COMPLEMENT Complement History Discovered in 1894 by Bordet

COMPLEMENT

Complement: History Discovered in 1894 by Bordet It was used to refer to a heat-labile serum component Its lytic activity destroyed when heated at 56°C for 30 min However it is now known that complement contributes to many other functions

Complement Functions • Host benefit: – – – opsonization to enhance phagocytosis phagocytes 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 v Complement comprises over 20 different serum proteins v These proteins are produced by a variety of cells: Hepatocytes, macrophages, gut epithelial cells v Some components can bind to Immunoglobulins v While others are proenzymes that when activated can cleave and activate other components

Proteins of the complement system (nomenclature) v C 1(q r s), C 2, C 3, C 4, C 5, C 6, C 7, C 8, C 9 v factors B, D, H and I, properdin (P) v mannose binding lectin (MBL), MBL associated serine proteases (MASP-1 MASP -2) v 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) 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 (CH 50): dilution of serum which lyses 50% of a standardized suspension of Ab-coated R. B. Cs • 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

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 into the microenvironment Letter “b” is usually added to the larger, membranebinding, 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) EXCEPTION: C 2 (the larger, membrane-binding moiety is C 2 a; the smaller one is C 2 b)

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

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

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

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

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

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 13

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 14

Components of mannose-binding (lectin) pathway v. Mannose Binding Lectin (MBL), which binds to bacterial surface with mannose-containing polysaccharides v. MBL associated serine proteases (MASP-1 MASP-2) 15

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

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

Components of the alternative pathway v. C 3 vfactors B & D vproperdin (P) 18

Control and regulation of the alternative pathway Factor I & H n DAF n Complement receptor 1 n 19

The alternative pathway can be activated by: v. Many gram-negative bacteria: Neisseria meningitidis and N. gonorrhoea v. Some gram-positive bacteria the result is lysis of these organisms v. Certain viruses and parasites v. Aggregated immunoglobulina(particularly Ig. A) 20

Activation of the alternative pathway n n n Spontaneous hydrolysis of C 3 to produce C 3 i cleaves Factor B into Bb The C 3 i. Bb complex acts as C 3 convertase(has very short half life) Once C 3 b is formed Factor B binds to it and becomes susceptible to cleavage by Factor D C 3 b. Bb is a more stable C 3 convertase which continues to generate more C 3 b(amplfication loop) 21

Components of the alternative pathway D C 3 B P

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

C 3 b stabilization and C 5 activation n n When C 3 b finds the appropriate surface it binds to factor B, which is cleaved by Factor D to produce C 3 convertase(C 3 b. Bb) (which is more stable) C 3 convertase(C 3 b. Bb) is further stabilized by Poperdin 24

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

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

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

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

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

Lytic pathway n n n C 5 -convertase (of the Classical and lectin Pathways or the Alternative Pathway) cleaves C 5 into C 5 a & C 5 b associates with C 6 &C 7 and insrets into the cell membrane Then C 8 binds, followed by several molecules of C 9 molecules form a pore in the membrane C 5 b. C 6 C 7 C 8 C 9 is MAC 30

Biological effects of C 5 a

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

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

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

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 35

Complement Deficiencies and Disease Lectin Pathway Component MBL Disease Susceptibility to bacterial infections in infants or immunosuppressed Mechanism Inability to initiate lectin pathway 36

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 37

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 38