《免疫学》（英文版） Chapter 13 The Complement System

chapter 13 The Complement System HE COMPLEMENT SYSTEM IS THE MAJOR EFFECTOR of the humoral branch of the immune system Research on complement began in the 1890s, when Jules bordet at the institut pasteur in paris showed that sheep antiserum to the bacterium vibrio cholerae caused lysis of the bacteria and that heating the antiserum destroyed its bacteriolytic activity. Surprisingly, the ability to lyse the bacteria was restored to the heated serum by adding fresh serum that contained no antibodies directed a The Functions of Complement against the bacterium and was unable to kill the bacterium by itself. Bordet correctly reasoned that bacteriolytic activ- a The Complement Components y requires two different substances: first, the specific an- n Complement Activation tibacterial antibodies, which survive the heating process, and a second, heat-sensitive component responsible for the a Regulation of the Complement System lytic activity. Bordet devised a simple test for the lytic ac- Biological Consequences of Complement tivity, the easily detected lysis of antibody-coated red blood Activation lls, called hemolysis. Paul Ehrlich in Berlin indepen- dently carried out similar experiments and coined the term a Complement Deficiencies complement, defining it as"the activity of blood serum that completes the action of antibody. In ensuing years, re- searchers discovered that the action of complement wa the result of interactions of a large and complex group of 3 This chapter describes the complement components and receptors with complement proteins controls B-cell activi- heir activation pathways, the regulation of the complement ties gives this system a role in the highly developed acquired system, the effector functions of various complement com- immune system. Thus we have a system that straddles in- ponents, and the consequences of deficiencies in them. a nate and acquired immunity, contributing to each in a vari Clinical Focus section describes consequences of a defect in ety of ways proteins that regulate complement activity. After initial activation, the various complement compo- nents interact, in a highly regulated cascade, to carry out a number of basic functions(Figure 13-1)including a Lysis of cells, bacteria, and viruses The Functions of Complement Opsonization, which promotes phagocytosis of Research on complement now includes more than 30 solu particulate antigens ble and cell-bound proteins. The biological activities of this system affect both innate and acquired immunity a Binding to specific complement receptors on cells of reach far beyond the original observations of antibody he immune system, triggering specific cell functions, mediated lysis of bacteria and red blood cells. Structural nflammation. and secretion of comparisons of the proteins involved in complement path- rays place the origin of this system in primitive organisms Immune clearance, which immune complexes from the circulation and deposits them in the spleen and

receptors with complement proteins controls B-cell activi￾ties gives this system a role in the highly developed acquired immune system. Thus we have a system that straddles in￾nate and acquired immunity, contributing to each in a vari￾ety of ways. After initial activation, the various complement compo￾nents interact, in a highly regulated cascade, to carry out a number of basic functions (Figure 13-1) including: ■ Lysis of cells, bacteria, and viruses ■ Opsonization, which promotes phagocytosis of particulate antigens ■ Binding to specific complement receptors on cells of the immune system, triggering specific cell functions, inflammation, and secretion of immunoregulatory molecules ■ Immune clearance, which removes immune complexes from the circulation and deposits them in the spleen and liver chapter 13 ■ The Functions of Complement ■ The Complement Components ■ Complement Activation ■ Regulation of the Complement System ■ Biological Consequences of Complement Activation ■ Complement Deficiencies The Complement System T       of the humoral branch of the immune system. Research on complement began in the 1890s, when Jules Bordet at the Institut Pasteur in Paris showed that sheep antiserum to the bacterium Vibrio cholerae caused lysis of the bacteria and that heating the antiserum destroyed its bacteriolytic activity. Surprisingly, the ability to lyse the bacteria was restored to the heated serum by adding fresh serum that contained no antibodies directed against the bacterium and was unable to kill the bacterium by itself. Bordet correctly reasoned that bacteriolytic activ￾ity requires two different substances: first, the specific an￾tibacterial antibodies, which survive the heating process, and a second, heat-sensitive component responsible for the lytic activity. Bordet devised a simple test for the lytic ac￾tivity, the easily detected lysis of antibody-coated red blood cells, called hemolysis. Paul Ehrlich in Berlin indepen￾dently carried out similar experiments and coined the term complement, defining it as “the activity of blood serum that completes the action of antibody.” In ensuing years, re￾searchers discovered that the action of complement was the result of interactions of a large and complex group of proteins. This chapter describes the complement components and their activation pathways, the regulation of the complement system, the effector functions of various complement com￾ponents, and the consequences of deficiencies in them. A Clinical Focus section describes consequences of a defect in proteins that regulate complement activity. The Functions of Complement Research on complement now includes more than 30 solu￾ble and cell-bound proteins. The biological activities of this system affect both innate and acquired immunity and reach far beyond the original observations of antibody￾mediated lysis of bacteria and red blood cells. Structural comparisons of the proteins involved in complement path￾ways place the origin of this system in primitive organisms possessing the most rudimentary innate immune systems. By contrast, the realization that interaction of cellular Poly-C9 Complex ART TO COME

300 paRt I Immune Effector Mechanisms LYSIS OPSONIZATION ACTIVATION OF INFLAMMATORY LEARANCE OF RESPONSE IMMUNE COMPLEXES Bacteria Complement complex Extravasation Blood Target cell P FIGURE 13-1 The multiple activities of the complement system. phagocytes: activation of inflammatory responses; and clearance of Serum complement proteins and membrane-bound complement circulating immune complexes by cells in the liver and spleen receptors partake in a number of immune activities: lysis of foreign Soluble complement proteins are schematically indicated by a trian- cells by antibody-dependent or antibody-independent pathways: gle and receptors by a semi-circle; no attempt is made to differenti opsonization or uptake of particulate antigens, including bacteria, by ate among individual components of the complement system here C5b, can occur by the classical pathway, the alternative The Complement Components pathway, or the lectin pathway. The final steps that lead to a membrane attack are the same in all pathways The proteins and glycoproteins that compose the complement system are synthesized mainly by liver hepatocytes, although signilicant amounts are also produced by blood monocytes, tis. The Classical Pathway Begins with sue macrophages, and epithelial clls of the gastrointestinal and Antigen-Antibody Binding weight) of the serum globulin fraction. Most circulate in the Complement activation by the classical pathway commonly serum in functionally inactive forms as proenzymes, or zymo- begins with the formation of soluble antigen-antibody com gens, which are inactive until proteolytic cleavage, which re- plexes(immune complexes)or with the binding of antibody moves an inhibitory fragment and exposes the active site. The to antigen on a suitable target, such as a bacterial cell IgM and complement-reaction sequence starts with an enzyme cascade. certain subclasses of IgG(human IgG1, IgG2, and igG3)can Complement components are designated by numerals activate the classical complement pathway. The initial stage of (C1-C9), by letter symbols (e.g, factor D), or by trivial activation involves C1, C2, C3, and CA, which are present in names(e.g, homologous restriction factor). Peptide frag- plasma in functionally inactive forms. Because the compo- ments formed by activation of a component are denoted by nents were named in order of their discovery and before their small letters. In most cases, the smaller fragment resulting functional roles had been determined, the numbers in their from cleavage of a component is designated"a"and the larger names do not always reflect the order in which they react. fragment designated"b"(e. g, C3a, C3b: note that C2 is an The formation of an antigen-antibody complex induces exception: C2a is the larger cleavage fragment). The larger conformational changes in the Fc portion of the igM mole fragments bind to the target near the site of activation, and cule that expose a binding site for the cl component of the the smaller fragments diffuse from the site and can initiate complement system. Cl in serum is a macromolecular com localized inflammatory responses by binding to specific re- plex consisting of Clq and two molecules each of Cir and ceptors. The complement fragments interact with one an- Cls, held together in a complex(C1qr2S2)stabilized by Ca other to form functional complexes. Those complexes that ions. The Clq molecule is composed of 18 polypeptide nated by a bar over the num- chains that associate to form six collagen-like triple helical ber or symbol (e.g, C4b2a, arms, the tips of which bind to exposed Clq- binding sites in the Ch2 domain of the antibody molecule(Figure 13-3, page 302). Each Clr and Cls monomer contains a catalytic Complement Activation domain and an interaction domain the latter facilitates in- teraction with Clq or with each other. Figure 13-2 on page 301 outlines the pathways of com Each C1 molecule must bind by its C1q globular heads to ment activation. The early steps, culminating in formation at least two Fc sites for a stable Cl-antibody interaction to

The Complement Components The proteins and glycoproteins that compose the complement system are synthesized mainly by liver hepatocytes, although significant amounts are also produced by blood monocytes, tis￾sue macrophages, and epithelial cells of the gastrointestinal and genitourinary tracts. These components constitute 5% (by weight) of the serum globulin fraction. Most circulate in the serum in functionally inactive forms as proenzymes, or zymo￾gens, which are inactive until proteolytic cleavage, which re￾moves an inhibitory fragment and exposes the active site. The complement-reaction sequence starts with an enzyme cascade. Complement components are designated by numerals (C1–C9), by letter symbols (e.g., factor D), or by trivial names (e.g., homologous restriction factor). Peptide frag￾ments formed by activation of a component are denoted by small letters. In most cases, the smaller fragment resulting from cleavage of a component is designated “a” and the larger fragment designated “b” (e.g., C3a, C3b; note that C2 is an exception: C2a is the larger cleavage fragment). The larger fragments bind to the target near the site of activation, and the smaller fragments diffuse from the site and can initiate localized inflammatory responses by binding to specific re￾ceptors. The complement fragments interact with one an￾other to form functional complexes. Those complexes that have enzymatic activity are designated by a bar over the num￾ber or symbol (e.g., C4b2a, C3bBb). Complement Activation Figure 13-2 on page 301 outlines the pathways of comple￾ment activation. The early steps, culminating in formation of 300 PART III Immune Effector Mechanisms C5b, can occur by the classical pathway, the alternative pathway, or the lectin pathway. The final steps that lead to a membrane attack are the same in all pathways. The Classical Pathway Begins with Antigen-Antibody Binding Complement activation by the classical pathway commonly begins with the formation of soluble antigen-antibody com￾plexes (immune complexes) or with the binding of antibody to antigen on a suitable target, such as a bacterial cell. IgM and certain subclasses of IgG (human IgG1, IgG2, and IgG3) can activate the classical complement pathway. The initial stage of activation involves C1, C2, C3, and C4, which are present in plasma in functionally inactive forms. Because the compo￾nents were named in order of their discovery and before their functional roles had been determined, the numbers in their names do not always reflect the order in which they react. The formation of an antigen-antibody complex induces conformational changes in the Fc portion of the IgM mole￾cule that expose a binding site for the C1 component of the complement system. C1 in serum is a macromolecular com￾plex consisting of C1q and two molecules each of C1r and C1s, held together in a complex (C1qr2s2) stabilized by Ca2 ions. The C1q molecule is composed of 18 polypeptide chains that associate to form six collagen-like triple helical arms, the tips of which bind to exposed C1q-binding sites in the CH2 domain of the antibody molecule (Figure 13-3, on page 302). Each C1r and C1s monomer contains a catalytic domain and an interaction domain; the latter facilitates in￾teraction with C1q or with each other. Each C1 molecule must bind by its C1q globular heads to at least two Fc sites for a stable C1-antibody interaction to FIGURE 13-1 The multiple activities of the complement system. Serum complement proteins and membrane-bound complement receptors partake in a number of immune activities: lysis of foreign cells by antibody-dependent or antibody-independent pathways; opsonization or uptake of particulate antigens, including bacteria, by phagocytes; activation of inflammatory responses; and clearance of circulating immune complexes by cells in the liver and spleen. Soluble complement proteins are schematically indicated by a trian￾gle and receptors by a semi-circle; no attempt is made to differenti￾ate among individual components of the complement system here. Complement receptor Blood Tissue Phagocyte Phagocyte Degranulation Target cell Ag-Ab complex Complement Extravasation Bacteria LYSIS OPSONIZATION ACTIVATION OF INFLAMMATORY RESPONSE CLEARANCE OF IMMUNE COMPLEXES

The Complement System CHAPTER 13 30 Mannose-binding Classical MBL-associated proteases (MASPl+2) bind MBL, Cl-like complex Cl binds gen-antibo Activa OD C3 convertase ①c2a5 C5b step C3 convertase( cabBy c3bBb3b 5 convertase Factor D Alternative (cabl FIGURE 13-2 Overview of the complement activation pathways. brane-attack complex(MAC)by a common sequence of terminal The classical pathway is initiated when C1 binds to antigen-antibody reactions. Hydrolysis of C3 is the major amplification step in all path- complexes. The alternative pathway is initiated by binding of spon- ways, generating large amounts of C3b, which forms part of C5 con- taneously generated C3b to activating surfaces such as microbial cell vertase C3b also can diffuse away from the activating surface and walls. The lectin pathway is initiated by binding of the serum protein bind to immune complexes or foreign cell surfaces, where it func- MBL to the surface of a pathogen. All three pathways generate C3 ons as an opsonin and c5 convertases and bound c5b, which is converted into a mem- occur. When pentameric IgM is bound to antigen on a target providing two attachment sites for Clg. This difference ac- surface it assumes the so-called"staple"configuration, in counts for the observation that a single molecule of igM which at least three binding sites for Clq are exposed. Circu- bound to ared blood cell can activate the classical complement lating IgM, however, exists as a planar configuration in which pathway and lyse the red blood cell while some 1000 mole the Clq-binding sites are not exposed(Figure 13-4, on page cules of igg are required to assure that two lgg molecules are 302)and therefore cannot activate the complement cascade. close enough to each other on the cell surface to initiate Clq An IgG molecule, on the other hand, contains only a single binding Clg-binding site in the Ch2 domain of the Fc, so that firm Clq The intermediates in the classical activation pathway binding is achieved only when two IgG molecules are within depicted schematically in Figure 13-5(page 303). Binding of 30-40 nm of each other on a target surface or in a complex, Clq to Fc binding sites induces a conformational change in (text continues on page 304)

occur. When pentameric IgM is bound to antigen on a target surface it assumes the so-called “staple” configuration, in which at least three binding sites for C1q are exposed. Circu￾lating IgM, however, exists as a planar configuration in which the C1q-binding sites are not exposed (Figure 13-4, on page 302) and therefore cannot activate the complement cascade. An IgG molecule, on the other hand, contains only a single C1q-binding site in the CH2 domain of the Fc, so that firm C1q binding is achieved only when two IgG molecules are within 30–40 nm of each other on a target surface or in a complex, The Complement System CHAPTER 13 301 providing two attachment sites for C1q. This difference ac￾counts for the observation that a single molecule of IgM bound to a red blood cell can activate the classical complement pathway and lyse the red blood cell while some 1000 mole￾cules of IgG are required to assure that two IgG molecules are close enough to each other on the cell surface to initiate C1q binding. The intermediates in the classical activation pathway are depicted schematically in Figure 13-5 (page 303). Binding of C1q to Fc binding sites induces a conformational change in FIGURE 13-2 Overview of the complement activation pathways. The classical pathway is initiated when C1 binds to antigen-antibody complexes. The alternative pathway is initiated by binding of spon￾taneously generated C3b to activating surfaces such as microbial cell walls. The lectin pathway is initiated by binding of the serum protein MBL to the surface of a pathogen. All three pathways generate C3 and C5 convertases and bound C5b, which is converted into a mem￾brane-attack complex (MAC) by a common sequence of terminal reactions. Hydrolysis of C3 is the major amplification step in all path￾ways, generating large amounts of C3b, which forms part of C5 con￾vertase. C3b also can diffuse away from the activating surface and bind to immune complexes or foreign cell surfaces, where it func￾tions as an opsonin. + C4 C2 C3b C3 C5 C5b C9 C8 C7 C6 C3 C3b C3bB Factor B Factor D C4b2a C3bBb C3bBb3b C4b2a3b C5 convertase C5 convertase C3 convertase C3 convertase Activated C1 MBL-associated proteases (MASP1 + 2) bind MBL, generate activated C1-like complex Mannose-binding lectin (MBL) binds foreign surface C1 binds antigen-antibody complex Classical pathway Alternative pathway Lectin pathway Major amplification step Membrane attack complex Spontaneous, slow, small amounts (text continues on page 304)

art 1 Immune effector mechanisms Heads Stalk IGURE 13-3 Structure of the C1 macromolecular complex. (a)Di- alytic domain with enzymatic activity and an interaction domain that agram of C1qr2S2 complex. A C1q molecule consists of 18 polypep- facilitates binding with Clq or with each other. (b)Electron micro- tide chains arranged into six triplets, each of which contains one A, graph of C1q molecule showing stalk and six globular heads /Part (b) one B, and one C chain. Each Clr and Cls monomer contains a cat- from H. R Knobel et al, 1975, Eur. I Immunol. 5: 78/ (b) FIGURE 13-4 Models of pentameric igM in planar form()and bound to flagella, showing the planar form(c)and staple form(d) staple"form(b). Several C1q- binding sites in the Fc region are /From A. Feinstein et al., 1981, Monogr. Allergy, 17: 28, and 1981 accessible in the staple form, whereas none are exposed in the pla- Ann. N.Y. Acad. Sci. 190: 1104. 1 nar form. Electron micrographs of IgM antiflagellum antibody

302 PART III Immune Effector Mechanisms (b) FIGURE 13-3 Structure of the C1 macromolecular complex. (a) Di￾agram of C1qr2s2 complex. A C1q molecule consists of 18 polypep￾tide chains arranged into six triplets, each of which contains one A, one B, and one C chain. Each C1r and C1s monomer contains a cat￾alytic domain with enzymatic activity and an interaction domain that facilitates binding with C1q or with each other. (b) Electron micro￾graph of C1q molecule showing stalk and six globular heads. [Part (b) from H. R. Knobel et al., 1975, Eur. J. Immunol. 5:78.] (a) C1r C1s Stalk Heads C1q FIGURE 13-4 Models of pentameric IgM in planar form (a) and “staple” form (b). Several C1q-binding sites in the Fc region are accessible in the staple form, whereas none are exposed in the pla￾nar form. Electron micrographs of IgM antiflagellum antibody bound to flagella, showing the planar form (c) and staple form (d). [From A. Feinstein et al., 1981, Monogr. Allergy, 17:28, and 1981, Ann. N.Y. Acad. Sci. 190:1104.] (a) (b) (c) (d)

The Complement System cHAPTER 13 VISUALIZING CONCEPTS Clq binds antigen-bound Clr ac The C3b component of C5 convertase binds C5, permitting nd Clr: bot CIs C4b2a to cleave c5 Ciqr252 Antibody clr. C5 convertase Cls cleaves C4 and C2. Cleaving C4 exposes the binding site for c2. C4 binds the surface near Cl and c2 binds C4 forming C3 convertase C5b binds C6, initiating the formation of the membrane- attack C2 C3 convertase C567 C5b678 C3 convertase hydrolyzes many C3 molecules. Some combine ith C3 convertase to form C5 convertase C5b678 Poly-C9 C3a Membrane attack comple C5 convertase FIGURE 13-5 Schematic diagram of intermediates in the classi- attack complex(MAC, bottom right) forms a large pore in th cal pathway of complement activation. The completed membrane- membrane

The Complement System CHAPTER 13 303 VISUALIZING CONCEPTS FIGURE 13-5 Schematic diagram of intermediates in the classi￾cal pathway of complement activation. The completed membrane￾attack complex (MAC, bottom right) forms a large pore in the membrane. Poly-C9 3 5 C5b binds C6, initiating the formation of the membrane-attack complex C1s cleaves C4 and C2. Cleaving C4 exposes the binding site for C2. C4 binds the surface near C1 and C2 binds C4, forming C3 convertase C3 convertase hydrolyzes many C3 molecules. Some combine with C3 convertase to form C5 convertase 4 The C3b component of C5 convertase binds C5, permitting C4b2a to cleave C5 1 C1q binds antigen-bound antibody. C1r activates auto￾catalytically and activates the second C1r; both activate C1s + C3 convertase C5 convertase C5 convertase + C5 C5b C5a C1q FC Antibody C1r2s2 C4b2a C4b2a C4b2a3b C1qr2s2 C4a C2 C2b C4 C3 C3b C3a 2 C9 C5b678 C8 C5b C567 C5b678 C6 C7 Membrane attack complex