The Vasculitides

© Dirk Biddle

1.7.12 Complement

Complement (C’) is the term given to the thermolabile group of proteins, the major components of which are labelled C1 to C9. In combination with antibodies complement causes the destruction (especially) of particulate antigens such as bacteria and foreign blood corpuscles. Compliment, so named because it “complements” the action of antibody, also helps clear (antibody/antigen) immune complexes (ICs) from the blood. It does this in a number of ways.

First complement can bind with antigens on the circulating immune complex, inhibiting precipitation or deposition and preventing its ability to damage blood vessel walls at the site of precipitation. When compliment binds to a circulating immune complex it initiates the classical complement cascade (see below), one of the end-products of which is the membrane attack complex (MAC). The MAC creates holes in the membrane of attacking bacteria, preventing them from maintaining osmotic balance in their cytoplasm, thereby killing them.

Second, complement clears the blood of circulating immune complexes by binding to receptors on cells such as erythrocytes, B-cells, T-cells, and macrophages. This enables immune complex transport through the circulation to the spleen or liver, where it can be destroyed by macrophage systems.

Finally, complement can also recognise and clear apoptotic (allergy causing) cells, decreasing the possibility that they may act as autoantigens that can provoke an autoimmune response to the hosts own tissues.

An important aspect of complement function is the complement cascade, which involves three distinct pathways: the classic, the alternative, and the lectin. Each pathway reacts to different aspects of a pathogen and utilises or produces different complement components. For example the classic pathway reacts mainly to IgM and IgG immune complexes and utilises C1, C2, and C4 (which are not part of the alternative pathway). C1 is a complex of three molecules: C1q, C1r, and C1s (C1s and C1r are proteases). When the Fab portion of IgM or IgG binds with an antigen, the Fc portion becomes receptive to the binding of C1q. C1q binding to antibody activates the protease function of C1r, which cuts a peptide bond in C1s to, in turn, activate its protease function. C1s then activates C2 and C4 by cutting a small peptide (C2a and C4a) from each. An active thioester bond on the remaining C4b is exposed and covalently binds to a molecule on the pathogen surface. C4b and the remaining C2b then associate together on the pathogen surface, leading to the formation of C3 convertase (C4b2b) that then proteolytically cleaves C3 into C3a (which stimulates inflammation) and C3b (which acts as an opsonin). Some C3b molecules also associate with C4b2b to form C5 convertase (C4b2b3b). The cleavage of C3 is where the three pathways converge. C5 is cleaved by C5 convertase into C5a and C5b. C5b combines with C6 and C7 in solution, and the C5b67 complex associates with the pathogen lipid membrane via hydrophobic sites on C7. C8 and several molecules of C9, which also have hydrophobic sites, join the membrane attack complex (MAC: C5b678poly9). Poly-C9 forms a pore in the membrane through which water and solutes can pass, resulting in osmotic lysis and cell death.

In the alternate pathway the complement cascade is the result of a reaction to the surface presentation of a pathogen (eg: some yeasts, fungi) or bacteria in the absence of antigen (although sometimes triggered by IgA immune complexes), thus giving the body a very rapid defence against certain pathogens. The alternative pathway utilises Factor D to cut Factor B to form active proteolytic Bb, which binds with C3b to form the alternative C3 convertase (C3bBb). The alternative C3 convertase then acts in the same manner as the classical convertase to form the alternative C5 convertase (C3bBb3b) which can also form, with C678 and poly-C9, the MAC.

The third pathway, the mannose-binding lectin (MBL) cascade, initiates as a result of lectin binding to mannan on the pathogen surface. MBL is made by the liver in response to macrophage cytokines produced in response to pathogen binding. MBL and two serum proteases function like C1 to activate C4 and C2 to form the MBL cascade C3 convertase. C3b then joins the complex to form MBL cascade C5 convertase. Like the alternative cascade, the MBL cascade is innate complement activity; like the classical cascade, the MBL cascade utilizes C4 and C2 to form C3 convertase. Just as in the classical and alternative cascade the MAC is formed.

Complement thus helps prevent the potential development of a vasculitis condition or an immune complex disease by decreasing the number of circulating immune complexes and apoptotic cells and by removing potentially damaging pathogens. Correspondingly, a deficiency in complement increases the risk of such disorders because the greater the concentration of immune complexes in the blood, the higher the likelihood that they will deposit in nearby tissues and cause an inflammatory response.

Deposition of immune complexes in local tissues with resultant inflammation can cause vascular damage. The combination of IgM or IgG antibodies with antigen activates the complement cascade, generating active peptides such as C5a, which, in addition to dilating capillaries and increasing vascular permeability, contracts smooth muscle and mobilizes phagocytic cells. The soluble split products C3a, C4a, and C5a are called anaphylatoxins because of their inflammatory activity. C5a is the most potent. Anaphylatoxins bind to receptors on various cell types to stimulate smooth muscle contraction, increase vascular permeability, and activate mast cells to release inflammatory mediators. C5a is also chemotactic for monocytes and neutrophils, and stimulates leukocyte adherence to blood vessel walls at the site of infection, phagocytosis, and bactericidal activities. C2a can be converted to C2 kinin, which regulates blood pressure by causing blood vessels to dilate. Binding of immune complexes to neutrophils and macrophages also activates the respiratory burst with generation of toxic oxygen products such as hydrogen peroxide, hydroxyl radical, hypochlorous acid, and chloramines. Lysosomal proteolytic enzymes, together with toxic oxygen products, produce a potent system that can damage protein and lead to blood vessel damage with haemorrhagic necrosis and local tissue destruction.

Low levels of compliment (hypocomplementemia) indicate complement consumption and this is an important marker for the presence of immune complexes, thus also indicating a classic complement cascade pathway. This is important in vasculitis condition diagnosis in that some vasculitides are recognised as complement consuming (eg; Cryoglobulinemia and Polyarteritis nodosa). Further, by examining which particular complement components are consumed, one may infer something about the type of antigen present, and thus a potential causal factor in the disease. Alternatively, raised levels of complement (hyperomplementemia) may be present in secondary vasculitis conditions (eg; Scleroderma, Sjögren's syndrome, Systemic Lupus Erythematosus and Urticaria). Hypercomplementemia results from a deficiency in complement cascade regulatory proteins (eg; C1q in the classic pathway), however the reasons for these deficiencies are not well understood.

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Complement: (C ) A term originally used to refer to the heat labile factor in serum that causes immune cytolysis , the lysis of antibody coated cells and now referring to the entire functionally related system comprising at least 20 distinct serum proteins that is the effector not only of immune cytolysis but also of other biologic functions. Complement activation occurs by two different sequences, the classic and alternative pathways. The proteins of the classic pathway are termed components of complement and are designated by the symbols C1 through C9. C1 is a calcium dependent complex of three distinct proteins C1q, C1r and C1s. The proteins of the alternative pathway (collectively referred to as the properdin system) and complement regulatory proteins are known by semisystematic or trivial names. Fragments resulting from proteolytic cleavage of complement proteins are designated with lower case letter suffixes, for example, C3a. Inactivated fragments may be designated with the suffix i, for example C3bi. Activated components or complexes with biological activity are designated by a bar over the symbol for example C1 or C4b, 2a. The classic pathway is activated by the binding of C1 to classic pathway activators, primarily antigen-antibody complexes containing IgM, IgG1, IgG3, C1q binds to a single IgM molecule or two adjacent IgG molecules.
The alternative pathway can be activated by IgA immune complexes and also by non-immunologic materials including bacterial endotoxins , microbial polysaccharides and cell walls. Activation of the classic pathway triggers an enzymatic cascade involving C1, C4, C2 and C3, activation of the alternative pathway triggers a cascade involving C3 and factors B, D and P. Both result in the cleavage of C5 and the formation of the membrane attack complex. Complement activation also results in the formation of many biologically active complement fragments that act as anaphylatoxins , opsonins or chemotactic factors. (OMD),
Thermolabile: Subject to alteration or destruction by heat.
Unstable when heated; specifically: subject to loss of characteristic properties on being heated to or above 55°C thermolabile enzymes and vitamins (M+),
Bacteria: One of the two major classes of prokaryotic organism (the other being the Cyanobacteria). Bacteria are small (linear dimensions of around 1 m), non-compartmentalised, with circular DNA and ribosomes of 70S. Protein synthesis differs from that of eukaryotes and many antibacterial antibiotics interfere with protein synthesis, but do not affect the infected host. Recently bacteria have been subdivided into Eubacteria and Archaebacteria , although some would consider the Archaebacteria to be a third kingdom, distinct from both Eubacteria and Eukaryotes. The Eubacteria can be further subdivided on the basis of their staining using Gram stain . Since the difference between gram-positiv e and gram-negative depends upon a fundamental difference in cell wall structure it is therefore more soundly based than classification on gross morphology alone (into cocci, bacilli, etc.). (OMD) +++ Any of a group (as kingdom Procaryotae or kingdom Monera or the former class Schizomycetes) of prokaryotic unicellular round, spiral, or rod-shaped single-celled microorganisms that are often aggregated into colonies or motile by means of flagella, that live in soil, water, organic matter, or the bodies of plants and animals, and that are autotrophic, saprophytic, or parasitic in nutrition and important because of their biochemical effects and pathogenicity. (M+),
Membrane Attack Complex: (MAC) - The C5-9 membrane attack complex is formed by the activation of C5 to form C5b by either the classical or alternative pathways. C5b sits in the target cell membrane and sequentially binds C6 to C8. The new complex then binds 8 or 9 molecules of C9 which compose the walls of a pore spanning the target cell membrane. The pore permits small molecules to pass to the inside with subsequent lysis and death of the target cell.,
Proteolytic: Pertaining to, characterised by or promoting proteolysis . (OMD) The enzyme involved is protease.,
Kinin: One of a number of widely differing substances having pronounced and dramatic physiological effects. Some (e.g., kallidin and bradykinin) are polypeptides, formed in blood by proteolysis secondary to some pathological process, that stimulate visceral smooth muscle but relax vascular smooth muscle, thus producing vasodilation. (OMD)

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