The complement system consists of a series of heat- labile serum proteins that are activated in turn. The components normally exist as soluble inactive precursors; once activated, a complement component may then act as an enzyme (Fig. 1), which cleaves several molecules of the next com ponent in the sequence (rather like the clotting cascade). Each precursor is cleaved into two or more fragments. The major fragment has two biologically active sites: one for binding to cell membranes or the triggering complex and the other for enzymatic cleavage of the next complement com ponent (Fig. 2). Control of the sequence involves spontaneous decay of any exposed attachment sites and specific inactivation by complement inhibitors. Minor fragments (usually prefixed ‘a’) generated by cleavage of components have important biological properties in the fluid phase, such as the chemotactic activity of C5a.
Fig1. Basic principle underlying the cleavage of complement components.
Fig2. Functions of complement pathways. MASP, MBL- associated serine protease; MBL, mannan- binding lectin.
The history of the discovery of the complement pathways has made the terminology confusing. Several of the components have numbers, but they are not necessarily activated in numerical order; the numbering coincides with the order of their discovery and not with their position in the sequence. Activated components are shown with a bar over the number of the component (e.g. C1 is activated to C1‒) and fragments of activated components by letters after the number (e.g. C3 is split initially into two fragments, C3a and C3b).
The major purpose of the complement pathways is to provide a means of removing or destroying antigen, regardless of whether or not it has become coated with antibody (Fig. 2). The lysis of whole invading microorganisms is a dramatic example of the activity of the complete sequence of complement activation, but that is not necessarily its most important role. The key function of complement is probably the opsonization of microorganisms and immune complexes; microorganisms coated (i.e. opsonized) with immunoglobulin and/or complement are more easily recognized by macrophages and more readily bound and phagocytosed through IgG: Fc and C3b receptors.
Similarly, immune complexes are opsonized by their activation of the classical complement pathway; individuals who lack one of the classical pathway components suffer from immune complex diseases. Soluble complexes are transported in the circulation from the inflammatory site by erythrocytes bearing CR1, which bind to the activated C3 (C3b) in the immune complex. Once in the spleen or liver, these complexes are removed from the red cells, which are then recycled (Fig. 3).
Fig3. Transport of immune complexes by erythrocytes to macrophages in liver and spleen.
Minor complement fragments are generated at almost every step in the cascade and contribute to the inflammatory response. Some increase vascular permeability (C3a), while others attract neutrophils and macrophages for subsequent opsonization and phagocytosis (C5a) (Fig. 2). C5a not only promotes leucocytosis in the bone marrow, but mobilizes and attracts neutrophils to the inflammatory site where it increases their adhesiveness; it also upregulates complement receptors CR1 and CR3 on neutrophils and macrophages to maximize phagocytosis.
Complement activation occurs in two phases: activation of the C3 component, followed by activation of the ‘attack’ or lytic sequence. The critical step is a cleavage of C3 by complement- derived enzymes termed ‘C3 convertases’. The cleavage of C3 is achieved by three routes, the classical, alternative and lectin pathways, all of which can generate C3 convertases but in response to different stimuli (Fig. 4). The pivotal role of C3 in complement activation is underlined by patients with a deficiency of C3, who cannot opsonize pathogens or immune complexes, predisposing them to bacterial infection as well as immune complex diseases.
Fig4. Complement pathways and their initiating factors. MBL, mannan- binding lectin.
The classical pathway was the first to be described. It is activated by a number of substances, the most widely recognized being antigen–antibody complexes where the antibody is either IgM or IgG (Fig. 4). The reaction of IgM or IgG with its antigen causes a conformational change in the Fc region of the antibody to reveal a binding site for the first component in the classical pathway, C1q. C1q is a remarkable, collagen- like protein composed of six subunits, resembling a ‘bunch of tulips’ when seen under the electron microscope. C1q reacts with Fc via its globular heads; attachment by two critically spaced binding sites is needed for activation. The Fc regions of pentameric IgM are spaced so that one IgM molecule can activate C1q; in contrast, IgG is relatively inefficient because the chance of two randomly sited IgG molecules being the critical distance apart to activate C1q is relatively low. IgA, IgD and IgE do not activate the classical pathway.
Once C1q is activated, C1r and C1s are sequentially bound to generate enzyme activity (C1 esterase) for C4 and C2 (see Fig. 2), splitting both molecules into ‘a’ and ‘b’ fragments. The complex C4b2b is the classical pathway C3 convertase. Other fragments released are C4a, C2a and a vasoactive peptide released from C2. C4b2b cleaves C3 into two fragments, C3a possessing anaphylatoxic and chemotactic activity and C3b that binds to the initiating complex and pro motes many of the biological properties of complement. The C 4b2b3b complex so generated is an enzyme, C5 convertase, which initiates the final lytic pathway (the ‘attack’ sequence).
The alternative pathway is phylogenetically older than the classical pathway. It is relatively inefficient in the tissues, and high concentrations of the various components are required. The central reaction in this pathway, as in the classical one, is the activation of C3, but the alternative pathway generates a C3 convertase without the need for antibody, C1, C4 or C2. Instead, the most important activators are bacterial cell walls and endotoxin (Fig. 4).
The initial cleavage of C3 in the alternative pathway hap pens continuously and spontaneously (see Fig. 4), generating a low level of C3b. C3b is an unstable substance and, if a suitable acceptor surface is not found, the attachment site in C3b decays rapidly and the molecule becomes inactive. If, however, an acceptor surface (bacterial cell walls and endo toxin) is nearby, the C3b molecules can bind and remain active. C3b is then able to use factors D and B of the alternative path way to produce the active enzyme ‘C3bBb’. This latter sub stance has two properties. It can break down more C3, providing still more C3b; this is known as the ‘positive feedback loop’ of the alternative pathway (Fig. 2). Alternatively, C3bBb becomes stabilized in the presence of properdin to form the C5 convertase of the alternative pathway.
There are thus two ways of producing C5 convertase. In the classical pathway C5 convertase is made up of C3b, C4b and C2b, while in the alternative pathway it is produced by C3b, Bb and properdin (Fig. 2).
The third pathway of complement activation is initiated by mannan- binding lectin (MBL, also known as mannan- binding protein), a surface receptor (see Fig. 2) shed into the circulation, binding avidly to carbohydrates on the surface of microorganisms. MBL is a member of the collectin family of C- type lectins, which also includes pulmonary surfactant proteins A and D. MBL is structurally related to C1q and activates complement through a serine protease known as MASP (MBL- associated serine protease), similar to C1r and C1s of the classical pathway. Inherited deficiency of MASP- 2 has been shown to predispose to recurrent pneumococcal infections and immune complex disease.
All pathways converge on a common final lytic pathway (‘attack’ sequence) of complement involving the sequential attachment of the components C5, C6, C7, C8 and C9 and resulting in lysis of the target cell such as an invading organ ism or a virally infected cell. The lytic pathway complex binds to the cell membrane and a transmembrane channel is formed. This can be seen by electron microscopy as a hollow, thin- walled cylinder through which salts and water flow, leading to the uptake of water by a cell, swelling and destruction. During the final lytic pathway, complement fragments are broken off. C5a and the activated complex C567 are both potent mediators of inflammation. C5a, along with C3a, are anaphylotoxins, i.e. they cause histamine release from mast cells with a resulting increase in vascular permeability. C5a also has the property of being able to attract neutrophils to the site of complement activation (i.e. it is chemotactic; see Fig. 2).
The control of any cascade sequence is extremely important, particularly when it results in the production of potentially self- damaging mediators of inflammation. The complement pathway is controlled by three mechanisms (see Box 1).
