The complement system consists of several plasma proteins that work together to opsonize microbes, to promote the recruitment of phagocytes to the site of infection, and, in some cases, to directly kill the microbes (Figure 1). The first step in activation of the complement system is recognition of molecules on microbial surfaces but not host cells, and this occurs in three ways, each referred to as a distinct pathway of complement activation.
Figure 1. C1, mannose-binding lectin, and ficolin (A). Three Pathways of complement activation (B).
The classical pathway, so called because it was discovered first, uses a plasma protein called C1q to detect antibodies bound to the surface of a microbe or other structure (Figure 1A). Once C1q binds to the Fc portion of the antibodies, two associated serine proteases, called C1r and C1s, become active and initiate a proteolytic cascade involving other complement proteins. The classical pathway is one of the major effector mechanisms of the humoral arm of adaptive immune responses. Innate immune system soluble proteins called pentraxins, can also bind C1q and initiate the classical pathway.
The alternative pathway, which was discovered later but is phylogenetically older than the classical pathway, is triggered when a complement protein called C3 directly recognizes certain microbial surface structures, such as bacterial LPS. C3 is also constitutively activated in solution at a low level and binds to cell surfaces, but it is then inhibited by regulatory molecules present on mammalian cells. Because microbes lack these regulatory proteins, the spontaneous activation can be amplified on microbial surfaces. Thus, this pathway can distinguish normal self from foreign microbes on the basis of the presence or absence of the regulatory proteins.
The lectin pathway is triggered by a plasma protein called mannose-binding lectin (MBL), which recognizes terminal mannose residues on microbial glycoproteins and glycolipids, similar to the mannose receptor on phagocyte membranes described earlier (see Figure 1A). MBL is a member of the collectin family (discussed later) with a hexameric structure similar to the C1q component of the complement system. After MBL binds to microbes, two zymogens called MASP1 (mannose-associated serine protease 1, or mannan-binding lectin-associated serine protease) and MASP2, with similar functions to C1r and C1s, associate with MBL and initiate downstream proteolytic steps identical to the classical pathway.
Recognition of microbes by any of the three complement pathways results in sequential recruitment and assembly of additional complement proteins into protease complexes. One of these complexes, called C3 convertase, cleaves the central protein of the complement system, C3, producing C3a and C3b. The larger C3b fragment becomes covalently attached to the microbial surface where the complement pathway was activated. C3b serves as an opsonin to promote phagocytosis of the microbes. The smaller fragment, C3a, is released and stimulates inflammation by acting as a chemoattractant for neutrophils. C3b binds other complement proteins to form a protease called C5 convertase that cleaves C5, generating a released peptide (C5a) and a larger fragment (C5b) that remains attached to the microbial cell membranes. C5a is also a chemoattractant; in addition, it induces changes in blood vessels that make them leak plasma proteins and fluid into sites of infections. C5b initiates the formation of a complex of the complement proteins C6, C7, C8, and C9, which are assembled into a membrane pore called the membrane attack complex (MAC) that causes lysis of the cells where complement is activated.
The complement system is an essential component of innate immunity, and patients with deficiencies in C3 are highly susceptible to recurrent, often lethal, bacterial infections. Genetic deficiencies in MAC formation (the terminal product of the classical pathway) increase susceptibility to only a limited number of microbes, notably Neisseria bacteria, which have thin cell walls that make them especially susceptible to the lytic action of the MAC.
Figure 2. The molecules involved in complement system.
Fifteen or more serum components constitute the complement system, the sequential activation and assembly into functional units of which leads to three main effects: release of peptides active in inflammation (Figure 2, top right); deposition of C3b, a powerful attachment promoter (or ’opsonin’) for phagocytosis, on cell membranes (Figure 2, bottom right); and membrane damage resulting in lysis (Figure 2, bottom left). Together these make it an important part of the defences against microorganisms. Deficiencies of some components can predispose to severe infections, particularly bacterial.
C1: A Ca2+-dependent union of three components: Clq (MW 400 000), a curious protein with six valencies for Ig linked by collagen-like fibrils, which activates in turn Clr (MW 170 000) and C1s (MW 80 000), a serine proteinase that goes on to attack C2 and C4.
C2: (MW 120 000), split by C1s into small (C2b) and large (C2a) fragments.
C4: (MW 240 000), likewise split into C4a (small) and C4b (large). C4b then binds to C2, and also, via a very unusual type of reactive thioester bond, to any local macromolecule, such as the antigen– antibody complex itself, or to the membrane in the case of a cell-bound antigen. This tethers the C4bC2 complex forming a ‘C3 convertase’. Note that some complementologists prefer to reverse the names of C2a and b, so that for both C2 and C4 the ‘a’ peptide is the smaller one.
C3: (MW 180 000), the central component of all complement reactions, split by its convertase into a small (C3a) and a large (C3b) fragment. Some of the C3b is deposited on the membrane, where it serves as an attachment site for phagocytic polymorphs and macrophages, which have receptors for it; some remains associated with C2a and C4b, forming a ‘C5 convertase’. Two ‘C3b inactivator’ enzymes rapidly inactivate C3b, releasing the fragment C3c and leaving membrane-bound C3d.
C5 (MW 180 000), split by its convertase into C5a, a small peptide that, together with C3a (anaphylatoxins), acts on mast cells, polymorphs and smooth muscle to promote the inflammatory response, and C5b, which initiates the assembly of C6, 7, 8 and 9 into the membrane damaging or ‘lytic’ unit.
Factor B: (MW 100 000), which complexes with C3b, whether produced via the classic pathway or the alternative pathway itself. It has both structural and functional similarities to C2, and both are coded for by genes within the very important major histocompatibility complex (see Fig. 11). In birds, which lack C2 and C4, C1 activates factor B.
Factor D: (MW 25 000), an enzyme that acts on the C3b–B complex to produce the active convertase, referred to in the language of complementologists as C3bBb.
Pr/Properdin: (MW 220 000), the first isolated component of the alternative pathway, once thought to be the actual initiator but now known merely to stabilize the C3b–B complex so that it can act on further C3. Thus, more C3b is produced which, with factors B and D, leads in turn to further C3 conversion, a ‘positive feedback’ loop with great amplifying potential (but restrained by the C3b inactivators factor H and factor I).
MBL: Mannose-binding lectin (also variously referred to as mannosebinding protein or mannan-binding protein), a C1q-like molecule that recognizes microbial components such as yeast mannan and activates C1r and C1s, and hence the rest of the classic pathway. MBL deficiency predisposes children to an increased incidence of some bacterial infections