Soluble complement receptor type I limits damage during revascularization of ischemic myocardium

Immunomodulation for optimal cardiac regeneration: insights from comparative analyses

Despite decades of research, regeneration of the infarcted human heart remains an unmet ambition. A significant obstacle facing experimental regenerative therapies is the hostile immune response which arises following a myocardial infarction (MI). Upon cardiac damage, sterile inflammation commences via the release of pro-inflammatory meditators, leading to the migration of neutrophils, eosinophils and monocytes, as well as the activation of local vascular cells and fibroblasts. This response is amplified by components of the adaptive immune system. Moreover, the physical trauma of the infarction and immune-mediated tissue injury provides a supply of autoantigens, perpetuating a cycle of autoreactivity, which further contributes to adverse remodelling. A gradual shift towards an immune-resolving environment follows, culminating in the formation of a collagenous scar, which compromises cardiac function, ultimately driving the development of heart failure. Comparing the human heart with those of animal models that are capable of cardiac regeneration reveals key differences in the innate and adaptive immune responses to MI. By modulating key immune components to better resemble those of regenerative species, a cardiac environment may be established which would, either independently or via the synergistic application of emerging regenerative therapies, improve functional recovery post-MI.

The use of TP10, soluble complement receptor 1, in cardiopulmonary bypass

Cardiopulmonary bypass (CPB) for cardiac surgery or lung transplantation initiates a systemic inflammatory response characterized by increased vascular permeability, generalized edema, abnormal lung function and oxygenation and impaired ventricular function. This post-CPB syndrome significantly contributes to postoperative morbidity and mortality. Activation of complement during CPB is a key component that initiates and augments this process. TP10, soluble complement receptor 1, is a novel complement inhibitor that is a potent inhibitor of C3 and C5 convertases, blocking activation of the complement cascade at the nexus of all three complement pathways. Recent controlled trials in humans have demonstrated that TP10 effectively inhibits complement activation during CPB. In high-risk adult patients, TP10 decreases the incidence of mortality and myocardial infarction in males but not in females following cardiac surgery. TP10 is also well tolerated and protects vascular function in infants undergoing CPB. In addition, TP10 leads to early extubation in adult lung transplant recipients. TP10 is currently positioned for clinical development in a male-only indication of cardiac surgery on CPB.

Targeted complement inhibition by C3d recognition ameliorates tissue injury without apparent increase in susceptibility to infection

Previous studies indicate a pivotal role for complement in mediating both local and remote injury following ischemia and reperfusion of the intestine. Here, we report on the use of a mouse model of intestinal ischemia/reperfusion injury to investigate the strategy of targeting complement inhibition to sites of complement activation by linking an iC3b/C3dg-binding fragment of mouse complement receptor 2 (CR2) to a mouse complement-inhibitory protein, Crry. We show that the novel CR2-Crry fusion protein targets sites of local and remote (lung) complement activation following intestinal ischemia and reperfusion injury and that CR2-Crry requires a 10-fold lower dose than its systemic counterpart, Crry-Ig, to provide equivalent protection from both local and remote injury. CR2-Crry has a significantly shorter serum half-life than Crry-Ig and, unlike Crry-Ig, had no significant effect on serum complement activity at minimum effective therapeutic doses. Furthermore, the minimum effective dose of Crry-Ig significantly enhanced susceptibility to infection in a mouse model of acute septic peritonitis, whereas the effect of CR2-Crry on susceptibility to infection was indistinguishable from that of PBS control. Thus, compared with systemic inhibition, CR2-mediated targeting of a complement inhibitor of activation improved bioavailability, significantly enhanced efficacy, and maintained host resistance to infection.

Activation of estrogen receptor-alpha protects the in vivo rabbit heart from ischemia-reperfusion injury

The estrogen receptor (ER) mediates estrogenic activity in a variety of organs, including those in the reproductive, cardiovascular, immune, and central nervous systems. Experimental studies have demonstrated that 17beta-estradiol (E2) protects the heart from ischemia-reperfusion injury. Two estrogen receptors, ER alpha and ER beta, mediate the actions of estrogen; however, it is not certain which ER mediates the cardioprotective effects of E2. In the present study, the ER-selective agonists 4,4',4''-[4-propyl-(1H)-pyrazole-1,3,5-triyl]tris-phenol (PPT; ER alpha) and 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN; ER beta) were assessed for their cardioprotective potential in an in vivo rabbit model of ischemia-reperfusion injury. Anesthetized female rabbits were administered PPT (3 mg/kg), DPN (3 mg/kg), E2 (20 microg/rabbit), or vehicle intravenously 30 min before a 30-min occlusion of the left anterior descending coronary artery followed by 4 h of reperfusion. Acute treatment with E2 (17.7 +/- 2.9%; P < 0.001) and PPT (18.1 +/- 2.9%; P < 0.001), but not DPN (45.3 +/- 2.4%) significantly decreased infarct size as a percent of area at risk compared with vehicle (45.3 +/- 2.4%). Coadministration of PPT or E2 with the ER antagonist ICI-182,780 limited the infarct size-sparing effect of the compounds (43.8 +/- 6.6% and 40.6 +/- 5.7% respectively, expressed as a percentage of risk region). PPT reduced the release of cardiac-specific troponin-I and reduced the tissue deposition of the membrane attack complex and C-reactive protein similar to that of E2. The results indicate that activation of ER alpha, but not ER beta, is required for the observed cardioprotective effects of E2.

Soluble human complement receptor 1 limits ischemic damage in cardiac surgery patients at high risk requiring cardiopulmonary bypass

BACKGROUND

This study was undertaken to determine whether soluble human complement receptor type 1 (TP10), a potent inhibitor of complement activation, would reduce morbidity and mortality in high-risk patients undergoing cardiac surgery on cardiopulmonary bypass (CPB).

METHODS

This was a randomized multicenter, prospective, placebo-controlled, double-blind study in which 564 high-risk patients undergoing cardiac surgery on CPB received an intravenous bolus of TP10 (1, 3, 5, 10 mg/kg) or placebo immediately before CPB. The primary endpoint was the composite events of death, myocardial infarction (MI), prolonged (> or =24 hours) intra-aortic balloon pump support (IABP), and prolonged intubation.

RESULTS

TP10 significantly inhibited complement activity after 10 to 15 minutes of CPB and this inhibition persisted for 3 days postoperatively. However, there was no difference in the primary endpoint between the 2 groups (33.7% placebo versus 31.4% TP10; P=0.31). The primary composite endpoint was, however, reduced in all male TP10 patients by 30% (P=0.025). TP10 reduced the incidence of death or MI in males by 36% (P=0.026), the incidence of death or MI in CABG males by 43% (P=0.043) and the need for prolonged IABP support in male CABG and valve patients by 100% (P=0.019). There was, however, no improvement seen in female TP10 patients. There were no significant differences in adverse events between the groups.

CONCLUSIONS

TP10 effectively inhibits complement activation during CPB; however, this was not associated with an improvement in the primary endpoint of the study. Nevertheless, TP10 did significantly decrease the incidence of mortality and MI in male patients.

Does Off-Pump Revascularization Reduce Coronary Endothelial Dysfunction?

BACKGROUND

Off-pump coronary revascularization (OPCAB) has been shown to reduce markers of acute inflammation but its effect on coronary endothelial function is unknown. This experimental study sought to determine whether OPCAB reduces endothelial dysfunction, compared to standard cardiopulmonary bypass (CPB) with and without the anticomplement agent soluble complement receptor-1 (sCR(1)).

METHODS

In 10 pigs, OPCAB was simulated by snaring the left anterior descending (LAD) artery for 15 minutes followed by 3 hours of reperfusion. On-pump revascularization was simulated in 20 pigs by 15 minutes of LAD occlusion on CPB with cold blood cardioplegic arrest followed by 3 hours of reperfusion. Ten of these animals received sCR(1) (10 mg/kg) prior to CPB. Inflammatory response was monitored by percent (%) lung water increase, wall motion scores (WMS) with transthoracic echocardiography where 4 = normal to -1 = dyskinesia, and endothelial function in the distal LAD with bradykinin-induced coronary artery relaxation using organ chamber methodology.

RESULTS

OPCAB had no effect on lung edema (% increase = 1.7 +/- 1.4 OPCAB vs. 3.4 +/- 0.5 CPB vs. 2.3 +/- 0.9 CPB + sCR(1)) and failed to prevent wall motion changes (WMS = 2.65 +/- 0.08 OPCAB vs. 2.70 +/- 0.04 CPB vs. 3.10 +/- 0.07* CPB + sCR(1), *p < 0.01) and coronary endothelial dysfunction (% relaxation = 41 +/- 9 OPCAB vs. 40 +/- 9 CPB vs. 78 +/- 8** CPB + sCR(1), **p < 0.001), which was best preserved with sCR(1).

CONCLUSIONS

This study suggests that agents which directly inhibit complement activation such as sCR(1) are more important in preventing endothelial dysfunction during coronary revascularization than merely avoiding CPB.

Pharmacokinetics and safety of TP10, soluble complement receptor 1, in infants undergoing cardiopulmonary bypass

BACKGROUND

Increase in vascular permeability and multiorgan dysfunction after cardiopulmonary bypass (CPB) are barriers to successful cardiac surgery in infants. Complement inhibition with TP10, a C3/C5 convertase inhibitor (AVANT Immunotherapeutics, Needham, Mass), blunts post-CPB organ dysfunction in the neonatal pig. Methods and results The pharmacokinetics and safety of TP10 in infants (age <1 year, n = 15) undergoing CPB were examined in a phase I/II open-label prospective trial. TP10 (10 mg/kg) was given intravenously before CPB and also added (10 mg/100 mL prime volume) to the CPB circuit. TP10 plasma levels correlated with C3a levels and measures of clinical course. All infants survived. No adverse events were attributed to TP10. TP10 plasma concentration fell to < or =60 microg/mL 12 hours after CPB. A 2-compartment model was fit to the TP10 blood levels as a function of time. Based on this model, an initial dose of 10 mg/kg over 0.5 hours followed by 10 mg/kg over 23.5 hours is the most appropriate for maintaining TP10 concentration between 100 microg/mL and 160 microg/mL for 24 hours after CPB. C3a was lower 12 hours after CPB than before CPB and still lower 24 hours after CPB. TP10 concentration was inversely correlated with the 12-hour post-CPB to pre-CPB ratio of C3a (Spearman rho -0.76, P = -.016), and with total (rho -0.56, P =.047) and net (rho -0.85, P =.0016) fluid and blood product administration/kg >24 hours after CPB.

CONCLUSIONS

TP10 administration to infants appears safe. Pharmacokinetic analysis generated an optimal dosing strategy to achieve effective TP10 levels for 24 hours after CPB. In the infant, TP10 appears to decrease CPB-induced complement activation and protect vascular function. These results support a phase III trial of TP10 in infants requiring CPB.

Functional activity of natural antibody is altered in Cr2-deficient mice

The major source of natural IgM Abs are B-1 cells, which differ from conventional B cells in their anatomic location, cell surface phenotype, restricted usage of particular V(H) genes and limited use of N-region addition during V-D-J rearrangement. The origin of B-1 cells is unclear. However, they are capable of self-renewal and their development is sensitive to signaling via the B cell receptor, as genetic defects that impair the strength of the signal often result in limited development. These findings suggest that B-1 cells require either an intrinsic signal, or contact with Ag, for positive selection and expansion and/or maintenance in the periphery. In support of interaction with cognate Ag, deficiency in the complement receptors CD21/CD35 results in a 30-40% decrease in the CD5(+) B-1 population. To determine whether this reduction reflects a loss of certain specificities or simply a proportional decline in the repertoire, we examined peritoneal B cells isolated from Cr2(+) and Cr2(def) mice for recognition of a B-1 cell Ag, i.e., phosphatidylcholine, and assayed for injury in an IgM natural Ab-dependent model of reperfusion injury. We found a similar frequency of phosphatidylcholine-specific CD5(+) B-1 cells in the two strains of mice. By contrast, the Cr2(def) mice have reduced injury in the IgM-dependent model of reperfusion injury. Reconstitution of the deficient mice with pooled IgM or adoptive transfer of Cr2(+) peritoneal B cells restored injury. These results suggest that complement receptors CD21/CD35 are important in maintenance of the B-1 cell repertoire to some, but not all, specificities.

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.

Reperfusion injury after focal myocardial ischaemia: polymorphonuclear leukocyte activation and its clinical implications

The only way to rescue ischaemic tissue is to re-instate the oxygen supply to the tissue. However reperfusion of the ischaemic area not only oxygenates the tissue but also initiates a cascade of processes, which may in some cases result in temporary dysfunction of the myocardium. In order to devise protective measures, it is essential to understand the mechanisms and the triggers of this reperfusion phenomenon. In this review we will mainly focus on the inflammatory response caused by reperfusion. We will cover the different steps of polymorphonuclear leukocyte activation and will briefly discuss the molecular biology of the receptors involved. The currently used pharmacological medications in acute cardiology will be reviewed and in particular their actions on polymorphonuclear leukocyte activation, adhesion and degranulation. This review is a compilation of the current knowledge in the field and the therapeutic progress in the prevention of reperfusion injury made today.

Total complement inhibition: an effective strategy to limit ischemic injury during coronary revascularization on cardiopulmonary bypass

BACKGROUND

Activation of complement during revascularization of ischemic myocardium accentuates myocardial dysfunction. Soluble human complement receptor type 1 (sCR1) is a potent inhibitor of complement, as are heparin-bonded (HB) cardiopulmonary bypass (CPB) circuits. This study sought to determine whether total complement inhibition with the combination of sCR1 and HB-CPB limits damage during the revascularization of ischemic myocardium.

METHODS AND RESULTS

In 40 pigs, the second and third diagonal coronary arteries were occluded for 90 minutes, followed by 45 minutes of cardioplegic arrest and 180 minutes of reperfusion. In 10 pigs, sCR1 (10 mg/kg) was infused 5 minutes after the onset of coronary occlusion (sCR1), 10 received HB-CPB only (HB-CPB), 10 received sCR1 and HB-CPB (sCR1+HB), and 10 received neither sCR1 or HB-CPB (unmodified). Addition of sCR1 to the HB group resulted in less myocardial tissue acidosis (DeltapH = -0.72+/-0.03 for unmodified; -0.46+/-0.05 for HB; -0.18+/-0.04 for sCR1; -0.13+/-0.01 for sCR1+HB), better recovery of wall motion scores (4 = normal to -1 = dyskinesia; 1.67+/-0.17 for unmodified; 2.80+/-0.08 for HB; 3.35+/-0.10 for sCR1; 3.59+/-0.08 for sCR1+HB), less lung water accumulation (5.46+/-0.28% for unmodified; 2.39+/-0.34% for HB; 1.22+/-0.07% for sCR1; 1.24+/-0.13% for sCR1+HB), and smaller infarct size (area necrosis/area risk = 44.6+/-0.7% for unmodified; 33.2+/-1.9% for HB; 19.0+/-2.4% for sCR1; 20+/-1.0% for sCR1+HB) (P<0.05 versus unmodified; P<0.05 versus unmodified and HB groups).

CONCLUSIONS

Total complement inhibition with sCR1 and sCR1+HB circuits optimizes recovery during the revascularization of ischemic myocardium.

Targeting the complement system

Interest has blossomed in the development of complement inhibitors, in parallel with a growth in our understanding of the biology of the complement cascade. The first generation of designed inhibitors was based on naturally occurring complement receptors and regulatory molecules. These agents provided useful tools for exploring the role of complement in experimental models of disease, but may have limited therapeutic application in humans because of their short half-lives, limited bioavailability and possible antigenicity. More recently, humanized antibodies and synthetic molecules that block the activation of complement have been developed, which look as though they may overcome some of these difficulties. The possibility for precision inhibition of a limited part of the complement cascade, or for inhibition confined to a single organ, may offer effective therapeutic results, while avoiding the disadvantages of nonselective complement blockade. This review examines the recent evidence that complement inhibition will reduce tissue damage resulting from organ transplantation, ischaemia-reperfusion injury, cancer, glomerulonephritis and the use of extracorporeal circuits.

Complement activation following oxidative stress

It is clear that complement plays an important role in the inflammatory process following oxidative stress in cellular and animal models. Clinical trials underway with novel complement inhibitors will establish the potential therapeutic benefit of complement inhibition in human disease. For as much as we understand about the role of complement in disease states, many questions remain. How is complement activated on endothelial cells following oxidative stress? What is the ligand for MBL on endothelial cells following oxidative stress? Will inhibition of MBL provide tissue protection to the extent observed with other complement inhibitors such as sCR1 or anti-C5 mAbs? These questions and more will undoubtedly be answered in the next millennium.

Neuronal protection in stroke by an sLex-glycosylated complement inhibitory protein

Glycoprotein adhesion receptors such as selectins contribute to tissue injury in stroke. Ischemic neurons strongly expressed C1q, which may target them for complement-mediated attack or C1qRp-mediated clearance. A hybrid molecule was used to simultaneously inhibit both complement activation and selectin-mediated adhesion. The extracellular domain of soluble complement receptor-1 (sCR1) was sialyl Lewis x glycosylated (sCR1sLex) to inhibit complement activation and endothelial-platelet-leukocyte interactions. sCR1 and sCR1sLex colocalized to ischemic cerebral microvessels and C1q-expressing neurons, inhibited neutrophil and platelet accumulation, and reduced cerebral infarct volumes. Additional benefit was conferred by sialyl Lewis x glycosylation of the unmodified parent sCR1 molecule.