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

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