Cardiolipin-protein complexes and initiation of complement activation after coronary artery occlusion

Double-Edged Sword: Exploring the Mitochondria-Complement Bidirectional Connection in Cellular Response and Disease

Mitochondria serve an ultimate purpose that seeks to balance the life and death of cells, a role that extends well beyond the tissue and organ systems to impact not only normal physiology but also the pathogenesis of diverse diseases. Theorized to have originated from ancient proto-bacteria, mitochondria share similarities with bacterial cells, including their own circular DNA, double-membrane structures, and fission dynamics. It is no surprise, then, that mitochondria interact with a bacterium-targeting immune pathway known as a complement system. The complement system is an ancient and sophisticated arm of the immune response that serves as the body's first line of defense against microbial invaders. It operates through a complex cascade of protein activations, rapidly identifying and neutralizing pathogens, and even aiding in the clearance of damaged cells and immune complexes. This dynamic system, intertwining innate and adaptive immunity, holds secrets to understanding numerous diseases. In this review, we explore the bidirectional interplay between mitochondrial dysfunction and the complement system through the release of mitochondrial damage-associated molecular patterns. Additionally, we explore several mitochondria- and complement-related diseases and the potential for new therapeutic strategies.

When Our Best Friend Becomes Our Worst Enemy: The Mitochondrion in Trauma, Surgery, and Critical Illness

Common for major surgery, multitrauma, sepsis, and critical illness, is a whole-body inflammation. Tissue injury is able to trigger a generalized inflammatory reaction. Cell death causes release of endogenous structures termed damage associated molecular patterns (DAMPs) that initiate a sterile inflammation. Mitochondria are evolutionary endosymbionts originating from bacteria, containing molecular patterns similar to bacteria. These molecular patterns are termed mitochondrial DAMPs (mDAMPs). Mitochondrial debris released into the extracellular space or into the circulation is immunogenic and damaging secondary to activation of the innate immune system. In the circulation, released mDAMPS are either free or exist in extracellular vesicles, being able to act on every organ and cell in the body. However, the role of mDAMPs in trauma and critical care is not fully clarified. There is a complete lack of knowledge how they may be counteracted in patients. Among mDAMPs are mitochondrial DNA, cardiolipin, N-formyl peptides, cytochrome C, adenosine triphosphate, reactive oxygen species, succinate, and mitochondrial transcription factor A. In this overview, we present the different mDAMPs, their function, release, targets, and inflammatory potential. In light of present knowledge, the role of mDAMPs in the pathophysiology of major surgery and trauma as well as sepsis, and critical care is discussed.

Complementary Sets of Autoantibodies Induced by SARS-CoV-2, Adenovirus and Bacterial Antigens Cross-React with Human Blood Protein Antigens in COVID-19 Coagulopathies

COVID-19 patients often develop coagulopathies including microclotting, thrombotic strokes or thrombocytopenia. Autoantibodies are present against blood-related proteins including cardiolipin (CL), serum albumin (SA), platelet factor 4 (PF4), beta 2 glycoprotein 1 (β2GPI), phosphodiesterases (PDE), and coagulation factors such as Factor II, IX, X and von Willebrand factor (vWF). Different combinations of autoantibodies associate with different coagulopathies. Previous research revealed similarities between proteins with blood clotting functions and SARS-CoV-2 proteins, adenovirus, and bacterial proteins associated with moderate-to-severe COVID-19 infections. This study investigated whether polyclonal antibodies (mainly goat and rabbit) against these viruses and bacteria recognize human blood-related proteins. Antibodies against SARS-CoV-2 and adenovirus recognized vWF, PDE and PF4 and SARS-CoV-2 antibodies also recognized additional antigens. Most bacterial antibodies tested (group A streptococci [GAS], staphylococci, Escherichia coli [E. coli], Klebsiella pneumoniae, Clostridia, and Mycobacterium tuberculosis) cross-reacted with CL and PF4. while GAS antibodies also bound to F2, Factor VIII, Factor IX, and vWF, and E. coli antibodies to PDE. All cross-reactive interactions involved antibody-antigen binding constants smaller than 100 nM. Since most COVID-19 coagulopathy patients display autoantibodies against vWF, PDE and PF4 along with CL, combinations of viral and bacterial infections appear to be necessary to initiate their autoimmune coagulopathies.

Inflammation in myocardial injury: mesenchymal stem cells as potential immunomodulators

Ischemic heart disease is a growing worldwide epidemic. Improvements in medical and surgical therapies have reduced early mortality after acute myocardial infarction and increased the number of patients living with chronic heart failure. The irreversible loss of functional cardiomyocytes puts these patients at significant risk of ongoing morbidity and mortality after their index event. Recent evidence suggests that inflammation is a key mediator of postinfarction adverse remodeling in the heart. In this review, we discuss the cardioprotective and deleterious effects of inflammation and its mediators during acute myocardial infarction. We also explore the role of mesenchymal stem cell therapy to limit secondary injury and promote myocardial healing after myocardial infarction.

Mitochondrial Dysfunction and the Aging Immune System

Mitochondria are ancient organelles that have co-evolved with their cellular hosts, developing a mutually beneficial arrangement. In addition to making energy, mitochondria are multifaceted, being involved in heat production, calcium storage, apoptosis, cell signaling, biosynthesis, and aging. Many of these mitochondrial functions decline with age, and are the basis for many diseases of aging. Despite the vast amount of research dedicated to this subject, the relationship between aging mitochondria and immune function is largely absent from the literature. In this review, three main issues facing the aging immune system are discussed: (1) inflamm-aging; (2) susceptibility to infection and (3) declining T-cell function. These issues are re-evaluated using the lens of mitochondrial dysfunction with aging. With the recent expansion of numerous profiling technologies, there has been a resurgence of interest in the role of metabolism in immunity, with mitochondria taking center stage. Building upon this recent accumulation of knowledge in immunometabolism, this review will advance the hypothesis that the decline in immunity and associated pathologies are partially related to the natural progression of mitochondrial dysfunction with aging.

Mitochondria in innate immune signaling

Mitochondria are functionally versatile organelles. In addition to their conventional role of meeting the cell's energy requirements, mitochondria also actively regulate innate immune responses against infectious and sterile insults. Components of mitochondria, when released or exposed in response to dysfunction or damage, can be directly recognized by receptors of the innate immune system and trigger an immune response. In addition, despite initiation that may be independent from mitochondria, numerous innate immune responses are still subject to mitochondrial regulation as discrete steps of their signaling cascades occur on mitochondria or require mitochondrial components. Finally, mitochondrial metabolites and the metabolic state of the mitochondria within an innate immune cell modulate the precise immune response and shape the direction and character of that cell's response to stimuli. Together, these pathways result in a nuanced and very specific regulation of innate immune responses by mitochondria.

NEDD4-1 protects against ischaemia/reperfusion-induced cardiomyocyte apoptosis via the PI3K/Akt pathway

Activation of the Akt pathway has been shown to protect the heart from ischaemia/reperfusion (I/R) injury. NEDD4-1 has been shown to positively regulate nuclear trafficking of the activated form of Akt. However, the role of NEDD4-1 in cardiac I/R injury remains to be elucidated. In the present study, Lentiviral vectors were constructed to overexpress or knockdown NEDD4-1 in H9c2 cardiomyocytes subjected to I/R injury or ischemic preconditioning (IPC). The results indicated that NEDD4-1 levels were decreased after I/R and increased after IPC in rat heart tissue and in H9c2 cardiomyocytes. Overexpression of NEDD4-1 activated the Akt pathway and regulated apoptosis-related proteins in H9c2 cardiomyocytes, attenuating SI/R-induced cell apoptosis and caspase 3/7 activities. Furthermore, in vivo overexpression of NEDD4-1 attenuated myocardial apoptosis following myocardial I/R. Our results demonstrated that NEDD4-1 protects the myocardium from I/R induced apoptosis by activating PI3K/Akt signaling.

Basic Biology of Oxidative Stress and the Cardiovascular System: Part 1 of a 3-Part Series

The generation of reactive oxygen species (ROS) is a fundamental aspect of normal human biology. However, when ROS generation exceeds endogenous antioxidant capacity, oxidative stress arises. If unchecked, ROS production and oxidative stress mediate tissue and cell damage that can spiral in a cycle of inflammation and more oxidative stress. This article is part 1 of a 3-part series covering the role of oxidative stress in cardiovascular disease. The broad theme of this first paper is the mechanisms and biology of oxidative stress. Specifically, the authors review the basic biology of oxidative stress, relevant aspects of mitochondrial function, and stress-related cell death pathways (apoptosis and necrosis) as they relate to the heart and cardiovascular system. They then explore telomere biology and cell senescence. As important regulators and sensors of oxidative stress, telomeres are segments of repetitive nucleotide sequence at each end of a chromosome that protect the chromosome ends from deterioration.

Ischemia/Reperfusion

Ischemic disorders, such as myocardial infarction, stroke, and peripheral vascular disease, are the most common causes of debilitating disease and death in westernized cultures. The extent of tissue injury relates directly to the extent of blood flow reduction and to the length of the ischemic period, which influence the levels to which cellular ATP and intracellular pH are reduced. By impairing ATPase-dependent ion transport, ischemia causes intracellular and mitochondrial calcium levels to increase (calcium overload). Cell volume regulatory mechanisms are also disrupted by the lack of ATP, which can induce lysis of organelle and plasma membranes. Reperfusion, although required to salvage oxygen-starved tissues, produces paradoxical tissue responses that fuel the production of reactive oxygen species (oxygen paradox), sequestration of proinflammatory immunocytes in ischemic tissues, endoplasmic reticulum stress, and development of postischemic capillary no-reflow, which amplify tissue injury. These pathologic events culminate in opening of mitochondrial permeability transition pores as a common end-effector of ischemia/reperfusion (I/R)-induced cell lysis and death. Emerging concepts include the influence of the intestinal microbiome, fetal programming, epigenetic changes, and microparticles in the pathogenesis of I/R. The overall goal of this review is to describe these and other mechanisms that contribute to I/R injury. Because so many different deleterious events participate in I/R, it is clear that therapeutic approaches will be effective only when multiple pathologic processes are targeted. In addition, the translational significance of I/R research will be enhanced by much wider use of animal models that incorporate the complicating effects of risk factors for cardiovascular disease. © 2017 American Physiological Society. Compr Physiol 7:113-170, 2017.

Cardiolipin-mediated procoagulant activity of mitochondria contributes to traumatic brain injury-associated coagulopathy in mice

Cardiolipin (CL) is an anionic phospholipid located exclusively in the mitochondrial inner membrane. Its presence in blood indicates mitochondrial damage and release from injured cells. Here, we report the detection of CL-exposed brain-derived mitochondrial microparticles (mtMPs) at 17 547 ± 2677/μL in the peripheral blood of mice subjected to fluid percussion injury to the brain. These mtMPs accounted for 55.2% ± 12.6% of all plasma annexin V-binding microparticles found in the acute phase of injury. They were also released from cultured neuronal and glial cells undergoing apoptosis. The mtMPs synergized with platelets to facilitate vascular leakage by disrupting the endothelial barrier. The disrupted endothelial barrier allowed the release of mtMPs into the systemic circulation to promote coagulation in both traumatically injured and mtMP- or CL-injected mice, leading to enhanced fibrinolysis, vascular fibrin deposition, and thrombosis. This mtMP-induced coagulation was mediated by CL transported from the inner to the outer mitochondrial membrane and was blocked by the scavenging molecule lactadherin. The mtMP-bound CL was ∼1600 times as active as purified CL in promoting coagulation. This study uncovered a novel procoagulant activity of CL and CL-exposed mitochondria that may contribute to traumatic brain injury-associated coagulopathy and identified potential pathways to block this activity.

Study of the anti-angiogenic effects of cardiolipin by the aortic ring assay

Cardiolipin (CL), a phospholipid found in the inner mitochondrial membrane in all cell types, is critical for the function of the electron transport chain. The role of CL is not fully understood, but it is assumed that the molecule maintains membrane potential and architecture and compensates for alterations in homeostasis that could affect the energy metabolism. The objective of this project was to determine the effects of increasing CL concentrations on angiogenic sprouting by using the aortic ring assay model. For this, 5-day-old C57Bl/6 pups were euthanized by cervical dislocation prior to removal of the aortas. The vessels were cleaned, cut in 0.5 mm wide rings, and placed in a collagen growth matrix supplemented with CL. The results revealed a highly significant reduction of sprout growth (both length and quantity) at low, physiological concentrations. In conclusion, the results of this study demonstrate that CL significantly reduces microvessel formation and that it could potentially provide an interesting novel therapeutic target for angiogenesis.

Neuronal injury from cardiac arrest: aging years in minutes

Cardiac arrest is a leading cause of death and permanent disability. Most victims succumb to the oxidative and inflammatory damage sustained during cardiac arrest/resuscitation, but even survivors typically battle long-term neurocognitive impairment. Although extensive research has delineated the complex mechanisms that culminate in neuronal damage and death, no effective treatments have been developed to interrupt these mechanisms. Of importance, many of these injury cascades are also active in the aging brain, where neurons and other cells are under persistent oxidative and inflammatory stress which eventually damages or kills the cells. In light of these similarities, it is reasonable to propose that the brain essentially ages the equivalent of several years within the few minutes taken to resuscitate a patient from cardiac arrest. Accordingly, cardiac arrest-resuscitation models may afford an opportunity to study the deleterious mechanisms underlying the aging process, on an accelerated time course. The aging and resuscitation fields both stand to gain pivotal insights from one another regarding the mechanisms of injury sustained during resuscitation from cardiac arrest and during aging. This synergism between the two fields could be harnessed to foster development of treatments to not only save lives but also to enhance the quality of life for the elderly.

Leucocyte expression of complement C5a receptors exacerbates infarct size after myocardial reperfusion injury

AIMS

Early reperfusion is mandatory for the treatment of acute myocardial infarction. This process, however, also induces additional loss of viable myocardium, called ischaemia-reperfusion (IR) injury. Complement activation plays an important role in IR injury, partly through binding of C5a to its major receptor (C5aR). We investigated the role of C5aR on infarct size and cardiac function in a model for myocardial IR injury.

METHODS AND RESULTS

BALB/c (WT) mice and C5aR(-/-) mice underwent coronary occlusion for 30 min, followed by reperfusion. Infarct size, determined 24 h after IR, was reduced in C5aR(-/-) mice compared with WT mice (28.5 ± 2.1 vs. 35.7 ± 2.5%, P = 0.017). Bone marrow (BM) chimaera experiments showed that this effect was due to the absence of C5aR on circulating leucocytes, since a similar reduction in infarct size was observed in WT mice with C5aR-deficient BM cells (25.3 ± 2.2 vs. 34.6 ± 2.8%, P < 0.05), but not in C5aR(-/-) mice with WT BM cells. Reduced infarct size was associated with fewer neutrophils, T cells, and macrophages in the infarcted area 24 h after IR in C5aR(-/-) mice, and also with lower levels of Caspase-3/7 indicating less inflammation and apoptosis. Echocardiography 4 weeks after IR showed an improved ejection fraction in C5aR(-/-) mice (25.8 ± 5.5 vs. 19.2 ± 5.4%, P < 0.001).

CONCLUSION

The absence of C5aR on circulating leucocytes reduces infarct size, is associated with reduced leucocyte infiltration and with less apoptosis in the infarcted myocardium, and improves cardiac function in a mouse model of myocardial IR injury. Selective blocking of C5aR might be a promising strategy to prevent myocardial IR injury.

High proportion of patients with end-stage heart failure regardless of aetiology demonstrates anti-cardiac antibody deposition in failing myocardium: humoral activation, a potential contributor of disease progression

AIMS

Various reports have raised the possibility of humoral immune responses as contributors for the progression of heart failure. Previous studies, however, have focused on the analysis of serum and documented circulating antibodies against a variety of cardiac proteins. However, there is little evidence on whether anti-cardiac antibodies are deposited in end-stage failing myocardium. Our objective was to determine whether or not there was evidence of deposition of anti-cardiac antibodies and/or activated complement components in end-stage failing human myocardium.

METHODS AND RESULTS

Myocardial samples were obtained from 100 end-stage heart failure patients and 40 donor control biopsies. Sections were cut and stained using standard fluorescent immunohistochemistry techniques with anti-human immunoglobulin G (IgG), IgG3, and C3c. Gel electrophoresis and protein identification by mass spectrometry were used to confirm the presence of IgG and its antigen. Immunoglobulin G was localized to the sarcolemma in 71% of patients, 48% of those being positive for the subtype IgG3. The proportion of patients with ischaemic heart disease that was positive for IgG was 65% and among those with non-ischaemic aetiologies was 76%. In a subgroup analysis, the presence of IgG and its subunits were confirmed by mass spectrometry and adenosine triphosphate synthase β subunit identified as an antigen. Complement was activated in 31% of all patients. The presence of IgG, IgG3, and C3c was directly correlated with the length of disease (r = 0.451, P = 0.006).

CONCLUSION

Evidence of anti-cardiac antibodies and complement activation was found in a large number of patients with end-stage cardiomyopathy regardless of the aetiology. Adenosine triphosphate synthase appears to be a new prominent antigenic stimulus; but more interestingly, the simultaneous co-existence of activated complement components suggests that this humoral mechanism may participate in disease progression.

CXC chemokine KC fails to induce neutrophil infiltration and neoangiogenesis in a mouse model of myocardial infarction

BACKGROUND

Chemokines and neutrophils, known as important players in the inflammatory cascade, also contribute to heart tissue recovery and scar formation after myocardial infarction (MI). The objective of this study was to determine the importance of ELR-containing CXC chemokine KC in neutrophil infiltration and neoangiogenesis, in a mouse model of chronic MI.

METHODS AND RESULTS

MI was induced in mice divided in four groups: control (untreated), anti-KC "later" (anti-KC antibody injections started 4 days after MI and then delivered every 72 hours for 3 weeks, to inhibit angiogenesis), anti-KC "earlier" (anti-KC antibody injections 1 day before and 1 day after MI, to block neutrophil infiltration), anti-KC (anti-KC antibody injections 1 day before and 1 day after MI, and then every 72 hours for 3 weeks). The efficiency of the anti-KC treatment was determined by the measurement of KC serum concentration and immunofluorescence staining, in each of the four groups. Surprisingly, we did not find any difference in neutrophil infiltration in the infarcted area between untreated and treated animals. Moreover, the heart function, infarct size, and neoangiogenesis were not different between the four groups. As expected, a comparable anti-CXCR2 treatment of mice before and after MI was able to significantly reduce neutrophil infiltration into the infarcted area and angiogenesis, but also to reduce the infarction size after long or "later" treatment.

CONCLUSIONS

The major finding of our study is that KC, a potent neutrophil chemoattractant and an established angiogenic factor, failed to interfere in the post-infarction inflammatory response, in wound healing and scar formation after MI. Therefore, these aspects need to be carefully taken into account when devising therapeutic strategies for myocardial infarction and ischemic cardiomyopathy.

Complement activation and cardiac surgery: a novel target for improving outcomes

Complement activation and the resulting inflammatory response is an important potential mechanism for multisystem organ injury in cardiac surgery. Novel therapeutic strategies using complement inhibitors may hold promise for improving outcomes for cardiac surgical patients by attenuating complement activation or its biologically active effector molecules. Recent clinical trials evaluating complement inhibitors have provided important data to further delineate the impact of complement activation and its inhibition on clinical outcomes. In this review we examine the role of complement activation and its inhibition as a therapeutic approach in cardiac surgery.

Cell biology of ischemia/reperfusion injury

Disorders characterized by ischemia/reperfusion (I/R), such as myocardial infarction, stroke, and peripheral vascular disease, continue to be among the most frequent causes of debilitating disease and death. Tissue injury and/or death occur as a result of the initial ischemic insult, which is determined primarily by the magnitude and duration of the interruption in the blood supply, and then subsequent damage induced by reperfusion. During prolonged ischemia, ATP levels and intracellular pH decrease as a result of anaerobic metabolism and lactate accumulation. As a consequence, ATPase-dependent ion transport mechanisms become dysfunctional, contributing to increased intracellular and mitochondrial calcium levels (calcium overload), cell swelling and rupture, and cell death by necrotic, necroptotic, apoptotic, and autophagic mechanisms. Although oxygen levels are restored upon reperfusion, a surge in the generation of reactive oxygen species occurs and proinflammatory neutrophils infiltrate ischemic tissues to exacerbate ischemic injury. The pathologic events induced by I/R orchestrate the opening of the mitochondrial permeability transition pore, which appears to represent a common end-effector of the pathologic events initiated by I/R. The aim of this treatise is to provide a comprehensive review of the mechanisms underlying the development of I/R injury, from which it should be apparent that a combination of molecular and cellular approaches targeting multiple pathologic processes to limit the extent of I/R injury must be adopted to enhance resistance to cell death and increase regenerative capacity in order to effect long-lasting repair of ischemic tissues.

Ischemia/reperfusion injury: The role of immune cells

Ischemia/reperfusion (I/R) injury is an inflammatory condition that is characterized by innate immunity and an adaptive immune response. This review is focused on the acute inflammatory response in I/R injury, and also the adaptive immunological mechanisms in chronic ischemic disease that lead to increased vulnerability during acute events, in relation to the cell types that have been shown to mediate innate immunity to an adaptive immune response in I/R, specifically myocardial infarction. Novel aspects are also highlighted in respect to the mechanisms within the cardiovascular system and cardiovascular risk factors that may be involved in the inflammatory response accompanying myocardial infarction. Experimental myocardial I/R has suggested that immune cells may mediate reperfusion injury. Specifically, monocytes, macrophages, T-cells, mast cells, platelets and endothelial cells are discussed with reference to the complement cascade, toll-like receptors, cytokines, oxidative stress, renin-angiotensin system, and in reference to the microvascular system in the signaling mechanisms of I/R. Finally, the findings of the data summarized in this review are most important for possible translation into clinical cardiology practice and possible avenues for drug development.

Induction of cardiac angiogenesis requires killer cell lectin-like receptor 1 and α4β7 integrin expression by NK cells

Recent findings indicate that NK cells are involved in cardiac repair following myocardial infarction. The aim of this study is to investigate the role NK cells in infarct angiogenesis and cardiac remodeling. In normal C57BL/6 mice, myelomonocytic inflammatory cells invaded infarcted heart within 24 h followed by a lymphoid/NK cell infiltrate by day 6, accompanied by substantial expression of IL-2, TNF-α, and CCL2. In contrast, NOD SCID mice had virtually no lymphoid cells infiltrating the heart and did not upregulate IL-2 levels. In vitro and in vivo, IL-2-activated NK cells promoted TNF-α-stimulated endothelial cell proliferation, enhanced angiogenesis and reduced fibrosis within the infarcted myocardium. Adoptive transfer of IL-2-activated NK cells to NOD SCID mice improved post-myocardial infarction angiogenesis. RNA silencing technology and neutralizing Abs demonstrated that this process involved α4β7 integrin/VCAM-1 and killer cell lectin-like receptor 1/N-cadherin-specific binding. In this study, we show that IL-2-activated NK cells reduce myocardial collagen deposition along with an increase in neovascularization following acute cardiac ischemia through specific interaction with endothelial cells. These data define a potential role of activated NK cells in cardiac angiogenesis and open new perspectives for the treatment of ischemic diseases.

Specific inhibition of the classical complement pathway with an engineered single-chain Fv to C1q globular heads decreases complement activation by apoptotic cells

Apoptotic cells are potent complement activators; and proposed mechanisms include IgM-mediated classical pathway activation, C-reactive protein (CRP)-mediated classical pathway activation, and IgM-mediated lectin pathway activation. While complement activation is beneficial in clearing apoptotic cells, the resulting complement-mediated inflammation may extend damage to the surrounding cells and tissues, as observed in ischemia/reperfusion injury. We previously engineered and characterized a single-chain Fv against C1q globular heads (scFv(QuVHVL)) that blocked C1q binding to immobilized IgG and to IgG-sensitized cells, and thereby inhibited IgG-mediated classical pathway activation [Hwang H.Y., Duvall M.R., Tomlinson S., Boackle R.J., 2008. Highly specific inhibition of C1q globular-head binding to human IgG: a novel approach to control and regulate the classical complement pathway using an engineered single-chain antibody variable fragment. Molecular Immunology 45, 2570-2580]. In the present study, this scFv(QuVHVL) was examined for its ability to restrict complement deposition on apoptotic cells in the presence of fresh normal human serum (NHS). Interestingly, the addition of scFv(QuVHVL) to NHS decreased C1-mediated C4b deposition on apoptotic cells by 60% as compared to appropriate buffer-treated control serum. By inhibiting initiation of the early complement components, the subsequent C3b and membrane attack complex depositions were inhibited by 70%. Apoptotic cells may acquire serum CRP, a known classical complement pathway activator. It was observed that scFv(QuVHVL) blocked C1 binding to CRP and blocked CRP-mediated classical pathway activation using an ELISA format. However, under the experimental conditions used, the addition of exogenous CRP to apoptotic cells did not further increase the levels of C4b, C3b, or MAC deposition significantly, suggesting predominance by other activation mechanisms, such as antibody-C1-mediated complement activation. In summary, the results indicated that C1-mediated classical pathway activation was a highly significant mechanism for complement activation by apoptotic cells. In the future, specific inhibition of classical complement pathway activation by a humanized form of scFv(QuVHVL) may be useful in reducing inadvertent damage to healthy bystander tissue in a variety of acute, complement-mediated inflammatory conditions, including ischemia/reperfusion injury.

The immune system and cardiac repair

Myocardial infarction is the most common cause of cardiac injury and results in acute loss of a large number of myocardial cells. Because the heart has negligible regenerative capacity, cardiomyocyte death triggers a reparative response that ultimately results in formation of a scar and is associated with dilative remodeling of the ventricle. Cardiac injury activates innate immune mechanisms initiating an inflammatory reaction. Toll-like receptor-mediated pathways, the complement cascade and reactive oxygen generation induce nuclear factor (NF)-kappaB activation and upregulate chemokine and cytokine synthesis in the infarcted heart. Chemokines stimulate the chemotactic recruitment of inflammatory leukocytes into the infarct, while cytokines promote adhesive interactions between leukocytes and endothelial cells, resulting in transmigration of inflammatory cells into the site of injury. Monocyte subsets play distinct roles in phagocytosis of dead cardiomyocytes and in granulation tissue formation through the release of growth factors. Clearance of dead cells and matrix debris may be essential for resolution of inflammation and transition into the reparative phase. Transforming growth factor (TGF)-beta plays a crucial role in cardiac repair by suppressing inflammation while promoting myofibroblast phenotypic modulation and extracellular matrix deposition. Myofibroblast proliferation and angiogenesis result in formation of highly vascularized granulation tissue. As the healing infarct matures, fibroblasts become apoptotic and a collagen-based matrix is formed, while many infarct neovessels acquire a muscular coat and uncoated vessels regress. Timely resolution of the inflammatory infiltrate and spatial containment of the inflammatory and reparative response into the infarcted area are essential for optimal infarct healing. Targeting inflammatory pathways following infarction may reduce cardiomyocyte injury and attenuate adverse remodeling. In addition, understanding the role of the immune system in cardiac repair is necessary in order to design optimal strategies for cardiac regeneration.

Role of novel mediators of inflammation in left ventricular remodeling in patients with acute myocardial infarction: do they affect the outcome of patients?

Left ventricular (LV) remodeling after acute myocardial infarction (AMI) is a major mechanism for cardiovascular death and disability. A significant number of post-MI patients develop progressive left ventricular enlargement and heart failure and many require heart transplantation and ventricular assist devices. Understanding of the basic mechanisms regulating the reaction to injury is crucial for the development of site-specific cell biological strategies of intervention to both reduce injury and promote repair. To determine whether there are new inflammatory markers having a role in structural remodeling after AMI in patients who applied to the emergency department of this hospital with severe chest pain at the first 12 hours, the authors measured the levels of C-reactive protein (CRP), macrophage colony-stimulating factor (M-CSF) and interleukin-3 (IL-3) in patients with AMI at hospital admission and on day 5. They measured plasma CRP concentrations by using highly sensitive CRP reagent with the immunonephelometric method, and plasma M-CSF and IL-3 concentrations with the help of a commercial enzyme-linked immunoassay test in 30 patients with AMI. Mean plasma CRP, M-CSF, and IL-3 concentrations at admission to the hospital were significantly higher than those on day 5 (5.0 -/+ 3.1 mg/dL, 119.4 -/+ 103.6 pg/mL, and 155.1 -/+ 83.4 ng/mL, respectively, p < 0.001 for each value). CRP, M-CSF, and IL-3 were all increased in patients with AMI. These findings suggest that these are new inflammatory markers, which may have important roles in LV remodeling after AMI.

Neutrophil ablation with anti-serum does not protect against phase 2 ventricular arrhythmias in anaesthetised rats with myocardial infarction

OBJECTIVES

Arrhythmias, including ventricular fibrillation (VF), occur in two phases after coronary obstruction, the first during the reversible stage of acute myocardial ischaemia (phase 1) and the second during evolution of the infarct (phase 2). We tested the hypothesis that phase 2 arrhythmias are mediated by actions of neutrophils accumulating within the infarct.

METHODS

Male rats (n=10 per group) were randomized to receive 2 ml/kg i.p. of either rabbit anti-rat neutrophil anti-serum or normal rabbit serum. After 17 h, single stage left coronary artery ligation was performed under pentobarbitone anaesthesia, and ischaemia was maintained for 240 min.

RESULTS

Anti-serum pretreatment caused almost total neutropenia, reducing neutrophils in circulating blood from 2096+/-274x10(3) to 8+/-8x10(3) per ml (p<0.05). It also blocked neutrophil accumulation in the infarct, reducing cardiac myeloperoxidase activity from 74.7+/-27.4 to 9+/-3 mU per mg protein (p<0.05). Despite this, there was no significant difference between control and anti-serum-treated rats in the incidence of phase 2 VF (30% in each group) tachycardia (VT; 60% vs 80%) or number of ventricular premature beats (VPBs).

CONCLUSION

Neutrophil accumulation within the evolving myocardial infarct does not mediate phase 2 VF.

Complement mediators in ischemia–reperfusion injury

BACKGROUND

Ischemia-reperfusion (I/R) injury occurs when a tissue is temporarily deprived of blood supply and the return of the blood supply triggers an intense inflammatory response. Pathologically, increased complement activity can cause substantial damage to blood vessels, tissues and also facilitate leukocyte activation and recruitment following I/R injury. Herein, previously published studies are reported and critically reviewed.

METHODS

Medline and the World Wide Web were searched and the relevant literature was classified under the following categories: (1) Complement pathways; (2) The complement system and the inflammatory response; (3) Complement in ischemia-reperfusion injuries; and (4) Therapeutic approaches against complement in I/R injuries.

RESULTS AND CONCLUSIONS

I/R injury is a common clinical event with the potential to seriously affect, and sometimes kill, the patient and is a potent inducer of complement activation that results in the production of a number of inflammatory mediators. Complement activation leads to the release of biologically active potent inflammatory complement substances including the anaphylatoxins (C3a and C5a) and the cytolytic terminal membrane attack complement complex C5b-9 (MAC). The use of specific complement inhibitors to block complement activation at various levels of the cascade has been shown to prevent or reduce local tissue injury after I/R. Several agents that inhibit all or part of the complement system, such as soluble complement receptor type 1 (sCR1), C1 inhibitor (C1-INH), C5a monoclonal antibodies, a C5a receptor antagonist and soluble CD59 (sCD59) have been shown to reduce I/R injury of various organs. The novel inhibitors of complement products may eventually find wide clinical application because there are no effective drug therapies currently available to treat I/R injuries.

The mechanistic basis of infarct healing

Myocardial infarction triggers an inflammatory cascade that results in healing and replacement of the damaged tissue with scar. Cardiomyocyte necrosis triggers innate immune mechanisms eliciting Toll-like receptor- mediated responses, activating the complement cascade and generating reactive oxygen species. Subsequent activation of NF-kappaB is a critical element in the regulation of cytokine, chemokine, and adhesion molecule expression in the ischemic myocardium. Chemokine induction mediates leukocyte recruitment in the myocardium. Pleiotropic proinflammatory cytokines, such as TNF-alpha, IL-1, and IL-6, are also upregulated in the infarct and exert a wide range of effects on a variety of cell types. Timely repression of proinflammatory gene synthesis is crucial for optimal healing; IL-10 and TGF-beta-mediated pathways may be important for suppression of chemokine and cytokine expression and for resolution of the leukocytic infiltrate. In addition, TGF-beta may be critically involved in inducing myofibroblast differentiation and activation, promoting extracellular matrix protein deposition in the infarcted area. The composition of the extracellular matrix plays an important role in regulating cell behavior. Both structural and matricellular proteins modulate cell signaling through interactions with specific surface receptors. The molecular and cellular changes associated with infarct healing directly influence ventricular remodeling and affect prognosis in patients with myocardial infarction.

Membrane complement regulatory proteins

A number of proteins anchored on the cell surface function to protect host tissues from bystander injury when complement is activated. In humans, they include decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46), complement receptor 1 (CR1, CD35) and CD59. Although disease conditions directly attributable to abnormal function of these proteins are relatively rare, it has become evident from recent studies using animal models that membrane complement regulatory proteins are important modulators of tissue injury in many autoimmune and inflammatory disease settings. Evidence is also emerging to support a role of these proteins in regulating cellular immunity. In this article, we highlight recent advances on the in vivo biology of membrane complement regulatory proteins and discuss their relevance in human disease pathogenesis and therapeutics.

Prevention of cardiac cell injury during acute myocardial infarction: possible role for complement inhibition

The purpose of this article is to describe mechanisms of cell death in patients with acute myocardial infarction, particularly the activation of the complement system. Various pro-inflammatory cytokines, released by the inflamed tissue, play a role in the activation of the complement system. Several complement inhibitors have been developed to reduce tissue damage following ischemia. According to animal studies the deleterious effects of activators of the complement system can be diminished by complement inhibition. Several clinical studies have been conducted for the potential treatment of cell injury during acute myocardial infarction. C1 inhibitor dose-dependently inhibited complement activation and appeared to reduce myocardial injury after reperfusion therapy in patients with acute myocardial infarction. C1 inhibitor dose-dependently reduced plasma levels of C4 activation fragments. In addition, cardiac enzymes (troponin T and creatine kinase-MB) returned to baseline levels more rapidly among patients treated with C1 inhibitor, compared with controls. Furthermore, preliminary results from a placebo-controlled trial indicate that treatment with intravenous pexelizumab (anti-C5 antibody) was well tolerated in a large number of patients undergoing coronary artery bypass graft surgery. Further, more randomized trials are necessary to clarify the clinical significance of this new and innovative treatment with complement inhibition.

Role of the complement system in ischaemic heart disease: potential for pharmacological intervention

The complement system is an innate, cytotoxic host defence system that normally functions to eliminate foreign pathogens. However, considerable evidence suggests that complement plays a key role in the pathophysiology of ischaemic heart disease (IHD). Experimental models of acute myocardial infarction (MI) and autopsy specimens taken from acute MI patients demonstrate that complement is selectively deposited in areas of infarction. Furthermore, inhibition of complement activation or depletion of complement components prior to myocardial reperfusion has been shown to reduce complement-mediated tissue injury in numerous animal models. IHD remains a leading cause of patient morbidity and mortality. Considerable effort in recent years has therefore been directed by biotechnology and pharmaceutical industries towards the development of novel, human complement inhibitors. Proposed anticomplement therapeutic strategies include the administration of naturally occurring or recombinant complement regulators, anticomplement monoclonal antibodies, and anticomplement receptor antagonists. Although data regarding the effectiveness of anticomplement therapy in humans is limited at present, a number of novel anticomplement therapeutic strategies are currently in clinical trials. The role of complement in IHD and potential for pharmacological intervention is reviewed.

Complement and its implications in cardiac ischemia/reperfusion: strategies to inhibit complement

Although reperfusion of the ischemic myocardium is an absolute necessity to salvage tissue from eventual death, it is also associated with pathologic changes that represent either an acceleration of processes initiated during ischemia or new pathophysiological changes that were initiated after reperfusion. This so-called "reperfusion injury" is accompanied by a marked inflammatory reaction, which contributes to tissue injury. In addition to the well known role of oxygen free radicals and white blood cells, activation of the complement system probably represents one of the major contributors of the inflammatory reaction upon reperfusion. The complement may be activated through three different pathways: the classical, the alternative, and the lectin pathway. During reperfusion, complement may be activated by exposure to intracellular components such as mitochondrial membranes or intermediate filaments. Two elements of the activated complement contribute directly or indirectly to damages: anaphylatoxins (C3a and C5a) and the membrane attack complex (MAC). C5a, the most potent chemotactic anaphylatoxin, may attract neutrophils to the site of inflammation, leading to superoxide production, while MAC is deposited over endothelial cells and smooth vessel cells, leading to cell injury. Experimental evidence suggests that tissue salvage may be achieved by inhibition of the complement pathway. As the complement is composed of a cascade of proteins, it provides numerous sites for pharmacological interventions during acute myocardial infarction. Although various strategies aimed at modulating the complement system have been tested, the ideal approach probably consists of maintaining the activity of C3 (a central protein of the complement cascade) and inhibiting the later events implicated in ischemia/reperfusion and also in targeting inhibition in a tissue-specific manner.

The phosphatidylserine receptor: a crucial molecular switch?

The uptake and removal of necrotic or lysed cells involves inflammation and an immune response, due in part to processes that involve members of the collectin family, surface calreticulin and CD91. Clearance of apoptotic cells, by contrast, does not induce either inflammation or immunity. Could the phosphatidylserine receptor be the molecular switch that determines what the outcome will be?

Therapeutic potential of complement inhibitors in myocardial ischaemia

Under normal conditions, the complement system functions to eradicate microbes and other membrane bound pathogens. In other situations, complement activation comprises a pivotal mechanism for mediating tissue demolition in inflammatory disorders, including ischaemia/reperfusion injury. Complement-mediated tissue damage has long been recognised as a significant contributor to myocardial reperfusion injury. However, clinical use of complement inhibitors to reduce the extent of irreversible tissue injury related to reperfusion, remains in the early stages of development. Activation of the complement system generates anaphylatoxins, opsonins and the lytic moiety known as the membrane attack complex (MAC). In addition, fragments of the complement cascade proteins (e.g., C3a and C5a) secondarily initiate processes deleterious to myocytes by recruiting and stimulating inflammatory cells, such as neutrophils and macrophages, within the area of reperfusion. Damaged tissue itself, is capable of upregulating the genes that encode the formation of complement proteins leading to assembly of the MAC, which in turn further advances tissue injury. All of these factors contribute to the development of myocardial infarction subsequent to ischaemia and reperfusion. This paper provides an overview of how the complement system operates and examines the various inhibitors, both endogenous and exogenous, that regulate the complement cascade. Activation and inhibition of the complement system will be discussed primarily in the context of myocardial ischaemia and reperfusion injury.

Complement activation induced by ischemia-reperfusion in humans: a study in patients undergoing partial hepatectomy

BACKGROUND/AIM

Activation of the complement system is induced by ischemia-reperfusion (I/R) in animal models. Whether I/R also induces complement activation in humans is not known. Here, we investigated complement activation in patients undergoing major liver resection.

METHODS

In 11 of 17 patients, the hepatoduodenal ligament was clamped, making the liver transiently ischemic (HEMI+; mean ischemia time, 42 +/- 18 min); 6 patients were operated without clamping (HEMI-). Activation at plasma level (circulating activation products) was studied in blood samples collected prior to surgery and 5, 24 and 48 h thereafter. Parameters analyzed were C4b/c and C3b/c, C4d and C3d, C3a, as well as complexes between complement and C-reactive protein (CRP), which reflect CRP-induced complement activation. Activation at tissue level (C3 and C4 fixation) was studied in liver biopsies obtained before and after resection.

RESULTS

In plasma, post-operative levels of C4b/c and C3b/c were not different from baseline levels in both groups. Mean plasma levels of C4b/c and C3b/c were significantly decreased at 24 h post-surgery in the HEMI+ group (p=0.02 and p=0.07). At the same time, levels of C4d-CRP and C3d-CRP were significantly increased (p<0.01 for both parameters). At tissue level, activated complement fragments were observed intracellularly in some pericentral hepatocytes. In I/R livers, large numbers of hepatocytes were positively stained for all complement activation products.

CONCLUSIONS

Our data show that in situ complement activation via the classical route occurred during liver resection and that ischemia and/or reperfusion may have contributed to activation. Levels of complement activation products in the circulation were low, showing that transient ischemia had no severe influence on systemic complement activation, suggesting a locally contained response.

Evaluation of the membrane attack complex of complement for the detection of a recent myocardial infarction in man

The diagnosis of an acute myocardial infarction (MI) can be cumbersome for pathologists. Even with a positive nitroblue tetrazolium (NBT) reaction, haematoxylin and eosin (H&E) evaluation of the myocardial tissue can remain inconclusive. Early signs presumed diagnostic for myocardial infarction, such as hypereosinophilia, waviness, and contraction band necrosis, have to be considered non-specific and are probably reversible signs of ischaemia. Several studies implicate the complement system, and especially complement factor C9, as part of the membrane attack factor (MAC), in cardiomyocyte damage during MI. In a post-mortem study on well-documented cardiological autopsies, we evaluated the use of complement factor C9 immunostaining as a marker for the detection of very recent MI. Forty-three tissue samples from 40 patients were obtained from the left ventricular free wall only, a region that can be specifically attributed to one corresponding coronary artery. As some patients presented with MIs of various stages in that perfusion area, in total 57 observations were possible. C9 immunostaining specifically detected irreversibly damaged (=infarcted) cardiomyocytes, as is implied by the lytic activity of C9/MAC binding to cell membranes. Most interesting was the group of clinically suspected, NBT-positive MIs resulting from very recent myocardial ischaemia. In this population, where H&E evaluation by (cardio-) experienced pathologists was not conclusive, C9 immunostaining clearly pointed towards myocardial infarction in 47% of the cases. In conclusion, C9 immunostaining, routinely practicable in the pathology laboratory, has an additional value in discriminating between reversible ischaemia and infarcted cardiomyocytes in very early MIs.

Cardiolipin enhances protein C pathway anticoagulant activity

The anticoagulant activity of activated protein C (APC) was studied using factor Xa-1-stage assays of both the procoagulant and anticoagulant activities of phospholipid vesicles containing phosphatidylserine or cardiolipin as active phospholipids. In the absence of APC, phosphatidylserine vesicles showed higher procoagulant activity than cardiolipin vesicles whereas cardiolipin vesicles supported APC-dependent anticoagulant activity better than phosphatidylserine vesicles. Enhancement of APC anticoagulant activity in plasma by cardiolipin was markedly stimulated by the APC cofactor protein S. In purified reaction mixtures, cardiolipin in phospholipid vesicles dose-dependently enhanced APC anticoagulant activity. This effect of cardiolipin was partially dependent on protein S, and immunoblotting studies showed that cardiolipin enhanced the APC-mediated cleavage of the factor Va heavy chain at Arg506 and Arg306. In solid-phase binding assays, increasing amounts of cardiolipin in multicomponent phospholipid vesicles increased the affinity for protein S and to a lesser extent APC. These data are consistent with the hypothesis that cardiolipin stimulates the anticoagulant protein C pathway by increasing the affinity of phospholipid surfaces for protein S:APC and by enhancing inactivation of factor Va by APC due to cleavages at Arg506 and Arg306 in factor Va. Based on this, it is further hypothesized that anti-cardiolipin or anti-oxidized cardiolipin antibodies may be thrombogenic because they inhibit phospholipid-dependent expression of the anticoagulant protein C pathway.

Contamination of lymph from the major prenodal cardiac lymphatic in dogs

Cannulation of the canine major prenodal cardiac lymphatic (MPCL) is the most common approach for the investigation of myocardial lymphatic function. However, the assumption that the MPCL drains pure cardiac lymph has been questioned. We studied variations of MPCL anatomy and investigated whether noncardiac lymph is drained by this lymphatic. After dye was injected into the lungs and left ventricular myocardium in 21 dogs, dissection of the cardiac lymphatic system yielded 3 anatomic variations. In variations 1 and 2 (81% of dogs), a mixture of cardiac and pulmonary lymph was drained via the MPCL. In variation 3 (19% of dogs) no connection was found between MPCL and pulmonary lymphatics. In variations 1 and 2, alteration of tidal volume resulted in significant changes of lymph flow rate. The pulmonary contribution to MPCL lymph flow was estimated as 34% in variation 2. We conclude that MPCL lymph may contain not only cardiac lymph but also significant pulmonary contamination. This finding should be considered in the interpretation of lymph data from cannulation of the canine MPCL.

Complement activation and regulator expression after anoxic injury of human endothelial cells

Complement activation is involved in the ischemia-reperfusion injury of various organs, but the mechanisms leading to activation of the complement system are incompletely understood. In this study we show that EA.hy 926 human endothelial cells cultured under anoxic conditions (24 or 48 h) become activators of the homologous complement system. Flow cytometric analysis indicated that C1q, C3c, C3d, C4, C5, C9 components of complement are deposited on anoxic but not on normoxic cells after incubation with normal human serum. Cell membrane-associated regulators of complement, membrane cofactor protein (CD46), decay-accelerating factor (CD55) and protectin (CD59) were expressed on EA.hy 926 cells grown under normal oxygen tension. Under anoxic conditions the expression of protectin was clearly decreased, whereas the expression of CD46 and CD55 diminished only slightly. Our results suggest that anoxia can convert human endothelial cells to activators of the complement system. The diminished expression of protectin, CD46 and CD55 can sensitize the cells to complement-mediated damage. Activation of the complement system due to the anoxic injury of human endothelial cells might be an important triggering mechanism in the pathogenesis of ischemia-reperfusion injury of human heart.