracellular contents and amplifying the inflammatory response. Therefore, increasing the rate of apoptosis, as a potentially anti-inflammatory strategy, must be matched by a mechanism that up-regulates macrophage phagocytic clearance capacity [57]. On this theme there are an increasing number of factors that aid the phagocytic clearance of apoptotic granulocytes. Ligation of the matrix receptor CD44, for instance, results in the rapid and specific internalisation of apoptotic PMN [58]. Besides controlling PMN trafficking, as mentioned earlier LXs and resolvins also stimulate monocyte chemotaxis and adherence. Certainly, this may seem dangerous for inflammation as too many monocyte-derived macrophages can be a bad thing, but these LX-chemoattracted macrophages along with resolvins accelerate resolution by enhancing phagocytosis of apoptotic PMNs in a non-phlogistic manner [59]. In addition to a role in granulocyte apoptosis, glucocorticoids facilitate the phagocytic response. It was found that exposure of peripheral blood monocytes to glucocorticoids during the first 24 h of the 5-day culture period induced a highly phagocytic monocyte-derived macrophage phenotype [60]. Functional and morphological homogeneity was matched by cell surface phenotype, including specific induction of expression of the haemoglobin scavenger receptor, CD163 following glucocorticoid treatment. A potentially pro-resolution role for CD163 was demonstrated recently in both in vitro and in vivo models of resolving inflammation [61]. Here, these authors showed that human peripheral blood monocyte-derived macrophages either in culture medium or in resolving phase cantharidin-induced skin blisters express CD163. These authors also found elevated levels of CD163 on circulating monocytes in cardiac surgical patients during the resolution phase of the systemic inflammatory response to cardiopulmonary bypass surgery. In each case, binding of the haemoglobin–haptoglobin complex to CD163-bearing cells elicited potent IL-10 secretion, which in turn enhanced hemeoxygenase 1, widely shown to have anti-inflammatory and tissue protective properties. Such induction of hemeoxygenase 1 was observed in vivo 24–48 h after the onset of cardiopulmonary bypass surgery. This is coincident with the observations of Willis et al. who showed that hemeoxygenase 1 was expressed during and essential for the resolution (24–48 h phase) of a rat carrageenin-induced pleurisy [62]. Thus, apoptosis and the phagocytosis of apoptotic cells are crucial to the resolution process, failure of which may predispose, as mentioned earlier, to chronic inflammation and possibly autoimmunity. This has been proposed in the case of systemic lupus erythematosus; an autoimmune syndrome that is associated with the presence of autoantibodies to endogenous antigens exposed on dead or dying cells, which failed to be cleared and disposed of. Studies in mice, for instance, have established a role for the complement receptor C1q on macrophages in the development of this disease, C1q was discovered to be important for the phagocytic


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clearance of apoptotic cells, in the absence of this receptor mice developed a lupus like syndrome [63]. The persistence of apoptotic cells and necrotic bodies led to the development of an inappropriate immune response to endogenous antigens. Evidence has also been established in human SLE patients for an association between C1q deficiency and disease [64]. Therefore, as phagocytosis of apoptotic cells has been suggested to play an important role in the negative regulation of macrophage activation, the mechanisms of apoptosis and the clearance of apoptotic cells may be critical in the development of chronic inflammation.

7. Other cells of the resolution cascade In mice, myeloid-derived suppressor cells (MDSCs) represent a heterogeneous group of GR-1+CD11b+ cells, comprised of both granulocytic and monocytic subsets – the precursors of PMNs, macrophages and DCs [65]. They are notable for their potent ability to suppress T-cell proliferation and responses, in particular Th1, through production of nitric oxide and reactive oxygen species (especially peroxynitrite), and for their expression upon activation of immune suppressive inducible nitric oxide synthase, arginase and a range of both pro and anti-inflammatory cytokines [65]. They may also induce the de novo development of T-regulatory cells (Treg) [66]. Like MDSCs, Tregs have an increasingly appreciated role in inflammatory resolution [67]. These CD4+ cells are characterised by high surface expression of CD25 (IL-2R␣ chain), the T-cell inhibitory receptor CTL-associated antigen 4 (CTLA-4), the glucocorticoidinduced TNF receptor (GITR) and the transcriptional regulator forkhead box p3 (Foxp3+), the latter correlating with Treg suppressive activity [68]. Treg may be subdivided into natural and induced (or adaptive) Tregs. The former arises during the normal process of maturation in the thymus, the latter developing from conventional CD4+CD25− T-cells in the periphery under specific stimulatory conditions elicited by infection/tissue damage [69]. At the site of inflammation, Tregs exert three major immunomodulatory effects: direct killing of cytotoxic cells, inhibition of pro-inflammatory cytokine secretion by cytotoxic cells (especially IL-2 and TNF-␣) and secretion of anti-inflammatory cytokines (notably TGF-␤ and IL-10) [70]. Classically studied in the setting of chronic infection, the potential for Treg numbers, activation and functional capacity to influence the outcome of pathogen/host interactions is well established [71]. Their role in the resolution of organ-specific autoimmune inflammation and ability to attenuate subsequent reactions has recently been described [72], while in LPS-induced ALI, depletion of Tregs has been associated with worse outcomes [73]. In some recent reports Cagliani and colleagues found that during various immune responses that CD4+ T cells that formerly expressed IL17A go on to acquire an anti-inflammatory phenotype. This transdifferentiation of Th17 cells, which are pathogenic in many human inflammatory diseases, into regulatory T cells was illustrated by a change in their signature transcriptional profile and the acquisition of potent regulatory capacity. Comparisons of the transcriptional profiles of pre- and post-conversion TH17 cells also revealed a role for TGF␤ signalling and consequently for the aryl hydrocarbon receptor in conversion of Th17 to Tregs. Thus, TH17 cells transdifferentiate into regulatory cells, and contribute to the resolution of inflammation suggesting that TH17 cell instability and plasticity is a therapeutic opportunity for inflammatory diseases [74]. In another recent study, Blazek and colleagues found that IL28A, also known and IFN-␭2, dramatically reduces numbers of pro-inflammatory IL17-producing Th17 and ␥␦T cells in the joints as well as lymph nodes of mice bearing collagen-induced arthritis (J Exp Med, 2015 doi: 10.1084/jem.20140995). IL-28A exerted its

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anti-inflammatory effect by restricting recruitment of PMNs, which expresses high levels of IL28 receptor ␣ and thereby targeted by IL-28A culminating in reduced joint pro-inflammatory cytokines levels, synovial thickening and bone destruction. These data highlight type three interferons or IL28RA agonists as putative new therapeutics for diseases driven by chronic inflammation. In general, we spend a great deal of time focusing on the importance of the haematopoietic component in inflammation and we tend to pay far less attention to the role stromal cells play in orchestrating the magnitude and duration of the response. While tissue-resident stromal cells such as fibroblasts, endothelial and epithelial cells define the microanatomy and architecture of organs and provide the appropriate microenvironment in which specialised immune functions can occur, these cells play an active role in governing the persistence of inflammatory disease, as well as enabling immunological memory to become established in a sitespecific manner [75]. Immune cells require stromal cells in order to home to and survive within the site of inflammation. Given that all inflammatory reactions take place within a defined background of specialised stromal cells, understanding the biology of both hematopoietic and non-hematopoietic cells is important in understanding how inflammatory cell infiltrates become established and persistent in chronic immune-mediated inflammatory diseases. 8. Resolution and adaptive immunity Remaining with the theme of macrophages, our group has described a novel resolution-phase macrophage (rM) possessing a hybrid phenotype of alternative activation, mannose receptor expression and synthesis of IL-10 and arginase 1 (classically M2, anti-inflammatory [76]) with high COX2 and iNOS expression (classically M1, pro-inflammatory) [77]. This rM phenotype, inducible by elevating intra-cellular cAMP, is vital in encouraging innate-type lymphocyte repopulation of inflamed cavities [77], a key step in controlling responses to secondary infective challenge [78]. Transcriptomic analysis of rM has suggested a potential role in antigen processing/presentation, T- and B-lymphocyte recruitment, termination of inflammatory cell trafficking and clearance but also highlighted the naivety of attempting to rigidly define and categorise such plastic cells in a dynamic environment [79]. We have taken this concept further by showing that resolution is not the end of the immune response to infection/injury but that it acts as a bridge between innate and adaptive immunity thereby adding a third phase to acute inflammation after onset and resolution, namely post-resolution [80]. Specifically, we found that innate immune-mediated responses to low-dose yeast cell wall extract (zymosan, i.p.) or bacteria (S. pneumoniae, i.p.) resolved spontaneously. Interestingly, these low-dose stimuli elicited a previously overlooked second wave of leucocyte influx into tissues that persisted for weeks. These cells comprised not just one type of rM as previously thought [77], but three separate populations including Ly6chi monocyte-derived macrophages bearing and M2-like phenotype including CD11B+ /CD49d+ /CD115+ /MHC-II+ MDSCs, F480lo /MHC-II+ /CD11c+ dendritic cells and F4-80int /CD11Bhi /CD11c− macrophages. Tissue-resident (yolk sac-derived) macrophages, which disappear during the inflammatory response, re-appear during resolution expressing high levels of TFG␤1. These diverse populations of macrophages were observed alongside lymph node expansion comprising blood Ly6chi monocyte-derived DCs and increased numbers of peripheral blood and peritoneal memory T and B lymphocytes. Based on our studies and supported by recent publications [81] we concluded that dendritic cells residing in naïve tissues take up antigens and migrate to local lymph nodes to drive lymphocyte expansion, aided by peripheral blood monocytederive DCs that traffic to the lymph node bypassing the peritoneum.




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During resolution, however, tissue-resident macrophages repopulate the cavity and preferentially phagocytose apoptotic PMNs and maintaining tolerance by generating iTregs. Ly6chi -monocyte derived macrophages remain in tissues for months after inflammation has resolved with a role in dictating the magnitude and duration of subsequent acute inflammatory stimulation. In contrast, injecting very high doses of zymosan into the peritoneum of mice resulted in a substantial inflammatory cell infiltrate comprising monocyte-derived M1-like macrophages [82]. Such an inflammatory insult was not associated with the appearance of MDSCs or regulatory macrophages while Tregs, lymph node expansion or memory lymphocyte re-population were less pronounced than that seen consequent to resolving inflammation. We concluded that while resolution of acute inflammation is important for leucocyte clearance/cytokine catabolism, it also has a hitherto unappreciated role in controlling adaptive immune responses and maintaining tolerance. Our findings are supported by those reported recently showing that dampening the persistent inflammatory response caused by lymphocytic choriomeningitis virus by blocking elevated type I interferon reduces innate immune activation and restores lymph node architecture resulting in enhanced virus clearance in a CD4 T cell-mediated manner [83,84]. It emerges that while early production of type 1 interferons promotes virus clearance, chronic exposure triggers immuno-suppressive IL-10, PD-L1 and indolamine as well as cause T cell apoptosis, collectively impairing the host’s ability to develop specific immunity [85,86]. Chronic TNF␣ exposure also attenuates T cell/adaptive immune responses as repeated injection of TNF␣ in mice dampens their T cells responsiveness to recall antigens, while chronic anti-TNF␣ therapy enhances T-cell reactivity [87]. Thus, while many early phase Th-1-type cytokines have evolved to drive inflammation, their persistence may paradoxically impair resolution resulting in impaired antigen clearance and the development of adaptive immunity/long-term memory. As a consequence, we no longer define resolution as simply a clearance of the traditional innate type leukocytes (PMNs and macrophages) and a decrement in pro-inflammatory cytokines leading to what was once termed restorative homeostasis [88]. Our data redefine resolution as the creation of a tissue microenvironment that facilitates interaction between the innate and adaptive arms of the immune system.

9. Summary Studies on inflammatory resolution have advanced our understanding of leucocyte trafficking, efferocytosis and proinflammatory leucocyte clearance as well as immune-suppressive eicosanoids, specialised immune-regulatory cells and cytokine catabolism. These pathways converge on the termination of acute inflammatory responses and contribute to the notion that chronic inflammation is avoided and wounds healed in an appropriate manner [88,4]. Implicit therein is that tolerance is not compromised making the host susceptible to autoimmunity. Recently we reported that this classic view of resolution needed to be expanded arising from our discovery in a murine model of zymosan-induced peritonitis of a third phase of inflammation following onset and classic resolution, which we called “post-resolution” [82]. We [82] and others [89] also found that Ly6chi -monocyte-derived macrophages remained in tissues for months after inflammation has resolved with a role in dictating the magnitude and duration of subsequent acute innate inflammatory stimulation; findings reminiscent of sustained maturation of DCs after resolution of Th2-mediated inflammation contributing to the phenomenon of polysensitization, a common feature in patients with allergies [90]. Therefore, our understanding of the cells, receptors, soluble

mediators and signalling pathways that are important to bring about resolution is evolving and it is appearing that pathways to resolution will be tissue and cause specific. As a result, it is unlikely that one single therapeutic intervention will be a panacea for all chronic inflammatory diseases. Instead, it will become increasingly imperative to focus on a specific disease and appreciate that even a single disease category is unlikely to be a homogenous in aetiology but comprised of many syndromes. In addition, we need greater emphasis on translational medicine in healthy human volunteers as well as patients whose disease we wish to understand coupled with the notion of unbiased mediator addition or receptor antagonism to improve clinical outcome. Acknowledgements DWG is funded by the Wellcome Trust as a Senior Research Fellow while RDM is funded by an MRC/CASE studentship. References [1] S.E. Headland, L.V. Norling, The resolution of inflammation: principles and challenges, Semin. Immunol. (2015), http://dx.doi.org/10.1016/j.smim.2015. 03.014 (published online Epub April 21). [2] G. Majno, I. Joris, Cells, Tissues and Disease. Principles of General Pathology, second edition, Oxford University Press, New York, 2004. [3] C.D. Buckley, D.W. Gilroy, C.N. Serhan, Proresolving lipid mediators and mechanisms in the resolution of acute inflammation, Immunity 40 (2014) 315–327, http://dx.doi.org/10.1016/j.immuni.2014.02.009 (published online Epub March 20). [4] C.D. Buckley, D.W. Gilroy, C.N. Serhan, B. Stockinger, P.P. Tak, The resolution of inflammation, Nat. Rev. Immunol. 13 (2013) 59–66, http://dx.doi.org/10.1038/ nri3362 (published online Epub January). [5] A.W. Segal, M. Geisow, R. Garcia, A. Harper, R. Miller, The respiratory burst of phagocytic cells is associated with a rise in vacuolar pH, Nature 290 (1981) 406–409 (published online Epub April 2). [6] D.W. Gilroy, P.R. Colville-Nash, S. McMaster, D.A. Sawatzky, D.A. Willoughby, T. Lawrence, Inducible cyclooxygenase-derived 15-deoxy(Delta)12-14PGJ2 brings about acute inflammatory resolution in rat pleurisy by inducing neutrophil and macrophage apoptosis, FASEB J. 17 (2003) 2269–2271 (published online Epub December). [7] W.R. Hansen, J.A. Keelan, S.J. Skinner, M.D. Mitchell, Key enzymes of prostaglandin biosynthesis and metabolism. Coordinate regulation of expression by cytokines in gestational tissues: a review, Prostaglandins Other Lipid Mediat. 57 (1999) 243–257 (published online Epub June). [8] R.J. Nibbs, G.J. Graham, Immune regulation by atypical chemokine receptors, Nat. Rev. Immunol. 13 (2013) 815–829 (published online Epub November). [9] A. Ariel, G. Fredman, Y.P. Sun, A. Kantarci, T.E. Van Dyke, A.D. Luster, C.N. Serhan, Apoptotic neutrophils and T cells sequester chemokines during immune response resolution through modulation of CCR5 expression, Nat. Immunol. 7 (2006) 1209–1216, http://dx.doi.org/10.1038/ni1392 (published online Epub November). [10] C. Serhan, P. Ward, D. Gilroy, Fundamentals of Inflammation, Cambridge University Press, 2010. [11] A.W. Segal, How neutrophils kill microbes, Ann. Rev. Immunol. 23 (2005) 197–223, http://dx.doi.org/10.1146/annurev.immunol.23.021704.115653 [12] C.N. Serhan, J. Savill, Resolution of inflammation: the beginning programs the end, Nat. Immunol. 6 (2005) 1191–1197 (published online Epub December). [13] M. Karin, T. Lawrence, V. Nizet, Innate immunity gone awry: linking microbial infections to chronic inflammation and cancer, Cell 124 (2006) 823–835 (published online Epub February 24). [14] J.R. Muppidi, J. Tschopp, R.M. Siegel, Life and death decisions: secondary complexes and lipid rafts in TNF receptor family signal transduction, Immunity 21 (2004) 461–465 (published online Epub October). [15] V. Baud, M. Karin, Signal transduction by tumor necrosis factor and its relatives, Trends Cell Biol. 11 (2001) 372–377 (published online Epub September). [16] R.H. Arch, R.W. Gedrich, C.B. Thompson, Tumor necrosis factor receptorassociated factors (TRAFs) – a family of adapter proteins that regulates life and death, Genes Dev. 12 (1998) 2821–2830 (published online Epub September 15). [17] N. Suzuki, S. Suzuki, G.S. Duncan, D.G. Millar, T. Wada, C. Mirtsos, H. Takada, A. Wakeham, A. Itie, S. Li, J.M. Penninger, H. Wesche, P.S. Ohashi, T.W. Mak, W.C. Yeh, Severe impairment of interleukin-1 and Toll-like receptor signalling in mice lacking IRAK-4, Nature 416 (2002) 750–756 (published online Epub April 18). [18] M.A. Kelliher, S. Grimm, Y. Ishida, F. Kuo, B.Z. Stanger, P. Leder, The death domain kinase RIP mediates the TNF-induced NF-kappaB signal, Immunity 8 (1998) 297–303 (published online Epub March). [19] I.E. Wertz, K.M. O’Rourke, H. Zhou, M. Eby, L. Aravind, S. Seshagiri, P. Wu, C. Wiesmann, R. Baker, D.L. Boone, A. Ma, E.V. Koonin, V.M. Dixit, Deubiquitination and ubiquitin ligase domains of A20 downregulate NF-kappaB signalling, Nature 430 (2004) 694–699 (published online Epub August 5).




[20] L. Chang, M. Karin, Mammalian MAP kinase signalling cascades, Nature 410 (2001) 37–40 (published online Epub March 1). [21] J.M. Kyriakis, J. Avruch, Mammalian mitogen-activated protein kinase signal transduction pathways activated by stress and inflammation, Physiol. Rev. 81 (2001) 807–869 (published online Epub April). [22] M. Karin, E. Gallagher, From JNK to pay dirt: jun kinases, their biochemistry, physiology and clinical importance, IUBMB Life 57 (2005) 283–295 (published online EpubApril–May). [23] T. Zarubin, J. Han, Activation and signaling of the p38 MAP kinase pathway, Cell Res. 15 (2005) 11–18 (published online Epub January). [24] S. Ghosh, M. Karin, Missing pieces in the NF-kappaB puzzle, Cell 109 (Suppl.) (2002) S81–S96 (published online Epub April). [25] M. Karin, The regulation of AP-1 activity by mitogen-activated protein kinases, J. Biol. Chem. 270 (1995) 16483–16486 (published online Epub July 14). [26] J.M. Park, F.R. Greten, A. Wong, R.J. Westrick, J.S. Arthur, K. Otsu, A. Hoffmann, M. Montminy, M. Karin, Signaling pathways and genes that inhibit pathogen-induced macrophage apoptosis-CREB and NF-kappaB as key regulators, Immunity 23 (2005) 319–329 (published online Epub September). [27] R. Winzen, G. Gowrishankar, F. Bollig, N. Redich, K. Resch, H. Holtmann, Distinct domains of AU-rich elements exert different functions in mRNA destabilization and stabilization by p38 mitogen-activated protein kinase or HuR, Mol. Cell Biol. 24 (2004) 4835–4847 (published online Epub June). [28] C.Y. Chen, F. Del Gatto-Konczak, Z. Wu, M. Karin, Stabilization of interleukin2 mRNA by the c-Jun NH2-terminal kinase pathway, Science 280 (1998) 1945–1949 (published online Epub June 19). [29] J.L. Dean, G. Sully, A.R. Clark, J. Saklatvala, The involvement of AU-rich elementbinding proteins in p38 mitogen-activated protein kinase pathway-mediated mRNA stabilisation, Cell Signal. 16 (2004) 1113–1121 (published online Epub October). [30] G. Bonizzi, M. Karin, The two NF-kappaB activation pathways and their role in innate and adaptive immunity, Trends Immunol. 25 (2004) 280–288 (published online Epub June). [31] Q. Li, I.M. Verma, NF-kappaB regulation in the immune system, Nat. Rev. Immunol. 2 (2002) 725–734 (published online Epub October). [32] J.J. O’Shea, M. Gadina, R.D. Schreiber, Cytokine signaling in 2002: new surprises in the Jak/Stat pathway, Cell 109 (Suppl.) (2002) S121–S131 (published online Epub April). [33] M. Martin, R.E. Schifferle, N. Cuesta, S.N. Vogel, J. Katz, S.M. Michalek, Role of the phosphatidylinositol 3 kinase-Akt pathway in the regulation of IL-10 and IL-12 by Porphyromonas gingivalis lipopolysaccharide, J. Immunol. 171 (2003) 717–725 (published online Epub July 15). [34] G. Stoecklin, P. Anderson, Posttranscriptional mechanisms regulating the inflammatory response, Adv. Immunol. 89 (2006) 1–37, http://dx.doi.org/10. 1016/S0065-2776(05)89001-7 [35] T. Hashiguchi, Y. Sakakibara, Y. Hara, T. Shimohira, K. Kurogi, R. Akashi, M.C. Liu, M. Suiko, Identification and characterization of a novel kaempferol sulfotransferase from Arabidopsis thaliana, Biochem. Biophys. Res. Commun. 434 (2013) 829–835, http://dx.doi.org/10.1016/j.bbrc.2013.04.022 (published online Epub May 17). [36] S. Divanovic, A. Trompette, S.F. Atabani, R. Madan, D.T. Golenbock, A. Visintin, R.W. Finberg, A. Tarakhovsky, S.N. Vogel, Y. Belkaid, E.A. Kurt-Jones, C.L. Karp, Negative regulation of Toll-like receptor 4 signaling by the Toll-like receptor homolog RP105, Nat. Immunol. 6 (2005) 571–578, http://dx.doi.org/10.1038/ ni1198 (published online Epub June). [37] P. Anderson, Post-transcriptional control of cytokine production, Nat. Immunol. 9 (2008) 353–359, http://dx.doi.org/10.1038/ni1584 (published online Epub April). [38] B.G. Neel, H. Gu, L. Pao, The ‘Shp’ing news: SH2 domain-containing tyrosine phosphatases in cell signaling, Trends Biochem. Sci. 28 (2003) 284–293 (published online Epub June). [39] H. Kamata, S. Honda, S. Maeda, L. Chang, H. Hirata, M. Karin, Reactive oxygen species promote TNFalpha-induced death and sustained JNK activation by inhibiting MAP kinase phosphatases, Cell 120 (2005) 649–661, http://dx.doi. org/10.1016/j.cell.2004.12.041 (published online Epub March 11). [40] W.S. Alexander, D.J. Hilton, The role of suppressors of cytokine signaling (SOCS) proteins in regulation of the immune response, Ann. Rev. Immunol. 22 (2004) 503–529. [41] D.L. Boone, E.E. Turer, E.G. Lee, R.C. Ahmad, M.T. Wheeler, C. Tsui, P. Hurley, M. Chien, S. Chai, O. Hitotsumatsu, E. McNally, C. Pickart, A. Ma, The ubiquitin-modifying enzyme A20 is required for termination of Toll-like receptor responses, Nat. Immunol. 5 (2004) 1052–1060 (published online Epub October). [42] T. Lawrence, M. Bebien, G.Y. Liu, V. Nizet, M. Karin, IKKalpha limits macrophage NF-kappaB activation and contributes to the resolution of inflammation, Nature 434 (2005) 1138–1143 (published online Epub April 28). [43] R.M. McLoughlin, J. Witowski, R.L. Robson, T.S. Wilkinson, S.M. Hurst, A.S. Williams, J.D. Williams, S. Rose-John, S.A. Jones, N. Topley, Interplay between IFN-gamma and IL-6 signaling governs neutrophil trafficking and apoptosis during acute inflammation, J. Clin. Investig. 112 (2003) 598–607 (published online Epub September). [44] S.M. Hurst, T.S. Wilkinson, R.M. McLoughlin, S. Jones, S. Horiuchi, N. Yamamoto, S. Rose-John, G.M. Fuller, N. Topley, S.A. Jones, Il-6 and its soluble receptor orchestrate a temporal switch in the pattern of leukocyte recruitment seen during acute inflammation, Immunity 14 (2001) 705–714 (published online Epub June).

7

[45] C.N. Serhan, Pro-resolving lipid mediators are leads for resolution physiology, Nature 510 (2014) 92–101, http://dx.doi.org/10.1038/nature13479 (published online Epub June 5). [46] B.D. Levy, C.B. Clish, B. Schmidt, K. Gronert, C.N. Serhan, Lipid mediator class switching during acute inflammation: signals in resolution, Nat. Immunol. 2 (2001) 612–619 (published online Epub August). [47] A. Papayianni, C.N. Serhan, H.R. Brady, Lipoxin A4 and B4 inhibit leukotrienestimulated interactions of human neutrophils and endothelial cells, J. Immunol. 156 (1996) 2264–2272 (published online Epub March 15). [48] L. Ziegler-Heitbrock, P. Ancuta, S. Crowe, M. Dalod, V. Grau, D.N. Hart, P.J. Leenen, Y.J. Liu, G. MacPherson, G.J. Randolph, J. Scherberich, J. Schmitz, K. Shortman, S. Sozzani, H. Strobl, M. Zembala, J.M. Austyn, M.B. Lutz, Nomenclature of monocytes and dendritic cells in blood, Blood 116 (2010) e74–e80, http://dx.doi.org/10.1182/blood-2010-02-258558 (published online Epub October 21). [49] R.T. Palframan, S. Jung, G. Cheng, W. Weninger, Y. Luo, M. Dorf, D.R. Littman, B.J. Rollins, H. Zweerink, A. Rot, U.H. von Andrian, Inflammatory chemokine transport and presentation in HEV: a remote control mechanism for monocyte recruitment to lymph nodes in inflamed tissues, J. Exp. Med. 194 (2001) 1361–1373 (published online Epub November 5). [50] F. Geissmann, S. Jung, D.R. Littman, Blood monocytes consist of two principal subsets with distinct migratory properties, Immunity 19 (2003) 71–82 (published online Epub July). [51] C. Auffray, D. Fogg, M. Garfa, G. Elain, O. Join-Lambert, S. Kayal, S. Sarnacki, A. Cumano, G. Lauvau, F. Geissmann, Monitoring of blood vessels and tissues by a population of monocytes with patrolling behavior, Science 317 (2007) 666–670, http://dx.doi.org/10.1126/science.1142883 (published online Epub August 3). [52] T. Vanden Berghe, A. Linkermann, S. Jouan-Lanhouet, H. Walczak, P. Vandenabeele, Regulated necrosis: the expanding network of non-apoptotic cell death pathways, Nat. Rev. Mol. Cell Biol. 15 (2014) 135–147, http://dx.doi.org/10. 1038/nrm3737 (published online Epub February). [53] Q. Remijsen, T.W. Kuijpers, E. Wirawan, S. Lippens, P. Vandenabeele, T. Vanden Berghe, Dying for a cause: NETosis, mechanisms behind an antimicrobial cell death modality, Cell Death Differ. 18 (2011) 581–588, http://dx.doi.org/10. 1038/cdd.2011.1 (published online Epub April). [54] S.J. Heasman, K.M. Giles, C. Ward, A.G. Rossi, C. Haslett, I. Dransfield, Glucocorticoid-mediated regulation of granulocyte apoptosis and macrophage phagocytosis of apoptotic cells: implications for the resolution of inflammation, J. Endocrinol. 178 (2003) 29–36 (published online Epub July). [55] V.A. Fadok, D.L. Bratton, P.M. Henson, Phagocyte receptors for apoptotic cells: recognition, uptake, and consequences, J. Clin. Investig. 108 (2001) 957–962 (published online Epub October). [56] M.L. Huynh, V.A. Fadok, P.M. Henson, Phosphatidylserine-dependent ingestion of apoptotic cells promotes TGF-beta1 secretion and the resolution of inflammation, J. Clin. Investig. 109 (2002) 41–50 (published online Epub January). [57] C. Ward, I. Dransfield, E.R. Chilvers, C. Haslett, A.G. Rossi, Pharmacological manipulation of granulocyte apoptosis: potential therapeutic targets, Trends Pharmacol. Sci. 20 (1999) 503–509 (published online Epub December). [58] J.C. McCutcheon, S.P. Hart, M. Canning, K. Ross, M.J. Humphries, I. Dransfield, Regulation of macrophage phagocytosis of apoptotic neutrophils by adhesion to fibronectin, J. Leukoc. Biol. 64 (1998) 600–607 (published online Epub November). [59] C. Godson, S. Mitchell, K. Harvey, N.A. Petasis, N. Hogg, H.R. Brady, Cutting edge: lipoxins rapidly stimulate nonphlogistic phagocytosis of apoptotic neutrophils by monocyte-derived macrophages, J. Immunol. 164 (2000) 1663–1667 (published online Epub February 15). [60] K.M. Giles, K. Ross, A.G. Rossi, N.A. Hotchin, C. Haslett, I. Dransfield, Glucocorticoid augmentation of macrophage capacity for phagocytosis of apoptotic cells is associated with reduced p130Cas expression, loss of paxillin/pyk2 phosphorylation, and high levels of active Rac, J. Immunol. 167 (2001) 976–986 (published online Epub July 15). [61] P. Philippidis, J.C. Mason, B.J. Evans, I. Nadra, K.M. Taylor, D.O. Haskard, R.C. Landis, Hemoglobin scavenger receptor CD163 mediates interleukin-10 release and heme oxygenase-1 synthesis: antiinflammatory monocyte-macrophage responses in vitro, in resolving skin blisters in vivo, and after cardiopulmonary bypass surgery, Circ. Res. 94 (2004) 119–126 (published online Epub January 9). [62] D. Willis, A.R. Moore, R. Frederick, D.A. Willoughby, Heme oxygenase: a novel target for the modulation of the inflammatory response, Nat. Med. 2 (1996) 87–90 (published online Epub January). [63] M. Botto, C. Dell’Agnola, A.E. Bygrave, E.M. Thompson, H.T. Cook, F. Petry, M. Loos, P.P. Pandolfi, M.J. Walport, Homozygous C1q deficiency causes glomerulonephritis associated with multiple apoptotic bodies, Nat. Genet. 19 (1998) 56–59 (published online Epub May). [64] M.J. Walport, K.A. Davies, M. Botto, C1q and systemic lupus erythematosus, Immunobiology 199 (1998) 265–285 (published online Epub August). [65] D.I. Gabrilovich, S. Nagaraj, Myeloid-derived suppressor cells as regulators of the immune system, Nat. Rev. Immunol. 9 (2009) 162–174, http://dx.doi.org/ 10.1038/nri2506 (published online Epub March). [66] B. Huang, P.Y. Pan, Q. Li, A.I. Sato, D.E. Levy, J. Bromberg, C.M. Divino, S.H. Chen, Gr-1+CD115+ immature myeloid suppressor cells mediate the development of tumor-induced T regulatory cells and T-cell anergy in tumor-bearing host, Cancer Res. 66 (2006) 1123–1131, http://dx.doi.org/10.1158/0008-5472.CAN05-1299 (published online Epub January 15).




[67] F. Venet, C.S. Chung, G. Monneret, X. Huang, B. Horner, M. Garber, A. Ayala, Regulatory, T cell populations in sepsis and trauma, J. Leukoc. Biol. 83 (2008) 523–535, http://dx.doi.org/10.1189/jlb.0607371 (published online Epub March). [68] D.J. Campbell, S.F. Ziegler, FOXP3 modifies the phenotypic and functional properties of regulatory T cells, Nat. Rev. Immunol. 7 (2007) 305–310, http://dx.doi. org/10.1038/nri2061 (published online Epub April). [69] J.A. Bluestone, A.K. Abbas, Natural versus adaptive regulatory T cells, Nat. Rev. Immunol. 3 (2003) 253–257, http://dx.doi.org/10.1038/nri1032 (published online Epub March). [70] A. Kessel, E. Bamberger, M. Masalha, E. Toubi, The role of T regulatory cells in human sepsis, J. Autoimmun. 32 (2009) 211–215, http://dx.doi.org/10.1016/j. jaut.2009.02.014 (published online Epub May–June). [71] Y. Belkaid, B.T. Rouse, Natural regulatory T cells in infectious disease, Nat. Immunol. 6 (2005) 353–360, http://dx.doi.org/10.1038/ni1181 (published online Epub April). [72] M.D. Rosenblum, I.K. Gratz, J.S. Paw, K. Lee, A. Marshak-Rothstein, A.K. Abbas, Response to self antigen imprints regulatory memory in tissues, Nature 480 (2011) 538–542, http://dx.doi.org/10.1038/nature10664 (published online Epub December 22). [73] F.R. D’Alessio, K. Tsushima, N.R. Aggarwal, E.E. West, M.H. Willett, M.F. Britos, M.R. Pipeling, R.G. Brower, R.M. Tuder, J.F. McDyer, L.S. King, CD4+CD25+Foxp3+ Tregs resolve experimental lung injury in mice and are present in humans with acute lung injury, J. Clin. Investig. 119 (2009) 2898–2913, http://dx.doi.org/10. 1172/JCI36498 (published online Epub October). [74] N. Gagliani, M.C. Vesely, A. Iseppon, L. Brockmann, H. Xu, N.W. Palm, M.R. de Zoete, P. Licona-Limon, R.S. Paiva, T. Ching, C. Weaver, X. Zi, X. Pan, R. Fan, L.X. Garmire, M.J. Cotton, Y. Drier, B. Bernstein, J. Geginat, B. Stockinger, E. Esplugues, S. Huber, R.A. Flavell, Th17 cells transdifferentiate into regulatory T cells during resolution of inflammation, Nature (2015), http://dx.doi.org/10. 1038/nature14452 (published online Epub April 29). [75] S.I. Hammerschmidt, M. Ahrendt, U. Bode, B. Wahl, E. Kremmer, R. Forster, O. Pabst, Stromal mesenteric lymph node cells are essential for the generation of gut-homing T cells in vivo, J. Exp. Med. 205 (2008) 2483–2490, http://dx.doi. org/10.1084/jem.20080039 (published online Epub October 27). [76] S. Gordon, Alternative activation of macrophages, Nat. Rev. Immunol. 3 (2003) 23–35, http://dx.doi.org/10.1038/nri978 (published online Epub January). [77] J. Bystrom, I. Evans, J. Newson, M. Stables, I. Toor, N. van Rooijen, M. Crawford, P. Colville-Nash, S. Farrow, D.W. Gilroy, Resolution-phase macrophages possess a unique inflammatory phenotype that is controlled by cAMP, Blood 112 (2008) 4117–4127, http://dx.doi.org/10.1182/blood-2007-12-129767 (published online Epub November 15). [78] R. Rajakariar, T. Lawrence, J. Bystrom, M. Hilliard, P. Colville-Nash, G. Bellingan, D. Fitzgerald, M.M. Yaqoob, D.W. Gilroy, Novel biphasic role for lymphocytes revealed during resolving inflammation, Blood 111 (2008) 4184–4192, http:// dx.doi.org/10.1182/blood-2007-08-108936 (published online Epub April 15). [79] M.J. Stables, S. Shah, E.B. Camon, R.C. Lovering, J. Newson, J. Bystrom, S. Farrow, D.W. Gilroy, Transcriptomic analyses of murine resolution-phase

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

[89]

[90]

macrophages, Blood 118 (2011) e192–e208, http://dx.doi.org/10.1182/blood2011-04-345330 (published online Epub December 22). J. Newson, M. Stables, E. Karra, F. Arce-Vargas, S. Quezada, M. Motwani, M. Mack, S. Yona, T. Audzevich, D.W. Gilroy, Resolution of acute inflammation bridges the gap between innate and adaptive immunity, Blood (2014), http:// dx.doi.org/10.1182/blood-2014-03-562710 (published online Epub July 8). H. Nakano, K.L. Lin, M. Yanagita, C. Charbonneau, D.N. Cook, T. Kakiuchi, M.D. Gunn, Blood-derived inflammatory dendritic cells in lymph nodes stimulate acute T helper type 1 immune responses, Nat. Immunol. 10 (2009) 394–402, http://dx.doi.org/10.1038/ni.1707 (published online Epub April). J. Newson, M. Stables, E. Karra, F. Arce-Vargas, S. Quezada, M. Motwani, M. Mack, S. Yona, T. Audzevich, D.W. Gilroy, Resolution of acute inflammation bridges the gap between innate and adaptive immunity, Blood 124 (2014) 1748–1764, http://dx.doi.org/10.1182/blood-2014-03-562710 (published online Epub September 11). E.B. Wilson, D.H. Yamada, H. Elsaesser, J. Herskovitz, J. Deng, G. Cheng, B.J. Aronow, C.L. Karp, D.G. Brooks, Blockade of chronic type I interferon signaling to control persistent LCMV infection, Science 340 (2013) 202–207, http://dx. doi.org/10.1126/science.1235208 (published online Epub April 12). J.R. Teijaro, C. Ng, A.M. Lee, B.M. Sullivan, K.C. Sheehan, M. Welch, R.D. Schreiber, J.C. de la Torre, M.B. Oldstone, Persistent LCMV infection is controlled by blockade of type I interferon signaling, Science 340 (2013) 207–211, http://dx.doi. org/10.1126/science.1235214 (published online Epub April 12). J.M. Gonzalez-Navajas, J. Lee, M. David, E. Raz, Immunomodulatory functions of type I interferons, Nat. Rev. Immunol. 12 (2012) 125–135, http://dx.doi.org/ 10.1038/nri3133 (published online Epub February). A. Boasso, A.W. Hardy, S.A. Anderson, M.J. Dolan, G.M. Shearer, HIV-induced type I interferon and tryptophan catabolism drive T cell dysfunction despite phenotypic activation, PLoS ONE 3 (2008) e2961, http://dx.doi.org/10.1371/ journal.pone.0002961 A.P. Cope, R.S. Liblau, X.D. Yang, M. Congia, C. Laudanna, R.D. Schreiber, L. Probert, G. Kollias, H.O. McDevitt, Chronic tumor necrosis factor alters T cell responses by attenuating T cell receptor signaling, J. Exp. Med. 185 (1997) 1573–1584 (published online Epub May 5). C.N. Serhan, S.D. Brain, C.D. Buckley, D.W. Gilroy, C. Haslett, L.A. O’Neill, M. Perretti, A.G. Rossi, J.L. Wallace, Resolution of inflammation: state of the art, definitions and terms, FASEB J. 21 (2007) 325–332, http://dx.doi.org/10.1096/ fj.06-7227rev (published online Epub February). S. Yona, K.W. Kim, Y. Wolf, A. Mildner, D. Varol, M. Breker, D. Strauss-Ayali, S. Viukov, M. Guilliams, A. Misharin, D.A. Hume, H. Perlman, B. Malissen, E. Zelzer, S. Jung, Fate mapping reveals origins and dynamics of monocytes and tissue macrophages under homeostasis, Immunity 38 (2013) 79–91, http://dx. doi.org/10.1016/j.immuni.2012.12.001 (published online Epub January 24). L.S. van Rijt, N. Vos, M. Willart, F. Muskens, P.P. Tak, C. van der Horst, H.C. Hoogsteden, B.N. Lambrecht, Persistent activation of dendritic cells after resolution of allergic airway inflammation breaks tolerance to inhaled allergens in mice, Am. J. Respir. Crit. Care Med. 184 (2011) 303–311, http://dx.doi.org/10.1164/ rccm.201101-0019OC (published online Epub August 1).


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