Wound macrophages as key regulators of repair: origin, phenotype, and function

Nanocarrier-mediated inhibition of macrophage migration inhibitory factor attenuates secondary injury after spinal cord injury

Spinal cord injury (SCI) can lead to permanent motor and sensory deficits. Following the initial traumatic insult, secondary injury mechanisms characterized by persistent heightened inflammation are initiated and lead to continued and pervasive cell death and tissue damage. Anti-inflammatory drugs such as methylprednisolone (MP) used clinically have ambiguous benefits with debilitating side effects. Typically, these drugs are administered systemically at high doses, resulting in toxicity and paradoxically increased inflammation. Furthermore, these drugs have a small time window postinjury (few hours) during which they need to be infused to be effective. As an alternative to MP, we investigated the effect of a small molecule inhibitor (Chicago sky blue, CSB) of macrophage migration inhibitory factor (MIF) for treating SCI. The pleiotropic cytokine MIF is known to contribute to upregulation of several pro-inflammatory cytokines in various disease and injury states. In vitro, CSB administration alleviated endotoxin-mediated inflammation in primary microglia and macrophages. Nanocarriers such as liposomes can potentially alleviate systemic side effects of high-dose therapy by enabling site-specific drug delivery to the spinal cord. However, the therapeutic window of 100 nm scale nanoparticle localization to the spinal cord after contusion injury is not fully known. Thus, we first investigated the ability of nanocarriers of different sizes to localize to the injured spinal cord up to 2 weeks postinjury. Results from the study showed that nanocarriers as large as 200 nm in diameter could extravasate into the injured spinal cord up to 96 h postinjury. We then formulated nanocarriers (liposomes) encapsulating CSB and administered them intravenously 48 h postinjury, within the previously determined 96 h therapeutic window. In vivo, in this clinically relevant contusion injury model in rats, CSB administration led to preservation of vascular and white matter integrity, improved wound healing, and an increase in levels of arginase and other transcripts indicative of a resolution phase of wound healing. This study demonstrates the potential of MIF inhibition in SCI and the utility of nanocarrier-mediated drug delivery selectively to the injured cord.

Azilsartan reduced TNF-α and IL-1β levels, increased IL-10 levels and upregulated VEGF, FGF, KGF, and TGF-α in an oral mucositis model

Oral mucositis (OM) is a common complication of treatments for head and neck cancer, particularly radiotherapy with or without chemotherapy. OM is characterised by oral erythema, ulceration, and pain. The aim of this study was to evaluate the effect of azilsartan (AZT), an angiotensin II receptor antagonist, on 5-fluorouracil (5-FU)-induced oral mucositis (OM) in Syrian hamsters. OM was induced by the intraperitoneal administration of 5-FU on experimental days 1 (60 mg/Kg) and 2 (40 mg/Kg). Animals were pretreated with oral AZT (1, 5, or 10 mg/kg) or vehicle 30 min before 5-FU injection and daily until day 10. Experimental treatment protocols were approved by the Animal Ethics Committee Use/CEUA (Number 28/2012) of the UFRN. Macroscopic analysis and cheek pouch samples were removed for histopathologic analysis. Myeloperoxidase (MPO), Malonyldialdehyde (MDA), interleukin-1 beta (IL-1β), interleukin-10 (IL-10), and tumour necrosis factor-alpha (TNF-α) were analysed by Enzyme Linked Immuno Sorbent Assay (ELISA). Vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), keratinocyte growth factor (KGF), and transforming growth factor (TGF)-α were measured by immunohistochemistry. Analysis of variance followed by Bonferroni's test was used to calculate the means of intergroup differences (p ≤ 0.05). Treatment with 1 mg/kg AZT reduced levels MPO (p<0.01), MDA (p<0.5) and histological inflammatory cell infiltration, and increased the presence of granulation tissue. AZT treatment at 1 mg/kg reduced the TNF-α (p<0.05) and IL-1β (p<0.05) levels, increased the cheek pouch levels of IL-10 (p<0.01), and upregulated VEGF, FGF, KGF, and TGF-α. Administration of AZT at higher doses (5 and 10 mg/kg) did not significantly reverse the OM. AZT at a dose of 1 mg/kg prevented the mucosal damage and inflammation associated with 5-FU-induced OM, increasing granulation and tissue repair.

The significance of macrophage polarization subtypes for animal models of tissue fibrosis and human fibrotic diseases

The systemic and organ-specific human fibrotic disorders collectively represent one of the most serious health problems world-wide causing a large proportion of the total world population mortality. The molecular pathways involved in their pathogenesis are complex and despite intensive investigations have not been fully elucidated. Whereas chronic inflammatory cell infiltration is universally present in fibrotic lesions, the central role of monocytes and macrophages as regulators of inflammation and fibrosis has only recently become apparent. However, the precise mechanisms involved in the contribution of monocytes/macrophages to the initiation, establishment, or progression of the fibrotic process remain largely unknown. Several monocyte and macrophage subpopulations have been identified, with certain phenotypes promoting inflammation whereas others display profibrotic effects. Given the unmet need for effective treatments for fibroproliferative diseases and the crucial regulatory role of monocyte/macrophage subpopulations in fibrogenesis, the development of therapeutic strategies that target specific monocyte/macrophage subpopulations has become increasingly attractive. We will provide here an overview of the current understanding of the role of monocyte/macrophage phenotype subpopulations in animal models of tissue fibrosis and in various systemic and organ-specific human fibrotic diseases. Furthermore, we will discuss recent approaches to the design of effective anti-fibrotic therapeutic interventions by targeting the phenotypic differences identified between the various monocyte and macrophage subpopulations.

Ontogeny and polarization of macrophages in inflammation: blood monocytes versus tissue macrophages

The explosion of new information in recent years on the origin of macrophages in the steady-state and in the context of inflammation has opened up numerous new avenues of investigation and possibilities for therapeutic intervention. In contrast to the classical model of macrophage development, it is clear that tissue-resident macrophages can develop from yolk sac-derived erythro-myeloid progenitors, fetal liver progenitors, and bone marrow-derived monocytes. Under both homeostatic conditions and in response to pathophysiological insult, the contribution of these distinct sources of macrophages varies significantly between tissues. Furthermore, while all of these populations of macrophages appear to be capable of adopting the polarized M1/M2 phenotypes, their respective contribution to inflammation, resolution of inflammation, and tissue repair remains poorly understood and is likely to be tissue- and disease-dependent. A better understanding of the ontology and polarization capacity of macrophages in homeostasis and disease will be essential for the development of novel therapies that target the inherent plasticity of macrophages in the treatment of acute and chronic inflammatory disease.

Macrophage activation and its role in repair and pathology after spinal cord injury

The injured spinal cord does not heal properly. In contrast, tissue repair and functional recovery occur after skin or muscle injuries. The reason for this dichotomy in wound repair is unclear but inflammation, and specifically macrophage activation, likely plays a key role. Macrophages have the ability to promote the repair of injured tissue by regulating transitions through different phase of the healing response. In the current review we compare and contrast the healing and inflammatory responses between spinal cord injuries and tissues that undergo complete wound resolution. Through this comparison, we identify key macrophage phenotypes that are inaptly triggered or absent after spinal cord injury and discuss spinal cord stimuli that contribute to this maladaptive response. Sequential activation of classic, pro-inflammatory, M1 macrophages and alternatively activated, M2a, M2b, and M2c macrophages occurs during normal healing and facilitates transitions through the inflammatory, proliferative, and remodeling phases of repair. In contrast, in the injured spinal cord, pro-inflammatory macrophages potentiate a prolonged inflammatory phase and remodeling is not properly initiated. The desynchronized macrophage activation after spinal cord injury is reminiscent of the inflammation present in chronic, non-healing wounds. By refining the role macrophages play in spinal cord injury repair we bring to light important areas for future neuroinflammation and neurotrauma research. This article is part of a Special Issue entitled SI: Spinal cord injury.

Brain-derived neurotrophic factor in chronic periodontitis

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophic factor family. Outside the nervous system, BDNF has been shown to be expressed in various nonneural tissues, such as periodontal ligament, dental pulp, and odontoblasts. Although a role for BDNF in periodontal regeneration has been suggested, a function for BDNF in periodontal disease has not yet been studied. The aim of this study was to analyze the BDNF levels in periodontal tissues of patients with chronic periodontitis (CP) and periodontally healthy controls (HC). All subjects were genotyped for the rs4923463 and rs6265 BDNF polymorphisms. Periodontal tissues were collected for ELISA, myeloperoxidase (MPO), and microscopic analysis from 28 CP patients and 29 HC subjects. BDNF levels were increased in CP patients compared to HC subjects. A negative correlation was observed when analyzing concentration of BDNF and IL-10 in inflamed periodontium. No differences in frequencies of BDNF genotypes between CP and HC subjects were observed. However, BDNF genotype GG was associated with increased levels of BDNF, TNF-α, and CXCL10 in CP patients. In conclusion, BDNF seems to be associated with periodontal disease process, but the specific role of BDNF still needs to be clarified.

Differential expression of granulocyte, macrophage, and hypoxia markers during early and late wound healing stages following transplantation of tissue-engineered skin substitutes of human origin

PURPOSE

Human pigmented tissue-engineered skin substitutes represent an advanced therapeutic option to treat skin defects. The inflammatory response is one of the major factors determining integration and long-term survival of such a graft in vivo. The aim of the present study was to investigate the spatiotemporal distribution of host-derived macrophage and granulocyte graft infiltration as well as hypoxia-inducible factor 1 alpha (HIF-1-alpha) expression in a (nu/nu) rat model.

METHODS

Keratinocytes, melanocytes, and fibroblasts derived from human skin biopsies were isolated, cultured, and expanded in vitro. Dermal fibroblasts were seeded into collagen type I hydrogels that were subsequently covered by keratinocytes and melanocytes in 5:1 ratio. These pigmented dermo-epidermal skin substitutes were transplanted onto full-thickness skin wounds on the back of immuno-incompetent rats and analyzed at early (1 and 3 weeks) and late (6 and 12 weeks) stages of wound healing. The expression of distinct inflammatory cell markers specific for granulocytes (HIS48) or macrophages (CD11b, CD68), as well as HIF-1-alpha were analyzed and quantified by immunofluorescence microscopy.

RESULTS

Our data demonstrate that granulocytes infiltrate the entire graft at 1 week post-transplantation. This was followed by monocyte/macrophage recruitment to the graft at 3-12 weeks. The macrophages were initially restricted to the borders of the graft (early stages), and were then found throughout the entire graft (late stages). We observed a time-dependent decrease of macrophages. Only a few graft-infiltrating granulocytes were found between 6-12 weeks, mostly at the graft borders. A heterogeneous expression of HIF-1-alpha was observed at both early and late wound healing stages.

CONCLUSIONS

Our findings demonstrate the spatiotemporal distribution of inflammatory cells in our transplants closely resembles the one documented for physiological wound healing.

Modeling the effects of systemic mediators on the inflammatory phase of wound healing

The normal wound healing response is characterized by a progression from clot formation, to an inflammatory phase, to a repair phase, and finally, to remodeling. In many chronic wounds there is an extended inflammatory phase that stops this progression. In order to understand the inflammatory phase in more detail, we developed an ordinary differential equation model that accounts for two systemic mediators that are known to modulate this phase, estrogen (a protective hormone during wound healing) and cortisol (a hormone elevated after trauma that slows healing). This model describes the interactions in the wound between wound debris, pathogens, neutrophils and macrophages and the modulation of these interactions by estrogen and cortisol. A collection of parameter sets, which qualitatively match published data on the dynamics of wound healing, was chosen using Latin Hypercube Sampling. This collection of parameter sets represents normal healing in the population as a whole better than one single parameter set. Including the effects of estrogen and cortisol is a necessary step to creating a patient specific model that accounts for gender and trauma. Utilization of math modeling techniques to better understand the wound healing inflammatory phase could lead to new therapeutic strategies for the treatment of chronic wounds. This inflammatory phase model will later become the inflammatory subsystem of our full wound healing model, which includes fibroblast activity, collagen accumulation and remodeling.

Regulatory T cells in central nervous system injury: a double-edged sword

Previous research investigating the roles of T effector (T(eff)) and T regulatory (T(reg)) cells after injury to the CNS has yielded contradictory conclusions, with both protective and destructive functions being ascribed to each of these T cell subpopulations. In this work, we study this dichotomy by examining how regulation of the immune system affects the response to CNS trauma. We show that, in response to CNS injury, T(eff) and T(reg) subsets in the CNS-draining deep cervical lymph nodes are activated, and surgical resection of these lymph nodes results in impaired neuronal survival. Depletion of T(reg), not surprisingly, induces a robust T(eff) response in the draining lymph nodes and is associated with impaired neuronal survival. Interestingly, however, injection of exogenous T(reg) cells, which limits the spontaneous beneficial immune response after CNS injury, also impairs neuronal survival. We found that no T(reg) accumulate at the site of CNS injury, and that changes in T(reg) numbers do not alter the amount of infiltration by other immune cells into the site of injury. The phenotype of macrophages at the site, however, is affected: both addition and removal of T(reg) negatively impact the numbers of macrophages with alternatively activated (tissue-building) phenotype. Our data demonstrate that neuronal survival after CNS injury is impaired when T(reg) cells are either removed or added. With this exacerbation of neurodegeneration seen with both addition and depletion of T(reg), we recommend exercising extreme caution when considering the therapeutic targeting of T(reg) cells after CNS injury, and possibly in chronic neurodegenerative conditions.

The immunology of the porcine skin and its value as a model for human skin

The porcine skin has striking similarities to the human skin in terms of general structure, thickness, hair follicle content, pigmentation, collagen and lipid composition. This has been the basis for numerous studies using the pig as a model for wound healing, transdermal delivery, dermal toxicology, radiation and UVB effects. Considering that the skin also represents an immune organ of utmost importance for health, immune cells present in the skin of the pig will be reviewed. The focus of this review is on dendritic cells, which play a central role in the skin immune system as they serve as sentinels in the skin, which offers a large surface area exposed to the environment. Based on a literature review and original data we propose a classification of porcine dendritic cell subsets in the skin corresponding to the subsets described in the human skin. The equivalent of the human CD141(+) DC subset is CD1a(-)CD4(-)CD172a(-)CADM1(high), that of the CD1c(+) subset is CD1a(+)CD4(-)CD172a(+)CADM1(+/low), and porcine plasmacytoid dendritic cells are CD1a(-)CD4(+)CD172a(+)CADM1(-). CD209 and CD14 could represent markers of inflammatory monocyte-derived cells, either dendritic cells or macrophages. Future studies for example using transriptomic analysis of sorted populations are required to confirm the identity of these cells.

Attempts to accelerate wound healing

Wound healing is a well-orchestrated process, where numerous factors are activated or inhibited in a sequence of steps. Immediately after the infliction of damage, the repair of wound stars. The initial step is an inflammatory change with activation of innate immunity, which is followed by proliferation phase, including fibroplasia, angiogenesis and re-epithelialization. Pathological impairment of wound healing process may lead to persistent ulceration as seen in diabetic patients. Various signaling pathways are involved in wound healing. TGFβ/Smad pathway is a representative and well known to participate in fibroplasia, however, its comprehensive effect on wound healing is controversial. Experimental and clinical remedies have been being tried to promote wound healing. Advancement of cell engineering allows us to use stem cells and living skin equivalents.

Rethinking regenerative medicine: a macrophage-centered approach

Regenerative medicine, a multi-disciplinary approach that seeks to restore form and function to damaged or diseased tissues and organs, has evolved significantly during the past decade. By adapting and integrating fundamental knowledge from cell biology, polymer science, and engineering, coupled with an increasing understanding of the mechanisms which underlie the pathogenesis of specific diseases, regenerative medicine has the potential for innovative and transformative therapies for heretofore unmet medical needs. However, the translation of novel technologies from the benchtop to animal models and clinical settings is non-trivial and requires an understanding of the mechanisms by which the host will respond to these novel therapeutic approaches. The role of the innate immune system, especially the role of macrophages, in the host response to regenerative medicine based strategies has recently received considerable attention. Macrophage phenotype and function have been suggested as critical and determinant factors in downstream outcomes. The constructive and regulatory, and in fact essential, role of macrophages in positive outcomes represents a significant departure from the classical paradigms of host-biomaterial interactions, which typically consider activation of the host immune system as a detrimental event. It appears desirable that emerging regenerative medicine approaches should not only accommodate but also promote the involvement of the immune system to facilitate positive outcomes. Herein, we describe the current understanding of macrophage phenotype as it pertains to regenerative medicine and suggest that improvement of our understanding of context-dependent macrophage polarization will lead to concurrent improvement in outcomes.

Joint haemorrhage partly accelerated immobilization-induced synovial adhesions and capsular shortening in rats

PURPOSE

To elucidate the effects of intra-articular haemorrhage on the joint capsule of immobilized knees in rats.

METHODS

The unilateral knee joints were immobilized using a plastic plate and screws. Sham operated rats had only screws inserted. A single injection of fresh autologous blood was given postoperatively into the knee joints of the immobilized blood injection (Im-B) and the Sham blood injection (Sm-B) groups. Normal saline was administered for the immobilized-saline injection (Im-S) group. Sagittal sections were prepared from the medial midcondylar region of the knee and assessed with histological, histomorphometric, and immunohistochemical methods. The range of motion (ROM) was measured, and the mechanical property of the capsule was assessed by scanning acoustic microscope.

RESULTS

Absorption of the injected blood was delayed and made severe adhesions in the Im-B group. The length of the synovial membrane in the Im-B group was significantly shorter than that of the other groups. The ROM was significantly restricted in the Im-B group compared with the other groups. The elasticity of the posterior capsule in the Im-B group was significantly lower than that in the Sm-B group. Iron deposition was observed in the Im-B and Sm-B groups. Strong immunoreactivities of CD68, TGF-β1, and α-SMA were observed in the adhesion area of the Im-B group. Joint immobilization with blood injection caused severe capsular adhesion and limited range of motion. Immunostaining related to fibrosis increased with joint haemorrhage.

CONCLUSION

Intra-articular haemorrhage with joint immobilization might be an accelerated risk factor for joint contracture. It is likely that leaving a haematoma inside an immobilized joint should be avoided.

Suppressed recruitment of alternatively activated macrophages reduces TGF-β1 and impairs wound healing in streptozotocin-induced diabetic mice

BACKGROUND

Diabetes mellitus inhibits wound-induced angiogenesis, impairing the wound healing process and leading to the development of chronic wounds. Impaired healing of diabetic wounds is caused by persistent pro-inflammatory macrophages recruited to the granulation tissue; however, little is known about the phenotype of the macrophages involved in diabetic wound healing. The present study was conducted to examine the involvement of macrophages in impaired wound healing using streptozotocin (STZ)-induced diabetic mice.

METHODS

Full-thickness skin wounds were created on the backs of mice treated with STZ or vehicle.

RESULTS

Compared with controls, wound healing and angiogenesis were suppressed in STZ-treated mice, with attenuated expression of vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF)-2 in wound granulation tissue. STZ-treated mice exhibited enhanced recruitment of classically activated macrophages (M1) expressing inducible nitric oxide synthase (iNOS) and suppressed recruitment of alternatively activated macrophages (M2) expressing transforming growth factor-beta-1 (TGF-β1). Treatment of diabetic mice with TGF-β1 restored wound healing and angiogenesis and normalized M1/M2 macrophage polarization in the granulation tissue.

CONCLUSIONS

These results suggest that an imbalance of macrophage phenotypes contributes to impaired wound healing in STZ-induced diabetic mice, and treatment with cytokines derived from M2 macrophages may be an effective therapeutic strategy to increase angiogenesis and promote healing of diabetic wounds.

In vivo remodelling of vascularizing engineered tissues

A critical aspect of creating vascularized tissues is the remodelling that occurs in vivo, driven in large part by the host response to the tissue construct. Rather than a simple inflammatory response, a beneficial tissue remodelling response results in the formation of vascularised tissue. The characteristics and dynamics of this response are slowly being elucidated, especially as they are modulated by the complex interaction between the biomaterial and cellular components of the tissue constructs and the host. This process has elements that are similar to both wound healing and tumour development, and its features are illustrated by reference to the bottom-up generation of a tissue using modular constructs. These modular constructs consist of mesenchymal stromal cells (MSC) embedded in endothelial cell (EC)-covered collagen gel rods that are a few hundred microns in size. Particular attention is paid to the role of hypoxia and macrophage recruitment, as well as the paracrine effects of the MSC and EC in this host response.

Uptake of neutrophil-derived Ym1 protein distinguishes wound macrophages in the absence of interleukin-4 signaling in murine wound healing

The determination of regenerative wound-healing macrophages as alternatively activated macrophages is currently questioned by the absence of IL-4 in wound tissue. Yet, murine wound tissue expressed high levels of Ym1 (chitinase 3-like 3), an established marker of the IL-4-induced alternatively activated macrophage phenotype. Ym1 was expressed in wound neutrophils but not in macrophages. Initially, Ym1-free wound-healing macrophages, invading from the wound margins, became gradually positive for the protein in the absence of IL-4 signaling and Stat6 activation, as they entered the neutrophil-populated wound regions. IL-4 failed to induce Ym1 protein in ex vivo-cultured wound tissue explants containing wound-healing macrophages. Recombinant Ym1 protein was selectively taken up by macrophages but not by keratinocytes and endothelial cells. Cultured macrophages lost the ability to take up the recombinant protein when four highly conserved residues and the 70-amino acid small α+β domain essential for Ym1 function were removed. The data suggest that the IL-4/Stat6-independent presence of Ym1 protein in wound-healing macrophages is of exogenous origin, with Ym1 taken up from wound neutrophils as the cellular source. The data suggest that in situ determination of wound-healing macrophages, often defined by Ym1, might not essentially describe an IL-4-dependent macrophage phenotype. Consequently, wound-healing macrophages should not be classified by the established categories of the well-accepted but simplified paradigm of M1/M2 macrophage activation.

The Role of MicroRNAs in Diabetic Complications-Special Emphasis on Wound Healing

Overweight and obesity are major problems in today’s society, driving the prevalence of diabetes and its related complications. It is important to understand the molecular mechanisms underlying the chronic complications in diabetes in order to develop better therapeutic approaches for these conditions. Some of the most important complications include macrovascular abnormalities, e.g., heart disease and atherosclerosis, and microvascular abnormalities, e.g., retinopathy, nephropathy and neuropathy, in particular diabetic foot ulceration. The highly conserved endogenous small non-coding RNA molecules, the micro RNAs (miRNAs) have in recent years been found to be involved in a number of biological processes, including the pathogenesis of disease. Their main function is to regulate post-transcriptional gene expression by binding to their target messenger RNAs (mRNAs), leading to mRNA degradation, suppression of translation or even gene activation. These molecules are promising therapeutic targets and demonstrate great potential as diagnostic biomarkers for disease. This review aims to describe the most recent findings regarding the important roles of miRNAs in diabetes and its complications, with special attention given to the different phases of diabetic wound healing.

Beyond canonical inflammasomes: emerging pathways in IL-1-mediated autoinflammatory disease

In recent years, non-communicable chronic diseases that are potentiated by sterile inflammation have replaced infectious diseases as the major threat to human health. Sterile inflammation that results from aberrant tissue damage plays pivotal roles in the pathogenesis of numerous acute and chronic inflammatory diseases including atherosclerosis, type 2 diabetes, cancer, obesity, and multiple neurodegenerative diseases. The cellular events and molecular signaling pathways that govern sterile inflammation currently remain poorly defined; however, emerging data suggest central roles for IL-1 in driving autoimmune and inflammatory disease pathogenesis. Improved characterization of the immunological pathways that contribute to sterile inflammation are desperately needed to develop effective therapeutics to treat these devastating diseases. In this review, we discuss recent advances in our understanding of how IL-1 is regulated in response to tissue damage. In particular, we highlight recent studies that describe novel roles for conventional cell death molecules in the regulation of IL-1β production.

Tsukushi is involved in the wound healing by regulating the expression of cytokines and growth factors

During the wound-healing process, macrophages, fibroblasts, and myofibroblasts play a leading role in shifting from the inflammation phase to the proliferation phase, although little is known about the cell differentiation and molecular control mechanisms underlying these processes. Previously, we reported that Tsukushi (TSK), a member of the small leucine-rich repeat proteoglycan family, functions as a key extracellular coordinator of multiple signalling networks. In this study, we investigated the contribution of TSK to wound healing. Analysis of wound tissue in heterozygous TSK-lacZ knock-in mice revealed a pattern of sequential TSK expression from macrophages to myofibroblasts. Quantitative PCR and in vitro cell induction experiments showed that TSK controls macrophage function and myofibroblast differentiation by inhibiting TGF-β1 secreted from macrophages. Our results suggest TSK facilitates wound healing by maintaining inflammatory cell quiescence.

Macrophages modulate adult zebrafish tail fin regeneration

Neutrophils and macrophages, as key mediators of inflammation, have defined functionally important roles in mammalian tissue repair. Although recent evidence suggests that similar cells exist in zebrafish and also migrate to sites of injury in larvae, whether these cells are functionally important for wound healing or regeneration in adult zebrafish is unknown. To begin to address these questions, we first tracked neutrophils (lyzC⁺, mpo⁺) and macrophages (mpeg1⁺) in adult zebrafish following amputation of the tail fin, and detailed a migratory timecourse that revealed conserved elements of the inflammatory cell response with mammals. Next, we used transgenic zebrafish in which we could selectively ablate macrophages, which allowed us to investigate whether macrophages were required for tail fin regeneration. We identified stage-dependent functional roles of macrophages in mediating fin tissue outgrowth and bony ray patterning, in part through modulating levels of blastema proliferation. Moreover, we also sought to detail molecular regulators of inflammation in adult zebrafish and identified Wnt/β-catenin as a signaling pathway that regulates the injury microenvironment, inflammatory cell migration and macrophage phenotype. These results provide a cellular and molecular link between components of the inflammation response and regeneration in adult zebrafish.

Wound administration of M2-polarized macrophages does not improve murine cutaneous healing responses

Macrophages play a crucial role in all stages of cutaneous wound healing responses and dysregulation of macrophage function can result in derailed wound repair. The phenotype of macrophages is influenced by the wound microenvironment and evolves during healing from a more pro-inflammatory (M1) profile in early stages, to a less inflammatory pro-healing (M2) phenotype in later stages of repair. The aim of the current study was to investigate the potential of exogenous administration of M2 macrophages to promote wound healing in an experimental mouse model of cutaneous injury. Bone marrow derived macrophages were stimulated in-vitro with IL-4 or IL-10 to obtain two different subsets of M2-polarized cells, M2a or M2c respectively. Polarized macrophages were injected into full-thickness excisional skin wounds of either C57BL/6 or diabetic db/db mice. Control groups were injected with non-polarized (M0) macrophages or saline. Our data indicate that despite M2 macrophages exhibit an anti-inflammatory phenotype in-vitro, they do not improve wound closure in wild type mice while they delay healing in diabetic mice. Examination of wounds on day 15 post-injury indicated delayed re-epithelialization and persistence of neutrophils in M2 macrophage treated diabetic wounds. Therefore, topical application of ex-vivo generated M2 macrophages is not beneficial and contraindicated for cell therapy of skin wounds.

Wound healing reaction: A switch from gestation to senescence

The repair of wounded tissue during postnatal life could be associated with the upregulation of some functions characteristic of the initial phases of embryonic development. The focusing of these recapitulated systemic functions in the interstitial space of the injured tissue is established through a heterogeneous endothelial barrier which has excretory-secretory abilities which in turn, would induce a gastrulation-like process. The repair of adult tissues using upregulated embryonic mechanisms could explain the universality of the inflammatory response against injury, regardless of its etiology. However, the early activation after the injury of embryonic mechanisms does not always guarantee tissue regeneration since their long-term execution is mediated by the host organism.

Macrophage plasticity in skeletal muscle repair

Macrophages are one of the first barriers of host defence against pathogens. Beyond their role in innate immunity, macrophages play increasingly defined roles in orchestrating the healing of various injured tissues. Perturbations of macrophage function and/or activation may result in impaired regeneration and fibrosis deposition as described in several chronic pathological diseases. Heterogeneity and plasticity have been demonstrated to be hallmarks of macrophages. In response to environmental cues they display a proinflammatory (M1) or an alternative anti-inflammatory (M2) phenotype. A lot of evidence demonstrated that after acute injury M1 macrophages infiltrate early to promote the clearance of necrotic debris, whereas M2 macrophages appear later to sustain tissue healing. Whether the sequential presence of two different macrophage populations results from a dynamic shift in macrophage polarization or from the recruitment of new circulating monocytes is a subject of ongoing debate. In this paper, we discuss the current available information about the role that different phenotypes of macrophages plays after injury and during the remodelling phase in different tissue types, with particular attention to the skeletal muscle.

A novel, visible light-induced, rapidly cross-linkable gelatin scaffold for osteochondral tissue engineering

Osteochondral injuries remain difficult to repair. We developed a novel photo-cross-linkable furfurylamine-conjugated gelatin (gelatin-FA). Gelatin-FA was rapidly cross-linked by visible light with Rose Bengal, a light sensitizer, and was kept gelled for 3 weeks submerged in saline at 37°C. When bone marrow-derived stromal cells (BMSCs) were suspended in gelatin-FA with 0.05% Rose Bengal, approximately 87% of the cells were viable in the hydrogel at 24 h after photo-cross-linking, and the chondrogenic differentiation of BMSCs was maintained for up to 3 weeks. BMP4 fusion protein with a collagen binding domain (CBD) was retained in the hydrogels at higher levels than unmodified BMP4. Gelatin-FA was subsequently employed as a scaffold for BMSCs and CBD-BMP4 in a rabbit osteochondral defect model. In both cases, the defect was repaired with articular cartilage-like tissue and regenerated subchondral bone. This novel, photo-cross-linkable gelatin appears to be a promising scaffold for the treatment of osteochondral injury.

The adenosine-dependent angiogenic switch of macrophages to an M2-like phenotype is independent of interleukin-4 receptor alpha (IL-4Rα) signaling

Murine macrophages are activated by interferon-γ (IFN-γ) and/or Toll-like receptor (TLR) agonists such as bacterial endotoxin (lipopolysaccharide [LPS]) to express an inflammatory (M1) phenotype characterized by the expression of nitric oxide synthase-2 (iNOS) and inflammatory cytokines such as tumor necrosis factor-α (TNF-α) and interleukin (IL)-12. In contrast, Th2 cytokines IL-4 and IL-13 activate macrophages by inducing the expression of arginase-1 and the anti-inflammatory cytokine IL-10 in an IL-4 receptor-α (IL-4Rα)-dependent manner. Macrophages activated in this way are designated as "alternatively activated" (M2a) macrophages. We have shown previously that adenosine A2A receptor (A(2A)R) agonists act synergistically with TLR2, TLR4, TLR7, and TLR9 agonists to switch macrophages into an "M2-like" phenotype that we have termed "M2d." Adenosine signaling suppresses the TLR-dependent expression of TNF-α, IL-12, IFN-γ, and several other inflammatory cytokines by macrophages and induces the expression of vascular endothelial growth factor (VEGF) and IL-10. We show here using mice lacking a functional IL-4Rα gene (IL-4Rα(-/-) mice) that this adenosine-mediated switch does not require IL-4Rα-dependent signaling. M2d macrophages express high levels of VEGF, IL-10, and iNOS, low levels of TNF-α and IL-12, and mildly elevated levels of arginase-1. In contrast, M2d macrophages do not express Ym1, Fizz1 (RELM-α), or CD206 at levels greater than those induced by LPS, and dectin-1 expression is suppressed. The use of these markers in vivo to identify "M2" macrophages thus provides an incomplete picture of macrophage functional status and should be viewed with caution.

The role of the extracellular matrix components in cutaneous wound healing

Wound healing is the physiologic response to tissue trauma proceeding as a complex pathway of biochemical reactions and cellular events, secreted growth factors, and cytokines. Extracellular matrix constituents are essential components of the wound repair phenomenon. Firstly, they create a provisional matrix, providing a structural integrity of matrix during each stage of healing process. Secondly, matrix molecules regulate cellular functions, mediate the cell-cell and cell-matrix interactions, and serve as a reservoir and modulator of cytokines and growth factors' action. Currently known mechanisms, by which extracellular matrix components modulate each stage of the process of soft tissue remodeling after injury, have been discussed.

Improved repair of dermal wounds in mice lacking microRNA-155

Wound healing is a well-regulated but complex process that involves haemostasis, inflammation, proliferation and maturation. Recent reports suggest that microRNAs (miRs) play important roles in dermal wound healing. In fact, miR deregulation has been linked with impaired wound repair. miR-155 has been shown to be induced by inflammatory mediators and plays a central regulatory role in immune responses. We have investigated the potential role of miR-155 in wound healing. By creating punch wounds in the skin of mice, we found an increased expression of miR-155 in wound tissue when compared with healthy skin. Interestingly, analysis of wounds of mice lacking the expression of miR-155 (miR-155(-/-) ) revealed an increased wound closure when compared with wild-type animals. Also, the accelerated wound closing correlated with elevated numbers of macrophages in wounded tissue. Gene expression analysis of wounds tissue and macrophages isolated from miR-155(-/-) mice that were treated with interleukin-4 demonstrated an increased expression of miR-155 targets (BCL6, RhoA and SHIP1) as well as, the finding in inflammatory zone-1 (FIZZ1) gene, when compared with WT mice. Moreover, the up-regulated levels of FIZZ1 in the wound tissue of miR-155(-/-) mice correlated with an increased deposition of type-1 collagens, a phenomenon known to be beneficial in wound closure. Our data indicate that the absence of miR-155 has beneficial effects in the wound healing process.

Macrophage activation and skeletal muscle healing following traumatic injury

Following injury to different tissues, macrophages can contribute to both regenerative and fibrotic healing. These seemingly contradictory roles of macrophages may be related to the markedly different phenotypes that macrophages can assume upon exposure to different stimuli. We hypothesized that fibrotic healing after traumatic muscle injury would be dominated by a pro-fibrotic M2a macrophage phenotype, with M1 activation limited to the very early stages of repair. We found that macrophages accumulated in lacerated mouse muscle for at least 21 days, accompanied by limited myofibre regeneration and persistent collagen deposition. However, muscle macrophages did not exhibit either of the canonical M1 or M2a phenotypes, but instead up-regulated both M1- and M2a-associated genes early after injury, followed by down-regulation of most markers examined. Particularly, IL-10 mRNA and protein were markedly elevated in macrophages from 3-day injured muscle. Additionally, though flow cytometry identified distinct subpopulations of macrophages based on high or low expression of TNFα, these subpopulations did not clearly correspond to M1 or M2a phenotypes. Importantly, cell therapy with exogenous M1 macrophages but not non-activated macrophages reduced fibrosis and enhanced muscle fibre regeneration in lacerated muscles. These data indicate that manipulation of macrophage function has potential to improve healing following traumatic injury.

In vitro and in vivo repair activities of undifferentiated and classically and alternatively activated macrophages

OBJECTIVE

Macrophages play a critical role in wound repair. However, the specific role of the different macrophage subtypes in wound repair remains incompletely understood. The aim of this study was to compare the wound repair activities of undifferentiated macrophages (M0), classically activated macrophages (M1) and alternatively activated (M2) macrophages.

METHODS

The macrophage repair activities of intestinal wounds were evaluated using in vitro and in vivo models.

RESULTS

All three macrophage subtypes enhanced wound closure in vitro, with the M2 macrophages demonstrating greater repair activities than the M0 and M1 macrophages. Injection of M0 and M2 macrophages into mice with experimental dextran sodium sulfate-induced colitis significantly enhanced ulcer repair when compared to control mice. In contrast, injection of M1 macrophages did not affect ulcer repair.

CONCLUSIONS

These results underscore the wound repair capacity of different macrophage subsets. Notably, wound repair activity is not restricted to M2 macrophages, as the current literature suggests.

The monocyte to macrophage transition in the murine sterile wound

The origin of wound repair macrophages is incompletely defined and was examined here in sterile wounds using the subcutaneous polyvinyl alcohol sponge implantation model in mice. Phenotypic analysis identified F4/80⁺Ly6C^hiCD64⁺MerTK^– monocytes and F4/80⁺Ly6C^lowCD64⁺MerTK⁺ macrophages in the wound. Circulating monocytes were the precursors of inflammatory Ly6C^hi wound monocytes. Ly6C^lowMerTK⁺ macrophages appeared later, expressed CD206, CD11c, and MHC class II, produced cytokines consistent with repair function, and lacked a gene expression profile compatible with mesenchymal transition or fibroblastic transdifferentiation. Data also demonstrated that Ly6C^hi wound cells were precursors of Ly6C^low macrophages, although monocytes did not undergo rapid maturation but rather persisted in the wound as Ly6C^hiMerTK^– cells. MerTK-deficient mice were examined to determine whether MerTK-dependent signals from apoptotic cells regulated the maturation of wound macrophages. MerTK-deficient mice had day 14 cell compositions that resembled more immature wounds, with a smaller proportion of F4/80⁺ cells and higher frequencies of Ly6G⁺ neutrophils and Ly6C^hi monocytes. The cytokine profile and number of apoptotic cells in day 14 wounds of MerTK-deficient mice was unaffected despite the alterations in cell composition. Overall, these studies identified a differentiation pathway in response to sterile inflammation in which monocytes recruited from the circulation acquire proinflammatory function, persist in the wound, and mature into repair macrophages.

Computational approach to characterize causative factors and molecular indicators of chronic wound inflammation

Chronic inflammation is rapidly becoming recognized as a key contributor to numerous pathologies. Despite detailed investigations, understanding of the molecular mechanisms regulating inflammation is incomplete. Knowledge of such critical regulatory processes and informative indicators of chronic inflammation is necessary for efficacious therapeutic interventions and diagnostic support to clinicians. We used a computational modeling approach to elucidate the critical factors responsible for chronic inflammation and to identify robust molecular indicators of chronic inflammatory conditions. Our kinetic model successfully captured experimentally observed cell and cytokine dynamics for both acute and chronic inflammatory responses. Using sensitivity analysis, we identified macrophage influx and efflux rate modulation as the strongest inducing factor of chronic inflammation for a wide range of scenarios. Moreover, our model predicted that, among all major inflammatory mediators, IL-6, TGF-β, and PDGF may generally be considered the most sensitive and robust indicators of chronic inflammation, which is supported by existing, but limited, experimental evidence.

A potential wound healing-promoting peptide from frog skin

Cutaneous wound healing is a dynamic, complex, and well-organized process that requires the orchestration of many different cell types and cellular processes. Transforming growth factor β1 is an important factor that plays a key role during wound healing. Amphibian skin has been proven to possess excellent wound healing ability, whilst no bioactive substrate related to it has ever been identified. Here, a potential wound healing-promoting peptide (AH90, ATAWDFGPHGLLPIRPIRIRPLCG) was identified from the frog skin of Odorrana grahami. It showed potential wound healing-promoting activity in a murine model with full thickness dermal wound. AH90 promoted release of transforming growth factor β1 through activation of nuclear factor-κB and c-Jun NH2-terminal kinase mitogen-activated protein kinases signaling pathways, while inhibitors of nuclear factor-κB and c-Jun NH2-terminal kinase inhibited the process. In addition, the effects of AH90 on Smads family proteins, key regulators in transforming growth factor β1 signaling pathways, could also be inhibited by transforming growth factor β1 antibody. Altogether, this indicated that AH90 promoted wound healing by inducing the release of transforming growth factor β1. This current study may facilitate the understanding of effective factors involved in the wound repair of amphibians and the underlying mechanisms as well. Considering its favorable traits as a small peptide that greatly promoting generation of endogenous wound healing agents (transforming growth factor β1) without mitogenic effects, AH90 might be an excellent template for the future development of novel wound-healing agents.

Resolvins, specialized proresolving lipid mediators, and their potential roles in metabolic diseases

Inflammation is associated with the development of diseases characterized by altered nutrient metabolism. Although an acute inflammatory response is host-protective and normally self-limited, chronic low-grade inflammation associated with metabolic diseases is sustained and detrimental. The resolution of inflammation involves the termination of neutrophil recruitment, counterregulation of proinflammatory mediators, stimulation of macrophage-mediated clearance, and tissue remodeling. Specialized proresolving lipid mediators (SPMs)-resolvins, protectins, and maresins-are novel autacoids that resolve inflammation, protect organs, and stimulate tissue regeneration. Here, we review evidence that the failure of resolution programs contributes to metabolic diseases and that SPMs may play pivotal roles in their resolution.

Macrophages promote epithelial repair through hepatocyte growth factor secretion

Macrophages play a critical role in intestinal wound repair. However, the mechanisms of macrophage-assisted wound repair remain poorly understood. We aimed to characterize more clearly the repair activities of murine and human macrophages. Murine macrophages were differentiated from bone marrow cells and human macrophages from monocytes isolated from peripheral blood mononuclear cells of healthy donors (HD) or Crohn's disease (CD) patients or isolated from the intestinal mucosa of HD. In-vitro models were used to study the repair activities of macrophages. We found that murine and human macrophages were both able to promote epithelial repair in vitro. This function was mainly cell contact-independent and relied upon the production of soluble factors such as the hepatocyte growth factor (HGF). Indeed, HGF-silenced macrophages were less capable of promoting epithelial repair than control macrophages. Remarkably, macrophages from CD patients produced less HGF than their HD counterparts (HGF level: 84 ± 27 pg/mg of protein and 45 ± 34 pg/mg of protein, respectively, for HD and CD macrophages, P < 0·009) and were deficient in promoting epithelial repair (repairing activity: 90·1 ± 4·6 and 75·8 ± 8·3, respectively, for HD and CD macrophages, P < 0·0005). In conclusion, we provide evidence that macrophages act on wounded epithelial cells to promote epithelial repair through the secretion of HGF. The deficiency of CD macrophages to secrete HGF and to promote epithelial repair might contribute to the impaired intestinal mucosal healing in CD patients.

Microdialysis sampling techniques applied to studies of the foreign body reaction

Implanted materials including drug delivery devices and chemical sensors undergo what is termed the foreign body reaction (FBR). Depending on the device and its intended application, the FBR can have differing consequences. An extensive scientific research effort has been devoted to elucidating the cellular and molecular mechanisms that drive the FBR. Important, yet relatively unexplored, research includes the localized tissue biochemistry and the chemical signaling events that occur throughout the FBR. This review provides an overview of the mechanisms of the FBR, describes how the FBR affects different implanted devices, and illustrates the role that microdialysis sampling can play in further elucidating the chemical communication processes that drive FBR outcomes.

The effect of polymer degradation time on functional outcomes of temporary elastic patch support in ischemic cardiomyopathy

Biodegradable polyurethane patches have been applied as temporary mechanical supports to positively alter the remodeling and functional loss following myocardial infarction. How long such materials need to remain in place is unclear. Our objective was to compare the efficacy of porous onlay support patches made from one of three types of biodegradable polyurethane with relatively fast (poly(ester urethane)urea; PEUU), moderate (poly(ester carbonate urethane)urea; PECUU), and slow (poly(carbonate urethane)urea; PCUU) degradation rates in a rat model of ischemic cardiomyopathy. Microporous PEUU, PECUU or PCUU (n = 10 each) patches were implanted over left ventricular lesions 2 wk following myocardial infarction in rat hearts. Infarcted rats without patching and age-matched healthy rats (n = 10 each) were controls. Echocardiography was performed every 4 wk up to 16 wk, at which time hemodynamic and histological assessments were performed. The end-diastolic area for the PEUU group at 12 and 16 wk was significantly larger than for the PECUU or PCUU groups. Histological analysis demonstrated greater vascular density in the infarct region for the PECUU or PCUU versus PEUU group at 16 wk. Improved left ventricular contractility and diastolic performance in the PECUU group was observed at 16 wk compared to infarction controls. The results indicate that the degradation rate of an applied elastic patch influences the functional benefits associated patch placement, with a moderately slow degrading PECUU patch providing improved outcomes.

Phenotypic transitions of macrophages orchestrate tissue repair

Macrophages are essential for the efficient healing of numerous tissues, and they contribute to impaired healing and fibrosis. Tissue repair proceeds through overlapping phases of inflammation, proliferation, and remodeling, and macrophages are present throughout this progression. Macrophages exhibit transitions in phenotype and function as tissue repair progresses, although the precise factors regulating these transitions remain poorly defined. In efficiently healing injuries, macrophages present during a given stage of repair appear to orchestrate transition into the next phase and, in turn, can promote debridement of the injury site, cell proliferation and angiogenesis, collagen deposition, and matrix remodeling. However, dysregulated macrophage function can contribute to failure to heal or fibrosis in several pathological situations. This review will address current knowledge of the origins and functions of macrophages during the progression of tissue repair, with emphasis on skin and skeletal muscle. Dysregulation of macrophages in disease states and therapies targeting macrophage activation to promote tissue repair are also discussed.

Identification of an adipogenic niche for adipose tissue remodeling and restoration

The regulatory events guiding progenitor activation and differentiation in adult white adipose tissue are largely unknown. We report that induction of brown adipogenesis by β3-adrenergic receptor (ADRB3) activation involves the death of white adipocytes and their removal by M2-polarized macrophages. Recruited macrophages express high levels of osteopontin (OPN), which attracts a subpopulation of PDGFRα+ progenitors expressing CD44, a receptor for OPN. Preadipocyte proliferation is highly targeted to sites of adipocyte clearance and occurs almost exclusively in the PDGFRα+ CD44+ subpopulation. Knockout of OPN prevents formation of crown-like structures by ADRB3 activation and the recruitment, proliferation, and differentiation of preadipocytes. The recruitment and differentiation of PDGFRα+ progenitors are also observed following physical injury, during matrix-induced neogenesis, and in response to high-fat feeding. Each of these conditions recruits macrophages having a unique polarization signature, which may explain the timing of progenitor activation and the fate of these cells in vivo.

Disclosure of the Culprits: Macrophages-Versatile Regulators of Wound Healing

SIGNIFICANCE

Macrophages are invariably present and tightly regulate all phases of adult wound healing, including inflammation, granulation tissue formation, and matrix deposition with the unavoidable outcome of scar formation. In response to environmental cues, macrophages mount a "classical" pro-inflammatory M1 activation as opposed to the "alternative" M2 phenotype, with wound macrophages having long been viewed as M2 macrophages.

RECENT ADVANCES

Recent studies rather point to large temporal and phenotypic variations of wound macrophages subsets. Therefore, a functional classification of macrophages according to wound-healing phases appears to better meet the in vivo complexity. In an ideal but simplistic scenario grossly reflecting normal wound healing, initial tissue injury induces inflammatory M1-like macrophages, which, upon engulfment of apoptotic neutrophils or in response to other inflammation dampening stimuli, switch toward anti-inflammatory M2-like macrophages and further toward growth factor-producing pro-fibrotic M2a-like macrophages. Although not yet documented for skin wounds, a subset of metalloproteinase-producing fibrolytic M2c-like macrophages may contribute to fibrosis resolution. Recent work identified a diversity of novel macrophage phenotypes associated with normal and pathologic wound healing, most of them ranging out of the M1/M2 paradigm. Iron-overloaded M1-like macrophages represent such a novel phenotypic subset driving the non-healing state of chronic venous leg ulcers.

CRITICAL ISSUES

Despite growing evidence that macrophage dysfunctions are, at least in part, responsible for pathologic wound healing, including nonhealing wounds and excessive scar formation, these are hardly specifically addressed even by modern therapeutic strategies.

FUTURE DIRECTIONS

If characterized in sufficient detail, distinct macrophage subsets and their impaired functions provide ideal targets for improving wound healing.

Small molecule screen yields inhibitors of Pseudomonas homoserine lactone-induced host responses

Pseudomonas aeruginosa infections are commonly associated with cystic fibrosis, pneumonias, neutropenia and burns. The P. aeruginosa quorum sensing molecule N-(3-oxo-dodecanoyl) homoserine lactone (C12) cause multiple deleterious host responses, including repression of NF-κB transcriptional activity and apoptosis. Inhibition of C12-mediated host responses is predicted to reduce P. aeruginosa virulence. We report here a novel, host-targeted approach for potential adjunctive anti-Pseudomonal therapy based on inhibition of C12-mediated host responses. A high-throughput screen was developed to identify C12 inhibitors that restore NF-κB activity in C12-treated, lipopolysaccharide (LPS)-stimulated cells. Triazolo[4,3-a]quinolines with nanomolar potency were identified as C12-inhibitors that restore NF-κB-dependent luciferase expression in LPS- and TNF-stimulated cell lines. In primary macrophages and fibroblasts, triazolo[4,3-a]quinolines inhibited C12 action to restore cytokine secretion in LPS-stimulated cells. Serendipitously, in the absence of an inflammatory stimulus, triazolo[4,3-a]quinolines prevented C12-mediated responses, including cytotoxicity, elevation of cytoplasmic calcium, and p38 MAPK phosphorylation. In vivo efficacy was demonstrated in a murine model of dermal inflammation involving intradermalC12 administration. The discovery of triazolo[4,3-a]quinolines provides a pharmacological tool to investigate C12-mediated host responses, and a potential host-targeted anti-Pseudomonal therapy.

Effects of systemic pretreatment with CpG oligodeoxynucleotides on skin wound healing in mice

Unmethylated CpG oligodeoxynucleotides (ODN) bind to the Toll-like receptor 9, thus stimulating the immune system. To study the effects of systemic pretreatment with CpG ODN on dermal regeneration, C57BL6/J Tyr mice were treated with CpG or control ODN 6 days prior to implantation of a dorsal skinfold chamber and skin wounding. Wound epithelialization was analyzed by planimetric microscopy. On day 18, wound tissues were taken for (immuno)histochemical staining. CpG ODN increased epithelialization compared with control ODN treatment. Histological analysis revealed reduced capillary density, reduced wound cellularity, and reduced numbers of infiltrating leukocytes, as well as reduced F4/80-positive macrophages, but increased numbers of RELM-α-positive M2 macrophages after CpG ODN treatment, reflecting a better quality of wound healing on day 18 compared with control ODN treatment. Reverse transcription-polymerase chain reaction analysis of Toll-like receptor 9 showed the receptor expression on both fibroblasts and keratinocytes. Fibroblasts showed an increase of migration upon increasing dosages of CpG and not control ODN, reaching ∼50% of the response of basic fibroblast growth factor-exposed cells. Keratinocytes dose-dependently responded to both CpG and control ODN up to values found in keratinocyte growth factor-exposed cells. In summary, CpG ODN support late tissue-remodeling processes that contribute to resolution of inflammation and solid wounds during skin regeneration.

Toll-like receptor 4 signaling regulates the acute local inflammatory response to injury and the fibrosis/neovascularization of sterile wounds

The role of Toll-like receptor 4 (TLR4) in the regulation of inflammation and fibrosis in sterile wounds was investigated in TLR4 signal-deficient (C3H/HeJ or TLR4(-/-) ) and control mice using the subcutaneously implanted polyvinyl alcohol sponge wound model. Total and differential wound cell counts 1, 3, and 7 days after injury did not differ between C3H/HeJ and C3H/HeOuJ animals. Blood monocytes from both strains expressed CCR2 equally. Day one wounds in C3H/HeJ mice contained fewer Gr-1(high) wound macrophages, CCL3, and CCL5, and more CCL17 than those in controls. The accumulation of CCL2, CX3CL1, tumor necrosis factor-α, interleukin (IL)-6, IL-10, IL-12, and interferon-γ in wound fluids was not TLR4 dependent. Wound macrophages from C3H/HeJ and C3H/HeOuJ mice expressed CCR4 and CCR5, but not CCR1 or CCR3. Wound macrophage recruitment was not altered in CCR5(-/-) mice or in C3H/HeOuJ animals injected with neutralizing anti-CCL3 and anti-CCL5 antibodies. Neutralization of the CCR4 ligand CCL17 in C3H/HeJ mice did not alter wound macrophage populations. There was a twofold increase in collagen content and number of neovessels in 21-day-old wounds in C3H/HeJ vs. C3H/HeOuJ mice. There were no differences between strains in the number of myofibroblasts in the wounds 7 or 21 days postwounding. The increased fibrosis and angiogenesis in wounds from /HeJ mice correlated with higher concentrations of transforming growth factor-β and fibroblast growth factor 2 in wound fluids from these animals. Wound fluids did not contain detectable lipopolysaccharide and did not induce IκBα degradation in J774.A1 macrophages. Results support a role for endogenous ligands of TLR4 in the regulation of inflammation and repair in sterile wounds.

Ceramide 1-phosphate mediates endothelial cell invasion via the annexin a2-p11 heterotetrameric protein complex

The bioactive sphingolipid, ceramide 1-phosphate (C-1-P), has been implicated as an extracellular chemotactic agent directing cellular migration in hematopoietic stem/progenitor cells and macrophages. However, interacting proteins that could mediate these actions of C-1-P have, thus far, eluded identification. We have now identified and characterized interactions between ceramide 1-phosphate and the annexin a2-p11 heterotetramer constituents. This C-1-P-receptor complex is capable of facilitating cellular invasion. Herein, we demonstrate in both coronary artery macrovascular endothelial cells and retinal microvascular endothelial cells that C-1-P induces invasion through an extracellular matrix barrier. By employing surface plasmon resonance, lipid-binding ELISA, and mass spectrometry technologies, we have demonstrated that the heterotetramer constituents bind to C-1-P. Although the annexin a2-p11 heterotetramer constituents do not bind the lipid C-1-P exclusively, other structurally similar lipids, such as phosphatidylserine, sphingosine 1-phosphate, and phosphatidic acid, could not elicit the potent chemotactic stimulation observed with C-1-P. Further, we show that siRNA-mediated knockdown of either annexin a2 or p11 protein significantly inhibits C-1-P-directed invasion, indicating that the heterotetrameric complex is required for C-1-P-mediated chemotaxis. These results imply that extracellular C-1-P, acting through the extracellular annexin a2-p11 heterotetrameric protein, can mediate vascular endothelial cell invasion.

Complement-triggered pathways orchestrate regenerative responses throughout phylogenesis

Adult tissue plasticity, cell reprogramming, and organ regeneration are major challenges in the field of modern regenerative medicine. Devising strategies to increase the regenerative capacity of tissues holds great promise for dealing with donor organ shortages and low transplantation outcomes and also provides essential impetus to tissue bioengineering approaches for organ repair and replacement. The inherent ability of cells to reprogram their fate by switching into an embryonic-like, pluripotent progenitor state is an evolutionary vestige that in mammals has been retained mostly in fetal tissues and persists only in a few organs of the adult body. Tissue regeneration reflects the capacity of terminally differentiated cells to re-enter the cell cycle and proliferate in response to acute injury or environmental stress signals. In lower vertebrates, this regenerative capacity extends to several organs and remarkably culminates in precise tissue patterning, through cellular transdifferentiation and complex morphogenetic processes that can faithfully reconstruct entire body parts. Many lessons have been learned from robust regeneration models in amphibians such as the newt and axolotl. However, the dynamic interactions between the regenerating tissue, the surrounding stroma, and the host immune response, as it adapts to the actively proliferating tissue, remain ill-defined. The regenerating zone, through a sequence of distinct molecular events, adopts phenotypic plasticity and undergoes rigorous tissue remodeling that, in turn, evokes a significant inflammatory response. Complement is a primordial sentinel of the innate immune response that engages in multiple inflammatory cascades as it becomes activated during tissue injury and remodeling. In this respect, complement proteins have been implicated in tissue and organ regeneration in both urodeles and mammals. Distinct complement-triggered pathways have been shown to modulate critical responses that promote tissue reprogramming, pattern formation, and regeneration across phylogenesis. This article will discuss the mechanistic insights underlying the crosstalk of complement with cytokine and growth factor signaling pathways that drive tissue regeneration and will provide a unified conceptual framework for considering complement modulation as a novel target for regenerative therapeutics.

Leukotriene B4 type-1 receptor signaling promotes liver repair after hepatic ischemia/reperfusion injury through the enhancement of macrophage recruitment

Recruited macrophages play a critical role in liver repair after acute liver injury. Leukotriene B4 (LTB4) is a potent chemoattractant for macrophages. In this study, we investigated the role of LTB4 receptor type 1 (BLT1) in liver repair during hepatic ischemia/reperfusion (I/R) injury. BLT1-knockout mice (BLT1(-/-)) or their wild-type counterparts (WT) were subjected to partial hepatic I/R. Compared with WT, BLT1(-/-) exhibited delayed liver repair and hepatocyte proliferation accompanied by a 70% reduction in the recruitment of macrophages and a 70-80% attenuation in hepatic expression of epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), and VEGF receptor 1 (VEGFR1). Disruption of BLT1 signaling also reduced the expression of EGF by 67% on recruited macrophages expressing VEGFR1 in the injured liver. Treatment of WT mice with an EGF-neutralizing antibody delayed liver repair and reduced macrophage recruitment, compared with control immunoglobulin G (IgG). BLT1 signaling enhanced the expression of VEGF, VEGFR1, and EGF in isolated peritoneal macrophages in vitro. These results indicate that BLT1 signaling plays a role in liver repair after hepatic I/R through enhanced expression of EGF in recruited macrophages and that the development of a specific agonist for BLT1 could be useful for liver recovery from acute liver injury.

Regulatory macrophages as therapeutic targets and therapeutic agents in solid organ transplantation

PURPOSE OF REVIEW

This review aims to provide a basic introduction to human macrophage biology and an appreciation of the diverse roles played by macrophage subsets in allograft damage and repair. Current and future strategies for therapeutically manipulating macrophage behaviour are discussed.

RECENT FINDINGS

Macrophages are extremely versatile effector cells that exert both immunostimulatory and immunosuppressive effects. This adaptability cannot be explained by differentiation into committed sublineages, but instead reflects the ability of macrophages to rapidly transition between states of functional polarisation. Consequently, categorisation of macrophage subpopulations is not straightforward and this, in turn, creates difficulties in studying their pathophysiology. Nevertheless, particular macrophage subpopulations have been implicated in exacerbating or attenuating ischaemia-reperfusion injury, rejection reactions and allograft fibrosis. Three general strategies for therapeutically targeting macrophages can be envisaged, namely, depletional approaches, in-situ repolarisation towards a regulatory or tissue-reparative phenotype, and ex-vivo generation of regulatory macrophages (M reg) as a cell-based therapy.

SUMMARY

As critical determinants of the local and systemic immune response to solid organ allografts, macrophage subpopulations represent attractive therapeutic targets. Rapid progress is being made in the implementation of novel macrophage-targeted therapies, particularly in the use of ex-vivo-generated M regs as a cell-based medicinal product.