Complement

OBJECTIVES: When you finish this section, you should be able to: 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. Discribe 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 refers, historically, to fresh serum capable of lysing antibody (Ab)-coated cells. This activity is destroyed (inactivated) by heating serum at 56EC for 30 minutes. The lytic activity of complement is decreased in certain diseases, e.g. SLE, serum sickness, chronic infections, complement deficiencies, etc.

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

Complement functions Host benefit: Host detriment: 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)

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) 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)

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 ƒ Activated by Ag-Ab binding (can also be activated by viruses, Mycoplasma, DNA).

ƒ 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. ƒ IgM more efficient complement binding (fixing) antibody than IgG. ƒ IgA and IgE lack C1q receptors and cannot activate complement. ƒ Activated C1q activates C1r which activates C1s.

Components of the Classical Pathway C1r C1s C1q C4 C2 C3 Chelating agents dismantle the C1 complex and are anti-complementary. Heat destroys the C2 component. Sample for C measurement should be drawn in a green-top vial (no EDTA), must be kept cold and tested as soon as possible. C1 complex

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

Classical Pathway Generation of C3-convertase C2b a C4a C1r C1s C1q _____ C4b2a is C3 convertase C4b

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

Biological Activities of Classical Pathway Components Biological Activity C2a(or b) Prokinin; cleaved by plasmin to yield kinin, which results in edema C3a 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 C3b Opsonin Activation of phagocytic cells C4a Anaphylaotoxin C4b

Control of Classical Pathway Components Regulation All C1-inhibitor (C1-INH); dissociates C1r and C1s from C1q C3a C3a-inactivator C3b Factors H and I; Factor H facilitates the degradation of C3b by Factor I C4a C3a-INH

C1-inhibitor deficiency: hereditary angioedema

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

Components of the lytic pathway 9

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

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

MANNAN-BINDING LECTIN COMPLEMENT PATHWAY ƒ Activated by binding of a serum protein -mannose-binding lectin (MBL) to mannose-containing proteins or to carbohydrates on bacteria or viruses. ƒ Activates the classical pathway in an (antibody and C1) independent manner. ƒ 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 ƒ 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 MASP2 C2 MASP1 MBL

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