Strategies of therapeutic complement inhibition

Inhibition of vertebrate complement system by hematophagous arthropods: inhibitory molecules, mechanisms, physiological roles, and applications

In arthropods, hematophagy has arisen several times throughout evolution. This specialized feeding behavior offered a highly nutritious diet obtained during blood feeds. On the other hand, blood-sucking arthropods must overcome problems brought on by blood intake and digestion. Host blood complement acts on the bite site and is still active after ingestion, so complement activation is a potential threat to the host's skin feeding environment and to the arthropod gut enterocytes. During evolution, blood-sucking arthropods have selected, either in their saliva or gut, anticomplement molecules that inactivate host blood complement. This review presents an overview of the complement system and discusses the arthropod's salivary and gut anticomplement molecules studied to date, exploring their mechanism of action and other aspects related to the arthropod-host-pathogen interface. The possible therapeutic applications of arthropod's anticomplement molecules are also discussed.

Identification of the key genes of tuberculosis and construction of a diagnostic model via weighted gene co-expression network analysis

BACKGROUND

Tuberculosis (TB) is an infectious disease with high mortality, and mining key genes for TB diagnosis is vital to raise the survival rate of patients.

METHODS

The whole microarray datasets GSE83456 (training set) and GSE19444 (validation set) of TB patients were downloaded from the Gene Expression Omnibus (GEO) database. Differential expression was conducted on genes between TB and normal samples (unconfirmed TB) in GSE83456 to yield TB-related differentially expressed genes (DEGs). DEGs were subjected to weighted gene co-expression network analysis (WGCNA) and clustered to form distinct gene modules. The immune scores of 25 kinds of immune cells were obtained by single-sample gene set enrichment analysis (ssGSEA) of TB samples, and Pearson correlation analysis was carried out between the 25 immune scores and diverse gene modules. The gene modules significantly associated with immune cells were retained as Target modules. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were performed on the genes in the modules (p-value <0.05). The protein-protein interaction (PPI) network was established utilizing the STRING database for genes in the Target module, and the selected key genes were intersected with immune-related genes in the ImmPort database. The obtained immune-related module genes were used for subsequent least absolute shrinkage and selection operator (LASSO) regression analysis and diagnostic models were constructed. Finally, the receiver operating characteristic (ROC) curve was utilized to validate the diagnostic model.

RESULTS

The turquoise and yellow modules had a high correlation with macrophages. LASSO regression analysis of immune-related genes in TB was carried on to finally construct a 5-gene diagnostic model composed of C5, GRN, IL1B, IL23A, and TYMP. As demonstrated by the ROC curves, the diagnostic efficiency of this diagnostic model was 0.957 and 0.944 in the training and validation sets, respectively. Therefore, the immune-related 5-gene model had a good diagnostic function for TB.

CONCLUSION

We identified 5 immune-related diagnostic markers that may play an important role in TB, and verified that this immune-related key gene model had a good diagnostic performance.

The case for complement component 5 as a target in neurodegenerative disease

INTRODUCTION

Complement-based drug discovery is undergoing a renaissance, empowered by new advances in structural biology, complement biology and drug development. Certain components of the complement pathway, particularly C1q and C3, have been extensively studied in the context of neurodegenerative disease, and established as key therapeutic targets. C5 also has huge therapeutic potential in this arena, with its druggability clearly demonstrated by the success of C5-inhibitor eculizumab.

AREAS COVERED

We will discuss the evidence supporting C5 as a target in neurodegenerative disease, along with the current progress in developing different classes of C5 inhibitors and the gaps in knowledge that will help progress in the field.

EXPERT OPINION

Validation of C5 as a therapeutic target for neurodegenerative disease would represent a major step forward for complement therapeutics research and has the potential to furnish disease-modifying drugs for millions of patients suffering worldwide. Key hurdles that need to be overcome for this to be achieved are understanding how C5a and C5b should be targeted to bring therapeutic benefit and demonstrating the ability to target C5 without creating vulnerability to infection in patients. This requires greater biological elucidation of its precise role in disease pathogenesis, supported by better chemical/biological tools.

The role of the complement system in kidney glomerular capillary thrombosis

The complement system is part of the innate immune system. The crucial step in activating the complement system is the generation and regulation of C3 convertase complexes, which are needed to generate opsonins that promote phagocytosis, to generate C3a that regulates inflammation, and to initiate the lytic terminal pathway through the generation and activity of C5 convertases. A growing body of evidence has highlighted the interplay between the complement system, coagulation system, platelets, neutrophils, and endothelial cells. The kidneys are highly susceptible to complement-mediated injury in several genetic, infectious, and autoimmune diseases. Atypical hemolytic uremic syndrome (aHUS) and lupus nephritis (LN) are both characterized by thrombosis in the glomerular capillaries of the kidneys. In aHUS, congenital or acquired defects in complement regulators may trigger platelet aggregation and activation, resulting in the formation of platelet-rich thrombi in the kidneys. Because glomerular vasculopathy is usually noted with immunoglobulin and complement accumulation in LN, complement-mediated activation of tissue factors could partly explain the autoimmune mechanism of thrombosis. Thus, kidney glomerular capillary thrombosis is mediated by complement dysregulation and may also be associated with complement overactivation. Further investigation is required to clarify the interaction between these vascular components and develop specific therapeutic approaches.

Bioactive zinc-doped sol-gel coating modulates protein adsorption patterns and in vitro cell responses

Zinc is an essential element with an important role in stimulating the osteogenesis and mineralization and suppressing osteoclast differentiation. In this study, new bioactive ZnCl₂-doped sol-gel materials were designed to be applied as coatings onto titanium. The biomaterials were physicochemically characterized and the cellular responses evaluated in vitro using MC3T3-E1 osteoblasts and RAW264.7 macrophages. The effect of Zn on the adsorption of human serum proteins onto the material surface was evaluated through nLC-MS/MS. The incorporation of Zn did not affect the crosslinking of the sol-gel network. A controlled Zn²⁺ release was obtained, reaching values below 10 ppm after 21 days. The materials were no cytotoxic and lead to increased gene expression of ALP, TGF-β, and RUNX2 in the osteoblasts. In macrophages, an increase of IL-1β, TGF-β, and IL-4 gene expression was accompanied by a reduced TNF-α liberation. Proteomic results showed changes in the adsorption patterns of proteins associated with immunological, coagulative, and regenerative functions, in a Zn dose-dependent manner. The variations in protein adsorption might lead to the downregulation of the NF-κB pathway, thus explain the observed biological effects of Zn incorporation into biomaterials. Overall, these coatings demonstrated their potential to promote bone tissue regeneration.

ANGPTL4 exacerbates pancreatitis by augmenting acinar cell injury through upregulation of C5a

Pancreatitis is the inflammation of the pancreas. However, little is known about the genes associated with pancreatitis severity. Our microarray analysis of pancreatic tissues from mild and severe acute pancreatitis mice models identified angiopoietin-like 4 (ANGPTL4) as one of the most significantly upregulated genes. Clinically, ANGPTL4 expression was also increased in the serum and pancreatic tissues of pancreatitis patients. The deficiency in ANGPTL4 in mice, either by gene deletion or neutralizing antibody, mitigated pancreatitis-associated pathological outcomes. Conversely, exogenous ANGPTL4 exacerbated pancreatic injury with elevated cytokine levels and apoptotic cell death. High ANGPTL4 enhanced macrophage activation and infiltration into the pancreas, which increased complement component 5a (C5a) level through PI3K/AKT signaling. The activation of the C5a receptor led to hypercytokinemia that accelerated acinar cell damage and furthered pancreatitis. Indeed, C5a neutralizing antibody decreased inflammatory response in LPS-activated macrophages and alleviated pancreatitis severity. In agreement, there was a significant positive correlation between C5a and ANGPTL4 levels in pancreatitis patients. Taken together, our study suggests that targeting ANGPTL4 is a potential strategy for the treatment of pancreatitis.

A possible use of melatonin in the dental field: Protein adsorption and in vitro cell response on coated titanium

Melatonin (MLT) is widely known for regulating the circadian cycles and has been studied for its role in bone regeneration and inflammation. Its application as a coating for dental implants can condition the local microenvironment, affecting protein deposition on its surface and the cellular and tissue response. Using sol-gel coatings as a release vehicle for MLT, the aim of this work was to assess the potential of this molecule in improving the osseointegration and inflammatory responses of a titanium substrate. The materials obtained were physicochemically characterized (scanning electron microscopy, contact angle, roughness, Fourier-transform infrared spectroscopy, nuclear magnetic resonance, Si release, MLT liberation, and degradation) and studied in vitro with MC3T3-E1 osteoblastic cells and RAW264.7 macrophage cells. Although MLT application led to an increased gene expression of RUNX2 and BMP2 in 10MTL, it did not improve ALP activity. On the other hand, MLT-enriched sol-gel materials presented potential effects in the adsorption of proteins related to inflammation, coagulation and angiogenesis pathways depending on the dosage used. Using LC-MS/MS, protein adsorption patterns were studied after incubation with human serum. Proteins related to the complement systems (CO7, IC1, CO5, CO8A, and CO9) were less adsorbed in materials with MLT; on the other hand, proteins with functions in the coagulation and angiogenesis pathways, such as A2GL and PLMN, showed a significant adsorption pattern.

A Novel Antitumor Strategy: Simultaneously Inhibiting Angiogenesis and Complement by Targeting VEGFA/PIGF and C3b/C4b

Therapeutic antibodies targeting vascular endothelial growth factor (VEGF) have become a critical regimen for tumor therapy, but the efficacy of monotherapy is usually limited by drug resistance and multiple angiogenic mechanisms. Complement proteins are becoming potential candidates for cancer-targeted therapy based on their role in promoting cancer progression and angiogenesis. However, the antitumor abilities of simultaneous VEGF and complement blockade were unknown. We generated a humanized soluble VEGFR-Fc fusion protein (VID) binding VEGFA/PIGF and a CR1-Fc fusion protein (CID) targeting C3b/C4b. Both VID and CID had good affinities to their ligands and showed effective bioactivities. In vitro, angiogenesis effects induced by VEGF and hemolysis induced by complement were inhibited by VID and CID, respectively. Further, VID and CID confer a synergetic therapeutic effect in a colitis-associated colorectal cancer (CAC) model and an orthotopic 4T1 breast cancer model. Mechanically, combination therapy inhibited tumor angiogenesis, cell proliferation, and MDSC infiltration in the tumor microenvironment and promoted tumor cell apoptosis. Our study offers a novel therapeutic strategy for anti-VEGF-resistant tumors and chronic-inflammation-associated tumors.

An inhibitor of complement C5 provides structural insights into activation

The complement system is a crucial antimicrobial system in the human body. However, controlling its regulation is essential, and failure to do so is implicated in a number of clinical inflammatory pathologies leading to great interest in therapeutic complement inhibition. We have identified and characterized a class of complement inhibitors from biting ticks. Utilizing both cryoelectron microscopy and X-ray crystallography we provide a comprehensive understanding of their mechanism of inhibition at the level of the terminal pathway of complement. We present a high-resolution cryoelectron microscopy structure of complement C5, the molecule targeted by the major therapeutic Eculizumab. In addition, we reveal the fold of the CirpT family of tick inhibitors and their unique mode of inhibition.

The complement system is a crucial part of innate immune defenses against invading pathogens. The blood-meal of the tick Rhipicephalus pulchellus lasts for days, and the tick must therefore rely on inhibitors to counter complement activation. We have identified a class of inhibitors from tick saliva, the CirpT family, and generated detailed structural data revealing their mechanism of action. We show direct binding of a CirpT to complement C5 and have determined the structure of the C5–CirpT complex by cryoelectron microscopy. This reveals an interaction with the peripheral macro globulin domain 4 (C5_MG4) of C5. To achieve higher resolution detail, the structure of the C5_MG4–CirpT complex was solved by X-ray crystallography (at 2.7 Å). We thus present the fold of the CirpT protein family, and provide detailed mechanistic insights into its inhibitory function. Analysis of the binding interface reveals a mechanism of C5 inhibition, and provides information to expand our biological understanding of the activation of C5, and thus the terminal complement pathway.

Early treatment with C1 esterase inhibitor improves weight but not memory deficits in a rat model of status epilepticus

BACKGROUND

Status epilepticus (SE) is a prolonged and continuous seizure that lasts for at least 5 min. An episode of SE in a healthy system can lead to the development of spontaneous seizures and cognitive deficits which may be accompanied by hippocampal injury and microgliosis. Although the direct mechanisms underlying the SE-induced pathophysiology remain unknown, a candidate mechanism is hyperactivation of the classical complement pathway (C1q-C3 signaling). We recently reported that SE triggered an increase in C1q-C3 signaling in the hippocampus that closely paralleled cognitive decline. Thus, we hypothesized that blocking activation of the classical complement pathway immediately after SE may prevent the development of SE-induced hippocampal-dependent learning and memory deficits.

METHODS

Because C1 esterase inhibitor (C1-INH) negatively regulates activation of the classical complement pathway, we used this drug to test our hypothesis. Two groups of male rats were subjected to 1 hr of SE with pilocarpine (280-300 mg/kg, i.p.), and treated with either C1-INH (SE+C1-INH, 20 U/kg, s.c.) or vehicle (SE+veh) at 4, 24, and 48 h after SE. Control rats were treated with saline. Body weight was recorded for up to 23 days after SE. At two weeks post SE, recognition and spatial memory were determined using Novel Object Recognition (NOR) and Barnes maze (BM), respectively, as well as locomotion and anxiety-like behaviors using Open Field (OF). Histological and biochemical methods were used to measure hippocampal injury including cell death, microgliosis, and inflammation.

RESULTS

One day after SE, both SE groups had a significant loss of body weight compared to controls (p<0.05). By day 14, the weight of SE+C1-INH rats was significantly higher than SE+veh rats (p<0.05), and was not different from controls (p>0.05). At 14 days post-SE, SE+C1-INH rats displayed higher mobility (distance travelled and average speed, p<0.05) and had reduced anxiety-like behaviors (outer duration, p<0.05) than control or SE+veh rats. In NOR, control rats spent significantly more time exploring the novel object vs. the familiar (p<0.05), while rats in both SE groups spent similar amount of time exploring both objects. During days 1-4 of BM training, the escape latency of the control group significantly decreased over time (p<0.05), whereas that of the SE groups did not improve (p>0.05). Compared to vehicle-treated SE rats, SE+C1-INH rats had increased levels of C3 and microglia in the hippocampus, but lower levels of caspase-3 and synaptic markers.

CONCLUSIONS

These findings suggest that acute treatment with C1-INH after SE may have some protective, albeit limited, effects on the physiological recovery of rats' weight and some anxiolytic effects, but does not attenuate SE-induced deficits in hippocampal-dependent learning and memory. Reduced levels of caspase-3 suggest that treatment with C1-INH may protect against cell death, perhaps by regulating inflammatory pathways and promoting phagocytic/clearance pathways.

Proteomic analysis of calcium-enriched sol-gel biomaterials

Calcium is an element widely used in the development of biomaterials for bone tissue engineering as it plays important roles in bone metabolism and blood coagulation. The Ca ions can condition the microenvironment at the tissue-material interface, affecting the protein deposition process and cell responses. The aim of this study was to analyze the changes in the patterns of protein adsorption on the silica hybrid biomaterials supplemented with different amounts of CaCl₂, which can function as release vehicles. This characterization was carried out by incubating the Ca-biomaterials with human serum. LC-MS/MS analysis was used to characterize the adsorbed protein layers and compile a list of proteins whose affinity for the surfaces might depend on the CaCl₂ content. The attachment of pro- and anti-clotting proteins, such as THRB, ANT3, and PROC, increased significantly on the Ca-materials. Similarly, VTNC and APOE, proteins directly involved on osteogenic processes, attached preferentially to these surfaces. To assess correlations with the proteomic data, these formulations were tested in vitro regarding their osteogenic and inflammatory potential, employing MC3T3-E1 and RAW 264.7 cell lines, respectively. The results confirmed a Ca dose-dependent osteogenic and inflammatory behavior of the materials employed, in accordance with the protein attachment patterns.

The effect of strontium incorporation into sol-gel biomaterials on their protein adsorption and cell interactions

It is known strontium can both inhibit the osteoclast formation and stimulate the osteoblast maturation, so biomaterials containing this element can favour bone structure stabilisation. The addition of Sr to biomaterials could affect their interactions with proteins and cells. Here, a silica-hybrid sol-gel network doped with different amounts of SrCl₂ and applied as coatings on titanium discs was examined. in vitro analysis was performed to determine the potential effect of Sr in the coatings, showing enhanced gene expression of osteogenic markers (alkaline phosphatase and transforming growth factor-β) in MC3T3-E1 incubated with Sr-doped biomaterials. The examination of inflammatory markers (tumour necrosis factor-α and interleukin 10) in RAW 264.7 macrophages revealed an anti-inflammatory potential of these materials. Proteins adsorbed onto the coatings incubated with human serum (3 h at 37 °C) were also analysed; mass spectrometry was used to characterise the proteins adhering to materials with different Sr content. Adding Sr to the coatings increased their affinity to APOE and VTNC proteins (associated with anti-inflammatory and osteogenic functions). Moreover, the proteins involved in coagulation processes, such as prothrombin, were more abundant on the coatings containing Sr than on the base sol-gel surfaces. Correlations between gene expression and proteomic results were also examined.

Evolution of compstatin family as therapeutic complement inhibitors

INTRODUCTION

Therapeutic modulation of complement activation is considered as a promising approach for the treatment of host tissue damage in several inflammatory and autoimmune diseases. Complement component protein C3 is a particularly attractive drug target for complement inhibitors, due to its central role in three pathways of complement activation cascade. Areas covered: The author provides a comprehensive review on compstatin family peptides which have been discovered and optimized as potent and selective C3 inhibitors via a combination of chemical, biophysical and computational approaches. New generations of the compstatin family with improved potency and therapeutic properties have been developed in recent years. Over two decades, compstatin demonstrated therapeutic potential as a first-of-its-kind complement inhibitor in a series of disease models, with encouraging efforts in clinical trials. Expert opinion: Compstatin holds promise for new therapeutic implications in blocking the effect of the complement cascade in a variety of disease conditions. The development of cost-effective treatment options with suitable dosing route and schedule will be critical for patients with complement mediated chronic diseases.

Bioactive potential of silica coatings and its effect on the adhesion of proteins to titanium implants

There is an ever-increasing need to develop dental implants with ideal characteristics to achieve specific and desired biological response in the scope of improve the healing process post-implantation. Following that premise, enhancing and optimizing titanium implants through superficial treatments, like silica sol-gel hybrid coatings, are regarded as a route of future research in this area. These coatings change the physicochemical properties of the implant, ultimately affecting its biological characteristics. Sandblasted acid-etched titanium (SAE-Ti) and a silica hybrid sol-gel coating (35M35G30T) applied onto the Ti substrate were examined. The results of in vitro and in vivo tests and the analysis of the protein layer adsorbed to each surface were compared and discussed. In vitro analysis with MC3T3-E1 osteoblastic cells, showed that the sol-gel coating raised the osteogenic activity potential of the implants (the expression of osteogenic markers, the alkaline phosphatase (ALP) and IL-6 mRNAs, increased). In the in vivo experiments using as model rabbit tibiae, both types of surfaces promoted osseointegration. However, the coated implants demonstrated a clear increase in the inflammatory activity in comparison with SAE-Ti. Mass spectrometry (LC-MS/MS) analysis showed differences in the composition of protein layers formed on the two tested surfaces. Large quantities of apolipoproteins were found attached predominantly to SAE-Ti. The 35M35G30T coating adsorbed a significant quantity of complement proteins, which might be related to the material intrinsic bioactivity, following an associated, natural and controlled immune response. The correlation between the proteomic data and the in vitro and in vivo outcomes is discussed on this experimental work.

Clusterin in the mouse epididymis: possible roles in sperm maturation and capacitation

Clusterin (CLU) is known as an extracellular chaperone for proteins under stress, thus preventing them from aggregation and precipitation. We showed herein that CLU, expressed by principal cells of the mouse caput epididymis, was present in high amounts in the lumen. In the cauda epididymis, CLU bound tightly to the sperm head surface and its amount on total sperm was similar to that in the bathing luminal fluid. In both immotile and motile caudal epididymal sperm, CLU was localized over the entire sperm head except at the convex ridge, although in the motile sperm population, the CLU immunofluorescence pattern was distinctively mottled with a lower intensity. However, when motile sperm became capacitated, CLU was relocalized to the head hook region, with immunofluorescence intensity being higher than that on the non-capacitated counterparts. Under a slightly acidic pH of the epididymal lumen, CLU may chaperone some luminal proteins and deliver them onto the sperm surface. Immunoprecipitation of epididymal fluid proteins indicated that CLU interacted with SED1, an important egg-binding protein present in a high amount in the epididymal lumen. In a number of non-capacitated sperm, fractions of SED1 and CLU co-localized, but after capacitation, SED1 and CLU dissociated from one another. While CLU moved to the sperm head hook, SED1 translocated to the head convex ridge, the egg-binding site. Overall, CLU localization patterns can serve as biomarkers of immotile sperm, and non-capacitated and capacitated sperm in mice. The chaperone role of CLU may also be important for sperm maturation and capacitation.

Medusa's Head: The Complement System in Traumatic Brain and Spinal Cord Injury

Traumatic brain injury (TBI) and spinal cord injury (SCI) are critical medical conditions and a public health problem for which limited therapeutic options are available. The complement cascade is activated after TBI and SCI, and the resulting effects have been investigated in gene-knockout and pharmacological models. Multiple experimental studies support a net detrimental role of C3 and C5 activation in the early stages of TBI and SCI. Less firm experimental evidence suggests that, downstream of C3/C5, effector mechanisms, including the generation of membrane-activated complex and direct damage to membranes and neutrophils infiltration, may bring about the direct damage of central nervous system tissue and enhancement of neuroinflammation. The role of upstream classical, alternative, or extrinsic complement activation cascades remains unclear. Although several issues remain to be investigated, current evidence supports the investigation of a number of complement-targeting agents targeting C3 or C5, such as eculizumab, for repurposing in TBI and SCI treatment.

Proteomic analysis of silica hybrid sol-gel coatings: a potential tool for predicting the biocompatibility of implants in vivo

The interactions of implanted biomaterials with the host organism determine the success or failure of an implantation. Normally, their biocompatibility is assessed using in vitro tests. Unfortunately, in vitro and in vivo results are not always concordant; new, effective methods of biomaterial characterisation are urgently needed to predict the in vivo outcome. As the first layer of proteins adsorbed onto the biomaterial surfaces might condition the host response, mass spectrometry analysis was performed to characterise these proteins. Four distinct hybrid sol-gel biomaterials were tested. The in vitro results were similar for all the materials examined here. However, in vivo, the materials behaved differently. Six of the 171 adsorbed proteins were significantly more abundant on the materials with weak biocompatibility; these proteins are associated with the complement pathway. Thus, protein analysis might be a suitable tool to predict the in vivo outcomes of implantations using newly formulated biomaterials.

Immobilization Strategies for Functional Complement Convertase Assembly at Lipid Membrane Interfaces

The self-assembly formation of complement convertases-essential biomacromolecular complexes that amplify innate immune responses-is triggered by protein adsorption. Herein, a supported lipid bilayer platform was utilized to investigate the effects of covalent and noncovalent tethering strategies on the self-assembly of alternative pathway C3 convertase components, starting with C3b protein adsorption followed bythe addition of factors B and D. Quartz crystal microbalance-dissipation (QCM-D) experiments measured the real-time kinetics of convertase assembly onto supported lipid bilayers. The results demonstrate that the nature of C3b immobilization onto supported lipid bilayers is a key factor governing convertase assembly. The covalent attachment of C3b to maleimide-functionalized supported lipid bilayers promoted the self-assembly of functional C3 convertase in the membrane-associated state and further enabled successful evaluation of a clinically relevant complement inhibitor, compstatin. By contrast, noncovalent attachment of C3b to negatively charged supported lipid bilayers also permitted C3b protein uptake, albeit membrane-associated C3b did not support convertase assembly in this case. Taken together, the findings in this work demonstrate that the attachment scheme for immobilizing C3b protein at lipid membrane interfaces is critical for downstream C3 convertase assembly, thereby offering guidance for the design and evaluation of membrane-associated biomacromolecular complexes.

Antibodies Against Complement Components: Relevance for the Antiphospholipid Syndrome-Biomarkers of the Disease and Biopharmaceuticals

PURPOSE OF REVIEW

Laboratory criterion for the diagnosis of antiphospholipid syndrome (APS) is the presence of antiphospholipid antibodies (aPL Abs). Complement system has a role in mediating aPL Abs-induced thrombosis in animal models. The importance of antibodies against complement components (potential biomarkers of APS) and the importance of antibodies with beneficial anti-complement effects in APS (as biopharmaceuticals) are reviewed.

RECENT FINDINGS

Antibodies against complement components described in APS patients, so far, are anti-C1q and anti-factor H Abs, although anti-factor B Abs and anti-C5a Abs were described in animal models of APS. Clinical studies in APS patients are limited to a small number of case reports. Studies that would confirm potential role of Abs against complement components (as potential biomarkers of APS) are lacking. Lack of randomized clinical trials (that would provide complete data for confirmation of beneficial effects of biopharmaceuticals in complement inhibition) in APS is alarming.

Dual inhibition of complement and Toll-like receptors as a novel approach to treat inflammatory diseases-C3 or C5 emerge together with CD14 as promising targets

Review of how targeting key upstream molecules at the recognition phase of innate immunity exert anti-inflammatory effects; a potential therapeutic regimen for inflammatory diseases.

The host is protected by pattern recognition systems, including complement and TLRs, which are closely cross-talking. If improperly activated, these systems might induce tissue damage and disease. Inhibition of single downstream proinflammatory cytokines, such as TNF, IL-1β, and IL-6, have failed in clinical sepsis trials, which might not be unexpected, given the substantial amounts of mediators involved in the pathogenesis of this condition. Instead, we have put forward a hypothesis of inhibition at the recognition phase by “dual blockade” of bottleneck molecules of complement and TLRs. By acting upstream and broadly, the dual blockade could be beneficial in conditions with improper or uncontrolled innate immune activation threatening the host. Key bottleneck molecules in these systems that could be targets for inhibition are the central complement molecules C3 and C5 and the important CD14 molecule, which is a coreceptor for several TLRs, including TLR4 and TLR2. This review summarizes current knowledge of inhibition of complement and TLRs alone and in combination, in both sterile and nonsterile inflammatory processes, where activation of these systems is of crucial importance for tissue damage and disease. Thus, dual blockade might provide a general, broad-acting therapeutic regimen against a number of diseases where innate immunity is improperly activated.

Structural basis for therapeutic inhibition of complement C5

Activation of complement C5 generates the potent anaphylatoxin C5a and leads to pathogen lysis, inflammation and cell damage. The therapeutic potential of C5 inhibition has been demonstrated by eculizumab, one of the world's most expensive drugs. However, the mechanism of C5 activation by C5 convertases remains elusive, thus limiting development of therapeutics. Here we identify and characterize a new protein family of tick-derived C5 inhibitors. Structures of C5 in complex with the new inhibitors, the phase I and phase II inhibitor OmCI, or an eculizumab Fab reveal three distinct binding sites on C5 that all prevent activation of C5. The positions of the inhibitor-binding sites and the ability of all three C5-inhibitor complexes to competitively inhibit the C5 convertase conflict with earlier steric-inhibition models, thus suggesting that a priming event is needed for activation.

The relative merits of therapies being developed to tackle inappropriate ('self'-directed) complement activation

The complement system is an enzyme cascade that helps defend against infection. Many complement proteins occur in serum as inactive enzyme precursors or reside on cell surfaces. Complement components have many biologic functions and their activation can eventually damage the plasma membranes of cells and some bacteria. Although a direct link between complement activation and autoimmune diseases has not been found, there is increasing evidence that complement activation significantly contributes to the pathogenesis of a large number of inflammatory diseases that may have autoimmune linkage. The inhibition of complement may therefore be very important in a variety of autoimmune diseases since their activation may be detrimental to the individual involved. However, a complete and long-term inhibition of complement may have some contra side effects such as increased susceptibility to infection. The site of complement activation will, however, determine the type of inhibitor to be used, its route of application and dosage level. Compared with conventional drugs, complement inhibitors may be the best option for treatment of autoimmune diseases. The review takes a critical look at the relative merits of therapies being developed to tackle inappropriate complement activation that are likely to result in sporadic autoimmune diseases or worsen already existing one. It covers the complement system, general aspects of complement inhibition therapy, therapeutic strategies and examples of complement inhibitors. It concludes by highlighting on the possibility that a better inhibitor of complement activation when found will help provide a formidable treatment for autoimmune diseases as well as preventing one.

Molecular insights into the surface-specific arrangement of complement C5 convertase enzymes

Background

Complement is a large protein network in plasma that is crucial for human immune defenses and a major cause of aberrant inflammatory reactions. The C5 convertase is a multi-molecular protease complex that catalyses the cleavage of native C5 into its biologically important products. So far, it has been difficult to study the exact molecular arrangement of C5 convertases, because their non-catalytic subunits (C3b) are covalently linked to biological surfaces through a reactive thioester. Through development of a highly purified model system for C5 convertases, we here aim to provide insights into the surface-specific nature of these important protease complexes.

Results

Alternative pathway (AP) C5 convertases were generated on small streptavidin beads that were coated with purified C3b molecules. Site-specific biotinylation of C3b via the thioester allowed binding of C3b in the natural orientation on the surface. In the presence of factor B and factor D, these C3b beads could effectively convert C5. Conversion rates of surface-bound C3b were more than 100-fold higher than fluid-phase C3b, confirming the requirement of a surface. We determine that high surface densities of C3b, and its attachment via the thioester, are essential for C5 convertase formation. Combining our results with molecular modeling explains how high C3b densities may facilitate intermolecular interactions that only occur on target surfaces. Finally, we define two interfaces on C5 important for its recognition by surface-bound C5 convertases.

Conclusions

We establish a highly purified model that mimics the natural arrangement of C5 convertases on a surface. The developed model and molecular insights are essential to understand the molecular basis of deregulated complement activity in human disease and will facilitate future design of therapeutic interventions against these critical enzymes in inflammation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0203-8) contains supplementary material, which is available to authorized users.

Therapeutic complement inhibition – from experimental to clinical medicine

Internationally, the use of the C5-inhibiting monoclonal antibody eculizumab has in the course of just a few years become the first choice of treatment of atypical haemolytic uraemic syndrome and the most severe phenotypes of paroxysmal nocturnal haemoglobinuria. At present eculizumab is the only complement inhibitor in ordinary clinical use. This despite the fact that there only exists one randomised, placebo-controlled trial of eculizumab for paroxysmal nocturnal haemoglobinuria and none for atypical haemolytic uraemic syndrome, and that the therapy is very costly. There is reason to believe that complement inhibition as therapy will increase in the future, and that other drugs will also prove to be effective.

Complement involvement in kidney diseases: From physiopathology to therapeutical targeting

Complement cascade is involved in several renal diseases and in renal transplantation. The different components of the complement cascade might represent an optimal target for innovative therapies. In the first section of the paper the authors review the physiopathology of complement involvement in renal diseases and transplantation. In some cases this led to a reclassification of renal diseases moving from a histopathological to a physiopathological classification. The principal issues afforded are: renal diseases with complement over activation, renal diseases with complement dysregulation, progression of renal diseases and renal transplantation. In the second section the authors discuss the several complement components that could represent a therapeutic target. Even if only the anti C5 monoclonal antibody is on the market, many targets as C1, C3, C5a and C5aR are the object of national or international trials. In addition, many molecules proved to be effective in vitro or in preclinical trials and are waiting to move to human trials in the future.

Isolation of an anti-complementary polysaccharide from the root of Bupleurum chinense and identification of its targets in complement activation cascade

AIM

To isolate and characterize the anti-complementary polysaccharide from the root of Bupleurum chinense.

METHODS

Bioactivity-guided fractionation and purification was used to obtain the anti-complementary polysaccharide from the hot-water extract of the root of Bupleurum chinense. The polysaccharide was characterized by various chemical and spectral analyses. The anti-complementary activities were evaluated by hemolytic assay in vitro. The action targets were identified in the system with individual complement-depleted sera.

RESULTS

A homogeneous polysaccharide BC-PS2 was isolated as an anti-complementary agent. It was identified as a branched polysaccharide with an average molecular weight about 2 000 KDa, composed of Glc, Ara, Gal, and Man in the ratio 3.5 : 2.4 : 2.0 : 1.0, respectively, along with a trace of Rha and Xyl, and only 1.11% of protein. The main linkages of the residues of BC-PS2 include terminal, 1, 6-linked, 1, 3-linked and 1, 3, 6-linked Glcp, terminal and 1, 5-linked Araf, terminal, 1, 4-linked, 1, 6-linked and 1, 4, 6-linked Galp, terminal, and, 1, 4-linked and 1, 4, 6-linked Manp. The bioassay experiments revealed that BC-PS2 inhibited complement activation on both the classical and alternative pathways, with CH50 and AP50 of (0.222 ± 0.013) and (0.356 ± 0.032) mg·mL(-1), respectively. Preliminary mechanism studies indicated that BC-PS2 interacted with C1q, C2, and C9 components.

CONCLUSION

The results demonstrated that BC-PS2 is an anti-complementary polysaccharide, and should be important constituent of the root of Bupleurum chinense for its application in the treatment of diseases associated with the excessive activation of complement system.

Neuroprotective effects of the SCR1-3 functional domain of CR1 on acute cerebral ischemia and reperfusion injury in rats

OBJECTIVE

Complement receptor type 1 (CR1), one of the most potent inhibitors in complement activation, shows a protective effect on cerebral ischemia/reperfusion (CI/R) injury due to its ability to bind C3b and C4b and to inactivate C3/C5 convertases. So far, no study assessed the effect of the first three short consensus repeats (SCR1-3) with low molecular weight, one of the most active functional domains of CR1, binding C4b with a powerful decay-acceleration effect on classical and alternative C3/C5 convertases pathways. Therefore, we aim to assess this effect on CI/R injury in the present study.

METHODS

Seventy-five adult male Sprague-Dawley rats were randomly divided into three groups: sham operation group (n = 15), CI/R group (n = 30), and CI/R group treated with CR1-SCR1-3 protein (n = 30). After middle cerebral artery occlusion (MCAO) for 1 hour and reperfusion for 24 hours, neurological motor deficits, cerebral infarct size, and biochemical parameters including myeloperoxidase (MPO), malondialdehyde (MDA), and superoxide dismutase (SOD) were assessed. Meanwhile, tissues in cerebral cortex were collected and processed for western blotting, immunohistochemistry, and HE staining.

RESULTS

CR1-SCR1-3 could improve neurological functions in brain with a 26.8% decrease in neurological motor deficit score and could lead to a 63.8% reduction in cerebral infarct size. Besides, pretreatment using CR1-SCR1-3 could prevent neutrophil infiltration and alleviate inflammation severity and subsequent tissue damage. Decreased C4b expression and action, as well as improved morphological changes, were also observed in cerebral tissues of CI/R+CR1-SCR1-3 rats.

CONCLUSION

CR1-SCR1-3 protein could possess a neuroprotective effect on acute CI/R injury.

Roles for endothelial cells in dengue virus infection

Dengue viruses cause two severe diseases that alter vascular fluid barrier functions, dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). The endothelium is the primary fluid barrier of the vasculature and ultimately the effects of dengue virus infection that cause capillary leakage impact endothelial cell (EC) barrier functions. The ability of dengue virus to infect the endothelium provides a direct means for dengue to alter capillary permeability, permit virus replication, and induce responses that recruit immune cells to the endothelium. Recent studies focused on dengue virus infection of primary ECs have demonstrated that ECs are efficiently infected, rapidly produce viral progeny, and elicit immune enhancing cytokine responses that may contribute to pathogenesis. Furthermore, infected ECs have also been implicated in enhancing viremia and immunopathogenesis within murine dengue disease models. Thus dengue-infected ECs have the potential to directly contribute to immune enhancement, capillary permeability, viremia, and immune targeting of the endothelium. These effects implicate responses of the infected endothelium in dengue pathogenesis and rationalize therapeutic targeting of the endothelium and EC responses as a means of reducing the severity of dengue virus disease.

Endothelial cells elicit immune-enhancing responses to dengue virus infection

Dengue viruses cause two severe diseases that alter vascular fluid barrier functions, dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). Preexisting antibodies to dengue virus disposes patients to immune-enhanced edema (DSS) or hemorrhagic (DHF) disease following infection by a discrete dengue virus serotype. Although the endothelium is the primary vascular fluid barrier, direct effects of dengue virus on endothelial cells (ECs) have not been considered primary factors in pathogenesis. Here, we show that dengue virus infection of human ECs elicits immune-enhancing EC responses. Our results suggest that rapid early dengue virus proliferation within ECs is permitted by dengue virus regulation of early, but not late, beta interferon (IFN-β) responses. The analysis of EC responses following synchronous dengue virus infection revealed the high-level induction and secretion of immune cells (T cells, B cells, and mast cells) as well as activating and recruiting cytokines BAFF (119-fold), IL-6/8 (4- to 7-fold), CXCL9/10/11 (45- to 338-fold), RANTES (724-fold), and interleukin-7 (IL-7; 128-fold). Moreover, we found that properdin factor B, an alternative pathway complement activator that directs chemotactic anaphylatoxin C3a and C5a production, was induced 34-fold. Thus, dengue virus-infected ECs evoke key inflammatory responses observed in dengue virus patients which are linked to DHF and DSS. Our findings suggest that dengue virus-infected ECs directly contribute to immune enhancement, capillary permeability, viremia, and immune targeting of the endothelium. These data implicate EC responses in dengue virus pathogenesis and further rationalize therapeutic targeting of the endothelium as a means of reducing the severity of dengue virus disease.

Advances in assay of complement function and activation

The main function of the complement system is pattern recognition of danger. Typical exogenous danger signals are pathogen associated molecular patterns inducing a protective inflammatory response. Other examples are exposure to foreign surfaces of biomedical materials including nanoparticles, which principally induce the same inflammatory response. If a surface is "foreign" to the host, it induces complement activation. Development of monoclonal antibodies to neoepitopes on complement activation products introduced an entirely new set of methods for assay of complement activation. Activation of complement by a surface occurs by impairment of the fine balance of the control system, e.g. by preferred binding of factor B at the expense of factor H. Sensitive methods to detect complement activation on surfaces and in the fluid phase are a prerequisite for investigation of the biocompatibility of artificial materials. This information can be used to develop new materials with enhanced biocompatibility. Here we review available methods to study human and animal complement function and activation in vitro and in vivo.

Neisseria meningitidis and Escherichia coli are protected from leukocyte phagocytosis by binding to erythrocyte complement receptor 1 in human blood

The initial interaction of Gram-negative bacteria with erythrocytes and its implications on leukocyte phagocytosis and oxidative burst in human whole blood were examined. Alexa-labeled Escherichia coli, wild-type H44/76 N. meningitidis and the H44/76lpxA lipopolysaccharide (LPS)-deficient mutant were incubated with whole blood using lepirudin as anticoagulant which has no adverse effects on complement. Bacteria free in plasma, bound to erythrocytes or phagocytized by granulocytes and monocytes were quantified using flow cytometry. The effects of the C3 inhibitor compstatin, a C5a receptor antagonist (C5aRa) and a complement receptor 1 (CR1)-blocking antibody (3D9) were examined. Most bacteria (80%) immediately bound to erythrocytes. The binding gradually declined over time, with a parallel increase in phagocytosis. Complement inhibition with compstatin reduced erythrocyte binding and bacterial C3 opsonization. In contrast, the C5aRa efficiently reduced phagocytosis, but did not affect the binding of bacteria to erythrocytes. The anti-CR1 blocking mAb dose-dependently reduced bacterial binding to erythrocytes to nil, with subsequent increased phagocytosis and oxidative burst. LPS had no effect on these processes since similar results were obtained using an LPS-deficient N. meningitidis mutant. In vivo experiments in a pig model of sepsis showed limited binding of bacteria to erythrocytes, consistent with the facts that erythrocyte CR1 receptors are absent in non-primates and that the bacteria were mainly found in the lungs. In conclusion, complement-dependent binding of Gram-negative bacteria to erythrocyte CR1 decreases phagocytosis and oxidative burst by leukocytes in human whole blood.

A new generation of potent complement inhibitors of the Compstatin family

Compstatin family peptides are potent inhibitors of the complement system and promising drug candidates against diseases involving under-regulated complement activation. Compstatin is a 13-residue cyclized peptide that inhibits cleavage of complement protein C3, preventing downstream complement activation. We present three new compstatin variants, characterized by tryptophan replacement at positions 1 and/or 13. Peptide design was based on physicochemical reasoning and was inspired by earlier work, which identified tryptophan substitutions at positions 1 and 13 in peptides with predicted C3c binding abilities [Bellows M.L., Fung H.K., Taylor M.S., Floudas C.A., López de Victoria A., Morikis D. (2010) Biophys J; 98: 2337-2346]. The new variants preserve distinct polar and nonpolar surfaces of compstatin, but have altered local interaction capabilities with C3. All three peptides exhibited potent C3 binding by surface plasmon resonance and potent complement inhibition by enzyme-linked immunosorbent assay. We also present enzyme-linked immunosorbent assay data and detailed surface plasmon resonance kinetic data of three peptides from previous computational design.

Use of DAF-displaying adenovirus vectors reduces induction of transgene- and vector-specific adaptive immune responses in mice

Adenovirus (Ad)-based vectors are attractive candidates for a variety of gene-transfer applications. In this study, we found that decay-accelerating factor (DAF)-displaying Ads induce significantly decreased cellular immune responses to transgenes expressed from the vectors in both Ad5-naive and Ad5-immune mice. Specifically, we found a diminished ability of splenocytes to secrete interferon-γ after recall exposure to multiple peptides derived from antigens expressed by DAF-displaying Ads. We also confirmed that DAF-displaying Ads induce decreased numbers of antigen-specific, CD8(+) effector memory and central memory CD8(+) T cells, thereby uncovering a unique role of complement in modulating the induction of robust memory T-cell responses. We also confirmed that DAF-displaying Ads generate significantly reduced titers of Ad capsid-specific neutralizing antibodies after gene transfer in vivo. In conclusion, DAF-displaying Ad5-based vectors exhibit decreased induction of complement-dependent, innate immune responses, resulting in both an improved safety profile and a decreased propensity to induce humoral and cellular adaptive immune responses to Ad capsid proteins and Ad vector-expressed transgene products. This attractive combination of features will be beneficial in a variety of clinically relevant gene-transfer applications.

Review: Complement and its regulatory proteins in kidney diseases

Complement is a part of the body's innate immune system that helps defend the host from microbial infection. It is tightly controlled by a number of cell surface and fluid-phase proteins so that under normal circumstances injury to autologous tissues is avoided. In many pathological settings, such as when the complement regulatory mechanisms are dysfunctional or overwhelmed, complement attack of autologous tissues can occur with severe, sometimes life-threatening consequences. The kidney appears to be particularly vulnerable to complement-mediated inflammatory injury and many kidney pathologies have been linked to abnormal complement activation. Clinical and experimental studies have shown that complement attack can be a primary cause in rare, genetically predisposed kidney diseases or a significant contributor to kidney injury caused by other etiological factors. Here we provide a brief review of recent advances on the activation and regulation of the complement system in kidney disease, with a particular emphasis on the relevance of complement regulatory proteins.

Natural IgM antibodies are involved in the activation of complement by hypoxic human tubular cells

Ischemia-reperfusion injury (IRI) has a major impact on graft survival after transplantation. Renal proximal tubular epithelial cells (PTEC) located at the corticomedullary zone are relatively susceptible to IRI and have been identified as one of the main targets of complement activation. Studies in mice have shown an important role for the alternative pathway of complement activation in renal IRI. However, it is unclear whether experimental data obtained in mice can be extrapolated to humans. Therefore, we developed an in vitro model to induce hypoxia-reoxygenation in human and mouse PTEC and studied the role of the different pathways of complement activation. Exposure of human PTEC to hypoxia followed by reoxygenation in human serum resulted in extensive complement activation. Inhibition studies using different complement inhibitors revealed no involvement of the alternative or lectin pathway of complement activation by hypoxic human PTEC. In contrast, complement activation by hypoxic murine PTEC was shown to be exclusively dependent on the alternative pathway. Hypoxic human PTEC induced classic pathway activation, supported by studies in C1q-depleted serum and the use of blocking antibodies to C1q. The activation of the classic pathway was mediated by IgM through interaction with modified phosphomonoesters exposed on hypoxic PTEC. Studies with different human sera showed a strong correlation between IgM binding to hypoxic human PTEC and the degree of complement activation. These results demonstrate important species-specific differences in complement activation by hypoxic PTEC and provide clues for directed complement inhibition strategies in the treatment and prevention of IRI in the human kidney.

The significance of the complement system for the pathogenesis of age-related macular degeneration - current evidence and translation into clinical application

BACKGROUND

Dysregulation of the complement system has been shown to play a major role in the pathogenesis of age-related macular degeneration (AMD).

METHODS

The current evidence from human studies derives from immunohistochemical and proteomic studies in donor eyes, genetic association studies, and studies of blood complement protein levels. These lines of evidence are corroborated by in vitro and animal studies.

RESULTS

In AMD donor eyes, detection of complement proteins in drusen suggested local inflammatory processes involving the complement system. Moreover, higher levels of complement proteins in the Bruch's membrane/choroid complex could be detected in AMD donor eyes compared to controls. A large number of independent genetic studies have consistently confirmed the association of AMD with risk or protective variants in genes coding for complement proteins, including complement factor H (CFH), CFH-related proteins 1 and 3, factor B/C2, C3 and factor I. Another set of independent studies detected increased levels of complement activation products in plasma of AMD patients, suggesting that AMD may be a systemic disease and the macula a vulnerable anatomic site of minimal resistance to complement activation. Genotype-phenotype correlations, including the impact of genetic variants on disease progression, gene-environment and pharmacogenetic interactions, have been investigated. There is evidence that complement gene variants may be associated with the progression from early to late forms of AMD, whereas they do not appear to play a significant role when late atrophic AMD has already developed. There are indications for an interaction between genetic variants and supplementation and dietary factors. Also, there is some evidence that variants in the CFH gene influence treatment effects in patients with neovascular AMD.

CONCLUSIONS

Such data suggest that the complement system may have a significant role for developing new prophylactic and therapeutic interventions in AMD. In fact, several compounds acting on the complement pathway are currently in clinical trials. Therapeutics that modulate the complement system need to balance inhibition with preservation of sufficient functional activity in order to maintain adequate immune responses and tissue homeostasis. Specifically, targeting the dysfunction appears more adequate than a global suppression of complement activation in chronic diseases such as AMD.

Novel analogues of the therapeutic complement inhibitor compstatin with significantly improved affinity and potency

Compstatin is a 13-residue disulfide-bridged peptide that inhibits a key step in the activation of the human complement system. Compstatin and its derivatives have shown great promise for the treatment of many clinical disorders associated with unbalanced complement activity. To obtain more potent compstatin analogues, we have now performed an N-methylation scan of the peptide backbone and amino acid substitutions at position 13. One analogue (Ac-I[CVW(Me)QDW-Sar-AHRC](NMe)I-NH(2)) displayed a 1000-fold increase in both potency (IC(50) = 62 nM) and binding affinity for C3b (K(D) = 2.3 nM) over that of the original compstatin. Biophysical analysis using surface plasmon resonance and isothermal titration calorimetry suggests that the improved binding originates from more favorable free conformation and stronger hydrophobic interactions. This study provides a series of significantly improved drug leads for therapeutic applications in complement-related diseases, and offers new insights into the structure-activity relationships of compstatin analogues.

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.

Early expression changes of complement regulatory proteins and C5A receptor (CD88) on leukocytes after multiple injury in humans

As a crucial element of innate immunity, the complement cascade becomes activated after severe trauma. Regulation of the complement cascade and protection against complement-mediated tissue destruction is provided by a selection of soluble and membrane-bound complement regulatory proteins (CRegs). To date, the leukocyte expression profile of CRegs in multiple injured patients is unknown. In the present study, expression of CRegs and the C5a receptor (CD88) was analyzed on neutrophils, monocytes, and lymphocytes by flow cytometry. Whole blood samples were obtained from healthy volunteers (n = 16) or multiple injured patients (n = 12) on admission in the emergency department and 4, 12, 24, 120, and 240 h after trauma. The content of CRegs and CD88 on leukocytes was significantly altered posttrauma: CD55 (decay accelerating factor) displayed a time-dependent, elevated expression pattern on neutrophils and monocytes, but not on lymphocytes. CD59 (membrane attack complex inhibitor) expression was significantly increased on neutrophils and monocytes at the time of admission and after 5 to 10 days in lymphocytes. CD46 (membrane cofactor protein) was significantly down-regulated in all three cell types posttrauma. CD35 (complement receptor 1) expression on neutrophils was initially decreased, whereas monocytes presented a significant increase in CD35 expression. CD35 on lymphocyte remained unchanged throughout the observation period. CD88 expression was considerably reduced on leukocytes between 0 and 240 h after injury. CD59, CD46, and CD88 expression values on neutrophils reversely correlated with severity of injury. In summary, expression profiles of CRegs and CD88 on leukocytes are specifically altered after polytrauma in humans, indicating a trauma-induced "complementopathy."

C3b and factor H: key components of the complement system

In all three complement pathways, the central molecule is C3, which, upon activation cleavage, forms the major opsonin C3b - the key component of complement. C3b is also essential for propagation of the complement cascade to the stage of the lytic terminal complement complexes. In order to prevent damage to self cells and tissues and restrict overconsumption of the complement components, C3b molecules need to be controlled by factor H. Defect in C3 functions leads to compromised microbial defence and increased susceptibility to certain autoimmune diseases. Deficiency of factor H, or a functional defect in its N terminus, often leads to membranoproliferative glomerulonephritis and complement depletion, owing to continuous overconsumption of C3. Defect in the factor H C terminus leads to a dramatically increased risk of atypical hemolytic uremic syndrome. In addition, recently, a polymorphism in the middle part of factor H (Y402H) has been shown to be the major risk factor for the most common cause of blindness in the industrialized world: age-related macular degeneration. In future, analysis of patient samples for defects in these key complement components may prove useful in diagnosis of these diseases and new therapeutic targets will certainly be the aim for use in the recently recognized factor H-related diseases.

Adenovirus capsid-display of the retro-oriented human complement inhibitor DAF reduces Ad vector-triggered immune responses in vitro and in vivo

Adenovirus (Ad) vectors are widely used in human clinical trials. However, at higher dosages, Ad vector-triggered innate toxicities remain a major obstacle to many applications. Ad interactions with the complement system significantly contribute to innate immune responses in several models of Ad-mediated gene transfer. We constructed a novel class of Ad vectors, genetically engineered to "capsid-display" native and retro-oriented versions of the human complement inhibitor decay-accelerating factor (DAF), as a fusion protein from the C-terminus of the Ad capsid protein IX. In contrast to conventional Ad vectors, DAF-displaying Ads dramatically minimized complement activation in vitro and complement-dependent immune responses in vivo. DAF-displaying Ads did not trigger thrombocytopenia, minimized endothelial cell activation, and had diminished inductions of proinflammatory cytokine and chemokine responses. The retro-oriented display of DAF facilitated the greatest improvements in vivo, with diminished activation of innate immune cells, such as dendritic and natural killer cells. In conclusion, Ad vectors can capsid-display proteins in a manner that not only retains the functionality of the displayed proteins but also potentially can be harnessed to improve the efficacy of this important gene transfer platform for numerous gene transfer applications.

Therapeutic targeting of classical and lectin pathways of complement protects from ischemia-reperfusion-induced renal damage

Ischemia-reperfusion injury is the major cause of delayed graft function in transplanted kidneys, an early event significantly affecting long-term graft function and survival. Several studies in rodents suggest that the alternative pathway of the complement system plays a pivotal role in renal ischemia-reperfusion injury. However, limited information is currently available from humans and larger animals. Here we demonstrated that 30 minutes of ischemia resulted in the induction of C4d/C1q, C4d/MLB, and MBL/MASP-2 deposits in a swine model of ischemia-reperfusion injury. The infusion of C1-inhibitor led to a significant reduction in peritubular capillary and glomerular C4d and C5b-9 deposition. Moreover, complement-inhibiting treatment significantly reduced the numbers of infiltrating CD163(+), SWC3a(+), CD4a(+), and CD8a(+) cells. C1-inhibitor administration led to significant inhibition of tubular damage and tubular epithelial cells apoptosis. Interestingly, we report that focal C4d-deposition colocalizes with C1q and MBL at the peritubular and glomerular capillary levels also in patients with delayed graft function. In conclusion, we demonstrated the activation and a pathogenic role of classical and lectin pathways of complement in a swine model of ischemia-reperfusion-induced renal damage. Therefore, inhibition of these two pathways might represent a novel therapeutic approach in the prevention of delayed graft function in kidney transplant recipients.

Therapeutic potential of complement modulation

The complement system is an essential component of innate immunity that has been more recently recognized as an unexpected player in various pathological states. These include age-related macular degeneration, atypical haemolytic uraemic syndrome, allergy, foetal loss, and axonal and myelin degradation after trauma. Its importance has also been recognized in physiological processes including haematopoietic stem cell homing to the bone marrow, liver regeneration and modulation of adaptive immune responses. Although the complement system has long been known to be involved in autoimmune and inflammatory diseases, few agents that target the complement system are currently approved for clinical use. However, renewed interest in modulating this system in various pathological conditions has emerged, and several agents are now in development.

Recent developments in low molecular weight complement inhibitors

As a key part of the innate immune system, complement plays an important role not only in defending against invading pathogens but also in many other biological processes. Inappropriate or excessive activation of complement has been linked to many autoimmune, inflammatory, and neurodegenerative diseases, as well as ischemia-reperfusion injury and cancer. A wide array of low molecular weight complement inhibitors has been developed to target various components of the complement cascade. Their efficacy has been demonstrated in numerous in vitro and in vivo experiments. Though none of these inhibitors has reached the market so far, some of them have entered clinical trials and displayed promising results. This review provides a brief overview of the currently developed low molecular weight complement inhibitors, including short peptides and synthetic small molecules, with an emphasis on those targeting components C1 and C3, and the anaphylatoxin receptors.