Suvasini Modi Complement System Activation of Membrane attacking complex (MAC) and its effect and regulation

Transcription

1 Figure- 1 Suvasini Modi Complement System Activation of Membrane attacking complex (MAC) and its effect and regulation

2 Content Introduction Activation of Membrane attacking complex (MAC) Effect and regulation of Complement Factors Role of Complement Receptors Role of Small Fragments Summary 2

3 Introduction The term "complement" was coined by Paul Ehrlich to describe the activity in serum, which could "complement" the ability of specific antibody to cause lysis of bacteria. Complement system is composed of more than 25 different proteins produced by hepatocytes, macrophages and intestinal epithelial cells. 3

4 Overview Clearance of apoptotic cells C3d C3dg B cell activation 4

5 Classical Pathway Antibody binds to specific antigen on pathogen surface Overview Lectin Pathway Mannose-binding protein binds pathogen surface Alternative Pathway Pathogen surface creates environment conducive to complement activation Clearance of Apoptotic Cells Complement Activation Formation of C3 and C5 convertases Membrane Attack Pathway Cytolysis of some pathogens Figure -3 Inflammatory response Opsonization & phagocytosis of some pathogens Clearance of Immune complexes 5

6 Formation of C3 convertase Once C1 is activated, it activates 2 other complement proteins, C2 and C4 by cleaving it C2 is cleaved into C2a and C2b C4 is cleaved into C4a and C4b Both C2a and C4b bind together on the surface of the bacteria C2b and C4a diffuse away This C4b2a bind together to form active C3 complex that cleaves the C3 into C3a and C3b. C4b2a3b is the C3 convertase C3a acts as an opsonin. 6

7 Regulation and stability of C3 convertase The Complement activation takes place in the surface of the pathogens or the damaged tissues and not on the host cells. Amplification of the complement system dependent on the stability of C3 convertase- C3bBb Stability controlled Positive Regulator Eg. Properdin (factorp) Negative Regulator Eg. Decay-accelerating factor (DAF), Complement Receptor 1 (CR1), Factor H, Membrane cofactor of Proteolysis 7

8 Negative Regulatory Proteins CR1 being negative regulator for the Complement proteins. DAF Factor H competes with Bb to bind to C3b Convertase formation prevented by cleaving it into inactive form ic3b, along with Factor I and MCP (membrane proteolysis cofactor). All the complement proteins are in zymogen forms except Factor D, as it is highly specific to its binding site C3b that is found only on pathogen and not host Figure 2.27 Immunobiology,9/e (Garland Science 2012) 8

9 Amplification of C3 convertase and formation of C5 convertase Together with other components, attached C3b forms C5 convertase C3b after the cleavage of the C3 into C3a and C3b; binds covalently through thioester bond on the pathogen surface or is inactivated by hydrolysis. Thus, C3 increases the number of C3b on the pathogen surface in order to carry out immunological reactions. C5 activates specific protease that cleave C5 into C5a and C5b (only if bound to C3b part). C5a also acts as an opsonin, that induces phagocytosis. Figure 2.25 Immunobiology,9/e (Garland Science 2012) 9

10 Membrane Attacking Complex (MAC) formation MAC Confirmational changes allow the C5b67 complex to insert itself into the membrane that further helps C8-C9 to form MAC. C8 C8β (binds to C5b that in turn insert the C8α-γ into the lipid layer) C8α-γ ( induces Polymerization to form MAC 10

11 MAC Formation MAC has Hydrophobic external face (allowing to associate with lipid layer) and hydrophilic internal channel (100 A 0 diameter) leading to homeostasis and disruption of proton gradient thus penetration of lysozymes, eventually destruction of the cell. 11

12 C5a C5 C5b C6 C7 C8 C9 MEMBRANE ATTACK COMPLEX C5-C9 MAC Lysis Bacterium IgG IgM Summery of Classical Pathway Activated C1 C1 C4b2a3b C3 CONVERTASE C4b2a C4 C2 C4a C4b C2a C2b C5 CONVERTASE C3 C3b C3a 12

13 Summery of Alternative Pathway C3a C3 C3b Ba Bb Alternative C5 convertase C3 C3b C3bB C3bBb C3bBb3b Alternative C3 convertase Bacterium Factor B Factor D Properdin Lysis C5-C9 MAC MEMBRANE ATTACK COMPLEX C9 C8 C7 C6 C5b C5a C5 13

14 C3aC3 C3b Anaphylatoxin C3 C5 Alternative Pathway C3-Convertase C5-Convertase C7 C8 C3a C3b C5b C5a D BbB Ba C9 C6 14

15 Role of Complement Receptors (CRs) The important role is to facilitate phagocytosis. There are several CRs: CR1(CD35), CR2 (CD21), CR3(CD11b:CD18), CR4 (CD11c:CD18), CRIg (complement receptor for immunoglobulin family), C5a and C3a CR1 expressed on the neutrophils and macrophages, when bind to C3b stimulates phagocytosis in presence of the immune mediators. Other complement receptors, bind to inactive form of CD3 that remain remain attached on pathogen surface. 15

16 Figure 2.32 Immunobiology,9/e (Garland Science 2012) C3b regulatory Protein ic3b (Recognized by CRs except CR1 to carry phagocytosis) Factor I and CR1 Cleave ic3b to C3c (leaves) and C3dg bound that activates phagocytosis via CR2 (Found on B-cells) 16

17 Figure 2.31 Immunobiology,9/e (Garland Science 2012) Anaphylatoxin C5a enhances phagocytosis of microrganisms opsonized in innate immune response 17

18 Figure 2.30 Immunobiology,9/e (Garland Science 2012) Distribution and function of cellsurfaced recepters for complement proteins 18

19 Effect of Complement Factors 1. Opsonization 19

20 2. Inflammation anaphylotoxins C3a, C4a---increased vascular permeability C5a chemoattraction C3a, C4a----activation 20

21 Inflammatory response by C3a and C5a C3a increases the inflammatory response by binding to mast cells and causing them to release histamine 21

22 Regulation of Complement activation 22

23 C1INH (C1 inhibitor)bind to active enzymes C1r:C1s that causes dissociation from C1q. 23

24 DAF and CR1 displace C2a from C4b2a Factor H Competes with Bb to bind to C3b and also acts as cofactor for Factor I MCP along with CR1 catalyzes C3b to ic3b 24

25 Factor H Competes with Bb to bind to C3b and also acts as cofactor for Factor I 25

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27 Functions of Complement proteins 27

28 SUMMARY POINTS Complement system can be activated directly or indirectly by pathogen bound antibody. There are positive and negative regulators of the Complement System. Activation of C3 convertase is the central activity in activation of complement system. Function of Complement Systema) Opsonization b) Inflammatory response c) Clearance of immune complexes d) Lysis 28

29 Thank you for Attention!!!! Any Questions? 29