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

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

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)

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 Definitions

Activation product of complement proteins (nomenclature) 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., C3b/C3a, C4b/C4a, C5b/C5a), EXCEPT C2 (the larger, membrane- binding moiety is C2a; the smaller one is C2b) Activated component are usually over-lined: e.g. C1qrs

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

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

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

C4b Mg ++ C4a Ca ++ C1r C1s C1q C2 C2b a C2 a _____ C4b2a is C3 convertase

Classical Pathway Generation of C5-convertase C4b Mg ++ C4a Ca ++ C1r C1s C1q C2b C2 a C3 C3a b ________ C4b2a3b is C5 convertase; it leads into the Membrane Attack Pathway

12 Biological Activities of Classical Pathway Components ComponentBiological Activity C2bProkinin; cleaved by plasmin to yield kinin, which results in edema C3aAnaphylotoxin; can activate basophils and mast cells to degranulate resulting in increased vascular permeability and contraction of smooth muscle cells, which may lead to anaphylaxis C3bOpsonin Activation of phagocytic cells C4aAnaphylaotoxin C4bOpsonin

13 Control of Classical Pathway Components ComponentRegulation AllC1-inhibitor (C1-INH); dissociates C1r and C1s from C1q C3aC3a-inactivator (C3a-INA; Carboxypeptidase B) C3bFactors H and I; Factor H facilitates the degradation of C3b by Factor I C4aC3a-INH C4bC4 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

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

Mannose-binding lectin pathway C4 MBL C4b C4a C4b C2 C2b C2a _____ C4b2a is C3 convertase; it will lead to the generation of C5 convertase MASP1 MASP2

Components of the alternative pathway C3 B D P

Spontaneous C3 activation C3 H2OH2O i B D Generation of C3 convertase C3iBb complex has a very short half life b C3 C3a b

B D b C3b If spontaneously-generated C3b is not degraded C3-activation the amplification loop C3 C3a b

B D BbBb C3b C3 b C3-activation the amplification loop C3b C3a b

BbBb C3b C3 BbBb B D b b C3a C3-activation the amplification loop C3b

C3a BbBb C3b BbBb BbBb C3a C3-activation the amplification loop C3b

C3a BbBb C3b BbBb BbBb C3a C3-activation the amplification loop C3b

Control of spontaneous C3 activation via DAF C3b DAF prevents the binding of factor B to C3b B Autologous cell membrane DAF CR1

Control of spontaneous C3 activation via DAF DAF dislodges C3b-bound factor Bb B bb C3b Autologous cell membrane DAF CR1 B b

Autologous cell membrane C3b B b H I iC3b Control of spontaneous C3 activation via CR1 B b I iC3b DAF CR1 DAF CR1

Degradation of spontaneously produced C3b C3b iC3b II C3dg C3c

C3b stabilization and C5 activation C3b C3b finds an activator (protector) membrane C3 C3a b B D b P This is stable C5 convertase of the alternative pathway

C3b regulation on self and activator surfaces C3b

C5-convertase of the two pathways C3b BbBb C5-convertase of the Alternative Pathway C4b C2a C3b C5-convertase of the Classical and lectin Pathways

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

Components of the lytic pathway C6 C9C9 C8 C7 C5

Lytic pathway C5-activation C3b C2 a C4b C5 b C5a

Lytic pathway assembly of the lytic complex C5 b C6 C7

Lytic pathway: insertion of lytic complex into cell membrane C5 b C6 C7 C8 C9C9 C9C9 C9C9 C9C9 C9C9 C9C9 C9C9 C9C9 C9C9

Neutrophil Activation Adhesion Neutrophil Chemotaxis Respiratory burst Monocyte Activation Cytokine production Mast cell Degranulation Vascular permeability Vascular wall transmigration Biological effects of C5a

ProductBiological EffectsRegulation Biological properties of C-activation products C2b (prokinin) edemaC1-INH C3a (anaphylatoxin) mast cell degranulation; enhanced vascular permeability; anaphylaxis carboxy- peptidase- B (C3-INA)

ProductBiological EffectsRegulation Biological properties of C-activation products as C3, but less potent (C3-INA) C4a (anaphylatoxin) opsonization; phagocytosis C4b (opsonin) C4-BP, factor I C3b (opsonin) opsonization; phagocyte activation factors H & I

ProductBiological EffectsRegulation Biological properties of C-activation products anaphylactic as C3, but much more potent; attracts & activates PMN causes neutrophil aggregation, stimulation of oxidative metabolism and leukotriene release C5a (chemotactic factor) carboxy- peptidase-B (C3-INA) C5b67protein-Schemotaxis, attaches to other membranes

39 Complement Deficiencies and Disease Classical Pathway Pathway ComponentDiseaseMechanism C1INHHereditary Angioedema Overproduction of C2b (prokinin) C1, C2, C4Predisposition to SLE Opsonization of immune complexes help keep them soluble, deficiency results in increased precipitation in tissues and inflammation

40 Complement Deficiencies and Disease Lectin Pathway Pathway ComponentDiseaseMechanism MBLSusceptibility to bacterial infections in infants or immunosuppressed Inability to initiate lectin pathway

41 Complement Deficiencies and Disease Alternative Pathway Pathway/ComponentDiseaseMechanism Factors B or DSusceptibility to pyogenic (pus-forming) bacterial infections Lack of sufficient opsonization of bacteria C3Susceptibility to bacterial infections Lack of opsonization and inability to utilize the membrane attack pathway C5, C6, C7 C8, or C9 Susceptibility to Gram- negative infections Inability to attack the outer membrane of Gram- negative bacteria

42 Complement Deficiencies and Disease Alternative Pathway cont. Pathway ComponentDiseaseMechanism Properdin (X-linked)Susceptibility meningococcal meningitis Lack of opsonization of bacteria Factors H or IC3 deficiency and susceptibility to bacterial infections Uncontrolled activation of C3 via alternative pathway resulting in depletion of C3

Opsonization and phagocytosis complement bacterium C3b iC3b C4b OPSONIZATION Complement receptor BINDING PHAGOCYTOSIS Phagocytic cell

C 1 I nh C1qrs breakdown C1r C1s C1q C1r C1s