Prepared by Dr. Dhafer Farhan PhD Cancer research/ UK
Today We are here* Presented by Dr. Minen Al-Kafajy PhD Microbiology USA
Complement system
*A defensive system consisting of over 30 proteins produced by the liver and found in circulating blood serum. (with exception of C1 which synthesize in the epithelial cells of the intestine & limited quantities of complement included C1q can be synthesize by activated monocytes or macrophage. 10% of the total serum proteins. Evolved as part of the innate immune system.
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They circulate in inactive forms as proenzymes or zymogens. Complement proteins are designated by numerals : C1 - C9 or by letters (factor D). Activation results in cleavage into 2 fragments a small fragment (a) which diffuse from the site & a large fragment (b) which bind to the target
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The complement system is one of the Ag-recognition molecules C1(C1q) is capable to recognized bacterial cell wall – polyanionic surfaces and lipoteichoic acid on Gram negative surfaces. * C3b is also capable to recognize sialic acid surface on pathogen’s surface. Complement system
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The complement system can be activated by classical , alternative or lectin pathways. The proteins of the system act in enzyme cascades. Where each step generates enzyme which act in the following step of the cascade. The three pathways generate enzymes which cleave C3 into two fragments, C3a and C3b as a central step in the process of complement activation.
Complement system
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is activated by immune complexes and the activation is initiated by the binding of C1 to domains in IgG (CH2-domain) or IgM (CH3-domain) which are complexed with Ag.
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The recognition unit of the classical pathway is the C1q which is a molecule with a six globular head group and when a specific Ab (IgG or IgM) interacts with its corresponding Ag , binding sites for the globular head group of the C1q are exposed on the Fc-region of the Ab –molecule.At least two molecules of IgG or one molecule of IgM are required for binding C1q. *
After binding to Fc-region of Ab molecule a conformational change occurs in C1q and this change in C1q causes the pro - enzyme C1r to become the enzymatically active C1r. The substrate for the enzyme C1r is C1s, which is then cleaved to become the serine estrase, C1s.
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Complement system
The activation unit, the active enzyme of C1s cleaves two proteins, C4 (into C4a & C4b) and C2 (into C2a & C2b), in a magnesium-dependent reaction. C4b and C2a combine to form an active enzyme, C4b2a, which is the classical pathway, C3-convertase which cleave many molecules of C3 (into C3a & C3b).*
Complement system
The C3b can bind to C4b2a to form C4b2a3b a C5-convertase, an enzyme cleaved C5 (into C5a & C5b). C5b binds to one molecule of C6 to form a stable bimolecular complex and its bind to C7 to form a trimolecular complex (C5b67), this trimolecular complex binds hydrophobically to a membrane since the complex is amphiphilic, this allow it to insert into cell membranes. C8 now joins the complex and unwinds into the cell membrane. Thus, forming a functional trans-membrane channel and itself cause disruption and lysis of membranes, an effect which is greatly enhanced by the incorporation of C9 and if more than six molecule of C9 enter the complex.*
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In presence of factor D and magnesium, C3b-like molecule can cleave factor B (into Ba and Bb), Bb bind to the C3b to form an alternative pathway C3-convertase C3bBb. The C3bBb is a very unstable and quickly inactivated by control proteins, unless its bound to activating surface and stabilized by P (properdin), the C3bBbP enzymatic complex can cleave additional molecules of C3. If a second C3b is inserted into the C3-convertase, it become C3bBb3bP, this becomes a C5-convertase that can cleave C5 into C5a and C5b. The membrane attack unite for the alternative pathway begins with C5b and progresses through C6,7,8 and C9 in exactly the same sequence as it does for the classical pathway.
Alternative pathway Activated near 'protected' surfaces, Such as bacterial or fungal cell walls, bacteraial lipopoly- sacharide, some virus-infected cells and rabbit RBCs.
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Schematic diagram of intermediates in the formation of bound C5b by the alternative pathway of complement activation
1.C3 hydrolyzes spontaneously, C3b fragment attaches to foreign surface2.Factor B binds C3a, exposes site acted on by Factor D. Cleavage generatesC3bBb, which has C3 convertase activity.3.Binding of properdin stabilizes convertase4.Convertase generates C3b; some binds to C3 convertase activating C5’ convertase. C5b binds to antigenic surface. *
Membrane attack complex
** Activated by bacterial carbohydrates. The molecule which initiates the pathway mannan - binding lectin (MBL). MBL, an acute phase protein, binds to mannose residues, and to certain other sugars on many pathogens. MBL, like C1q, is a two- to six-headed molecule that forms a complex with two protease zymogens (MASP-1 and MASP-2).
Lectin pathway
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When MBL binds to terminal mannose group on bacterial carbohydrates it activates MASP-1 & 2 which go on to activate the classical pathway in an Ab - independent fashion. But, it has been shown that activated MASP -2 cleaves C4 and C2 while activated MASP - 1 cleave C3 and C2.
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The biological consequences of complement activation
*Anaphylaxis: C3a, C5a a biologically active peptides and these anaphylatoxin mediate inflammation by inducing the release of mediators from basophiles and mast cells, causing smooth muscle contraction and increase vascular permeability.- Immune adherence: it’s the covalent bonding between C3b and nearby soluble immune complexes or particulate surfaces. Since C3b has a receptors on human erythrocyte, B-lymphocytes, monocytes, glomerular epithelial cells and mast cells. One biologic purpose for immune adherence is to facilitate removal of soluble immune complexes and immune adherence provides a mechanism for the soluble complexes to bind to erythrocytes, facilitating removal of these complexes by the reticuloendothelial system. *
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Several proteins serve as inhibitors or inactivators of specific reactions or products involved in the complement cascade like: *C1-inhibitor (C1INH): form irreversible complex with both C1r and C1s and block their enzyme activities and dissociate them from C1q. *Factor H & I : serve to control tightly the enzyme that cleave C3 and C5 (factor I: Inactivates C3b and C4b. Whereas, factor H accelerates the decay of alternative C3-convertase C3bBb3b by dissociating Bb from the enzyme.).
The first mean of control is if activated complement (enzyme) does not combine with its substance within milliseconds, the activity is lost.
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Control of complement activation
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C4-binding protein ( C4BP ): bind to and inactivate C4.•Anaphylatoxin inactivators (carboxypeptidase) effect C4a, C3a, C5a by removal of a single amino acid (carboxyterminal arginine). *MAC inhibitor: a serum protein can bind to fluid phase C5b67, prevent its attachment to membrane protein or C8.* Inhibition of assembly: like the (protectin, CD59) and (delay accelerating factors, DAF). *
Control proteins of the classical and Alternative pathways
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C1INH-deficiency or ANGIONEUROTIC EDEMA :Deficiency of C1INH lead to uncontrolled activation of the classical PW. Thus, C1s continues to cleavage C4 and C2 resulting in C2a release leading to kinin like activity and C4a release leading to anaphylactic reaction Activity. Both producing swelling ( extra-ordinary amounte of fluid in tissue spaces as a result of increase vascular Permeability and smooth muscle contraction ).No immunodeficiency, and decreased C2 & C4, with Normal C3 (downstream C4BP works).•The heridetery form called HEREDITARY ANGIOEDEMA (HAE), its autosomal dominant trait. Whereas, acquired form associated with lympho-proliferative diseases.* Treatment is either with Danazol or anabolic steroids. Clinical diseases associated with complement activity:
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It is a clonal disorder of hemopoietic cells, in which there is increased susceptability to damage by complement due to deficiency of glycosyl phosphatidyl inositol (GPI) which binds delay accelerating factors (DAF) and protectin (CD59) to membrane. The clinical feature of this illness is the episodic hemoglobinuria mainly at morning, hemolytic anemia, increased susceptibility to infection due to decreased number or function of WBC.
Paroxysmal nocturnal haemoglobinuria (PNH)
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Nephritic factors (NF)
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