Mummification of the infarcted myocardium by high dose corticosteroids

Macrophage heterogeneity in myocardial infarction: Evolution and implications for diverse therapeutic approaches

Given the extensive participation of myeloid cells (especially monocytes and macrophages) in both inflammation and resolution phases post-myocardial infarction (MI) owing to their biphasic role, these cells are considered as crucial players in the disease pathogenesis. Multiple studies have agreed on the significant contribution of macrophage polarization theory (M2 vs. M1) while determining the underlying reasons behind the observed biphasic effects; nevertheless, this simplistic classification attracts severe drawbacks. The advent of multiple advanced technologies based on OMICS platforms facilitated a successful path to explore comprehensive cellular signatures that could expedite our understanding of macrophage heterogeneity and plasticity. While providing an overall basis behind the MI disease pathogenesis, this review delves into the literature to discuss the current knowledge on multiple macrophage clusters, including the future directions in this research arena. In the end, our focus will be on outlining the possible therapeutic implications based on the emerging observations.

Unveiling macrophage diversity in myocardial ischemia-reperfusion injury: identification of a distinct lipid-associated macrophage subset

Background and objective

Macrophages play a crucial and dichotomous role cardiac repair following myocardial ischemia-reperfusion, as they can both facilitate tissue healing and contribute to injury. This duality is intricately linked to environmental factors, and the identification of macrophage subtypes within the context of myocardial ischemia-reperfusion injury (MIRI) may offer insights for the development of more precise intervention strategies.

Methods

Specific marker genes were used to identify macrophage subtypes in GSE227088 (mouse single-cell RNA sequencing dataset). Genome Set Enrichment Analysis (GSEA) was further employed to validate the identified LAM subtypes. Trajectory analysis and single-cell regulatory network inference were executed using the R packages Monocle2 and SCENIC, respectively. The conservation of LAM was verified using human ischemic cardiomyopathy heart failure samples from the GSE145154 (human single-cell RNA sequencing dataset). Fluorescent homologous double-labeling experiments were performed to determine the spatial localization of LAM-tagged gene expression in the MIRI mouse model.

Results

In this study, single-cell RNA sequencing (scRNA-seq) was employed to investigate the cellular landscape in ischemia-reperfusion injury (IRI). Macrophage subtypes, including a novel Lipid-Associated Macrophage (LAM) subtype characterized by high expression of Spp1, Trem2, and other genes, were identified. Enrichment and Progeny pathway analyses highlighted the distinctive functional role of the SPP1+ LAM subtype, particularly in lipid metabolism and the regulation of the MAPK pathway. Pseudotime analysis revealed the dynamic differentiation of macrophage subtypes during IRI, with the activation of pro-inflammatory pathways in specific clusters. Transcription factor analysis using SCENIC identified key regulators associated with macrophage differentiation. Furthermore, validation in human samples confirmed the presence of SPP1+ LAM. Co-staining experiments provided definitive evidence of LAM marker expression in the infarct zone. These findings shed light on the role of LAM in IRI and its potential as a therapeutic target.

Conclusion

In conclusion, the study identifies SPP1+ LAM macrophages in ischemia-reperfusion injury and highlights their potential in cardiac remodeling.

Pre-hospital pulse glucocorticoid therapy in patients with ST-segment elevation myocardial infarction transferred for primary percutaneous coronary intervention: a randomized controlled trial (PULSE-MI)

BACKGROUND

Inflammation in ST-segment elevation myocardial infarction (STEMI) is an important contributor to both acute myocardial ischemia and reperfusion injury after primary percutaneous coronary intervention (PCI). Methylprednisolone is a glucocorticoid with potent anti-inflammatory properties with an acute effect and is used as an effective and safe treatment of a wide range of acute diseases. The trial aims to investigate the cardioprotective effects of pulse-dose methylprednisolone administered in the pre-hospital setting in patients with STEMI transferred for primary PCI.

METHODS

This trial is a randomized, blinded, placebo-controlled prospective clinical phase II trial. Inclusion will continue until 378 patients with STEMI have been evaluated for the primary endpoint. Patients will be randomized 1:1 to a bolus of 250 mg methylprednisolone intravenous or matching placebo over a period of 5 min in the pre-hospital setting. All patients with STEMI transferred for primary PCI at Rigshospitalet, Copenhagen University Hospital, Denmark, will be screened for eligibility. The main eligibility criteria are age ≥ 18 years, acute onset of chest pain with < 12 h duration, STEMI on electrocardiogram, no known allergy to glucocorticoids or no previous coronary artery bypass grafting, previous acute myocardial infarction in assumed culprit, or a history with previous maniac/psychotic episodes. Primary outcome is final infarct size measured by late gadolinium enhancement on cardiac magnetic resonance (CMR) 3 months after STEMI. Secondary outcomes comprise key CMR efficacy parameters, clinical endpoints at 3 months, the peak of cardiac biomarkers, and safety.

DISCUSSION

We hypothesize that pulse-dose methylprednisolone administrated in the pre-hospital setting decreases inflammation and thus reduces final infarct size in patients with STEMI treated with primary PCI.

TRIAL REGISTRATION

EU-CT number: 2022-500762-10-00; Submitted May 5, 2022.

CLINICALTRIALS

gov Identifier: NCT05462730; Submitted July 7, 2022, first posted July 18, 2022.

Inflammation in Myocardial Ischemia/Reperfusion Injury: Underlying Mechanisms and Therapeutic Potential

Acute myocardial infarction (MI) occurs when blood flow to the myocardium is restricted, leading to cardiac damage and massive loss of viable cardiomyocytes. Timely restoration of coronary flow is considered the gold standard treatment for MI patients and limits infarct size; however, this intervention, known as reperfusion, initiates a complex pathological process that somewhat paradoxically also contributes to cardiac injury. Despite being a sterile environment, ischemia/reperfusion (I/R) injury triggers inflammation, which contributes to infarct expansion and subsequent cardiac remodeling and wound healing. The immune response is comprised of subsets of both myeloid and lymphoid-derived cells that act in concert to modulate the pathogenesis and resolution of I/R injury. Multiple mechanisms, including altered metabolic status, regulate immune cell activation and function in the setting of acute MI, yet our understanding remains incomplete. While numerous studies demonstrated cardiac benefit following strategies that target inflammation in preclinical models, therapeutic attempts to mitigate I/R injury in patients were less successful. Therefore, further investigation leveraging emerging technologies is needed to better characterize this intricate inflammatory response and elucidate its influence on cardiac injury and the progression to heart failure.

The Protective Role of TREM2 in the Heterogenous Population of Macrophages during Post-Myocardial Infarction Inflammation

Advances in interventions after myocardial infarction (MI) have dramatically increased survival, but MI remains the leading cause of heart failure due to maladaptive ventricular remodeling following ischemic damage. Inflammation is crucial in both the initial response to ischemia and subsequent wound healing in the myocardium. To date, preclinical and clinical efforts have been made to elucidate the deleterious effects of immune cells contributing to ventricular remodeling and to identify therapeutic molecular targets. The conventional concept classifies macrophages or monocytes into dichotomous populations, while recent studies support their diverse subpopulations and spatiotemporal dynamicity. The single-cell and spatial transcriptomic landscapes of macrophages in infarcted hearts successfully revealed the heterogeneity of cell types and their subpopulations post-MI. Among them, subsets of Trem2hi macrophages were identified that were recruited to infarcted myocardial tissue in the subacute phase of MI. The upregulation of anti-inflammatory genes was observed in Trem2hi macrophages, and an in vivo injection of soluble Trem2 during the subacute phase of MI significantly improved myocardial function and the remodeling of infarcted mice hearts, suggesting the potential therapeutic role of Trem2 in LV remodeling. Further investigation of the reparative role of Trem2 in LV remodeling would provide novel therapeutic targets for MI.

lncRNA260 siRNA Accelerates M2 Macrophage Polarization and Alleviates Oxidative Stress via Inhibiting IL28RA Gene Alternative Splicing

The macrophage transformation of inflammatory M1 to anti-inflammatory M2 could be promoted by activating PI3K/AKT signaling pathway. In our previous study, it was found that downregulation of lncRNA260 could ameliorate hypoxic cardiomyocyte injury by regulating IL28RA through the activation of PI3K/AKT signaling pathways. It was suggested that lncRNA260 siRNA could promote the macrophages toward M2 polarization by regulating IL28RA. In this study, lncRNA260 siRNA was used to observe its effect on the polarization of murine bone marrow-derived macrophages (BMDM) and investigate its related mechanisms. lncRNA 260 specific siRNA were designed and synthesized which were transfected into murine BMDM with liposomes. The experiment was divided into three groups: Hypoxia group, Hypoxia+lncRNA 260-specific siRNA transfection group, and Normoxia group. The CD206-APC/CD11b-FITC or CD206-FITC/CD107b (Mac-3) double positive proportions were used to compare the M2 polarization proportions in the hypoxia process by using the immunofluorescence staining method. The p-AKT, Arg 1, PI3KCG, IL28RAV1, and IL28RAV2 protein expression changes were observed by using the western blot method. Compared with the Normoxia group, the M2 proportions were significantly decreased in the Hypoxia group (P < 0.05). Compared with the hypoxia group, the M2 proportions were significantly increased in the Hypoxia+lncRNA260 siRNA transfection group (P < 0.05). In the Hypoxia group, the ratios of Arg 1/β-Actin, p-AKT/β-Actin, PI3KCG/β-Actin, and IL28RAV1/β-Actin were significantly lower than those in the Normoxia group (P < 0.05). After transfection with lncRNA260 siRNA, the ratios of Arg1/β-Actin, p-AKT/β-Actin, PI3KCG/β-Actin, and IL28RAV1/β-Actin were significantly higher than those in the Hypoxia group (P < 0.05). Compared with the Normoxia group, the IL28RAV2/β-Actin in the Hypoxia group was significantly increased (P < 0.05). After transfection with lncRNA260 siRNA, the ratio of IL28RAV2/β-Actin was significantly decreased than that in the Hypoxia group (P < 0.05). lncRNA260 siRNA could promote the M2 polarization of the hypoxia macrophages by reducing the IL28RAV2 alternative splicing variant, which might be related to the activation of the JAK-STAT and PI3K/AKT signaling pathways. It will provide a new strategy for the anti-inflammation, antioxidative stress therapy, and cardiac remodeling after AMI.

Secretome of Stressed Peripheral Blood Mononuclear Cells Alters Transcriptome Signature in Heart, Liver, and Spleen after an Experimental Acute Myocardial Infarction: An In Silico Analysis

Simple Summary

Acute myocardial infarction is characterized by impaired coronary blood flow, which leads to cardiac ischemia and, ultimately, compromised heart function. Damage and cellular responses are not limited to the non-perfused area, but rather affect the entire heart, as well as distal organs, such as the liver and spleen. We found that the therapeutic secretome of stressed white blood cells improved short-term and long-term cardiac performance in a porcine infarction model. In order to unravel the molecular events governing secretome-mediated tissue regeneration, we performed transcriptional analyses of the non-perfused, transition, and perfused heart, as well as the liver and spleen 24 h after myocardial infarction. We observed a highly tissue-specific effect of the secretome and, except for the transition zone, a uniform downregulation of pro-inflammatory factors and pathways. Simultaneously, the secretome strongly promoted the expression of genes that are essential for heart function in the non-perfused area. In the liver and spleen, different metabolic processes were induced. Together, our data suggest several plausible mechanisms by which the secretome improves heart function after cardiac ischemia. Deepening our understanding of the molecular processes identified here might uncover further pharmacologic strategies aiming at delimiting adverse cardiac remodeling and sequelae after myocardial infarction.

Abstract

Acute myocardial infarction (AMI) is a result of cardiac non-perfusion and leads to cardiomyocyte necrosis, inflammation, and compromised cardiac performance. Here, we showed that the secretome of γ-irradiated peripheral blood mononuclear cells (PBMCsec) improved heart function in a porcine AMI model and displayed beneficial long- and short-term effects. As an AMI is known to strongly affect gene regulation of the ischemia non-affected heart muscle and distal organs, we employed a transcriptomics approach to further study the immediate molecular events orchestrated using the PBMCsec in myocardium, liver, and spleen 24 h post ischemia. In the infarcted area, the PBMCsec mainly induced genes that were essential for cardiomyocyte function and simultaneously downregulated pro-inflammatory genes. Interestingly, genes associated with pro-inflammatory processes were activated in the transition zone, while being downregulated in the remote zone. In the liver, we observed a pronounced inhibition of immune responses using the PBMCsec, while genes involved in urea and tricarboxylic cycles were induced. The spleen displayed elevated lipid metabolism and reduced immunological processes. Together, our study suggested several types of pharmacodynamics by which the PBMCsec conferred immediate cardioprotection. Furthermore, our data supported the assumption that an AMI significantly affects distal organs, suggesting that a holistic treatment of an AMI, as achieved by PBMCsec, might be highly beneficial.

The Role of Immune Cells in Cardiac Remodeling After Myocardial Infarction

Myocardial infarction (MI) is an irreversible damage of the heart muscle, which often leads to adverse cardiac remodeling and progressive heart failure. After MI, immune cells play a vital role in the clearance of the dying tissue and cardiac remodeling. Post-MI events include the release of danger signals by necrotic cardiomyocytes and the migration of the inflammatory cells, such as dendritic cells, neutrophils, monocytes, and macrophages, into the site of the cardiac injury to digest the cell debris and secrete a variety of inflammatory factors activating the inflammatory response. In this review, we focus on the role of immune cells in the cardiac remodeling after MI and the novel immunotherapies targeting immune cells.

Post-ischemic Myocardial Inflammatory Response: A Complex and Dynamic Process Susceptible to Immunomodulatory Therapies

Following acute occlusion of a coronary artery causing myocardial ischemia and implementing first-line treatment involving rapid reperfusion, a dynamic and balanced inflammatory response is initiated to repair and remove damaged cells. Paradoxically, restoration of myocardial blood flow exacerbates cell damage as a result of myocardial ischemia-reperfusion (MI-R) injury, which eventually provokes accelerated apoptosis. In the end, the infarct size still corresponds to the subsequent risk of developing heart failure. Therefore, true understanding of the mechanisms regarding MI-R injury, and its contribution to cell damage and cell death, are of the utmost importance in the search for successful therapeutic interventions to finally prevent the onset of heart failure. This review focuses on the role of innate immunity, chemokines, cytokines, and inflammatory cells in all three overlapping phases following experimental, mainly murine, MI-R injury known as the inflammatory, reparative, and maturation phase. It provides a complete state-of-the-art overview including most current research of all post-ischemic processes and phases and additionally summarizes the use of immunomodulatory therapies translated into clinical practice.

Inflammatory Response Mechanisms of the Dentine-Pulp Complex and the Periapical Tissues

The macroscopic and microscopic anatomy of the oral cavity is complex and unique in the human body. Soft-tissue structures are in close interaction with mineralized bone, but also dentine, cementum and enamel of our teeth. These are exposed to intense mechanical and chemical stress as well as to dense microbiologic colonization. Teeth are susceptible to damage, most commonly to caries, where microorganisms from the oral cavity degrade the mineralized tissues of enamel and dentine and invade the soft connective tissue at the core, the dental pulp. However, the pulp is well-equipped to sense and fend off bacteria and their products and mounts various and intricate defense mechanisms. The front rank is formed by a layer of odontoblasts, which line the pulp chamber towards the dentine. These highly specialized cells not only form mineralized tissue but exert important functions as barrier cells. They recognize pathogens early in the process, secrete antibacterial compounds and neutralize bacterial toxins, initiate the immune response and alert other key players of the host defense. As bacteria get closer to the pulp, additional cell types of the pulp, including fibroblasts, stem and immune cells, but also vascular and neuronal networks, contribute with a variety of distinct defense mechanisms, and inflammatory response mechanisms are critical for tissue homeostasis. Still, without therapeutic intervention, a deep carious lesion may lead to tissue necrosis, which allows bacteria to populate the root canal system and invade the periradicular bone via the apical foramen at the root tip. The periodontal tissues and alveolar bone react to the insult with an inflammatory response, most commonly by the formation of an apical granuloma. Healing can occur after pathogen removal, which is achieved by disinfection and obturation of the pulp space by root canal treatment. This review highlights the various mechanisms of pathogen recognition and defense of dental pulp cells and periradicular tissues, explains the different cell types involved in the immune response and discusses the mechanisms of healing and repair, pointing out the close links between inflammation and regeneration as well as between inflammation and potential malignant transformation.

The Role of Extracellular DNA and Histones in Ischaemia-Reperfusion Injury of the Myocardium

Despite an increase in the rates of survival in patients suffering myocardial infarction, as yet there is no therapy specifically targeting ischaemia and reperfusion injury of the myocardium. With a greater understanding of immune activation during infarction, more potential treatment targets are now being identified. The innate immune system is believed to play an important role in the myocardium after ischaemia-driven cardiomyocyte death. The release of intracellular contents including DNA into the extracellular space during necrosis and cell rupture is now believed to create a pro-inflammatory milieu which propagates the inflammatory process. DNA and DNA fragments have been shown to activate the innate immune system by acting as Danger-Associated Molecular Patterns (DAMPs), which act as ligands on toll-like receptors (TLRs). Stimulation of TLRs, in turn, can activate intracellular cell death pathways such as pyroptosis. Here, we review the role of DNA fragments during ischaemia and reperfusion, and assess their potential as a target in the quest to preserve cardiomyocyte viability following myocardial infarction.

Ischemia/Reperfusion Injury: Pathophysiology, Current Clinical Management, and Potential Preventive Approaches

Myocardial ischemia reperfusion syndrome is a complex entity where many inflammatory mediators play different roles, both to enhance myocardial infarction-derived damage and to heal injury. In such a setting, the establishment of an effective therapy to treat this condition has been elusive, perhaps because the experimental treatments have been conceived to block just one of the many pathogenic pathways of the disease, or because they thwart the tissue-repairing phase of the syndrome. Either way, we think that a discussion about the pathophysiology of the disease and the mechanisms of action of some drugs may shed some clarity on the topic.

ROS-responsive polyurethane fibrous patches loaded with methylprednisolone (MP) for restoring structures and functions of infarcted myocardium in vivo

Reactive oxygen species (ROS) play an important role in the pathogenesis of numerous diseases including atherosclerosis, diabetes, inflammation and myocardial infarction (MI). In this study, a ROS-responsive biodegradable elastomeric polyurethane containing thioketal (PUTK) linkages was synthesized from polycaprolactone diol (PCL-diol ), 1,6-hexamethylene diisocyanate (HDI), and ROS-cleavable chain extender. The PUTK was electrospun into fibrous patches with the option to load glucocorticoid methylprednisolone (MP), which were then used to treat MI of rats in vivo. The fibrous patches exhibited suitable mechanical properties and high elasticity. The molecular weight of PUTK was decreased significantly after incubation in 1 mM H₂O₂ solution for 2 weeks due to the degradation of thioketal bonds on the polymer backbone. Both the PUTK and PUTK/MP fibrous patches showed good antioxidant property in an oxidative environment in vitro. Implantation of the ROS-responsive polyurethane patches in MI of rats in vivo could better protect cardiomyocytes from death in the earlier stage (24 h) than the non ROS-responsive ones. Implantation of the PUTK/MP fibrous patches for 28 days could effectively improve the reconstruction of cardiac functions including increased ejection fraction, decreased infarction size, and enhanced revascularization of the infarct myocardium.

A sequential interferon gamma directed chemotactic cellular immune response determines survival and cardiac function post-myocardial infarction

AIMS

Myelomonocytic cells are critical in injury and healing post-myocardial infarction (MI). Mechanisms of regulation, however, are incompletely understood. The aim of the study was to elucidate the role of interferon gamma (IFN-γ) in the orchestrated inflammatory response in a murine model of MI.

METHODS AND RESULTS

MI was induced in 8- to 12-week-old male mice (C57BL/6 background) by permanent ligation of the left anterior descending (LAD) coronary artery. Lysozyme M (LysM)+ cell-depleted LysMiDTR transgenic mice displayed a reduced influx of CD45.2+/CD3-/CD11b+/Gr-1high neutrophils into infarcted myocardium 1 day post-MI compared with infarcted controls, paralleled by decreased cardiac mRNA levels of IFN-γ and tumour necrosis factor alpha (TNF-α). Mortality after MI was significantly increased in LysM+ cell-depleted mice within 28 days post-MI. To more specifically address the role of neutrophils, we depleted C57BL/6 mice with a monoclonal anti-Gr-1 antibody and found increased mortality, deteriorated cardiac function as well as decreased cardiac IFN-γ mRNA expression early after MI. Ccl2, Cxcl1, Cx3cl1, and Il12b mRNA were reduced 3 days after MI, as was the amount of CD11b+/Ly-6G-/Ly-6Chigh inflammatory monocytes. LAD-ligated Cramp-/- mice lacking cathelicidin important in neutrophil-dependent monocyte chemotaxis as well as IFNγ-/- and TNFα-/- mice phenocopied Gr-1+ cell-depleted mice, supporting a regulatory role of IFN-γ impacting on both the sequence of inflammatory cell invasion and cardiac outcome early after MI. The use of conditional IFN-γ receptor deficient mice indicated a direct effect of IFN-γ on LysM+ cells in cardiac injury post-MI. Using IFN-γ reporter mice and flow cytometry, we identified cardiac lymphoid cells (CD4+ and CD8+ T cells and natural killer cells) as primary source of this cytokine in the cardiac inflammatory response post-MI.

CONCLUSION

IFN-γ directs a sequential chemotactic cellular immune response and determines survival and cardiac function post-MI.

C5L2 Regulates DMP1 Expression during Odontoblastic Differentiation

The presence of stem cells within the dental-pulp tissue as well as their differentiation into a new generation of functional odontoblast-like cells constitutes an important step of the dentin-pulp regeneration. Recent investigations demonstrated that the complement system activation participates in 2 critical steps of dentin-pulp regeneration: pulp progenitor's recruitment and pulp nerve sprouting. Surprisingly, its implication in odontoblastic differentiation has not been addressed yet. Since the complement receptor C5a receptor-like 2 (C5L2) is expressed by different stem cells, the aim of this study is to investigate if the dental pulp stem cells express C5L2 and if this receptor participates in odontoblastic differentiation. Immunohistochemistry performed on human third molar pulp sections showed a perivascular co-localization of the mesenchymal stem cell markers STRO1 and C5L2. In vitro immunofluorescent staining confirmed that hDPSCs express C5L2. Furthermore, we determined by real-time polymerase chain reaction that the expression of C5L2 is highly modulated in human dental pulp stem cells (hDPSCs) undergoing odontoblastic differentiation. Moreover, we showed that this odontogenesis-regulated expression of C5L2 is specifically potentiated by the proinflammatory cytokine TNFα. Using a C5L2-siRNA silencing strategy, we provide direct evidence that C5L2 constitutes a negative regulator of the dentinogenic marker DMP1 (dentin matrix protein 1) expression by hDPSCs. Our findings suggest a direct correlation between the odontoblastic differentiation and the level of C5L2 expression in hDPSCs and identify C5L2 as a negative regulator of DMP1 expression by hDPSCs during the odontoblastic differentiation and inflammation processes. This work is the first to demonstrate the involvement of C5L2 in the biological function of stem cells, provides an important knowledge in understanding odontoblastic differentiation of dental pulp stem cells, and may be useful in future dentin-pulp engineering strategies.

Immunomodulatory interventions in myocardial infarction and heart failure: a systematic review of clinical trials and meta-analysis of IL-1 inhibition

Following a myocardial infarction (MI), the immune system helps to repair ischaemic damage and restore tissue integrity, but excessive inflammation has been implicated in adverse cardiac remodelling and development towards heart failure (HF). Pre-clinical studies suggest that timely resolution of inflammation may help prevent HF development and progression. Therapeutic attempts to prevent excessive post-MI inflammation in patients have included pharmacological interventions ranging from broad immunosuppression to immunomodulatory approaches targeting specific cell types or factors with the aim to maintain beneficial aspects of the early post-MI immune response. These include the blockade of early initiators of inflammation including reactive oxygen species and complement, inhibition of mast cell degranulation and leucocyte infiltration, blockade of inflammatory cytokines, and inhibition of adaptive B and T-lymphocytes. Herein, we provide a systematic review on post-MI immunomodulation trials and a meta-analysis of studies targeting the inflammatory cytokine Interleukin-1. Despite an enormous effort into a significant number of clinical trials on a variety of targets, a striking heterogeneity in study population, timing and type of treatment, and highly variable endpoints limits the possibility for meaningful meta-analyses. To conclude, we highlight critical considerations for future studies including (i) the therapeutic window of opportunity, (ii) immunological effects of routine post-MI medication, (iii) stratification of the highly diverse post-MI patient population, (iv) the potential benefits of combining immunomodulatory with regenerative therapies, and at last (v) the potential side effects of immunotherapies.

Anti-inflammatory therapies in myocardial infarction: failures, hopes and challenges

In the infarcted heart, the damage-associated molecular pattern proteins released by necrotic cells trigger both myocardial and systemic inflammatory responses. Induction of chemokines and cytokines and up-regulation of endothelial adhesion molecules mediate leukocyte recruitment in the infarcted myocardium. Inflammatory cells clear the infarct of dead cells and matrix debris and activate repair by myofibroblasts and vascular cells, but may also contribute to adverse fibrotic remodelling of viable segments, accentuate cardiomyocyte apoptosis and exert arrhythmogenic actions. Excessive, prolonged and dysregulated inflammation has been implicated in the pathogenesis of complications and may be involved in the development of heart failure following infarction. Studies in animal models of myocardial infarction (MI) have suggested the effectiveness of pharmacological interventions targeting the inflammatory response. This article provides a brief overview of the cell biology of the post-infarction inflammatory response and discusses the use of pharmacological interventions targeting inflammation following infarction. Therapy with broad anti-inflammatory and immunomodulatory agents may also inhibit important repair pathways, thus exerting detrimental actions in patients with MI. Extensive experimental evidence suggests that targeting specific inflammatory signals, such as the complement cascade, chemokines, cytokines, proteases, selectins and leukocyte integrins, may hold promise. However, clinical translation has proved challenging. Targeting IL-1 may benefit patients with exaggerated post-MI inflammatory responses following infarction, not only by attenuating adverse remodelling but also by stabilizing the atherosclerotic plaque and by inhibiting arrhythmia generation. Identification of the therapeutic window for specific interventions and pathophysiological stratification of MI patients using inflammatory biomarkers and imaging strategies are critical for optimal therapeutic design.

The Open Artery Hypothesis: Beneficial Effects and Long-term Prognostic Importance of Patency of the Infarct-Related Coronary Artery

There seem to be additional mechanisms of benefit in patients receiving late reperfusion therapy in a time when the opportunity for myocardial salvage has been missed. Previous studies have demonstrated that the restoration of blood flow in the infarct-related coronary artery in patients with acute myocardial infarction improves left ventricular function and reduces mortality. Initially, it was thought that survival was improved because viable myocardium was salvaged. However, data obtained over the past several years have suggested that the restoration of antegrade flow in the infarct-related artery may improve survival via a mechanism independent of the influence on left ventricular function. Clinical interest in the open artery hypothesis has recently resurfaced owing to a substantial improvement in technical aspects of percutaneous coronary interventions (PCI). Observational data suggest a role for late intervention as safer and more effective mechanical reperfusion practices have emerged. Long-term clinical benefits have been shown from balloon angioplasty late after myocardial infarction (MI). Therefore, patients with failed thrombolysis or those with late-presenting MI may still benefit from PCI by mechanisms independent of myocardial salvage. There is accumulative evidence on this matter. Possible mechanisms include reduction of ventricular remodeling, diminished ventricular instability reducing the incidence of arrhythmias, and provision of collaterals to other territories in the event of further coronary artery occlusion. However, caution must be exercised in interpreting the results of studies examining the open artery hypothesis. This hypothesis can be tested in its purest sense in animal experiments; however, the clinical situation is much more complex. Patients may have acute-on-chronic coronary artery occlusion in the presence of multivessel disease and well-developed collateral channels. The pattern of necrosis may also be different with areas of necrosis separated by islands of ischemic, stunned, hibernating, or normal cells. Therefore, the patency of the infarct-related coronary artery in single or multivessel disease days to weeks after infarction markedly influences long-term prognosis unrelated to improvement of left ventricular function. Current technology has made it feasible to open and maintain patency of most occluded infarct-related arteries. However, the hypothesis that late mechanical reperfusion in patients with asymptomatic occluded infarct-related artery will improve long-term clinical outcomes remains to be proved and is currently being tested in a large randomized trial.

Nerve Growth Factor Secretion From Pulp Fibroblasts is Modulated by Complement C5a Receptor and Implied in Neurite Outgrowth

Given the importance of sensory innervation in tooth vitality, the identification of signals that control nerve regeneration and the cellular events they induce is essential. Previous studies demonstrated that the complement system, a major component of innate immunity and inflammation, is activated at the injured site of human carious teeth and plays an important role in dental-pulp regeneration via interaction of the active Complement C5a fragment with pulp progenitor cells. In this study, we further determined the role of the active fragment complement C5a receptor (C5aR) in dental nerve regeneration in regards to local secretion of nerve growth factor (NGF) upon carious injury. Using ELISA and AXIS co-culture systems, we demonstrate that C5aR is critically implicated in the modulation of NGF secretion by LTA-stimulated pulp fibroblasts. The NGF secretion by LTA-stimulated pulp fibroblasts, which is negatively regulated by C5aR activation, has a role in the control of the neurite outgrowth length in our axon regeneration analysis. Our data provide a scientific step forward that can guide development of future therapeutic tools for innovative and incipient interventions targeting the dentin-pulp regeneration process by linking the neurite outgrowth to human pulp fibroblast through complement system activation.

Sources of dentin-pulp regeneration signals and their modulation by the local microenvironment

Many aspects of dentin pulp tissue regeneration have been investigated, and it has been shown that dentin pulp has a high regeneration capacity. This seems to be because of the presence of progenitor cells and inductive regeneration signals from different origins. These signals can be liberated after the acidic dissolution of carious dentin as well as from pulp fibroblasts and endothelial cells in cases of traumatic injury. Thus, both carious lesions and pulp cells provide the required mediators for complete dentin-pulp regeneration including reparative dentin secretion, angiogenesis, and innervation. Additionally, all dentin pulp insults including carious "infection," traumatic injuries, application of restorative materials on the injured dentin pulp, and subsequent apoptosis are known activators of the complement system. This activation leads to the production of several biologically active fragments responsible for the vascular modifications and the attraction of immune cells to the inflammatory/injury site. Among these, C5a is involved in the recruitment of pulp progenitor cells, which express the C5a receptor. Thus, in addition to dentin and pulp cells, plasma should be considered as an additional source of regeneration signals.

Pulp Fibroblasts Control Nerve Regeneration through Complement Activation

Dentin-pulp regeneration is closely linked to the presence of nerve fibers in the pulp and to the healing mechanism by sprouting of the nerve fiber’s terminal branches beneath the carious injury site. However, little is known about the initial mechanisms regulating this process in carious teeth. It has been recently demonstrated that the complement system activation, which is one of the first immune responses, contributes to tissue regeneration through the local production of anaphylatoxins such as C5a. While few pulp fibroblasts in intact teeth and in untreated fibroblast cultures express the C5a receptor (C5aR), here we show that all dental pulp fibroblasts, localized beneath the carious injury site, do express this receptor. This observation is consistent with our in vitro results, which showed expression of C5aR in lipoteichoic acid–stimulated pulp fibroblasts. The interaction of C5a, produced after complement synthesis and activation from pulp fibroblasts, with the C5aR of these cells mediated the local brain-derived neurotropic factor (BDNF) secretion. Overall, this activation guided the neuronal growth toward the lipoteichoic acid–stimulated fibroblasts. Thus, our findings highlight a new mechanism in one of the initial steps of the dentin-pulp regeneration process, linking pulp fibroblasts to the nerve sprouting through the complement system activation. This may provide a useful future therapeutic tool in targeting the fibroblasts in the dentin-pulp regeneration process.

Myocardial fibrosis seen through the lenses of T-cell biology

Lymphocytes came recently into focus as modulators of non-infectious myocardial diseases. Several lines of experimental evidence now indicate that CD4(+) T-cells can influence the healing and scarring processes that follow a myocardial infarction episode. Furthermore, such heart-directed T-cell activity has also been implicated in the pathogenesis cardiac remodeling that develops in response to chronic pressure-overload conditions. Mechanistically, different T-cell subsets can secrete several mediators and growth factors that influence the myocardial molecular milieu and directly interfere with the macrophages' and fibroblasts' activity. Therefore, the present review summarizes the current experimental evidence on the role of T-cells in myocardial scar formation after infarction and myocardial fibrosis as central mechanism of ventricular remodeling.

Inflammation as a therapeutic target in myocardial infarction: learning from past failures to meet future challenges

In the infarcted myocardium, necrotic cardiomyocytes release danger signals, activating an intense inflammatory response. Inflammatory pathways play a crucial role in regulation of a wide range of cellular processes involved in injury, repair, and remodeling of the infarcted heart. Proinflammatory cytokines, such as tumor necrosis factor α and interleukin 1, are markedly upregulated in the infarcted myocardium and promote adhesive interactions between endothelial cells and leukocytes by stimulating chemokine and adhesion molecule expression. Distinct pairs of chemokines and chemokine receptors are implicated in recruitment of various leukocyte subpopulations in the infarcted myocardium. For more than the past 30 years, extensive experimental work has explored the role of inflammatory signals and the contributions of leukocyte subpopulations in myocardial infarction. Robust evidence derived from experimental models of myocardial infarction has identified inflammatory targets that may attenuate cardiomyocyte injury or protect from adverse remodeling. Unfortunately, attempts to translate the promising experimental findings to clinical therapy have failed. This review article discusses the biology of the inflammatory response after myocardial infarction, attempts to identify the causes for the translational failures of the past, and proposes promising new therapeutic directions. Because of their potential involvement in injurious, reparative, and regenerative responses, inflammatory cells may hold the key for design of new therapies in myocardial infarction.

Pathophysiology Underlying the Bimodal Edema Phenomenon After Myocardial Ischemia/Reperfusion

BACKGROUND

Post-ischemia/reperfusion (I/R) myocardial edema was recently shown to follow a consistent bimodal pattern: an initial wave of edema appears on reperfusion and dissipates at 24 h, followed by a deferred wave that initiates days after infarction, peaking at 1 week.

OBJECTIVES

This study examined the pathophysiology underlying this post-I/R bimodal edematous reaction.

METHODS

Forty instrumented pigs were assigned to different myocardial infarction protocols. Edematous reaction was evaluated by water content quantification, serial cardiac magnetic resonance T2-mapping, and histology/immunohistochemistry. The association of reperfusion with the initial wave of edema was evaluated in pigs undergoing 40-min/80-min I/R and compared with pigs undergoing 120-min ischemia with no reperfusion. The role of tissue healing in the deferred wave of edema was evaluated by comparing pigs undergoing standard 40-min/7-day I/R with animals subjected to infarction without reperfusion (chronic 7-day coronary occlusion) or receiving post-I/R high-dose steroid therapy.

RESULTS

Characterization of post-I/R tissue changes revealed maximal interstitial edema early on reperfusion in the ischemic myocardium, with maximal content of neutrophils, macrophages, and collagen at 24 h, day 4, and day 7 post-I/R, respectively. Reperfused pigs had significantly higher myocardial water content at 120 min and T2 relaxation times on 120 min cardiac magnetic resonance than nonreperfused animals. Permanent coronary occlusion or high-dose steroid therapy significantly reduced myocardial water content on day 7 post-infarction. The dynamics of T2 relaxation times during the first post-infarction week were altered significantly in nonreperfused pigs compared with pigs undergoing regular I/R.

CONCLUSIONS

The 2 waves of the post-I/R edematous reaction are related to different pathophysiological phenomena. Although the first wave is secondary to reperfusion, the second wave occurs mainly because of tissue healing processes.

Cross talk of the first-line defense TLRs with PI3K/Akt pathway, in preconditioning therapeutic approach

Toll-like receptor family (TLRs), pattern recognition receptors, is expressed not only on immune cells but also on non-immune cells, including cardiomyocytes, fibroblasts, and vascular endothelial cells. One main function of TLRs in the non-immune system is to regulate apoptosis. TLRs are the central mediators in hepatic, pulmonary, brain, and renal ischemic/reperfusion (I/R) injury. Up-regulation of TLRs and their ligation by either exogenous or endogenous danger signals plays critical roles in ischemia/reperfusion-induced tissue damage. Conventional TLR-NF-κB pathways are markedly activated in failing and ischemic myocardium. Recent studies have identified a cross talk between TLR activation and the PI3K/Akt pathway. The activation of TLRs is proposed to be the most potent preconditioning method after ischemia, to improve the cell survival via the mechanism involved the PI3K/Akt signaling pathway and to attenuate the subsequent TLR-NF-κB pathway stimulation. Thus, TLRs could be a great target in the new treatment approaches for myocardial I/R injury.

Cross talk of the first-line defense TLRs with PI3K/Akt pathway, in preconditioning therapeutic approach

Toll-like receptor family (TLRs), pattern recognition receptors, is expressed not only on immune cells but also on non-immune cells, including cardiomyocytes, fibroblasts, and vascular endothelial cells. One main function of TLRs in the non-immune system is to regulate apoptosis. TLRs are the central mediators in hepatic, pulmonary, brain, and renal ischemic/reperfusion (I/R) injury. Up-regulation of TLRs and their ligation by either exogenous or endogenous danger signals plays critical roles in ischemia/reperfusion-induced tissue damage. Conventional TLR-NF-κB pathways are markedly activated in failing and ischemic myocardium. Recent studies have identified a cross talk between TLR activation and the PI3K/Akt pathway. The activation of TLRs is proposed to be the most potent preconditioning method after ischemia, to improve the cell survival via the mechanism involved the PI3K/Akt signaling pathway and to attenuate the subsequent TLR-NF-κB pathway stimulation. Thus, TLRs could be a great target in the new treatment approaches for myocardial I/R injury.

Interleukin-3 greatly expands non-adherent endothelial forming cells with pro-angiogenic properties

Circulating endothelial progenitor cells (EPCs) provide revascularisation for cardiovascular disease and the expansion of these cells opens up the possibility of their use as a cell therapy. Herein we show that interleukin-3 (IL3) strongly expands a population of human non-adherent endothelial forming cells (EXnaEFCs) with low immunogenicity as well as pro-angiogenic capabilities in vivo, making their therapeutic utilisation a realistic option. Non-adherent CD133(+) EFCs isolated from human umbilical cord blood and cultured under different conditions were maximally expanded by day 12 in the presence of IL3 at which time a 350-fold increase in cell number was obtained. Cell surface marker phenotyping confirmed expression of the hematopoietic progenitor cell markers CD133, CD117 and CD34, vascular cell markers VEGFR2 and CD31, dim expression of CD45 and absence of myeloid markers CD14 and CD11b. Functional experiments revealed that EXnaEFCs exhibited classical properties of endothelial cells (ECs), namely binding of Ulex europaeus lectin, up-take of acetylated-low density lipoprotein and contribution to EC tube formation in vitro. These EXnaEFCs demonstrated a pro-angiogenic phenotype within two independent in vivo rodent models. Firstly, a Matrigel plug assay showed increased vascularisation in mice. Secondly, a rat model of acute myocardial infarction demonstrated reduced heart damage as determined by lower levels of serum creatinine and a modest increase in heart functionality. Taken together, these studies show IL3 as a potent growth factor for human CD133(+) cell expansion with clear pro-angiogenic properties (in vitro and in vivo) and thus may provide clinical utility for humans in the future.

Steroid use in kidney transplant recipients presented with acute myocardial infarction

Suppression of the hypothalamic-pituitary-adrenal axis due to chronic exogenous steroid use is the most common cause of secondary adrenal insufficiency. Most kidney transplant recipients receive steroid therapy for immunosuppression; they are also at high risk for acute coronary events which can increase their physiological stress. Use of steroids early in the course of acute myocardial infarction (MI) raises concerns about the possibility of an increased risk of aneurysm formation and myocardial rupture. We present six case reports of kidney transplant recipients. Two of these recipients developed adrenal insufficiency after acute anterior MI; the life-threatening situation was successfully managed with corticosteroid administration. Four of these kidney transplant recipients presented with acute anterior MI; in these patients prophylactic steroid therapy prevented adrenal insufficiency, without any complication of the MI. We recommend the use of prophylactic corticosteroids for kidney transplant recipients to prevent adrenal insufficiency in the early course of acute MI.

Anti-inflammatory strategies for ventricular remodeling following ST-segment elevation acute myocardial infarction

Acute myocardial infarction (AMI) leads to molecular, structural, geometric, and functional changes in the heart in a process known as ventricular remodeling. An intense organized inflammatory response is triggered after myocardial ischemia and necrosis and involves all components of the innate immunity, affecting both cardiomyocytes and noncardiomyocyte cells. Inflammation is triggered by tissue injury; it mediates wound healing and scar formation and affects ventricular remodeling. Many therapeutic attempts aimed at reducing inflammation in AMI during the past 3 decades presented issues of impaired healing or increased risk of cardiac rupture or failed to show any additional benefit in addition to standard therapies. More recent strategies aimed at selectively blocking one of the key factors upstream rather than globally suppressing the response downstream have shown some promising results in pilot trials. We herein review the pathophysiological mechanisms of inflammation and ventricular remodeling after AMI and the results of clinical trials with anti-inflammatory strategies.

Structural composition of myocardial infarction scar in middle-aged male and female rats: does sex matter?

The present study was designed to determine whether the structural composition of the scar in middle-aged post-myocardial infraction (MI) rats is affected by the biological sex of the animals. A large MI was induced in 12-month-old male (M-MI) and female (F-MI) Sprague-Dawley rats by ligation of the left coronary artery. Four weeks after the MI, rats with transmural infarctions, greater than 50% of the left ventricular (LV) free wall, were evaluated. The extent of LV remodeling and fractional volumes of fibrillar collagen (FC), myofibroblasts, vascular smooth muscle (SM) cells, and surviving cardiac myocytes (CM) in the scars were compared between the two sexes. The left ventricle of post-MI male and female rats underwent a similar degree of remodeling as evidenced by the analogous scar thinning ratio (0.46 ± 0.02 vs. 0.42 ± 0.05) and infarct expansion index (1.06 ± 0.07 vs. 1.12 ± 0.08), respectively. Most important, the contents of major structural components of the scar revealed no evident difference between M-MI and F-MI rats (interstitial FC, 80.74 ± 2.08 vs. 82.57 ± 4.53; myofibroblasts, 9.59 ± 1.68 vs.9.56 ± 1.15; vascular SM cells, 2.27 ± 0.51 vs. 3.38 ± 0.47; and surviving CM, 3.26 ± 0.39 vs. 3.05 ± 0.38, respectively). Our data are the first to demonstrate that biological sex does not influence the structural composition of a mature scar in middle-aged post-MI rats.

Depression after myocardial infarction: TNF-α-induced alterations of the blood-brain barrier and its putative therapeutic implications

Patients experiencing an acute myocardial infarction (AMI) have a three times higher chance to develop depression. Vice versa, depressive symptoms increase the risk of cardiovascular events. The co-existence of both conditions is associated with substantially worse prognosis. Although the underlying mechanism of the interaction is largely unknown, inflammation is thought to be of pivotal importance. AMI-induced peripheral cytokines release may cause cerebral endothelial leakage and hence induces a neuroinflammatory reaction. The neuroinflammation may persist even long after the initial peripheral inflammation has subsided. Among those selected brain regions that are prone to blood-brain barrier dysfunction, the paraventricular nucleus of the hypothalamus (PVN), a major center for cardiovascular autonomic regulation, is indicated to play a mediating role. Optimal cardiovascular therapy improves cardiovascular prognosis without major effects on depression. By the same token, antidepressant therapy in cardiovascular disease is associated with modest improvement in depressive symptoms, however without improvement in cardiac outcome. The failure of current antidepressants and the growing number of patients suffering from both conditions legitimize the search for better antidepressive therapies, from patients as well as society perspectives. Though we appreciate the mutual character of the interaction between depression and AMI, the present review focuses on the side of AMI induced depression and discusses the role of inflammation, represented by the proinflammatory cytokine TNF-α, as potential underlying mechanism. It is conceivable that inhibition of the inflammatory response post-AMI, through targeted anti-inflammatory pharmacotherapeutical agents may prevent the development of depressive symptoms and ultimately may improve cardiovascular outcomes.

Complement anaphylatoxins C3a and C5a induce a failing regenerative program in cardiac resident cells. Evidence of a role for cardiac resident stem cells other than cardiomyocyte renewal

Cardiac healing, which follows myocardial infarction, is a complex process guided by intricate interactions among different components. Some resident cell populations with a potential role in cardiac healing have already been described in cardiac tissues. These non-cardiomyocyte cell subsets, globally described as cardiac pluripotent/progenitor cells (CPCs), are able to differentiate into all three major cardiac cell lineages (endothelial, smooth muscle and cardiomyocyte cells) in experimental settings. Nevertheless, physiological cardiac healing results in a fibrous scar, which remains to be fully modelled experimentally. Since a role for complement anaphylatoxins (C3a and C5a) has been described in several regeneration/repair processes, we examined the effects that C3a and C5a exert on a defined population of CPCs. We found that C3a and C5a are able to enhance CPC migration and proliferation. In vitro studies showed that this effect is linked to activation of telomerase mRNA and partial preservation of telomere length, in an NFκB-dependent manner. In addition, anaphylatoxin signalling modulates the CPC phenotype, increasing myofibroblast differentiation and reducing endothelial and cardiac gene expression. These findings may denote that C3a and C5a are able to maintain/increase the cardiac stem cell pool within the heart, whilst simultaneously facilitating and modulating resident cell differentiation. We found that this modulation was directed towards scar forming cells, which increased fibroblast/myofibroblast generation and suggests that both these anaphylatoxins could play a relevant role in the damage-coupled activation of resident cells, and regulation of the cardiac healing process after injury.

Effects of xenon and isoflurane on apoptosis and inflammation in a porcine myocardial infarction model

Volatile anaesthetics can reduce the infarction size in myocardial tissue when administered before and during experimentally induced ischaemia. The aim of this study was to investigate whether xenon is beneficial compared to isoflurane in limiting myocardial tissue apoptosis and inflammation induced by experimental ischaemia-reperfusion injury in a porcine right ventricular infarction model. Twenty-one animals used for this study randomly received isoflurane, xenon or thiopental, (n=6-8 per group). Myocardial infarction was induced for 90min, followed by reperfusion for 120min. Tissues from the left and right ventricles were removed from the sites of infarction, reperfusion and remote areas, and processed for immunohistochemistry. Apoptosis (caspase-3 staining) and neutrophilic infiltration (naphthol AS-D chloroacetate-specific esterase) were assessed and evaluated. Statistical analysis was performed using an ANOVA of repeated measures. Density of apoptotic cells were higher in tissues from animals that were anesthetized with xenon. This effect was significant in comparison to isoflurane (p=0.0177). Neutrophilic infiltration was significantly higher in the right compared to the left ventricle (p<0.001), whereas no significant differences in the number of granulocytes based on the anaesthetic regime or the different tissue areas were found. We conclude that xenon, in the early phase of ischaemia and reperfusion, induces a significant increase in apoptosis compared to isoflurane. Therefore, clinical use of this anaesthetic in cardiocompromised patients should be taken with care until more long-term studies have been carried out. The increased neutrophilic infiltration in the right vs. the left ventricle indicates the right ventricle being more susceptible to ischaemia-reperfusion injury.

The future of regenerating the myocardium

PURPOSE OF REVIEW

Stem cell therapy for cardiac disease may be facing two major problems nowadays: although vasculogenesis likely occurs as a result of cell therapy, its clinical applications are limited and significant, integrated cardiomyogenesis has not demonstratively been shown to occur, even in the experimental setting, with any other source than embryonic or other pluripotent stem cells.

RECENT FINDINGS

In this article, we highlight several factors that will need to be optimized if we are to achieve clinically effective cardiomyogenesis, such as the identification of optimal stem cell populations, and the ideal time and methods for cell transplantation. So far, educated attempts at achieving transplanted stem cell-induced myogenesis have largely failed outside of the embryonic stem cell realm, and we present the rationale for also considering acellular techniques, which may enhance the potential of endogenous progenitor populations.

SUMMARY

In today's cardiovascular field, once a cardiomyocyte is lost it is lost for good, without any form of direct therapeutic option. For these reasons, cell therapy justifies our continued attention and efforts, and may constitute the holy grail of cardiovascular therapeutics.

Model-based design of mechanical therapies for myocardial infarction

The mechanical properties of healing myocardial infarcts are a critical determinant of pump function and the transition to heart failure. Recent reports suggest that modifying infarct mechanical properties can improve function and limit ventricular remodeling. However, little attempt has been made to identify the specific infarct material properties that would optimize left ventricular (LV) function. We utilized a finite-element model of a large anteroapical infarct in a dog heart to explore a wide range of infarct mechanical properties. Isotropic stiffening of the infarct reduced end-diastolic (EDV) and end-systolic (ESV) volumes, improved LV contractility, but had little effect on stroke volume. A highly anisotropic infarct, with high longitudinal stiffness but low circumferential stiffness coefficients, produced the best stroke volume by increasing diastolic filling, without affecting contractility or ESV. Simulated infarcts in two different locations displayed different transmural strain patterns. Our results suggest that there is a general trade-off between acutely reducing LV size and acutely improving LV pump function, that isotropically stiffening the infarct is not the only option of potential therapeutic interest, and that customizing therapies for different infarct locations may be important. Our model results should provide guidance for design and development of therapies to improve LV function by modifying infarct mechanical properties.

Intravenous clusterin administration reduces myocardial infarct size in rats

BACKGROUND

Clusterin (Apolipoprotein J), a plasma protein with cytoprotective and complement-inhibiting activities, localizes in the infarcted heart during myocardial infarction (MI). Recently, we have shown a protective effect of exogenous clusterin in vitro on ischaemically challenged cardiomyocytes independent of complement. We therefore hypothesized that intravenous clusterin administration would reduce myocardial infarction damage.

METHODS

Wistar rats undergoing experimental MI, induced by 40 min ligation of a coronary vessel, were treated with either clusterin (n=15) or vehicle (n=13) intravenously, for 3 days post-MI. After 4 weeks, hearts were analysed. The putative role of megalin, a clusterin receptor, was also studied.

RESULTS

Administration of human clusterin significantly reduced both infarct size (with 75 ± 5%) and death of animals (23% vehicle group vs. 0% clusterin group). Importantly, histochemical analysis showed no signs of impaired wound healing in the clusterin group. In addition, significantly increased numbers of macrophages were found in the clusterin group. We also found that the clusterin receptor megalin was present on cardiomyocytes in vitro which, however, was not influenced by ischaemia. Human clusterin co-localized with this receptor in vitro, but not in the human heart. In addition, using a megalin inhibitor, we found that clusterin did not exert its protective effect on cardiomyocytes through megalin.

CONCLUSIONS

Our results thus show that clusterin has a protective effect on cardiomyocytes after acute myocardial infarction in vivo, independent of its receptor megalin. This indicates that clusterin, or a clusterin derivate, is a potential therapeutic agent in the treatment of MI.

Irradiated cultured apoptotic peripheral blood mononuclear cells regenerate infarcted myocardium

BACKGROUND

Acute myocardial infarction (AMI) is followed by post AMI cardiac remodelling, often leading to congestive heart failure. Homing of c-kit+ endothelial progenitor cells (EPC) has been thought to be the optimal source for regenerating infarcted myocardium.

METHODS

Immune function of viable peripheral blood mononuclear cells (PBMC) was evaluated after co-culture with irradiated apoptotic PBMC (IA-PBMC) in vitro. Viable PBMC, IA-PBMC and culture supernatants (SN) thereof were obtained after 24 h. Reverse transcription polymerase chain reaction and enzyme-linked immunosorbent assay were utilized to quantify interleukin-8 (IL-8), vascular endothelial growth factor, matrix metalloproteinase-9 (MMP9) in PBMC, SN and SN exposed fibroblasts. Cell suspensions of viable- and IA-PBMC were infused in an experimental rat AMI model. Immunohistological analysis was performed to detect inflammatory and pro-angiogenic cells within 72 h post-infarction. Functional data and determination of infarction size were quantified by echocardiography and Elastica van Gieson staining.

RESULTS

The IA-PBMC attenuated immune reactivity and resulted in secretion of pro-angiogenic IL-8 and MMP9 in vitro. Fibroblasts exposed to viable and IA-PBMC derived SN caused RNA increment of IL-8 and MMP9. AMI rats that were infused with IA-PBMC cell suspension evidenced enhanced homing of endothelial progenitor cells within 72 h as compared to control (medium alone, viable-PBMC). Echocardiography showed a significant reduction in infarction size and improvement in post AMI remodelling as evidenced by an attenuated loss of ejection fraction.

CONCLUSION

These data indicate that infusion of IA-PBMC cell suspension in experimental AMI circumvented inflammation, caused preferential homing of regenerative EPC and replaced infarcted myocardium.

Toll-like receptor signaling: a critical modulator of cell survival and ischemic injury in the heart

Toll-like receptors (TLRs) represent the first line of host defense against microbial infection and play a pivotal role in both innate and adaptive immunity. TLRs recognize invading pathogens through molecular pattern recognition, transduce signals via distinct intracellular pathways involving a unique set of adaptor proteins and kinases, and ultimately lead to the activation of transcription factors and inflammatory responses. Among 10 TLRs identified in humans, at least two exist in the heart, i.e., TLR2 and TLR4. In addition to the critical role of these in mediating cardiac dysfunction in septic conditions, emerging evidence suggests that the TLRs can also recognize endogenous ligands and may play an important role in modulating cardiomyocyte survival and in ischemic myocardial injury. In animal models of ischemia-reperfusion injury or in hypoxic cardiomyocytes in vitro, the administration of a sublethal dose of lipopolysaccharide, which signals through TLR4, reduces subsequent myocardial infarction, improves cardiac functions, and attenuates cardiomyocyte apoptosis. By contrast, a systemic deficiency of TLR2, TLR4, or myeloid differentiation primary-response gene 88, an adaptor critical for all TLR signaling, except TLR3, leads to an attenuated myocardial inflammation, a smaller infarction size, a better preserved ventricular function, and a reduced ventricular remodeling after ischemic injury. These loss-of-function studies suggest that both TLRs contribute to myocardial inflammation and ischemic injury in the heart although the exact contribution of cardiac (vs. circulatory cell) TLRs remains to be defined. These recent studies demonstrate an emerging role for TLRs as a critical modulator in both cell survival and tissue injury in the heart.

Improvement in cardiac function after bone marrow cell thearpy is associated with an increase in myocardial inflammation

The mechanisms for the beneficial impact of bone marrow cell (BMC) therapy after myocardial infarction (MI) are ill defined. We hypothesized that the implanted cells improve function by attenuating post-MI inflammation and repair. In mice, 3 x 10(5) fresh BMCs were implanted immediately after coronary ligation. Cardiac function was evaluated over time. Inflammatory cytokines and cells were measured, and their impacts on the (myo)fibroblastic repair response, angiogenesis, and scar formation were determined. All differences below had P values of <0.05. BMC implantation reduced the decline in fractional shortening and ventricular dilation. Invasive hemodynamics confirmed a difference in systolic function at day 7 and diastolic function at day 28 favoring the BMC group. Interestingly, BMC implantation caused a 1.6-fold increase in the number of macrophages infiltrating the infarct but did not affect neutrophils. This increase was associated with a 1.9-fold higher myocardial TNF-alpha level. The heightened inflammatory response was associated with a 1.4-fold induction of transforming growth factor-beta and a 1.3-fold induction of basic fibroblast growth factor. These changes resulted in a 1.6-fold increase in alpha-smooth muscle actin and a 1.9-fold increase in total discoidin domain receptor 2-expressing cells in the BMC group. These two markers are expressed by cardiac (myo)fibroblasts. Capillary density in the border zone increased 2.0-fold. Consistent with a more robust repair-mediated scar "contracture," the final scar size was 0.7-fold smaller in the BMC group. In conclusion, after MI, BMC therapy induced a more robust inflammatory response that improved the "priming" of the (myo)fibroblast repair phase. Enhancing this response may further improve the beneficial impact of cellular therapy.

Innate immune adaptor MyD88 mediates neutrophil recruitment and myocardial injury after ischemia-reperfusion in mice

MyD88 is an adaptor protein critical for innate immune response against microbial infection and in certain noninfectious tissue injury. The present study examined the role of MyD88 in myocardial inflammation and injury after ischemia-reperfusion (I/R). I/R was produced by coronary artery ligation for 30 min followed by reperfusion. The ratios of area at risk to left ventricle (LV) were similar between wild-type (WT) and MyD88-deficient (MyD88-/-) mice. However, 24 h after I/R, the ratios of myocardial infarction to area at risk were 58% less in MyD88(-/-) than in WT mice (14 +/- 2% vs. 33 +/- 6%, P = 0.01). Serial echocardiographic studies demonstrated that there was no difference in baseline LV contractile function between the two groups. Twenty-four hours after I/R, LV ejection fraction (EF) and fractional shortening (FS) in WT mice were reduced by 44% and 62% (EF, 51 +/- 2%, and FS, 22 +/- 1%, P < 0.001), respectively, and remained depressed on the seventh day after I/R. In comparison, EF and FS in MyD88(-/-) mice were 67 +/- 3% and 33 +/- 2%, respectively, after I/R (P < 0.001 vs. WT). Similarly, LV function, as demonstrated by invasive hemodynamic measurements, was better preserved in MyD88(-/-) compared with WT mice after I/R. Furthermore, when compared with WT mice, MyD88(-/-) mice subjected to I/R had a marked decrease in myocardial inflammation as demonstrated by attenuated neutrophil recruitment and decreased expression of the proinflammatory mediators keratinocyte chemoattractant, monocyte chemoattractant protein-1, and ICAM-1. Taken together, these data suggest that MyD88 modulates myocardial inflammatory injury and contributes to myocardial infarction and LV dysfunction during I/R.