Molecular organization of C9 within the membrane attack complex of complement. Induction of circular C9 polymerization by the C5b-8 assembly

The Interactions of the Complement System with Human Cytomegalovirus

The complement system is an evolutionarily ancient component of innate immunity that serves as an important first line of defense against pathogens, including viruses. In response to infection, the complement system can be activated by three distinct yet converging pathways (classical, lectin, and alternative) capable of engaging multiple antiviral host responses to confront acute, chronic, and recurrent viral infections. Complement can exert profound antiviral effects via multiple mechanisms including the induction of inflammation and chemotaxis to sites of infection, neutralization/opsonization of viruses and virally infected cells, and it can even shape adaptive immune responses. With millions of years of co-evolution and the ability to establish life-long infections, herpesviruses have evolved unique mechanisms to counter complement-mediated antiviral defenses, thus enabling their survival and replication within humans. This review aims to comprehensively summarize how human herpesviruses engage with the complement system and highlight our understanding of the role of complement in human cytomegalovirus (HCMV) infection, immunity, and viral replication. Herein we describe the novel and unorthodox roles of complement proteins beyond their roles in innate immunity and discuss gaps in knowledge and future directions of complement and HCMV research.

Protein profile of circulating extracellular vesicles reveals biomarker candidates for diagnosis of post-traumatic deep vein thrombosis

BACKGROUND AND OBJECTIVE

Deep vein thrombosis (DVT) is a common complication after trauma and mostly without specific symptoms. Timely diagnosis and early appropriate treatment measures can prevent further development of thrombosis for patients with traumatic lower extremity fractures. Although extracellular vesicles (EVs) are confirmed as promising disease biomarkers, little is known about the role of altered levels and composition in the diagnosis of post-traumatic DVT.

METHOD

The levels of circulating EVs subgroups were measured using flow cytometry. Isolated EVs were characterized and subjected to proteomics analysis to screen for differentially expressed proteins (DEPs) between DVT and non-DVT patients. Regularized logistic regression analysis based on L2 penalty terms using R's caret package was applied to build a model for DVT diagnosis.

RESULTS

Compared to non-DVT patients, DVT patients had higher circulating hepatocyte-derived EVs (hEVs) with good predictive value for post-traumatic DVT diagnosis. The results of the proteomic analysis showed that differentially expressed proteins (DEPs) of circulating EVs between the DVT group and non-DVT group were enriched in the complement and coagulation cascade. Finally, an integrated model of five biomarkers including SERPING1, C8G, CFH, FIX, and hEVs level was established for post-traumatic DVT diagnosis with robust identification of the traumatic patients with and without DVT (AUC 0.972).

CONCLUSION

Post-traumatic DVT patients had changed levels and composition of circulating EVs compared to non-DVT patients and healthy controls. Circulating EVs may acquire pathological protein signatures and become potential biomarkers for identifying subjects' post-traumatic DVT.

The Role of the Complement System in the Pathogenesis of Infectious Forms of Hemolytic Uremic Syndrome

Hemolytic uremic syndrome (HUS) is an acute disease and the most common cause of childhood acute renal failure. HUS is characterized by a triad of symptoms: microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. In most of the cases, HUS occurs as a result of infection caused by Shiga toxin-producing microbes: hemorrhagic Escherichia coli and Shigella dysenteriae type 1. They account for up to 90% of all cases of HUS. The remaining 10% of cases grouped under the general term atypical HUS represent a heterogeneous group of diseases with similar clinical signs. Emerging evidence suggests that in addition to E. coli and S. dysenteriae type 1, a variety of bacterial and viral infections can cause the development of HUS. In particular, infectious diseases act as the main cause of aHUS recurrence. The pathogenesis of most cases of atypical HUS is based on congenital or acquired defects of complement system. This review presents summarized data from recent studies, suggesting that complement dysregulation is a key pathogenetic factor in various types of infection-induced HUS. Separate links in the complement system are considered, the damage of which during bacterial and viral infections can lead to complement hyperactivation following by microvascular endothelial injury and development of acute renal failure.

Underlying Genetics of aHUS: Which Connection with Outcome and Treatment Discontinuation?

Atypical hemolytic uremic syndrome (aHUS) is a rare disease caused by a genetic dysregulation of the alternative complement pathway, characterized by thrombocytopenia, hemolytic anemia, and acute kidney injury, and included in the group of thrombotic microangiopathies. With the introduction of humanized monoclonal antibodies that inhibit C5 activation, the natural history of aHUS completely changed, with a better prognosis, a quick recovery of renal function, and a significant reduction of end-stage renal disease incidence. Nowadays, there is an increasing interest in the molecular and genetic bases of this severe disease. The aim of this narrative review is to provide readers with a practical guide about different possible involved genes, elucidating the specific role of each transcribed protein in the pathogenesis of aHUS. Moreover, we analyzed the main current evidence about the relationship among genetic mutations, outcomes, and the risk of recurrence of this manifold disease.

Subversion of the Complement System by Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen heavily implicated in chronic diseases. Immunocompromised patients that become infected with P. aeruginosa usually are afflicted with a lifelong chronic infection, leading to worsened patient outcomes. The complement system is an integral piece of the first line of defense against invading microorganisms. Gram-negative bacteria are thought to be generally susceptible to attack from complement; however, P. aeruginosa can be an exception, with certain strains being serum resistant. Various molecular mechanisms have been described that confer P. aeruginosa unique resistance to numerous aspects of the complement response. In this review, we summarize the current published literature regarding the interactions of P. aeruginosa and complement, as well as the mechanisms used by P. aeruginosa to exploit various complement deficiencies and the strategies used to disrupt or hijack normal complement activities.

Multifaceted Activities of Seven Nanobodies against Complement C4b

Cleavage of the mammalian plasma protein C4 into C4b initiates opsonization, lysis, and clearance of microbes and damaged host cells by the classical and lectin pathways of the complement system. Dysregulated activation of C4 and other initial components of the classical pathway may cause or aggravate pathologies, such as systemic lupus erythematosus, Alzheimer disease, and schizophrenia. Modulating the activity of C4b by small-molecule or protein-based inhibitors may represent a promising therapeutic approach for preventing excessive inflammation and damage to host cells and tissue. Here, we present seven nanobodies, derived from llama (Lama glama) immunization, that bind to human C4b (Homo sapiens) with high affinities ranging from 3.2 nM to 14 pM. The activity of the nanobodies varies from no to complete inhibition of the classical pathway. The inhibiting nanobodies affect different steps in complement activation, in line with blocking sites for proconvertase formation, C3 substrate binding to the convertase, and regulator-mediated inactivation of C4b. For four nanobodies, we determined single-particle cryo-electron microscopy structures in complex with C4b at 3.4-4 Å resolution. The structures rationalize the observed functional effects of the nanobodies and define their mode of action during complement activation. Thus, we characterized seven anti-C4b nanobodies with diverse effects on the classical pathway of complement activation that may be explored for imaging, diagnostic, or therapeutic applications.

On-target IgG hexamerisation driven by a C-terminal IgM tail-piece fusion variant confers augmented complement activation

The majority of depleting monoclonal antibody (mAb) drugs elicit responses via Fc-FcγR and Fc-C1q interactions. Optimal C1q interaction is achieved through hexameric Fc:Fc interactions at the target cell surface. Herein is described an approach to exploit the tailpiece of the naturally multimeric IgM to augment hexamerisation of IgG. Fusion of the C-terminal tailpiece of IgM promoted spontaneous hIgG hexamer formation, resulting in enhanced C1q recruitment and complement-dependent cytotoxicity (CDC) but with off-target complement activation and reduced in-vivo efficacy. Mutation of the penultimate tailpiece cysteine to serine (C575S) ablated spontaneous hexamer formation, but facilitated reversible hexamer formation after concentration in solution. C575S mutant tailpiece antibodies displayed increased complement activity only after target binding, in-line with the concept of 'on-target hexamerisation', whilst retaining efficient in-vivo efficacy and augmented target cell killing in the lymph node. Hence, C575S-tailpiece technology represents an alternative format for promoting on-target hexamerisation and enhanced CDC.

Complement Inhibitors in Age-Related Macular Degeneration: A Potential Therapeutic Option

Age-related macular degeneration (AMD) is a multifactorial disease, which can culminate in irreversible vision loss and blindness in elderly. Nowadays, there is a big gap between dry AMD and wet AMD on treatment. Accounting for nearly 90% of AMD, dry AMD still lacks effective treatment. Numerous genetic and molecular researches have confirmed the significant role of the complement system in the pathogenesis of AMD, leading to a deeper exploration of complement inhibitors in the treatment of AMD. To date, at least 14 different complement inhibitors have been or are being explored in AMD in almost 40 clinical trials. While most complement inhibitors fail to treat AMD successfully, two of them are effective in inhibiting the rate of GA progression in phase II clinical trials, and both of them successfully entered phase III trials. Furthermore, recently emerging complement gene therapy and combination therapy also offer new opportunities to treat AMD in the future. In this review, we aim to introduce genetic and molecular associations between the complement system and AMD, provide the updated progress in complement inhibitors in AMD on clinical trials, and discuss the challenges and prospects of complement therapeutic strategies in AMD.

OMPdb: A Global Hub of Beta-Barrel Outer Membrane Proteins

OMPdb (www.ompdb.org) was introduced as a database for β-barrel outer membrane proteins from Gram-negative bacteria in 2011 and then included 69,354 entries classified into 85 families. The database has been updated continuously using a collection of characteristic profile Hidden Markov Models able to discriminate between the different families of prokaryotic transmembrane β-barrels. The number of families has increased ultimately to a total of 129 families in the current, second major version of OMPdb. New additions have been made in parallel with efforts to update existing families and add novel families. Here, we present the upgrade of OMPdb, which from now on aims to become a global repository for all transmembrane β-barrel proteins, both eukaryotic and bacterial.

Going full circle: Determining the structures of complement component 9

Pore forming proteins (PFPs) undergo dramatic conformational changes to punch holes in the target membrane. These PFPs have the ability to self-assemble, by way of oligomerization, and have the capacity to transform from a water soluble state (commonly referred to as fluid phase) to a membrane adhered form. Accordingly, PFPs are metastable, that is they are inert until the right conditions cause the release of potential energy stored in the conformational fold leading to a vast structural rearrangement into a membrane-inserted oligomeric form. However, the metastable state of PFPs poses a problem of leading to aggregation and precipitation in conditions typically required for structural biology techniques. Here, we discuss the protein chemistry of the MACPF protein complement component 9 (C9). C9 is part of a larger complex assembly known as the membrane attack complex (MAC) that has been studied extensively for its ability to form pores in bacteria. An unusual artifact of human C9 is the ability to form a soluble oligomeric state of the channel portion of the MAC, called polyC9. PolyC9 formation does not require the presence of membranes or other complement factors. It is only in recent years that structural studies of the MAC have become successful owing to improved recombinant DNA expression systems and the improvement of high-resolution techniques (both X-ray crystallography and single particle cryo-EM). We discuss the expression and purification of recombinant C9, crystallization of the soluble monomeric form of C9 and the preparation of the oligomeric polyC9.

The Challenges and Promise of Complement Therapeutics for Ocular Diseases

Ocular inflammation is a defining feature of sight threating diseases and its dysregulation can catalyze and or propagate ocular neurodegenerative maladies such as age-related macular degeneration (AMD). The complement system, an intrinsic component of the innate immunity, has an integral role in maintaining immune-surveillance and homeostasis in the ocular microenvironment; however, overstimulation can drive ocular inflammatory diseases. The mechanism for complement disease propagation in AMD is not fully understood, although there is accumulating evidence showing that targeted modulation of complement-specific proteins has the potential to become a viable therapeutic approach. To date, a major focus of complement therapeutics has been on targeting the alternative complement system in AMD. Recent studies have outlined potential complement cascade inhibitors that might mitigate AMD disease progression. First-in-class complement inhibitors target the modulation of complement proteins C3, C5, factor B, factor D, and properdin. Herein, we will summarize ocular inflammation in the context of AMD disease progression, current clinical outcomes and complications of complement-mediated therapeutics. Given the need for additional therapeutic approaches for ocular inflammatory diseases, targeted complement modulation has emerged as a leading candidate for eliminating inflammation-driven ocular maladies.

CryoEM reveals how the complement membrane attack complex ruptures lipid bilayers

The membrane attack complex (MAC) is one of the immune system's first responders. Complement proteins assemble on target membranes to form pores that lyse pathogens and impact tissue homeostasis of self-cells. How MAC disrupts the membrane barrier remains unclear. Here we use electron cryo-microscopy and flicker spectroscopy to show that MAC interacts with lipid bilayers in two distinct ways. Whereas C6 and C7 associate with the outer leaflet and reduce the energy for membrane bending, C8 and C9 traverse the bilayer increasing membrane rigidity. CryoEM reconstructions reveal plasticity of the MAC pore and demonstrate how C5b6 acts as a platform, directing assembly of a giant β-barrel whose structure is supported by a glycan scaffold. Our work provides a structural basis for understanding how β-pore forming proteins breach the membrane and reveals a mechanism for how MAC kills pathogens and regulates cell functions.

Perforin-2 Breaches the Envelope of Phagocytosed Bacteria Allowing Antimicrobial Effectors Access to Intracellular Targets

Perforin-2, the product of the MPEG1 gene, limits the spread and dissemination of bacterial pathogens in vivo. It is highly expressed in murine and human phagocytes, and macrophages lacking Perforin-2 are compromised in their ability to kill phagocytosed bacteria. In this study, we used Salmonella enterica serovar Typhimurium as a model intracellular pathogen to elucidate the mechanism of Perforin-2's bactericidal activity. In vitro Perforin-2 was found to facilitate the degradation of Ags contained within the envelope of phagocytosed bacteria. In contrast, degradation of a representative surface Ag was found to be independent of Perforin-2. Consistent with our in vitro results, a protease-sensitive, periplasmic superoxide dismutase (SodCII) contributed to the virulence of S. Typhimurium in Perforin-2 knockout but not wild-type mice. In aggregate, our studies indicate that Perforin-2 breaches the envelope of phagocytosed bacteria, facilitating the delivery of proteases and other antimicrobial effectors to sites within the bacterial cell.

Osmotic and hypoxic induction of the complement factor C9 in cultured human retinal pigment epithelial cells: Regulation of VEGF and NLRP3 expression

Purpose

Variants of complement factor genes, hypoxia and oxidative stress of the outer retina, and systemic hypertension affect the risk of age-related macular degeneration. Hypertension often results from the high intake of dietary salt that increases extracellular osmolarity. We determined the effects of extracellular hyperosmolarity, hypoxia, and oxidative stress on the expression of complement genes in cultured (dedifferentiated) human RPE cells and investigated the effects of C9 siRNA and C9 protein on RPE cells.

Methods

Hyperosmolarity was induced by adding 100 mM NaCl or sucrose to the culture medium. Hypoxia was induced by culturing cells in 1% O₂ or by adding the hypoxia mimetic CoCl₂. Oxidative stress was induced by adding H₂O₂. Gene and protein expression levels were determined with real-time RT-PCR, western blot, and ELISA analyses. The expression of the nuclear factor of activated T cell 5 (NFAT5) and complement factor (C9) was knocked down with siRNA.

Results

Extracellular hyperosmolarity, hypoxia, and oxidative stress strongly increased the transcription of the C9 gene, while the expression of the C3, C5, CFH, and CFB genes was moderately altered or not altered at all. Hyperosmolarity also induced a moderate increase in the cytosolic C9 protein level. The hyperosmotic C9 gene expression was reduced by inhibitors of the p38 MAPK, ERK1/2, JNK, and PI3K signal transduction pathways and of the transcription factors STAT3 and NFAT5. The hypoxic C9 gene expression was reduced by a STAT3 inhibitor. The knockdown of C9 with siRNA decreased the hypoxic vascular endothelial growth factor (VEGF) and NLRP3 gene expression, the hypoxic secretion of VEGF, and the hyperosmotic expression of the NLRP3 gene. Exogenous C9 protein inhibited the hyperosmotic expression of the C9 gene, the hypoxic and hyperosmotic VEGF gene expression, and the hyperosmotic expression of the NLRP3 gene. Both C9 siRNA and C9 protein inhibited inflammasome activation under hyperosmotic conditions, as indicated by the decrease in the cytosolic level of mature IL-1β.

Conclusions

The expression of the C9 gene in cultured RPE cells is highly induced by extracellular hyperosmolarity, hypoxia, and oxidative stress. The data may support the assumption that C9 gene expression may stimulate the expression of inflammatory (NLRP3) and angiogenic growth factors (VEGF) in RPE cells. Extracellular C9 protein may attenuate this effect, in part via negative regulation of the C9 mRNA level.

Molecular cell biology of complement membrane attack

The membrane attack complex (MAC) is the pore-forming toxin of the complement system, a relatively early evolutionary acquisition that confers upon complement the capacity to directly kill pathogens. The MAC is more than just a bactericidal missile, having the capacity when formed on self-cells to initiate a host of cell activation events that can have profound consequences for tissue homeostasis in the face of infection or injury. Although the capacity of complement to directly kill pathogens has been recognised for over a century, and the pore-forming killing mechanism for at least 50 years, there remains considerable uncertainty regarding precisely how MAC mediates its killing and cell activation activities. A recent burst of new information on MAC structure provides context and opportunity to re-assess the ways in which MAC kills bacteria and modulates cell functions. In this brief review we will describe key aspects of MAC evolution, function and structure and seek to use the new structural information to better explain how the MAC works.

HUS and atypical HUS

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy characterized by intravascular hemolysis, thrombocytopenia, and acute kidney failure. HUS is usually categorized as typical, caused by Shiga toxin-producing Escherichia coli (STEC) infection, as atypical HUS (aHUS), usually caused by uncontrolled complement activation, or as secondary HUS with a coexisting disease. In recent years, a general understanding of the pathogenetic mechanisms driving HUS has increased. Typical HUS (ie, STEC-HUS) follows a gastrointestinal infection with STEC, whereas aHUS is associated primarily with mutations or autoantibodies leading to dysregulated complement activation. Among the 30% to 50% of patients with HUS who have no detectable complement defect, some have either impaired diacylglycerol kinase ε (DGKε) activity, cobalamin C deficiency, or plasminogen deficiency. Some have secondary HUS with a coexisting disease or trigger such as autoimmunity, transplantation, cancer, infection, certain cytotoxic drugs, or pregnancy. The common pathogenetic features in STEC-HUS, aHUS, and secondary HUS are simultaneous damage to endothelial cells, intravascular hemolysis, and activation of platelets leading to a procoagulative state, formation of microthrombi, and tissue damage. In this review, the differences and similarities in the pathogenesis of STEC-HUS, aHUS, and secondary HUS are discussed. Common for the pathogenesis seems to be the vicious cycle of complement activation, endothelial cell damage, platelet activation, and thrombosis. This process can be stopped by therapeutic complement inhibition in most patients with aHUS, but usually not those with a DGKε mutation, and some patients with STEC-HUS or secondary HUS. Therefore, understanding the pathogenesis of the different forms of HUS may prove helpful in clinical practice.

The complement system: an evolution in progress

The complement system, which consists of three independent but interacting pathways, constitutes a powerful arm of innate immunity. Its major function is to recognize and destroy pathogenic microorganisms as well as eliminate modified self-antigens. Although it is a fine-tuned system with innate capacity to discriminate self from non-self as well as danger from non-danger signals, an unwarranted activation can nonetheless occur and cause tissue destruction. To prevent such activation, specific regulators present both in plasma and on the cell surface tightly control it. Data accumulated over the past four decades have also shown that the complement system is capable of not only cross-talk with the activation cascades of plasma––i.e. blood coagulation, contact activation, and the kinin/kallikrein system––but also serving as a bridge between innate and adaptive immunity. It is for these reasons that the various activation steps of the complement system have been recently targeted for therapy to treat diseases in which the role of complement is beyond doubt. This trend will certainly continue for years to come, especially as novel concepts guiding the field into areas never contemplated before are continuing to be discovered.

Complement system activation contributes to the ependymal damage induced by microbial neuraminidase

Background

In the rat brain, a single intracerebroventricular injection of neuraminidase from Clostridium perfringens induces ependymal detachment and death. This injury occurs before the infiltration of inflammatory blood cells; some reports implicate the complement system as a cause of these injuries. Here, we set out to test the role of complement.

Methods

The assembly of the complement membrane attack complex on the ependymal epithelium of rats injected with neuraminidase was analyzed by immunohistochemistry. Complement activation, triggered by neuraminidase, and the participation of different activation pathways were analyzed by Western blot. In vitro studies used primary cultures of ependymal cells and explants of the septal ventricular wall. In these models, ependymal cells were exposed to neuraminidase in the presence or absence of complement, and their viability was assessed by observing beating of cilia or by trypan blue staining. The role of complement in ependymal damage induced by neuraminidase was analyzed in vivo in two rat models of complement blockade: systemic inhibition of C5 by using a function blocking antibody and testing in C6-deficient rats.

Results

The complement membrane attack complex immunolocalized on the ependymal surface in rats injected intracerebroventricularly with neuraminidase. C3 activation fragments were found in serum and cerebrospinal fluid of rats treated with neuraminidase, suggesting that neuraminidase itself activates complement. In ventricular wall explants and isolated ependymal cells, treatment with neuraminidase alone induced ependymal cell death; however, the addition of complement caused increased cell death and disorganization of the ependymal epithelium. In rats treated with anti-C5 and in C6-deficient rats, intracerebroventricular injection of neuraminidase provoked reduced ependymal alterations compared to non-treated or control rats. Immunohistochemistry confirmed the absence of membrane attack complex on the ependymal surfaces of neuraminidase-exposed rats treated with anti-C5 or deficient in C6.

Conclusions

These results demonstrate that the complement system contributes to ependymal damage and death caused by neuraminidase. However, neuraminidase alone can induce moderate ependymal damage without the aid of complement.

Therapeutic targeting of complement to modify disease course and improve outcomes in neurological conditions

The recognition that complement proteins are abundantly present and can have pathological roles in neurological conditions offers broad scope for therapeutic intervention. Accordingly, an increasing number of experimental investigations have explored the potential of harnessing the unique activation pathways, proteases, receptors, complexes, and natural inhibitors of complement, to mitigate pathology in acute neurotrauma and chronic neurodegenerative diseases. Here, we review mechanisms of complement activation in the central nervous system (CNS), and explore the effects of complement inhibition in cerebral ischemic-reperfusion injury, traumatic brain injury, spinal cord injury, Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease and Huntington's disease. We consider the challenges and opportunities arising from these studies. As complement therapies approach clinical translation, we provide perspectives on how promising complement-targeted therapeutics could become part of novel and effective future treatment options to improve outcomes in the initiation and progression stages of these debilitating CNS disorders.

Quantitative Modeling of the Alternative Pathway of the Complement System

The complement system is an integral part of innate immunity that detects and eliminates invading pathogens through a cascade of reactions. The destructive effects of the complement activation on host cells are inhibited through versatile regulators that are present in plasma and bound to membranes. Impairment in the capacity of these regulators to function in the proper manner results in autoimmune diseases. To better understand the delicate balance between complement activation and regulation, we have developed a comprehensive quantitative model of the alternative pathway. Our model incorporates a system of ordinary differential equations that describes the dynamics of the four steps of the alternative pathway under physiological conditions: (i) initiation (fluid phase), (ii) amplification (surfaces), (iii) termination (pathogen), and (iv) regulation (host cell and fluid phase). We have examined complement activation and regulation on different surfaces, using the cellular dimensions of a characteristic bacterium (E. coli) and host cell (human erythrocyte). In addition, we have incorporated neutrophil-secreted properdin into the model highlighting the cross talk of neutrophils with the alternative pathway in coordinating innate immunity. Our study yields a series of time-dependent response data for all alternative pathway proteins, fragments, and complexes. We demonstrate the robustness of alternative pathway on the surface of pathogens in which complement components were able to saturate the entire region in about 54 minutes, while occupying less than one percent on host cells at the same time period. Our model reveals that tight regulation of complement starts in fluid phase in which propagation of the alternative pathway was inhibited through the dismantlement of fluid phase convertases. Our model also depicts the intricate role that properdin released from neutrophils plays in initiating and propagating the alternative pathway during bacterial infection.

Heterogeneous MAC Initiator and Pore Structures in a Lipid Bilayer by Phase-Plate Cryo-electron Tomography

Pore formation in membranes is important for mammalian immune defense against invading bacteria. Induced by complement activation, the membrane attack complex (MAC) forms through sequential binding and membrane insertion of C5b6, C7, C8, and C9. Using cryo-electron tomography with a Volta phase plate and subtomogram averaging, we imaged C5b-7, C5b-8, and C5b-9 complexes and determined the C5b-9 pore structure in lipid bilayers. The in situ C5b-9 pore structure at 2.3-nm resolution reveals a 10- to 11.5-nm cone-shaped pore starting with C5b678 and multiple copies of C9 that is poorly closed, yielding a seam between C9 and C6 substituting for the shorter β strands in C6 and C7. However, large variations of composite pore complexes are apparent in subtomograms. Oligomerized initiator complexes C5b-7 and C5b-8 show stages of membrane binding, deformation, and perforation that yield ∼3.5-nm-wide pores. These data indicate a dynamic process of pore formation that likely adapts to biological membranes under attack.

Genetic Association of the Porcine C9 Complement Component with Hemolytic Complement Activity

The complement system is a part of the natural immune regulation mechanism against invading pathogens. Complement activation from three different pathways (classical, lectin, and alternative) leads to the formation of C5-convertase, an enzyme for cleavage of C5 into C5a and C5b, followed by C6, C7, C8, and C9 in membrane attack complex. The C9 is the last complement component of the terminal lytic pathway, which plays an important role in lysis of the target cells depending on its self-polymerization to form transmembrane channels. To address the association of C9 with traits related to disease resistance, the complete porcine C9 cDNA was comparatively sequenced to detect single nucleotide polymorphisms (SNPs) in pigs of the breeds Hampshire (HS), Duroc (DU), Berlin miniature pig (BMP), German Landrace (LR), Pietrain (PIE), and Muong Khuong (Vietnamese potbelly pig). Genotyping was performed in 417 F2 animals of a resource population (DUMI: DU×BMP) that were vaccinated with Mycoplasma hyopneumoniae, Aujeszky diseases virus and porcine respiratory and reproductive syndrome virus at 6, 14 and 16 weeks of age, respectively. Two SNPs were detected within the third exon. One of them has an amino acid substitution. The European porcine breeds (LR and PIE) show higher allele frequency of these SNPs than Vietnamese porcine breed (MK). Association of the substitution SNP with hemolytic complement activity indicated statistically significant differences between genotypes in the classical pathway but not in the alternative pathway. The interactions between eight time points of measurement of complement activity before and after vaccinations and genotypes were significantly different. The difference in hemolytic complement activity in the both pathways depends on genotype, kind of vaccine, age and the interaction to the other complement components. These results promote the porcine C9 (pC9) as a candidate gene to improve general animal health in the future.

Congenital CD59 Deficiency

The severe clinical symptoms of inherited CD59 deficiency confirm the importance of CD59 as essential complement regulatory protein for protection of cells against complement attack, in particular protection of hematopoietic cells and human neuronal tissue. Targeted complement inhibition might become a treatment option as suggested by a case report. The easy diagnostic approach by flow cytometry and the advent of a new treatment option should increase the awareness of this rare differential diagnosis and lead to further studies on their pathophysiology.

The making of a macromolecular machine: assembly of the membrane attack complex

The complement terminal pathway clears pathogens by generating cytotoxic membrane attack complex (MAC) pores on target cells. For more than 40 years, biochemical and cellular assays have been used to characterize the lytic nature of the MAC and to define its protein composition. Although models for pore formation have been inferred from structures of bacterial cytolysins, it was only recently that we were able to visualize how complement components come together during MAC assembly. This review highlights structural analyses of terminal pathway complexes to explore molecular mechanisms underlying MAC formation.

Structural biology of the membrane attack complex

The complement system is an intricate network of serum proteins that mediates humoral innate immunity through an amplification cascade that ultimately leads to recruitment of inflammatory cells or opsonisation or killing of pathogens. One effector arm of this network is the terminal pathway of complement, which leads to the formation of the membrane attack complex (MAC) composed of complement components C5b, C6, C7, C8 and C9. Upon formation of C5 convertases via the classical or alternative pathways of complement activation, C5b is generated from C5 by proteolytic cleavage, nucleating a series of association and polymerisation reactions of the MAC-constituting complement components that culminate in pore formation of pathogenic membranes. Recent structures of MAC components and homologous proteins significantly increased our understanding of oligomerisation, membrane association and integration, shedding light onto the molecular mechanism of this important branch of the innate immune system.

Perforin evolved from a gene duplication of MPEG1, followed by a complex pattern of gene gain and loss within Euteleostomi

Background

The pore-forming protein perforin is central to the granule-exocytosis pathway used by cytotoxic lymphocytes to kill abnormal cells. Although this mechanism of killing is conserved in bony vertebrates, cytotoxic cells are present in other chordates and invertebrates, and their cytotoxic mechanism has not been elucidated. In order to understand the evolution of this pathway, here we characterize the origins and evolution of perforin.

Results

We identified orthologs and homologs of human perforin in all but one species analysed from Euteleostomi, and present evidence for an earlier ortholog in Gnathostomata but not in more primitive chordates. In placental mammals perforin is a single copy gene, but there are multiple perforin genes in all lineages predating marsupials, except birds. Our comparisons of these many-to-one homologs of human perforin show that they mainly arose from lineage-specific gene duplications in multiple taxa, suggesting acquisition of new roles or different modes of regulation. We also present evidence that perforin arose from duplication of the ancient MPEG1 gene, and that it shares a common ancestor with the functionally related complement proteins.

Conclusions

The evolution of perforin in vertebrates involved a complex pattern of gene, as well as intron, gain and loss. The primordial perforin gene arose at least 500 million years ago, at around the time that the major histocompatibility complex-T cell receptor antigen recognition system was established. As it is absent from primitive chordates and invertebrates, cytotoxic cells from these lineages must possess a different effector molecule or cytotoxic mechanism.

Assembly and regulation of the membrane attack complex based on structures of C5b6 and sC5b9

Activation of the complement system results in formation of membrane attack complexes (MACs), pores that disrupt lipid bilayers and lyse bacteria and other pathogens. Here, we present the crystal structure of the first assembly intermediate, C5b6, together with a cryo-electron microscopy reconstruction of a soluble, regulated form of the pore, sC5b9. Cleavage of C5 to C5b results in marked conformational changes, distinct from those observed in the homologous C3-to-C3b transition. C6 captures this conformation, which is preserved in the larger sC5b9 assembly. Together with antibody labeling, these structures reveal that complement components associate through sideways alignment of the central MAC-perforin (MACPF) domains, resulting in a C5b6-C7-C8β-C8α-C9 arc. Soluble regulatory proteins below the arc indicate a potential dual mechanism in protection from pore formation. These results provide a structural framework for understanding MAC pore formation and regulation, processes important for fighting infections and preventing complement-mediated tissue damage.

Structure of complement C6 suggests a mechanism for initiation and unidirectional, sequential assembly of membrane attack complex (MAC)

The complement membrane attack complex (MAC) is formed by the sequential assembly of C5b with four homologous proteins as follows: one copy each of C6, C7, and C8 and 12-14 copies of C9. Together these form a lytic pore in bacterial membranes. C6 through C9 comprise a MAC-perforin domain flanked by 4-9 "auxiliary" domains. Here, we report the crystal structure of C6, the first and longest of the pore proteins to be recruited by C5b. Comparisons with the structures of the C8αβγ heterodimer and perforin show that the central domain of C6 adopts a "closed" (perforin-like) state that is distinct from the "open" conformations in C8. We further show that C6, C8α, and C8β contain three homologous subdomains ("upper," "lower," and "regulatory") related by rotations about two hinge points. In C6, the regulatory segment includes four auxiliary domains that stabilize the closed conformation, inhibiting release of membrane-inserting elements. In C8β, rotation of the regulatory segment is linked to an opening of the central β-sheet of its clockwise partner, C8α. Based on these observations, we propose a model for initiation and unidirectional propagation of the MAC in which the auxiliary domains play key roles: in the assembly of the C5b-8 initiation complex; in driving and regulating the opening of the β-sheet of the MAC-performin domain of each new recruit as it adds to the growing pore; and in stabilizing the final pore. Our model of the assembled pore resembles those of the cholesterol-dependent cytolysins but is distinct from that recently proposed for perforin.

The role of the complement system and the activation fragment C5a in the central nervous system

The complement system is a pivotal component of the innate immune system which protects the host from infection and injury. Complement proteins can be induced in all cell types within the central nervous system (CNS), where the pathway seems to play similar roles in host defense. Complement activation produces the C5 cleavage fragment C5a, a potent inflammatory mediator, which recruits and activates immune cells. The primary cellular receptor for C5a, the C5a receptor (CD88), has been reported to be on all CNS cells, including neurons and glia, suggesting a functional role for C5a in the CNS. A second receptor for C5a, the C5a-like receptor 2 (C5L2), is also expressed on these cells; however, little is currently known about its potential role in the CNS. The potent immune and inflammatory actions of complement activation are necessary for host defense. However, if over-activated, or left unchecked it promotes tissue injury and contributes to brain disease pathology. Thus, complement activation, and subsequent C5a generation, is thought to play a significant role in the progression of CNS disease. Paradoxically, complement may also exert a neuroprotective role in these diseases by aiding in the elimination of aggregated and toxic proteins and debris which are a principal hallmark of many of these diseases. This review will discuss the expression and known roles for complement in the CNS, with a particular focus on the pro-inflammatory end-product, C5a. The possible overarching role for C5a in diseases of the CNS is reviewed, and the therapeutic potential of blocking C5a/CD88 interaction is evaluated.

An abnormal but functionally active complement component C9 protein found in an Irish family with subtotal C9 deficiency

Two independently segregating C9 genetic defects have previously been reported in two siblings in an Irish family with subtotal C9 deficiency. One defect would lead to an abnormal C9 protein, with replacement of a cysteine by a glycine (C98G). The second defect is a premature stop codon at amino acid 406 which would lead to a truncated C9. However, at least one of two abnormal proteins was present in the circulation of the proband at 0.2% of normal C9 concentration. In this study, the abnormal protein was shown to have a molecular weight approximately equal to that of normal C9, and to carry the binding site for monoclonal antibody (mAb) Mc42 which is known to react with an epitope at amino acid positions 412-426, distal to 406. Therefore, the subtotal C9 protein carries the C98G defect. The protein was incorporated into the terminal complement complex, and was active in haemolytic, bactericidal and lipopolysaccharide release assays. A quantitative haemolytic assay indicated even slightly greater haemolytic efficiency than normal C9. Epitope mapping with six antihuman C9 mAbs showed the abnormal protein to react to these antibodies in the same way as normal C9. However, none of these mAbs have epitopes within the lipoprotein receptor A module, where the C98G defect is located. The role of this region in C9 functionality is still unclear. In conclusion, we have shown that the lack of a cysteine led to the production of a protein present in the circulation at very much reduced levels, but which was fully functionally active.

Complement component C9 enhances the capacity of beta-lactam antibiotics to kill Escherichia coli in vitro and in vivo

Complement component C9 is required for rapid complement-mediated killing of Escherichia coli. In this report, the influence of supplemental C9 on the bactericidal and protective effects of beta-lactam antibiotics in neonates was assessed. By rocket immunoelectrophoresis, the intrinsic C9 concentrations of pooled serum from both human and rat neonates was less than 20% of adult levels. Supplemental C9 purified from human plasma enhanced the capacity of ampicillin-treated serum from human neonates to impair the survival of E coli O7:K1:NM (P < 0.02). Similarly, supplemental C9 enhanced the capacity of cefotaxime-treated neonatal rat serum to impair the survival of E coli O1:K1:NM (P < 0.05). Moreover, the intraperitoneal administration of C9 enhanced the survival of cefotaxime-treated neonatal rats that were septic with E coli (P < 0.05). These observations may contribute to the development of new strategies, such as augmentation of complement component serum concentrations, to reduce the morbidity and mortality of neonatal E coli sepsis.

The administration of complement component C9 enhances the survival of neonatal rats with Escherichia coli sepsis

To determine the significance of neonatal C9 deficiency, an animal model was developed in the rat. By rocket immunoelectrophoresis, the concentration of C9 in pooled adult rat serum was 224 +/- 7.2 microg/mL. In contrast, the concentration of C9 in pooled serum from 1-d-old rats was only 43 +/- 3.8 microg/mL and increased during the first 3 wk of life to 170 +/- 20 microg/mL. Similarly, the capacities of neonatal rat serum to kill two pathogenic strains of Escherichia coli and to lyse sensitized sheep erythrocytes were diminished compared with adult serum but increased during the first 3 wk of life. Supplemental human C9 significantly enhanced the bactericidal and hemolytic activity of neonatal rat serum. The capacity of neonatal rats to survive after the intrapulmonary injection of E. coli was positively correlated with the serum C9 concentration, bactericidal activity, and hemolytic activity. In 2-d-old rats infected with E. coli, the intraperitoneal administration of human C9 significantly enhanced survival and also enhanced the protective effect of intraperitoneal human IgG antibodies. The data indicate that C9 deficiency predisposed neonatal rats to invasion by E. coli. The neonatal rat appears to be a suitable model with which to investigate the significance of C9 deficiency.

The kinetics and distribution of C9 and SC5b-9 in vivo: effects of complement activation

Many diseases associated with complement activation are characterized by tissue deposition of components of the terminal complement complex (TCC). The ninth component of complement (C9) plays an important role in the cytolytic effects, and may contribute to the non-lethal cell-regulating functions of the TCC. In this study we examined the behaviour of radiolabelled human C9 and its soluble complexed form SC5b-9 in vivo in order to determine the effects of complement activation on its turnover, distribution and molecular size. In normal rabbits the metabolic parameters of 125I-C9 (median and range) were: plasma half-life (t1/2) 25.9 (20.6-29.5) h, fractional catabolic rate (FCR) 5.7 (5.3-7.0)%/h, and extravascular/intravascular ratio (EV/IV) 0.7 (0.6-1.1). The distribution of radiolabelled C9 amongst body tissues was similar to that observed for rabbit serum albumin (RSA). Activation of the complement cascade with i.v. injection of cobra venom factor (CVF) resulted in rapid disappearance of C9 from the plasma and accumulation of protein-bound radiolabeled in the spleen (exceeding the plasma concentration) and the liver. RSA metabolism and distribution were unaffected by CVF. Fine performance liquid chromatography (FPLC) gel filtration of plasma samples suggested that monomeric C9 was the only major radiolabelled protein present during normal turnovers, whereas CVF administration was accompanied by the prompt appearance of a high mol. wt species consistent in size with SC5b-9. When injected directly, 125I-SC5b-9 disappeared rapidly from the plasma, falling by 50% in 0.7 (0.6-0.8) h, and less than 15% remaining after 4 h with accumulation of protein-bound label in the spleen and liver. These results demonstrate the complexity of C9 metabolism during complement activation.

The size, shape and stability of complement component C9

Electron microscopy of specimens of C9 tilted through 90 degrees visualized this protein to be a globular ellipsoid with dimensions of 77 x 70 x 52 A. To check the congruence of this observation with physical properties of the molecule, hydrodynamic parameters for C9 were determined. From this work a frictional ratio of 1.32 was calculated. C9 was compared with several other proteins of similar frictional ratios whose tertiary structures are known. All examples found of such proteins whose frictional ratios were between 1.26 and 1.37 are either heart-shaped or globular ellipsoids, but none are prolate ellipsoids. By comparison the size and shape of C9 determined by electron microscopy are congruent with its hydrodynamic parameters. Both electron microscopy and physical measurements suggest that the length (110-120 A) of C9 determined by neutron and X-ray scattering experiments is an overestimate. The source of the discrepancy was identified by the demonstration that the high concns of C9 employed in neutron and X-ray scattering work lead to aggregation of the protein. Thus, investigations involving neutron and X-ray scattering were measuring polydisperse solutions of C9. The deduced value of the radius of gyration from that work (33-35 A) is now recognized as being statistical and significantly higher than the correct value of monomeric C9 (26 A), which was calculated from electron microscopy measurements. Also high-resolution electron microscopy clearly visualized poly(C9) to be a barrel-stave construct. These results suggest that monomeric C9 must undergo a major conformational alteration to extend by 55-70 A in order to self-associate laterally in order to fashion the cylindrical poly(C9).

Small angle neutron scattering studies of C8 and C9 and their interactions in solution

Small angle neutron scattering (SANS) results revealed that contrary to most reports C9 is not a globular protein. Its radius of gyration (Rg) at pH 8 and an ionic strength of 0.5 is 32.2 +/- 1.4 A increasing to 35 A at physiologic ionic strength. In contrast, C8, which has a 2.2-fold larger mass, has a similar Rg value [34.6 +/- 1.6 A]. Calibration plots of Rg vs. M(r) indicate that native C8 is a spherical protein whereas native C9 is elongated. From previous reports it was known that native C8 and C9 associate in solutions of low ionic strength. SANS results confirmed this observation but also demonstrated that C8-C9 heterodimers are already formed at physiologic ionic strength. The dimeric complex is globular [Rg = 40 +/- 0.8 A] indicating that the proteins associate side-by-side rather than end-to-end. In contrast, in presence of the drug Suramin, a potent inhibitor of the assembly of the C5b-9 complex, C9 forms a complex with twice the molecular mass that is still elongated (Rg = 48.8 +/- 0.8 A), suggesting that in this case the protein dimerizes end-to-end via a bridging Suramin molecule.

Clusterin in renal tissue: preferential localization with the terminal complement complex and immunoglobulin deposits in glomeruli

The membrane attack complex (MAC) of complement is activated by immune and non-immune mechanisms in the kidney. MAC has been found associated with glomerular immune deposits, but also to cell remnants, particularly along tubules and in vessel walls. Clusterin and S-protein (vitronectin) bind to MAC, rendering it cytolytically inactive. Both have been found associated with MAC in renal tissue. Here we analysed the deposition of clusterin and S-protein in 118 renal biopsies relative to the localization of the MAC using MoAbs. Statistical analysis was performed comparing no or little versus evident or strong staining by immunofluorescence (IF). In glomeruli, out of the 92 biopsies where both MAC and immunoglobulins were evaluated, deposits of MAC were found in the presence (32 out of 41) but also in the absence of immunoglobulins (20/51). Clusterin and S-protein deposits were seen, respectively, in 25 out of 61 and 36 out of 61 biopsies containing glomerular MAC, and almost never in its absence (one out of 50 for both). The association of the two inhibitors with MAC was observed mainly in glomeruli containing immunoglobulin deposits (respectively, 21 out of 32 and 25 out of 32), but not when immunoglobulins were absent (three out of 20 and seven out of 20) (coefficient of concordance, K = 0.47 and 0.43). The localization of MAC along tubules and in vessels was easily identified in most biopsies (93 out of 118) and was accompanied by S-protein in most cases (tubules, 86 out of 93; vessels, 82 out of 93) (K = 0.58 and 0.57 respectively) but not by clusterin (28 out of 93 and 24 out of 93). These results suggest that clusterin does not co-localize with MAC whenever there is formation and fixation of the MAC. It seems that clusterin has a particular affinity for MAC which is associated with immunoglobulin. This observation should help to distinguish between the different forms of MAC, and might indicate that MAC associated with immunoglobulin is essentially in its cytolytically inactive form.

Trichinella spiralis: activation of complement by infective larvae, adults, and newborn larvae

The ability of Trichinella spiralis to activate complement (C) has been addressed by several investigators. However, these investigators employed methods in which either detection of C fragments on the parasite surface or the adherence of leukocytes to the parasite was considered an indication of C activation. The present studies were undertaken to examine: (a) whether activation of C occurs via the classical and/or alternative pathway, (b) at which stage(s) of the parasite C activating capacity is acquired, and (c) what molecular entities of the epicuticle and/or cuticle are responsible for initiating C activation. Our studies indicate that T. spiralis activates C primarily via the alternative pathway (and weakly via the classical pathway) since incubation of parasites obtained from infected mice with either normal human serum (NHS) or Mg.EGTA-NHS, followed by incubation (1 hr, 37 degrees C) with antibody-sensitized sheep erythrocytes or rabbit erythrocytes, respectively, showed a time-and parasite number-dependent depletion of C. Although the three stages of T. spiralis, i.e., infective larvae, adults and newborn larvae, are capable of activating C, the newborn appears to be the most potent activator, especially when parasite number and size are taken into consideration. Further evidence of C activation is obtained from SDS-PAGE and Western blot analysis in which homogenates of parasites preincubated with NHS showed the presence of C3, C9, and C1q, whereas controls without serum were negative. Since isolated C1q was also capable of directly binding to the surface of adults and infective larvae, it is postulated that their cuticle and/or epicuticle may possess surface structures which serve as binding sites for C1q.(ABSTRACT TRUNCATED AT 250 WORDS)

Pore-forming peptide of pathogenic Entamoeba histolytica

A polypeptide that causes pore formation in target-cell membranes is implicated in the potent cytolytic activity of pathogenic Entamoeba histolytica. Pore-forming material was purified to apparent homogeneity by a multistep procedure, and its analysis by NaDodSO4/PAGE revealed one peptide of 4-5 kDa under nonreducing or under reducing conditions. Pore-forming activity was measured by depolarization of liposome membrane potential and was found to be optimally expressed at low pH. Active material preferentially inserted into negatively charged lipid vesicles. Treatment of purified amoeba peptide in solution or bound to liposomes with glutaraldehyde revealed oligomers upon NaDodSO4/PAGE, suggesting functionally relevant peptide-peptide interactions. The NH2-terminal amino acid sequence of the amoeba peptide was determined by protein sequencing and revealed a structural similarity to melittin, the membranolytic peptide of bee venom.

Detection of the terminal fluid-phase complement complex, SC5b-9, in the plasma of patients with insulin-dependent (type I) diabetes mellitus. Relation to increased urinary albumin excretion and plasma von Willebrand factor

An ELISA was used to measure the fluid-phase complement complex in the plasma of 54 patients with insulin-dependent (type I) diabetes mellitus. Sixty-seven per cent of the diabetic patients had increased levels of SC5b-9. In individual diabetic patients, increased SC5b-9 was found to be significantly associated with the occurrence of anti-heparan sulphate cross-reactive anti-ssDNA antibodies and in some cases with circulating immune complexes. There was a significant correlation between levels of SC5b-9 and those of urinary albumin excretion rate (AER) (r = 0.39, P less than 0.01). Levels of AER were 8.4 +/- 2.26 micrograms/min and 2.04 +/- 0.35 micrograms/min in the SC5b-9 positive and negative patients, respectively (P less than 0.01). A relationship was also found between SC5b-9 and plasma von Willebrand Factor (r = 0.45, P less than 0.02), von Willebrand factor was 189.2 +/- 19.3% and 132.3 +/- 19.6% in SC5b-9 positive and negative patients, respectively (P less than 0.05). It may be that the abnormalities found in this study play a role in the pathogenesis of the late diabetic vascular complications.

Functional size of complement and perforin pores compared by confocal laser scanning microscopy and fluorescence microphotolysis

Confocal laser scanning microscopy and fluorescence microphotolysis (also referred to as fluorescence photobleaching recovery) were employed to study the transport of hydrophilic fluorescent tracers through complement and perforin pores. By optimizing the confocal effect it was possible to determine the exclusion limit of the pores in situ, i.e. without separation of cells and tracer solution. Single-cell flux measurements by fluorescence microphotolysis yielded information on the sample population distribution of flux rates. By these means a direct comparison of complement and perforin pores was made in sheep erythrocyte membranes. In accordance with previous studies employing a variety of different techniques complement pores were found to have a functional radius of approx. 50 A when generated at high complement concentrations. The flux rate distribution indicated that pore size heterogeneity was rather small under these conditions. Perforin pores, generated in sheep erythrocyte membranes at high perforin concentrations, were found to have a functional size very similar to complement pores. Furthermore, the functional size of the perforin pore seemed to be relatively independent of the dynamic properties of the target membrane since in two cell membranes which are very different in this regard, the human erythrocyte membrane and the plasma membrane of erythroleukemic cells, the functional radius of the perforin pore was also close to 50 A. A perforin-specific antibody reduced the functional radius of perforin pores to 45 A.

Taenia taeniaeformis: evasion of complement-mediated lysis by early larval stages following activation of the alternative pathway

Activation of the alternative pathway of complement by T. taeniaeformis oncospheres and early stage metacestodes, although a factor in host defense against primary infection, does not directly lead to the killing of the parasite larvae observed prior to day 6 post-infection in innately resistant BALB/cByJ inbred mice. Immunogold labelling techniques clearly demonstrated tegument-associated C3 on in vitro-activated oncospheres incubated with non-immune mouse sera. However, C5, a protease necessary for the assembly of the membrane attack complex, was not detected. Early stage larvae cultured from in vitro-activated oncospheres escaped membrane damage and survived incubation in non-immune sera from both BALB/cByJ and taeniid-susceptible C3H/HeDub mice. Comparisons of cobra venom factor-treated and untreated C5-deficient B10.D2osn mice revealed no significant differences in parasite burden and local eosinophil infiltration at 6 days post-infection, suggesting that the terminal arm of the complement system is necessary for the previously reported role of complement in resistance to primary infection in BALB/cByJ and C3H/HeDub mice. An in vivo test of chemotaxis indicated that although both complement-intact mouse strains examined responded to intraperitoneal injections of inulin, there were lower numbers of eosinophils in C3H/HeDub mice than in BALB/cByJ mice, perhaps pointing to possible mouse strain differences in C5a generation/catabolism or eosinophil ability to respond to C5a. Lectin-binding studies showed an affinity of PNA for the exposed surface of taeniid oncospheres and 4-day post-infection metacestodes; however, binding of lectin to the carbohydrate moiety did not inhibit complement activation.

Localization and molecular modelling of the membrane-inserted domain of the ninth component of human complement and perforin

Upon interaction with the membrane-bound C5b-8 complex, the ninth component of complement (C9) unfolds and inserts into the membrane of cells on which surface complement has been activated. Consequently C9 oligomerization occurs and transmembrane channels of varying sizes are formed. The domain of the unfolded protein interacting with the cell membrane has so far not been identified since, unlike many integral membrane proteins, the C9 sequence does not contain a continuous stretch of hydrophobic amino acids. We studied the interaction of C9 with the lipid bilayer using the membrane-restricted photoaffinity label 3-(trifluoromethyl)-3-(m[125I]iodophenyl)diazirine (125I-TID). C9 was assembled on liposomes and after photoactivation, several labeled and non-labeled peptides, obtained by chemical and enzymatic cleavage or the 125I-TID-labeled C9, were analyzed. The segment from 176 to 345 was identified as the region containing the membrane-interacting structure. By means of secondary structure predictions, we identified two amphipathic alpha-helices (292-308 and 313-333) separated by a turn (309-312). Based on these results, we constructed a molecular model for the membrane-spanning region of C9. By analogy, we also constructed a model for this domain in perforin/cytolysin, a pore-forming protein found in the cytoplasmic granules of cytotoxic T-lymphocytes.