Sites within the complement C3b/C4b receptor important for the specificity of ligand binding

Mechanistic insights into complement pathway inhibition by CR1 domain duplication

Complement receptor 1 (CR1) is a membrane glycoprotein with a highly duplicated domain structure able to bind multiple ligands such as C3b and C4b, the activated fragments of complement components C3 and C4, respectively. We have previously used our knowledge of this domain structure to identify CSL040, a soluble extracellular fragment of CR1 containing the long homologous repeat (LHR) domains A, B, and C. CSL040 retains the ability to bind both C3b and C4b but is also a more potent complement inhibitor than other recombinant CR1-based therapeutics. To generate soluble CR1 variants with increased inhibitory potential across all three complement pathways, or variants with activity skewed to specific pathways, we exploited the domain structure of CR1 further by generating LHR domain duplications. We identified LHR-ABCC, a soluble CR1 variant containing a duplicated C3b-binding C-terminal LHR-C domain that exhibited significantly enhanced alternative pathway inhibitory activity in vitro compared to CSL040. Another variant, LHR-BBCC, containing duplications of both LHR-B and LHR-C with four C3b binding sites, was shown to have reduced classical/lectin pathway inhibitory activity compared to CSL040, but comparable alternative pathway activity. Interestingly, multiplication of the C4b-binding LHR-A domain resulted in only minor increases in classical/lectin pathway inhibitory activity. The CR1 duplication variants characterized in these in vitro potency assays, as well as in affinity in solution C3b and C4b binding assays, not only provides an opportunity to identify new therapeutic molecules but also additional mechanistic insights to the multiple interactions between CR1 and C3b/C4b.

Complement receptor 1 is the human erythrocyte receptor for Plasmodium vivax erythrocyte binding protein

The discovery that Africans were resistant to infection by Plasmodium vivax (P. vivax) led to the conclusion that P. vivax invasion relied on the P. vivax Duffy Binding Protein (PvDBP) interacting with the Duffy Antigen Receptor for Chemokines (DARC) expressed on erythrocytes. However, the recent reporting of P. vivax infections in DARC-negative Africans suggests that the parasite might use an alternate invasion pathway to infect DARC-negative reticulocytes. To identify the parasite ligands and erythrocyte receptors that enable P. vivax invasion of both DARC-positive and -negative erythrocytes, we expressed region II containing the Duffy Binding-Like (DBL) domain of P. vivax erythrocyte binding protein (PvEBP-RII) and verified that the DBL domain binds to both DARC-positive and -negative erythrocytes. Furthermore, an AVidity-based EXtracelluar Interaction Screening (AVEXIS) was used to identify the receptor for PvEBP among over 750 human cell surface receptor proteins, and this approach identified only Complement Receptor 1 (CR1, CD35, or C3b/C4b receptor) as a PvEBP receptor. CR1 is a well-known receptor for P. falciparum Reticulocyte binding protein Homology 4 (PfRh4) and is present on the surfaces of both reticulocytes and normocytes, but its expression decreases as erythrocytes age. Indeed, PvEBP-RII bound to a subpopulation of both reticulocytes and normocytes, and this binding was blocked by the addition of soluble CR1 recombinant protein, indicating that CR1 is the receptor of PvEBP. In addition, we found that the Long Homology Repeat A (LHR-A) subdomain of CR1 is the only subdomain responsible for mediating the interaction with PvEBP-RII.

The Molecular Mechanisms of Complement Receptor 1-It Is Complicated

Human complement receptor 1 (CR1) is a membrane-bound regulator of complement that has been the subject of recent attempts to generate soluble therapeutic compounds comprising different fragments of its extracellular domain. This review will focus on the extracellular domain of CR1 and detail how its highly duplicated domains work both separately and together to mediate binding to its main ligands C3b and C4b, and to inhibit the classical, lectin, and alternative pathways of the complement cascade via the mechanisms of decay acceleration activity (DAA) and co-factor activity (CFA). Understanding the molecular basis of CR1 activity is made more complicated by the presence not only of multiple ligand binding domains within CR1 but also the fact that C3b and C4b can interact with CR1 as both monomers, dimers, and heterodimers. Evidence for the interaction of CR1 with additional ligands such as C1q will also be reviewed. Finally, we will bring the mechanistic understanding of CR1 activity together to provide an explanation for the differential complement pathway inhibition recently observed with CSL040, a soluble CR1-based therapeutic candidate in pre-clinical development.

C3-dependent effector functions of complement

C3 is the central effector molecule of the complement system, mediating its multiple functions through different binding sites and their corresponding receptors. We will introduce the C3 forms (native C3, C3 [H₂ O], and intracellular C3), the C3 fragments C3a, C3b, iC3b, and C3dg/C3d, and the C3 expression sites. To highlight the important role that C3 plays in human biological processes, we will give an overview of the diseases linked to C3 deficiency and to uncontrolled C3 activation. Next, we will present a structural description of C3 activation and of the C3 fragments generated by complement regulation. We will proceed by describing the C3a interaction with the anaphylatoxin receptor, followed by the interactions of opsonins (C3b, iC3b, and C3dg/C3d) with complement receptors, divided into two groups: receptors bearing complement regulatory functions and the effector receptors without complement regulatory activity. We outline the molecular architecture of the receptors, their binding sites on the C3 activation fragments, the cells expressing them, the diversity of their functions, and recent advances. With this review, we aim to give an up-to-date analysis of the processes triggered by C3 activation fragments on different cell types in health and disease contexts.

Complement Receptors and Their Role in Leukocyte Recruitment and Phagocytosis

The complement system is deeply embedded in our physiology and immunity. Complement activation generates a multitude of molecules that converge simultaneously on the opsonization of a target for phagocytosis and activation of the immune system via soluble anaphylatoxins. This response is used to control microorganisms and to remove dead cells, but also plays a major role in stimulating the adaptive immune response and the regeneration of injured tissues. Many of these effects inherently depend on complement receptors expressed on leukocytes and parenchymal cells, which, by recognizing complement-derived molecules, promote leukocyte recruitment, phagocytosis of microorganisms and clearance of immune complexes. Here, the plethora of information on the role of complement receptors will be reviewed, including an analysis of how this functionally and structurally diverse group of molecules acts jointly to exert the full extent of complement regulation of homeostasis.

A novel soluble complement receptor 1 fragment with enhanced therapeutic potential

Human complement receptor 1 (HuCR1) is a pivotal regulator of complement activity, acting on all three complement pathways as a membrane-bound receptor of C3b/C4b, C3/C5 convertase decay accelerator, and cofactor for factor I-mediated cleavage of C3b and C4b. In this study, we sought to identify a minimal soluble fragment of HuCR1, which retains the complement regulatory activity of the wildtype protein. To this end, we generated recombinant, soluble, and truncated versions of HuCR1 and compared their ability to inhibit complement activation in vitro using multiple assays. A soluble form of HuCR1, truncated at amino acid 1392 and designated CSL040, was found to be a more potent inhibitor than all other truncation variants tested. CSL040 retained its affinity to both C3b and C4b as well as its cleavage and decay acceleration activity and was found to be stable under a range of buffer conditions. Pharmacokinetic studies in mice demonstrated that the level of sialylation is a major determinant of CSL040 clearance in vivo. CSL040 also showed an improved pharmacokinetic profile compared with the full extracellular domain of HuCR1. The in vivo effects of CSL040 on acute complement-mediated kidney damage were tested in an attenuated passive antiglomerular basement membrane antibody-induced glomerulonephritis model. In this model, CSL040 at 20 and 60 mg/kg significantly attenuated kidney damage at 24 h, with significant reductions in cellular infiltrates and urine albumin, consistent with protection from kidney damage. CSL040 thus represents a potential therapeutic candidate for the treatment of complement-mediated disorders.

Novel potential inhibitors of complement system and their roles in complement regulation and beyond

The complement system resembles a double-edged sword since its activation can either benefit or harm the host. Thus, regulation of this system is of utmost importance and performed by several circulating and membrane-bound complement inhibitors. The pool of well-established regulators has recently been enriched with proteins that either share structural homology to known complement inhibitors such as Sushi domain-containing (SUSD) protein family and Human CUB and Sushi multiple domains (CSMD) families or extracellular matrix (ECM) macromolecules that interact with and modulate complement activity. In this review, we summarize the current knowledge about newly discovered complement inhibitors and discuss their implications in complement regulation, as well as in processes beyond complement regulation such cancer development. Understanding the behavior of these proteins will introduce new mechanisms of complement regulation and may provide new avenues in the development of novel therapies.

C1q and Mannose-Binding Lectin Interact with CR1 in the Same Region on CCP24-25 Modules

Complement receptor type 1 (CR1) is a multi modular membrane receptor composed of 30 homologous complement control protein modules (CCP) organized in four different functional regions called long homologous repeats (LHR A, B, C, and D). CR1 is a receptor for complement-opsonins C3b and C4b and specifically interacts through pairs of CCP modules located in LHR A, B, and C. Defense collagens such as mannose-binding lectin (MBL), ficolin-2, and C1q also act as opsonins and are involved in immune clearance through binding to the LHR-D region of CR1. Our previous results using deletion variants of CR1 mapped the interaction site for MBL and ficolin-2 on CCP24-25. The present work aimed at deciphering the interaction of C1q with CR1 using new CR1 variants concentrated around CCP24-25. CR1 bimodular fragment CCP24-25 and CR1 CCP22-30 deleted from CCP24-25 produced in eukaryotic cells enabled to highlight that the interaction site for both MBL and C1q is located on the same pair of modules CCP24-25. C1q binding to CR1 shares with MBL a main common interaction site on the collagen stalks but also subsidiary sites most probably located on C1q globular heads, contrarily to MBL.

No Evidence that Knops Blood Group Polymorphisms Affect Complement Receptor 1 Clustering on Erythrocytes

Clustering of Complement Receptor 1 (CR1) in the erythrocyte membrane is important for immune-complex transfer and clearance. CR1 contains the Knops blood group antigens, including the antithetical pairs Swain-Langley 1 and 2 (Sl1 and Sl2) and McCoy a and b (McC^a and McC^b), whose functional effects are unknown. We tested the hypothesis that the Sl and McC polymorphisms might influence CR1 clustering on erythrocyte membranes. Blood samples from 125 healthy Kenyan children were analysed by immunofluorescence and confocal microscopy to determine CR1 cluster number and volume. In agreement with previous reports, CR1 cluster number and volume were positively associated with CR1 copy number (mean number of CR1 molecules per erythrocyte). Individuals with the McC^b/McC^b genotype had more clusters per cell than McC^a/McC^a individuals. However, this association was lost when the strong effect of CR1 copy number was included in the model. No association was observed between Sl genotype, sickle cell genotype, α+thalassaemia genotype, gender or age and CR1 cluster number or volume. Therefore, after correction for CR1 copy number, the Sl and McCoy polymorphisms did not influence erythrocyte CR1 clustering, and the effects of the Knops polymorphisms on CR1 function remains unknown.

How novel structures inform understanding of complement function

During the last decade, the complement field has experienced outstanding advancements in the mechanistic understanding of how complement activators are recognized, what C3 activation means, how protein complexes like the C3 convertases and the membrane attack complex are assembled, and how positive and negative complement regulators perform their function. All of this has been made possible mostly because of the contributions of structural biology to the study of the complement components. The wealth of novel structural data has frequently provided support to previously held knowledge, but often has added alternative and unexpected insights into complement function. Here, we will review some of these findings focusing in the alternative and terminal complement pathways.

Protection of host cells by complement regulators

The complement cascade is an ancient immune-surveillance system that not only provides protection from pathogen invasion but has also evolved to participate in physiological processes to maintain tissue homeostasis. The alternative pathway (AP) of complement activation is the evolutionarily oldest part of this innate immune cascade. It is unique in that it is continuously activated at a low level and arbitrarily probes foreign, modified-self, and also unaltered self-structures. This indiscriminate activation necessitates the presence of preformed regulators on autologous surfaces to spare self-cells from the undirected nature of AP activation. Although the other two canonical complement activation routes, the classical and lectin pathways, initiate the cascade more specifically through pattern recognition, their activity still needs to be tightly controlled to avoid excessive reactivity. It is the perpetual duty of complement regulators to protect the self from damage inflicted by inadequate complement activation. Here, we review the role of complement regulators as preformed mediators of defense, explain their common and specialized functions, and discuss selected cases in which alterations in complement regulators lead to disease. Finally, rational engineering approaches using natural complement inhibitors as potential therapeutics are highlighted.

High-resolution Melting PCR for Complement Receptor 1 Length Polymorphism Genotyping: An Innovative Tool for Alzheimer's Disease Gene Susceptibility Assessment

Complement receptor 1 (CR1), a transmembrane glycoprotein that plays a key role in the innate immune system, is expressed on many cell types, but especially on red blood cells (RBCs). As a receptor for the complement components C3b and C4b, CR1 regulates the activation of the complement cascade and promotes the phagocytosis of immune complexes and cellular debris, as well as the amyloid-beta (Aβ) peptide in Alzheimer's disease (AD). Several studies have confirmed AD-associated single nucleotide polymorphisms (SNPs), as well as a copy-number variation (CNV) in the CR1 gene. Here, we describe an innovative method for determining the length polymorphism of the CR1 receptor. The receptor includes three domains, called long homologous repeats (LHR)-LHR-A, LHR-C, and LHR-D-and an n domain, LHR-B, where n is an integer between 0 and 3. Using a single pair of specific primers, the genetic material is used to amplify a first fragment of the LHR-B domain (the variant amplicon B) and a second fragment of the LHR-C domain (the invariant amplicon). The variant amplicon B and the invariant amplicon display differences at five nucleotides outside of the hybridization areas of said primers. The numbers of variant amplicons B and of invariant amplicons is deduced using a quantitative tool (high-resolution melting (HRM) curves), and the ratio of the variant amplicon B to the invariant amplicon differs according to the CR1 length polymorphism. This method provides several advantages over the canonical phenotype method, as it does not require fresh material and is cheaper, faster, and therefore applicable to larger populations. Thus, the use of this method should be helpful to better understand the role of CR1 isoforms in the pathogenesis of diseases such as AD.

Red Blood Cells: A Source of Extracellular Vesicles

During their lifetime, like all other cell types, red blood cells (RBCs) release both exosomes and plasma membrane derived EVs (ectosomes). RBC exosomes are formed only during the development of RBCs in bone marrow, and are released following the fusion of microvesicular bodies (MVB) with the plasma membrane. On the other hand, RBC EVs are generated during normal aging of RBCs in circulation by budding of the plasma membrane due to complement -mediated calcium influx, followed by vesicle shedding. This makes red blood cells and stored red cells a reliable source of EVs for basic and clinical research.

Creating functional sophistication from simple protein building blocks, exemplified by factor H and the regulators of complement activation

Complement control protein modules (CCPs) occur in numerous functionally diverse extracellular proteins. Also known as short consensus repeats (SCRs) or sushi domains each CCP contains approximately 60 amino acid residues, including four consensus cysteines participating in two disulfide bonds. Varying in length and sequence, CCPs adopt a β-sandwich type fold and have an overall prolate spheroidal shape with N- and C-termini lying close to opposite poles of the long axis. CCP-containing proteins are important as cytokine receptors and in neurotransmission, cell adhesion, blood clotting, extracellular matrix formation, haemoglobin metabolism and development, but CCPs are particularly well represented in the vertebrate complement system. For example, factor H (FH), a key soluble regulator of the alternative pathway of complement activation, is made up entirely from a chain of 20 CCPs joined by short linkers. Collectively, therefore, the 20 CCPs of FH must mediate all its functional capabilities. This is achieved via collaboration and division of labour among these modules. Structural studies have illuminated the dynamic architectures that allow FH and other CCP-rich proteins to perform their biological functions. These are largely the products of a highly varied set of intramolecular interactions between CCPs. The CCP can act as building block, spacer, highly versatile recognition site or dimerization mediator. Tandem CCPs may form composite binding sites or contribute to flexible, rigid or conformationally 'switchable' segments of the parent proteins.

Regulators of complement activity mediate inhibitory mechanisms through a common C3b-binding mode

Regulators of complement activation (RCA) inhibit complement‐induced immune responses on healthy host tissues. We present crystal structures of human RCA (MCP, DAF, and CR1) and a smallpox virus homolog (SPICE) bound to complement component C3b. Our structural data reveal that up to four consecutive homologous CCP domains (i–iv), responsible for inhibition, bind in the same orientation and extended arrangement at a shared binding platform on C3b. Large sequence variations in CCP domains explain the diverse C3b‐binding patterns, with limited or no contribution of some individual domains, while all regulators show extensive contacts with C3b for the domains at the third site. A variation of ~100° rotation around the longitudinal axis is observed for domains binding at the fourth site on C3b, without affecting the overall binding mode. The data suggest a common evolutionary origin for both inhibitory mechanisms, called decay acceleration and cofactor activity, with variable C3b binding through domains at sites ii, iii, and iv, and provide a framework for understanding RCA disease‐related mutations and immune evasion.

Role of complement receptor 1 (CR1; CD35) on epithelial cells: A model for understanding complement-mediated damage in the kidney

The regulators of complement activation gene cluster encodes a group of proteins that have evolved to control the amplification of complement at the critical step of C3 activation. Complement receptor 1 (CR1) is the most versatile of these inhibitors with both receptor and regulatory functions. While expressed on most peripheral blood cells, the only epithelial site of expression in the kidney is by the podocyte. Its expression by this cell population has aroused considerable speculation as to its biologic function in view of many complement-mediated renal diseases. The goal of this investigation was to assess the role of CR1 on epithelial cells. To this end, we utilized a Chinese hamster ovary cell model system. Among our findings, CR1 reduced C3b deposition by ∼ 80% during classical pathway activation; however, it was an even more potent regulator (>95% reduction in C3b deposition) of the alternative pathway. This inhibition was primarily mediated by decay accelerating activity. The deposited C4b and C3b were progressively cleaved with a t½ of ∼ 30 min to C4d and C3d, respectively, by CR1-dependent cofactor activity. CR1 functioned intrinsically (i.e, worked only on the cell on which it was expressed). Moreover, CR1 efficiently and stably bound but didn't internalize C4b/C3b opsonized immune complexes. Our studies underscore the potential importance of CR1 on an epithelial cell population as both an intrinsic complement regulator and an immune adherence receptor. These results provide a framework for understanding how loss of CR1 expression on podocytes may contribute to complement-mediated damage in the kidney.

More than just immune evasion: Hijacking complement by Plasmodium falciparum

Malaria remains one of the world's deadliest diseases. Plasmodium falciparum is responsible for the most severe and lethal form of human malaria. P. falciparum's life cycle involves two obligate hosts: human and mosquito. From initial entry into these hosts, malaria parasites face the onslaught of the first line of host defence, the complement system. In this review, we discuss the complex interaction between complement and malaria infection in terms of hosts immune responses, parasite survival and pathogenesis of severe forms of malaria. We will focus on the role of complement receptor 1 and its associated polymorphisms in malaria immune complex clearance, as a mediator of parasite rosetting and as an entry receptor for P. falciparum invasion. Complement evasion strategies of P. falciparum parasites will also be highlighted. The sexual forms of the malaria parasites recruit the soluble human complement regulator Factor H to evade complement-mediated killing within the mosquito host. A novel evasion strategy is the deployment of parasite organelles to divert complement attack from infective blood stage parasites. Finally we outline the future challenge to understand the implications of these exploitation mechanisms in the interplay between successful infection of the host and pathogenesis observed in severe malaria.

Using mutagenesis and structural biology to map the binding site for the Plasmodium falciparum merozoite protein PfRh4 on the human immune adherence receptor

To survive and replicate within the human host, malaria parasites must invade erythrocytes. Invasion can be mediated by the P. falciparum reticulocyte-binding homologue protein 4 (PfRh4) on the merozoite surface interacting with complement receptor type 1 (CR1, CD35) on the erythrocyte membrane. The PfRh4 attachment site lies within the three N-terminal complement control protein modules (CCPs 1-3) of CR1, which intriguingly also accommodate binding and regulatory sites for the key complement activation-specific proteolytic products, C3b and C4b. One of these regulatory activities is decay-accelerating activity. Although PfRh4 does not impact C3b/C4b binding, it does inhibit this convertase disassociating capability. Here, we have employed ELISA, co-immunoprecipitation, and surface plasmon resonance to demonstrate that CCP 1 contains all the critical residues for PfRh4 interaction. We fine mapped by homologous substitution mutagenesis the PfRh4-binding site on CCP 1 and visualized it with a solution structure of CCPs 1-3 derived by NMR and small angle x-ray scattering. We cross-validated these results by creating an artificial PfRh4-binding site through substitution of putative PfRh4-interacting residues from CCP 1 into their homologous positions within CCP 8; strikingly, this engineered binding site had an ∼30-fold higher affinity for PfRh4 than the native one in CCP 1. These experiments define a candidate site on CR1 by which P. falciparum merozoites gain access to human erythrocytes in a non-sialic acid-dependent pathway of merozoite invasion.

Deciphering complement receptor type 1 interactions with recognition proteins of the lectin complement pathway

Complement receptor type 1 (CR1) is a membrane receptor expressed on a wide range of cells. It is involved in immune complex clearance, phagocytosis, and complement regulation. Its ectodomain is composed of 30 complement control protein (CCP) modules, organized into four long homologous repeats (A-D). In addition to its main ligands C3b and C4b, CR1 was reported to interact with C1q and mannan-binding lectin (MBL) likely through its C-terminal region (CCP22-30). To decipher the interaction of human CR1 with the recognition proteins of the lectin complement pathway, a recombinant fragment encompassing CCP22-30 was expressed in eukaryotic cells, and its interaction with human MBL and ficolins was investigated using surface plasmon resonance spectroscopy. MBL and L-ficolin were shown to interact with immobilized soluble CR1 and CR1 CCP22-30 with apparent dissociation constants in the nanomolar range, indicative of high affinity. The binding site for CR1 was located at or near the MBL-associated serine protease (MASP) binding site in the collagen stalks of MBL and L-ficolin, as shown by competition experiments with MASP-3. Accordingly, the mutation of an MBL conserved lysine residue essential for MASP binding (K55) abolished binding to soluble CR1 and CCP22-30. The CR1 binding site for MBL/ficolins was mapped to CCP24-25 of long homologous repeat D using deletion mutants. In conclusion, we show that ficolins are new CR1 ligands and propose that MBL/L-ficolin binding involves major ionic interactions between conserved lysine residues of their collagen stalks and surface exposed acidic residues located in CR1 CCP24 and/or CCP25.

Lack of evidence from studies of soluble protein fragments that Knops blood group polymorphisms in complement receptor-type 1 are driven by malaria

Complement receptor-type 1 (CR1, CD35) is the immune-adherence receptor, a complement regulator, and an erythroid receptor for Plasmodium falciparum during merozoite invasion and subsequent rosette formation involving parasitized and non-infected erythrocytes. The non-uniform geographical distribution of Knops blood group CR1 alleles Sl1/2 and McC^a/b may result from selective pressures exerted by differential exposure to infectious hazards. Here, four variant short recombinant versions of CR1 were produced and analyzed, focusing on complement control protein modules (CCPs) 15–25 of its ectodomain. These eleven modules encompass a region (CCPs 15–17) key to rosetting, opsonin recognition and complement regulation, as well as the Knops blood group polymorphisms in CCPs 24–25. All four CR1 15–25 variants were monomeric and had similar axial ratios. Modules 21 and 22, despite their double-length inter-modular linker, did not lie side-by-side so as to stabilize a bent-back architecture that would facilitate cooperation between key functional modules and Knops blood group antigens. Indeed, the four CR1 15–25 variants had virtually indistinguishable affinities for immobilized complement fragments C3b (K_D = 0.8–1.1 µM) and C4b (K_D = 5.0–5.3 µM). They were all equally good co-factors for factor I-catalysed cleavage of C3b and C4b, and they bound equally within a narrow affinity range, to immobilized C1q. No differences between the variants were observed in assays for inhibition of erythrocyte invasion by P. falciparum or for rosette disruption. Neither differences in complement-regulatory functionality, nor interactions with P. falciparum proteins tested here, appear to have driven the non-uniform geographic distribution of these alleles.

Complement receptor 1 and the molecular pathogenesis of malaria

Malaria is a pathogenic infection caused by protozoa of the genus plasmodium. It is mainly confined to sub-Saharan Africa, Asia and South America. This disease claims the life of over 1.5 to 2.7 million people per year. Owing to such a high incidence of malarial infections, there is an urgent need for the development of suitable vaccines. For the development of ideal vaccines, it is essential to understand the molecular mechanisms of malarial pathogenesis and the factors that lead to malaria infection. Genetic factors have been proposed to play an important role in malarial pathogenesis. Complement receptor 1 (CR1) is an important host red blood cell protein involved in interaction with malarial parasite. Various polymorphic forms of CR1 have been found to be involved in conferring protection or increasing susceptibility to malaria infections. Low-density allele (L) of CR1 gave contradictory results in different set of studies. In addition, Knops polymorphic forms Sl (a⁺) and McC (a) have been found to contribute more towards the occurrence of cerebral malaria in malaria endemic regions compared to individuals with Sl (a^-) / McC (a/b) genotype. This article reviews the research currently going on in this area and throws light on as yet unresolved mysteries of the role of CR1 in malarial pathogenesis

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.

Plasmodium falciparum uses a key functional site in complement receptor type-1 for invasion of human erythrocytes

The Plasmodium falciparum adhesin PfRh4 binds to complement receptor type-1 (CR1) on human erythrocytes and mediates a glycophorin-independent invasion pathway. CR1 is a complement regulator and immune-adherence receptor on erythrocytes required for shuttling of C3b/C4b-opsonized particles to liver and spleen for phagocytosis. Using recombinant CR1 constructs, we mapped the recognition site for PfRh4 to complement control protein modules 1 to 3 (CCP1-3) at the membrane-distal amino terminus of CR1. This region of CR1 binds to C4b and C3b and accelerates decay of both classic pathway and alternative pathway C3 and C5 convertases. CCP1-3 competed for PfRh4 binding to erythroid CR1 and inhibited the PfRh4-CR1 invasion pathways across a wide range of P falciparum strains. PfRh4 did not bind significantly to other CR1 constructs, including CCP15-17, which is 85% identical to CCP1-3. PfRh4 binding to CR1 did not affect its C3b/C4b binding capability, and we show evidence for a ternary complex between CCP1-3, C4b, and PfRh4. PfRh4 binding specifically inhibited CR1's convertase decay-accelerating activity, whereas there was no effect on factor H-mediated decay-accelerating activity. These results increase our understanding of the functional implications of CR1 engagement with PfRh4 and highlight the interplay between complement regulation and infection.

Complement receptor 1 expression on mouse erythrocytes mediates clearance of Streptococcus pneumoniae by immune adherence

Complement-containing immune complexes can be presented to phagocytes by human erythrocytes bearing complement receptor 1 (CR1). Although this has long been assumed to be a mechanism by which humans are able to protect themselves from "extracellular" bacteria such as pneumococci, there is little direct evidence. In these studies we have investigated this question by comparing results for erythrocytes from transgenic mice expressing human CR1 on their erythrocytes to the results for wild-type mouse erythrocytes that do not express CR1. We demonstrate that human CR1 expression on murine erythrocytes allows immune adherence to beads opsonized with either mouse or human serum as a source of complement. The role of CR1 in immune adherence was supported by studies showing that it was blocked by the addition of antibody to human CR1. Furthermore, human CR1 expression enhances the immune adherence of opsonized pneumococci to erythrocytes in vitro, and the pneumococci attached to erythrocytes via CR1 can be transferred in vitro to live macrophages. Even more importantly, we observed that if complement-opsonized pneumococci are injected intravenously with CR1(+) mouse erythrocytes into wild-type mice (after a short in vitro incubation), they are cleared faster than opsonized pneumococci similarly injected with wild-type mouse erythrocytes. Finally, we have shown that the intravenous (i.v.) injection of pneumococci into CR1(+) mice also results in more rapid blood clearance than in wild-type mice. These data support that immune adherence via CR1 on erythrocytes likely plays an important role in the clearance of opsonized bacteria from human blood.

An engineered complement receptor 1 composed of two functional domains can protect against immune-mediated hemolysis

Complement receptor type 1 (CR1) is a versatile inhibitor of both classical and alternative pathway C3 and C5 convertases with an ability to accelerate decay activity and act as a co-factor in C3b/C4b cleavage. In order to develop a short form of CR1 with similar biological activities to the full-length human CR1, we combined functional domain 1, located in the long homologous repeat (LHR) A, with functional domain 2, located in LHR C. We expressed the two-domain, two-function protein with an enterokinase site at the N-terminus and a termination codon at the C-terminus in Escherichia coli. The fusion protein was purified on a Ni-NTA-agarose column. After subsequent refolding, the recombinant CR1-derivative protein was obtained by enterokinase cleavage and subsequent purification. In vitro, the recombinant CR1-derivative reduced hemolysis, C5a release and surface C3 deposition. It was also effective in prolonging survival of transfused incompatible red blood cells in vivo. Our results indicate that the CR1-based protein may be a model for developing smaller and more potent complement inhibitors for future therapeutics.

Ligation of erythrocyte CR1 induces its clustering in complex with scaffolding protein FAP-1

The primary identified function of complement receptor 1 (CR1/CD35) on primate erythrocytes is to bind complement-tagged inflammatory particles including microbes and immune complexes. When erythrocytes circulate through liver and spleen, sinusoidal phagocytes remove CR1-adherent particles and erythrocytes return to the circulation. This process of immune adherence clearance is important for host defense and prevention of autoimmunity. CR1 was previously described as clustered in the human erythrocyte membrane, which was thought to be necessary for binding complement-opsonized particles. In contrast, we demonstrate that on erythrocytes CR1 is not clustered, but dispersed, and able to bind complement-tagged particles. When fresh erythrocytes are solubilized by nonionic detergent, CR1 partitions to the cytoskeleton fraction. Using a PDZ-peptide array, CR1's cytoplasmic tail, which contains 2 PDZ-motifs, binds PDZ domains 2, 3, and 5 of Fas-associated phosphatase 1 (FAP-1), a scaffolding protein. We show that FAP-1, not previously recognized as an erythroid protein, is expressed on circulating erythrocytes. CR1 and FAP-1 coimmunoprecipitate, which confirms their molecular association. Disperse CR1 on erythrocytes may be advantageous for capturing immune-complexes, while ligation-induced CR1 clustering may prevent ingestion of the erythrocyte during the immune-complex transfer to the macrophages by keeping the opsonic stimulus localized thus preventing phagocyosis.

A polymorphism in the type one complement receptor (CR1) involves an additional cysteine within the C3b/C4b binding domain that inhibits ligand binding

The type one complement receptor (CR1) contains a variable number of binding domains for C3b and C4b, formed through a nearly identical set of repeating units known as short consensus repeats (SCRs). Each SCR contains four cysteines that, by forming two disulfide bonds, impart a conformation critical for function. In this study, we identified a CR1 single nucleotide polymorphism (1597C>T) that results in an additional cysteine (483R>C) in SCR 8 of the N-terminal C3b/C4b binding domain, and occurring sporadically in corresponding SCRs of other repeated C3b/C4b binding domains. The normal carrier frequency for 483-C was 6.3% in 175 African Americans, and 2.4% in 153 Caucasians. In expression constructs containing one C3b/C4b binding domain, the 483-C residue reduced binding to C3b, C3bi, and C4b by over 80% (each p<0.0001), versus the wildtype construct. Full-length CR1 from 483-C carriers also exhibited reduced binding to C3b and C4b, although the effect was influenced by the total number of binding domains present. Race-matched comparisons between SLE patients (86 African Americans, 228 Caucasians) and the normal cohort showed that 483-C carrier status alone is not a risk factor for SLE or lupus nephritis. The physiological role of this polymorphism remains to be determined.

Identification of a complement receptor 1 peptide for inhibition of immune hemolysis

Complement sensitization of red blood cells (RBCs) can cause life-threatening hemolytic anemias. We have previously shown that complement receptor 1 (CR1) derivatives specifically the N-terminal region with decay accelerating activity (DAA) for inactivation of a key enzyme in the complement cascade can reduce complement-mediated RBC destruction in vitro and in an in vivo mouse model of hemolytic transfusion reaction. In the present study, we have modeled the N-terminal CR1 molecule based on the X-ray crystal structure of decay accelerating factor and the NMR structure of a homologous CR1 domain. Based on the homology model, we identified a 34-mer peptide encompassing the putative DAA which in vitro reduced hemolysis, C3a release and surface C3 deposition. More importantly, this peptide at 0.6 mM was effective in prolonging survival of transfused incompatible RBCs in vivo. Our results indicate that CR1-based structure-function studies may provide insights for developing structure-derived transfusion therapeutics in the future.

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

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

Large-scale modelling as a route to multiple surface comparisons of the CCP module family

Numerous mammalian proteins are constructed from a limited repertoire of module-types. Proteins belonging to the regulators of complement activation family--crucial for ensuring a complement-mediated immune response is targeted against infectious agents--are composed solely of complement control protein (CCP) modules. In the current study, CCP module sequences were grouped to allow selection of the most appropriate experimentally determined structures to serve as templates in an automated large-scale structure modelling procedure. The resulting 135 individual CCP module models, valuable in their own right, are available at the online database http://www.bru.ed.ac.uk/~dinesh/ccp-db.html. Comparisons of surface properties within a particular family of modules should be more informative than sequence alignments alone. A comparison of surface electrostatic features was undertaken for the first 28 CCP modules of complement receptor type 1 (CR1). Assignments to clusters based on surface properties differ from assignments to clusters based on sequences. This observation might reflect adaptive evolution of surface-exposed residues involved in protein-protein interactions. This illustrative example of a multiple surface-comparison was indeed able to pinpoint functional sites in CR1.

A novel Sushi domain-containing protein of Plasmodium falciparum

Using bioinformatics analyses of the completed malaria genome sequence, we have identified a novel protein with a potential role in erythrocyte invasion. The protein (PFD0295c, ) has a predicted signal sequence and transmembrane domain and a sequence near the C-terminus of the protein shows significant similarity with Sushi domains. These domains, which exist in a wide variety of complement and adhesion proteins, have previously been shown to be involved in protein-protein and protein-ligand interactions. Orthologous genes have also been identified in the genomes of several other Plasmodium species, suggesting a conserved function for this protein in Plasmodium. Our results show that this protein is located in apical organelles and we have therefore designated the protein apical Sushi protein (ASP). We show that the expression of ASP is tightly regulated in the intraerythrocytic stages of the parasite and that it undergoes post-translational proteolytic processing. Based on our observations of timing of expression, location and proteolytic processing, we propose a role for ASP in erythrocyte invasion.

A transgenic mouse model for studying the clearance of blood-borne pathogens via human complement receptor 1 (CR1)

Complement receptor 1 (CR1) on the surface of human erythrocytes facilitates intravascular clearance of complement-opsonized pathogens. The need for complement activation can be circumvented by directly coupling the organism to CR1 using a bispecific monoclonal antibody heteropolymer (HP). Lack of a functional homologue to CR1 on mouse erythrocytes has made it difficult to study HP-dependent clearance of pathogens in small animals. We have developed a transgenic mouse that expresses human CR1 on erythrocytes. CR1 antigen is of appropriate size and in a clustered distribution as confirmed by immunoblotting and fluorescence microscopy, respectively. HP that immobilized bacteriophage PhiX174 prototype pathogen to erythrocyte CR1 of the transgenic mice increased the rate of clearance of the virus compared with HP that bound bacteriophage, but not CR1. This transgenic mouse model will allow evaluation of different HPs for their in vivo efficacy and potential as human therapeutics.

Characterization of complement receptor 1 domains for prevention of complement-mediated red cell destruction

BACKGROUND

Complement activation resulting in intravascular hemolysis can cause transfusion-associated mortality. We recently showed that a recombinant soluble form of complement receptor 1 (CR1) effectively reduces complement-mediated red blood cell (RBC) destruction in vitro and more importantly prolongs the survival of transfused human RBCs in mice. To determine CR1-active sites that prevent RBC destruction, structure-function analysis of its extracellular 1930-amino-acid domain has been performed.

STUDY DESIGN AND METHODS

Several CR1-truncated soluble proteins were prepared and tested for their ability to prevent complement-mediated RBC destruction in vitro and in mice.

RESULTS

A 250-amino-acid region in CR1 that possesses antihemolytic activity and is effective in prolonging survival of transfused RBCs in vivo was identified. Mutation of two critical residues (D109N and E116K) in this 250-amino-acid domain, previously shown to improve complement-inhibitory functions of CR1 derivatives, resulted in a more potent inhibition of complement activation in vitro. In vivo, however, the activity of mutant proteins was comparable to the wild-type molecules.

CONCLUSION

Our structure-function studies have characterized smaller CR1-based complement inhibitors for future development of structure-derived transfusion therapeutics. Our studies underscore the importance of testing CR1 inhibitors in vivo.

Backbone dynamics of complement control protein (CCP) modules reveals mobility in binding surfaces

The regulators of complement activation (RCA) are critical to health and disease because their role is to ensure that a complement-mediated immune response to infection is proportionate and targeted. Each protein contains an uninterrupted array of from four to 30 examples of the very widely occurring complement control protein (CCP, or sushi) module. The CCP modules mediate specific protein-protein and protein-carbohydrate interactions that are key to the biological function of the RCA and, paradoxically, provide binding sites for numerous pathogens. Although structural and mutagenesis studies of CCP modules have addressed some aspects of molecular recognition, there have been no studies of the role of molecular dynamics in the interaction of CCP modules with their binding partners. NMR has now been used in the first full characterization of the backbone dynamics of CCP modules. The dynamics of two individual modules-the 16th of the 30 modules of complement receptor type 1 (CD35), and the N-terminal module of membrane cofactor protein (CD46)-as well as their solution structures, are compared. Although both examples share broadly similar three-dimensional structures, many structurally equivalent residues exhibit different amplitudes and timescales of local backbone motion. In each case, however, regions of the module-surface implicated by mutagenesis as sites of interactions with other proteins include several mobile residues. This observation suggests further experiments to explore binding mechanisms and identify new binding sites.

Kinetic analysis of the interactions between vaccinia virus complement control protein and human complement proteins C3b and C4b

The vaccinia virus complement control protein (VCP) is an immune evasion protein of vaccinia virus. Previously, VCP has been shown to bind and support inactivation of host complement proteins C3b and C4b and to protect the vaccinia virions from antibody-dependent complement-enhanced neutralization. However, the molecular mechanisms involved in the interaction of VCP with its target proteins C3b and C4b have not yet been elucidated. We have utilized surface plasmon resonance technology to study the interaction of VCP with C3b and C4b. We measured the kinetics of binding of the viral protein to its target proteins and compared it with human complement regulators factor H and sCR1, assessed the influence of immobilization of ligand on the binding kinetics, examined the effect of ionic contacts on these interactions, and sublocalized the binding site on C3b and C4b. Our results indicate that (i) the orientation of the ligand is important for accurate determination of the binding constants, as well as the mechanism of binding; (ii) in contrast to factor H and sCR1, the binding of VCP to C3b and C4b follows a simple 1:1 binding model and does not involve multiple-site interactions as predicted earlier; (iii) VCP has a 4.6-fold higher affinity for C4b than that for C3b, which is also reflected in its factor I cofactor activity; (iv) ionic interactions are important for VCP-C3b and VCP-C4b complex formation; (v) VCP does not bind simultaneously to C3b and C4b; and (vi) the binding site of VCP on C3b and C4b is located in the C3dg and C4c regions, respectively.

Complement inhibitor C4b-binding protein-friend or foe in the innate immune system?

The complement system constitutes an important component of the defence against foreign organisms, functioning both in innate and adaptive immune systems. It is potentially harmful also to the own organism and is therefore tightly regulated by a number of membrane-bound and soluble factors. C4b-binding protein (C4BP) is a potent circulating soluble inhibitor of the classical and lectin pathways of complement. In recent years, the relationships between the structure of C4BP and its functions have been elucidated using a combination of computer-based molecular analysis and recombinant DNA technologies. Moreover, two novel functions have recently been ascribed to C4BP. One is the ability of C4BP to localize complement regulatory activity to the surface of apoptotic cells via its interaction with the membrane-binding vitamin K-dependent protein S. The other is the ability of C4BP to act as a survival factor for B cells due to an interaction with CD40. The complement regulatory activity of C4BP is not only beneficial because it is also explored by pathogens such as Neisseria gonorrhoeae, Bordetella pertussis, Streptococcus pyogenes, Escherichia coli K1, and Candida albicans, that bind C4BP to their surfaces. This contributes to the serum resistance and the pathogenicity of these bacteria. In this review, the structural requirements and functional importance of the interactions between C4BP and its various ligands are discussed.

The C4A and C4B isotypic forms of human complement fragment C4b have the same intrinsic affinity for complement receptor 1 (CR1/CD35)

Several previous reports concluded that the C4b fragment of human C4A (C4Ab) binds with higher affinity to CR1 than does C4Bb. Because the isotypic residues, (1101)PCPVLD and (1101)LSPVIH in C4A and C4B, respectively, are located within the C4d region, one may have expected a direct binding contribution of C4d to the interaction with CR1. However, using surface plasmon resonance as our analytical tool, with soluble rCR1 immobilized on the biosensor chip, we failed to detect significant binding of C4d of either isotype. By contrast, binding of C4c was readily detectable. C4A and C4B, purified from plasma lacking one of the isotypes, were Cs converted to C4Ab and C4Bb. Spontaneously formed disulfide-linked dimers were separated from monomers and higher oligomers by sequential chromatographic steps. The binding sensorgrams of C4Ab and C4Bb monomers as analytes reached steady state plateaus, and these equilibrium data yielded essentially superimposable saturation curves that were well fit by a one-site binding model. Although a two-site model was required to fit the equilibrium-binding data for the dimeric forms of C4b, once again there was little difference in the K(D) values obtained for each isotype. Independent verification of our surface plasmon resonance studies came from ELISA-based inhibition experiments in which monomers of C4Ab and C4Bb were equipotent in inhibiting the binding of soluble CR1 to plate-bound C4b. Although divergent from previous reports, our results are consistent with recent C4Ad structural data that raised serious doubts about there being a conformational basis for the previously reported isotypic differences in the C4b-CR1 interaction.

Two cDNAs from the purple sea urchin, Strongylocentrotus purpuratus , encoding mosaic proteins with domains found in factor H, factor I, and complement components C6 and C7

The vertebrate complement system is composed of about 30 serum and cell surface proteins that make up three activation pathways, a lytic pathway, and a set of proteins that regulate complement. Regulatory proteins are required for host protection against autologous complement attack and to control the amplification feedback loop of the alternative pathway. Purple sea urchin, Strongylocentrotus purpuratus, homologues of complement C3 (SpC3) and factor B (SpBf) have been identified, suggesting the presence of an alternative complement pathway. This implies that echinoderms require a complement regulatory system for the same reasons that it is required in higher vertebrates. Two cDNAs, Sp5 and Sp5013, have been characterized from coelomocytes and the deduced structures of the encoded mosaic proteins, SpCRL ( S. p urpuratus complement related protein, long form) and SpCRS ( short form), have domains that are also found in regulatory proteins such as factor H and factor I and the terminal pathway components C6 and C7. These domains include multiple short consensus repeats, a fucolectin domain, Ser/Thr/Pro-rich regions, a Cys-rich region, and a factor I-membrane attack complex domain. The genes are constitutively expressed in all tissues of the sea urchin and are not induced in response to immune challenge. Multiple bands of varying intensity on both genome blots and RNA blots suggest that Sp5 and Sp5013 are members of a small gene family and that they might undergo alternative splicing. Based on the domains present in SpCRL and SpCRS, they might be either examples of complement regulatory proteins or members of the terminal pathway of complement.

Mutations in alpha-chain of C4BP that selectively affect its factor I cofactor function

C4b-binding protein (C4BP) inhibits all pathways of complement activation, acting as a cofactor to the serine protease factor I (FI) in the degradation of activated complement factors C4b and C3b. C4BP is a disulfide-linked polymer of seven alpha-chains and a unique beta-chain, the alpha- and beta-chains being composed of eight and three complement control protein (CCP) domains, respectively. In previous studies we have localized cofactor activity and binding of C4b to alpha-chain CCP1-3 of C4BP, whereas the binding of C3b required additionally CCP4. Likewise, introduced point mutations that decreased binding of C4b/C3b caused a decrease in cofactor activity. In the present study, we describe two mutants of C4BP, K126Q/K128Q and F144S/F149S, clustered on alpha-chain CCP3, which selectively lost their ability to act as cofactors in the cleavage of both C4b and C3b. Both mutants show the same binding affinity for C4b/C3b as measured by surface plasmon resonance and have the same inhibitory effect on formation and decay of the classical pathway C3-convertase as the wild type C4BP. It appears that C4b and C3b do not undergo the same conformational changes upon binding to the C4BP mutants as during the interaction with the wild type C4BP, which then results in the observed loss of the cofactor activity.

Variola virus immune evasion proteins

Variola virus, the causative agent of smallpox, encodes approximately 200 proteins. Over 80 of these proteins are located in the terminal regions of the genome, where proteins associated with host immune evasion are encoded. To date, only two variola proteins have been characterized. Both are located in the terminal regions and demonstrate immunoregulatory functions. One protein, the smallpox inhibitor of complement enzymes (SPICE), is homologous to a vaccinia virus virulence factor, the vaccinia virus complement-control protein (VCP), which has been found experimentally to be expressed early in the course of vaccinia infection. Both SPICE and VCP are similar in structure and function to the family of mammalian complement regulatory proteins, which function to prevent inadvertent injury to adjacent cells and tissues during complement activation. The second variola protein is the variola virus high-affinity secreted chemokine-binding protein type II (CKBP-II, CBP-II, vCCI), which binds CC-chemokine receptors. The vaccinia homologue of CKBP-II is secreted both early and late in infection. CKBP-II proteins are highly conserved among orthopoxviruses, sharing approximately 85% homology, but are absent in eukaryotes. This characteristic sets it apart from other known virulence factors in orthopoxviruses, which share sequence homology with known mammalian immune regulatory gene products. Future studies of additional variola proteins may help illuminate factors associated with its virulence, pathogenesis and strict human tropism. In addition, these studies may also assist in the development of targeted therapies for the treatment of both smallpox and human immune-related diseases.

Complement receptor 1 inhibitors for prevention of immune-mediated red cell destruction: potential use in transfusion therapy

Activation of complement cascade via the antibody-mediated classical pathway can initiate red blood cell (RBC) destruction, causing transfusion reactions and hemolytic anemia. In the present study, we have assessed the ability of a human recombinant soluble form of complement receptor 1 (sCR1) to inhibit complement-mediated RBC destruction in vitro and in vivo. Using an in vitro alloimmune incompatibility model, sCR1 inhibited complement activation and prevented hemolysis. Following transfusion of human group O RBCs into mice lacking detectable pre-existing antibodies against the transfused RBCs, systemic coadministration of 10 mg/kg sCR1, a dose well tolerated in human subjects for prevention of tissue injury, completely inhibited the in vivo clearance of the transfused RBCs and surface C3 deposition in the first hour after transfusion, correlating with the half-life of sCR1 in the circulation. Treatment with sCR1 increased the survival of transfused human group A RBCs in the circulation of mice with pre-existing anti-A for 2 hours after transfusion by 50%, reduced intravascular hemolysis, and lowered the levels of complement deposition (C3 and C4), but not immunoglobulin G (IgG) or IgM, on the transfused cells by 100-fold. We further identified potential functional domains in CR1 that can act to limit complement-mediated RBC destruction in vitro and in vivo. Collectively, our data highlight a potential use of CR1-based inhibitors for prevention of complement-dependent immune hemolysis.

A CR1 polymorphism associated with constitutive erythrocyte CR1 levels affects binding to C4b but not C3b

The erythrocyte type one complement receptor (E-CR1) mediates erythrocyte binding of complement-opsonized immune complexes (IC), and helps protect against random deposition of circulating IC. Two linked CR1 polymorphisms occur in binding domains, at I643T and Q981H. In Caucasians, the variant alleles (643T, 981H) are associated with low constitutive E-CR1 expression levels. This study was conducted to determine if these polymorphisms affect ligand binding, and if so, represent risk factors for the autoimmune IC disease, systemic lupus erythematosus (SLE). In an ELISA comparing relative ligand binding differences, E-CR1 from individuals homozygous for the variant residues (643TT/981HH) exhibited greater binding to C4b, but not C3b, than homozygous wild-type E-CR1. Analysis of single-binding domain CR1 constructs demonstrated that the 981H residue imparted this enhanced C4b binding. No differences were observed in the 981H allele frequency between Caucasian controls (0.170, n = 100) and SLE patients (0.130, n = 150, P = 0.133), or between African American controls (0.169, n = 71) and SLE patients (0.157, n = 67). In a subset of individuals assessed for CR1 size, excluding from this analysis those expressing at least one B allele revealed a trend for over-representation of the 981H allele in Caucasian controls (0.231 frequency, n = 26) versus SLE patients (0.139, n = 83, P = 0.089), but again no difference between African American controls (0.188, n = 24) and SLE patients (0.191, n = 34). These data suggest that the 981H residue compensates for low constitutive expression of E-CR1 in Caucasians by enhancing C4b binding. This may contribute protection against SLE.

Viral mimicry of the complement system

The complement system is a potent innate immune mechanism consisting of cascades of proteins which are designed to fight against and annul intrusion of all the foreign pathogens. Although viruses are smaller in size and have relatively simple structure, they are not immune to complement attack. Thus, activation of the complement system can lead to neutralization of cell-free viruses, phagocytosis of C3b-coated viral particles, lysis of virus-infected cells, and generation of inflammatory and specific immune responses. However, to combat host responses and succeed as pathogens, viruses not only have developed/adopted mechanisms to control complement, but also have turned these interactions to their own advantage. Important examples include poxviruses, herpesviruses, retroviruses, paramyxoviruses and picornaviruses. In this review, we provide information on the various complement evasion strategies that viruses have developed to thwart the complement attack of the host. A special emphasis is given on the interactions between the viral proteins that are involved in molecular mimicry and the complement system.

Human complement receptor type 1 (CR1) binds to a major malarial adhesin

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), a major adhesin molecule expressed on Plasmodium-falciparum-infected erythrocytes, interacts with several receptors on endothelial cells and uninfected erythrocytes. This 'stickiness', known as rosetting, is a strategy used by the parasite to remain sequestered in the microvasculature to avoid destruction in the spleen and liver. Erythrocyte rosetting causes obstruction of the blood flow in microcapillaries. Recent data suggest a direct interaction between PfEMP1 and a functional site of complement receptor type 1 (CR1; CD35) on uninfected erythrocytes. Consistent with the hypothesis that CR1 is important in malaria pathogenesis is a 40-70-fold increase in the frequency of two CR1 blood-group antigens (at least one of which might rosette less efficiently) in malaria-exposed African populations. Furthermore, structural differences in erythrocyte CR1 between human and non-human primates are probably explained by the selective pressure of malaria.

Variola virus immune evasion design: expression of a highly efficient inhibitor of human complement

Variola virus, the most virulent member of the genus Orthopoxvirus, specifically infects humans and has no other animal reservoir. Variola causes the contagious disease smallpox, which has a 30-40% mortality rate. Conversely, the prototype orthopoxvirus, vaccinia, causes no disease in immunocompetent humans and was used in the global eradication of smallpox, which ended in 1977. However, the threat of smallpox persists because clandestine stockpiles of variola still exist. Although variola and vaccinia share remarkable DNA homology, the strict human tropism of variola suggests that its proteins are better suited than those of vaccinia to overcome the human immune response. Here, we demonstrate the functional advantage of a variola complement regulatory protein over that of its vaccinia homologue. Because authentic variola proteins are not available for study, we molecularly engineered and characterized the smallpox inhibitor of complement enzymes (SPICE), a homologue of a vaccinia virulence factor, vaccinia virus complement control protein (VCP). SPICE is nearly 100-fold more potent than VCP at inactivating human C3b and 6-fold more potent at inactivating C4b. SPICE is also more human complement-specific than is VCP. By inactivating complement components, SPICE serves to inhibit the formation of the C3/C5 convertases necessary for complement-mediated viral clearance. SPICE provides the first evidence that variola proteins are particularly adept at overcoming human immunity, and the decreased function of VCP suggests one reason why the vaccinia virus vaccine was associated with relatively low mortality. Disabling SPICE may be therapeutically useful if smallpox reemerges.

Structure of the C3b binding site of CR1 (CD35), the immune adherence receptor

Complement receptor type 1 (CR1 or CD35) is a multiple modular protein that mediates the immune adherence phenomenon, a fundamental event for destroying microbes and initiating an immunological response. It fulfills this role through binding C3b/C4b-opsonized foreign antigens. The structure of the principal C3b/C4b binding site (residues 901-1095) of CR1 is reported, revealing three complement control protein modules (modules 15-17) in an extended head-to-tail arrangement with flexibility at the 16-17 junction. Structure-guided mutagenesis identified a positively charged surface region on module 15 that is critical for C4b binding. This patch, together with basic side chains of module 16 exposed on the same face of CR1, is required for C3b binding. These studies reveal the initial structural details of one of the first receptor-ligand interactions to be identified in immunobiology.

Localization of Knops system antigens in the long homologous repeats of complement receptor 1

BACKGROUND

The Sl(a) (Knops system) located on complement receptor 1 (CR1) has been associated with malarial rosetting, a process associated with severe malarial infections. Moreover, the long homologous repeats (LHRs) B and C of CR1 were implicated in rosette formation. As a step toward mapping the location of Knops system antigens, truncated CR1 proteins have been expressed and their ability to inhibit antibodies to the high-incidence Knops system antigens was assessed.

STUDY DESIGN AND METHODS

Individual LHRs (A, B, C, and D) of CR1 of the common CR1*1 (F) allotype were expressed as secreted forms in 293T cells. Their abilities to specifically neutralize Knops system antibodies were tested by both hemagglutination and flow cytometry.

RESULTS

Three examples of anti-Kn(a) (n = 6) were almost completely inhibited by LHR-C and three by LHR-D. Two examples of anti-McC(a) (n = 2) and seven examples of anti-Sl(a) (n = 8) were inhibited by LHR-D. Both examples of anti-Yk(a) (n = 2) were partially inhibited by LHR-D.

CONCLUSION

The high-incidence Knops system antigens reside within LHR-D and to a lesser extent within LHR-C. Because of the role of Sl(a) antigen in malaria rosetting, these results indicate that LHR-D may represent an additional malaria interaction region in CR1.

Structural Requirements for the Complement Regulatory Activities of C4BP

C4b-binding protein (C4BP) is a regulator of the classical complement pathway C3 convertase (C4bC2a complex). It is a disulfide-linked polymer of seven alpha-chains and a unique beta-chain; the alpha- and beta-chains are composed of eight and three complement control protein (CCP) domains, respectively. To elucidate the importance of the polymeric nature of C4BP and the structural requirements for the interaction between C4b and the alpha-chain, 19 recombinant C4BP variants were created. Six truncated monomeric variants, nine polymeric variants in which individual CCPs were deleted, and finally, four variants in which double alanine residues were introduced between CCPs were functionally characterized. The smallest truncated C4BP variant still active in regulating fluid phase C4b comprised CCP1-3. The monomeric variants were less efficient than polymeric C4BP in degrading C4b on cell surfaces. All three N-terminal CCP domains contributed to the binding of C4b and were important for full functional activity; CCP2 and CCP3 were the most important. The spatial arrangements of the first CCPs were found to be important, as introduction of alanine residues between CCPs 1 and 2, CCPs 2 and 3, and CCPs 3 and 4 resulted in functional impairment. The results presented here elucidate the structural requirements of individual CCPs of C4BP, as well as their spatial arrangements within and between subunits for expression of full functional activity.