Pathways of Complement Activation and Regulation PDF

Pathways of complement activation and regulation Learning Objectives On completion of this session you should be able to 1) Comprehend the classical, lectin, and alternative pathways of complement activation, including their unique triggers and components involved. 2) Explain the sequence of...

Pathways of complement activation and regulation Learning Objectives On completion of this session you should be able to 1) Comprehend the classical, lectin, and alternative pathways of complement activation, including their unique triggers and components involved. 2) Explain the sequence of events in the complement activation cascade for each pathway and how they converge at the formation of C3 convertase. 3) Recognise the key proteins and complexes involved in each pathway, and their roles in the activation process. Complement Activation 1) Three pathways by which complement is activated: Classical Pathway - activated by antibody:antigen (Ab:Ag) complexes (adaptive) and pentraxins Mannan binding lectin (MBL) pathway by binding to carbohydrate on pathogen surface (innate) Alternative Pathway - part of innate immune response and amplifies classical and MBL pathways 2) Final steps leading to membrane-attack complex (MAC) are identical in all 3 pathways Overview MB-LECTIN CLASSICAL ALTERNATIVE PATHWAY AB:AG COMPLEX PATHOGEN SURFACE ACTIVATION OF COMPLEMENT PROTEINS RECRUITMENT FORMATION OF OF OPSONISATION MEMBRANE INFLAMMATORY OF PATHOGEN ATTACK CELLS, COMPLEX ACTIVATION OF (MAC) ENDOTHELIUM ETC Classical Pathway 1) Activated by Ab:Ag interactions (adaptive response, most commonly IgM and IgG) 2) First step is the binding of C1 to antibody on the surface of the pathogen 3) C1 has three subunits - C1q, C1r and C1s 4) One C1q is associated with two molecules each of C1r and C1s i.e. (C1r:C1s)2 5) C1q binds to antibody and undergoes a conformational change Classical Pathway 1) This change activates C1r which cleaves C1s 2) C1s is activated and cleaves C4 to give C4b and C4a 3) C4b binds covalently to the pathogen surface 4) Bound C4b binds C2, localising it on the pathogen surface and allowing it to be cleaved by C1s 5) This generates the complex C4b2a which is the C3 convertase of the classical pathway Classical Pathway Classical Pathway Classical Pathway Classical Pathway Classical Pathway C3 1) Most abundant complement protein in plasma 2) Up to 1000 molecules can bind in vicinity of C3 convertase 3) Main effect of complement activation is the deposition of large quantities of C3b on pathogen surface (opsonin) 4) C3b binds C3 convertase allowing it to bind and activate C5 (i.e generates C5 convertases - C4b2a3b and C3bBb3b) 5) C5b initiates formation of MAC 6) Thus, all three pathways result in the activation of C3 and, to a lesser extent C5 7) These components are central to the effector mechanisms of the complement pathway Membrane Attack Complex (MAC) 1) Final stage of classical pathway involves formation of MAC which is also called lytic unit 2) Form pores in cell membrane 3) Ions and small molecules can freely pass through pores 4) Cells cannot maintain osmotic stability, leading to swelling and cell lysis Membrane Attack Complex (MAC) MBL Pathway 1) Antibody-Independent 2) However, proceeds more like classical pathway 3) Uses C4 and C2 4) MBL is present in low concentrations in plasma and increased during acute phase response 5) MBL forms a complex with two enzymes called MASP-1 and MASP-2 (mannan binding lectin associated serine proteases) 6) First step is the binding of MBL to mannose residues on the surface of the pathogen 7) On binding MBL undergoes a conformational change that activates MASP-2 MBL Pathway 1) MASP-2 cleaves a second MASP-2 to generate active protease 2) The role of MASP-1 is unclear 3) Activated MASP-2 cleaves plasma protein C4 to give C4b which binds covalently to the surface of the pathogen 4) C4b binds C2 making it susceptible to cleavage by MASP-2 5) MASP-2 cleaves C2 to give C2b which remains bound to C4b 6) The complex C4b2b is also the C3 convertase of the MB lectin pathway 7) Role of C3 convertase is activation of C3 to C3a and C3b 8) Thus pathogen becomes coated with C3b 9) C3 convertase is covalently bound to pathogen so activation of C3 is localised Alternative Pathway 1) Antibody-Independent 2) Initiated by cell surface constituents foreign to host 3) E.g. Gram+ or Gram- bacteria Alternative Pathway 1) Activated by C3b bound to pathogen 2) C3b generated by classical pathway, MB lectin pathway or spontaneous cleavage 3) Bound C3b binds factor B which induces its cleavage to generate Bb, by the protease factor D 4) C3bBb is the C3 convertase of the alternative pathway 5) C3bBb cleaves more C3 to coat pathogen with C3b 6) Again, cleavage is localised Alternative Pathway Complement Activation 1) Three pathways by which complement is activated: 1. Classical Pathway - activated by antibody:antigen (Ab:Ag) complexes (adaptive) and pentraxins 2. Mannan binding lectin (MBL) pathway by binding to carbohydrate on pathogen surface (innate) 3. Alternative Pathway - part of innate immune response and amplifies classical and MBL pathways 2) Final steps leading to membrane-attack complex (MAC) are identical in all 3 pathways Complement Regulation 1) Via control proteins that regulate all three pathways 2) Regulatory proteins can be soluble or cell-surface associated 3) Soluble proteins prevent excessive activation of complement by circulating immune complexes or by spontaneous activation of complement proteins 4) Cell surface inhibitors protect host cells from complement effector mechanisms 5) C3b and C4b can covalently bind to host cell surfaces Regulation of Classical Pathway 1) Regulation exerted at the first step - activation of CLASSICAL PATHWAY C1 1. ACTIVE C1 2) Controlled by C1inh, a serpin or serine protease HOST CELL inhibitor (soluble inhibitor) C1INH 3) C1inh is the only plasma inhibitor of activated C1 (C1r:C1s)2 C1q 4) C1inh binds C1r:C1s complex and promotes dissociation from C1q 2*. C4b2b 5) C1inh limits the time in which active C1s can DAF/C4BP/CR1 cleave C4 and C2 C2b C4b 6) C1inh limits spontaneous activation of C1 in plasma FACTOR I + (C4BP/CR1/MCP) 7) C1inh deficiency has been described and causes C4c + C4d peripheral inflammation (hereditary angioedema - HAE) *ALSO IN MB LECTIN PATHWAY Regulation of Classical & MBL Pathways 1) The C3 convertase (C4b2b) is CLASSICAL PATHWAY targeted 1. ACTIVE C1 2) C4 Binding Protein (C4BP) HOST CELL C1INH (soluble factor), CR1 (CD35) and (C1r:C1s)2 C1q Decay Accelerating Factor (DAF – CD55) can all bind C4b and 2*. C4b2b DAF/C4BP/CR1 displace C2b thereby inactivating C2b C4b C3 convertase FACTOR I + (C4BP/CR1/ MCP) 3) C4b bound to C4BP, membrane C4c + C4d co-factor of proteolysis (MCP – *ALSO IN MB LECTIN PATHWAY CD46) or CR1 can be cleaved by Factor I to generate inactive C4c and C4d Regulation of Alternative Pathways 1) C3 convertase (C3bBb) is ALTERNATIVE PATHWAY targeted C3 -> C3b 2) Factor H, DAF or CR1 can all inactivate C3bBb by binding C3b HOST CELL FACTOR B & D and displacing Bb Ba 3) In the presence of Factor H, C3bBb CR1 or MCP, C3b is inactivated DAF + FACTOR H by Factor I (iC3b) C3b + Bb FACTOR I (cofactors H + MCP) iC3b Regulation of All Pathways 1) CD59 (protectin) - prevents ALL PATHWAYS formation of MAC by inhibiting binding of C9 to C5b678 complex 2) These inhibitors limit the C5b678 spontaneous activation of complement proteins and protect C9 CD59 host cells 3) Host cells express inhibitors DAF, MCP, CD59 (membrane bound) C5b678:CD59 4) Other inhibitors are serum + C9 proteins Summary 1. Complement activation encompasses three main pathways: classical, lectin, and alternative, each triggered by distinct mechanisms and involving specific components. 2. In the classical pathway, activation is initiated by the binding of antibodies to antigens, leading to the activation of the C1 complex, subsequent cleavage of C4 and C2, and the formation of the C3 convertase. 3. The lectin pathway is triggered by the recognition of carbohydrate patterns on pathogens by molecules like MBL, leading to the activation of MASP enzymes, cleavage of C4 and C2, and C3 convertase formation. 4. The alternative pathway is constitutively active at low levels due to the spontaneous hydrolysis of C3, leading to the binding of Factor B, cleavage by Factor D, and assembly of the C3 convertase. 5. Understanding these pathways provides insights into immune defence mechanisms, inflammation, and the development of therapeutic interventions for immune-related disorders and infectious diseases.