Complement Lecture Outline PDF

SENSING DANGER II: COMPLEMENT FUNCTION, ACTIVATION AND REGULATION Daniel Quinn, PhD [email protected]  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Comp...

SENSING DANGER II: COMPLEMENT FUNCTION, ACTIVATION AND REGULATION Daniel Quinn, PhD [email protected]  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  Learning Objectives Differentiate between the three pathways of complement activation Discuss the biological properties of the products of complement activation – Split Products Discuss the regulation of complement activation and its products: Describe the role of the complement system in host resistance to infection, inflammation, and immunopathology​ Discuss the consequences of complement deficiencies and of defects in regulation of complement activation Reserved for captions  Introduction to The Complement System I Complement consists of >20 plasma and cell surface proteins that interact with one another Most components of complement are indicated by an upper- case C followed by a number e.g., C1, C2, C3 etc. Complement is activated, by Pattern Recognition, during inflammation and trauma Complement activation involves sequential proteolytic activation of zymogens (inactive proenzymes) to yield active “split products” Split products are indicated by a suffix, e.g., C3a and C3b are split products of C3 Reserved for captions  Introduction to The Complement System II Some complement split products covalently bind to microbes, immune complexes (Ab-Ag complexes) and apoptotic bodies and promote their elimination from the body Some complement split products promote inflammation by attracting leukocytes and by activating tissue mast cells All pathways of complement activation result in the formation of a Membrane Attack Complex (MAC) which can perforate membranes and kill susceptible pathogens Complement activation is inhibited by proteins expressed by host cells – Regulators of Complement Activation (RCA) Reserved for captions  Overview of Complement Functions Gram neg. bacteria are more susceptible to lysis by MAC than are Gram pos.- bacteria. 1. Membrane Attack MAC will perforate MEMBRANES Complex (MAC)  Overview of Complement Functions Gramneg. Gram neg.bacteria bacteriaare aremore moresusceptible susceptibletoto lysisbybyMAC lysis MACthan thanareareGram Grampos.- pos.-bacteria. bacteria. MACwill MAC willperforate perforateMEMBRANES MEMBRANES 1. Membrane Attack Complex (MAC) 2. Opsonization and Phagocytosis Most important Most important Without With function of Opsonin Opsonin function of complement complement Opsonized by Ab and/or Complement  Overview of Complement Functions Gram neg. bacteria are more susceptible to lysis by MAC than are Gram pos.- bacteria. 1. Membrane Attack MAC will perforate MEMBRANES Complex (MAC) 2. Opsonization and Phagocytosis Most important 3. Inflammation function of complement  Overview of Complement Functions Gram neg. bacteria are more susceptible to lysis by MAC than are Gram pos.- bacteria. 1. Membrane Attack MAC will perforate MEMBRANES Complex (MAC) 2. Opsonization and Phagocytosis Most important 3. Inflammation function of 4. Clearance of complement Immune Complexes (Ab-Ag complexes) 5. B-cell co-receptor (will be discussed later in module)  Review of Antibody Structure 1. Fab region – binds to antigen 2. Fc region – binds to complement and to Fc receptors on cells 3. Heavy chains (two) – made up of CH1, hinge, CH2 and CH3 CH2 4. Light chains (two) – made up of VL and CL CH3 5. Antigen binding site – contains “hypervariable regions” 6. Hinge regions – confer flexibility  Antibody Classes (Isotypes) Monomeric – IgD, IgE and IgG Dimeric – Secretory IgA (sIgA) Pentameric – IgM (5 Fc regions) Polymeric immunoglobulins (IgA and IgM) are held together by the J chain  Immune Complex (IC) Formation Immune complexes (ICs or Ab-Ag complexes) are formed when antibody binds to antigen Complement can become activated by ICs (see details later in this class) ICs bind to complement receptors on surface of RBCs and are transported to the spleen and liver Splenic macrophages and Kupffer Antibody cells bind to the Fc portion of Abs in ICs and “strip” the immune Antigen complexes off the RBCs.  Immune Complex (IC) Elimination Activation of Complement ICs bind to complement receptors on surface of RBCs (CR1) and are transported to the spleen and liver Splenic macrophages and Kupffer cells bind to the Fc portion of Abs in ICs using high affinity Fc receptors (FcR) ICs are “stripped” from the surface of RBCs and destroyed by phagocytosis  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  Pathways of Complement Activation Three distinct pathways of complement activation Alternative pathway – spontaneous – happening all the time (“tickover”) Lectin pathway – requires recognition of e.g., microbial surface carbohydrates by plasma lectins Classical pathway – initiated by recognition of IgM or IgG bound to antigen Functions of Complement Activation: Alert the immune system to presence of danger by detecting pathogens Central Event: Activation of C3 to generate C3a and the opsonin C3b Recruit phagocytes (C5a) and activate mast cells (C3a/C5a) Kill pathogens (form pores in membranes) C5b6789(n) Reserved for captions  Pathways of Complement Activation Reserved for captions  The Alternative Pathway The alternative pathway is initiated by spontaneous hydrolysis (“tickover”) of plasma C3 to generate C3b which binds via a highly reactive thioester to the pathogen surface Generation of surface-bound C3 convertase results in amplification of pathway with generation of high levels of C3a and C3b. Reserved for captions  Role of Properdin (Factor P) Properdin (Factor P) binds to and stabilizes alternative pathway C3 convertase (C3bBb) and prevents its inactivation by the plasma proteases Factor H and Factor I Alternative pathway C3 convertase is C3bBb Without Trauma or Infection C3b is Inactivated If a pathogen surface is unavailable, C3b will rapidly be inactivated by combining with a molecule of H2O.  Opsonization by C3b A major consequence of complement activation is to target bacteria for phagocytosis by “coating” the bacterial surface with the opsonin C3b Other opsonins include C-reactive protein (CRP), mannose binding lectin (MBL) and IgG “Fixation” of complement is another term for “activation” of complement Opsonization and Pathogen Elimination Opsonization enhances the ability of phagocytes to engulf and destroy pathogens Phagocytosis of opsonized material is highly efficient because of the high affinity binding of the opsonin to its receptor on the phagocyte surface.  Opsonization by C3b and IgG Opsonization enhances phagocytosis by coating particles with molecules for which phagocytes have receptors that can mediate internalization Pathogenic bacteria that have developed capsules to protect themselves from uptake can be taken up after they have been opsonized The main opsonins are IgG and the complement components C3b and C4b that bind to the surface of microbes A variety of different receptors can mediate internalization of antibody and complement-opsonized particles, respectively. Encapsulated pathogens (e.g. bacteria, CR1 is a receptor that binds C3b and fungi) are resistant to phagocytosis C4b unless they are opsonized. Reserved for captions Some of the C3b binds to C3 Convertase to generate C5 Convertase Bb C3b C3b C5 convertase cleaves C5 into C5a and C5b C5a attracts neutrophils C5b initiates the assembly of the Membrane Attack Complex (MAC)  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  C3b, C4b C5a>C3a>>C4a Membrane Attack Complex (MAC) [C5b6789(n)] The lectin pathway uses soluble receptors that recognize microbial surfaces to activate the complement cascade MASPs are serine The lectin pathway is proteases that also activated by cleave C4 and C2 cytokeratin that is exposed when endothelial cells become damaged. MASP - MBL-Associated Serine Protease Binding specificity of MBL allows detection of Pathogens  Lectin-Activated Cleavage of C4, C2 and C3 MASPs are serine proteases that cleave C4 and C2 C3 convertase is C4b2a  Generation of C5 Convertase C3 convertase deposits large amounts of the opsonin C3b on pathogen surfaces Some of the C3b generated binds to C3 convertase and converts this enzyme into C5 convertase C5 convertase cleaves C5 to give C5a and C5b C5a attracts neutrophils C5b initiates the assembly of the Membrane Attack Complex (MAC)  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  C3b, C4b C5a>C3a>>C4a Membrane Attack Complex (MAC) [C5b6789(n)]  Activation of the Classical Pathway The Classical Pathway is activated by binding of C1 to the Fc portion of antibodies that have bound to antigen. C1 consists of 1 molecule of C1q and 2 molecules each of C1r and C1s (C1qr2s2). C1 Complex C1q binds here Activation of the Classical Pathway C1q must bind to two Fc regions to activate the Classical Pathway IgM is the most efficient immunoglobulin isotype at activating Complement  C1: C1qC1r2s2 Requires 2 or more IgG molecules close together so C1q can “bridge the gap” between them Reserved for captions  Similarity between Lectin and Classical Pathways Following the initial recognition event, the Lectin and Classical Pathways are identical C3 convertase C5 convertase  Review Slide: All 3 Pathways Reserved for captions  Late Steps of Complement Activation: the Membrane Attack Complex (MAC) C5b binding to C6 initiates MAC formation C7 then binds followed by C8, and finally C9 binds to C5b678 MAC = C5b6789(n) i.e., variable numbers of molecules of C9 form a cylindrical pore in membranes Reserved for captions  MAC Attack Starts with C5b Ends with C5b6789(n) pore Action of Complement “Split Products” – Anaphylatoxins C5a and C3a are “anaphylatoxins” and (1) cause an increase in vascular permeability, (2) activate mast cells and (3) attract leukocytes (esp. neutrophils). They bind to receptors on vascular endothelial cells and cause the cells to contract so that spaces develop between adjacent cells. When mast cells are activated, they release substances that also increase vascular permeability as well as attract and activate leukocytes.  Biologic Functions of Complement Split Products C3b is a potent opsonin. Because Deficiencies in late complement of this, C3 deficiency is the most components resulting in failure to serious of the complement assemble the MAC only results in deficiencies and leads to serious increased susceptibility to Neisserial recurrent bacterial infections due infections, e.g., Neisseria to a lack of C3b. meningitidis.  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  Receptors for Complement Proteins Receptor Function Keywords Binds to Complement Receptor 1 (CR1) Phagocytosis by activated phagocytes; Immune C3b, C4b, complex elimination; Dissociation of C3 convertase C1q, MBL Complement Receptor 2 (CR2) B cell coreceptor; Also expressed on follicular iC3b, C3dg, dendritic cells (FDC) in B cell follicles C3d Epstein-Barr virus receptor Discussed later in module Complement Receptor 3 (CR3, Phagocytosis; Leukocyte adhesion iC3b, CAMs MAC-1)  chain is CD18 Complement Receptor 4 (CR4) Phagocytosis; Cell adhesion iC3b, CAMs  chain is CD18 C3a receptor Chemotaxis; vascular permeability; mast cell C3a degranulation C5a receptor Chemotaxis; vascular permeability; mast cell C5a degranulation CAMs: Cell Adhesion Molecules Reserved for captions  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  Regulators of Complement Activation Complement control proteins regulate all three pathways of complement activation and protect the host from the adverse effects of complement activation.  Regulators of Complement Activation Properdin: increases complement activation by preventing degradation of alternative pathway C3 convertase Factor H: decreases complement activation by binding C3b enabling cleavage of C3b to iC3b by Factor I (also decreases complement activation) Decay-accelerating Factor (DAF): dissociates C3b from convertase Membrane Cofactor Protein (MCP): same as DAF plus helps Factor I Reserved for captions  C1-INH and CD59 C1 inhibitor CD59 inhibits MAC Reserved for captions  Review: Complement Activation and Function Reserved for captions  Deficiencies of Complement C3 deficiency: most serious; leads to serious recurrent bacterial infections Deficiencies in terminal complex components → failure to assemble MAC → increased susceptibility to Neisserial infections Factor D and P (alternative pathway): also susceptibility to Neisserial infections Reserved for captions  Neisserial Infections Neisseria meningitidis Neisseria gonorrhoeae (meningococcus) (gonococcus) Pyogenic gram-negative Causative agent of gonorrhea diplococcus Sexual contact or baby’s birth Bacterial meningitis canal One of the most commonly reported sexually transmitted diseases in the USA Reserved for captions  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  Hereditary Angioedema - HAE C1-INH also inhibits proteinases of fibrinolytic, clotting, and kinin pathways Deficiency permits plasma kallikrein activation → dysregulated production of vasoactive peptide bradykinin Deficient levels C1INH results in HAE Edema due to excessive levels of bradykinin which increases vascular permeability – Fluid that leaks out of the vasculature causes swelling of tissues Reduced serum levels of C2 & C4 observed  Paroxysmal nocturnal hemoglobinuria - PNH Acquired, rather than inherited, genetic changes Many patients present with unexplained in the PIGA gene hemolytic anemia, fatigue, jaundice, or DAF(CD55), CD59, and MCP(CD46) are GPI- red/pink/dark urine every morning anchored proteins Reserved for captions  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  Pathologic Effects of the Complement System Tissue damage associated with bacterial infections and complement-mediated inflammatory responses Intravascular thrombosis can lead to ischemic injury to tissues Immune complexes in autoimmune diseases: systemic vasculitis and/or glomerulonephritis Therapeutic agent: Eculizumab (humanized monoclonal antibody against C5) is used to treat paroxysmal nocturnal hemoglobinuria (PNH), atypical hemolytic uremic syndrome, generalized myasthenia gravis, and neuromyelitis optica. Reserved for captions  Introduction Learning Objectives Introduction to Complement Functions of Complement Pathways of Complement Activation LECTURE Complement Receptors OUTLINE Regulation of Complement Activation Deficiencies of Complement Defects of Complement Regulation Pathologic Effects Summary  First Aid: Complement Reserved for captions  First Aid: Complement II Reserved for captions