as N‐acetylglucosamine or N‐acetylgalactosamine, on microbial surfaces.
Mannan‐binding lectin and ficolin are found in the circulation complexed with proteases, known as the mannose‐associated serine proteases (MASPs). Once bound to the bacterium, one of the proteases, MASP‐2, sequentially cleaves C4 and C2 to form C4b2a on the surface of the bacterium. As we discussed previously, C4b2a is also formed in the classical pathway; it is the C3 convertase that cleaves the next component in the pathway, C3. Thus, the lectin and classical pathways converge at this point.
Alternative Pathway
Activators.
The alternative pathway of complement activation can be triggered by almost any foreign substance, and in the absence of specific antibody. Thus, the alternative pathway is a key part of the innate immune defenses, involved early in the response to pathogens. The most widely studied activators include lipopolysaccharides from the cell walls of Gram‐negative bacteria (which are endotoxins), the cell walls of some yeasts, and cobra venom factor, a protein present in some snakes. Some agents that activate the classical pathway—viruses, aggregated immunoglobulins, and necrotic cells—also trigger the alternative pathway.
Early Steps in the Alternative Pathway That Lead to C3 Cleavage.
The deposition of C3b on the cell surface initiates the alternative pathway (see Figure 4.2C). C3b is generated in the circulation in small amounts by the spontaneous cleavage of a reactive thiol group in C3; this “preformed” C3b can bind to proteins and carbohydrates expressed on cell surfaces, either of a pathogen or of a host (mammalian) cell. (If C3b does not bind to one of these surfaces, it is rapidly inactivated.)
Thus, in a sense, the alternative pathway is always “on,” and continual activation could damage cells of the host. However, as we describe in more detail subsequently, mammalian cells regulate the progression of the alternative pathway. Microbial cells lack such regulators and cannot prevent the development of subsequent steps in the alternative pathway.
Following the deposition of C3b, the serum protein factor B combines with C3b on the cell surface to form a complex, C3bB. Factor D then cleaves factor B in the cell surface‐associated C3bB complex, generating fragments Ba, which is released into the fluid phase, and Bb, which remains attached to C3b. C3bBb is the alternative pathway C3 convertase, which cleaves C3 into C3a and C3b.
Steps Shared by All Pathways: Activation of C3 and C5
C3 cleavage is the first step that is common to all three complement pathways (Figure 4.3). In the classical and lectin pathways (Figure 4.3A), the C3 convertase C4b2a cleaves C3 into two fragments, C3a and C3b. In the alternative pathway (Figure 4.3B), the C3 convertase C3bBb cleaves C3 into the same two fragments. The smaller fragment, C3a, is released into the fluid phase, and the larger one, C3b, continues the complement activation cascade by binding covalently to the cell surface around the site of complement activation.
Note a unique feature of the alternative pathway shown in Figure 4.3B: binding of the serum protein properdin (also known as factor P) stabilizes C3bBb on the pathogen surface. As a result, C3bBb rapidly cleaves further C3 molecules, resulting in the huge build‐up of C3b on the surface of the pathogen. As we described above, the deposition of C3b on a cell surface is the initiating step in the activation of the alternative pathway. Thus, deposition on the cell surface of these rapidly produced and increased levels of C3b results in an almost explosive triggering of the alternative pathway. As we describe below, properdin’s ability to activate this amplification loop is balanced by negative or regulatory molecules. Consequently, under normal conditions, the alternative pathway is not continually activated.
Figure 4.3. Cleavage of C3 by C3 convertase and C5 by C5 convertase. (A) Classical and lectin pathways. (B) Alternative pathway. In all pathways, C3 is cleaved to C3b, which deposits on the cell surface, and C3a, which is released into the fluid phase. Similarly, C5 is cleaved into C5b, which deposits on the cell surface, and C5a, which is released into the fluid phase. In the alternative pathway, the stabilization of C3bBb by properdin increases C3b deposition on the cell surface and amplification of complement activation.
C3b binding to either the classical/lectin or alternative pathway C3 convertases allows the next component in the sequence, C5, to bind and be cleaved (middle section of Figure 4.3A,B). For this reason, the C3 convertases with bound C3b are referred to as C5 convertases—C4b2a3b in the classical/lectin pathways, C3bBb3b in the alternative pathway. The cleavage of C5 produces two fragments: C5a is released into the fluid phase and has potent anaphylatoxic properties, and C5b binds to the cell surface and forms the nucleus for the binding of the terminal complement components.
Terminal Pathway
The terminal components of the complement cascades—C5b, C6, C7, C8, and C9—are common to the three complement activation pathways. These components bind to one another and form a membrane attack complex (MAC) that results in the death (lysis) of the cell on which they deposit (Figure 4.4).
The first step in MAC formation is C6 binding to C5b on the cell surface. C7 then binds to C5b and C6, with C7 inserting into the outer membrane of the cell. The subsequent binding of C8 to C5b67 results in the complex penetrating deeper into the cell’s membrane. C5b‐C8 on the cell membrane acts as a receptor for C9, a perforin‐like molecule that binds to C8. Additional C9 molecules interact with the C9 molecule in the complex to form polymerized C9 (poly‐C9). Poly‐C9 forms a transmembrane channel that disturbs the cell’s osmotic equilibrium: ions pass through the channel and water enters the cell. The cell swells and the membrane becomes permeable to macromolecules, which then escape from the cell. The result is cell lysis.
Figure 4.4. Formation of membrane attack complex (MAC). Late‐stage complement components C5b–C9 bind sequentially to form a complex on the cell surface. Multiple C9 components bind to this complex and polymerize to form poly‐C9, creating a channel that disrupts the cell membrane.
REGULATION OF COMPLEMENT ACTIVITY
Uncontrolled complement activation can rapidly deplete complement components, leaving the host unable to defend against subsequent
