Complement: History. Discovered in 1894 by Bordet. It represents lytic activity of fresh serum

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2 Complement: History Discovered in 1894 by Bordet It represents lytic activity of fresh serum Its lytic activity destroyed when heated at 56C for 30 min

3 Complement functions Host benefit: opsonization to enhance phagocytosis phagocyte attraction and activation lysis of bacteria and infected cells regulation of antibody responses clearance of immune complexes clearance of apoptotic cells Host detriment: Inflammation, anaphylaxis

4 Proteins of the complement system (nomenclature) C1(qrs), C2, C3, C4, C5, C6, C7, C8, C9 factors B, D, H and I, properdin (P) mannose binding lectin (MBL), MBL associated serine proteases (MASP-1 MASP-2) C1 inhibitor (C1-INH, serpin), C4-binding protein (C4-BP), decay accelerating factor (DAF), Complement receptor 1 (CR1), protein- S (vitronectin)

5 Definitions C-activation: alteration of C proteins such that they interact with the next component C-fixation: utilization of C by Ag-Ab complexes Hemolytic units (CH50): dilution of serum which lyses 50% of a standardized suspension of Ab-coated r.b.c C-inactivation: denaturation (usually by heat) of an early C-component resulting in loss of hemolytic activity Convertase/esterase: altered C-protein which acts as a proteolytic enzyme for another C-component

6 Activation product of complement proteins (nomenclature) Activated component are usually over-lined: e.g. C1qrs When enzymatically cleaved, the larger moiety, binds to the activation complex or membrane and the smaller peptide is released in the microenvironment Letter b is usually added to the larger, membrane-binding, peptide and a to the smaller peptide (e.g., /C3a, C4b/C4a, C5b/C5a), EXCEPT C2 (the larger, membranebinding moiety is C2a; the smaller one is C2b)

7 Pathways of complement activation CLASSICAL PATHWAY LECTIN PATHWAY ALTERNATIVE PATHWAY antibody dependent antibody independent Activation of C3 and generation of C5 convertase activation of C5 LYTIC ATTACK PATHWAY

8 Components of the Classical Pathway C1r C1s Ca ++ C1q C2 C3 C4 C1 complex

9 Classical Pathway Generation of C3-convertase C4a C1r C1s C1q Ca ++ b C4

10 Classical Pathway Generation of C3-convertase C4a C1r C1s C1q Ca ++ Mg ++ a C2 C2b C4b2a is C3 convertase C4b C2a

11 Classical Pathway Generation of C5-convertase C4a C1r C1s C1q Ca ++ Mg ++ C2b C3a C4b2a3b is C5 convertase; it leads into the Membrane Attack Pathway C4b C2a C3 b

12 Biological Activities of Classical Pathway Components Component C2b C3a C4a Biological Activity Prokinin; cleaved by plasmin to yield kinin, which results in edema Anaphylotoxin; can activate basophils and mast cells to degranulate resulting in increased vascular permeability and contraction of smooth muscle cells, which may lead to anaphylaxis Opsonin Activation of phagocytic cells Anaphylaotoxin C4b Opsonin 12

13 Control of Classical Pathway Components Component All Regulation C1-inhibitor (C1-INH); dissociates C1r and C1s from C1q C3a C3a-inactivator (C3a-INA; Carboxypeptidase B) C4a C4b Factors H and I; Factor H facilitates the degradation of by Factor I C3a-INH C4 binding protein (C4-BP) and Factor I; C4-BP facilitates degradation of C4b by Factor I; C4-BP also prevents the association of C2a with C4b thus blocking formation of C3 convertase 13

14 C1-inhibitor deficiency: hereditary angioedema

15 Components of mannose-binding lectin pathway C4 MASP2 MBL C2 MASP1

16 Mannose-binding lectin pathway C4a MASP1 C2b MASP2 MBL C4b C2 C2a C4b2a is C3 convertase; it will lead to the generation of C5 convertase C4b C2a

17 Components of the alternative pathway D C3 B P

18 Spontaneous C3 activation Generation of C3 convertase H 2 O D C3 i B b C3 b C3a C3iBb complex has a very short half life

19 C3-activation the amplification loop If spontaneously-generated is not degraded D C3a B b C3 b

20 C3-activation the amplification loop D C3 b B b C3a C3a Bb

21 C3-activation the amplification loop D C3 b B b Bb C3a C3a C3a Bb

22 C3-activation the amplification loop Bb Bb C3a C3a C3a Bb

23 C3-activation the amplification loop Bb Bb C3a C3a C3a Bb

24 Control of spontaneous C3 activation via DAF DAF prevents the binding of factor B to B DAF CR1 Autologous cell membrane

25 Control of spontaneous C3 activation via DAF DAF dislodges -bound B b B b factor Bb DAF CR1 Autologous cell membrane

26 Control of spontaneous C3 activation via CR1 B b H Bb DAF I CR1 i DAF Autologous cell membrane i CR1 I

27 Degradation of spontaneously produced C3c I C3dg i C3c I C3dg i

28 stabilization and C5 activation finds an activator (protector) membrane P D C3a This is stable C5 convertase of the alternative pathway B b C3 b

29 regulation on self and activator surfaces

30 C5-convertase of the two pathways C5-convertase of the Classical and lectin Pathways C5-convertase of the Alternative Pathway C4b C2a Bb

31 Lytic pathway Generation of C5 convertase leads to the activation of the Lytic pathway

32 Components of the lytic pathway C5 C6 C7 C8 C 9

33 Lytic pathway C5-activation C5a C5 b C4b C2 a

34 Lytic pathway assembly of the lytic complex C6 C7 C5 b

35 Lytic pathway: insertion of lytic complex into cell membrane C6 C8 C7 C5 b C 9 C 9 C 9 C 9 C 9 C 9 C 9 C 9 C 9

36 Biological effects of C5a

37 Biological properties of C-activation products Product Biological Effects Regulation C2b (prokinin) C3a (anaphylatoxin) edema mast cell degranulation; enhanced vascular permeability; anaphylaxis C1-INH carboxypeptidase- B (C3-INA)

38 Biological properties of C-activation products Product Biological Effects Regulation (opsonin) C4a (anaphylatoxin) C4b (opsonin) opsonization; phagocyte activation as C3, but less potent opsonization; phagocytosis factors H & I (C3-INA) C4-BP, factor I

39 Biological properties of C-activation products Product Biological Effects Regulation C5a (chemotactic factor) C5b67 anaphylactic as C3, but much more potent; attracts & activates PMN causes neutrophil aggregation, stimulation of oxidative metabolism and leukotriene release chemotaxis, attaches to other membranes carboxypeptidase-b (C3-INA) protein-s

40 Complement Deficiencies and Disease Classical Pathway Pathway Component Disease Mechanism C1INH Hereditary Angioedema Overproduction of C2b (prokinin) C1, C2, C4 Predisposition to SLE Opsonization of immune complexes help keep them soluble, deficiency results in increased precipitation in tissues and inflammation 40

41 Complement Deficiencies and Disease Lectin Pathway Pathway Component Disease Mechanism MBL Susceptibility to bacterial infections in infants or immunosuppressed Inability to initiate lectin pathway 41

42 Complement Deficiencies and Disease Alternative Pathway Pathway/Component Disease Mechanism Factors B or D C3 C5, C6, C7 C8, or C9 Susceptibility to pyogenic (pus-forming) bacterial infections Susceptibility to bacterial infections Susceptibility to Gramnegative infections Lack of sufficient opsonization of bacteria Lack of opsonization and inability to utilize the membrane attack pathway Inability to attack the outer membrane of Gramnegative bacteria 42

43 Complement Deficiencies and Disease Alternative Pathway cont. Pathway Component Disease Mechanism Properdin (X-linked) Susceptibility meningococcal meningitis Lack of opsonization of bacteria Factors H or I C3 deficiency and susceptibility to bacterial infections Uncontrolled activation of C3 via alternative pathway resulting in depletion of C3 43