Complement Pathways and Functions
Classical Pathway
Initiated when the complement component C1 recognizes a microbial surface directly or binds to antibodies bound to the surface of a pathogen. C1 is comprised of a recognition particle of C1q which is associated with the serine proteases C1r and C1s. C1r and C1s interact non-covalently forming C1r:C1s pairs, two or more of these pairs fold into the arms of C1q. C1q binds directly to the pathogen’s surface or Fc region of antibody bound to the microbial surface. The recognition function of C1q resides in its six globular heads. When two or more of these heads interact with a ligand, it causes a conformational change in the C1r:C1s complex leading to activation of C1r which then cleaves its adjacent C1s to activate it. C1s initially cleaves C4 into C4b which binds to C1s and C4a which is a mediator of inflammation. With C4b bound, C1s can now cleave C2 into C2a which binds to C4b and C2b. C4b2a is a C3 convertase and acts to cleave C3b into C3a. C3b is an opsonin and enhances phagocytosis by binding to a C3b receptor present on phagocytes termed CR1. Binding of C3b alone is not sufficient in promoting phagocytosis as the presence of other mediators are required such as C5a binding to the C5a receptor. C5a is produced by the cleavage of C5 by the C5 convertase. This convertase is produced by C3b binding to the C2 convertase C4b2a producing C4b2a3b which cleaves C5 into C5a and C5b which is involved in initiating the formation of the MAC. The MAC is formed by C5b binding one molecule of C6 forming C5b6, this complex then binds one molecule of C7 creating C5b67, the binding of C7 leading to conformational change on the constitutive molecules thus exposing the hydrophobic site on C7 allowing it to insert itself into the lipid bilayer. C8 then binds to the complex creating C5b678. C8 is a complex of two proteins C8B and C8y. The C8B protein binds to the complex exposing the hydrophobic region of C8y to insert into the membrane. The C8a-y protein then induces polymerization of 10-16 molecules of C9 which bind to the complex to form a pore-forming structure C56789. The diameter of the channel is about 100 angstroms and allows the free passage of solutes and water across the lipid bilayer, this disruption leads to cellular destruction.
Lectin Pathway
Follows the same path however the initiation is different as instead of C1 recognizing patterns present on the surface they are recognized by mannose-binding lectins or ficolins. These recognition molecules form a complex with serine proteases MASP-1 and MASP-2. When MBL or ficolin binds to a ligand, a conformational change occurs in MASP-2 activating it causing it to cleave a second MASP-2 present in the same complex. This activated MASP-2 molecule can go on to cleave C4 and C2 as seen in the classical pathway by C1s. The pathway then follows the same pattern as seen in the classical pathway ultimately leading to the formation of the MAC.
Alternative Pathway
Slightly different and does not have a recognition complex of its own to initiate the pathway. This pathway can be spontaneously activated by possessing its own unique C3 convertase composed of C3b bound to Bb which is a cleavage fragment of the plasma protein factor B. This pathway can be activated in two different ways; the first is by activation of either of the other pathways leading to the generation of C3b covalently linked to a microbial surface. This C3b can then bind to factor B. This binding causes a conformational change in factor B allowing its cleavage by the protease factor D into Ba and Bb, which remains stably attached to C3b forming the C3 convertase C3bBb. This pathway can also be initiated by the spontaneous hydrolysis of C3 forming C3fiH2O which can bind factor B, which is thus cleaved by factor D and producing the short-lived fluid-phase C3 convertase C3fiH2OBb which, although produced in small amounts, can cleave many C3 molecules. This pathway follows the same trend as the other pathways however cleavage of C3 by C3bBb creating C3a and C3b results in the formation of a special C5 convertase C3bBb3b. This convertase can go on to cleave C5 into C5a and C5b which is followed by the formation of the MAC.
Non-lytic Function
Several complement components have non-lytic functions, as mentioned previously C3b and C5a are involved in opsonization acting to enhance phagocytosis of the pathogen. Other components act as anaphylatoxins, these include C3a, C4a, and C5a and act to induce degranulation of mast cells leading to the release of histamine which binds to H2 receptors of vascular smooth muscle promoting vasodilation. The anaphylatoxins also increase capillary permeability promoting extravasation. These components also have chemotactic properties attracting phagocytes and neutrophils to the site of infection. C5a acts directly on neutrophils and monocytes increasing their adherence to vessel walls. Many pathogens have evolved defense mechanisms against the complement system. Neisseria meningitis produces a favor H binding protein. Factor H acts to protect self-cells from complement it works by binding to C3b on the surface of self-cells, it can only bind when sialic acid is present, and prevents the formation of the C3 convertase. N. meningitis produce a protein which can bind factor H and thus prevent the formation of the MAC complex. Other pathogens have evolved to be encapsulated such as streptococcus pneumoniae, the capsule provides a physical barrier between the C3b bound to the cell membrane and CR1 present on phagocytic cells thus preventing the opsonization characteristic of C3b.