《动物免疫学》课程教学资源（文献资料）补体 Complement（英文）2.pdf_大学文库

Complement J Oriol Sunyer, University of Pennsylvania, Philadelphia, Pennsylvania, USA John D Lambris, University of Pennsylvania, Philadelphia, Pennsylvania, USA The mammalian complement system consists of a complex group of more than 30 soluble proteins and receptors that play an important role in innate and acquired host defence mechanisms against infection, and participate in various immunoregulatory processes. The functions mediated by complement activation products include phagocytosis, cytolysis, inflammation, solubilization of immune complexes and promotion of humoral immune responses. Introduction The existence of the complement system was first recognized near the end of the nineteenth century, when normal sheep blood was found to possess a mild bactericidal activity that was lost when the blood was heated to 558C. This labile bactericidal activity was later termed alexin by Bordet. By 1900 Paul Ehrlich had proposed a scheme for humoral immunity in which he identified the heat-stable immune sensitizer component of serum as ‘amboreceptor’ (antibody), while the heat-labile factor in serum (Bordet’s alexin) was termed ‘complement’. Since then an impressive number of complement components have been, and are being, added to the list of molecules that make up the complement system as we know it today. The complement system is activated by three different pathways, termed the classical, alternative and lectin pathways. All three pathways are activated in a sequential manner, with activation of one component leading to the activation of the next (Figure 1). Activation of complement through any of the three pathways leads to activation of C3, the central protein of the complement system. C3 is a fascinating molecule that has the capacity to interact with more than 20 different proteins of complement and noncomplement origin. Native C3 is not a functional molecule, and all of the ligand-binding sites on C3 are hidden until the molecule is activated. As we shall see in the following sections, native C3 contains a thioester group that upon activation makes C3 a functional protein that is able to interact with its ligands. After activation the C3 molecule can either be inactivated to avoid selfdamage, or Article Contents Introductory article . Introduction . Alternative Pathway . Classical Pathway . Lectin Pathway . Lytic Pathway . Components of the Complement System . Components Involved in the Activation Sequences . Regulatory Proteins . Complement Receptors . Functions of the Complement System . Complement Deficiencies C6 Classical Ab C1 C4 C1 C4a C2 C1 C4b C1 Lectin MBL MASP MBL MBL, MASP C3 C3b B, D Alternative C3a C3 C3iBb C3bBb D C3iB B C3i H2O C3 Initiation C4b2a C3b C3a C1 C4b2a3b C5b C5a C5 C7 C8 C9 C5-9 C3 convertase C5 convertase C3bBb3b C3 convertase C5 convertase Antibody Activating particle C1, Activated C1 Figure 1 Pathways of complement activation. ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1

Complement ntial activation of the s molecules.Like C ntains nd that (MAC). can attach covalently to surfaces via their hydroxyl or result of this antibody-mediated omp agregauo Alternative Pathway C2 a othe The alternative pathway is activated by a variety of orotease.C2 binds to C4b in a Mg -dependent manner and is cleaved by CIs to C2a and C2b. he association c bac ria,fungi and C2a(which ontas the catalyticsite)with C has been shown to enhance the activation process.The This enzy alternative pathway is kept in a low level of steady state which is then deposited in large amounts onto the target surface.I his depo nize the targe t of hydr nativ s of the itate its aing th al s C3continuously available and ready to react with potential (C4b.2a.3b).With the C2a fragr the pathogens.It is also necessary because the half-life of the catalytic subunit,C5 convertase cleaves C5 to Csb.which is then added to the complex. acoecoromoS5ho able to bind to rB. proteolytically activated and cleaved to Bb(66kDa)and Lectin Pathway Ba(33 kDa)fragments by a second serine protease acto also directly activate the complement system (including viruses.bacteria and fungi)via its metastable serine protease (MASPI and MASP2).Once MBL binds to thioester bond.Most of this resulting C3b,as well as the C3(H2O),is i ntaneously inactivated by or c MCP The Cih that is not inactivated (the Ci ho d to and ctiv SPng C3 activating surfaces)is involved in an amplification loop of ve pathway Bb Lytic Pathway 、urther din.Close another C3b molecule with the C3b.Bb complex form All three pathways of complement activation converge enzyme C5 onvertase (C3b.Bb.3b).which cleaves C5 into with the production ofa C5 convertase which cleaves C5 to C5a and C5b(Figure1 Activation of the pathway is very MAC CSC the self-assembly of the whether amplification (hinding of factor B to C3b)or the Components of the Complement System It has now become clear that the complement system arose Classical Pathway first in invertebrate species more than 600 million years Activation of the syste by the classical complement genes ha ave already b oned they ene In t出 ve even more primitive species.the complement sstem m complex via the Clqsubunit (Figure1).Binding ofantibody have emerged as a simple system comprising a small and factor B and Dand/o ons(perhaps onl ENCYCLOPEDIA OF LIFE SCIENCES/2001 Nature Publishing Group /www.els.net

it can be covalently attached to target surfaces where it leads to either opsonization, or cytolysis through the sequential activation of the membrane attack complex (MAC). Alternative Pathway The alternative pathway is activated by a variety of microorganisms including viruses, bacteria, fungi and protozoans. Although the initiation of activation is essentially antibody-independent, aggregated antibody has been shown to enhance the activation process. The alternative pathway is kept in a low level of steady state activation as a result of hydrolysis of the thioester group of native C3, which forms C3(H2O) (hydrolysed C3). This low-level activation is essential since it makes ‘functional’ C3 continuously available and ready to react with potential pathogens. It is also necessary because the half-life of the active form of C3 is very short. Once formed, C3(H2O) is able to bind to factor B, the catalytic subunit of the alternative pathway. Factor B is proteolytically activated and cleaved to Bb (66 kDa) and Ba (33 kDa) fragments by a second serine protease, factor D, to generate the enzyme C3 convertase (C3b,Bb). C3 convertase is able to cleave native C3 to C3a and C3b; C3b can then covalently bind to the surface of nearby particles (including viruses, bacteria and fungi) via its metastable thioester bond. Most of this resulting C3b, as well as the C3(H2O), is instantaneously inactivated by factor I in the presence of cofactor regulatory molecules (CR1, factor H, MCP). The C3b that is not inactivated (the C3b bound to activating surfaces) is involved in an amplification loop of the activation process, an essential feature of the activation of the alternative pathway. Binding of factor B to the newly generated C3b forms a new C3 convertase (C3b,Bb), which is further stabilized by properdin. Close association of another C3b molecule with the C3b,Bb complex forms the enzyme C5 convertase (C3b,Bb,3b), which cleaves C5 into C5a and C5b (Figure 1). Activation of the pathway is very much dependent upon the microenvironment surrounding the C3b bound molecule; conditions therefore determine whether amplification (binding of factor B to C3b) or abrogation of the pathway (binding of a regulator molecule to C3b) will occur. Classical Pathway Activation of the system by the classical pathway is primarily dependent on the immunoglobulin M (IgM) or IgG present in immune complexes, which binds to the C1 complex via the C1q subunit (Figure1). Binding of antibody induces conformational changes in the C1 complex and leads to the activation of its C1r and C1s serine protease subunits. Once activated, the C1s can cleave C4 to C4a and C4b; one molecule of activated C1s can cleave many C4 molecules. Like C3b, C4b contains a thioester bond that can attach covalently to surfaces via their hydroxyl or amino groups. The result of this antibody-mediated activation of the C1 complex is an aggregation of C4 molecules surrounding the antibody–C1 site. The next protein to bind to the complex is C2, another serine protease. C2 binds to C4b in a Mg2+-dependent manner and is cleaved by C1s to C2a and C2b. The association of C2a (which contains the catalytic site) with C4b leads to the formation of the classical pathway C3 convertase (C4b,2a). This enzyme is capable of binding and cleaving C3 to C3b, which is then deposited in large amounts onto the target surface. This deposition of C3 serves to opsonize the target surface and facilitate its phagocytosis, while initiating the terminal reaction sequence by forming C5 convertase (C4b,2a,3b). With the C2a fragment again supplying the catalytic subunit, C5 convertase cleaves C5 to C5b, which is then added to the complex. Lectin Pathway Lectins can also directly activate the complement system, through the binding of mannans to the complex compound of mannose-binding lectin (MBL) and theMBL-associated serine protease (MASP1 andMASP2). OnceMBL binds to mannans on the surface of various microorganisms, MASP becomes capable of cleaving and activating C3, C4 and C2. The distinct roles ofMASP1 andMASP2 are as yet unclear. Lytic Pathway All three pathways of complement activation converge with the production of a C5 convertase which cleaves C5 to C5b and C5a. C5b then initiates the self-assembly of the MAC C5b–C9(see below). Components of the Complement System It has now become clear that the complement system arose first in invertebrate species more than 600 million years ago, since complement genes have already been cloned, and the molecules they encode have been purified, from echinoderms and tunicates. In these animals, or perhaps even more primitive species, the complement system may have emerged as a simple system comprising a small number of components (perhaps C3, factor B and D and/or C3, MASP and MBL) with limited functions (perhaps only Complement 2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

Complement the opsonization of foreign material.a combination of other serine protease domain-containing molecules are gene duplication in combination with exon shuffling. 8rtooa8emtaladhtonodetiono6teealpe protein mosaics that are made up of different domains or the N-terminus d the pr wo non CIs/UEGF/BMPI (CUB)D characterizes the complement system in vertebrate spe cellular proteins playing roles indevelopmental processes) (Table 1). an epidermal growth factor(EGF)-like domain,two short From a functional point of view,complement proteins consensus repeat(SCR)domains nto three major ulat the ave of th e fo (2)those involved in the regulation of these activation ancestor.since they share very similar domain structures sequences,and (3)those that act as receptors for complement proteins Several e complement proteins Is ar ne proteas hin the compl tein) ded hy in haptoglobin 2 molecule also contains two CCP (compl ment control protein)modules.Therefore.an evolutionary happears toist between haptoglobin and the Components Involved in the Activation and are localized in tandem within the maior histocompat Sequences the SCR structur Complement components Clq and MBL (mannose ng lecun)st cular architect d lectin The CL similaritiesit is believed that factor Band C2 have arisen b molecule has a hexameric structure.and each of the six ule nas sea urcmi chin has ve nature and is thus by far the lobular head region.Modules with AmmcaoecaaoionoTheABsnCeh athway characterized in animal sp cies.It is not known in cule was are also present in the erminal regions of nonco e the C2 molecule.However mple ment protei human type V VIII and ein that s ns to play the ha olved in the binding of Cla to the Fc regionso IgM and IgG antibodies Components C3/C4/C5 tho Inc trast to most of the other complement co subunits is composed of three identical chains.Th C3.C4 and C5 contain neither repeating structures nor terminal region of each chain forms a triple-helical evident modules. the prmary sequences anc collagen-like struct tbchadthcCtcminalreioneonlains co nponer are ver to th ca te-rece mannan gr to the nam cestral molecule C3 C4 and cs are similar in size(200kDa),subunit structure,and order of their chains (a-B in C3 and C5 and a-B-y in C4);all three arginine ween chal th C3 Complement components with serine protease domains The thioesters located in the C3d and C4d region of C3 and C4.respectively,are responsible for the covalent due at p domain but this con group of regulatory proteins.Except for factor D.all the the presence of His 1126 is believed to be essential for the ENCYCLOPEDIA OF LIFE SCIENCES/2001N els.net

the opsonization of foreign material). A combination of gene duplication in combination with exon shuffling, including sequential addition or deletion of several types of modules or domains from various proteins, has produced the functional and structural complexity that characterizes the complement system in vertebrate species (Table 1). From a functional point of view, complement proteins can be grouped into three major categories: (1) those involved in the activation sequences of the three pathways, (2) those involved in the regulation of these activation sequences, and (3) those that act as receptors for complement proteins. Several of the complement proteins may belong to more than one of these three groups, since they have several functional roles within the complement system. Components Involved in the Activation Sequences Complement components C1q and MBL (mannosebinding lectin) share a similar molecular architecture, and they play analogous roles in the activation of the classical and lectin pathways, respectively. The C1q molecule has a hexameric structure, and each of the six subunits is made of three different chains, A, B and C. The N-terminal portion of each chain has a collagen-like sequence with a triple-helical structure: the C-terminal portion contains a globular head region. Modules with similarity to C-terminal portion of the A, B and C chains are also present in the C-terminal regions of noncomplement proteins, including human type VIII and X collagens and precerebellin. The globular head region of the C1q chains is involved in the binding of C1q to the Fc regions of IgM and IgG antibodies. MBL is a collectin with collagen-like stalks similar to those of C1q; like C1q, MBL also has a hexameric structure. However, in contrast to C1q, each of its six subunits is composed of three identical chains. The Nterminal region of each chain forms a triple-helical collagen-like structure, and the C-terminal region contains a C-type lectin carbohydrate-recognition domain that is involved in the binding of the MBL molecule to the mannan groups on the surfaces of microorganisms. Complement components with serine protease domains The complement components that contain a serine protease domain are factor D, C1r, C1s, MASP1, MASP2, factor B and C2. Factor I also has a serine protease domain, but this complement component falls within the group of regulatory proteins. Except for factor D, all the other serine protease domain-containing molecules are protein mosaics that are made up of different domains or modules. Beginning from the N-terminus, C1r, C1s, MASP1 and MASP2 consist of two noncontiguous C1r/ C1s/UEGF/BMPI (CUB) modules (found also in extracellular proteins playing roles in developmental processes), an epidermal growth factor (EGF)-like domain, two short consensus repeat (SCR) domains and a serine protease domain at the C-terminal part of the molecule. The four molecules appear to have descended from a common ancestor, since they share very similar domain structures and functional activities. It is interesting to note that MASP2, C1r and C1s are similar to the serine protease domain of haptoglobin (a haemoglobin-binding serum protein) in being encoded by a single exon; in addition, the haptoglobin 2 molecule also contains two CCP (complement control protein) modules. Therefore, an evolutionary relationship appears to exist between haptoglobin and the MASP2, C1r and C1s molecules. Factor B and C2 share the same genomic organization and are localized in tandem within the major histocompatibility complex (MHC) class III region. They also share the same domain structure and are both composed of three SCR domains, a Von Willebrand domain and the serine protease domain (N- to C-terminus). Because of their similarities it is believed that factor B and C2 have arisen by gene duplication from a common ancestor. A factor B-like molecule has been cloned from the sea urchin (echinoderm), suggesting that the alternative pathway is very primitive in nature and is thus by far the most ancient pathway characterized in animal species. It is not known in which animal species the ancestral factor B molecule was duplicated and diverged to give the C2 molecule. However, teleost fish (trout) and birds (chickens) are known to have a protein that seems to play the roles of both factor B and C2. Components C3/C4/C5 In contrast to most of the other complement components, C3, C4 and C5 contain neither repeating structures nor evident modules. However, the primary sequences and genomic organizations of the three components are very similar, leading to the belief that the three proteins arose from a common ancestral molecule. C3, C4 and C5 are similar in size ( 200 kDa), subunit structure, and order of their chains (a-b in C3 and C5 and a-b-g in C4); all three also possess arginine linkers between chains, and both C3 and C4 contain an active thioester site in the a chain. The thioesters located in the C3d and C4d region of C3 and C4, respectively, are responsible for the covalent binding of these components to their acceptor molecules. The His residue at position 1126 of human C3, as well as the Gln located two amino acids upstream, is very important in determining the binding specificity of the thioester; indeed, the presence of His 1126 is believed to be essential for the Complement ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 3

Table 1 Alternative, classical and lectin pathway proteins Protein Structure Concentration (µg mL–1) Cellular sources Key function Alternative pathway (AP) Factor B 93 kDa 210 Hepatocytes, mononuclear phagocytes, epithelial and endothelial cells, adipocytes, fibroblasts Catalytic subunit of AP C3 convertase, forms part of the C5 convertase Factor D 24 kDa 1–2 Mononuclear phagocytes, adipocytes Cleaves factor B that is bound to C3b or C3(H2O) Properdin 55–220 kDa monomer to tetramer 25 Mononuclear phagocytes Stabilizes AP C3 convertase C3 (185 kDa) 110 kDa α chain 75 kDa β chain 1300 Hepatocytes, mononuclear phagocytes, epithelial and endothelial cells, adipocytes, fibroblasts Activated C3 (C3b) covalently binds to activating surfaces. It forms part of the C3 and C5 convertases. Forms part of both alternative and classical pathways Factor H 150 kDa 500 Hepatocytes, mononuclear phagocytes, epithelial and endothelial cells, fibroblasts, B cells, keratinocytes, myoblasts Accelerates the dissociation of AP C3 convertase. Cofactor for factor I Factor I 88 kDa 35 Hepatocytes, mononuclear phagocytes, myoblasts, adipocytes, fibroblasts, B cells C4b/C3b inactivator Classical pathway (CP) C1q (462 kDa) Hexamer. Subunit contains: (( ×1)A+( ×1)B+( ×1)C) (×6) 26.5 kDa A chains (×6) 26.5 kDa B chains (×6) 24 kDa C chains 80 Hepatocytes, mononuclear phagocytes, fibroblasts, gastrointestinal epithelial cells Binds to IgM or IgG or C-reactive protein (CRP) and initiates the classical pathway C1r 83 kDa 50 Hepatocytes, mononuclear phagocytes, fibroblasts, gastrointestinal epithelial cells Cleaves C1s C1s 83 kDa 50 Hepatocytes, mononuclear phagocytes, fibroblasts, gastrointestinal epithelial cells Cleaves C4 and C2 C4 (205 kDa) 97 kDa, α chain 75 kDa, β chain 33 kDa, γ chain 600 Hepatocytes, mononuclear phagocytes, fibroblasts, genitourinary and alveolar type II epithelial cells Activated C4 (C4b) covalently binds to activating surfaces. Forms part of classical C3 convertase C2 110 kDa 20 Hepatocytes, mononuclear phagocytes, fibroblasts, genitourinary and alveolar type II epithelial cells Catalytic subunit of the CP C3 convertase. Forms part of the C5 convertase Complement 4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

Complement binding of C3 Regulatory Proteins s C4 exists as two isoforms.C4A and C4B Serum-soluble protein Although the two molecules have very few amino acid es the classical f CI way ains Asp instead components ofCl.It is partof the serine p eas inhibitor hydroxyl gro structure. molecule of teleost fish is encoded by regulates eiomat8nPortaimCbpAandC46pe various gene C3 protein present in C4b by factor I.There are three isoforn ificities to a variety of complement-activatn surfaces major form is composed of seven chains (each 70kDa) It has been hypothesized that this functional and structural ndone Bchain(75kDa)that are linked by disulfide bonds C3 diversity allows these nals to expand their innate 01 2 m contains SCRs The and 6 chains are composed of eight rtases to produe and three SCRs,respectively and both C3 and C4 are inactivated by factor I in the acto polypeptide chain of 150kDa gene ate rmina egulates ampl cep or fluid-phase C3b and accelerating the decay of the um proteinase inhibito C3b.Bb convertase,or by acting as a cofactor for factor macroglobulin.suggesting that the ancestral molecule '器eeompemetpCopeaipicatio Properdin is required fo the stabilization of the C3b,B e 1 (TSPD n ong e at type Factor I is an 88-kDa serine protease with an unusual structure:from N-tern inus to its C-terminus itis Late components factor /mem comp ich ass A(LDLRA)modules and a serine protease domain. with the C5b initiating the self assembly of the membrane attack complex (MAC). za that co 0 oluble plasma inhibitors of MAC formation that bind to be for creating the transmembrane channels that lead to the C5b-7 complexes and prevent their insertion into cell cell lysis.The assembly of MAC and its insertion into the nembr -protein glycoprotein th contains m-l nd e indin on and hat a alad u hin th thrombospondin-like N-terminal domain insertion of the C5b-C7 complex into a discrete site on Carboxypeptidase N.also termed anaphylatoxin inact target membrane and it now serves as a receptor for C8. C5 C8 complex can then bind multiple in the MAC.Although polymerization of C9 is not esse Membrane-bound proteins lysis of erythrocytes and nucleated cells,it is believed to be Decay-accelerating factor(DAF or CD55)and membrane killing of bacteria.Like the classical and ve path hanne gctoLPoteinMCporCD4,haeasimilartnucture ent acu and membrane regulatory proteins. cont lvcoprotein that contains four SCRs followed by a serine threonine/proline(STP)domain.The C-terminal portion ends with tha pecomes anchore serve as the cell ENCYCLOPEDIA OF LIFE SCIENCES/2001 Nature Publishing Group/www.els.net

binding of C3 and C4 to hydroxyl groups. This binding preference is clearly illustrated in the case of mammalian C4; in humans C4 exists as two isoforms, C4A and C4B. Although the two molecules have very few amino acid differences (13 substitutions in 1722 residues), C4A (which contains an Asp instead of the His residue) binds preferentially to surfaces carrying amino groups, whereas C4B (which contains the His) binds to those containing hydroxyl groups. It is noteworthy that, unlike any other species, the C3 molecule of teleost fish is encoded by various genes and that the multiple C3 proteins present in an individual fish have the ability to bind with different specificities to a variety of complement-activating surfaces. It has been hypothesized that this functional and structural C3 diversity allows these animals to expand their innate immune recognition capabilities. C3, C4 and C5 are cleaved by their respective convertases to produce C3a, C4a and C5a anaphylotoxins, and both C3 and C4 are inactivated by factor I in the presence of cofactors to generate similar cleavage products. Except for the C-terminal region, the overall structure and gene organization of C3, C4 and C5 very much resemble that of the serum proteinase inhibitor a2- macroglobulin, suggesting that the ancestral molecule from which the three complement components were derived might have been originated by gene duplication from an ancestral a2-macroglobulin-like molecule. Late components All three pathways of activation converge to the production of a C5 convertase which cleaves C5 to C5b and C5a, with the C5b initiating the self assembly of the membrane attack complex (MAC). The MAC is a supramolecular organization of molecules that contains C5b, C6, C7, C8, together with numerous molecules of C9, and is responsible for creating the transmembrane channels that lead to cell lysis. The assembly of MAC and its insertion into the cell membrane occurs by the following sequence of events. The C5b–C6 complex binds C7 and exposes hydrophobic sites that are concealed within the molecule. This allows insertion of the C5b–C7 complex into a discrete site on target membrane and it now serves as a receptor for C8. The C5b–C8 complex can then bind multiple molecules of C9(n= 1–18) and the polymerization of C9molecules results in the membrane lesions that are characteristic of MAC. Although polymerization of C9is not essential for lysis of erythrocytes and nucleated cells, it is believed to be required for the killing of bacteria. Like the classical and alternative pathways of complement activation, channel formation by the MAC is also under the control of serum and membrane regulatory proteins. Regulatory Proteins Serum-soluble proteins C1 inhibitor (C1-inh) regulates the classical pathway by blocking the proteolytic activity of C1r and C1s subcomponents of C1. It is part of the serine protease inhibitor or serpin superfamily and lacks any characteristic modular structure. C4b-binding protein (C4bpA and C4bpB) regulates the formation of the classical pathway C3 convertase by serving as a cofactor in the inactivation of C4b by factor I. There are three isoforms of C4bp. The major form is composed of seven a chains (each 70 kDa) and one b chain (75 kDa) that are linked by disulfide bonds to form a spider-like structure. As is true of many of the regulatory C3b- and C4b-binding proteins, C4bp also contains SCRs. The a and b chains are composed of eight and three SCRs, respectively. Factor H is a single polypeptide chain of 150 kDa composed of 20 SCRs. It downregulates amplification of the alternative pathway by binding either to surface-bound or fluid-phase C3b and accelerating the decay of the C3b,Bb convertase, or by acting as a cofactor for factor I. Properdin is required for the stabilization of the C3b,Bb convertase, allowing rapid amplification of surface-bound C3b. It contains six thrombospondin repeat type 1 (TSP1) domains. Factor I is an 88-kDa serine protease with an unusual structure; from its N-terminus to its C-terminus it is composed of a factor I/membrane attack complex (FIMAC) module, a scavenger receptor cysteine-rich (SRCR) module, two low-density lipoprotein receptor class A (LDLRA) modules and a serine protease domain. Factor I cleaves C3b and C4b (in the presence of various cofactor molecules) at several positions in the molecules. S-protein (vitronectin) and clusterin (SP-40-40) are soluble plasma inhibitors of MAC formation that bind to the C5b-7 complexes and prevent their insertion into cell membranes. S-protein is a glycoprotein that contains a fibronectin-like integrin-binding region and a heparanbinding domain. Clusterin is a heterodimeric protein with a thrombospondin-like N-terminal domain. Carboxypeptidase N, also termed anaphylatoxin inactivator, inactivates C3a, C4a and C5a by removing their Cterminal arginyl residue. Membrane-bound proteins Decay-accelerating factor (DAF or CD55) and membrane cofactor protein (MCP or CD46) have a similar structure, and both downregulate complement activation to protect self tissue from attack by complement. DAF is a 75-kDa glycoprotein that contains four SCRs followed by a serine/ threonine/proline (STP) domain. The C-terminal portion ends with 24 hydrophobic residues that serve as a membrane anchor. DAF becomes anchored to the cell Complement 6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

Complement membrane via a covalent linkage to a glycosylphosphati- to ic3b.C3dg and C3d fragments of C3.It also binds to dylinositol(GPD)and may be involved in signal transduc gp350.an Epstein-Barr virus envelope glycoprotein,and tion.The role of DAF is to d the binding of the ell surtg CZa or iva 45 to 75kDa.depending on the degree of glycosylation. erminal centres.CR2is implicated in the regulation of B cell responses and is involved in antibody responses to T- cell-dependent and-independent a -me DaF MCP is devoid of ded accelerating activity g to the leucocyte-integrin family CD59 is an 18-20-kDa GPI-linked membrane proteir s not contain any SCR modules which bindstond C and inhibits the formation ofthe (LFA-Dand(CR).whereas (CD11b)is a type I transmembrane glycoprotein that is valentlv with the B chain.CR3 binds Complement Receptors to the form of C3 in a diy C1q receptors h ide.zymosan.soluble Fey receptor Leis/imania s have be ops on an A (SP A interacts with cla roxide reseh gqR characterized receptor,is a Complement receptor type 4(CR4.CDIle/CD18)is kDa type I membr From the N adhesion EGF-like modules and s very s a cytoplasmic tyrosine phosphorylation motif that is role of CRis not clear.but its prop rties may be similar to activation signals those of CR3 MBL orSP-A-cmtaining C3a and C5a anaphylotoxin receptors are members of or involved in C3/C4 receptors release of lysosomal contents CD35)is a type presents four different allotypes with different molecula Functions of the Complement System 5 y as a tor for of the s biomp ent syst pManhi ngly than doe system plays very important roles in the following. Opsonization of foreign material (1)As a regu iatedry prote This s involves the c and it also has decay. activity for the C3 convertase.(2)As a receptor,CRI promotes the binding The opsonized surface becomes a target that can then be and phagocytosis of C3 and C4 phagocytic cel and also invo arance or type 2(CR2.CD21 or C3d/EBV receptor)is a 140-kDa membrane glycoprotein that binds ENCYCLOPEDIA OF LIFE SCIENCES/62001 N www.els.net >

membrane via a covalent linkage to a glycosylphosphatidylinositol (GPI) and may be involved in signal transduction. The role of DAF is to dissociate the C3 and C5 convertases by releasing C2a or Bb from the convertases. MCP is a single-chain glycoprotein that ranges in size from 45 to 75 kDa, depending on the degree of glycosylation. MCP is present in four different isoforms in humans. Like DAF,MCP contains four SCRs, but it differs from DAF in that it serves as a cofactor for factor I-mediated cleavage of C3b and C4b deposited on self tissue; also in contrast to DAF, MCP is devoid of decay-accelerating activity. CD59is an 18–20-kDa GPI-linked membrane protein which binds to C8 and C9and inhibits the formation of the MAC on host cells. Complement Receptors C1q receptors Three molecules have been shown to act as C1q receptors. cC1qR is a calreticulin-like molecule that also binds to MBL or surfactant protein A (SP-A). C1qR02 only interacts with C1q and appears to trigger superoxide production. C1qRp, the best characterized receptor, is a 66.5-kDa type I membrane glycoprotein. From the Nterminus to C-terminus C1qRp is composed of a C-type lectin domain (CRD domain), five EGF-like modules, and a cytoplasmic tyrosine phosphorylation motif that is involved in transducing cellular activation signals. C1qRp is also important in mediating phagocytic processes following binding of C1q, MBL or SP-A-containing complexes. C3/C4 receptors Complement receptor type 1 (CR1 or CD35) is a type I transmembrane glycoprotein of 220 kDa that contains 30 SCRs and is present in a wide variety of cells (Table 2). CR1 presents four different allotypes with different molecular masses of 160 (C form), 190 (A form), 220 (B form) and 250 (D form) kDa. It functions mainly as a receptor for C3b and C4b, although it binds with lesser affinity to iC3b and C3c. Multivalent C3b binds more strongly than does monovalent C3b, a difference which might have physiological relevance for the CR1-mediated functions. CR1 has two functions: (1) As a regulatory protein, it serves as a cofactor for the factor I-mediated cleavage of C3b or C4b, and it also has decay-accelerating activity for the C3 convertase. (2) As a receptor, CR1 promotes the binding and phagocytosis of C3b- and C4b-coated particles by phagocytic cells, and is also involved in the clearance of immune complexes. Complement receptor type 2 (CR2, CD21 or C3d/EBV receptor) is a 140-kDa membrane glycoprotein that binds to iC3b, C3dg and C3d fragments of C3. It also binds to gp350, an Epstein–Barr virus envelope glycoprotein, and mediates the binding of the virions at the cell surface. CD23, a low-affinity receptor for IgE, also binds to CR2, an interaction that may influence the survival of B cells in germinal centres. CR2 is implicated in the regulation of Bcell responses and is involved in antibody responses to Tcell-dependent and -independent antigens. Complement receptor type 3 (CR3, CD11b/CD18), with a 170-kDa a chain and a 95-kDa b chain, is an adhesion molecule belonging to the leucocyte–integrin family. Unlike CR1 or CR2, it does not contain any SCR modules. The b chain of CR3 (CD18) is common to other members of the leucocyte–integrin family such as CD11a/CD18 (LFA-1) and CD11c/CD18 (CR4), whereas the a chain (CD11b) is a type I transmembrane glycoprotein that is associated noncovalently with the b chain. CR3 binds specifically to the iC3b form of C3 in a divalent cationdependent manner. It also binds to several other molecules, such as coagulation factor X, fibrinogen, lipopolysaccharide, zymosan, soluble Fcg receptor III, Leishmania promastigote surface glycoprotein gp63, and others. When iC3b serves as the ligand, CR3 mediates opsonization and phagocytosis of microorganisms, as well as enhancement of natural killer (NK) cell activity for C3-coated targets. Complement receptor type 4 (CR4, CD11c/CD18) is also an adhesion molecule of the leucocyte–integrin family. It is very similar to CR3, and it also binds iC3b in a divalent cation-dependent manner. The physiological role of CR4 is not clear, but its properties may be similar to those of CR3. C3a and C5a anaphylotoxin receptors are members of the superfamily of rhodopsin-type receptors, which contain seven transmembrane loops. C3aR (95 kDa) and C5aR (43 kDa) are involved in a variety of processes, including chemotaxis, cell aggregation and adhesion, and release of lysosomal contents. Functions of the Complement System Activation of the complement system results in the initiation of various biological processes. The complement system plays very important roles in the following. Opsonization of foreign material This process involves the covalent binding of C3b and/or C4b to viruses, bacteria, fungi and other microorganisms. The opsonized surface becomes a target that can then be recognized and engulfed by phagocytic leucocytes bearing C3b or C4b receptors on their surfaces. Complement ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 7

Table 2 Complement receptors CP, classical pathway; AP, alternative pathway; RCA, regulators of complement activation; ADCC, antibody-dependent cellular cytotoxicity; NK, natural killer; DAF, decay-accelerating factor; MCP, membrane cofactor protein. Protein Specificity Structure Cell type(s) Key features Cell surface proteins CR1 C3b, C4b, iC3b, C3c 4 allotypes: 160 kDa 190 kDa 220 kDa 250 kDa Erythrocytes, eosinophils, monocytes, macrophages, neutrophils, B and some T lymphocytes, glomerular podocytes, follicular dendritic cells, mast cells, polymorphonuclear cells Member of RCA family Accelerates dissociation of CP and AP C3 convertases Cofactor for factor I Helps to process immune complexes Involved in phagocytosis CR2 iC3b, C3dg, EBV gp350 140 kDa B cells, T cells, follicular dendritic cells Member of RCA family Plays a role in immunoregulation CR3 iC3b, C3dg 170 kDa α chain 95 kDa β chain Polymorphonuclear cells, monocytes, natural killer cells, some B and T lymphocytes Member of the leucocyte–integrin family Involved in phagocytosis of iC3b-coated particles, adhesion of neutrophils, cytotoxicity of cells bearing activated complement components CR4 (p150, 95) iC3b 150 kDa α chain 95 kDa β chain Monocytes, macrophages, NK and ADCC effector lymphocytes, neutrophils Functions in cell adhesion C3aR C3a, C4a 95 kDa Mast cells, neutrophils, basophils, monocytes, T lymphocytes, eosinophils Depending on cell type, functions include chemotaxis, chemokinesis, cell aggregation and adhesion, release of lysosomal contents, may play a role in immunoregulation C5aR C5a 43 kDa Neutrophils, eosinophils, monocytes, macrophages, liver parenchymal cells, lung vascular smooth muscle and endothelial cells, bronchial and alveolar epithelial cells, astrocytes Depending on cell type, functions include directed chemotaxis, cell adhesion and aggregation, release of granular enzymes and histamine Augments the humoral and cellular responses C3eR C3dk, C3e Not characterized Neutrophils, monocytes Lysosomal enzyme release, leucocytosis DAF C3b,Bb, C4b,2a 75 kDa Erythrocytes, all leucocytes, platelets Accelerates decay of CP and AP C3 convertases MCP C3b, iC3b, C4b 45–70 kDa Neutrophils, monocytes, platelets, reticulocytes, most lymphocytes, granulocytes, endothelial cells, epithelial cells, mesenchymal cells Member of RCA family Cofactor for factor I Does not accelerate decay of C3 convertases CD59 C8, C9 18–20 kDa Inhibits MAC on host cells Undefined C3 (β chain) Neutrophils, eosinophils Eosinophil cytotoxicity inhibitor Inhibitor of neutrophil adherence Complement 8 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net

Complement Lysis of foreign material CR2) present on many ant including B lymp After deposition of C3b or C4b onto the surface of a present them on MHC class II molecules to T cells microorganism.the microorganism can be lysed by the insertion of the MAC into its cell membrane.This process Complement is important for the formation of memory B the of the cell lipid membrane bilayer cells,as has been shown by the CR2-mediated localization rupt killing the microorganism by osmotic lysis and retention of immune complexes by germinal centres.It has also been demonstrated that coligation of the C21(CR2)/CD19/TAPA-1 corece Pto with the B-cell Inflammatory processes ant ugen recepto (BCR)lowers 10-to 100-fold the Complement activation results in the generation of the quantity of antigen required for activation. example,when an antigen is coupled to the C3d fragment C3a,C4a and C5a anaphylotoxins.The effect of these (the protein of C3 containing the thioester site),the molecules is mediated through specific receptors present on antibody response is dependent upon binding of C3d to the surface of various cell types(Table 2).The overall role of CR2 and is greatly enhanced:in fact.C3d acts as a natural these anaphylotoxins is to recruit cells that play particular adjuvant,bridging the innate with the acquired immune roles in inflammation.as well as to trigger their r gh CR2.Comple ase in vascular ment,furthe C3a,C4a and C5a generally induce a dsmothmlciecogce permeability(all to reach the e place of injury) tolerance. e contraction.Activation of mast cells by these three anaphylotoxins results in the release of various mediators with similar activities.C5a is much more active than C3a and C4a and induces degranulation and respiratory burst activation in neutrophils.In addition. C5a pr notes the production of prostaglandins and Complement Deficiencies ds Deficiencies of specific complement components have been identified in the classical,alternative and terminal path- Solubilization and clearance of immune ways,as well as in regulatory proteins and complement complexes receptors. Clearance of immune complexes is necessary to preven damage to autol gous tiss e resulting from Deficiencies in classical pathway complement act vation. fcomplement components pathway inhibits the fo rmation of precipitating antigen- Deficiencies in Clq.CIr,Cls,C2(the most common)or C4 antibody complexes.The alternative pathway is respon- sible for the solubilization of precipitated antigen-anti produce deficiencies in classical pathway activation.A body complexes.In humans the immune complexes with common disorder that is associated with of all these deposited C3 are cleared from the circulation by erythro- deficiencies is the autoimmune disease systemic lupus cytes through binding to CRI present on their surfaces erythematosus(SLE).SLE is thought to develop as Ma consequenc f the defe from the liv ct in cle mpiexe on with cla erythrocytes without affecting the erythrocyte integrity.In eficiencies are not nece nly correlate with increased infections,implying that the alternative some autoimmune diseases(such as lupus erythematosus) the formation and deposition of immune complexes can be and/or lectin pathways are sufficient for the elimination of massive,and in such cases the action of complement can most foreign microorganisms.However,deficiencies in C3 are normally associated with a higher susceptibility to damage the surface of the cells on which the complexes are nfection In addition c3 deficienc s are also associated present. with olom rulonephritis,a patholo gic condition charac ter zed by kidr ney damage resulti from compleme Bridging innate and adaptive immune activation that has been stimulated by the presence of responses immune complexes in the basement membranes of blood vessels in the renal glomerulus.This pathology refects the Complement plays a fundamental role in mediating and importance of C3 in immune complex clearance.In enhancing humoral immunity.As a result of complement general.C3 deficiencies (and any deficiency that results in n for ire C3b or C3 fra ag a defect in C3activation n or depositio on forei rticles' are bound their surfaces the particles are taken up by complement receptors(C l-independent antigens ENCYCLOPEDIA OF LIFE SCIENCES/2001 Nature Publishing Group/www.els.net 9

Lysis of foreign material After deposition of C3b or C4b onto the surface of a microorganism, the microorganism can be lysed by the insertion of the MAC into its cell membrane. This process disrupts the integrity of the cell lipid membrane bilayer, killing the microorganism by osmotic lysis. Inflammatory processes Complement activation results in the generation of the C3a, C4a and C5a anaphylotoxins. The effect of these molecules is mediated through specific receptors present on the surface of various cell types (Table 2). The overall role of these anaphylotoxins is to recruit cells that play particular roles in inflammation, as well as to trigger their responses. C3a, C4a and C5a generally induce an increase in vascular permeability (allowing cells to reach the place of injury) and smooth muscle contraction. Activation of mast cells by these three anaphylotoxins results in the release of various mediators with similar activities. C5a is much more active than C3a and C4a and induces degranulation and respiratory burst activation in neutrophils. In addition, C5a promotes the production of prostaglandins and eicosanoids. Solubilization and clearance of immune complexes Clearance of immune complexes is necessary to prevent damage to autologous tissue resulting from complement activation. Activation of complement through the classical pathway inhibits the formation of precipitating antigen– antibody complexes. The alternative pathway is responsible for the solubilization of precipitated antigen–antibody complexes. In humans the immune complexes with deposited C3 are cleared from the circulation by erythrocytes through binding to CR1 present on their surfaces. Macrophages from the liver and spleen remove and degrade the complexes present on the surface of the erythrocytes without affecting the erythrocyte integrity. In some autoimmune diseases (such as lupus erythematosus) the formation and deposition of immune complexes can be massive, and in such cases the action of complement can damage the surface of the cells on which the complexes are present. Bridging innate and adaptive immune responses Complement plays a fundamental role in mediating and enhancing humoral immunity. As a result of complement activation, foreign particles acquire C3b or C3 fragments which are covalently bound to their surfaces. These particles are taken up by complement receptors (CR1, CR2) present on many antigen-presenting cells (APCs), including B lymphocytes, which process the antigens and present them on MHC class II molecules to T cells. Complement is important for the formation of memory B cells, as has been shown by the CR2-mediated localization and retention of immune complexes by germinal centres. It has also been demonstrated that coligation of the C21(CR2)/CD19/TAPA-1 coreceptor with the B-cell antigen receptor (BCR) lowers by 10- to 100-fold the quantity of antigen required for B-cell activation. For example, when an antigen is coupled to the C3d fragment (the protein of C3 containing the thioester site), the antibody response is dependent upon binding of C3d to CR2 and is greatly enhanced; in fact, C3d acts as a natural adjuvant, bridging the innate with the acquired immune response, through CR2. Complement, furthermore, has also been found to play an important role in B-cell tolerance. Complement Deficiencies Deficiencies of specific complement components have been identified in the classical, alternative and terminal pathways, as well as in regulatory proteins and complement receptors. Deficiencies in classical pathway complement components Deficiencies in C1q, C1r, C1s, C2 (the most common) or C4 produce deficiencies in classical pathway activation. A common disorder that is associated with of all these deficiencies is the autoimmune disease systemic lupus erythematosus (SLE). SLE is thought to develop as a consequence of the defect in clearing immune complexes that occur in individuals with classical pathway deficiencies. C2 and C4 deficiencies are not necessarily correlated with increased infections, implying that the alternative and/or lectin pathways are sufficient for the elimination of most foreign microorganisms. However, deficiencies in C3 are normally associated with a higher susceptibility to infection. In addition, C3 deficiencies are also associated with glomerulonephritis, a pathologic condition characterized by kidney damage resulting from complement activation that has been stimulated by the presence of immune complexes in the basement membranes of blood vessels in the renal glomerulus. This pathology reflects the importance of C3 in immune complex clearance. In general, C3 deficiencies (and any deficiency that results in a defect in C3 activation or deposition on foreign particles), produce an impairment of the immune response to T-celldependent and T-cell-independent antigens. Complement ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 9

Complement Deficiencies in alternative pathway of C3 from plasma,as a result of a continuous formation of fluid-phase C3 convertase.Consequently,individuals These deficiencies are less common than those of classical become more susceptible to infection by pyogenic bacteria. pathway components.Deficiencies in properdin (the most Impairment in the clearance of immune complexes in these common of the alternative pathway deficiencies)and deficiencies leads to glomerulonephritis. factor D result in abnormal activation of the alternative Deficiencies in membrane regulators such as DAF and pathway.Recurrent infections are not common in CD59 produce a deregulation of C3 convertase activity individuals with a deficiency of only one protein of the and a higher susceptibility of erythrocytes to complement- alternative pathway,but have been observed in individuals mediated lysis.Paroxysmal nocturnal haemoglobinuria with various factor D deficiencies.Meningococcal infec- (PNH)is a disease associated with DAF and CD59 tions are the most frequently detected in alternative deficiencies and is characterized by erythrocyte lysis pathway deficiencies. throughout the vascular system,leading to chronic haemolytic anaemia and venous thrombosis.among other Deficiencies in late components disorders.Deficiencies in CR3 and CR4 are associated with the disease known as leucocyte adhesion deficiency,which Deficiencies of any of the late complement components results in recurrent pyogenic infections. leads to inability to form the MAC,which results in failure to kill foreign pathogens by complement-mediated lysis. The infections most frequently associated with deficiencies of late components,with the exception of C9,are Further Reading meningococcal or gonococcal infections. Carroll MC(1998)The role of complement and complement receptors in Deficiencies in complement regulatory induction and regulation of immunity.Annual Review of Immunology 16:545-568. proteins and complement receptors Lambris JD(ed.)(1990)The third component of complement.Current Topics in Microbiology and Immunology 153:1-251. Deficiencies in Cl inhibitor,factor I and factor H are Sahu A.Sunyer JO,Moore WT,Souvlika A.Sarrias MR and Lambris commonly associated with regulation problems in the JD(1998)Structure,functions,and evolution of the third complement alternative or classical pathways of complement activa- component and viral molecular mimicry.Immunological Research 17: tion,and in C3 consumption.Cl inhibitor deficiency is 109-121. associated with the development of hereditary angioneuro- Sunyer JO,Zarkadis IK and Lambris JD(1998)Complement diversity:a mechanism for generating immune diversity?Immrology Today 19: tic oedema,which is characterized by the accumulation of 519-523. oedema fluid in skin and mucosa.Factor I and H Volanakis JE and Frank M(eds)(1998)The Human Complement System deficiencies are characterized by a complete consumption in Health and Disease,Ist edn.New York:Marcel Dekker. 10 ENCYCLOPEDIA OF LIFE SCIENCES/2001 Nature Publishing Group/www.els.net

Deficiencies in alternative pathway These deficiencies are less common than those of classical pathway components. Deficiencies in properdin (the most common of the alternative pathway deficiencies) and factor D result in abnormal activation of the alternative pathway. Recurrent infections are not common in individuals with a deficiency of only one protein of the alternative pathway, but have been observed in individuals with various factor D deficiencies. Meningococcal infections are the most frequently detected in alternative pathway deficiencies. Deficiencies in late components Deficiencies of any of the late complement components leads to inability to form the MAC, which results in failure to kill foreign pathogens by complement-mediated lysis. The infections most frequently associated with deficiencies of late components, with the exception of C9, are meningococcal or gonococcal infections. Deficiencies in complement regulatory proteins and complement receptors Deficiencies in C1 inhibitor, factor I and factor H are commonly associated with regulation problems in the alternative or classical pathways of complement activation, and in C3 consumption. C1 inhibitor deficiency is associated with the development of hereditary angioneurotic oedema, which is characterized by the accumulation of oedema fluid in skin and mucosa. Factor I and H deficiencies are characterized by a complete consumption of C3 from plasma, as a result of a continuous formation of fluid-phase C3 convertase. Consequently, individuals become more susceptible to infection by pyogenic bacteria. Impairment in the clearance of immune complexes in these deficiencies leads to glomerulonephritis. Deficiencies in membrane regulators such as DAF and CD59produce a deregulation of C3 convertase activity and a higher susceptibility of erythrocytes to complementmediated lysis. Paroxysmal nocturnal haemoglobinuria (PNH) is a disease associated with DAF and CD59 deficiencies and is characterized by erythrocyte lysis throughout the vascular system, leading to chronic haemolytic anaemia and venous thrombosis, among other disorders. Deficiencies in CR3 and CR4 are associated with the disease known as leucocyte adhesion deficiency, which results in recurrent pyogenic infections. Further Reading Carroll MC (1998) The role of complement and complement receptors in induction and regulation of immunity. Annual Review of Immunology 16: 545–568. Lambris JD (ed.) (1990) The third component of complement. Current Topics in Microbiology and Immunology 153: 1–251. Sahu A, Sunyer JO, Moore WT, Souvlika A, Sarrias MR and Lambris JD (1998) Structure, functions, and evolution of the third complement component and viral molecular mimicry. Immunological Research 17: 109–121. Sunyer JO, Zarkadis IK and Lambris JD (1998) Complement diversity: a mechanism for generating immune diversity? Immunology Today 19: 519–523. Volanakis JE and Frank M (eds) (1998) The Human Complement System in Health and Disease, 1st edn. New York: Marcel Dekker. Complement 10 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net