Leukocyte HMGB1 is required for vessel remodeling in regenerating muscles

Long-Term Controlled Growth Factor Release Using Layer-by-Layer Assembly for the Development of In Vivo Tissue-Engineered Blood Vessels

The development of a well-designed tissue-engineered blood vessel (TEBV) still remains a challenge. In recent years, approaches in which the host response to implanted biomaterials is used to generate vascular constructs within the patient's body have gained increasing interest. The delivery of growth factors to these in situ-engineered vascular grafts might enhance myofibroblast recruitment and the secretion of essential extracellular matrix proteins, thereby optimizing their functional properties. Layer-by-layer (LbL) coating has emerged as an innovative technology for the controlled delivery of growth factors in tissue engineering applications. In this study, we combined the use of surface-etched polymeric rods with LbL coatings to control the delivery of TGF-β1, PDGF-BB, and IGF-1 and steer the foreign body response toward the formation of a functional vascular graft. Results showed that the regenerated tissue is composed of elastin, glycosaminoglycans, and circumferentially oriented collagen fibers, without calcification or systemic spill of the released growth factors. Functional controlled delivery was observed, whereas myofibroblast-rich tissue capsules were formed with enhanced collagen and elastin syntheses using TGF-β1 and TGF-β1/PDGF-BB releasing rods, when compared to control rods that were solely surface-engineered by chloroform etching. By combining our optimized LbL method and surface-engineered rods in an in vivo bioreactor approach, we could regulate the fate and ECM composition of in situ-engineered vascular grafts to create a successful in vivo vascular tissue-engineered replacement.

Platelet HMGB1 in Platelet-Rich Plasma (PRP) promotes tendon wound healing

Platelet-rich plasma (PRP) is a widely used autologous treatment for tendon injuries in clinics. Platelets (PLTs) are a major source of high mobility group box1 (HMGB1) that is gaining attention as a chemoattractant that can recruit stem cells to the wound area to enhance healing of injured tissues; however, the contribution of PLT HMGB1 in wounded tendon healing remains unexplored. This study investigated the effect of PLT HMGB1 within PRP on tendon healing using PLT HMGB1 knockout (KO) and GFP mice. A window defect was created in the patellar tendons of both groups of mice, and wounds were treated with either saline, PRP isolated from PLT HMGB1-KO mice, or PRP isolated from GFP mice. Seven days post-treatment, animals were sacrificed and analyzed by gross inspection, histology, and immunostaining for characteristic signs of tendon healing and repair. Our results showed that in comparison to mice treated with PRP from PLT HMGB1-KO mice, wounds treated with PRP from GFP mice healed faster and exhibited a better organization in tendon structure. Mice treated with PRP from PLT HMGB1-KO mice produced tendon tissue with large premature wound areas and low cell densities. However, wounds of PLT HMGB1-KO mice showed better healing with PRP from HMGB1-KO mice compared to saline treatment. Moreover, wounds treated with PRP from GFP mice had increased extracellular HMGB1, decreased CD68, increased stem cell markers CD146 and CD73, and increased collagen III protein expression levels compared to those treated with PRP from PLT HMGB1-KO mice. Thus, PLT HMGB1 within PRP plays an important role in tendon wound healing by decreasing inflammation, increasing local HMGB1 levels, and recruiting stem cells to the wound area in the tendon. Our findings also suggest that the efficacy of PRP treatment for tendon injuries in clinics may depend on PLT HMGB1 within PRP preparations.

Exosomal HMGB1 derived from hypoxia-conditioned bone marrow mesenchymal stem cells increases angiogenesis via the JNK/HIF-1α pathway

Mesenchymal stem cells (MSCs) have been described to induce angiogenesis in various tissues and have been used for the development of novel cell‐based therapies. Increasing evidence suggests that MSCs execute their paracrine function via the secretion of exosomes, especially under hypoxic conditions. However, the mechanisms by which MSC‐derived exosomes secreted under hypoxia enhance angiogenesis still remain unclear. To study exosome physiology under hypoxic or normoxic conditions, we isolated exosomes from bone marrow mesenchymal stem cells (BMSCs). Furthermore, we detected the uptake of exosomes by human umbilical vein endothelial cells (HUVECs) by immunofluorescence staining. In addition, we determined the effects of exosomes on cell viability, migration and tube formation in HUVECs by Cell Counting Kit‐8, migration and tube formation assays, respectively. We examined the expression of key proteins related to exosome‐induced angiogenesis by BMSCs cultured under hypoxic conditions by western blot. Exosomes released by BMSCs cultured under hypoxic conditions enhanced cell proliferation, migration and angiogenesis of HUVECs. Hypoxia induced the expression of high mobility group box 1 protein (HMGB1) in BMSC‐derived exosomes, and silencing of HMGB1 abolished the angiogenic effect in HUVECs. Furthermore, exosomal HMGB1 activated the JNK signaling pathway and induced hypoxia‐inducible factor‐1α/vascular endothelial growth factor expression, consequently enhancing angiogenesis in HUVECs. Our data reveal that exosomal HMGB1 promotes angiogenesis via JNK/hypoxia‐inducible factor‐1α signaling. Therefore, BMSC exosomes derived under hypoxia may have potential for development of novel treatment strategies for angiogenesis‐related diseases.

Oxidation of HMGB1 Is a Dynamically Regulated Process in Physiological and Pathological Conditions

Acute inflammation is a complex biological response of tissues to harmful stimuli, such as pathogens or cell damage, and is essential for immune defense and proper healing. However, unresolved inflammation can lead to chronic disorders, including cancer and fibrosis. The High Mobility Group Box 1 (HMGB1) protein is a Damage-Associated Molecular Pattern (DAMP) molecule that orchestrates key events in inflammation by switching among mutually exclusive redox states. Fully reduced HMGB1 (frHMGB1) supports immune cell recruitment and tissue regeneration, while the isoform containing a disulphide bond (dsHMGB1) promotes secretion of inflammatory mediators by immune cells. Although it has been suggested that the tissue itself determines the redox state of the extracellular space and of released HMGB1, the dynamics of HMGB1 oxidation in health and disease are unknown. In the present work, we analyzed the expression of HMGB1 redox isoforms in different inflammatory conditions in skeletal muscle, from acute injury to muscle wasting, in tumor microenvironment, in spleen, and in liver after drug intoxication. Our results reveal that the redox modulation of HMGB1 is tissue-specific, with high expression of dsHMGB1 in normal spleen and liver and very low in muscle, where it appears after acute damage. Similarly, dsHMGB1 is highly expressed in the tumor microenvironment while it is absent in cachectic muscles from the same tumor-bearing mice. These findings emphasize the accurate and dynamic regulation of HMGB1 redox state, with the presence of dsHMGB1 tightly associated with leukocyte infiltration. Accordingly, we identified circulating, infiltrating, and resident leukocytes as reservoirs and transporters of dsHMGB1 in tissue and tumor microenvironment, demonstrating that the redox state of HMGB1 is controlled at both tissue and cell levels. Overall, our data point out that HMGB1 oxidation is a timely and spatially regulated process in physiological and pathological conditions. This precise modulation might play key roles to finetune inflammatory and regenerative processes.

Macrophages in inflammation, repair and regeneration

Tissue injury triggers a complex series of cellular responses, starting from inflammation activated by tissue and cell damage and proceeding to healing. By clearing cell debris, activating and resolving inflammation and promoting fibrosis, macrophages play key roles in most, if not all, phases of the response to injury. Recent studies of the mechanisms underlying the initial inflammation and later tissue regeneration and repair revealed that macrophages bridge these processes in part by supporting and activating stem/progenitor cells, clearing damaged tissue, remodeling extracellular matrix to prepare scaffolding for regeneration and promoting angiogenesis. However, macrophages also have a central role in the development of pathology induced by failed resolution (e.g. chronic inflammation) and excessive scarring. In this review, we summarize the activities of macrophages in inflammation and healing in response to acute injury in tissues with differing regenerative capacities. While macrophages lead similar processes in response to tissue injury in these tissues, their priorities and the consequences of their activities differ among tissues. Moreover, the magnitude, nature and duration of injury also greatly affect cellular responses and healing processes. In particular, continuous injury and/or failed resolution of inflammation leads to chronic ailments in which macrophage activities may become detrimental.

Melatonin decreases and cytokines increase in women with placental insufficiency

Purpose: To investigate the levels of melatonin, proinflammatory and anti-inflammatory cytokines in pregnant women with placental insufficiency (PI).Materials and Methods: The PI was manifested as the intrauterine growth restriction syndrome of fetus (IUGR) in the third pregnancy trimester. The control group consisted of 20 women with uncomplicated pregnancy in the same term. The blood concentrations of melatonin, proinflammatory cytokines, such as tumor necrotizing factor-α (TNF-α), interleukin-1-β (IL-1-β), interleukin-6 (IL-6), and anti-inflammatory cytokines, such as interleukin-4 (IL-4), and interleukin-10 (IL-10), were studied.Results: The concentration of melatonin was found to decrease significantly if pregnancy was complicated by intrauterine fetal growth retardation (study group -126.87 ± 14.87 pg/ml, control group -231.25 ± 21.56 pg/ml, p < .001). The levels of proinflammatory cytokines in the study group were significantly higher as compared with the control group (TNF-α: study group -10.05 ± 1.35 pg/ml, control group -5.60 ± 1.50 pg/ml, p < .05; IL-1-β: study group -14.67 ± 2.13 pg/ml, control group -3.96 ± 0.92 pg/ml, p < .001; IL-6: study group -6.91 ± 0.99 pg/ml, control group -2.69 ± 0.99 pg/ml, p < .05). The same is true about anti-inflammatory cytokines (IL-4: study group -5.97 ± 0.50 pg/ml, control group -3.74 ± 0.62 pg/ml, p < .05; IL-10: study group -11.40 ± 1.50 pg/ml, control group -4.70 ± 3.20 pg/ml, p < .001). A moderate negative correlation between melatonin and IL-1-β in the group with PI (r = -0.3776, p = .0097), a closed negative correlation between the same indexes in the control group (r = -0.6785, p = .001), and a moderate negative correlation between melatonin and TNF-α (r = -0.4908, p = .02) were found.Conclusions: The blood level of melatonin significantly decreases in case of placental insufficiency, manifested as intrauterine fetal growth restriction. Strengthening of the proinflammatory immunity shown as the increasing of the levels of TNF-α, IL-1-β, and IL-6 levels is also present in case of IUGR. Increase of the serum concentration of the anti-inflammatory cytokines, such as IL-4 and IL-10, in our opinion, can be explained by activation of compensatory mechanisms, which decrease the risk of premature labor.

Origin and Consequences of Necroinflammation

When cells undergo necrotic cell death in either physiological or pathophysiological settings in vivo, they release highly immunogenic intracellular molecules and organelles into the interstitium and thereby represent the strongest known trigger of the immune system. With our increasing understanding of necrosis as a regulated and genetically determined process (RN, regulated necrosis), necrosis and necroinflammation can be pharmacologically prevented. This review discusses our current knowledge about signaling pathways of necrotic cell death as the origin of necroinflammation. Multiple pathways of RN such as necroptosis, ferroptosis, and pyroptosis have been evolutionary conserved most likely because of their differences in immunogenicity. As the consequence of necrosis, however, all necrotic cells release damage associated molecular patterns (DAMPs) that have been extensively investigated over the last two decades. Analysis of necroinflammation allows characterizing specific signatures for each particular pathway of cell death. While all RN-pathways share the release of DAMPs in general, most of them actively regulate the immune system by the additional expression and/or maturation of either pro- or anti-inflammatory cytokines/chemokines. In addition, DAMPs have been demonstrated to modulate the process of regeneration. For the purpose of better understanding of necroinflammation, we introduce a novel classification of DAMPs in this review to help detect the relative contribution of each RN-pathway to certain physiological and pathophysiological conditions.

Exercise alarms the immune system: A HMGB1 perspective

The "danger" model of immunology states that the immune system detects and responds to danger by releasing endogenous molecules called alarmins. Strenuous exercise perturbs physiological homeostasis, increasing circulating alarmins to drive the inflammatory response. We describe a working concept of exercise-induced High Mobility Group Box (HMGB)1, a prototypical alarmin, in modulating immune responses and adaptations.

High-mobility group box 1 protein orchestrates responses to tissue damage via inflammation, innate and adaptive immunity, and tissue repair

A single protein, HMGB1, directs the triggering of inflammation, innate and adaptive immune responses, and tissue healing after damage. HMGB1 is the best characterized damage-associated molecular pattern (DAMP), proteins that are normally inside the cell but are released after cell death, and allow the immune system to distinguish between antigens that are dangerous or not. Notably, cells undergoing severe stress actively secrete HMGB1 via a dedicated secretion pathway: HMGB1 is relocated from the nucleus to the cytoplasm and then to secretory lysosomes or directly to the extracellular space. Extracellular HMGB1 (either released or secreted) triggers inflammation and adaptive immunological responses by switching among multiple oxidation states, which direct the mutually exclusive choices of different binding partners and receptors. Immune cells are first recruited to the damaged tissue and then activated; thereafter, HMGB1 supports tissue repair and healing, by coordinating the switch of macrophages to a tissue-healing phenotype, activation and proliferation of stem cells, and neoangiogenesis. Inevitably, HMGB1 also orchestrates the support of stressed but illegitimate tissues: tumors. Concomitantly, HMGB1 enhances the immunogenicity of mutated proteins in the tumor (neoantigens), promoting anti-tumor responses and immunological memory. Tweaking the activities of HMGB1 in inflammation, immune responses and tissue repair could bring large rewards in the therapy of multiple medical conditions, including cancer.

The Neutrophil's Choice: Phagocytose vs Make Neutrophil Extracellular Traps

Neutrophils recognize particulate substrates of microbial or endogenous origin and react by sequestering the cargo via phagocytosis or by releasing neutrophil extracellular traps (NETs) outside the cell, thus modifying and alerting the environment and bystander leukocytes. The signals that determine the choice between phagocytosis and the generation of NETs are still poorly characterized. Neutrophils that had phagocytosed bulky particulate substrates, such as apoptotic cells and activated platelets, appear to be “poised” in an unresponsive state. Environmental conditions, the metabolic, adhesive and activation state of the phagocyte, and the size of and signals associated with the tethered phagocytic cargo influence the choice of the neutrophils, prompting either phagocytic clearance or the generation of NETs. The choice is dichotomic and apparently irreversible. Defects in phagocytosis may foster the intravascular generation of NETs, thus promoting vascular inflammation and morbidities associated with diseases characterized by defective phagocytic clearance, such as systemic lupus erythematosus. There is a strong potential for novel treatments based on new knowledge of the events determining the inflammatory and pro-thrombotic function of inflammatory leukocytes.

The STAT3-IL-10-IL-6 Pathway Is a Novel Regulator of Macrophage Efferocytosis and Phenotypic Conversion in Sterile Liver Injury

The disposal of apoptotic bodies by professional phagocytes is crucial to effective inflammation resolution. Our ability to improve the disposal of apoptotic bodies by professional phagocytes is impaired by a limited understanding of the molecular mechanisms that regulate the engulfment and digestion of the efferocytic cargo. Macrophages are professional phagocytes necessary for liver inflammation, fibrosis, and resolution, switching their phenotype from proinflammatory to restorative. Using sterile liver injury models, we show that the STAT3-IL-10-IL-6 axis is a positive regulator of macrophage efferocytosis, survival, and phenotypic conversion, directly linking debris engulfment to tissue repair.

Endogenous DAMPs, Category I: Constitutively Expressed, Native Molecules (Cat. I DAMPs)

This chapter provides the reader with a collection of endogenous DAMPs in terms of constitutively expressed native molecules. The first class of this category refers to DAMPs, which are passively released from necrotic cells, and includes the most prominent subclasses of high mobility group box I and heat shock proteins. Further subclasses of DAMPs that are passively released from necrotic cells include S100 proteins, nucleic acids, histones, pro-forms of interleukin-1-family members, mitochondria-derived N-formylated peptides, F-actin, and heme. A particular subclass of these passively released DAMPs are molecules, which indirectly activate the inflammasome, including adenosine-5′-triphosphate, monosodium urate crystals, cholesterol crystals, some lipolytic species, and beta-amyloid. All these passively released DAMPs are characterized by their capability to promote necroinflammatory responses. The second class of this Category I refers to molecules, which are exposed on the surface of stressed cells. They include the subclass of phagocytosis-facilitating molecules such as calreticulin, as well as the subclass of MHC-I-related molecules such as MHC-I-related molecule A and B. These DAMPs are capable of inducing the activation of innate lymphoid cells and unconventional T cells. One of these DAMPs, the major histocompatibility complex I-related molecule A, is shown to act as a bona fide transplantation antigen. In sum, the endogenous constitutively expressed native molecules represent an impressive category of DAMPs with extraordinary properties, which play a critical role in the pathogenesis of many human diseases.

Increased expression of high mobility group box protein 1 and vascular endothelial growth factor in placenta previa

Placenta previa is often associated with preterm delivery, reduced birth weight, a higher frequency of placental accreta and postpartum haemorrhage, and increased likelihood of blood transfusion. The present study aimed to examine the expression of high mobility group box protein 1 (HMGB1) in the placenta of women with or without placenta previa. The study group consisted of placental tissues obtained from women with or without placenta previa. The expression levels of HMGB1 and vascular endothelial growth factor (VEGF) were evaluated in the placental tissues using reverse transcription‑quantitative polymerase chain reaction, western blotting and immunohistochemistry. The mRNA expression levels of HMGB1 and VEGF were significantly increased in the placenta previa group compared with in the normal group. In addition, the placenta previa group exhibited increased HMGB1 and VEGF staining in vascular endothelial cells and trophoblasts. There were no significant differences in the expression of HMGB1 or VEGF between groups with or without placenta accreta or postpartum haemorrhage. The present study hypothesised that the increased expression of HMGB1 in the placenta may be associated with the pathogenesis of placenta previa by regulating the expression of the proangiogenic factor VEGF.

Strategies to develop endogenous stem cell-recruiting bioactive materials for tissue repair and regeneration

A leading strategy in tissue engineering is the design of biomimetic scaffolds that stimulate the body's repair mechanisms through the recruitment of endogenous stem cells to sites of injury. Approaches that employ the use of chemoattractant gradients to guide tissue regeneration without external cell sources are favored over traditional cell-based therapies that have limited potential for clinical translation. Following this concept, bioactive scaffolds can be engineered to provide a temporally and spatially controlled release of biological cues, with the possibility to mimic the complex signaling patterns of endogenous tissue regeneration. Another effective way to regulate stem cell activity is to leverage the inherent chemotactic properties of extracellular matrix (ECM)-based materials to build versatile cell-instructive platforms. This review introduces the concept of endogenous stem cell recruitment, and provides a comprehensive overview of the strategies available to achieve effective cardiovascular and bone tissue regeneration.

High-mobility group box-1 contributes tumor angiogenesis under interleukin-8 mediation during gastric cancer progression

Many soluble factors are involved in tumor angiogenesis. Thus, it is valuable to identify novel soluble factors for effective control of tumor angiogenesis in gastric cancer (GC). We investigated the role of extracellular high‐mobility group box‐1 (HMGB1) and its associated soluble factors in the tumor angiogenesis of GC. Clinically, we measured serum levels of HMGB1 and GC‐associated cytokines/chemokines using GC serum samples (n = 120), and calculated microvessel density (MVD) by CD34 immunostaining using human GC tissues (n = 27). Then we analyzed the correlation of serum HMGB1 levels with MVD or that with cytokine/chemokine levels by linear regression. As in vitro angiogenesis assay for HMGB1, HUVEC migration and capillary tube formation assay were carried out using different histological types of human GC cells (N87 and KATOIII). CD34‐positive microvessels were detected from early GC, but MVD increased according to GC stages, and were closely correlated with serum HMGB1 levels (R = 0.608, P = 0.01). The HUVECs cultured in conditioned media derived from rhHMGB1‐treated or HMGB1‐TF GC cells showed remarkably enhanced migration and tube formation activities. These effects were abrogated by anti‐HMGB1 antibody or HMGB1 siRNA in both N87 and KATOIII cells (all P < 0.05). Among tested cytokines/chemokines, interleukin‐8 (IL‐8) was the most remarkable cytokine correlated with serum HMGB1 (P < 0.001), and enhanced HUVEC migration and tube formation activities by rhHMGB1 or HMGB1‐TF were significantly reversed by IL‐8 inhibition. These results indicate overexpressed HMGB1 contributes to tumor angiogenesis through IL‐8 mediation, and combined targeting of HMGB1 and IL‐8 can control tumor angiogenesis in GC.

Autologous serum supplement favours in vitro regenerative paracrine factors synthesis

OBJECTIVES

Foetal bovine serum (FBS) is often the serum supplement of choice for in vitro human cell culture. This study compares the effect of FBS and autologous human serum (AuHS) supplement in human peripheral blood mononuclear cell (PBMC) culture to prepare secretome.

MATERIALS AND METHODS

The PBMC (n = 7) were cultured either in RPMI-1640 containing L-glutamine and 50 units/ml Penicillin-Streptomycin (BM) or in BM with either AuHS or FBS. Viability, proliferation and differentiation of PBMC were evaluated. Paracrine factors present in the secretomes (n = 6) were analysed using ProcartaPlex Human Cytokine panel (17 plex). Ingenuity Pathway Analysis (IPA) was performed to predict activation or inhibition of biological functions related to tissue regeneration.

RESULTS

The viability of PBMC that were cultured with FBS supplement was significantly reduced at 96 h compared to those at 0 and 24 h (P < .05). While the reduction of the viability of PBMC that were cultured with AuHS supplement was not significantly different compared to those at 0 and 24 h. The FBS secretomes prepared at 24 h was found to contain significantly higher amount of EGF (P < .05) compared to that in AuHS or BM secretome. The AuHS secretomes contained significantly higher amount of HGF at 24 (P < .05) and 96 h (P < .01), and VEGF-A at 24 h (P < .05) compared to those in the FBS secretomes. SDF-1 was not detected in the FBS secretomes prepared at either 24 or 96 hours. Double immunocytochemical staining revealed a marked increase in co-localization of SDF-1 and its receptor in PBMC that were cultured with AuHS supplement compared to that cultured with FBS supplement.

CONCLUSION

In secretome preparation, AuHS supplement favours synthesis of paracrine factors that are needed for regenerative therapy.

Skeletal muscle inflammation and atrophy in heart failure

Heart failure represents a systemic disease with profound effects on multiple peripheral tissues including skeletal muscle. Within the context of heart failure, perturbations in skeletal muscle physiology, structure, and function strongly contribute to exercise intolerance and the morbidity of this devastating disease. There is growing evidence that chronic heart failure imparts specific pathological changes within skeletal muscle beds resulting in muscle dysfunction and tissue atrophy. Mechanistically, systemic and local inflammatory responses drive critical aspects of this pathology. In this review, we will discuss pathological mechanisms that drive skeletal muscle inflammation and highlight emerging roles for distinct innate immune subsets that reside within damage muscle tissue focusing on the recently described embryonic and monocyte-derived macrophage lineages. Within this context, we will discuss how immune mechanisms can be differentially targeted to stimulate skeletal muscle inflammation, catabolism, fiber atrophy, and regeneration.

The clearance of cell remnants and the regeneration of the injured muscle depend on soluble pattern recognition receptor PTX3

OBJECTIVE

The signals causing the resolution of muscle inflammation are only partially characterized. The long pentraxin PTX3, which modulates leukocyte recruitment and activation, could contribute.

METHODS

We analysed the expression of ptx3 after muscle injury and verified whether hematopoietic precursors are a source of the protein. The kinetics of regeneration and leukocytes infiltration, the accumulation of cell remnants and anti-histidyl-t-RNA synthetase autoantibodies were compared in wild-type and ptx3-deficient mice.

RESULTS

Ptx3 expression was up-regulated three-five days after injury and restricted to the extracellular matrix. Cellular debris and leukocytes persisted in the muscle of ptx3-deficient mice for a long time after wild-type animals had healed. ptx3-deficient macrophages expressed receptors involved in apoptotic cell clearance and engulfed dead cells in vitro. Accumulation of cell debris in a pro-inflammatory microenvironment was not sufficient to elicit autoantibodies.

CONCLUSION

PTX3 generated in response to muscle injury prompts the clearance of debris and the termination of the inflammatory response.

The Effect of High-Mobility Group Box 1 in Rat Steatotic and Nonsteatotic Liver Transplantation From Donors After Brain Death

High-mobility group box 1 (HMGB1) has been described in different inflammatory disorders, and the deleterious effects of brain death (BD) may counteract the protection conferred by ischemic preconditioning (IP), the only surgical strategy that is being applied in clinical liver transplantation. Our study examined how HMGB1 may affect preconditioned and unpreconditioned steatotic and nonsteatotic liver grafts from donors after BD (DBDs) for transplantation. HMGB1 was pharmacologically modulated in liver grafts from DBDs, and HMGB1-underlying mechanisms were characterized. We found that BD decreased HMGB1 in preconditioned and unpreconditioned livers and was associated with inflammation and damage. Exogenous HMGB1 in DBDs activates phosphoinositide-3-kinase and Akt and reduces hepatic inflammation and damage, increasing the survival of recipients. Combination of IP and exogenous HMGB1 shows additional benefits compared with HMGB1 alone. This study provides new mechanistic insights into the pathophysiology of BD-derived liver graft damage and contributes to the development of novel and efficient strategies to ultimately improve liver graft quality.

Key Role of DAMP in Inflammation, Cancer, and Tissue Repair

PURPOSE

This review aimed to take stock of the current status of research on damage-associated molecular pattern (DAMP) protein. We discuss the Janus-faced role of DAMP molecules in inflammation, cancer, and tissue repair. The high-mobility group box (HMGB)-1 and adenosine triphosphate proteins are well-known DAMP molecules and have been primarily associated with inflammation. However, as we shall see, recent data have linked these molecules to tissue repair. HMGB1 is associated with cancer-related inflammation. It activates nuclear factor kB, which is involved in cancer regulation via its receptor for advanced glycation end-products (RAGE), Toll-like receptors 2 and 4. Proinflammatory activity and tissue repair may lead to pharmacologic intervention, by blocking DAMP RAGE and Toll like receptor 2 and 4 role in inflammation and by increasing their concentration in tissue repair, respectively.

METHODS

We conducted a MEDLINE search for articles pertaining to the various issues related to DAMP, and we discuss the most relevant articles especially (ie, not only those published in journals with a higher impact factor).

FINDINGS

A cluster of remarkable articles on DAMP have appeared in the literature in recent years. Regarding inflammation, several strategies have been proposed to target HMGB1, from antibodies to recombinant box A, which interacts with RAGE, competing with the full molecule. In tissue repair, it was reported that the overexpression of HMGB1 or the administration of exogenous HMGB1 significantly increased the number of vessels and promoted recovery in skin-wound, ischemic injury.

IMPLICATIONS

Due to the bivalent nature of DAMP, it is often difficult to explain the relative role of DAMP in inflammation versus its role in tissue repair. However, this point is crucial as DAMP-related treatments move into clinical practice.

Vascular Remodelling and Mesenchymal Transition in Systemic Sclerosis

Fibrosis of the skin and of internal organs, autoimmunity, and vascular inflammation are hallmarks of Systemic Sclerosis (SSc). The injury and activation of endothelial cells, with hyperplasia of the intima and eventual obliteration of the vascular lumen, are early features of SSc. Reduced capillary blood flow coupled with deficient angiogenesis leads to chronic hypoxia and tissue ischemia, enforcing a positive feed-forward loop sustaining vascular remodelling, further exacerbated by extracellular matrix accumulation due to fibrosis. Despite numerous developments and a growing number of controlled clinical trials no treatment has been shown so far to alter SSc natural history, outlining the need of further investigation in the molecular pathways involved in the pathogenesis of the disease. We review some processes potentially involved in SSc vasculopathy, with attention to the possible effect of sustained vascular inflammation on the plasticity of vascular cells. Specifically we focus on mesenchymal transition, a key phenomenon in the cardiac and vascular development as well as in the remodelling of injured vessels. Recent work supports the role of transforming growth factor-beta, Wnt, and Notch signaling in these processes. Importantly, endothelial-mesenchymal transition may be reversible, possibly offering novel cues for treatment.

Cell death, clearance and immunity in the skeletal muscle

The skeletal muscle is an immunologically unique tissue. Leukocytes, virtually absent in physiological conditions, are quickly recruited into the tissue upon injury and persist during regeneration. Apoptosis, necrosis and autophagy coexist in the injured/regenerating muscles, including those of patients with neuromuscular disorders, such as inflammatory myopathies, dystrophies, metabolic and mitochondrial myopathies and drug-induced myopathies. Macrophages are able to alter their function in response to microenvironment conditions and as a consequence coordinate changes within the tissue from the early injury throughout regeneration and eventual healing, and regulate the activation and the function of stem cells. Early after injury, classically activated macrophages ('M1') dominate the picture. Alternatively activated M2 macrophages predominate during resolution phases and regulate the termination of the inflammatory responses. The dynamic M1/M2 transition is increasingly felt to be the key to the homeostasis of the muscle. Recognition and clearance of debris originating from damaged myofibers and from dying stem/progenitor cells, stromal cells and leukocytes are fundamental actions of macrophages. Clearance of apoptotic cells and M1/M2 transition are causally connected and represent limiting steps for muscle healing. The accumulation of apoptotic cells, which reflects their defective clearance, has been demonstrated in various tissues to prompt autoimmunity against intracellular autoantigens. In the muscle, in the presence of type I interferon, apoptotic myoblasts indeed cause the production of autoantibodies, lymphocyte infiltration and continuous cycles of muscle injury and regeneration, mimicking human inflammatory myopathies. The clearance of apoptotic cells thus modulates the homeostatic response of the skeletal muscle to injury. Conversely, defects in the process may have deleterious local effects, guiding maladaptive tissue remodeling with collagen and fat accumulation and promoting autoimmunity itself. There is strong promise for novel treatments based on new knowledge of cell death, clearance and immunity in the muscle.

Leukocytes recruited by tumor-derived HMGB1 sustain peritoneal carcinomatosis

The factors that determine whether disseminated transformed cells in vivo yield neoplastic lesions have only been partially identified. We established an ad hoc model of peritoneal carcinomatosis by injecting colon carcinoma cells in mice. Tumor cells recruit inflammatory leukocytes, mostly macrophages, and generate neoplastic peritoneal lesions. Phagocyte depletion via clodronate treatment reduces neoplastic growth. Colon carcinoma cells release a prototypic damage-associated molecular pattern (DAMP)/alarmin, High Mobility Group Box1 (HMGB1), which attracts leukocytes. Exogenous HMGB1 accelerates leukocyte recruitment, macrophage infiltration, tumor growth and vascularization. Lentiviral-based HMGB1 knockdown or pharmacological interference with its extracellular impair macrophage recruitment and tumor growth. Our findings provide a preclinical proof of principle that strategies based on preventing HMGB1-driven recruitment of leukocytes could be used for treating peritoneal carcinomatosis.

Potential role of High mobility group box 1 in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide and is characterized as a typical inflammation-related carcinoma. High mobility group box protein 1 (HMGB1), a non-histone DNA-binding protein, is identified as a potent proinflammatory mediator when presents extracellularly. Recently, a growing body of evidence indicates that HMGB1 plays a potential role in HCC, but many questions remain unanswered about the relationship between HMGB1 and HCC formation and development. This review focuses on the biological effect of HMGB1, and discusses the association of HMGB1 with HCC and potential use of strategies targeting HMGB1 in HCC treatment.

DAMPs from Cell Death to New Life

Our body handles tissue damage by activating the immune system in response to intracellular molecules released by injured tissues [damage-associated molecular patterns (DAMPs)], in a similar way as it detects molecular motifs conserved in pathogens (pathogen-associated molecular patterns). DAMPs are molecules that have a physiological role inside the cell, but acquire additional functions when they are exposed to the extracellular environment: they alert the body about danger, stimulate an inflammatory response, and finally promote the regeneration process. Beside their passive release by dead cells, some DAMPs can be secreted or exposed by living cells undergoing a life-threatening stress. DAMPs have been linked to inflammation and related disorders: hence, inhibition of DAMP-mediated inflammatory responses is a promising strategy to improve the clinical management of infection- and injury-elicited inflammatory diseases. However, it is important to consider that DAMPs are not only danger signals but also central players in tissue repair. Indeed, some DAMPs have been studied for their role in tissue healing after sterile or infection-associated inflammation. This review is focused on two exemplary DAMPs, HMGB1 and adenosine triphosphate, and their contribution to both inflammation and tissue repair.

Fat deposition and accumulation in the damaged and inflamed skeletal muscle: cellular and molecular players

The skeletal muscle has the capacity to repair damage by the activation and differentiation of fiber sub-laminar satellite cells. Regeneration impairment due to reduced satellite cells number and/or functional capacity leads to fiber substitution with ectopic tissues including fat and fibrous tissue and to the loss of muscle functions. Muscle mesenchymal cells that in physiological conditions sustain or directly contribute to regeneration differentiate in adipocytes in patients with persistent damage and inflammation of the skeletal muscle. These cells comprise the fibro-adipogenic precursors, the PW1-expressing cells and some interstitial cells associated with vessels (pericytes, mesoangioblasts and myoendothelial cells). Resident fibroblasts that are responsible for collagen deposition and extracellular matrix remodeling during regeneration yield fibrotic tissue and can differentiate into adipose cells. Some authors have also proposed that satellite cells themselves could transdifferentiate into adipocytes, although recent results by lineage tracing techniques seem to put this theory to discussion. This review summarizes findings about muscle resident mesenchymal cell differentiation in adipocytes and recapitulates the molecular mediators involved in intramuscular adipose tissue deposition.

Required role of apoptotic myogenic precursors and toll-like receptor stimulation for the establishment of autoimmune myositis in experimental murine models

OBJECTIVE

Muscle regeneration is a hallmark of the idiopathic inflammatory myopathies (IIMs), a group of autoimmune disorders that are characterized by leukocyte infiltration and dysfunction of the skeletal muscle. Despite detailed studies describing the clinical and histopathologic features of IIMs, the immunopathogenesis of these disorders remains undefined. The aim of this study was to investigate the immunopathologic processes of autoimmune myositis in experimental murine models.

METHODS

Expression of the autoantigen histidyl-transfer RNA synthetase (HisRS) was analyzed in mice with acutely injured or dystrophic muscles, in inflammatory leukocytes, and in purified satellite cells. Anti-HisRS antibodies and myositis induction were assessed in mice after muscle injury and immunization with apoptotic satellite cells or C2C12 myoblasts, in the presence or absence of the Toll-like receptor 7 (TLR-7) agonist R848.

RESULTS

Muscle necrosis, leukocyte infiltration, and myofiber regeneration induced by toxic agents (cardiotoxin or glycerol) or promoted by genetic disruption of the α-sarcoglycan/dystrophin complex in mice were uniformly associated with up-regulated expression of HisRS. Although regenerating myofibers and purified satellite cells are known to show increased expression of HisRS in these settings, anti-HisRS antibodies were not detectable. However, intramuscular immunization with ultraviolet B-irradiated, HisRS-expressing apoptotic myoblasts in the presence of R848 triggered the production of anti-HisRS IgG antibodies as well as persistent lymphocyte infiltration and prolonged/delayed muscle regeneration. Conversely, intramuscular administration of R848 alone or in combination with living or postapoptotic/necrotic myoblasts failed to generate this myositis phenotype.

CONCLUSION

In the presence of TLR/adjuvant signals and underlying muscle injury, apoptotic myogenic precursors expressing high levels of autoantigen can provoke autoantibody formation and lymphocytic infiltration of muscle tissue, effectively replicating the features of IIM.

Activated platelets present high mobility group box 1 to neutrophils, inducing autophagy and promoting the extrusion of neutrophil extracellular traps

BACKGROUND

Increasing evidence implicates both platelets and neutrophils in the formation, stabilization, and growth of peripheral and coronary thrombi. Neutrophil extracellular traps (NETs) play a key role. The early events in the deregulated cross-talk between platelets and neutrophils are poorly characterized.

OBJECTIVES

To identify at the molecular level the mechanism through which platelets induce the generation of NETs in sterile conditions.

PATIENTS/METHODS

The presence of NETs was determined in 26 thrombi from patients with acute myocardial infarction by immunohistochemistry and immunofluorescence and markers of NETs assessed in the plasma. In vitro NET generation was studied in static and in physiological flow conditions.

RESULTS

Coronary thrombi mainly consist of activated platelets, neutrophils, and NETs in close proximity of platelets. Activated platelets commit neutrophils to NET generation. The event abates in the presence of competitive antagonists of the high mobility group box 1 (HMGB1) protein. Hmgb1(-/-) platelets fail to elicit NETs, whereas the HMGB1 alone commits neutrophils to NET generation. Integrity of the HMGB1 receptor, Receptor for Advanced Glycation End products (RAGE), is required for NET formation, as assessed using pharmacologic and genetic tools. Exposure to HMGB1 prevents depletion of mitochondrial potential, induces autophagosome formation, and prolongs neutrophil survival. These metabolic effects are caused by the activation of autophagy. Blockade of the autophagic flux reverts platelet HMGB1-elicited NET generation.

CONCLUSIONS

Activated platelets present HMGB1 to neutrophils and commit them to autophagy and NET generation. This chain of events may be responsible for some types of thromboinflammatory lesions and indicates novel paths for molecular intervention.

Danger signals - damaged-self recognition across the tree of life

Multicellular organisms suffer injury and serve as hosts for microorganisms. Therefore, they require mechanisms to detect injury and to distinguish the self from the non-self and the harmless non-self (microbial mutualists and commensals) from the detrimental non-self (pathogens). Danger signals are "damage-associated molecular patterns" (DAMPs) that are released from the disrupted host tissue or exposed on stressed cells. Seemingly ubiquitous DAMPs are extracellular ATP or extracellular DNA, fragmented cell walls or extracellular matrices, and many other types of delocalized molecules and fragments of macromolecules that are released when pre-existing precursors come into contact with enzymes from which they are separated in the intact cell. Any kind of these DAMPs enable damaged-self recognition, inform the host on tissue disruption, initiate processes aimed at restoring homeostasis, such as sealing the wound, and prepare the adjacent tissues for the perception of invaders. In mammals, antigen-processing and -presenting cells such as dendritic cells mature to immunostimulatory cells after the perception of DAMPs, prime naïve T-cells and elicit a specific adaptive T-/B-cell immune response. We discuss molecules that serve as DAMPs in multiple organisms and their perception by pattern recognition receptors (PRRs). Ca(2+)-fluxes, membrane depolarization, the liberation of reactive oxygen species and mitogen-activated protein kinase (MAPK) signaling cascades are the ubiquitous molecular mechanisms that act downstream of the PRRs in organisms across the tree of life. Damaged-self recognition contains both homologous and analogous elements and is likely to have evolved in all eukaryotic kingdoms, because all organisms found the same solutions for the same problem: damage must be recognized without depending on enemy-derived molecules and responses to the non-self must be directed specifically against detrimental invaders.