THE COMPLEMENT SYSTEM OBJECTIVES:

Dr Mohammed Al- ani

THE COMPLEMENT SYSTEM OBJECTIVES: When you finish this section, you should be able to: 1. Describe the effects of complement activation. 2. Outline the Classical, Mannan-Binding (MB) Lectin and Alternative pathways of complement activation, including their activators. 3. Discuss the major regulatory points in complement activation and the consequences of deficiencies in complement or complement regulators. 4. Discuss the functions of the various complement receptors. 5. Discuss the biologic activities of complement and identify the components involved.

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 Complex series of plasma proteins (C1-C9). Heat labile (inactivated at 56 C for 30 min). Synthesized in vivo by liver macrophages, hepatocytes and intestinal epithelia. C3 is most abundant (1g/L of plasma) and important complement component. Plays a major role in innate and adaptive immunity.

Complement functions Host benefit: opsonization to enhance phagocytosis phagocyte attraction and activation lysis of bacteria and infected cells 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), decay accelerating factor (DAF)

Definitions C-activation: alteration of C proteins such that they interact with the next component C-fixation: utilization of C by Ag-Ab complexes 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. 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., C3b/C3a, C4b/C4a, C5b/C5a), EXCEPT C2 (the larger, membranebinding moiety is C2a; the smaller one is C2b)

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

THE CLASSICAL COMPLEMENT PATHWAY Activated by Ag-Ab binding (can also be activated by viruses, Mycoplasma DNA).

One IgM molecule can stimulate C1 since it is a pentamer with 5 Fc regions. At least 2 IgG molecules are required to activate C1. IgM more efficient complement binding (fixing) antibody than IgG. IgA and IgE lack C1q receptors and cannot activate complement. Only Ag-Ab complexes and not free or soluble Abs activate complement. Activated C1q activates C1r which activates C1s.

Biological effects of C5a

Components of the Classical Pathway C3 C4 C1 complex

Classical Pathway Generation of C3-convertase

Classical Pathway Generation of C3-convertase C4b2a is C3 convertase C4b

Classical Pathway Generation of C5-convertase C4b2a3b is C5 convertase; it leads into the Membrane Attack Pathway C4b C3b

Control of Classical Pathway Components Component All Regulation C1-inhibitor (C1-INH); dissociates C1r and C1s from C1q 25

C1-inhibitor deficiency: hereditary angioedema

Biological properties of C-activation products Product Biological Effects Regulation C2a (prokinin) C3a (anaphylatoxin) edema mast cell degranulation; enhanced vascular permeability; anaphylaxis C1-INH (C3-INA)

Biological properties of C-activation products Product Biological Effects Regulation C3b (opsonin) C4a (anaphylatoxin) C4b (opsonin) opsonization; phagocyte activation as C3, but less potent opsonization; phagocytosis factors H & I (C3-INA)

Biological Activities of Classical Pathway Components Component Biological Activity C2a C3a C3b C4a 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 30

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 protein-s

MANNAN-BINDING LECTIN COMPLEMENT PATHWAY Activated by binding of a serum protein -mannose-binding lectin (MBL) to mannose-containing proteins or to carbohydrates on bacteria or viruses. Activates the classical pathway in an antibody and C1qindependent manner. MBL is structurally similar to C1q. Instead of C1r and C1s, MBL associates with mannose-binding lectin-associated serum proteases (MASP-1 and MASP-2) to activate C4 and C2 Activation by MBL:MASP complex generates C3 convertase which progresses as in the classical pathway to produce the membrane attack complex.

Components of mannose-binding lectin pathway MBL MASP1

Mannose-binding lectin pathway MASP1 C4b2a is C3 convertase; it will lead to the generation of C5 convertase MBL

THE ALTERNATIVE COMPLEMENT PATHWAY Activated in the absence of antibody. except (IgA) Activated by bacterial products (lipopolysaccharide {LPS}), cell wall (teichoic acid of Gram+ bacteria), yeast and fungal cell wall (zymosan), viruses, parasites (trypanosomes). Depends on the presence of small amounts of C3b in serum due to proteolysis of C3 by plasmin or thrombin

C3 Components of the alternative pathway

C3-activation the amplification loop C3b b C3 b

C3-activation the amplification loop C3 b b C3b C3b

C3-activation the amplification loop C3 b b C3b C3b C3b

C3-activation the amplification loop C3b C3b C3b

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

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

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

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

Components of the lytic pathway C6 C7 C 9

ALTERNATIVE PATHWAY Factor H binds to C3b preventing binding of Factor B and renders it (C3b) susceptible to cleavage by Factor I. Also promotes dissociation of B from C3b..

Lytic pathway C5-activation b C4b C3b

Lytic pathway assembly of the lytic complex C6 C7 b

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

CLASSICAL PATHWAY 1. INHIBITION OF PROTEOLYTIC ACTIVITY OF C1 C1 inhibitor (C1INH), a serine protease inhibitor, binds to and inactivates C1r, C1s. Most of the C1 in blood is bound to C1 INH to prevent spontaneous complement activation. Binding of C1q to Ag-Ab complex releases C1 from C1 INH inhibition

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) C3b C4a Factors H and I; Factor H facilitates the degradation of C3b by Factor I C3a-INH 54

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

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 57

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 59