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1、Complement- outline of lecture Components of the complement systemComplement activation pathways Immunobiology of complementSongwe雷达物位计 Complement- outline of lecture Components of the complement systemComplement activation pathways Immunobiology of complementHistory and nomenclatureHeat-labile prin

2、ciple in serum that complemented antibody activity 30 proteins, 3 g/liter, 15% of the serum globulin fractionMainly synthesized in the liver, also by epithelial cells and monocytes etcActivation of complement is essentially a proteolytic chain reaction There are three activation pathways: classical,

3、 alternative and MBLWalport Figure 1History and nomenclatureProteins in the classical pathway are given numerical numbers, C1-C9Proteins in the alternative pathway are referred to as Factors, e.g Factor B, Factor D etc Activation of complement does not always follow the numerical order When a partic

4、ular component is cleaved, the large fragment is usually referred to as the b fragment and the smaller fragment the a fragment, e.g. C3b and C3a. The only exception is with the cleavage of C2. Basic features of complement proteins and activation Some of the complement components are zymogens- inacti

5、ve serine proteases that are activated after cleavage or conformational change (C1, C2, Factor B). Factor D is constitutively active.Other complement proteins are structural proteins that may (C4, C3 and C5) or may not (C6, C7, C8, C9) require cleavage to be activated.C4 and C3 are proteins that con

6、tain internal thioester bonds that can cross-link covalently to hydroxyl groups on the cell surfaces after activation.Basic features of complement proteins and activation Some of the complement components are zymogens- inactive serine proteases that are activated after cleavage or conformational cha

7、nge (C1, C2, Factor B). Factor D is constitutively active.Other complement proteins are structural proteins that may (C4, C3 and C5) or may not (C6, C7, C8, C9) require cleavage to be activated.C4 and C3 are proteins that contain internal thioester bonds that can cross-link covalently to hydroxyl gr

8、oups on the cell surfaces after activation.C1 is a special component in that it is not a single protein but rather a complex of proteins composed of one C1q, two each of C1r and C1s molecules. C1q in turn is composed of 18 polypeptides organized into 6 triple helices made up of A, B, C chains. C1 is

9、 a complex of multiple proteinsComplement- outline of lecture Components of the complement systemComplement activation pathways Immunobiology of complementWalport Figure 1Alternative pathway: low level tick overInitial discovery was controversialThe internal thioester bond in C3 can be hydrolyzed sp

10、ontaneously to generate an activated form of C3: C3(H2O). C3(H2O) can bind Factor B to form C3(H2O)B which after cleavage by Factor D, forms an initiating C3 convertase C3(H2O)Bb. C3(H2O)Bb will cleave C3 and generate C3b which can cross-link to cell surfaces. Immobilized C3b will bind more Factor B

11、 and therefore start the amplification loopWalport Figure 1Major points of activation pathwaysAll pathways converge at the C3 cleavage step C3 activation initiates a self-amplification loop Classical Pathway always activates the Alternative PathwayClassical Pathway requires Ca+ Alternative Pathway r

12、equires Mg+ C4bC2a and C3bBb are two C3 convertasesAlternative pathway is initiated by spontaneous tick over C4bC2aC3b and C3bBbC3b are C5 convertasesC2 and Factor B are analogous to each other. Which are zymogens, constitutively active protease(s) and structural proteins? Which structural proteins

13、need to be cleaved and which do not need to be cleaved? Complement- outline of lecture Components of the complement systemComplement activation pathways Immunobiology of complementImmunobiology of complementHost defense against infection Anaphylatoxin production (C5a and C3a) OpsonizationDirect lysi

14、s Interface between innate and adaptive immunityDisposal of wasteWalport Figure 1OpsonizationC3 and C4 ligands and their receptorso CR1 (CD35): C3b iC3b C4b o CR2 (CD21): iC3b, C3dg o CR3 (Mac-1, CD18/CD11b): iC3bo CR4 (CD18/CD11c): iC3bC3 C3b iC3b C3dg C3dC3aC3fC3cC3gC5b-9 complex: direct cell lysi

15、s Immunobiology of complementHost defense against infection Interface between innate and adaptive immunityLowering the threshold of B cell activation B cell survival in germinal centersT cell function Disposal of wasteLowering the threshold of B cell activationC3d-bound antigen could be 10,000 fold

16、more potent than native antigen in stimulating a certain level of antibody response. B cell survival in germinal centersT cell functionImmunobiology of complementHost defense against infection Interface between innate and adaptive immunityDisposal of wasteImmune complex clearance Complement and apop

17、totic cell clearance Paradoxical roles of complement Pro-inflammatory (C3 consumption and lupus flare-ups) C1q, C1r/C1s and C4 deficiency predisposes individuals to SLE (97%, 57% and 75% penetrance)C1q, IgM, Serum amyloid P and DNase I KO mice all develop autoimmunityHypothesis I: Innate immunity is

18、 important for peripheral tolerance Innate immune system including complement, IgM, SAP, DNase I helps in the removal of potential autoantigens derived from apoptotic cells and therefore:Complement plays a physiological role in maintaining peripheral tolerance Hypothesis II: Complement plays a role

19、in central toleranceBecause complement is important for B cell coreceptor activation in mature B cells in the periphery, it may also play a similar role in B cell development in the bone morrow where B cell receptor ligation with self-antigens occurs. Therefore: In the absence of complement, self-re

20、active B cell are not deleted and escape to the periphery to cause autoimmunity. Complement regulators Why need inhibitors- to confer specificity to pathogens Two types of inhibitors: Membrane-bound inhibitorsMembrane Co-factor protein (MCP, CD46Decay accelerating factor (DAF, CD55)Complement recept

21、or 1 (CR1, CD35)CD59Fluid phase inhibitors C1 Inhibitor (C1INH): C1r, C1s and MASPCarboxypeptidase N: C3a, C5aFactor IFactor H (for alternative pathway)C4 Binding protein (C4BP) (for classical pathway) 30 SCRDAF MCP CR1 CD59Host cells are protected from complement host cellhost cellComplement regula

22、tors Why need inhibitors- to confer specificity to pathogens Two types of inhibitors: Membrane-bound inhibitorsMembrane Co-factor protein (MCP, CD46Decay accelerating factor (DAF, CD55)Complement receptor 1 (CR1, CD35)CD59Fluid phase inhibitors C1 Inhibitor (C1INH): C1r, C1s and MASPCarboxypeptidase

23、 N: C3a, C5aFactor IFactor H (for alternative pathway)C4 Binding protein (C4BP) (for classical pathway) Clinical implications of complement regulatorsC1 INH. Autosomal dominant mutation leads to insufficient C1 INH production, increased vascular permeability and plasma leakage-edema (hereditary angi

24、oedema). DAF and CD59: human PNH syndrome Factor H and MCP: membranoproliferative glomerulonephritis and atypical hemolytic uremic syndrome (aHUS) and age-related macular degeneration (AMD).Eculizumab and PNHEffect of eculizumab on hemolysis and transfusion requirements in patients with paroxysmal n

25、octurnal hemoglobinuria.N Engl J Med. 2004 Feb 5;350(6):552-9. The complement inhibitor eculizumab in paroxysmal nocturnal hemoglobinuria.N Engl J Med. 2006 Sep 21;355(12):1233-43. Copyright 2007 Rockefeller UniversityPressaHUSAMD (Y402H)Age-related macular degeneration (AMD) (Y402H): inflammation of the eyeAtypical hemolytic uremic syndrome (aHUS): microangiopathic hemolytic anemia