N-Acetylheparin pretreatment reduces infarct size in the rabbit

Revisiting the role of the complement system in intracerebral hemorrhage and therapeutic prospects

Intracerebral hemorrhage (ICH) is a stroke subtype characterized by non-traumatic rupture of blood vessels in the brain, resulting in blood pooling in the brain parenchyma. Despite its lower incidence than ischemic stroke, ICH remains a significant contributor to stroke-related mortality, and most survivors experience poor outcomes that significantly impact their quality of life. ICH has been accompanied by various complex pathological damage, including mechanical damage of brain tissue, hematoma mass effect, and then leads to inflammatory response, thrombin activation, erythrocyte lysis, excitatory amino acid toxicity, complement activation, and other pathological changes. Accumulating evidence has demonstrated that activation of complement cascade occurs in the early stage of brain injury, and the excessive complement activation after ICH will affect the occurrence of secondary brain injury (SBI) through multiple complex pathological processes, aggravating brain edema, and pathological brain injury. Therefore, the review summarized the pathological mechanisms of brain injury after ICH, specifically the complement role in ICH, and its related pathological mechanisms, to comprehensively understand the specific mechanism of different complements at different stages after ICH. Furthermore, we systematically reviewed the current state of complement-targeted therapies for ICH, providing a reference and basis for future clinical transformation of complement-targeted therapy for ICH.

The role of complement in brain injury following intracerebral hemorrhage: A review

Intracerebral hemorrhage (ICH) is a significant cause of death and disability and current treatment is limited to supportive measures to reduce brain edema and secondary hematoma expansion. Current evidence suggests that the complement cascade is activated early after hemorrhage and contributes to brain edema/injury in multiple ways. The aim of this review is to summarize the most recent literature about the role of the complement cascade after ICH. Primary literature demonstrating complement mediated brain edema and neurologic injury through the membrane attack complex (MAC) as well as C3a and C5a are reviewed. Further, attenuation of brain edema and improved functional outcomes are demonstrated after inhibition of specific components of the complement cascade. Conversely, complement also plays a significant role in neurologic recovery after ICH and in other neurologic disorders. We conclude that the role of complement after ICH is complex. Understanding the role of complement after ICH is essential and may elucidate possible interventions to reduce brain edema and injury.

Hurdles to Cardioprotection in the Critically Ill

Cardiovascular disease is the largest contributor to worldwide mortality, and the deleterious impact of heart failure (HF) is projected to grow exponentially in the future. As heart transplantation (HTx) is the only effective treatment for end-stage HF, development of mechanical circulatory support (MCS) technology has unveiled additional therapeutic options for refractory cardiac disease. Unfortunately, despite both MCS and HTx being quintessential treatments for significant cardiac impairment, associated morbidity and mortality remain high. MCS technology continues to evolve, but is associated with numerous disturbances to cardiac function (e.g., oxidative damage, arrhythmias). Following MCS intervention, HTx is frequently the destination option for survival of critically ill cardiac patients. While effective, donor hearts are scarce, thus limiting HTx to few qualifying patients, and HTx remains correlated with substantial post-HTx complications. While MCS and HTx are vital to survival of critically ill cardiac patients, cardioprotective strategies to improve outcomes from these treatments are highly desirable. Accordingly, this review summarizes the current status of MCS and HTx in the clinic, and the associated cardiac complications inherent to these treatments. Furthermore, we detail current research being undertaken to improve cardiac outcomes following MCS/HTx, and important considerations for reducing the significant morbidity and mortality associated with these necessary treatment strategies.

Complement Inhibition Attenuates Early Erythrolysis in the Hematoma and Brain Injury in Aged Rats

Background and Purpose- Early erythrolysis in the hematoma contributes to brain injury after intracerebral hemorrhage (ICH). This study investigated the effects of N-acetylheparin, a complement inhibitor, and aurin tricarboxylic acid, a membrane attack complex inhibitor, on early erythrolysis, brain iron deposition, and brain injury in aged rats. Methods- There were 3 parts in the study. First, aged (18 months old) male Fischer 344 rats had an ICH. The time course of erythrolysis in the hematoma was determined by T2* weighted magnetic resonance imaging, and the expression of CD163 was examined. Second, aged rats had an ICH with N-acetylheparin or vehicle. Rats were euthanized at days 1, 3, and 28 after magnetic resonance imaging (T2-, T2*-weighted, and T2* array) and behavioral tests. Brains were used for immunohistochemistry. Third, aged rats had an ICH with avaurin tricarboxylic acid or vehicle. The rats had magnetic resonance imaging and behavioral tests and were euthanized at day 3. Brains were used for immunohistochemistry. Results- Early erythrolysis occurred within the clot in aged F344 rats. There were increased numbers of CD163-positive cells after ICH. Almost all perihematomal CD163-positive cells were microglia/macrophages, while positive neurons were found more distant from the hematoma. Coinjection of N-acetylheparin attenuated erythrolysis, iron accumulation, CD163 expression, microglia activation, brain swelling, and neuronal death in the acute phase, as well as reducing brain atrophy and neurological deficits in the chronic phase. Coinjection of aurin tricarboxylic acid also reduced erythrolysis and ICH-induced brain injury. Conclusions- Inhibiting complement activation resulted in less erythrolysis and brain injury after ICH.

A novel fibroblast growth factor-1 ligand with reduced heparin binding protects the heart against ischemia-reperfusion injury in the presence of heparin co-administration

AIMS

Fibroblast growth factor 1 (FGF1), a heparin/heparan sulfate-binding growth factor, is a potent cardioprotective agent against myocardial infarction (MI). The impact of heparin, the standard of care for MI patients entering the emergency room, on cardioprotective effects of FGF1 is unknown, however.

METHODS AND RESULTS

To address this, a rat model of MI was employed to compare cardioprotective potentials (lower infarct size and improve post-ischemic function) of native FGF1 and an engineered FGF1 (FGF1ΔHBS) with reduced heparin-binding affinity when given at the onset of reperfusion in the absence or presence of heparin. FGF1 and FGF1ΔHBS did not alter heparin's anticoagulant properties. Treatment with heparin alone or native FGF1 significantly reduced infarct size compared to saline (P < 0.05). Surprisingly, treatment with FGF1ΔHBS markedly lowered infarct size compared to FGF1 (P < 0.05). Both native and modified FGF1 restored contractile and relaxation function (P < 0.05 versus saline or heparin). Furthermore, FGF1ΔHBS had greater improvement in cardiac function compared to FGF1 (P < 0.05). Heparin negatively impacted the cardioprotective effects (infarct size, post-ischemic recovery of function) of FGF1 (P < 0.05) but not of FGF1ΔHBS. Heparin also reduced the biodistribution of FGF1, but not FGF1ΔHBS, to the left ventricle. FGF1 and FGF1ΔHBS bound and triggered FGFR1-induced downstream activation of ERK1/2 (P < 0.05); yet, heparin co-treatment decreased FGF1-produced ERK1/2 activation, but not that activated by FGF1ΔHBS.

CONCLUSION

These findings demonstrate that modification of the heparin-binding region of FGF1 significantly improves the cardioprotective efficacy, even in the presence of heparin, identifying a novel FGF ligand available for therapeutic use in ischemic heart disease.

Cardioprotection by polysaccharide sulfate against ischemia/reperfusion injury in isolated rat hearts

AIM

Polysaccharide sulfate (PSS) is a new type of heparinoid synthesized with alginic acid as the basic material and then by chemical introduction of effective groups. Although PSS is successfully applied in ischemic cardio-cerebrovascular disease, its effect on cardiac function after ischemia/reperfusion (I/R) injury has previously not been investigated. The aim of the present study was to investigate whether PSS can protect the heart from I/R injury and the underlying mechanism of protection.

METHODS

Isolated rat hearts were perfused (Langendorff) and subjected to 20 min global ischemia followed by 60 min reperfusion with Kreb's Henseleit solution or PSS (0.3-100 mg/L). Myocardial contractile function was continuously recorded. Creatine kinase (CK) and lactate dehydrogenase (LDH) leakage were measured. Tumor necrosis factor-alpha (TNF-alpha) expression in cardiomyocytes was investigated. Western blot analysis for extracellular regulated kinases (ERKs), c-jun amino-terminal kinase (JNKs) and p38 mitogen-activated protein kinase (MAPK) activity was performed.

RESULTS

After I/R, cardiac contractility decreased, CK and LDH levels increased in the coronary effluent, and TNF-alpha expression increased in cardiomyocytes. PSS administration at concentrations of 1-30 mg/L improved cardiac contractility, reduced CK and LDH release and inhibited TNF-alpha production. Phosphorylated-p38MAPK (p-p38MAPK) and p-p54/p46-JNK increased in I/R rat hearts but diminished in PSS (1-30 mg/L) treated hearts. P-p44/p42-ERK levels were unchanged. In contrast, high concentrations of PSS (100 mg/L) had adverse effects that caused a worsening of heart function.

CONCLUSION

PSS has dose-dependent cardioprotective effects on the rat heart after I/R injury. The beneficial effects may be mediated through normalization of the activity of p38 MAPK and JNK pathways as well as controlling the level of TNF-alpha expression.

Sulodexide: a renewed interest in this glycosaminoglycan

Glycosaminoglycans (GAGs) are the most abundant group of heteropolysaccharides found in the body. These long unbranched molecules contain a repeating disaccharide unit. GAGs are located primarily in the extracellular matrix or on the surface of cells. These molecules serve as lubricants in the joints while at the same time providing structural rigidity to cells. Sulodexide is a highly purified glycosaminoglycan composed of a fast mobility heparin fraction as well as dermatan sulfate. Sulodexide differs from other glycosaminoglycans, like heparin, by having a longer half-life and a reduced effect on systemic clotting and bleeding. In addition, sulodexide demonstrates a lipolytic activity that is increased in comparison to heparin. Oral administration of sulodexide results in the release of tissue plasminogen activator and an increase in fibrinolytic activities. An increasing body of research has demonstrated the safety and efficacy of sulodexide in a wide range of vascular pathologies.

Dermatan disulfate (Intimatan) prevents complement-mediated myocardial injury in the human-plasma-perfused rabbit heart

Intimatan (dermatan 4,6-O-disulfate), a heparin cofactor II agonist, is a highly sulfated negatively charged semisynthetic polysaccharide. The present study examined the hypothesis that Intimatan reduces complement-mediated myocardial injury. The rabbit isolated heart was perfused with 4% normal human plasma (NHP) as a source of complement in the absence or presence of Intimatan (5 microM). Heat-inactivated human plasma (HIHP) was used as a negative control. Previous studies demonstrated that contact of rabbit tissue with human plasma results in activation of the alternative pathway of the human complement system, leading to irreversible myocardial injury. In the presence of NHP, left ventricular end-diastolic pressure (LVEDP) was increased significantly to 61.8+/-11.7 mm Hg compared to a value of 17.2+/-6.1 mm Hg in hearts perfused in the presence of HIHP. Left ventricular developed pressure (LVDP) was reduced significantly upon exposure to NHP, 19.3+/-10.2 (NHP) vs. 54.0+/-8.0 mm Hg (HIHP). Functional impairment in the presence of NHP was accompanied by a significant release of cardiac troponin I (cTnI; 131.8+/-20.3 ng/ml) as compared to hearts exposed to HIHP (0.8+/-0.8). Intimatan treatment improved cardiac function and maintained viability of cardiac myocytes (LVEDP 14.6+/-5.6, LVDP 58.0+/-8.1 mm Hg and cTnI 6.7+/-5.2 ng/ml). Immunohistochemical staining demonstrated that Intimatan pretreatment prevented deposition of the human membrane attack complex (MAC) in hearts exposed to NHP. The results indicate that Intimatan, a glycosaminoglycan (GAG), can reduce tissue injury and preserve organ function that otherwise would be compromised during activation of the human complement cascade.

M1/70 attenuates blood-borne neutrophil oxidants, activation, and myofiber damage following stretch injury

The purpose of this study was to determine the role of the CD11b-dependent respiratory burst in neutrophil oxidant generation and activation, interleukin-8 (IL-8) production, and myofiber damage after muscle stretch injury by using the monoclonal antibody M1/70 to block this pathway. Twelve male New Zealand White rabbits were randomly assigned to a treatment group: M1/70 (n = 6), IgG isotype control (n = 3), or saline control (n = 3). After intravenous injection of the assigned agent under gas anesthesia, a standardized single-stretch injury was created in the right tibialis anterior, whereas the left tibialis anterior underwent a sham surgery. Blood-borne neutrophil oxidant generation and CD11b receptor density and plasma IL-8 levels were measured pre- and 24 h postinjury. Damage was assessed histologically at the hematoma site by counting torn myofibers. M1/70 group demonstrated decreased blood-borne neutrophil oxidant generation (P < 0.05) and CD11b receptor density (P < 0.05), an increase in plasma IL-8 concentration (P < 0.01), and less torn myofibers (P < 0.01) compared with IgG isotype or saline control groups. These data indicate that 1). CD11b-dependent respiratory burst is a major source of oxidants produced by the neutrophil, and that treatment with M1/70 2). attenuates neutrophil activation status, 3). increases plasma IL-8 concentration, and 4). minimizes myofiber damage 24 h postmuscle stretch injury.

Endothelial transcytosis of myeloperoxidase confers specificity to vascular ECM proteins as targets of tyrosine nitration

Nitrotyrosine formation is a hallmark of vascular inflammation, with polymorphonuclear neutrophil-derived (PMN-derived) and monocyte-derived myeloperoxidase (MPO) being shown to catalyze this posttranslational protein modification via oxidation of nitrite (NO(2)(-)) to nitrogen dioxide (NO(2)(*)). Herein, we show that MPO concentrates in the subendothelial matrix of vascular tissues by a transcytotic mechanism and serves as a catalyst of ECM protein tyrosine nitration. Purified MPO and MPO released by intraluminal degranulation of activated human PMNs avidly bound to aortic endothelial cell glycosaminoglycans in both cell monolayer and isolated vessel models. Cell-bound MPO rapidly transcytosed intact endothelium and colocalized abluminally with the ECM protein fibronectin. In the presence of the substrates hydrogen peroxide (H(2)O(2)) and NO(2)(-), cell and vessel wall-associated MPO catalyzed nitration of ECM protein tyrosine residues, with fibronectin identified as a major target protein. Both heparin and the low-molecular weight heparin enoxaparin significantly inhibited MPO binding and protein nitrotyrosine (NO(2)Tyr) formation in both cultured endothelial cells and rat aortic tissues. MPO(-/-) mice treated with intraperitoneal zymosan had lower hepatic NO(2)Tyr/tyrosine ratios than did zymosan-treated wild-type mice. These data indicate that MPO significantly contributes to NO(2)Tyr formation in vivo. Moreover, transcytosis of MPO, occurring independently of leukocyte emigration, confers specificity to nitration of vascular matrix proteins.

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.

Conditioned blood reperfusion during angioplasty (CoBRA) treatment of acute myocardial infarction

Acute myocardial infarct (MI) results in ischemia distal to lesions which puts heart muscle at risk for reperfusion injury (RI). Neutrophils, platelets and complement are putative mediators of RI. Recent advances in filtration technology provide integrated neutrophil and platelet removal together with complement-attenuating properties in a single blood-conditioning device. The present study characterizes the properties of a blood-conditioning filter and describes its clinical effect when used in conjunction with active hemoperfusion for acute MI. The filter reduces leukocytes by 99.9998 +/- 0.0002% (p<0.0001) and platelets by 99.9934 +/- 0.0069% (p<0.0001). Human plasma, derived from heparinized blood that was 'conditioned' by filtration, was studied using the Langendorff isolated rabbit heart preparation. The deposition of membrane attack complex and the resultant functional myocardial impairments [reflected in hemodynamic and biochemical measurements, including developed pressure, coronary blood flow, lymph-derived myocardial creatine kinase (CK)] are significantly attenuated by blood conditioning. Integration of the blood-conditioning filter into an active hemoperfusion system during primary percutaneous transluminal coronary angioplasty (PTCA) for acute MI (n=8) did not delay the procedure or cause any complications. Reperfusion of occluded coronary arteries with 300 cm3 of conditioned blood led to significant improvement in echocardiographic global wall motion scores (in standard deviations) following treatment (-1.64 +/- 0.18 to -1.45 +/- 0.15, p=0.02). Initial reperfusion of totally occluded coronary arteries with conditioned blood leads to acutely improved ventricular function. Collectively, these data provide a strong indication for continued investigation of conditioned blood reperfusion in angioplasty following acute MI for the long-term effect upon recovery of salvagable myocardium.

Systemic complement depletion diminishes perihematomal brain edema in rats

BACKGROUND AND PURPOSE

The complement cascade is activated after experimental intracerebral hemorrhage (ICH). It remains unclear, however, whether depleting the complement system will improve injury resulting from ICH. This study investigated the effects of systemic complement depletion on brain edema formation after ICH.

METHODS

Fifty-six pentobarbital-anesthetized Sprague-Dawley rats were used. Treatment animals were complement-depleted with cobra venom factor (CVF) (intraperitoneally). Control rats received an equal volume of saline injection (intraperitoneally). In both treatment and control rats, autologous blood (100 microL) was infused stereotaxically into the right basal ganglia. Rats were killed 2, 24, or 72 hours later for brain water, ion, and tumor necrosis factor-alpha (TNF-alpha) measurements, for Western blot analysis, and for immunohistochemical studies. Brain edema was quantitated by wet/dry weight. TNF-alpha levels were measured by enzyme-linked immunosorbent assay. Western blot analysis was applied for C9 semiquantification. Immunohistochemistry was used to detect complement C3d, C5a, C9, and myeloperoxidase.

RESULTS

Perihematomal brain edema was reduced by systemic complement depletion at 24 hours (78.8+/-0.6% versus 81.5+/-0.8% in control, P:<0.01) and 72 hours (81.5+/-1.5% versus 83.6+/-0.9% in control, P:<0.05), while cerebellar water content was unaffected (78.2+/-0.3% versus 78.0+/-0. 1%). Complement depletion reduced TNF-alpha production 2 hours after ICH. Immunocytochemistry showed that complement depletion significantly reduced perihematomal C9 deposition, C3d production, and the number of C5a- and myeloperoxidase-positive cells.

CONCLUSIONS

Complement depletion by CVF attenuates brain edema in ICH, indicating that complement activation plays an important role in ICH-induced brain edema. Preventing complement activation may be effective in the treatment of ICH.

Inhibition of selectin-mediated cell adhesion and prevention of acute inflammation by nonanticoagulant sulfated saccharides. Studies with carboxyl-reduced and sulfated heparin and with trestatin a sulfate

Selectins play a major role in the inflammatory reaction by initiating neutrophil attachment to activated vascular endothelium. Some heparin preparations can interact with L- and P-selectin; however, the determinants required for inhibiting selectin-mediated cell adhesion have not yet been characterized. We now report that carboxyl-reduced and sulfated heparin (prepared by chemical modifications of porcine intestinal mucosal heparin leading to the replacement of carboxylates by O-sulfate groups) and trestatin A sulfate (obtained by sulfation of trestatin A, a non-uronic pseudo-nonasaccharide extracted from Streptomyces dimorphogenes) exhibit strong anti-P-selectin and anti-L-selectin activity while lacking antithrombin-mediated anticoagulant activity. In vitro experiments revealed that both compounds inhibited P-selectin- and L-selectin-mediated cell adhesion under laminar flow conditions. Moreover, carboxyl-reduced and sulfated heparin and trestatin A sulfate were also active in vivo, as assessed by experiments showing 1) that microinfusion of trestatin A sulfate reduced by 96% leukocyte rolling along rat mesenteric postcapillary venules and 2) that both compounds inhibited (by 58-81%) neutrophil migration into thioglycollate-inflamed peritoneum of BALB/c mice. These results indicate that nonanticoagulant sulfated saccharides targeted at P-selectin and L-selectin may have therapeutic potential in inflammatory disorders.

Preconditioning reduces myocardial complement gene expression in vivo

This investigation examined the effect of preconditioning in an in vivo model of ischemia-reperfusion injury. Anesthetized New Zealand White rabbits underwent 30 min of regional myocardial ischemia followed by 2 h of reperfusion. Hearts preconditioned with two cycles of 5 min ischemia-10 min reperfusion (IPC) or with the ATP-sensitive K (K(ATP)) channel opener, diazoxide (10 mg/kg), exhibited significantly (P < 0.05) smaller infarcts compared with control. These treatments also significantly (P < 0.001 to P < 0.05) reduced C1q, C1r, C3, C8, and C9 mRNA in the areas at risk (AAR). The K(ATP) channel blocker 5-hydroxydecanoate (5-HD; 10 mg/kg) attenuated infarct size reduction elicited by IPC and diazoxide treatment. 5-HD partially reversed the decrease in complement expression caused by IPC but not diazoxide. There were no significant differences in complement gene expression in the nonrisk regions and livers of all groups. Western blot analysis revealed that IPC also reduced membrane attack complex expression in the AAR. The data demonstrate that preconditioning significantly decreases reperfusion-induced myocardial complement expression in vivo.

Complement activation in the brain after experimental intracerebral hemorrhage

OBJECT

Brain edema formation following intracerebral hemorrhage (ICH) appears to be partly related to erythrocyte lysis and hemoglobin release. Erythrocyte lysis may be mediated by the complement cascade, which then triggers parenchymal injury. In this study the authors examine whether the complement cascade is activated after ICH and whether inhibition of complement attenuates brain edema around the hematoma.

METHODS

This study was divided into three parts. In the first part, 100 microl of autologous blood was infused into the rats' right basal ganglia, and the animals were killed at 24 and 72 hours after intracerebral infusion. Their brains were tested for complement factors C9, C3d, and clusterin (a naturally occurring complement inhibitor) by using immunohistochemical analysis. In the second part of the study, the rats were killed at 24 or 72 hours after injection of 100 microl of blood. The C9 and clusterin proteins were quantitated using Western blot analysis. In the third part, the rats received either 100 microl of blood or 100 microl of blood plus 10 microg of N-acetylheparin (a complement activation inhibitor). Then they were killed 24 or 72 hours later for measurement of brain water and ion contents. It was demonstrated on Western blot analysis that there had been a sixfold increase in C9 around the hematoma 24 hours after the infusion of 100 microl of autologous blood. Marked perihematomal C9 immunoreactivity was detected at 72 hours. Clusterin also increased after ICH and was expressed in neurons 72 hours later. The addition of N-acetylheparin significantly reduced brain edema formation in the ipsilateral basal ganglia at 24 hours (78.5 +/- 0.5% compared with 81.6 +/- 0.8% in control animals, p < 0.001) and at 72 hours (80.9 +/- 2.2% compared with 83.6 +/- 0.9% in control animals, p < 0.05) after ICH.

CONCLUSIONS

It was found that ICH causes complement activation in the brain. Activation of complement and the formation of membrane attack complex contributes to brain edema formation after ICH. Blocking the complement cascade could be an important step in the therapy for ICH.

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.