Mammalian lectins in activation and clearance mechanisms involving the complement system

Is the Complement System Dysregulated in Preeclampsia Comorbid with HIV Infection?

South Africa is the epicentre of the global HIV pandemic, with 13.9% of its population infected. Preeclampsia (PE), a hypertensive disorder of pregnancy, is often comorbid with HIV infection, leading to multi-organ dysfunction and convulsions. The exact pathophysiology of preeclampsia is triggered by an altered maternal immune response or defective development of maternal tolerance to the semi-allogenic foetus via the complement system. The complement system plays a vital role in the innate immune system, generating inflammation, mediating the clearance of microbes and injured tissue materials, and a mediator of adaptive immunity. Moreover, the complement system has a dual effect, of protecting the host against HIV infection and enhancing HIV infectivity. An upregulation of regulatory proteins has been implicated as an adaptive phenomenon in response to elevated complement-mediated cell lysis in HIV infection, further aggravated by preeclamptic complement activation. In light of the high prevalence of HIV infection and preeclampsia in South Africa, this review discusses the association of complement proteins and their role in the synergy of HIV infection and preeclampsia in South Africa. It aims to identify women at elevated risk, leading to early diagnosis and better management with targeted drug therapy, thereby improving the understanding of immunological dysregulation.

Contributions of animal models to mechanistic understandings of antibody-dependent disease and roles of the amplification loop

The complement system plays an important pathophysiologic role in human diseases associated with immune or ischemic insults. In addition to understanding the effector mechanisms that are important for the biological effects of the system, substantial efforts have gone into understanding which specific complement activation pathways generate these potent effects. These approaches include the use of gene-targeted mice and specific pathway inhibitors, as well as the integration of human disease genetic and biomarker studies. In some disease states, it is quite clear that the alternate pathway plays a unique role in the initiation of the complement system. However, although initially a widely unexpected finding, it has now been shown in many tissue-based disease models and in initial human studies that engagement of the amplification loop is also essential for tissue injury when the classical and/or lectin pathways initiate pathway activation through pathogenic autoantibodies. This review provides evidence for such a conclusion through animal models, focusing on pathogenic antibody passive transfer models but also other relevant experimental systems. These data, along with initial biomarkers and clinical trial outcomes in human diseases that are associated with pathogenic autoantibodies, suggest that targeting the alternative pathway amplification loop may have near-universal therapeutic utility for tissue-based diseases.

Trypanosoma cruzi Evades the Complement System as an Efficient Strategy to Survive in the Mammalian Host: The Specific Roles of Host/Parasite Molecules and Trypanosoma cruzi Calreticulin

American Trypanosomiasis is an important neglected reemerging tropical parasitism, infecting about 8 million people worldwide. Its agent, Trypanosoma cruzi, exhibits multiple mechanisms to evade the host immune response and infect host cells. An important immune evasion strategy of T. cruzi infective stages is its capacity to inhibit the complement system activation on the parasite surface, avoiding opsonizing, immune stimulating and lytic effects. Epimastigotes, the non-infective form of the parasite, present in triatomine arthropod vectors, are highly susceptible to complement-mediated lysis while trypomastigotes, the infective form, present in host bloodstream, are resistant. Thus T. cruzi susceptibility to complement varies depending on the parasite stage (amastigote, trypomastigotes or epimastigote) and on the T. cruzi strain. To avoid complement-mediated lysis, T. cruzi trypomastigotes express on the parasite surface a variety of complement regulatory proteins, such as glycoprotein 58/68 (gp58/68), T. cruzi complement regulatory protein (TcCRP), trypomastigote decay-accelerating factor (T-DAF), C2 receptor inhibitor trispanning (CRIT) and T. cruzi calreticulin (TcCRT). Alternatively, or concomitantly, the parasite captures components with complement regulatory activity from the host bloodstream, such as factor H (FH) and plasma membrane-derived vesicles (PMVs). All these proteins inhibit different steps of the classical (CP), alternative (AP) or lectin pathways (LP). Thus, TcCRP inhibits the CP C3 convertase assembling, gp58/68 inhibits the AP C3 convertase, T-DAF interferes with the CP and AP convertases assembling, TcCRT inhibits the CP and LP, CRIT confers ability to resist the CP and LP, FH is used by trypomastigotes to inhibit the AP convertases and PMVs inhibit the CP and LP C3 convertases. Many of these proteins have similar molecular inhibitory mechanisms. Our laboratory has contributed to elucidate the role of TcCRT in the host-parasite interplay. Thus, we have proposed that TcCRT is a pleiotropic molecule, present not only in the parasite endoplasmic reticulum, but also on the trypomastigote surface, participating in key processes to establish T. cruzi infection, such as inhibition of the complement system and serving as an important virulence factor. Additionally, TcCRT interaction with key complement components, participates as an anti-angiogenic and anti-tumor molecule, inhibiting at least in important part, tumor growth in infected animals.

Targeting mechanisms at sites of complement activation for imaging and therapy

The complement system plays a key role in many acute injury states as well as chronic autoimmune and inflammatory diseases. Localized complement activation and alternative pathway-mediated amplification on diverse target surfaces promote local recruitment of pro-inflammatory cells and elaboration of other mediators. Despite a general understanding of the architecture of the system, though, many of the mechanisms that underlie site-specific complement activation and amplification in vivo are incompletely understood. In addition, there is no capability yet to measure the level of local tissue site-specific complement activation in patients without performing biopsies to detect products using immunohistochemical techniques. Herein is reviewed emerging evidence obtained through clinical research studies of human rheumatoid arthritis along with translational studies of its disease models which demonstrate that several parallel mechanisms are involved in site-specific amplification of activation of the complement system in vivo. Among these processes are de-regulation of the alternative pathway, effector pathway-catalyzed amplification of proximal complement activation, recognition of injury-associated ligands by components of the lectin pathway, and engagement of pathogenic natural antibodies that recognize a limited set of injury-associated neoepitopes. Studies suggest that each of these inter-related processes can play key roles in amplification of complement-dependent injury on self-tissues in vivo. These findings, in addition to development of an imaging strategy described herein designed to quantitatively measure local complement C3 fixation, have relevance to therapeutic and diagnostic strategies targeting the complement system.

Complement and its receptors: new insights into human disease

Although new activation and regulatory mechanisms are still being identified, the basic architecture of the complement system has been known for decades. Two major roles of complement are to control certain bacterial infections and to promote clearance of apoptotic cells. In addition, although inappropriate complement activation has long been proposed to cause tissue damage in human inflammatory and autoimmune diseases, whether this is indeed true has been uncertain. However, recent studies in humans, especially those using newly available biological therapeutics, have now clearly demonstrated the pathophysiologic importance of the complement system in several rare diseases. Beyond these conditions, recent genetic studies have strongly supported an injurious role for complement in a wide array of human inflammatory, degenerative, and autoimmune diseases. This review includes an overview of complement activation, regulatory, and effector mechanisms. It then focuses on new understandings gained from genetic studies, ex vivo analyses, therapeutic trials, and animal models as well as on new research opportunities.

Purification, characterization and functional analysis of a novel β-1, 3-glucan binding protein from green tiger shrimp Penaeus semisulcatus

A β-1, 3-Glucan binding protein (β-GBP) was isolated from green tiger shrimp Penaeus semisulcatus and purified using laminarin precipitation and affinity chromatography on laminarin-Sepharose 6B column respectively. P. semisulcatus β-GBP exhibits a single band with a molecular weight of 112 kDa on SDS-PAGE and pI of 5.9 in isoelectric focusing (IEF). Negative staining of P. semisulcatus β-GBP showed large aggregates with crystalline surface when viewed by Electron Microscopy. Circular dichroism spectra of P. semisulcatus β-GBP showed broad negative minimum wavelength extending from 200 to 250 nm can be attributed to the presence of β-sheets in its secondary structure. P. semisulcatus β-GBP comprises the specific binding affinity with the polysaccharide β-1, 3-glucans (laminarin), this recognition and binding leads to the activation of prophenoloxidase cascade. Interestingly, P. semisulcatus β-GBP also involved in the agglutination of baker's yeast, bacteria, erythrocytes (RBCs) and enhances the PO activity. Herein, we have investigated the importance of β-GBP in innate immune response of P. semisulcatus and they implicate the evolutionary link with similar proteins found in other invertebrates.

Linking complement and anti-dsDNA antibodies in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus is a severe autoimmune disease that affects multiple organ systems resulting in diverse symptoms and outcomes. It is characterized by antibody production to a variety of self-antigens, but it is specifically associated with those against anti-dsDNA. Anti-dsDNA antibodies are present before the onset of clinical disease and are associated with severe manifestations of lupus such as glomerulonephritis. Their levels fluctuate with changes in disease activity and, in combination with the levels of complement proteins C3 and C4, are strong indicators of disease flare and treatment response in patients with lupus. The decreased complement levels that are noted during flares of lupus activity are believed to be secondary to increased autoantibody production and immune complex formation that results in tissue damage; however, recent data suggest that complement activation can also drive development of these pathogenic autoantibodies. This review will explore the various roles of complement in the development and pathogenesis of anti-dsDNA antibodies.

CR2-mediated targeting of complement inhibitors: bench-to-bedside using a novel strategy for site-specific complement modulation

Recent approval of the first human complement pathway-directed therapeutics, along with high-profile genetic association studies, has catalyzed renewed biopharmaceutical interest in developing drugs that modulate the complement system. Substantial challenges remain, however, that must be overcome before widespread application of complement inhibitors in inflammatory and autoimmune diseases becomes possible. Among these challenges are the following: (1) defining the complement pathways and effector mechanisms that cause tissue injury in humans and determining whether the relative importance of each varies by disease, (2) blocking or modulating, using traditional small molecule or biologic approaches, the function of complement proteins whose circulating levels are very high and whose turnover rates are relatively rapid, especially in the setting of acute and chronic autoimmune diseases, and (3) avoiding infectious complications or impairment of other important physiological functions of complement when using systemically active complement-blocking agents. This chapter will review data that address these challenges to therapeutic development, with a focus on the development of a novel strategy of blocking specific complement pathways by targeting inhibitors using a recombinant portion of the human complement receptor type 2 (CR2/CD21) which specifically targets to sites of local complement C3 activation where C3 fragments are covalently fixed. Recently, the first of these CR2-targeted proteins has entered human phase I studies in the human disease paroxysmal nocturnal hemoglobinuria. The results of murine translational studies using CR2-targeted inhibitors strongly suggest that a guiding principle going forward in complement therapeutic development may well be to focus on developing strategies to modulate the pathway as precisely as possible by physically localizing therapeutic inhibitory effects.

The structure, genetic polymorphisms, expression and biological functions of complement receptor type 1 (CR1/CD35)

The complement system is comprised of soluble and cell surface associated proteins that recognize exogenous, altered, or potentially harmful endogenous ligands. In recent years, the complement system--particularly component C3 and its receptors--have been demonstrated to be a key link between innate and adaptive immunity. Complement receptor type 1 (CR1), the receptor for C3b/C4b complement peptides, has emerged as a molecule of immense interest in gaining insight to the susceptibility, pathophysiology, diagnosis, prognosis and therapy of such diseases. In this review, we wish to briefly bring forth the structure, genetic polymorphisms, expression and biological functions of CR1.

Complement receptor 2, natural antibodies and innate immunity: Inter-relationships in B cell selection and activation

Complement receptor type 2 (CR2) is a receptor that serves as an important interface between the complement system and adaptive immunity. Recent studies have shown that CR2 is also centrally involved in innate immunity, and one key area is the development of potentially pathogenic natural antibodies that target neo-epitopes revealed in ischemic tissue undergoing reperfusion. Mice lacking either total immunoglobulins or CR2 alone are protected from the development of ischemia-reperfusion injury, and this effect can be reversed by introducing CR2-sufficient B-1 cells or by transferring polyclonal natural IgM antibody from wild type mice as well as monoclonal antibodies that recognize phospholipids, DNA or non-muscle myosin. We will report at the XXI ICW an additional membrane-associated protein to which pathogenic IgM antibodies are directed. Whether B cells producing these natural antibodies are differentially selected in CR2-deficient mice is as yet not well understood, and the complement-related mechanism(s) whereby this differential repertoire selection process could occur have yet to be explored in any detail. In addition to this important role in innate immunity, CR2 can also act as a receptor for other components or activators of innate immunity. One such component is interferon-alpha, an anti-viral cytokine that binds CR2 and induces a component of its mRNA signature in B cells through this receptor. Other potential CR2 ligands are DNA and DNA-containing complexes such as chromatin. The biologic role of these CR2 interactions with interferon-alpha and DNA-containing complexes is not well understood, but may be important in the development of the autoimmune disease systemic lupus erythematosus that is characterized by enhanced interferon-alpha levels and loss of self tolerance to DNA-containing self antigens.

The central role of the alternative complement pathway in human disease

The complement system is increasingly recognized as important in the pathogenesis of tissue injury in vivo following immune, ischemic, or infectious insults. Within the complement system, three pathways are capable of initiating the processes that result in C3 activation: classical, alternative, and lectin. Although the roles that proinflammatory peptides and complexes generated during complement activation play in mediating disease processes have been studied extensively, the relative contributions of the three activating pathways is less well understood. Herein we examine recent evidence that the alternative complement pathway plays a key and, in most instances, obligate role in generating proinflammatory complement activation products in vivo. In addition, we discuss new concepts regarding the mechanisms by which the alternative pathway is activated in vivo, as recent clinical findings and experimental results have provided evidence that continuous active control of this pathway is necessary to prevent unintended targeting and injury to self tissues.

Complement receptors and the shaping of the natural antibody repertoire

Complement and complement receptors have been known for several decades to play important roles in immune effector mechanisms related to pathogen elimination and tissue inflammation. In addition, studies over the last 10 years have clearly demonstrated a key role for the complement C3d activation fragment receptor designated CR2 (complement receptor type 2) in the switched-isotype, high-affinity and memory humoral immune responses to T-dependent foreign antigens. More recent studies have extended those observations to include a key role for CR2 and C3d in the humoral immune response to T-independent foreign antigens. Conversely, as these studies have proceeded, a parallel series of analyses have linked defects in expression or function of complement C4 and other classical pathway activation pathway proteins, as well as CR2 and the closely related CR1, to the loss of self tolerance to nuclear antigens such as double-stranded DNA and chromatin in systemic lupus erythematosus. With regard to the topic of this issue, it is now becoming increasingly clear that CR2 also plays a major role in the development of the natural antibody repertoire. Specifically, in the absence of this receptor natural IgM and IgG develop in the naïve animal that demonstrate clearly altered recognition patterns for specific natural antibody targets. This repertoire change is important physiologically in at least one setting because these CR2-dependent natural antibodies are necessary for the recognition of ischemic self tissues. In addition, it is possible that certain of the phenotypes manifest by CR2-deficient mice may be strongly influenced not only by effects on later stages of B cell activation and maturation, as commonly thought, but also by alterations in the pre-existing pool of natural antibodies that are influenced by this receptor. This review will examine the evidence that has accumulated over the last few years supporting these hypotheses.

The complement system in regulation of adaptive immunity

The serum complement system, which represents a chief component of innate immunity, not only participates in inflammation but also acts to enhance the adaptive immune response. Specific activation of complement via innate recognition proteins or secreted antibody releases cleavage products that interact with a wide range of cell surface receptors found on myeloid, lymphoid and stromal cells. This intricate interaction among complement activation products and cell surface receptors provides a basis for the regulation of both B and T cell responses. This review highlights fundamental events, explaining how complement links innate and adaptive immunity as well as describing more recent studies on how this large family of proteins functions locally in peripheral lymph nodes to enhance B and T cell responses.

Complement and autoimmunity

The complement system is comprised of a number of serum and membrane-bound proteins that play an important role in the elimination of foreign microorganisms while protecting the host organism from complement-related damage. Complement has also been shown to participate in the generation of normal humoral immune responses to foreign antigens. Recent studies suggest that the functions of complement may be extended to include the maintenance of B cell tolerance. Complement receptor 2 (CR2/CD21) has been implicated in lupus susceptibility in both humans and animal models of disease. Located primarily on B cells and follicular dendritic cells, CR2 binds C3 degradation products that have become covalently bound to antigen or immune complexes in the process of complement activation. The mechanism by which CR2 might regulate B cell reactivity to autoantigens has not been elucidated, but may involve direct effects on B cell tolerance or indirect effects on T cell tolerance.

Complementation of pulmonary abnormalities in SP-D(-/-) mice with an SP-D/conglutinin fusion protein

Surfactant protein D (SP-D) and serum conglutinin are closely related members of the collectin family of host defense lectins. Although normally synthesized at different anatomic sites, both proteins participate in the innate immune response to microbial challenge. To discern the roles of specific domains in the function of SP-D in vivo, a fusion protein (SP-D/Cong(neck+CRD)) consisting of the NH(2)-terminal and collagenous domains of rat SP-D (rSP-D) and the neck and carbohydrate recognition domains (CRDs) of bovine conglutinin (Cong) was expressed in the respiratory epithelium of SP-D gene-targeted (SP-D(-/-)) mice. While SP-D/Cong(neck+CRD) fusion protein did not affect lung morphology and surfactant phospholipid levels in the lungs of wild type mice, the chimeric protein substantially corrected the increased lung phospholipids in SP-D(-/-) mice. The SP-D/Cong(neck+CRD) fusion protein also completely corrected defects in influenza A clearance and inhibited the exaggerated inflammatory response that occurs following viral infection. However, the chimeric protein did not ameliorate the ongoing lung inflammation, enhanced metalloproteinase expression, and alveolar destruction that characterize this model of SP-D deficiency. By contrast, a single arm mutant (RrSP-D(Ser15,20)) partially restored antiviral activity but otherwise failed to rescue the deficient phenotype. Our findings directly implicate the CRDs of both SP-D and conglutinin in host defense in vivo. Our findings also strongly suggest that the molecular mechanisms underlying impaired pulmonary host defense and abnormal lipid metabolism are distinct from those that promote ongoing inflammation and the development of emphysema.

Rat T cells express neither CD55 nor CD59 and are dependent on Crry for protection from homologous complement

All human blood cells express decay-accelerating factor (DAF, CD55), CD59, and, with the exception of erythrocytes, membrane cofactor protein (MCP, CD46) to protect themselves from damage by the constant low-level activation of complement in serum. In rats and mice MCP is expressed only in testis, whereas DAF and CD59 are broadly distributed. Rats and mice also express a unique complement regulator, Crry. Previously we have shown that DAF was absent from at least 75% of rat T cells. To further investigate this surprising finding, we assessed the expression levels of DAF, CD59 and Crry on all blood cell types in the rat. We found that Crry was abundantly expressed on all blood cells. CD59 was expressed abundantly on erythrocytes and granulocytes but was absent from all T cellsand platelets and a minority of B cells and NK cells. Double staining and depletion studies showed that T cells in all rat strains tested were DAF-CD59-. Neutralization of Crry using a blocking monoclonal antibody rendered T cells susceptible to lysis by homologous complement, indicating that Crry was solely responsible for protecting DAF-CD59- T cells from complement damage in the rat.

Complement inhibitors: a resurgent concept in anti-inflammatory therapeutics

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

Aerosolized endotoxin is immediately bound by pulmonary surfactant protein D in vivo

Collectins are carbohydrate binding proteins that are implicated in innate host defense. The lung collectins, surfactant proteins A and D (SP-A and SP-D), bind a variety of pathogens in vitro and influence phagocytosis by alveolar macrophages. In this report we show that SP-D binds endotoxin (lipopolysaccharide, LPS) in vivo in a rat model of acute respiratory distress syndrome (ARDS). Intratracheal aerosolization of LPS in rats resulted in the typical features of human ARDS. Total amounts of SP-D, as well as the carbohydrate binding properties of SP-D were measured in lung lavage as a function of time. The amount of SP-D did not change during 24 h. Interestingly, SP-D in lung lavage isolated from rats during the first 2 h after LPS treatment, was not able to bind to carbohydrate. Further analysis revealed that the carbohydrate binding sites of SP-D were occupied by LPS, suggesting that SP-D is an LPS scavenging molecule in vivo. Electron microscopic analysis indicated that, 1 h after LPS aerosolization, aggregates of SP-D with LPS were found in lysosomal structures in alveolar macrophages. We conclude that the lung collectin SP-D binds inhaled endotoxin in vivo, which may help to protect the lung from endotoxin-induced disease.

Mannose-binding protein B allele confers protection against tuberculous meningitis

Inhalation is the principal mode of entry for Mycobacterium tuberculosis in humans. Primary infection is usually restricted to the lungs and contiguous lymph nodes. In a subset of infected individuals, predominantly children, the infection is spread hematogenously to the meninges. The host factors that influence the development of tuberculous meningitis have not been well elucidated. The mannose-binding protein (MBP), a serum protein, is considered as an "ante-antibody." MBP has been shown to bind mycobacteria and acts as an opsonin in vitro. Although MBP plays a role in first-line host defense, it may under certain circumstances be deleterious to the host. In tuberculosis (TB), MBP may assist the spread of this intracellular pathogen. Therefore, we hypothesized that MBP genotypes that result in a phenotype of low MBP levels might be protective. We studied a well-defined South African population in which TB has reached epidemic levels. We found that the MBP B allele (G54D), which disrupts the collagen region of the protein and results in low MBP levels, was found in 22 of 79 (28%) of the TB-negative controls from the same community, compared with 12 of 91 (13%) of the patients with pulmonary TB (p < 0.017), and 5 of 64 (8%) of patients with tuberculous meningitis (p < 0.002). In addition, we found significantly lower serum MBP concentrations in TB-negative controls compared with postacute phase, fully recovered TB patients (p < 0.004). These findings suggest that the MBP B allele affords protection against tuberculous meningitis.

Collectins and pulmonary host defense

The surfactant-associated proteins SP-A and SP-D are members of a family of collagenous host defense lectins, designated collectins. There is increasing evidence that these pulmonary epithelial-derived proteins are important components of the innate immune response to microbial challenge, and that they participate in other aspects of immune and inflammatory regulation within the lung. The collectins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms and certain other organic particles in vitro. Although binding may facilitate microbial clearance through aggregation or other direct effects on the organism, SP-A and SP-D also have the capacity to modulate leukocyte function and, in some circumstances, to enhance their killing of microorganisms. The biologic activity of cell wall components, such as gram-negative bacterial polysaccharides, may be altered by interactions with collectins. Complementary or cooperative interactions between SP-A and SP-D could contribute to the efficiency of this defense system. Collectins may play particularly important roles in settings of inadequate or impaired specific immunity. Acquired or genetic alterations in the levels of active proteins within the airspaces and distal airways may increase susceptibility to infection.

The role of complement and complement receptors in induction and regulation of immunity

Covalent attachment of activated complement C3 (C3d) to antigen links innate and adaptive immunity by targeting antigen to follicular dendritic cells (FDC) and B cells via specific receptors CD21 and CD35. Recent characterization of knockout mice deficient in complement components C3, C4, or the receptors CD21 and CD35 as well as biochemical studies of the CD21/CD19/Tapa-1 coreceptor on B cells have helped to elucidate the mechanism of complement regulation of both B-1 and B-2 lymphocytes. Interestingly, natural antibody of the adaptive immune system provides a major recognition role in activation of the complement system, which in turn enhances activation of antigen-specific B cells. Enhancement of the primary and secondary immune response to T-dependent antigens is mediated by coligation of the coreceptor and the B cell antigen receptor, which dramatically increases follicular retention and B cell survival within the germinal center. Most recent evidence suggests that complement also regulates elimination of self-reactive B cells, as breeding of mice that are deficient in C4 or CD21/CD35 with the lupus-prone strain of lpr mice demonstrates an exacerbation of disease due to an increase in autoantibodies.

Solution structure of Compstatin, a potent complement inhibitor

The third component of complement, C3, plays a central role in activation of the classical, alternative, and lectin pathways of complement activation. Recently, we have identified a 13-residue cyclic peptide (named Compstatin) that specifically binds to C3 and inhibits complement activation. To investigate the topology and the contribution of each critical residue to the binding of Compstatin to C3, we have now determined the solution structure using 2D NMR techniques; we have also synthesized substitution analogues and used these to study the structure-function relationships involved. Finally, we have generated an ensemble of a family of solution structures of the peptide with a hybrid distance geometry-restrained simulated-annealing methodology, using distance, dihedral angle, and 3J(NH-Halpha)-coupling constant restraints. The Compstatin structure contained a type I beta-turn comprising the segment Gln5-Asp6-Trp7-Gly8. Preference for packing of the hydrophobic side chains of Val3, Val4, and Trp7 was observed. The generated structure was also analyzed for consistency using NMR parameters such as NOE connectivity patterns, 3J(NH-Halpha)-coupling constants, and chemical shifts. Analysis of Ala substitution analogues suggested that Val3, Gln5, Asp6, Trp7, and Gly8 contribute significantly to the inhibitory activity of the peptide. Substitution of Gly8 caused a 100-fold decrease in inhibitory potency. In contrast, substitution of Val4, His9, His10, and Arg11 resulted in minimal change in the activity. These findings indicate that specific side-chain interactions and the beta-turn are critical for preservation of the conformational stability of Compstatin and they might be significant for maintaining the functional activity of Compstatin.

The collectins in innate immunity

The collectins are proteins with collagen tails and globular lectin domains that appear to play an important role in mammalian first line host defense. Recent insights have clarified the structural basis of ligand recognition, the interactions of collectins with complement cascades, and the association with disease susceptibility.