2 Chapter 6 ComplementThe end of 19 centuryJules Bordet ( )
3 Fresh serum containing an antibacterial antibody was added to the bacteria at physiologic temperature (37℃），bacteria were lysed.If the serum was heated to 56℃ or more , it lost its lytic capacity.This loss of lytic capacity was not due to decay of antibody activity because antibodies are heat-stable and even heated serum was capable of agglutinating the bacteria.Bordet concluded that the serum must contain another , heat-labile component that assists the lytic function of antibodies, and this component was later given the name “complement”
4 Definition of complement system： A system of serum and cell surface proteins （including more than 30 proteins ) that interact with one another and with other molecules of the immune system to generate important effectors of innate and adaptive immune response.
5 Contents PartⅠ The components and properties of complement system PartⅡ Activation of complement systemPartⅢ Regulation of complement systemPart Ⅳ Complement receptorsPartⅤ Biological functions of complementPartⅥ Complement and disease
6 PartⅠ The components and properties of complement system (I) The components and nomenclatureof complement system(II) Physical and chemical features of complement
8 (I). The components and nomenclature of complement system Components participating complement activationClassical pathway: C1 (C1q,C1r,C1s)，C2， C3， C4MBL（mannan-binding lectin) pathway: MBL, MASP( MBL-associated serine protease)Alternative pathway: factor B, factor DCommon terminal pathway：C5，C6，C7，C8，C9
9 2. Regulatory components of complement system factor I, factor H, S protein, properdin(factor P), C1 inhibitor(C1 INH) , C4-binding protein(C4BP), SP40/40, membrane cofactor protein(MCP), decay accelerating factor(DAF), homologous restriction factor(HRF), membrane inhibitor of reactive lysis(MIRL)3. Complement receptorsCR1~CR5, C3aR, C2aR, C4aR, etc
10 Nomenclature of the complement system Intrinsic components in classical pathway:C1~C9Intrinsic components in alternative pathway: factor DRegulatory proteins:C1INH,C4BPCleaved fragments:C3a, C3b; C2a, C2bActivated components:C1Inactivated components:iC3bComplement receptor:CR
11 （Ⅱ）. The physical and chemical features of complement 1. Synthesized sites: liver, macrophage, small intestine epithelium2. The concentration of complement in serum is stable( 10% of serum proteins) , C3 is the highest in all of complement components:1~2 g/L3. Heat–labile feature:56℃ 30min—inactivation0~10℃ for 3~5 days4. The concentration of complement is the highest in the serumof guinea pig.
13 PartⅡ Activation of the complement system The soluble proteins of the complement system are synthesized in the liver and are secreted as non-active forms called zymogens.Zymogen: The cleavage is required for activationThe cleavage of a zymogen usually produces a large active fragment with enzymatic activity and a small fragment with inflammatory effects.
14 Three different pathways of complement activation： 1.Classical pathway: from C1 by Ag-Ab2.Alternative pathway: from C3 by the surface of microbe3.MBL pathway: from C4 and C2 by binding of MBL mannan on the surface of microbesTerminal pathway : formation of MAC (membrane attack complex), same in the three pathways
16 Ⅰ. The Classical pathway of complement activation 1.Initiating substances: antigen-antibody complex or immune complex2.Components: C1q ,C1r,C1s,C4,C2,C3,5,6,7,8,93.Process:C1q,C1r,C1s C4,C2,C3,5,6,7,8,9
17 The process of complement activation in classical pathway: 1. Initiation step: recognizing unit (C1qrs),activated C12. Activation step: activating unit( C4,C2,C3),C3 convertase and C5 convertase3. Effector step: membrane-attack complex (MAC) ,C5~9
18 1. Initiation steprecognizing unit (C1qrs) --- activated C1IgG1~3 and IgM can activate complement by classical pathwayThe complement component C1 binds to the Fc part of the antibodies(CH2 of IgG or CH3 of IgM), and then is turned into activated C1 (C1)
26 IgGThe C1 must bind to at least two IgG molecules that are close enough together so that it can bind to both of them at the same time.IgM The C1 must bind at least 2 CH3 domains of one IgM molecule to be activated.IgM is the best complement activator because it is a pentamer.
49 Classical pathway IgM/IgG –Ag complex C1q : r : s C4 C4b + C2 C4a C2b C4b2aC3C3bC3aCa++Mg++（C3 convertase）C4b2a3b（C5 convertase）
50 3. Effector step: Common terminal pathway Formation of the Membrane Attack Complex (MAC)MAC: a lytic complex of the terminal components of the complement cascade, including C5,6,7,8 and multiple copies of C9, that forms in the membrane of target cells. The MAC causes lethal ionic and osmotic changes in cells.
62 Effect of MAC On the surface of cell: lyse the cell In the serum:SC5b~7, SC5b~8, SC5b~9
63 Ⅱ. Alternative pathway 1.The initiating substances: some components of microbial cell surfaceaggregated IgA or IgG4---providing a surface for binding of complement2.Components and process: factor D, factor B, C3,5,6,7,8,93.Function: participate in non–specific immunity
64 Normally, C3 in plasma is being continuously cleaved at a low rate to generate C3b in a process. A small amount of the C3b in the fluid phaseis unstable and inactive.C3b may become covalently attached to the surfaces of cells, including microbes.
65 The process of complement activation in alternative pathway: 1. Initiation step: C3b binds to microbial surface,binds factor B, and forms C3 convertase.2. Activation step: form C5 convertase3. Effector step: membrane-attack complex (MAC) ,C5-9
71 D factor P factor C3 C3bBbP C3b C3bBb （C3 convertase） C3b C3bnBb B factorC3Spontaneous conversion or from classical pathwayC3bBbPC3bC3bBb（C3 convertase）C3bC3bnBb（C5 convertase）enlarge
72 positive feedback loop of C3 C3bD factorBbC3C3bBC3bB factor
73 Important characteristics In alternative pathway, complement can recognize self from nonself----complement regulatory proteinAlternative pathway is the important enlarge mechanism of complement
74 Ⅲ. MBL or Lectin Pathway1.Initiating substances: MBL combine with mannose on the surface of microbe.2.Components: Mannose-binding lectin (MBL) , MBL-associated serine protease (MASP)， C4,C2,C3,5,6,7,8,93.Process:MBL-mannose-MASP----C4,C2,C3,5,6,7,8,9
75 The MBL and c-reactive protein were produced by liver after microbe infection MBL binds to mannose residues on polysaccharide of microbeThe MBL, structurally similar to C1q.The MBL activate MASP( mannan-associated serine protease) and then MASP activate C4,C2(MASP is similar to C1r and C1s)
76 Mannose-binding lectin MBL is Structurally similar to C1q
77 MBL binds to mannose on glycoproteins on the surface of microorganisms. Then MASPsbind to it.MASP-1MASP-1MASP-2MASP-2MASP = mannose associated serine protease
85 2. Regulation of C3 convertase formation Factor H removes Bb from the alternative pathway C3 convertase,breaking the positive feedback loop.Factor I inactivates C3b and C4bDAF -----binds to C4b and C2a, preventingformation of C3 convertase
89 3. Regulation of MACHomologous restriction factor(HRF):HRF(C8bp )------interferes with binding of C9 and C8, prevents formation of MACMembrane inhibitor of reactive lysis(MIRL): MIRL(CD59)------interferes with binding of C5b6 complex and C7,C8, and prevents formation of MAC