Complement System PDF

COMPLEMENT SYSTEM Selim BADUR, PhD 4th lecture of Immunology  From the previous lecture… All multicellular organisms have intrinsic mechanisms of defense against infections, which constitute innate immunity. Toll-like receptors (TLRs), expressed...

COMPLEMENT SYSTEM Selim BADUR, PhD 4th lecture of Immunology  From the previous lecture… All multicellular organisms have intrinsic mechanisms of defense against infections, which constitute innate immunity. Toll-like receptors (TLRs), expressed on plasma membranes and endosomal membranes of many cell types, are an innate immune system receptors that recognize different microbial products The principal components of innate immunity are: epithelial barrier cells in skin, gastrointestinal tract, and respiratory tract; phagocytes; dendritic cells; mast cells; natural killer cells; cytokines; and plasma proteins, including the proteins of the complement system Epithelia provide physical barriers against microbes; produce antimicrobial peptides, including defensins and cathelicidins; and contain lymphocytes that may prevent infections In inflammation, phagocytes are recruited from the circulation to sites of infection and tissue damage. The leukocytes are activated, and they ingest and destroy microbes and damaged cells. The components of the innate immune system include: - Epithelial cells - Circulating and recruited phagocytes (monocytes and neutrophils) - Sentinel cells in tissues (macrophages, dendritic cells, mast cells, and others) - Innate lymphoid cells - NK cells - and a number of plasma proteins (Complement system)  The complement system-1 Part of innate immunity Acts as a bridge between innate and acquired arms of the Immune system A defensive system consisting > 30 proteins produced by the liver and found in circulating blood serum Components of the Complement system: - Heat labile serum proteins (Complement) - Complement Receptors - Regulatory serum proteins  The complement system-2 These proteins are not immunoglobulins and their concentration in serum do not increase after immunization The term complement refers to the ability of these proteins to complement the activity of antibodies in destroying cells, including microbes The alphanumeric naming system uses a combination of uppercase ‘C’ (complement), a number 1 – 11 to designate the subtype of complement enzymes and a lowercase ’a’ or ‘b’ to identify a cleaved fragment of the enzyme.  Discovery of complement system Complement was discovered by Jules Bordet as a heat-labile component of normal plasma that causes the opsonisation and killing of bacteria In 1890, and in Institut Pasteur, Paris he observed: - Sheep antiserum to Vibrio cholerae caused lysis of the bacteria - Heating the antiserum destroyed its bacteriolytic activity - Addition of fresh normal serum, (no Antibodies against the bacterium and was unable to kill the bacterium by itself), restored the NO FREE FREE ability to lyse the bacteria by the heated antiserum COMPLEMENT COMPLEMENT Complement was discovered as a heat-labile component of normal plasma that allows antibodies to kill some bacteria. This activity was said to 'complement' the antibacterial activity of antibody, hence the name.  Pathways of complement activation The activation of the complement system involves sequential proteolytic cleavage of complement proteins, leading to the generation of effector molecules that participate in eliminating microbes in different ways The activation of the complement system may be initiated by three distinct pathways, all of which lead to the production of C3b. The three pathways of complement activation differ in how they are initiated, but they share the late steps and perform the same effector functions C3b initiates the late steps of complement activation, culminating in the formation of a multiprotein complex called the membrane attack complex (MAC), which is a transmembrane channel that causes lysis of microbes/cells.  The complement system activation-1 Many complement proteins are proteolytic enzymes, and complement activation involves the sequential activation of these enzymes The complement cascade may be initiated by any of three pathway: - The alternative pathway is triggered when some complement proteins are activated on microbial surfaces - The classical pathway is most often triggered by antibodies that bind to microbes or other antigens - The lectin pathway is activated when a carbohydrate-binding plasma protein, mannose- binding lectin (MBL), binds to its carbohydrate ligands on microbes.  The complement system activation-2 Activated complement proteins function as proteolytic enzymes to cleave other complement proteins The central component of all three complement pathways is a plasma protein called C3, which is cleaved by enzymes generated in the early steps The major proteolytic fragment of C3, called C3b, becomes covalently attached to microbes and is able to recruit and activate downstream complement proteins on the microbial surface  Overview of the classical Complement pathway - C1 cleaves C4 and C2 - C4b and C2a bound to the surface of pathogen, forming C3 convertase - C3 convertase cleaves C3 - C3b binds to the pathogen near the C3 convertase forming C5 convertase - C5 convertase cleaves C5 - C5b joins C6, C7, C8 and C9 to form MAC, penetrating the pathogen cell membrane Early steps of complement activation Although the sequence of events is similar, the three pathways differ in their requirement for antibody and the proteins used. Late steps of complement activation The late steps of complement activation start after the formation of the C5 convertase and are identical in the alternative and classical pathways.  1 Activation of complement system by classic pathway 2 3 4 5 6 7 8 9 MAC formation 1 Activation of complement system by alternative pathway 2 3 4 5 6 Early steps components of complement system Late steps components of complement system The activated molecule is divided in 2 parts: 1st part with enzimatic activity 2nd part with biologic activity Biologically active intermediates are released  Creating MAC is not the most important activity of the system * In addition to promoting microorganism destruction by MAC (membrane attack complex), the complement system exerts a wide-ranging influence on the activities of cells involved in the immune response Thus, complement is instrumental in - the direction of blood leucocytes to a site of inflammation; - the induction of granule release and synthesis of cytotoxic oxygen- and nitrogen-containing compounds by leucocytes of the myeloid lineage; - the promotion of particle phagocytosis by these cells; - the clearance of soluble immune complexes (ICs) from the circulation; - and the induction of a primary B-cell response to antigen The functions of complement The complement system serves three main functions in host defense: 1- Opsonization and phagocytosis. C3b coats microbes and promotes the binding of these microbes to phagocytes by virtue of receptors for C3b that are expressed on the phagocytes: OPSONIZATION. Thus, microbes that are coated with complement proteins are rapidly ingested and destroyed by phagocytes. 2- Cell lysis. Complement activation culminates in the formation of a polymeric protein complex (MAC) that inserts into the microbial cell membrane, disturbing the permeability barrier and causing osmotic lysis 3- Inflammation. Some proteolytic fragments of complement proteins, especially C5a and C3a, are chemoattractants for leukocytes and they also are activators of endothelial cells and mast cells. Effects of anaphylatoxins Neutrophiles and monocytes Increased vascular Adhesion and permeability chemotaxis Chemokines Activation Smooth musle contraction Vasoactive amines Mast cell A)- Lysis: Complement activation and the generation of C5 convertases lead to the liberation of the C5 product, C5b. C5b forms the basis of the MAC assembly: form a stably inserted pore with ~10nm diameter. Formation of the pore leads to a dysregulation of ion concentrations across the membrane and loss of mitochondrial polarity. B)- Inflammation: Anaphylatoxins are potent proinflammatory molecules generated from the cleavage of C4, C3, and C5 into C4a, C3a, and C5a, respectively. Binding of anaphylatoxins to the N-terminal region of their cognate receptors, C3aR and C5aR, allows conformational changes to the intracellular domains to induce G-protein coupling and downstream signaling. C)- Opsonization / Phagocytosis: Generation of the C3b fragments by C3 convertases of all three activation pathways initiates the opsonization pathway of complement. Opsonic fragments, such as C3b and its cleavage products, are recognized by complement receptors. Fc receptors bind to the Fc region of antibody. Binding of the complement receptors to opsonized bodies mediates their sequestration and uptake by phagocytic cells, most commonly macrophages and neutrophils. In summary… The classical complement pathway typically requires Ag-Ab complexes (ICs) for activation The alternative pathway can be activated by spontaneous C3 hydrolysis, foreign material, pathogens, or damaged cells. The lectin pathway can be activated by C3 hydrolysis or antigens without the presence of antibodies. In all three pathways, C3-convertase cleaves and activates component C3, creating C3a and C3b, and causes a cascade of further cleavage and activation events. C3b binds to the surface of pathogens, leading to greater internalization by phagocytic cells by opsonization.  Receptors of Complement components (CR) - These effects are exerted through interaction of the activation products of complement factors C1, C3 or C5 with specific receptors (CR) on the responding cells. - CR is a group of membrane proteins expressed on the surface of immune cells, which is an important connection between complement components and cellular functions CRs involve in the regulation of multiple bio-activities, such as phagocytosis, immunoregulation, adhesion, inflammation, immune complex (IC) clearance, etc.  Regulation of complement activation Mammalian cells express regulatory proteins that inhibit complement activation, thus preventing complement- mediated damage to host cell C1 inhibitor (C1 INH) prevents the assembly of the C1 complex, which consists of C1q, C1r, and C1s proteins, thereby blocking complement activation by the classical pathway. The lipid-linked cell surface protein decay-accelerating factor (DAF) and the type 1 complement receptor (CR1) interfere with the formation of the C3 convertase by blocking the binding of Bb (in the alternative pathway) or C2a (in the classical pathway). Membrane cofactor protein (or CD46) and CR1 serve as cofactors for cleavage of C3b by a plasma enzyme called factor I, thus destroying any C3b that may be formed Major regulatory components of complement system Summary The complement system is a collection of circulating and cell surface proteins that play important roles in host defense. The complement system may be activated on microbial surfaces without antibodies (alternative and lectin pathways, mechanisms of innate immunity) and after the binding of antibodies to antigens (classical pathway, a mechanism of adaptive humoral immunity). Complement proteins are sequentially cleaved, and active components, in particular C4b and C3b, become covalently attached to the surfaces on which complement is activated. The late steps of complement activation lead to the formation of the cytolytic MAC. Different products of complement activation promote phagocytosis of microbes, induce cell lysis, and stimulate inflammation. Mammals express cell surface and circulating regulatory proteins that prevent inappropriate complement activation on host cells