COMPLEMENT SYSTEM

SUGANDH CHAUHANMSC. BIOTECHNOLOGY

CENTRAL UNIVERSITY OFHARYANA

TOPICS• Definition • Pathway for complement activation • Lectin pathway•Membrane attack complex -Regulation of complement system• Biological effects of complement activation• Summary

DEFINITIONComplement is defined as the activity of blood serum that completes the action of antibody.

Consist of more than 30 soluble and cell bound proteins.

THREE PATHWAYS FOR COMPLEMENT ACTIVATION1.Classical pathway2.Alternative pathway3.Lectin or MBL pathway

LECTIN PATHWAY

• Independent of antibodies for its activation.• Lectins are proteins that recognize and bind to specific

carbohydrate targets.• Activated by Mannose binding lectin (MBL) – lectin that binds to

mannose residues on glycoproteins on surface of microbes .• MBL is an acute phase protein produced in inflammatory

responses.• Also known as MBL pathway.• MBL is similar to C1q in structure and function.

ACTIVATION OF LECTIN PATHWAY• First step is binding of Mannose binding lectin (MBL) to

mannose residues on the surface of microbes.• To the MBL bound to microbe , MBL Associated Serine

Proteases MASP-1 and MASP-2 will bind.•MASP-1 and 2 is similar in structure and function to C1s and

C1r.• This complex causes cleavage and activation of C4 and C2.• Activation of C2 - C4 lead to formation of C5 convertase.

MEMBRANE ATTACK COMPLEX• C5b , C6 ,C7 , C8 and C9 interact to form a structure called

membrane attack complex (MAC)•MAC forms a large channel through the membrane , enabling

ions and small molecules to diffuse freely across the membrane.• End result of activating these three pathways is production of an

active C5 convertase.• C5 convertase cleaves C5 (comprises of two chains α and β) at

the amino terminus of α chain generating C5a and C5b fragment.

-C5b binds to the surface of target cell , providing the binding site for subsequent components of MAC. C5a diffuses away.

-C5b binds to C6

-C5b6 binds toC7

-Complex undergoes a hydrophilic – amphiphilic structural transition exposing hydrophobic regions , which serve as binding sites for membrane phospholipids.

-If reaction occurs on target cell membrane hydrophobic binding sites enable the complex to insert the phospholipid bilayer.

CONT….• If reaction occurs on an immune complex then the

hydrophobic binding sites cannot anchor the complex and is released.

• Released C5b67 complex insert into membrane of nearby cells and mediate lysis.

• Regulator proteins prevent this from occurring.

-C8 binds to membrane bound C5b67 and undergoes a hydrophobic – amphiphilic transition, exposing hydrophobic regions which interact with PM

-C5b678 complex creates a pore leading to lysis of red blood cells.

-Final step is binding and polymerization of C9 , a perforin like molecule to C5b678 complex.

-10-17 molecules of C9 can be bound and polymerized by a single C5b678 complex.

-C9 molecule undergo a hydrophilic amphiphilic transition so cannot insert into the membrane.

CONT….• MAC has a tubular form and pore size of 70-100 A.

• Since ions and small molecules can diffuse freely through the central channel of MAC ,the cell cannot maintain its osmotic stability and is killed by an influx of water and loss of electrolytes.

REGULATION OF COMPLEMENT SYSTEM• Many elements of the complement system are capable of attacking

host cells as well as foreign cells and microorganisms.• Regulatory mechanisms have evolved to restrict complement activity

to designated targets. • Mechanism of regulation in all complement pathways is the inclusion

of highly labile components that undergo spontaneous inactivation if they are not stabilized by reaction with other components.• A series of regulatory proteins can inactivate various complement

components.

CONT….• For example, the glycoprotein C1 inhibitor (C1Inh) can form a complex with

C1r2s2, causing it to dissociate from C1q and preventing further activation of C.

• Regulatory proteins all contain repeating amino acid sequences of about 60 residues, termed short consensus repeats (SCRs).

• All these proteins are encoded at a single location on chromosome 1 in humans, known as the regulators of complement activation (RCA) gene cluster.

CONT….• In the classical and lectin pathways, three structurally distinct RCA

proteins act similarly to prevent assembly of C3 convertase.

• Regulatory proteins include soluble C4b-binding protein (C4bBP) and two membrane-bound proteins, complement receptor type 1 (CR1) and membrane cofactor protein (MCP).

• Each of these regulatory proteins binds to C4b and prevents its association with C2a. Once C4bBP, CR1, or MCP is bound to C4b, another regulatory protein, factor I, cleaves the C4b into bound C4d and soluble C4c.

CONT…..• Decay accelerating factor (DAF or CD55) which is a glycoprotein

anchored covalently to a glycophospholipid membrane, has the ability to dissociate C3 convertase.• After dissociation of C3 convertase , factor I cleaves membrane bound

C4b or C3b component irreversibly inactivating the convertase.• Regulatory proteins also operate at the level of MAC.• Serum protein S bind to released C5b67 including a hydrophilic

transition and prevent its insertion into membrane of nearby cells.

CONT…• Membrane proteins ( HRF and MIRL ) block MAC formation.

• Both protect cells from complement mediated lysis by binding to C8 preventing assembly of poly C9 and its insertion into the PM.

BIOLOGICAL EFFECTS OF COMPLEMENT SYSTEM-MAC mediate lysis.

Other components participate in-• Inflammatory response• Virus neutralization• Opsonization of antigen ( enhance phagocytosis)• Clearance of immune complexes

CELL LYSIS • MAC lyse gram negative bacteria , viruses , parasites , erythrocytes and nucleated

cells.

• These pathways serve as important innate defense against infectious microorganisms . Antibody and complement play role in defense against viruses.

• Herpesviruses , orthomoxyviruses , paramoxyviruses and retroviruses are susceptible to complement mediated lysis.

• Gram positive bacteria are resistant to lysis due to thick peptidoglycan layer that prevents insertion of MAC.

CONT….• Complement system is effective in lysing gram negative bacteria. A

few gram negative bacteria develop resistance to complement mediated lysis.

• E.coli and Salmonella show resistance to lysis b due to presence of long polysaccharide chains in cell wall LPS which prevent insertion of MAC into membrane.

OPSONIZATION• C3b is major opsonin of

complement system.

• C3b and C4b binding facilitates opsonization.

• Enhance phagocytosis.

CONT….• C3 activation result in a coating of C3b on immune complexes and

particulate antigens. Phagocytic cells express complement receptors (CR1 ,CR3 and CR4) that bind C3b ,C4b or iC3b.

• Antigen coated with C3b binds to cells bearing CR1 facilitating phagocytosis of C3b coated antigens.

• C3b targets the antigen to the phagocyte enhancing the initiation of antigen processing and accelerating antibody production.

CLEAVAGE PRODUCT MEDIATE INFLAMMATION• Smaller fragments resulting from complement cleavage C3a , C4a , C5a called

anaphylatoxin bind to receptors on mast cells and basophils and induce degranulation with release of histamine.

-It also induce smooth muscle contraction and increased vascular permeability.

-Activation of CS result in influx of fluid that carries antibody to site of antigen entry. Activity of highly reactive anaphylatoxins are regulated by a serum protease called carboxypeptidase N.

-C3a ,C5a and C5b67induce monocytes and neutrophils to adhere to vascular endothelial cells , extravasate through the endothelial lining of the capillary and migrate toward the site of complement activation.

COMPLEMENT SYSTEM NEUTRALIZES VIRAL INFECTIVITY• Some viruses eg.retroviruses , can activate all the three pathway in

absence of antibody.• Neutralization is achieved through the formation of larger viral

aggregates and reduce the net number of infectious viral particles.• C3b component facilitates aggregate formation in presence of

antibody. For example , polyoma virus coated with antibody is neutralized when serum containing activated C3 is added.• Binding of antibody to surface of viral particle creates a thick protein

coating.

CONT….• Coating neutralizes viral infectivity by blocking attachment to

susceptible host cells.

• Complement is effective in lysing of most of the enveloped viruses resulting in fragmentation of envelope and disintegration of nucleocapsid.

CLEARS IMMUNE COMPLEXES

• Patients with autoimmune disease systemic lupus erythematosus (SLE) produce large quantities of immune complexes and suffer tissue damage as a result of lysis.

• Deficiencies in C1 ,C2 ,C4 and CR1 develop SLE and interfere with clearance of immune complexes.

CONT…..• Coating of soluble immune complexes with C3b facilitate their binding

to CR1 on erythrocytes.

• Erythrocytes play important role in binding C3b coated immune complexes and carry these complexes to liver and spleen.

• In these organs immune complexes are stripped from RBCs and are phagocytosed and prevent deposition in tissues.

SUMMARY• Complement system comprises a group of serum proteins that

complement the action of antibody.

• Complement activation occurs by classical , alternative and lectin pathway.

• Activation of lectin pathway is antibody dependent and the end result is cell lysis and complement system mediate opsonization of bacteria , activation of inflammation and clearance of immune complexes.

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