Structure of C3b in complex with CRIg gives insights into regulation of complement activation

The complement system is a key part of the innate immune system, and is required for clearance of pathogens from the bloodstream. After exposure to pathogens, the third component of the complement system, C3, is cleaved to C3b which, after recruitment of factor B, initiates formation of the alternative pathway convertases. CRIg, a complement receptor expressed on macrophages, binds to C3b and iC3b mediating phagocytosis of the particles, but it is unknown how CRIg selectively recognizes proteolytic C3-fragments and whether binding of CRIg to C3b inhibits convertase activation. Here we present the crystal structure of C3b in complex with CRIg and, using CRIg mutants, provide evidence that CRIg acts as an inhibitor of the alternative pathway of complement. The structure shows that activation of C3 induces major structural rearrangements, including a dramatic movement (>80 A) of the thioester-bond-containing domain through which C3b attaches to pathogen surfaces. We show that CRIg is not only a phagocytic receptor, but also a potent inhibitor of the alternative pathway convertases. The structure provides insights into the complex macromolecular structural rearrangements that occur during complement activation and inhibition. Moreover, our structure-function studies relating the structural basis of complement activation and the means by which CRIg inhibits the convertases provide important clues to the development of therapeutics that target complement.     

The structure of complement C3b provides insights into complement activation and regulation

The human complement system is an important component of innate immunity. Complement-derived products mediate functions contributing to pathogen killing and elimination. However, inappropriate activation of the system contributes to the pathogenesis of immunological and inflammatory diseases. Complement component 3 (C3) occupies a central position because of the manifold biological activities of its activation fragments, including the major fragment, C3b, which anchors the assembly of convertases effecting C3 and C5 activation. C3 is converted to C3b by proteolysis of its anaphylatoxin domain, by either of two C3 convertases. This activates a stable thioester bond, leading to the covalent attachment of C3b to cell-surface or protein-surface hydroxyl groups through transesterification. The cleavage and activation of C3 exposes binding sites for factors B, H and I, properdin, decay accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46), complement receptor 1 (CR1, CD35) and viral molecules such as vaccinia virus complement-control protein. C3b associates with these molecules in different configurations and forms complexes mediating the activation, amplification and regulation of the complement response. Structures of C3 and C3c, a fragment derived from the proteolysis of C3b, have revealed a domain configuration, including six macroglobulin domains (MG1-MG6; nomenclature follows ref. 5) arranged in a ring, termed the beta-ring. However, because neither C3 nor C3c is active in complement activation and regulation, questions about function can be answered only through direct observations on C3b. Here we present a structure of C3b that reveals a marked loss of secondary structure in the CUB (for 'complement C1r/C1s, Uegf, Bmp1') domain, which together with the resulting translocation of the thioester domain provides a molecular basis for conformational changes accompanying the conversion of C3 to C3b. The total conformational changes make many proposed ligand-binding sites more accessible and create a cavity that shields target peptide bonds from access by factor I. A covalently bound N-acetyl-l-threonine residue demonstrates the geometry of C3b attachment to surface hydroxyl groups.     

Crystallography: crystallographic evidence for deviating C3b structure

Activation of the protein C3 into C3b in the complement pathway is a crucial step in the complement immune response against pathogenic, immunogenic and apoptotic particles. Ajees et al. describe a crystal structure for C3b that deviates from the one reported by Janssen et al. and by Wiesmann et al.. We have reanalysed the data deposited by Ajees et al. and have discovered features that are inconsistent with the known physical properties of macromolecular structures and their diffraction data. Our findings therefore call into question the crystal structure for C3b reported by Ajees et al..     

Structural/functional similarity between proteins involved in complement- and cytotoxic T-lymphocyte-mediated cytolysis

Cytolysis mediated by complement or cytolytic lymphocytes results in the formation of morphology similar lesions in the target membrane. These lesions, formed by the polymerization of C9 or perforin respectively, contribute the major killing action by causing osmotic lysis of the target cell. Following the suggestion of Mayer that the mechanisms of humoral and cell-mediated cytotoxicity might be related, studies into the morphology of the membrane lesions formed, and the proteins responsible for causing the lesions, have shown several similarities. While the lesion caused by natural and T-killer cells is a little larger than that caused by complement, its overall shape is similar and in both cases the cylindrical pore is formed by polymerization of a monomeric subunit, C9 (relative molecular mass, Mr = 71,000) for complement, and perforin (Mr = 66,000) for cell-mediated cytotoxicity. C9 has an absolute requirement for a receptor in the target membrane formed by the earlier membrane attack complex components, C5b, C6, C7 and C8 (ref. 8). For perforin, polymerization in a target membrane requires no receptor, specificity being derived from the specific recognition between killer and target cell. Both proteins can be made to polymerize in vitro by the addition of divalent cations (Zn2+ for C9 (ref. 16) and Ca2+ for perforin) and the resultant complexes closely resemble their physiological counterparts. Antibodies raised against lymphocyte-killed targets have also been shown to cross-react with complement proteins, but the antigenically related proteins were not determined in these studies. We show here using purified proteins that perforin, C9 and complexes involving C7 and C8 share a common antigenic determinant which is probably involved in polymerization.     

Monoclonal antibodies demonstrate protection of polymorphonuclear leukocytes against complement attack

Studies on erythrocytes have shown that the formation of the membrane attack complex on a cell surface inevitably results in lysis. However, it is known that nucleated cells are much more difficult to kill with complement, although the molecular basis of this resistance has never been established. We have shown that a very early intracellular event, occurring within seconds of formation of the attack complex in the membrane, is a rise in cytoplasmic Ca2+, which can activate cell responses without cell death 5,6. Here we report the use of a monoclonal antibody to the terminal complement component C9, quantified by 125I and visualized by fluorescein, to demonstrate a protection mechanism in polymorphonuclear leukocytes (PMNs) attacked by complement, involving removal of the attack complex by vesiculation. Concomitantly, there is a Ca2+-dependent activation of reactive oxygen metabolite production without cell lysis. These findings have important implications in the evolutionary and pathological significance of the terminal components of the complement pathway.     

Structure of C3b reveals conformational changes that underlie complement activity

Resistance to infection and clearance of cell debris in mammals depend on the activation of the complement system, which is an important component of innate and adaptive immunity. Central to the complement system is the activated form of C3, called C3b, which attaches covalently to target surfaces to amplify complement response, label cells for phagocytosis and stimulate the adaptive immune response. C3b consists of 1,560 amino-acid residues and has 12 domains. It binds various proteins and receptors to effect its functions. However, it is not known how C3 changes its conformation into C3b and thereby exposes its many binding sites. Here we present the crystal structure at 4-A resolution of the activated complement protein C3b and describe the conformational rearrangements of the 12 domains that take place upon proteolytic activation. In the activated form the thioester is fully exposed for covalent attachment to target surfaces and is more than 85 A away from the buried site in native C3 (ref. 5). Marked domain rearrangements in the alpha-chain present an altered molecular surface, exposing hidden and cryptic sites that are consistent with known putative binding sites of factor B and several complement regulators. The structural data indicate that the large conformational changes in the proteolytic activation and regulation of C3 take place mainly in the first conversion step, from C3 to C3b. These insights are important for the development of strategies to treat immune disorders that involve complement-mediated inflammation.     

The third component of complement (C3) is responsible for the intracellular survival of Leishmania major

Leishmania are obligate intracellular parasites of mononuclear phagocytes. We and others have shown that the promastigote form of all species of leishmania activates complement from non-immune serum and that this activation can result in parasite lysis. This work, as well as earlier in vivo studies, suggested that complement is an important component of host defence against leishmaniasis. We now present evidence that parasite complement fixation, in addition to increasing parasite phagocytosis, is required for the intracellular survival of leishmania in macrophages. We specifically show a strong correlation between parasite C3 fixation and intracellular survival. We attribute this survival, in part, to a decrease in the magnitude of the macrophage respiratory burst which is triggered by complement-coated, as opposed to uncoated, parasites.     

Sequence and topology of a model intracellular membrane protein, E1 glycoprotein, from a coronavirus

In the eukaryotic cell, both secreted and plasma membrane proteins are synthesized at the endoplasmic reticulum, then transported, via the Golgi complex, to the cell surface. Each of the compartments of this transport pathway carries out particular metabolic functions, and therefore presumably contains a distinct complement of membrane proteins. Thus, mechanisms must exist for localizing such proteins to their respective destinations. However, a major obstacle to the study of such mechanisms is that the isolation and detailed analysis of such internal membrane proteins pose formidable technical problems. We have therefore used the E1 glycoprotein from coronavirus MHV-A59 as a viral model for this class of protein. Here we present the primary structure of the protein, determined by analysis of cDNA clones prepared from viral mRNA. In combination with a previous study of its assembly into the endoplasmic reticulum membrane, the sequence reveals several unusual features of the protein which may be related to its intracellular localization.     

The genome of Cryptosporidium hominis

Cryptosporidium species cause acute gastroenteritis and diarrhoea worldwide. They are members of the Apicomplexa--protozoan pathogens that invade host cells by using a specialized apical complex and are usually transmitted by an invertebrate vector or intermediate host. In contrast to other Apicomplexans, Cryptosporidium is transmitted by ingestion of oocysts and completes its life cycle in a single host. No therapy is available, and control focuses on eliminating oocysts in water supplies. Two species, C. hominis and C. parvum, which differ in host range, genotype and pathogenicity, are most relevant to humans. C. hominis is restricted to humans, whereas C. parvum also infects other mammals. Here we describe the eight-chromosome approximately 9.2-million-base genome of C. hominis. The complement of C. hominis protein-coding genes shows a striking concordance with the requirements imposed by the environmental niches the parasite inhabits. Energy metabolism is largely from glycolysis. Both aerobic and anaerobic metabolisms are available, the former requiring an alternative electron transport system in a simplified mitochondrion. Biosynthesis capabilities are limited, explaining an extensive array of transporters. Evidence of an apicoplast is absent, but genes associated with apical complex organelles are present. C. hominis and C. parvum exhibit very similar gene complements, and phenotypic differences between these parasites must be due to subtle sequence divergence.     

Interferon amplifies complement activation by Burkitt's lymphoma cells

Interferon was originally described as an antiviral agent produced shortly after onset of infection with most viruses. However, in addition to inducing an antiviral state, interferon inhibits cell division, increases the expression of cell-surface antigens, boosts the cytotoxic activity of natural killer (NK) cells and modulates several immune functions of lymphocytes and macrophages. Moreover, a special class of interferon (immune interferon or IFN-gamma) is produced by T cells following stimulation with antigen or interaction with mitogens. The different methods by which interferon is induced and its multiple effects suggest that it may be part of a first-line defence system controlling the spread of virus infections and the proliferation of modified 'self' cells that have been affected by virus infection or neoplastic transformation. The ability of certain human lymphoma cells to activate the alternative pathway of complement is well established. Here we show that monoclonal antibody-purified interferon can amplify the ability of certain tumour cells to activate complement via the alternative pathway. This demonstration may reflect an additional, as yet unknown, role of interferon in inducing non-specific anti-tumour immunity.     

Protochordate amphioxus is an emerging model organism for comparative immunology

Protochordate amphioxus is an extant invertebrate regarded quite recently as a basal chordate. It has a vertebrate-like body plan including a circulation system with an organization similar to that of vertebrates. However, amphioxus is less complex than vertebrates for having a genome uncomplicated by extensive genomic duplication, and lacking lymphoid organs and free circulating blood cells. Recent studies on immunity have demonstrated the presence in amphioxus of both the constituent elements of key molecules involved in adaptive immunity such as proto-major histocompatibility complex (proto-MHC), V region-containing chitin-binding protein (VCBP) and V and C domain-bearing protein (VCP), and the complement system operating via the alternative and lectin pathways resembling those seen in vertebrates. In addition, the acute phase response profile in amphioxus has been shown to be similar to that observed in vertebrates. These findings together with the relative structural and genomic simplicity make amphioxus an ideal organism for gaining insights into the origin and evolution of the vertebrate immune system, especially adaptive immunity, and the composition and mechanisms of the vertebrate innate immunity.     

Ca2+-dependent and Ca2+-independent phagocytosis in human neutrophils

The phagocytic function of neutrophils is a crucial element in host defence against invading microorganisms. Two main specific receptor-mediated mechanisms operate in the phagocyte plasma membrane, one recognizing the C3b/bi fragment of complement and the other the Fc domain of immunoglobulin G (ref. 1). There is evidence that phagocytosis mediated by these receptors differs in the number and nature of the intracellular signals generated. However, the mechanisms by which receptor binding is transduced into a signal that generates the formation of the phagocyte pseudopod is not known, although extensive biochemical evidence has allowed the postulate that calcium ion gradients in the peripheral cytoplasm, by interacting with calcium-sensitive contractile proteins, initiate the process of engulfment. Using the high-affinity fluorescent calcium indicator quin2 both to measure and to buffer intracellular calcium ([Ca2+]i), we show here that in human neutrophils two mechanisms of phagocytosis coexist: a [Ca2+]i-dependent and modulated phagocytosis, triggered by activation of the Fc receptor, and a [Ca2+]i-independent mechanism triggered by the activation of the C3b/bl receptors.     

L-谷氨酸-5-甲酯金属铜化合物的合成、表征及生物活性

通过L-谷氨酸-5-甲酯与Cu（CH3COO）2.H2O反应,合成了相应的金属铜化合物[Cu（C6H10NO4）2].对L-谷氨酸-5-甲酯金属铜化合物的晶体进行了X-射线单晶衍射测定.结果表明：L-谷氨酸-5-甲酯金属铜化合物属单斜晶系,P2（1）空间群,其中a=9.5883（19）,b=5.1521（10）,c=15.562（3）;β=98.65（3）°.抗菌筛选数据表明,L-谷氨酸-5-甲酯金属铜化合物对枯草芽胞杆菌具有较好的抑制作用.     

Growth control of activated, synchronized murine B cells by the C3d fragment of human complement

Three restriction points control the cell cycle of activated B lymphocytes. The first occurs directly after mitosis and is controlled by the occupancy of surface-bound immunoglobulin. The second is observed approximately 4 h after mitosis in the G1 phase of the cycle, that is, before DNA replication, and is controlled by growth factors that are produced by macrophages which we have previously classified as alpha-type factors. The third restriction point occurs in the G2 phase, 2-4 h before mitosis, and is controlled by beta-type growth factors probably produced by helper T lymphocytes. The third component of complement, C3, has long been implicated in the control of B-cell responses. C3 is secreted by monocytes and macrophages. We have found recently that crosslinked, but not soluble, human C3 stimulates activated, but not resting, murine B cells to thymidine uptake. Here we investigate the role of C3b and C3d in the progression of the cell cycle of activated, synchronized murine B cells. We find that crosslinked C3d replaces the action of alpha-factors within the cell cycle of these cells and allows entry into S phase. In contrast, soluble C3d inhibits the action of alpha-factors. This implies that a C3d-specific receptor, probably the murine analogue to the human complement receptor CR2, is a growth factor receptor on activated B cells that will give the cell a growth-positive signal when it is crosslinked, while occupancy by the soluble form of C3d will result in inhibition of the action of alpha-factors or of crosslinked C3b or C3d. A stretch of weak homology between the cDNA sequence of murine C3d and those of murine growth factors indicates that an insulin-like growth factor could be the active principle of C3d that controls the cell cycle of activated B cells.     

A role for the C3a anaphylatoxin receptor in the effector phase of asthma

Asthma is a chronic inflammatory disease of the airways and lung mucosa with a strong correlation to atopy and acquired (IgE) immunity. However, many features of bronchial asthma, such as smooth muscle contraction, mucus secretion and recruitment of inflammatory cells, are consistent with the actions of complement anaphylatoxins, in particular C3a and C5a. Complement activation forms a central core of innate immune defence against mucosal bacteria, viruses, fungi, helminths and other pathogens. As a system of 'pattern-recognition molecules', foreign surface antigens and immune complexes lead to a proteolytic cascade culminating in a lytic membrane attack. The anaphylatoxins C3a and C5a are liberated as activation byproducts and are potent pro-inflammatory mediators that bind to specific cell surface receptors and cause leukocyte activation, smooth muscle contraction and vascular permeability. Here we show that in a murine model of allergic airway disease, genetic deletion of the C3a receptor protects against the changes in lung physiology seen after allergen challenge. Furthermore, human asthmatics develop significant levels of ligand C3a following intra-pulmonary deposition of allergen, but not saline. We propose that, in addition to acquired immune responses, the innate immune system and complement (C3a in particular) are involved in the pathogenesis of asthma.     

A mechanosensory complex that mediates the endothelial cell response to fluid shear stress

Shear stress is a fundamental determinant of vascular homeostasis, regulating vascular remodelling, cardiac development and atherogenesis, but the mechanisms of transduction are poorly understood. Previous work showed that the conversion of integrins to a high-affinity state mediates a subset of shear responses, including cell alignment and gene expression. Here we investigate the pathway upstream of integrin activation. PECAM-1 (which directly transmits mechanical force), vascular endothelial cell cadherin (which functions as an adaptor) and VEGFR2 (which activates phosphatidylinositol-3-OH kinase) comprise a mechanosensory complex. Together, these receptors are sufficient to confer responsiveness to flow in heterologous cells. In support of the relevance of this pathway in vivo, PECAM-1-knockout mice do not activate NF-kappaB and downstream inflammatory genes in regions of disturbed flow. Therefore, this mechanosensing pathway is required for the earliest-known events in atherogenesis.     

黄鳝SOD，AKP，补体C3、C4组织分布研究

以在水温28±3℃下饲养的黄鳝(Monopterus ablus)(37.35±0.89 g)为研究对象,以酶学方法研究黄鳝不同免疫组织(血液、体表粘液、肝脏、肠道、肾脏、脾脏)中非特异性免疫相关酶(超氧化物歧化酶(SOD)、酸性和碱性磷酸酶(ACP、AKP)、补体C3、C4)的分布情况。研究结果显示,组织中SOD活性依次为;肝脏>肾脏、脾脏>粘液>肠道>血液(P<0.05);ACP活性依次为:脾脏>肾脏>肝脏>肠道>血液、粘液;AKP活性依次为:肾脏>肝脏>脾脏>肠道>粘液>血液;补体C3的含量依次为:肝脏>肾脏>脾脏>肠道>血液、粘液;补体C4的含量依次为:肝脏>脾脏>肾脏>肠道>血液、粘液。由此可见,超氧化物歧化酶和补体C3、C4主要分布于肝脏中,碱性磷酸酶主要分布于肾脏中,酸性磷酸酶主要分布于脾脏中;而各种非特异性酶和物质在肠道、血液和粘液中也均有少量分布。     

CD3-negative natural killer cells express zeta TCR as part of a novel molecular complex

Natural killer (NK) cells are large granular lymphocytes capable of killing tumour cells in a non-MHC restricted manner. NK cells do not express cell-surface CD3, or any known target recognition structure analogous to the T cell antigen receptor (TCR) heterodimers (alpha beta or gamma delta). Consistent with their lack of expression of a CD3-TCR complex, NK cells do not require prior sensitization or antigen presentation by accessory cells to specifically recognize their tumour targets. Although NK cells do not express CD3-TCR, they do express CD2, the target of an alternative activation pathway which is functional in both T cells and NK cells. In T cells, this alternative activation pathway utilizes some component of the CD3-TCR complex as a transducer molecule that is required for mitogenesis. The fact that NK cells are activated by this alternative pathway suggested that they might express a related subunit of the CD3-TCR complex capable of transducing the CD2-mediated signal. Here we show that human NK cells express the zeta-chain of the TCR complex in association with additional structures not included in CD3-TCR.