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

ART III Immune Effector mechanisms Clr that converts Clr to an active serine protease enzyme, The Alternative Pathway ls crr, which then cleaves Cls to a similar active enzyme, C Cis has two substrates, CA and C2. The C4 component is a Antibody-Independent glycoprotein containing three polypeptide chains a, B, and y. The alternative pathway generates bound C5b, the same C4 is activated when Cis hydrolyzes a small fragment( C4a) product that the classical pathway generates, but it does so from the amino terminus of the a chain, exposing a binding without the need for antigen-antibody complexes for initia site on the larger fragment(C4b). The CAb fragment attaches tion. Because no antibody is required, the alternative path to the target surface in the vicinity of Cl, and the C2 proen- way is a component of the innate immune system. This yme then attaches to the exposed binding site on C4b, where major pathway of complement activation involves four the C2 is then cleaved by the neighboring CIs; the smaller serum proteins: C3, factor B, factor D, and properdin. The al- fragment( C2b)diffuses away. The resulting C4b2a complex ternative pathway is initiated in most cases by cell-surface is called C3 convertase, referring to its role in converting the constituents that are foreign to the host(Table 13-1).For ex 3 into an active form. The smaller fragment from C4 cleav- ample both gram-negative and gram-positive bacteria have age, C4a, is an anaphylatoxin, or mediator of inflammation, cell-wall constituents that can activate the alternative pat which does not participate directly in the complement cas- way. The intermediates in the alternative pathway for gener cade; the anaphylatoxins, which include the smaller frag- ating C5b are shown schematically in Figure 13-7(page 306) ments of C4. C3 and C5 are described below In the classical pathway, C3 is rapidly cleaved to C3a and The native C3 component consists of two polypeptide C3b by the enzymatic activity of the C3 convertase. In the al hains, a and B. Hydrolysis of a short fragment( C3a) from ternative pathway, serum C3, which contains an unstable the amino terminus of the a chain by the C3 convertase gen- thioester bond, is subject to slow spontaneous hydrolysis to erates C3b(Figure 13-6). A single C3 convertase molecule can yield C3a and C3b. The C3b component can bind to foreign generate over 200 molecules of C3b, resulting in tremendous surface antigens(such as those on bacterial cells or viral par amplification at this step of the sequence. Some of the C3b ticles) or even to the host's own cells(see Figure 13-6c).The binds to CAb2a to form a trimolecular complex C4b2a3b membranes of most mammalian cells have high levels of called C5 convertase. The C3b component of this complex sialic acid, which contributes to the rapid inactivation of binds C5 and alters its conformation, so that the C4b2a com- bound C3b molecules on host cells; consequently this bind ponent can cleave C5 into C5a, which diffuses away, and C5b, ing rarely leads to further reactions on the host cell mem- which attaches to C6 and initiates formation of the membrane- brane. Because many foreign antigenic surfaces(e. g, bac- attack complex in a sequence described later. Some of the terial cell walls, yeast cell walls, and certain viral envelopes) C3b generated by C3 convertase activity does not associate have only low levels of sialic acid, C3b bound to these sur- with C4b2a; instead it diffuses away and then coats immune faces remains active for a longer time. The C3b present on the complexes and particulate antigens, functioning as an opsonin surface of the foreign cells can bind another serum protein s described in the Clinical Focus C3b may also bind directly called factor b to form a complex stabilized by Mg2+Bind- to cell membranes ing to C3b exposes a site on factor B that serves as the sub 0 Cell membrane C4b2a O=C SH Activated c3b FIGURE 13-6 Hydrolysis of C3 by C3 convertase C4b2a(a)Native fragment to bind to free hydroxyl or amino groups(R)on a cell mem- C3. (b)Activated C3 showing site of cleavage by C4b2a resulting in brane. Bound C3b exhibits various biological activities, including production of the C3a and C3b fragments. (c)A labile intemal binding of C5 and binding to C3b receptors on phagocytic cells thioester bond in C3 is activated as C3b is formed, allowing the C3b

C1r that converts C1r to an active serine protease enzyme, C1r, which then cleaves C1s to a similar active enzyme, C1s. C1s has two substrates, C4 and C2. The C4 component is a glycoprotein containing three polypeptide chains ,, and . C4 is activated when C1s hydrolyzes a small fragment (C4a) from the amino terminus of the chain, exposing a binding site on the larger fragment (C4b). The C4b fragment attaches to the target surface in the vicinity of C1, and the C2 proen￾zyme then attaches to the exposed binding site on C4b, where the C2 is then cleaved by the neighboring C1s; the smaller fragment (C2b) diffuses away. The resulting C4b2a complex is called C3 convertase, referring to its role in converting the C3 into an active form. The smaller fragment from C4 cleav￾age, C4a, is an anaphylatoxin, or mediator of inflammation, which does not participate directly in the complement cas￾cade; the anaphylatoxins, which include the smaller frag￾ments of C4, C3, and C5 are described below. The native C3 component consists of two polypeptide chains, and . Hydrolysis of a short fragment (C3a) from the amino terminus of the chain by the C3 convertase gen￾erates C3b (Figure 13-6). A single C3 convertase molecule can generate over 200 molecules of C3b, resulting in tremendous amplification at this step of the sequence. Some of the C3b binds to C4b2a to form a trimolecular complex C4b2a3b, called C5 convertase. The C3b component of this complex binds C5 and alters its conformation, so that the C4b2a com￾ponent can cleave C5 into C5a, which diffuses away, and C5b, which attaches to C6 and initiates formation of the membrane￾attack complex in a sequence described later. Some of the C3b generated by C3 convertase activity does not associate with C4b2a ; instead it diffuses away and then coats immune complexes and particulate antigens, functioning as an opsonin as described in the Clinical Focus. C3b may also bind directly to cell membranes. The Alternative Pathway Is Antibody-Independent The alternative pathway generates bound C5b, the same product that the classical pathway generates, but it does so without the need for antigen-antibody complexes for initia￾tion. Because no antibody is required, the alternative path￾way is a component of the innate immune system. This major pathway of complement activation involves four serum proteins: C3, factor B, factor D, and properdin. The al￾ternative pathway is initiated in most cases by cell-surface constituents that are foreign to the host (Table 13-1). For ex￾ample, both gram-negative and gram-positive bacteria have cell-wall constituents that can activate the alternative path￾way. The intermediates in the alternative pathway for gener￾ating C5b are shown schematically in Figure 13-7 (page 306). In the classical pathway, C3 is rapidly cleaved to C3a and C3b by the enzymatic activity of the C3 convertase. In the al￾ternative pathway, serum C3, which contains an unstable thioester bond, is subject to slow spontaneous hydrolysis to yield C3a and C3b. The C3b component can bind to foreign surface antigens (such as those on bacterial cells or viral par￾ticles) or even to the host’s own cells (see Figure 13-6c). The membranes of most mammalian cells have high levels of sialic acid, which contributes to the rapid inactivation of bound C3b molecules on host cells; consequently this bind￾ing rarely leads to further reactions on the host cell mem￾brane. Because many foreign antigenic surfaces (e.g., bac￾terial cell walls, yeast cell walls, and certain viral envelopes) have only low levels of sialic acid, C3b bound to these sur￾faces remains active for a longer time. The C3b present on the surface of the foreign cells can bind another serum protein called factor B to form a complex stabilized by Mg2. Bind￾ing to C3b exposes a site on factor B that serves as the sub- 304 PART III Immune Effector Mechanisms FIGURE 13-6 Hydrolysis of C3 by C3 convertase C4b2a (a) Native C3. (b) Activated C3 showing site of cleavage by C4b2a resulting in production of the C3a and C3b fragments. (c) A labile internal thioester bond in C3 is activated as C3b is formed, allowing the C3b fragment to bind to free hydroxyl or amino groups (R) on a cell mem￾brane. Bound C3b exhibits various biological activities, including binding of C5 and binding to C3b receptors on phagocytic cells. (a) (c) (b) C O S S S S S S S S S S S S S α β C3 C4b2a C3a C + O – S Activated C3b C O SH R O Bound C3b Cell membrane

The Complement System cHAPTER 13 305 Initiators of the alternative pathway ment binds to mannose residues, some authors designate this TABLE 13-1 of complement activation the MBLectin pathway or mannan-binding lectin pathway. The lectin pathway, like the alternative pathway, does not de- PATHOGENS AND PARTICLES OF MICROBIAL ORIGIN pend on antibody for its activation. However, the mechanism is more like that of the classical pathway, because after initia Many strains of gram-negative bacteria tion, it proceeds, through the action of C4 and C2, to pro- Lipopolysaccharides from gram-negative bacteria duce a C5 convertase(see Figure 13-2) Many strains of gram-positive bacteria The lectin pathway is activated by the binding of man- nose-binding lectin (MBL) to mannose residues on glyce proteins or carbohydrates on the surface of microorganisms Fungal and yeast cell walls(zymosa including certain Salmonella, Listeria, and Neisseria strains, Some viruses and virus-infected cells ell as Cryptococcus neoformans and Candida albicans. Some tumor cells(Raji) MBL is an acute phase protein produced in inflammatory Its function in the way is sin Parasites(trypanosomes to that of C1q, which it resembles in structure. After MBL NONPATHOGENS binds to the surface of a cell or pathogen, MBL-associated serine proteases, MASP-1 and MASP-2, bind to MBL. The ac Human igG, IgA and ige in complexes tive complex formed by this association causes cleavage and Rabbit and guinea pig IgG in complexes activation of C4 and C2. The MASP-1 and-2 proteins have Cobra venom factor structural similarity to CIr and Cls and mimic their activi- ties. This means of activating the C2-C4 components to Heterologous erythrocytes(rabbit, mouse, chicken) form a C5 convertase without need for specific antibody Anionic polymers(dextran sulfate) binding represents an important innate defense mechanism Pure carbohydrates(agarose, inulin) able to the alternative pathway but utilizing the ele- ments of the classical pathway except for the Cl proteins SOURCE: Adapted from M.K. Pangburn, 1986, in Immunobiology of the Complement System, Academic Press The Three Complement Pathways Converge at the Membrane-Attack Complex The terminal sequence of complement activation involves strate for an enzymatically active serum protein called factor C5b, C6, C7, C8, and C9, which interact sequentially to form D Factor d cleaves the C3b-bound factor B, releasing a small a macromolecular structure called the membrane-attack fragment(Ba)that diffuses away and generating C3bBb. The complex (MAC). This complex forms a large channel C3bBb complex has C3 convertase activity and thus is analo- through the membrane of the target cell, enabling ions and gous to the C4b2a complex in the classical pathway. The C3 small molecules to diffuse freely across the membrane convertase activity of C3b Bb has a half-life of only 5 minutes The end result of activating the classical, alternative, or unless the serum protein properdin binds to it, stabilizing lectin pathways is production of an active C5 convertase it and extending the half-life of this convertase activity to his enzyme cleaves C5, which contains two protein chains, 30 minutes a and B After binding of C5 to the nonenzymatic C3b com The C3bBb generated in the alternative pathway can acti- ponent of the convertase, the amino terminus of the a chain te unhydrolyzed C3 to generate more C3b autocatalytically. is cleaved. This generates the small C5a fragment, which dif- As a result, the initial steps are repeated and amplified, so fuses away, and the large C5b fragment, which binds to the that more than 2 x 106 molecules of C3b can be deposited surface of the target cell and provides a binding site for the on an antigenic surface in less than 5 minutes. The C3 con- subsequent components of the membrane-attack complex vertase activity of c3bbb generates the C3bBb36 complex, see Figure 13-5, step 5). The C5b component is extremely la which exhibits C5 convertase activity, analogous to the bile and becomes inactive within 2 minutes unless c6 binds C4b2a3b col to it and stabilizes its activity matic C3b component binds C5, and the Bb component Up to this point, all the complement reactions take place C5b(see Figure 13-7); the latter binds to the antigenic surface. complexes in the fluid phase. As C5b6 binds to C7, the result ing complex undergoes a hydrophilic-amphiphilic structural The Lectin Pathway Originates With Host transition that exposes hydrophobic regions, which serve as Proteins Binding Microbial Surfaces binding sites for membrane phospholipids. If the occurs on a target-cell membrane, the hydrophobic Lectins are proteins that recognize and bind to specific car- sites enable the C5b67 complex to insert into the phospho- bohydrate targets. (Because the lectin that activates comple- lipid bilayer. If, however, the reaction occurs on an immune

strate for an enzymatically active serum protein called factor D. Factor D cleaves the C3b-bound factor B, releasing a small fragment (Ba) that diffuses away and generating C3bBb . The C3bBb complex has C3 convertase activity and thus is analo￾gous to the C4b2a complex in the classical pathway. The C3 convertase activity of C3bBb has a half-life of only 5 minutes unless the serum protein properdin binds to it, stabilizing it and extending the half-life of this convertase activity to 30 minutes. The C3bBb generated in the alternative pathway can acti￾vate unhydrolyzed C3 to generate more C3b autocatalytically. As a result, the initial steps are repeated and amplified, so that more than 2  106 molecules of C3b can be deposited on an antigenic surface in less than 5 minutes. The C3 con￾vertase activity of C3bBb generates the C3b Bb3b complex, which exhibits C5 convertase activity, analogous to the C4b2a 3b complex in the classical pathway. The nonenzy￾matic C3b component binds C5, and the Bb component subsequently hydrolyzes the bound C5 to generate C5a and C5b (see Figure 13-7); the latter binds to the antigenic surface. The Lectin Pathway Originates With Host Proteins Binding Microbial Surfaces Lectins are proteins that recognize and bind to specific car￾bohydrate targets. (Because the lectin that activates comple￾ment binds to mannose residues, some authors designate this the MBLectin pathway or mannan-binding lectin pathway.) The lectin pathway, like the alternative pathway, does not de￾pend on antibody for its activation. However, the mechanism is more like that of the classical pathway, because after initia￾tion, it proceeds, through the action of C4 and C2, to pro￾duce a C5 convertase (see Figure 13-2). The lectin pathway is activated by the binding of man￾nose-binding lectin (MBL) to mannose residues on glyco￾proteins or carbohydrates on the surface of microorganisms including certain Salmonella, Listeria, and Neisseria strains, as well as Cryptococcus neoformans and Candida albicans. MBL is an acute phase protein produced in inflammatory responses. Its function in the complement pathway is similar to that of C1q, which it resembles in structure. After MBL binds to the surface of a cell or pathogen, MBL-associated serine proteases, MASP-1 and MASP-2, bind to MBL. The ac￾tive complex formed by this association causes cleavage and activation of C4 and C2. The MASP-1 and -2 proteins have structural similarity to C1r and C1s and mimic their activi￾ties. This means of activating the C2–C4 components to form a C5 convertase without need for specific antibody binding represents an important innate defense mechanism comparable to the alternative pathway, but utilizing the ele￾ments of the classical pathway except for the C1 proteins. The Three Complement Pathways Converge at the Membrane-Attack Complex The terminal sequence of complement activation involves C5b, C6, C7, C8, and C9, which interact sequentially to form a macromolecular structure called the membrane-attack complex (MAC). This complex forms a large channel through the membrane of the target cell, enabling ions and small molecules to diffuse freely across the membrane. The end result of activating the classical, alternative, or lectin pathways is production of an active C5 convertase. This enzyme cleaves C5, which contains two protein chains, and . After binding of C5 to the nonenzymatic C3b com￾ponent of the convertase, the amino terminus of the chain is cleaved. This generates the small C5a fragment, which dif￾fuses away, and the large C5b fragment, which binds to the surface of the target cell and provides a binding site for the subsequent components of the membrane-attack complex (see Figure 13-5, step 5). The C5b component is extremely la￾bile and becomes inactive within 2 minutes unless C6 binds to it and stabilizes its activity. Up to this point, all the complement reactions take place on the hydrophilic surface of membranes or on immune complexes in the fluid phase. As C5b6 binds to C7, the result￾ing complex undergoes a hydrophilic-amphiphilic structural transition that exposes hydrophobic regions, which serve as binding sites for membrane phospholipids. If the reaction occurs on a target-cell membrane, the hydrophobic binding sites enable the C5b67 complex to insert into the phospho￾lipid bilayer. If, however, the reaction occurs on an immune The Complement System CHAPTER 13 305 TABLE 13-1 Initiators of the alternative pathway of complement activation PATHOGENS AND PARTICLES OF MICROBIAL ORIGIN Many strains of gram-negative bacteria Lipopolysaccharides from gram-negative bacteria Many strains of gram-positive bacteria Teichoic acid from gram-positive cell walls Fungal and yeast cell walls (zymosan) Some viruses and virus-infected cells Some tumor cells (Raji) Parasites (trypanosomes) NONPATHOGENS Human IgG, IgA, and IgE in complexes Rabbit and guinea pig IgG in complexes Cobra venom factor Heterologous erythrocytes (rabbit, mouse, chicken) Anionic polymers (dextran sulfate) Pure carbohydrates (agarose, inulin) SOURCE: Adapted from M. K. Pangburn, 1986, in Immunobiology of the Complement System, Academic Press.