Complement activation by the classical pathway commonly begins with the formation of soluble antigen-antibody complexes (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 components 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 molecule that expose a binding site for the C1 component of the complement system. C1 in serum is a macromolecular complex 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 interaction 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
C1 binds antigen-antibody complex
Mannose-binding lectin (MBL) binds foreign surface
MBL-associated proteases (MASP1 + 2) bind MBL, generate activated C1-like complex
C3 convertase Q^j C3bBb
QGO C3bBb3b C5 convertase
QGO C3bBb3b C5 convertase
C6 C7 C8 C9
Q Factor B
Membrane attack complex
Spontaneous, slow, small amounts
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 spontaneously 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 pathways, generating large amounts of C3b, which forms part of C5 convertase. C3b also can diffuse away from the activating surface and bind to immune complexes or foreign cell surfaces, where it functions as an opsonin.
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. Circulating IgM, however, exists as a planar configuration in which the Clq-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 Clq-binding site in the CH2 domain of the Fc, so that firm Clq binding is achieved only when two IgG molecules are within 30-40 nm of each other on a target surface or in a complex, providing two attachment sites for Clq. This difference accounts 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 molecules 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
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Structure of the C1 macromolecular complex. (a) Diagram of C1 qr2s2 complex. A C1 q molecule consists of 18 polypeptide chains arranged into six triplets, each of which contains one A, one B, and one C chain. Each C1 r 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 micrograph of C1 q molecule showing stalk and six globular heads. [Part (b) from H. R. Knobel et al, 1975, Eur. J. Immunol. 5:78]
Models of pentameric IgM in planar form (a) and "staple" form (b). Several C1 q-binding sites in the Fc region are accessible in the staple form, whereas none are exposed in the planar 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.]
C1q binds antigen-bound antibody. C1r activates auto-catalytically and activates the second C1r; both activate C1s
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
C4b2a3b C5 convertase
C4b2a3b C5 convertase
The C3b component of C5 convertase binds C5, permitting C4b2a to cleave C5
C5b binds C6, initiating the formation of the membrane-attack complex
Membrane attack complex
Schematic diagram of intermediates in the classi- attack complex (MAC, bottom right) forms a large pore in the cal pathway of complement activation. The completed membrane- membrane.
C1r that converts C1r to an active serine protease enzyme, Clr, which then cleaves C1s to a similar active enzyme, Cls. Cls has two substrates, C4 and C2. The C4 component is a glycoprotein containing three polypeptide chains a, p, and 7. C4 is activated when Cls hydrolyzes a small fragment (C4a) from the amino terminus of the a 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 proenzyme then attaches to the exposed binding site on C4b, where the C2 is then cleaved by the neighboring Cls; 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 cleavage, C4a, is an anaphylatoxin, or mediator of inflammation, which does not participate directly in the complement cascade; the anaphylatoxins, which include the smaller fragments of C4, C3, and C5 are described below.
The native C3 component consists of two polypeptide chains, a and p. Hydrolysis of a short fragment (C3a) from the amino terminus of the a chain by the C3 convertase generates 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 C4bla3b, called C5 convertase. The C3b component of this complex binds C5 and alters its conformation, so that the C4bla component 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 C4bla; 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.
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