Review

Inflammation and metabolism in tissue repair and regeneration

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Science  09 Jun 2017:
Vol. 356, Issue 6342, pp. 1026-1030
DOI: 10.1126/science.aam7928

Abstract

Tissue repair after injury is a complex, metabolically demanding process. Depending on the tissue’s regenerative capacity and the quality of the inflammatory response, the outcome is generally imperfect, with some degree of fibrosis, which is defined by aberrant accumulation of collagenous connective tissue. Inflammatory cells multitask at the wound site by facilitating wound debridement and producing chemokines, metabolites, and growth factors. If this well-orchestrated response becomes dysregulated, the wound can become chronic or progressively fibrotic, with both outcomes impairing tissue function, which can ultimately lead to organ failure and death. Here we review the current understanding of the role of inflammation and cell metabolism in tissue-regenerative responses, highlight emerging concepts that may expand therapeutic perspectives, and briefly discuss where important knowledge gaps remain.

Effective tissue repair is critical for the survival of all living organisms (1). After injury, necrotic debris, the clotting reaction, and any invading microbes collectively activate an inflammatory response that is propagated by the local release of chemotactic factors. Neutrophils, monocytes, and other innate immune cells are recruited to the wound site to clear cell debris and infectious organisms and subsequently help orchestrate the tissue repair response. The degree and duration of the response varies, and this influences the final outcome. There are major benefits of raising an inflammatory response, but there are also negative consequences, including the activation of a fibrotic response, which is defined by the excessive and aberrant accumulation of collagenous connective tissues that can debilitate tissue function and, in some cases, lead to organ failure.

Inflammation after injury is normally modular in nature, with three distinct phases facilitating the restoration of normal tissue architecture. These stages include an early proinflammatory step, in which elements of the innate immune response initiate the repair response by mobilizing the recruitment of key inflammatory cells. In the second major phase, the proinflammatory response begins to subside, with key inflammatory cells such as macrophages switching to a reparative phenotype. In the final stage, tissue homeostasis is restored when the inflammatory cells either exit the site of injury or are eliminated through apoptosis.

This Review will explore some of the newer concepts of inflammation in repair and regeneration and how they offer insights for potential therapeutic modulation to improve healing of tissues in the clinic. Because of recent advances in live imaging of translucent model organisms such as zebrafish larvae, the dynamic behavior of inflammatory cells can now be viewed as they are drawn to wounds and interact with local cell populations. These approaches, along with recent mammalian tissue repair studies in multiple organs from skin to liver, have revealed how leukocytic cells undergo behavioral switching from a proinflammatory to an anti-inflammatory state and have demonstrated how this affects their dynamic interaction with host tissue cells to either drive fibrosis or mediate its resolution (2). Beyond elucidating spatial and temporal cues that govern immune cell function, recent studies have also identified previously unappreciated molecular determinants that orchestrate immune cell responses. In this context, evidence in the nascent field of immunometabolism has shown how metabolic adaptation is not simply regulated by nutrient availability but is also controlled by immune signals that govern immune cell function (3). Therefore, the rapid response of immune cells to tissue injury and the need to adjust their metabolic reprogramming during the repair response may, in turn, regulate immune cell function at the wound site. Thus, targeting metabolic pathways in immune cells may offer new opportunities to therapeutically modulate the inflammatory response and improve the overall outcome of wound-healing responses.

New insights emerging from live imaging of the wound response

The immune response to tissue damage and infection was first observed by Metchnikoff in his studies of translucent waterborne creatures in the early 20th century (4). His classic experiment of puncturing a starfish embryo with a rose thorn provided the first evidence for a “wandering” population of cells that are drawn to invading foreign bodies (Fig. 1A). In recent years, fluorescent tagging of macrophages in Drosophila embryos and neutrophils and macrophages in zebrafish larvae, which are both translucent but more genetically tractable than starfish, has enabled us to gain profound new live imaging insights into the biology of the wound inflammatory response (Fig. 1B).

Fig. 1 Live imaging of the wound inflammatory response.

(A) Metchnikoff’s discovery of damage-triggered inflammation in starfish embryos (4). (B) Shown on the left is a time-lapse series after laser wounding of the Drosophila pupal wing epithelium, revealing macrophage (green with red nuclei) recruitment to the wound. The schematic on the right illustrates those aspects of the wound inflammatory response that can be analyzed by dynamic imaging studies in Drosophila and zebrafish. (C) Neutrophil diapedezing through the outer pericyte layer of a mouse vessel. Migratory tracks in (B) and (C) are shown in blue.

Graphic: ADAPTED BY K. SUTLIFF/SCIENCE; (B, left) courtesy H. Weavers, University of Bristol; (C) courtesy of T. Girbl and S. Nourshargh, Queen Mary University of London

Real-time fluorescent imaging of green fluorescent protein–tagged neutrophils has revealed the dynamic influx and resolution of these cells in mammalian skin wounds (5). In mice, studies have also shown that not all wound inflammatory cells are tissue-resident or come from the bloodstream. A recent sterile liver injury study using intravital microscopy identified the peritoneum as an important source of macrophages, and the authors showed that this pool of macrophages was essential to repair (6). Other murine imaging studies have captured leukocytes as they engage with and then breach the venular wall [Fig. 1C and (7)] or have identified previously unknown in vivo responses after tissue damage, including neutrophil “swarming” behaviors after skin wounding (8).

Model organism studies reveal some of the damage attractants and how inflammatory cells respond

At a fundamental cell biology level, we know that immune cells respond to attractant cues through Rho family small guanosine triphosphatase (GTPase) regulation of their actin cytoskeletons. If Rac is knocked down in Drosophila macrophages, they fail to make proper lamellae and efficiently migrate to the wound. If Rho is knocked down, macrophage-directed migration is unperturbed, but the cells cannot contract and detach their uropod or tail end and thus remain tethered to the spot (9). Spatial activation of these small GTPase switches thus enables cells to migrate toward cues, as highlighted, for example, in zebrafish larval experiments in which light activation of a genetically encoded Rac is used to artificially turn a neutrophil in vivo (10).

The damage attractants identified in these models include small molecules such as H2O2 (11), but these are clearly not the only attractants. Mathematical modeling of macrophage response behaviors in a Drosophila pupal wound reveals the nature of the key attractant(s) that trigger these cells to turn toward the wound; the signals travel back through wound fluids and tissues much too slowly to be small molecules such as adenosine triphosphate (ATP) or H2O2 (12). Immune cell attractants must therefore involve other signals, which likely include growth factors and chemokines, as well as complement components and metabolites of polyunsaturated fatty acids. Naive immune cells cannot respond to a wound attractant; they first need to be primed by other cues that include previous engulfment of apoptotic corpses (13). Studies in flies and fish are beginning to elucidate the mechanisms by which inflammatory cells detect and integrate multiple priming and guidance cues in space and time en route to the damage site. The translation of these findings will be important in mammalian wound scenarios where the distances travelled are greater and more complex than for the fly and fish model systems.

Resolving the inflammatory response

After healing is complete, the resolution of inflammation is not simply a passive process. Although there is clear evidence in mammalian wounds that many neutrophils undergo apoptosis at the site of inflammation, there is also evidence for the reverse migration of zebrafish neutrophils from sites of inflammation. Tracking studies have suggested that neutrophils that have already been exposed to one damage site can remain responsive to secondary insults (14). Other studies have suggested that neutrophils play an active role in the resolution of inflammation by depleting the chemoattractants that initially drew them to the wound site (15). These findings, along with our increasing understanding of pro-resolving molecules produced by macrophages and other cells at sites of inflammation, offer new therapeutic angles for modulating the trafficking of inflammatory cells (16).

Reliance of tissue repair and regeneration on the wound inflammatory response

The role of the inflammatory response, and specifically the function of macrophages, is not clear-cut. Knockdown of macrophages in rabbits with anti-macrophage serum led to severely impaired healing (17), whereas no major impact was observed when neutrophils were depleted. Still, other studies have suggested that repair can happen independently of any inflammatory response. For example, wound re-epithelialization is so rapid in adult zebrafish that it is almost complete before the inflammatory response is initiated. Similar observations have also been reported for injured mammalian embryos that lack a mature inflammatory response (1). Neonatal mice that are genetically missing innate immune cell lineages repair injured tissues faster than controls and typically without any evidence of a scar (1). However, the situation is dramatically different in the adult mouse, where innate immune cell lineages play critical roles during the repair and regeneration process. In parallel studies of injured liver (which can fully regenerate) and skin (which can repair but retains a scar), macrophages play distinct roles, depending on at what stage of repair they are depleted. In the liver, macrophage depletion during early repair reduces scar formation; however, if these cells are depleted after the extracellular matrix is laid down, the normal resolution of fibrosis is impaired (18). In the skin, early macrophage depletion after injury prevents normal granulation tissue formation and re-epithelialization, whereas midstage depletion results in reduced normal vessel development, leading to hemorrhaging; late-stage depletion of macrophages alters the pattern of scar formation (19).

Regenerative inflammation: An evolutionarily conserved mechanism

In lower vertebrates that regenerate whole appendages, such as fins and limbs, and even regions of damaged central nervous system, most studies suggest that inflammation is necessary. For example, if macrophages are depleted during the period of salamander limb blastema formation, regeneration fails, and the site of the amputated limb simply heals (20). Tail fin regeneration is similarly blocked in adult zebrafish if macrophages are genetically ablated during the period of blastema proliferation. Once blastema formation is complete, however, macrophages are no longer critical (21).

Whereas neuroinflammation in the damaged mammalian brain is generally considered to be a negative influence because it triggers glial scarring that hinders axon rewiring, the situation in zebrafish is completely different. The adult zebrafish brain and heart can fully regenerate after injury, but, in both cases, the repair is dependent on inflammation (22, 23). Macrophages also influence the repair of vascular structures in the brain after injury. For example, live imaging of zebrafish brains showed that macrophages facilitate the ligation of injured vessels by positioning themselves between the two severed ends (24). The capacity to regenerate damaged cardiac tissue is also seen in mice, but only until 5 days after birth. In this neonatal period, macrophage influx is again essential for repair (25). The difference between this neonatal period, when regeneration is scar-free, and afterward, when hearts heal with a scar, lies in the source and phenotype of the inflammatory macrophages. In the neonate, they are an expanded population of embryonic-derived macrophages, whereas in the damaged adult heart, they are supplemented by proinflammatory monocyte-derived macrophages (26). Interestingly, inhibition of the monocyte-derived source of macrophages enhances adult cardiac repair (26), as does increasing the numbers of macrophages activated by interleukin-4 and -13 (IL-4 and IL-13) during post-infarct repair (27), supporting the view that IL-4– and IL-13–mediated polarization of macrophages is the more pro-regenerative phenotypic state. Nevertheless, emerging data have shown that macrophage activation is complex and influenced by ontogeny, local environmental factors, and epigenetic changes that enable profound transcriptional reprogramming and influence macrophages’ functional plasticity (28).

Dysregulated tissue repair and inflammation promote pathological fibrosis

Although the wound inflammatory response drives many aspects of tissue repair and regeneration, it can become dysregulated or chronic, and this may lead to the development of pathological fibrosis or scarring that can disrupt normal tissue architecture and function (29). As discussed above, it is becoming increasingly apparent that monocytes and macrophages may play distinct roles in tissue repair, with recruited monocytes often contributing to collateral tissue injury (30), whereas the tissue-resident macrophage population appears to exert more beneficial properties by exhibiting pro-resolving, anti-inflammatory, and pro-regenerative activities (31). Nevertheless, there is substantial overlap between the two populations. Some studies have shown that inflammatory monocytes can quickly adapt to a homeostatic mode. In some injured tissues—the most extreme example being toxic liver injury—the recruited monocytes may supplement or permanently replace the resident macrophage population (32). Because the transformation of proinflammatory monocytes and macrophages to the pro-resolving phenotype is believed to be critical for normal wound repair and reduction of fibrosis, recent studies have focused on elucidating the mechanisms that license these distinct activation states (33).

Core immunological pathways of repair and fibrosis in mammals

Numerous mechanisms can contribute to the development of fibrosis, but two key immunological drivers of tissue fibrogenesis are transforming growth factor–β (TGF-β) and the type 2 cytokines IL-4 and IL-13 (34). TGF-β1 has long been identified as a critical mediator of tissue repair and fibrosis after type 1– and type 17–driven inflammatory responses, which are characterized by the production of the proinflammatory cytokines interferon-γ (IFN-γ) and IL-17A, respectively. Moreover, several studies have concluded that the IL-17A–IL-1β–TGF-β1 axis is important to the development of fibrosis (35, 36). In addition, TGF-β1 promotes tissue repair and fibrosis by two distinct mechanisms. It suppresses the production of proinflammatory mediators that can worsen tissue injury, while simultaneously activating myofibroblasts that facilitate wound closure but also deposit aberrant levels and patterns of collagen at the site of repair (37). IL-13 exhibits similar and overlapping activity in that it functions as both a potent anti-inflammatory cytokine and a driver of myofibroblast activation. In contrast to TGF-β1, however, IL-13 serves as the dominant mediator of tissue repair and fibrosis during a sustained type 2–polarized inflammatory response, which is characterized by IL-4, IL-5, and IL-13. Consequently, dysregulated type 1 and 17 and type 2 inflammatory responses can each lead to the development of pathological fibrosis; however, distinct mechanisms dominate in each case (38) (Fig. 2). The mechanisms that initiate each response also appear to be distinct, with IL-1 and IL-17A activating the TGF-β1 pathway and IL-25, IL-33, and thymic stromal lymphopoietin (TSLP) serving as key initiators of type 2–dependent fibrosis (36, 39). Expansion of IL-13–producing type 2 innate lymphoid cells and CD4+ T helper 2 cells by IL-25, IL-33, and TSLP has been shown to promote fibrosis in multiple tissues, including the skin, lung, and liver (4043). CD11b+ macrophages also contribute to the development of IL-13–driven fibrosis by serving as key sources of inflammatory chemokines (44). Nevertheless, although macrophages are important for the initiation of type 2 fibrosis, studies dissecting the specific contribution of IL-4– and IL-13–polarized macrophages concluded that this subset primarily suppresses the development of fibrosis in the lung and liver (45). A more recent study of skin injury determined that type 2–activated macrophages also influence collagen fibril assembly in fibroblasts by inducing lysyl hydroxylase 2 that directs collagen cross-linking and the stabilization of collagen (46). In contrast to the liver, the dermal component of the skin has limited regenerative capacity in mammals, and healing leaves a scar. Hence, the studies in different organs corroborate the principal tissue-protective role of type 2 immunity, but the different outcome and quality of the healing-promoting activity is likely tissue-specific.

Fig. 2 Distinct mechanisms contribute to pathological tissue remodeling during highly polarized type 1 and type 2 responses.

Sustained type 1 responses (IFN-γ and IL-17A) lead to substantial tissue damage. The injury, in turn, activates TGF-β, which suppresses the inflammatory response while activating extracellular matrix production by myofibroblasts that contribute to fibrosis. During a polarized type 2 response, IL-13 serves as an important driver of fibrosis, with the IL-13 decoy receptor (IL-13Rα2) and IL-10 exhibiting negative regulatory activity. Effective tissue regeneration is typically associated with less polarized immune responses.

GRAPHIC: ADAPTED BY N. CARY/SCIENCE

Fibrosis and tissue regeneration are distinctly regulated by type 2 immunity

IL-4– and IL-13–activated macrophages are important producers of a variety of growth factors, including TGF-β1, insulin-like growth factor 1, vascular endothelial growth factor (VEGF), platelet-derived growth factor, and Relm (31). Therefore, they have long been implicated in tissue repair, regeneration, and fibrosis (47, 48). In addition, other nonimmune cells, including fibroblasts, epithelial cells, hepatocytes, and various stem and tissue progenitor cells, have emerged as critical targets of IL-4 and IL-13 signaling after injury. For example, a recent study exploring the mechanisms of liver regeneration showed that IL-4 receptor signaling in hepatocytes is required for proliferation after injury, with eosinophils delivering the IL-4 (49). Another study exploring the mechanisms of hepatic fibrosis after infection identified an important role for IL-4Rα expression in fibroblasts (50), with IL-13, rather than IL-4, identified as the critical regenerative mediator; instead of promoting hepatocyte proliferation, IL-13 targeted bile duct epithelial cells (cholangiocytes) and hepatic progenitor cells directly, leading to increased numbers of bile ducts (50). A similar pro-regenerative role for IL-4 has also been reported in a model of skeletal muscle injury, in which muscle damage triggered the recruitment of IL-4–producing eosinophils, which activated the regenerative actions of muscle-resident fibro-adipocyte progenitor cells that support myogenesis (51).

Linking metabolism to immune cell repair functions

Why some immune pathways exhibit tissue-protective activity and others play pathogenic roles remains unclear. Recent discoveries in the field of immunometabolism have generated considerable excitement because they suggest that immune cell metabolic signatures may affect cell plasticity and function and thus might provide new mechanistic insights into how the immune system regulates repair (52).

As outlined above, macrophages and other cells of the innate and adaptive immune response contribute to tissue repair. However, the various mechanisms that modulate macrophage functional complexity during the different stages of healing are not fully understood. Local microenvironmental conditions at sites of injury may be inhospitable to effective repair because of a combination of infection, hypoxia, and accumulating metabolites (Fig. 3). Although macrophages are known to adapt rapidly to these demanding conditions to combat infection and facilitate tissue repair, their exact bioenergetic needs in these scenarios are still poorly understood. These bioenergetic needs are characterized by the activation of multiple metabolic pathways, including glycolysis, the tricarboxylic acid (TCA) cycle, the pentose phosphate pathway, and fatty acid synthesis and oxidation (Fig. 3) (53, 54). Nevertheless, the impacts of these pathways on tissue repair, regeneration, and fibrosis remain largely unknown.

Fig. 3 Integrated perspective of potential activation phenotypes and metabolic pathways in wound macrophages.

HIF-1α activation and glycolysis are hallmarks of inflammatory activation. In contrast, mitochondrial biogenesis, FA degradation, an intact TCA cycle, and oxidative phosphorylation lead to efficient production of large amounts of ATP that sustain the secretory and resolution phenotype of type 2–activated macrophages. How the dynamics of macrophage polarization phenotypes (marker genes are depicted in triangles in the middle panel) and cellular metabolic pathways instruct the sequence of specific repair mechanisms at a tissue level remains undetermined. NADPH, nicotinamide adenine dinucleotide; e, electron; PPP, pentose phosphate pathway; PRR, pathogen recognition receptor; PAMP, pathogen-associated molecular pattern; DAMP, damage-associated molecular pattern; TCA, tricarboxylic acid; SDH, succinate dehydrogenase; TAG, triacylglycerol; FFA, free fatty acids; LDL, low-density lipoprotein; VLDL, very-low-density lipoprotein.

GRAPHIC: ADAPTED BY N. CARY/SCIENCE

Glycolysis mediates antimicrobial and inflammatory activity of type 1 immune responses

The transcription factor hypoxia-inducible factor 1α (HIF-1α) was identified as a key regulator that coordinates the transcriptional programs of glycolytic metabolism and proinflammatory cytokine production in type 1–activated macrophages (55). Recent work has unraveled the regulatory network linking HIF-1α, energetic metabolism, and inflammation. Interestingly, succinate that accumulates as a consequence of a broken TCA cycle under inflammatory conditions stabilizes HIF-1α, which sustains the proinflammatory phenotype (56). In addition, mitochondrial succinate oxidation by succinate dehydrogenase (SDH) maintains lipopolysaccharide (LPS)–induced inflammation (57). Along these lines, itaconate, an abundant metabolite of LPS-activated macrophages, dampens inflammation by inhibiting SDH (58). Collectively, these studies highlight succinate metabolism as a critical mechanism controlling the macrophage inflammatory state. Inactivation of HIF-1α in myeloid cells was shown to dramatically reduce expression of the glucose transporter GLUT1 and initiation of the glycolytic cascade. Accordingly, production of lactate and ATP in response to LPS was reduced in HIF-1α–deficient macrophages, resulting in compromised antimicrobial activity (59). These findings demonstrated that type 1–activated macrophages exploit glycolysis to fuel their short-lived antimicrobial and proinflammatory functions. In addition, synthesis of IL-1β and VEGF, both factors that are integrally linked to the initiation of the wound-healing response, was also shown to be reduced in HIF-1α–deficient macrophages (55). Together, these studies revealed how a single transcription factor serves as a key regulatory node and critically controls antimicrobial activity, energy metabolism, inflammation, and key tissue repair mechanisms such as angiogenesis.

Oxidative phosphorylation supports tissue-protective type 2 immune responses

A recent study showed that type 2 cytokines serve as key activators of oxidative phosphorylation (OxPhos) in macrophages (3). OxPhos produces high-energy phosphates in mitochondria that are needed for energetically demanding cell replication, cell growth, and synthesis of macromolecules (Fig. 3). Microarray and metabolic analyses of type 1– and type 2–activated macrophages revealed that genes important for fatty acid oxidation (FAO) are preferentially expressed in IL-4– and IL-13–activated macrophages. Consistently, IL-4–stimulated macrophages showed a higher rate of FAO, mitochondrial mass, and mitochondrial respiration. Inhibition of OxPhos dramatically attenuated the activation of the type 2–activated phenotype and resulted in increased expression of inflammatory markers. The type 2–activated phenotype could be restored by transgenic expression of PPARγ coactivator 1β, an important mediator of FAO and mitochondrial oxidative metabolism. Collectively, this study (3) demonstrated that the metabolic shift from glycolysis toward OxPhos is paralleled by the transition from an inflammatory toward a pro-resolution phenotype, thus revealing new therapeutic options to modulate inflammation. An important unanswered question is whether, under certain conditions, oxidative metabolism might promote unsuccessful healing.

The in vitro analyses discussed above are now supported by in vivo research showing that pharmaceutical inhibition of lysosomal lipolysis in murine macrophages results in a blockade of IL-4Rα–mediated macrophage activation (60). This study investigated the source of FA and discovered a previously unappreciated role of lysosomal acid lipase (LAL) in cell-intrinsic lipolysis in macrophages. LAL supported FAO, thus revealing an integral role for this enzyme in the activation of the type 2 phenotype in macrophages. This study also showed that proper mitochondrial function is required for type 2–mediated release of Relm-α, a factor involved in type 2–mediated skin fibrosis, from macrophages (46). Last, a recent study also identified the Akt-mTORC1 axis as a key node calibrating the metabolic demands of type 2 cytokine–activated macrophages (61).

Emerging concepts in profibrotic mediator blockade

Because type 2 immunity and TGF-β1 pathway activation both serve as core drivers of tissue repair and fibrogenesis, numerous therapeutic strategies that modulate specific components of each pathway are under development (29, 62). However, these pathways are also critically involved in normal tissue regeneration after injury (29, 50). IL-13 and TGF-β1 are well-known anti-inflammatory cytokines; therefore, therapeutics targeting these pathways may unleash rebound inflammatory responses that could impair healing, exacerbate tissue injury, and activate autoimmune disease (63, 64). Any successful antifibrotic strategy must mitigate these potential negative outcomes.

Whereas the clinical use of therapeutics that interfere with TGF-β signaling has been approved for treatment of a few fibrotic diseases, agents specifically targeting type 2 immunity have not yet been licensed. Instead, they are being investigated in the clinic for other repair-related indications, including for atopic dermatitis (AD), idiopathic pulmonary fibrosis, and asthma. Phase 3 trials have demonstrated the efficacy and tolerance of an IL-4Rα–blocking antibody in AD (65), and these beneficial outcomes in AD therapy may facilitate the investigation of type 2 immunity–targeting agents as antifibrotic therapies. Along these lines, a bispecific antibody that neutralizes IL-4 and IL-13 is currently being tested in a clinical trial for scleroderma (https://clinicaltrials.gov; NCT02921971), a connective tissue disease for which no disease-modifying treatment is approved as yet.

Conclusions and future directions

It is still unclear how immune cells signal to stromal cells and what the cell and molecular changes are that allow responding cells to proliferate and cause scarring. There is growing evidence that monocyte and macrophage polarization dynamics are critical to instruct immune and nonimmune tissue-resident cells to both initiate and terminate healing responses. We next need to better understand the hierarchy of molecular determinants that orchestrate these divergent phenotypes that lead to different outcomes in the repair response.

Much has been learned about immune cell polarization and tissue regeneration from defined responses in experimental prototypic settings [e.g., helminth infection (62)], yet future studies need to clarify whether these mechanisms can be generalized. Current evidence suggests that the repair response triggered by core immune pathways is context-dependent and is a function of a combination of multiple determinants, including the nature of the damage (e.g., pathogen-induced, sterile injury, or mixed injury pattern), the amplitude and duration of different polarization states, and the intrinsic regenerative capacity of different organs and tissues.

In translational terms, the interaction between immune cells and stromal cells is an obvious therapeutic target for dampening the scar response. Tissue repair and fibrosis may also be influenced by directly modulating the inflammatory response and by manipulating known endogenous profibrotic mediators such as TGF-β1, osteopontin, hedgehog signaling, and type 2 cytokines (37, 48), among others. In addition, augmenting the actions of pro-resolving molecules such as IL-10 and resolvins at the wound site might be another way to encourage resolution after the essential functions of neutrophils and macrophages have been accomplished (11). In vitro and in vivo screens are beginning to reveal small molecules and drugs that may trigger early resolution (66) and enable better-controlled regeneration of the wound inflammatory response. Additionally, it may be possible to enhance tissue-protective mechanisms that are up-regulated at the wound site, including, for example, the enzymes downstream of NRF2 signaling that sequester reactive oxygen species (ROS) and shield repairing tissue from some of the negative consequences of sustained inflammation (67). All such therapeutic strategies must be engineered in a way that does not negatively affect pro-regenerative pathways.

Immune cell metabolism is also a heretofore unexplored area for therapeutic intervention at sites of tissue repair. To date, little detailed information is known about how immune cell metabolism affects repair mechanisms. In this respect, it is intriguing to speculate that the distinct regenerative capacities of different tissues and organs may be explained in part by their particular metabolic demands. It will also be important to understand whether scarring or failed regeneration results from aberrant shifts in metabolic programming in immune or nonimmune cells. Relatedly, a recent systems biology study found parallel metabolic and transcriptional pathways that control macrophage polarization (68), revealing potential pharmacological control points.

Last, we still do not fully understand why scars are capable of resolving in some tissues, such as liver, but not in others, such as skin. Moreover, why organisms such as zebrafish display excellent regenerative ability but mammals do not also remains unclear. Future studies should therefore address whether differences in macrophage origin (28), recruitment, and activation may be an important contributing mechanism explaining this variation between different organs and species. Understanding precisely how these key inflammatory cells are influenced by the wound environment, and how they themselves influence how stromal cells deposit matrix in the repairing tissue, will undoubtedly guide us toward therapeutic strategies for managing and improving healing in the clinic.

References and Notes

  1. Acknowledgments: The authors would like to thank K. Vannella, T. Gieseck, K. Hart, H. Weavers, R. Richardson, and S. Willenborg for helpful comments. T.A.W. is supported by the intramural research program of NIH’s National Institute of Allergy and Infectious Diseases. P.M. is supported by investigator/program grants from the Wellcome Trust, Cancer Research UK, and the Medical Research Council. S.A.E. is supported by grants from the Deutsche Forschungsgemeinschaft (SFB829, FOR 2240, and CECAD) and the Center for Molecular Medicine Cologne.

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