Complement History Discovered in 1894 by Bordet It

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

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 3

Proteins of the complement system (nomenclature) • C 1(qrs), C 2, C 3, C 4, C 5, C 6, C 7, C 8, C 9 • factors B, D, H and I, properdin (P) • mannose binding lectin (MBL), MBL associated serine proteases (MASP-1 MASP-2) • C 1 inhibitor (C 1 -INH, serpin), C 4 -binding protein (C 4 -BP), decay accelerating factor (DAF), Complement receptor 1 (CR 1), protein. S (vitronectin) 4

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 (CH 50): 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 5

Activation product of complement proteins (nomenclature) Activated component are usually over-lined: e. g. C 1 qrs 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. , C 3 b/C 3 a, C 4 b/C 4 a, C 5 b/C 5 a), EXCEPT C 2 (the larger, membranebinding moiety is C 2 a; the smaller one is C 2 b) 6

Pathways of complement activation CLASSICAL PATHWAY antibody dependent LECTIN PATHWAY ALTERNATIVE PATHWAY antibody independent Activation of C 3 and generation of C 5 convertase activation of C 5 LYTIC ATTACK PATHWAY 7

Components of the Classical Pathway C 1 r C 1 s Ca++ C 1 q C 2 C 3 C 4 C 1 complex 8

Classical Pathway Generation of C 3 -convertase C 1 r C 1 s Ca++ C 1 q C 4 b C 4 a 9

Classical Pathway Generation of C 3 -convertase C 4 a C 1 r C 1 s Ca++ C 1 q C 2 b a 2 C _____ Mg++ C 4 b 2 a is C 3 convertase C 4 b C 2 a 10

Classical Pathway Generation of C 5 -convertase C 4 a C 1 r C 1 s Ca++ C 1 q Mg++ C 2 b C 3 a ____ C 4 b 2 a 3 b is C 5 convertase; it leads into the Membrane Attack Pathway C 4 b C 2 a C 3 b 11

Biological Activities of Classical Pathway Components Component Biological Activity C 2 b Prokinin; cleaved by plasmin to yield kinin, which results in edema C 3 a 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 C 3 b Opsonin Activation of phagocytic cells C 4 a Anaphylaotoxin C 4 b Opsonin 12

Control of Classical Pathway Components Component Regulation All C 1 -inhibitor (C 1 -INH); dissociates C 1 r and C 1 s from C 1 q C 3 a-inactivator (C 3 a-INA; Carboxypeptidase B) C 3 b Factors H and I; Factor H facilitates the degradation of C 3 b by Factor I C 4 a C 3 a-INH C 4 b C 4 binding protein (C 4 -BP) and Factor I; C 4 -BP facilitates degradation of C 4 b by Factor I; C 4 -BP also prevents the association of C 2 a with C 4 b thus blocking formation of C 3 convertase 13

Components of mannose-binding lectin pathway C 4 MASP 2 MBL C 2 MASP 1 14

Mannose-binding lectin pathway C 2 b C 4 a MASP 1 MASP 2 MBL _____ C 4 b 2 a is C 3 convertase; it will lead to the generation of C 5 convertase C 4 b C 4 a 2 CC 2 C 4 b C 2 a 15

Components of the alternative pathway D C 3 B P 16

Spontaneous C 3 activation Generation of C 3 convertase H 2 O C 3 i D Bb C 3 a C 3 i. Bb complex has a very short half life 17

C 3 -activation the amplification loop If spontaneously-generated C 3 b is not degraded D C 3 a C 3 b Bb C 3 b 18

C 3 -activation the amplification loop D C 3 b C 3 a C 3 b Bb C 3 a 19

C 3 -activation the amplification loop D Bb C 3 a Bb C 3 b Bb C 3 a 20

C 3 -activation the amplification loop Bb C 3 a Bb C 3 b Bb C 3 a 21

C 3 -activation the amplification loop Bb C 3 a Bb C 3 b Bb C 3 a 22

Control of spontaneous C 3 activation via DAF prevents C 3 b factor B to C 3 b B DAF the binding of CR 1 Autologous cell membrane 23

Control of spontaneous C 3 activation via DAF dislodges factor Bb C 3 b Bb DAF C 3 b-bound Bb CR 1 Autologous cell membrane 24

Control of spontaneous C 3 activation via CR 1 Bb H Bb C 3 b DAF I i. C 3 b CR 1 DAF C 3 b I i. C 3 b CR 1 Autologous cell membrane 25

Degradation of spontaneously produced C 3 b C 3 c I C 3 b C 3 dg i. C 3 b 26

C 3 b stabilization and C 5 activation C 3 a C 3 b finds an activator (protector) membrane P C 3 b This is stable C 5 convertase D of the alternative pathway Bb C 3 b 27

C 3 b regulation on self and activator surfaces C 3 b 28

C 5 -convertase of the two pathways C 5 -convertase of the Classical and lectin Pathways C 4 b C 2 a C 3 b C 5 -convertase of the Alternative Pathway C 3 b Bb C 3 b 29

Lytic pathway Generation of C 5 convertase leads to the activation of the Lytic pathway 30

Components of the lytic pathway C 7 C 6 C 5 C 8 C 9 31

Lytic pathway C 5 -activation C 5 a C 5 b C 4 b C 2 a C 3 b 32

Lytic pathway assembly of the lytic complex C 6 C 7 C 5 b 33

Lytic pathway: insertion of lytic complex into cell membrane C 6 C 8 C 7 C 5 b CC C 9 9 9 9 C 9 C C C 9 9 34

Biological effects of C 5 a Adhesion Neutrophil Vascular wall transmigration Chemotaxis Neutrophil Mast cell Monocyte Activation Degranulation Neutrophil Activation Cytokine Vascular permeability production Respiratory burst 35

Biological properties of C-activation products Product C 2 b (prokinin) C 3 a (anaphylatoxin) Biological Effects edema Regulation C 1 -INH mast cell degranulation; carboxypeptidase- B enhanced vascular (C 3 -INA) permeability; anaphylaxis 36

Biological properties of C-activation products Product Biological Effects Regulation C 3 b (opsonin) opsonization; phagocyte activation factors H & I C 4 a as C 3, but less (anaphylatoxin) potent (C 3 -INA) C 4 b (opsonin) C 4 -BP, factor I opsonization; phagocytosis 37

Biological properties of C-activation products Product Biological Effects Regulation C 5 a (chemotactic factor) anaphylactic as C 3, but much more potent; attracts & activates PMN causes neutrophil aggregation, stimulation of oxidative metabolism and leukotriene release carboxypeptidase-B (C 3 -INA) C 5 b 67 chemotaxis, attaches to other membranes protein-S 38

Complement Deficiencies and Disease Classical Pathway Component Disease Mechanism C 1 INH Hereditary Angioedema Overproduction of C 2 b (prokinin) C 1, C 2, C 4 Predisposition to SLE Opsonization of immune complexes help keep them soluble, deficiency results in increased precipitation in tissues and inflammation 39

Complement Deficiencies and Disease Lectin Pathway Component MBL Disease Susceptibility to bacterial infections in infants or immunosuppressed Mechanism Inability to initiate lectin pathway 40

Complement Deficiencies and Disease Alternative Pathway/Component Disease Mechanism Factors B or D Susceptibility to pyogenic (pus-forming) bacterial infections Lack of sufficient opsonization of bacteria C 3 Susceptibility to bacterial infections Lack of opsonization and inability to utilize the membrane attack pathway C 5, C 6, C 7 C 8, or C 9 Susceptibility to Gramnegative infections Inability to attack the outer membrane of Gramnegative bacteria 41

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 C 3 deficiency and susceptibility to bacterial infections Uncontrolled activation of C 3 via alternative pathway resulting in depletion of C 3 42