Recombinant glycoproteins that inhibit complement activation and also bind the selectin adhesion molecules

Sialylation-dependent pharmacokinetics and differential complement pathway inhibition are hallmarks of CR1 activity in vivo

Human Complement Receptor 1 (HuCR1) is a potent membrane-bound regulator of complement both in vitro and in vivo, acting via interaction with its ligands C3b and C4b. Soluble versions of HuCR1 have been described such as TP10, the recombinant full-length extracellular domain, and more recently CSL040, a truncated version lacking the C-terminal long homologous repeat domain D (LHR-D). However, the role of N-linked glycosylation in determining its pharmacokinetic (PK) and pharmacodynamic (PD) properties is only partly understood. We demonstrated a relationship between the asialo-N-glycan levels of CSL040 and its PK/PD properties in rats and non-human primates (NHPs), using recombinant CSL040 preparations with varying asialo-N-glycan levels. The clearance mechanism likely involves the asialoglycoprotein receptor (ASGR), as clearance of CSL040 with a high proportion of asialo-N-glycans was attenuated in vivo by co-administration of rats with asialofetuin, which saturates the ASGR. Biodistribution studies also showed CSL040 localisation to the liver following systemic administration. Our studies uncovered differential PD effects by CSL040 on complement pathways, with extended inhibition in both rats and NHPs of the alternative pathway compared to the classical and lectin pathways that were not correlated with its PK profile. Further studies showed that this effect was dose dependent and observed with both CSL040 and the full-length extracellular domain of HuCR1. Taken together, our data suggests that sialylation optimization is an important consideration for developing HuCR1-based therapeutic candidates such as CSL040 with improved PK properties and shows that CSL040 has superior PK/PD responses compared to full-length soluble HuCR1.

Complement: A primer for the coming therapeutic revolution

The complement system is an important part of the innate and adaptive immune systems. Originally characterized as a single serum component contributing to the killing of bacteria, we now know that there are close to sixty complement proteins, multiple activation pathways and a wide range of effector functions mediated by complement. The system plays a critical role in host defense against bacteria, viruses, fungi and other pathogens. However, inappropriate complement activation contributes to the pathophysiology of autoimmune diseases and many inflammatory syndromes. Over the last several decades, therapeutic approaches to inhibit complement activation at various steps in the pathways have met with initial success, particularly at the level of the terminal pathway. This success, combined with insight from animal model studies, has lead to an unprecedented effort by biotech and pharmaceutical companies to begin developing complement inhibitors. As a result, complement has been brought for the first time to the attention of pharmacologists, toxicologists, project managers and others in the drug development industry, as well as those in the investment world. The purpose of this primer is to provide a broad overview of complement immunobiology to help those new to complement understand the rationale behind the current therapeutic directions and the investment potential of these new therapeutics.

The Immune Response to Acute Focal Cerebral Ischemia and Associated Post-stroke Immunodepression: A Focused Review

It is currently well established that the immune system is activated in response to transient or focal cerebral ischemia. This acute immune activation occurs in response to damage, and injury, to components of the neurovascular unit and is mediated by the innate and adaptive arms of the immune response. The initial immune activation is rapid, occurs via the innate immune response and leads to inflammation. The inflammatory mediators produced during the innate immune response in turn lead to recruitment of inflammatory cells and the production of more inflammatory mediators that result in activation of the adaptive immune response. Under ideal conditions, this inflammation gives way to tissue repair and attempts at regeneration. However, for reasons that are just being understood, immunosuppression occurs following acute stroke leading to post-stroke immunodepression. This review focuses on the current state of knowledge regarding innate and adaptive immune activation in response to focal cerebral ischemia as well as the immunodepression that can occur following stroke. A better understanding of the intricate and complex events that take place following immune response activation, to acute cerebral ischemia, is imperative for the development of effective novel immunomodulatory therapies for the treatment of acute stroke.

Membrane-bound complement regulatory proteins as biomarkers and potential therapeutic targets for SLE

For the last two decades, there had been remarkable advancement in understanding the role of complement regulatory proteins in autoimmune disorders and importance of complement inhibitors as therapeutics. Systemic lupus erythematosus is a prototype of systemic autoimmune disorders. The disease, though rare, is potentially fatal and afflicts women at their reproductive age. It is a complex disease with multiorgan involvement, and each patient presents with a different set of symptoms. The diagnosis is often difficult and is based on the diagnostic criteria set by the American Rheumatology Association. Presence of antinuclear antibodies and more specifically antidouble-stranded DNA indicates SLE. Since the disease is multifactorial and its phenotypes are highly heterogeneous, there is a need to identify multiple noninvasive biomarkers for SLE. Lack of validated biomarkers for SLE disease activity or response to treatment is a barrier to the efficient management of the disease, drug discovery, as well as development of new therapeutics. Recent studies with gene knockout mice have suggested that membrane-bound complement regulatory proteins (CRPs) may critically determine the sensitivity of host tissues to complement injury in autoimmune and inflammatory disorders. Case-controlled and followup studies carried out in our laboratory suggest an intimate relation between the level of DAF, MCP, CR1, and CD59 transcripts and the disease activity in SLE. Based on comparative evaluation of our data on these four membrane-bound complement regulatory proteins, we envisaged CR1 and MCP transcripts as putative noninvasive disease activity markers and the respective proteins as therapeutic targets for SLE. Following is a brief appraisal on membrane-bound complement regulatory proteins DAF, MCP, CR1, and CD59 as biomarkers and therapeutic targets for SLE.

New insights of an old defense system: structure, function, and clinical relevance of the complement system

The complement system was discovered a century ago as a potent defense cascade of innate immunity. After its first description, continuous experimental and clinical research was performed, and three canonical pathways of activation were established. Upon activation by traumatic or surgical tissue damage, complement reveals beneficial functions of pathogen and danger defense by sensing and clearing injured cells. However, the latest research efforts have provided a more distinct insight into the complement system and its clinical subsequences. Complement has been shown to play a significant role in the pathogenesis of various inflammatory processes such as sepsis, multiorgan dysfunction, ischemia/reperfusion, cardiovascular diseases and many others. The three well-known activation pathways of the complement system have been challenged by newer findings that demonstrate direct production of central complement effectors (for example, C5a) by serine proteases of the coagulation cascade. In particular, thrombin is capable of producing C5a, which not only plays a decisive role on pathogens and infected/damaged tissues, but also acts systemically. In the case of uncontrolled complement activation, “friendly fire” is generated, resulting in the destruction of healthy host tissue. Therefore, the traditional research that focuses on a mainly positive-acting cascade has now shifted to the negative effects and how tissue damage originated by the activation of the complement can be contained. In a translational approach including structure-function relations of this ancient defense system, this review provides new insights of complement-mediated clinical relevant diseases and the development of complement modulation strategies and current research aspects.

Complement-targeted therapeutics

The complement system is a central component of innate immunity and bridges the innate to the adaptive immune response. However, it can also turn its destructive capabilities against host cells and is involved in numerous diseases and pathological conditions. Modulation of the complement system has been recognized as a promising strategy in drug discovery, and a large number of therapeutic modalities have been developed. However, successful marketing of complement-targeted drugs has proved to be more difficult than initially expected, and many strategies have been discontinued. The US Food and Drug Administration's approval of the first complement-specific drug, an antibody against complement component C5 (eculizumab; Soliris), in March 2007, was a long-awaited breakthrough in the field. Approval of eculizumab validates the complement system as therapeutic target and might facilitate clinical development of other promising drug candidates.

Emerging therapies for treatment of acute lung injury and acute respiratory distress syndrome

Acute lung injury/acute respiratory distress syndrome (ALI/ARDS) is a life-threatening form of respiratory failure that affects a heterogeneous population of critically ill patients. Although overall mortality appears to be decreasing in recent years due to improvements in supportive care, there are presently no proven, effective pharmacological therapies to treat ARDS and prevent its associated complications. The most common cause of death in ARDS is not hypoxemia or pulmonary failure, but rather multiple organ dysfunction syndrome (MODS), suggesting that improving survival in patients with ARDS may be linked to decreasing the incidence or severity of MODS. The key to developing novel treatments depends, in part, on identifying and understanding the mechanisms by which ARDS leads to MODS, although the heterogeneity and complexity of this disorder certainly poses a challenge to investigators. Novel therapies in development for treatment of ALI/ARDS include exogenous surfactant, therapies aimed at modulating neutrophil activity, such as prostaglandin and complement inhibitors, and treatments targeting earlier resolution of ARDS, such as beta-agonists and granulocyte macrophage colony-stimulating factor. From a clinical perspective, identifying subpopulations of patients most likely to benefit from a particular therapy and recognising the appropriate stage of illness in which to initiate treatment could potentially lead to better outcomes in the short term.

Complement-Dependent P-Selectin Expression and Injury following Ischemic Stroke

The mechanisms that contribute to inflammatory damage following ischemic stroke are poorly characterized, but studies indicate a role for both complement and P-selectin. In this study, we show that compared with wild-type mice, C3-deficient mice showed significant improvement in survival, neurological deficit, and infarct size at 24 h after middle cerebral artery occlusion and reperfusion. Furthermore, P-selectin protein expression was undetectable in the cerebral microvasculature of C3-deficient mice following reperfusion, and there was reduced neutrophil influx, reduced microthrombus formation, and increased blood flow postreperfusion in C3-deficient mice. We further investigated the use of a novel complement inhibitory protein in a therapeutic paradigm. Complement receptor 2 (CR2)-Crry inhibits complement activation at the C3 stage and targets to sites of complement activation. Treatment of normal mice with CR2-Crry at 30 min postreperfusion resulted in a similar level of protection to that seen in C3-deficient mice in all of the above-measured parameters. The data demonstrate an important role for complement in cerebrovascular thrombosis, inflammation, and injury following ischemic stroke. P-selectin expression in the cerebrovasculature, which is also implicated in cerebral ischemia and reperfusion injury, was shown to be distal to and dependent on complement activation. Data also show that a CR2-targeted approach of complement inhibition provides appropriate bioavailability in cerebral injury to enable complement inhibition at a dose that does not significantly affect systemic levels of serum complement activity, a potential benefit for stroke patients where immunosuppression would be undesirable due to significantly increased susceptibility to lung infection.

Strategies of therapeutic complement inhibition

The involvement of complement in the pathogenesis of a great number of partly life threatening diseases defines the importance to develop inhibitors which specifically interfere with its deleterious action. Endogenous soluble complement-inhibitors, antibodies or low molecular weight antagonists, either blocking key proteins of the cascade reaction or neutralizing the action of the complement-derived anaphylatoxins have successfully been tested in various animal models over the past years. Promising results consequently led to first clinical trials. This review is focused on different approaches for the development of inhibitors, on their site of action in the cascade, on possible indications for complement inhibition based on experimental animal data, and on potential side effects of such treatment.

In vivo correction of complement regulatory protein deficiency with an inhibitor targeting the red blood cell membrane

Because of the complement system's involvement in many human diseases and potential complications associated with its systemic blockade, site-specific regulation of this effector system is an attractive concept. We report on further developments of such an approach using a single-chain Ab fragment as a vehicle to deliver complement regulatory proteins to a defined cell type. In a model system in which RBCs deficient in complement receptor 1-related gene/protein y (Crry) are rapidly cleared after injection into wild-type animals by a complement-dependent mechanism, we selectively reconstituted these cells with N- and C-terminally targeted recombinant forms of Crry. Transfusion of Crry-coated knockout RBCs into C57BL/6 mice extended their in vivo half-life from <5 min to approximately 2 days. Maintenance of protective levels of Crry (by a combined treatment of donor and recipient RBCs) led to nearly normal RBC survival. Uniform in vitro and in vivo coating of the RBCs and the more efficient complement inhibitory capacity of C-terminally tagged Crry were other interesting features of this experimental system. These results suggest the possibility of using the single-chain Ab fragment-mediated targeting concept of complement regulatory proteins to restrict complement inhibition to the site of its excessive activation.

Synergy between two active sites of human complement receptor type 1 (CD35) in complement regulation: implications for the structure of the classical pathway C3 convertase and generation of more potent inhibitors

The extracellular domain of the complement receptor type 1 (CR1; CD35) consists entirely of 30 complement control protein repeats (CCPs). CR1 has two distinct functional sites, site 1 (CCPs 1-3) and two copies of site 2 (CCPs 8-10 and CCPs 15-17). In this report we further define the structural requirements for decay-accelerating activity (DAA) for the classical pathway (CP) C3 and C5 convertases and, using these results, generate more potent decay accelerators. Previously, we demonstrated that both sites 1 and 2, tandemly arranged, are required for efficient DAA for C5 convertases. We show that site 1 dissociates the CP C5 convertase, whereas the role of site 2 is to bind the C3b subunit. The intervening CCPs between two functional sites are required for optimal DAA, suggesting that a spatial orientation of the two sites is important. DAA for the CP C3 convertase is increased synergistically if two copies of site 1, particularly those carrying DAA-increasing mutations, are contained within one protein. DAA in such constructs may exceed that of long homologous repeat A (CCPs 1-7) by up to 58-fold. To explain this synergy, we propose a dimeric structure for the CP C3 convertase on cell surfaces. We also extended our previous studies of the amino acid requirements for DAA of site 1 and found that the CCP 1/CCP 2 junction is critical and that Phe82 may contact the C3 convertases. These observations increase our understanding of the mechanism of DAA. In addition, a more potent decay-accelerating form of CR1 was generated.

Development of complement therapeutics for inhibition of immune-mediated red cell destruction

A major objective of my National Blood Foundation (NBF)-funded proposal was to produce recombinant soluble forms of a complement regulatory protein called complement receptor 1 (CR1) that carries the Knops blood group system antigens to perform antibody neutralization studies. By generating these recombinant proteins, we were able to inhibit several Knops antibodies in patient serum samples, thereby demonstrating their usefulness for clinical use. Interestingly, the recombinant CR1 proteins generated through NBF funding were also found to strongly reduce complement-mediated red cell destruction in a mouse hemolytic transfusion model. In this review, I will outline our NBF-funded studies, give an overview of recent advances from our group and others in the development of complement therapeutics, and highlight their potential use in the transfusion medicine setting.

Controlling the complement system for prevention of red cell destruction

PURPOSE OF REVIEW

Complement sensitization of red blood cells (RBCs) can lead to both intravascular and extravascular red cell destruction. Altered levels of naturally occurring complement regulatory proteins on red cells can result in hemolysis, while defective expression of these proteins on immune cells can cause breakdown of tolerance to self antigens and is associated with autoimmune disease.

RECENT FINDINGS

To date several complement inhibitors, including recombinant forms of complement regulatory proteins, humanized antibodies, and synthetic molecules have been described that limit complement activation by interfering with different steps in the complement cascade. However, few have been evaluated for prevention of complement-mediated RBC destruction. In this review, possible applications of these complement inhibitors for treatment of complement-mediated hemolysis in specific disease states are described. Furthermore, the implication of the regulatory role of complement in the development of autoimmune hemolytic anemia is discussed.

SUMMARY

Complement therapeutics has potential for effective and safe prophylactic use and treatment of hemolytic transfusion reactions and complement-mediated hemolytic diseases. Furthermore, the regulatory function of complement may be exploited to prevent and treat autoimmune hemolytic anemia.

The administration of cobra venom factor reduces post-ischemic cerebral injury in adult and neonatal rats

The role of complement in post-ischemic cerebral injury is incompletely understood. Therefore, experiments were designed to test the effect of complement depletion on cerebral infarct volume in adult rats and cerebral atrophy in neonatal rats. Cerebral infarcts were induced in adult rats by transient filamentous occlusion of the right middle cerebral artery (MCAO). Cerebral atrophy was induced by subjecting 7-day-old rats to ligation of the right common carotid artery followed by 2.5h of hypoxia (8% O2). Forty-eight hours after MCAO, coronal sections of adult brains were obtained and stained with 2,3,5-triphenyl tetrazolium chloride. The infant rat brains were removed for analysis 6 weeks after the hypoxic-ischemic insult. Volumes of infarcts and normal hemispheric parenchyma were quantified by computer-based planimetry. Twenty-four hours prior to MCAO (adults) or hypoxia-ischemia (neonates), each animal received an i.p. injection of either 1 mcg/g body weight cobra venom factor (CVF; adult n=11; neonatal n=20) or normal saline (adult n=12; neonatal n=24). In the neonates, a second dose of CVF or saline was administered 2 days after hypoxia-ischemia. The administration of CVF significantly reduced: (1) post-ischemic cerebral infarct volume in the adults and (2) post-hypoxic-ischemic cerebral atrophy in the neonates. Therefore, complement activation augmented post-ischemic cerebral injury in adult and neonatal rats. Complement depletion induced by CVF significantly reduced post-ischemic cerebral infarct volume and atrophy in adult and neonatal rats.

Complement Regulatory Protein C1 Inhibitor Binds to Selectins and Interferes with Endothelial-Leukocyte Adhesion

C1 inhibitor (C1INH), a member of the serine proteinase inhibitor (serpin) family, is an inhibitor of proteases in the complement system, the contact system of kinin generation, and the intrinsic coagulation pathway. It is the most heavily glycosylated plasma protein, containing 13 definitively identified glycosylation sites as well as an additional 7 potential glycosylation sites. C1INH consists of two distinct domains: a serpin domain and an amino-terminal domain. The serpin domain retains all the protease-inhibitory function, while the amino-terminal domain bears most of the glycosylation sites. The present studies test the hypothesis that plasma C1INH bears sialyl Lewis(x)-related moieties and therefore binds to selectin adhesion molecules. We demonstrated that plasma C1INH does express sialyl Lewis(x)-related moieties on its N-glycan as detected using mAb HECA-452 and CSLEX1. The data also show that plasma C1INH can bind to P- and E-selectins by FACS and immunoprecipitation experiments. In a tissue culture model of endothelial-leukocyte adhesion, C1INH showed inhibition in a dose-dependent manner. Significant inhibition (>50%) was achieved at a concentration of 250 micro g/ml or higher. This discovery may suggest that C1INH plays a role in the endothelial-leukocyte interaction during inflammation. It may also provide another example of the multifaceted anti-inflammatory effects of C1INH in various animal models and human diseases.

Complement receptor 2-mediated targeting of complement inhibitors to sites of complement activation

In a strategy to specifically target complement inhibitors to sites of complement activation and disease, recombinant fusion proteins consisting of a complement inhibitor linked to a C3 binding region of complement receptor (CR) 2 were prepared and characterized. Natural ligands for CR2 are C3 breakdown products deposited at sites of complement activation. Fusion proteins were prepared consisting of a human CR2 fragment linked to either the N terminus or C terminus of soluble forms of the membrane complement inhibitors decay accelerating factor (DAF) or CD59. The targeted complement inhibitors bound to C3-opsonized cells, and all were significantly more effective (up to 20-fold) than corresponding untargeted inhibitors at protecting target cells from complement. CR2 fusion proteins also inhibited CR3-dependent adhesion of U937 cells to C3 opsonized erythrocytes, indicating a second potential anti-inflammatory mechanism of CR2 fusion proteins, since CR3 is involved in endothelial adhesion and diapedesis of leukocytes at inflammatory sites. Finally, the in vivo validity of the targeting strategy was confirmed by the demonstration that CR2-DAF, but not soluble DAF, targets to the kidney in mouse models of lupus nephritis that are associated with renal complement deposition.

Bacterial expression and membrane targeting of the rat complement regulator Crry: a new model anticomplement therapeutic

Inappropriate or unregulated activation of complement can contribute to pathology in inflammatory diseases. Previous studies have shown that soluble recombinant regulators of complement are effective in animal models and some human diseases. However, limitations include cost, rapid clearance, and unwanted systemic effects. To avoid some of these problems, bacterial expression of regulators has been optimized and methods for the addition of a membrane-targeting moiety to the complement regulator developed. When administered directly to sites of inflammation, membrane-targeted human regulators are retained and inhibit complement-activation locally. To test the efficacy of membrane-targeted complement regulators in vivo, we have undertaken the expression and membrane targeting of the rat-complement regulator Crry. A soluble recombinant form of Crry, containing only the first four short consensus repeats, was expressed in a mammalian expression system and shown to be functional as a fluid phase regulator. To generate the quantities required for testing in vivo, Crry was expressed in bacteria and refolded successfully. Refolded protein had full-complement regulatory activity in vitro. Attachment of a membrane address tag conferred membrane-binding capacity and greatly increased complement regulatory function in vitro. This novel anticomplement agent can now be applied to rat models of arthritis and other inflammatory diseases.

Use of complement inhibitors in tissue injury

A great deal of information has accumulated implicating the complement system in several human disease processes. Although some of this information is circumstantial, protein inhibitors of the complement system have been developed and applied successfully to experimental disease models in animals. Two inhibitors, soluble complement receptor 1 (sCR1) and anti-C5 monoclonal antibody, are now being investigated in a variety of clinical conditions such as systemic lupus erythematosus and rheumatoid arthritis (RA), diseases for which current therapy has changed little and remains unsatisfactory. Preliminary successes in Phase II clinical trials of RA have provided optimism that complement inhibition might prove useful in these diseases and become part of standard medical therapy.

Sialyl Lewis(x) hybridized complement receptor type 1 moderates acid aspiration injury

The potentially enhanced anti-inflammatory effects of the sialyl Lewis(x) (sLe(x))-decorated version of soluble complement receptor type 1 (sCR1) in moderating acid aspiration injury are examined. HCl was instilled in tracheostomy tubes placed in mice, and extravasation of (125)I-labeled albumin in bronchoalveolar lavage (BAL) fluid was used to calculate the vascular permeability index (PI). Neutrophil counts in BAL fluid and immunohistochemistry were performed. PI was moderated by 82% after treatment with sCR1sLe(x) compared with 54% in sCR1-untreated mice (P < 0.05). Respective reductions in PI in mice treated 0.5 and 1 h after acid aspiration with sCR1sLe(x) of 70 and 57% were greater than the decreases in PI of 45 and 38% observed in respective sCR1-treated groups (P < 0.05). BAL fluid neutrophil counts in sCR1sLe(x)-treated mice were significantly less than those in sCR1-treated animals, which were similar to those in untreated mice. Immunohistochemistry stained for sCR1 only on the pulmonary vascular endothelium of sCR1sLe(x)- but not sCR1-treated mice. In conclusion, sCR1sLe(x) moderates permeability by antagonizing complement activation and neutrophil adhesion. Delayed complement and neutrophil antagonism significantly reduces injury.

Moderation of skeletal muscle reperfusion injury by a sLe(x)-glycosylated complement inhibitory protein

The role of the sialyl Lewis(x) (sLe(x))-decorated version of soluble complement receptor type 1 (sCR1) in moderating skeletal muscle reperfusion injury, by antagonizing neutrophil endothelial selectin interaction and complement activation, is examined. Mice underwent 2 h of hindlimb ischemia and 3 h of reperfusion. Permeability index (PI) was assessed by extravasation of 125I-labeled albumin. Neutrophil depletion and complement inhibition with sCR1 reduced permeability by 72% (PI 0.81 +/- 0.10) compared with a 42% decrease (PI 1.53 +/- 0.08) observed in neutropenic mice, indicating that part of the complement-mediated injury is neutrophil independent. sCR1sLe(x) treatment reduced PI by 70% (PI 0.86 +/- 0.06), an additional 20% decrease compared with sCR1 treatment (PI 1.32 +/- 0.08). Treatment with sCR1sLe(x) 0.5 and 1 h after reperfusion reduced permeability by 63% (PI 0.09 +/- 0.07) and 52% (PI 1.24 +/- 0.09), respectively, compared with the respective decreases of 41% (PI 1.41 +/- 0.10) and 32% (PI 1.61 +/- 0.07) after sCR1 treatment. Muscle immunohistochemistry stained for sCR1 only on the vascular endothelium of sCR1sLe(x)-treated mice. In conclusion, sCR1sLe(x) is more effective than sCR1 in moderating skeletal muscle reperfusion injury.

sCR1sLe ameliorates ischemia/reperfusion injury in experimental lung transplantation

BACKGROUND

The nonspecific immune response with activation of the complement system and polymorphonuclear leukocytes is important for the mediation of reperfusion injury after lung transplantation. In this study, we investigated the combined blockade of the complement system and leukocyte adhesion by a novel drug combining soluble complement receptor type 1 (sCR1, CD35) with the selectin ligand sialyl Lewis X (sLe(X), CD15s) synthesized to sCR1sLe(X). Both sCR1 and sCR1sLe(X) were supplied by AVANT Immunotherapeutics, Inc, Needham, Massachusetts.

METHODS

Orthotopic allogeneic single left lung transplantation was performed in male rats (Brown Norway to Fischer F344; n = 5 in all groups) after a total ischemic time of 20 hours. Recipients received either no specific treatment (control) or administration of sCR1 (10 mg/kg) or sCR1sLe(X) (10 mg/kg) 15 minutes before reperfusion by intracardiac injection. Twenty-four hours after reperfusion, the native contralateral lung was occluded to assess gas exchange of the graft only. In additional animals (5 per group), lung tissue was frozen 24 hours after reperfusion and assessed for myeloperoxidase activity as a measurement of neutrophil migration into the graft and thiobarbituric acid reactive substances to quantify lipid peroxidation.

RESULTS

Graft function as assessed by arterial PO (2) in recipients treated with sCR1sLeX was superior not only to that of controls (383 +/- 53 vs 56 +/- 7 mm Hg, P =. 000095) but also to that of animals treated with sCR1 (243 +/- 45 mm Hg, P =.031). This improvement was confirmed by significant reduction of neutrophil migration (0.33 +/- 0.05 vs control, 1.0 +/- 0.09 DeltaOD/mg/min, P =.0000024) and lipid peroxidation (6.2 +/- 0. 38 vs control, 10.6 +/- 0.54 pmol/g, P =.00021).

CONCLUSIONS

Our data indicate that combined inhibition of complement activation and leukocyte adhesion with sCR1sLe(X) reduces reperfusion injury significantly and that both mechanisms are effectively inhibited in this model.

Complement inhibitors: a resurgent concept in anti-inflammatory therapeutics

In addition to its essential role in immune defense, the complement system contributes to tissue damage in many clinical conditions. Thus, there is a pressing need to develop therapeutically effective complement inhibitors to prevent these adverse effects. This concept, though old, received little scientific attention until recently. Data from animal models of diseases that have been produced using complement-deficient, knockout, and transgenic animals, as well as data demonstrating that complement proteins are produced in many important tissue sites (including the brain) have attracted the interest of many basic research scientists and applied scientists from the biotechnology field and larger pharmaceutical firms. This resurgence of interest has generated a wealth of new information in the field of complement inhibition. In this article, we comprehensively review up-to-date information in the field of complement inhibitors.

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.

alpha(1,2)-fucosylation prevents sialyl Lewis x expression and E-selectin-mediated adhesion of fucosyltransferase VII-transfected cells

E-selectin is a cytokine-inducible, calcium-dependent endothelial cell adhesion molecule that plays a critical role in the leucocyte-endothelium interaction during inflammation and is thought to contribute to the metastatic dissemination of tumour cells. Like the other selectins, E-selectin binds to ligands carrying the tetrasaccharide sialyl-Lewis x (NeuAcalpha2,3Galbeta1,4[Fucalpha1, 3]GlcNAc)1 or its isomer sialyl-Lewis a (NeuAcalpha2, 3Galbeta1, 3[Fucalpha1,4]GlcNAc). We examined the effect of expressing the H-type alpha(1,2)-fucosyltransferase or the alpha(2, 6)-sialyltransferase on the synthesis of sialyl-Lewis x by alpha(1, 3)fucosyltransferase. We found that H-type alpha(1, 2)-fucosyltransferase but not alpha(2,6)-sialyltransferase, strongly inhibited sialyl-Lewis x expression and E-selectin adhesion. We assume that H-type alpha(1,2)-fucosyltransferase competes with the endogenous alpha(2,3)-sialyltransferase for the N-acetyllactosamine structures assigned to further serve as acceptors for alpha(1, 3)fucosyltransferase.

Reduction of myocardial infarct size with sCR1sLe(x), an alternatively glycosylated form of human soluble complement receptor type 1 (sCR1), possessing sialyl Lewis x

1 This study investigated the effects of soluble complement receptor type 1 (sCR1) or sCR1sLex, agents which function as a complement inhibitor or as a combined complement inhibitor and selectin adhesion molecule antagonist, respectively, on the infarct size and cardiac troponin T (cTnT) release caused by regional myocardial ischaemia and reperfusion in the rat. 2 Eighty-two, male Wistar rats were subjected to 30 min occlusion of the left anterior descending coronary artery (LAD) followed by 2 h of reperfusion. Haemodynamic parameters were continuously recorded and at the end of the experiments infarct size (with p-nitro-blue tetrazolium) and cTnT release were determined. 3 Infusion of sCR1 (1, 5 or 15 mg kg-1, each n=7) or sCR1sLe(x) (1, 5 or 15 mg kg-1, n=7, 13 or 13, respectively) 5 min prior to LAD-reperfusion caused a reduction in infarct size from 59+/-2% (PBS - control, n=12) to 46+/-6%, 25+/-9% and 37+/-6% or 42+/-6%, 35+/-6% and 35+/-4%, respectively. 4 Infusion of sCR1 (15 mg kg-1, n=5) or sCR1sLe(x) (15 mg kg-1, n=5) also reduces the myocardial TnT release from 80+/-20 ng ml-1 (control) to 13+/-7 or 4+/-1 ng ml-1, respectively. 5 Thus, sCR1 or sCRsLe(x) significantly reduce infarct size and cardiac TnT release caused by 30 min of regional myocardial ischaemia and 2 h of reperfusion in the rat. The mechanisms of the cardioprotective effects of sCR1 or sCR1sLe(x) are not entirely clear, but may be due complement inhibition and/or prevention of the adhesion and activation of neutrophils.

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.

Endothelial Targeting and Enhanced Antiinflammatory Effects of Complement Inhibitors Possessing Sialyl Lewisx Moieties

The complement inhibitor soluble complement receptor type 1 (sCR1) and a truncated form of sCR1, sCR1[desLHR-A], have been generated with expression of the selectin-reactive oligosaccharide moiety, sialyl Lewisx (sLex), as N-linked oligosaccharide adducts. These modified proteins, sCR1sLex and sCR1[desLHR-A]sLex, were assessed in the L-selectin- and P-selectin-dependent rat model of lung injury following systemic activation of complement by cobra venom factor and in the L-selectin-, P-selectin-, and E-selectin-dependent model of lung injury following intrapulmonary deposition of IgG immune complexes. In the cobra venom factor model, sCR1sLex and sCR1[desLHR-A]sLex caused substantially greater reductions in neutrophil accumulation and in albumin extravasation in lung when compared with the non-sLex-decorated forms. In this model, increased lung vascular binding of sCR1sLex and sCR1[desLHR-A]sLex occurred in a P-selectin-dependent manner, in contrast to the absence of any increased binding of sCR1 or sCR1[desLHR-A]. In the IgG immune complex model, sCR1[desLHR-A]sLex possessed greater protective effects relative to sCR1[desLHR-A], based on albumin extravasation and neutrophil accumulation. Enhanced protective effects correlated with greater lung vascular binding of sCR1[desLHR-A]sLex as compared with the non-sLex-decorated form. In TNF-α-activated HUVEC, substantial in vitro binding occurred with sCR1[desLHR-A]sLex (but not with sCR1[desLHR-A]). This endothelial cell binding was blocked by anti-E-selectin but not by anti-P-selectin. These data suggest that sLex-decorated complement inhibitors have enhanced antiinflammatory effects and appear to have enhanced ability to localize to the activated vascular endothelium.