complement system
The complement system comprises an assembly of liver-manufactured, soluble and cell-bound proteins that participate in innate and adaptive immunity. Activation of the complement cascade by protease cleavage leads to chemotaxis (C5a), inflammation and increased capillary permeability (C3a, C5a), opsonization (C3b), and cytolysis.
▼activation : alternative pathway : amplification by C3 : anaphylatoxins C3a, C5a : antigen uptake : C1 : C2 : C3 : C4 : C5 : C6 : C7 : C8 : C9 : C1INH : CD59 : chemotaxis : classical pathway : complement cascade : complement control proteins : convertases C3, C3/C5, C5 : disorders : evolution : Factor B : Factor H : Factor I : ficolins : immunoglobulins and complement activation : inhibitory proteins : lectin pathway : MBL -MAPS : mannose-binding lectin pathway : membrane attack complex : opsonin : pathways : perforin : phagocytosis : pore : proteases : regulation : serine proteases : sialic acid ▼
Sequential activation of the protein components of the complement cascade upon cleavage by a protease, leads to each component's becoming, in its turn, a protease. Three pathways are involved in complement attack upon pathogens:
● classical pathway
● alternative pathway
● mannose-binding lectin pathway (MBL -MAPS)
The classical pathway utilizes C1, which is activated by binding of an antibody to its cognate antigen.
Inactive C1 circulates as a serum molecular complex comprising 6 C1q molecules, 2 C1r molecules, and 2 C1s molecules. Constant regions in some immunoglobulins specifically bind C1q, activating C1r and C1s. The mu chains of IgM and some gamma chains of IgG contain specific binding sites, though IgM is far more effective than IgG.
Activated C1s is a serine protease that cleaves C4 and C2 into small inactive fragments (C4a, C2a) and larger active fragments, C4b and C2b. The active component C4b binds to the sugar moieties of surface glycoproteins and binds noncovalently to C2b, forming another serine protease C4b•C2b, which is called C3 convertase because it cleaves C3, releasing an active C3b opsonin fragment.
Macrophages and neutrophils possess receptors for C3b, so cells coated with C3b are targetted for phagocytosis (opsonization). The small C3a fragment is released into solution where it can bind to basophils and mast cells, triggering histamine release and, as an anaphylatoxin, potentially participating in anaphylaxis.
C3 amplifies the humoral response because of its abundance and its ability to auto-activate (as a C3 convertase). Breakdown of C3b generates an antigen-binding C3d fragment that enhances antigen uptake by dendritic cells and B cells .
Binding of C3b to C5 induces an allosteric change that exposes C3b•C5 to cleavage by C4b•C2b, which is now acting as C3/C5 convertase. The alternative pathway possesses a distinct C5 convertase, so the two pathways converge through C5.
Cleavage of C5 by the C3/C5 convertase releases:
● anaphylotoxic C5a, which promotes chemotaxis of neutrophils
● C5b, which complexes with one molecule of each of C6, C7, and C8. The resultant C5b•6•7•8 complex assists polymerization of as many as 18 C9 molecules to form a cytolysis-promoting pore (membrane attack complex, tem) through the plasma membrane of the target cell, which then suffers osmosis-induced cytolysis.
Another cytolytic mediator utilized by CTLs and NK cells is perforin, which is a 534 aa glycoprotein with sequence homology to the membrane attack component of complement C9. Like C9, perforin integrates into the target cell membrane, forming polyprotein pores up to 20nm in diameter comprising 12—18 perforin monomers, which breach membrane integrity and permit cytolytic cell death.
The alternative pathway is not activated by antigen-antibody binding, but instead relies upon spontaneous conversion of C3 to C3b, which is rapidly inactivated by its binding to inhibitory proteins and sialic acid on the cell's surface. Because bacteria and other foreign materials lack these inhibitory proteins and sialic acid, the C3b is not inactivated and it forms the C3b•Bb complex with Factor B. The C3b.Bb complex acts as a C3 convertase, forming C3b•Bb•C3b, which acts as a C5 convertase that can ititiate assembly of the membrane attack complex. C3b•Bb, acting as a C3 convertase, provides a positive feedback loop that amplifies production of C3.
The lectin pathway (MBL - MASP) is homologous to the classical pathway, but utilizes opsonin, mannan-binding lectin (MBL, MBP) and ficolins rather than C1q. Binding of mannan-binding lectin to mannose residues on the pathogen surface activates the MBL-associated serine proteases, MASP-1, MASP-2, MASP-3, which cleave C4 into C4b and C2 into C2b. As in the classical pathway, C4b and C2b bind to form the C4b•C2b C3 convertase. Ficolins are homologous to MBL and function through MASPs. Diversified ficolins are of particular importance in invertebrates, which lack the adaptive immune response that evolved some 500 million years ago in jawed vertebrates.
Several complement control proteins regulate activity of the complement system, including:
● C1 inhibitor (C1INH), which eliminates the proteolytic activity of activated C1r and C1s. Following C1 activation by antigen-antibody complexes, C1INH permits only a brief interval during which activated C1 can cleave C4 and C2.
● Factor I, which inactivates C3b
● Factor H, which removes Bb, thus interrupting the C3 convertase feedback loop within the alternative pathway
● CD59, which inhibits C9 polymerization during assembly of the membrane attack complex.
Dysregulation of the complement system manifests variously as immune complex disorders (C2 deficiency), susceptibility to bacterial infections (C3 deficiency), the autoimmune disorder SLE (early component or C2 deficiency), hereditary angioneurotic edema (HANE) (C1INH deficiency).
▲ activation : alternative pathway : amplification by C3 : anaphylotoxins C3a, C5a ф antibodies ф antigen : antigen uptake : C1 : C2 : C3 : C4 : C5 : C6 : C7 : C8 : C9 : C1INH : CD59 : chemotaxis : classical pathway : complement cascade : complement control proteins : convertases C3, C3/C5, C5 : disorders : evolution : Factor B : Factor H : Factor I : ficolins : immunoglobulins and complement activation : inhibitory proteins : lectin pathway : MBL -MAPS : mannose-binding lectin pathway : membrane attack complex : opsonin : pathways : perforin : phagocytosis : pore : proteases : regulation : serine proteases : sialic acid ▲
Tables Fc receptors Immune Cytokines Immunoglobulins
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tags [Immunology][complement]
