Lecture 4: The Complement System PDF

Lecture 4 The Complement System 09/17/2024 Learning objectives What is the What happens Pathways of complement when complement complement system and how is system...

Lecture 4 The Complement System 09/17/2024 Learning objectives What is the What happens Pathways of complement when complement complement system and how is system activation it activated. malfunctions What is the complement system? Complement system System of soluble plasma proteins (aka Produced mainly by the liver complement proteins) that act to opsonize and lyse pathogens. Opsonization: coating of a pathogen with antibodies and/or complement proteins so that it can be more readily taken and destroyed by phagocytic cells. Discovered in 1890s by Jules Bordet. Composed of >30 proteins. Circulate in inactive forms. Essential component of the innate immune system. ‘Activated’ in the presence of Enhances or complements our immunity. pathogens or antibodies bound to pathogens Complement Activation Pathways Three different proteolytic pathways lead to complement activation Contact with Alternative Lectin Classical pathogen pathway pathway pathway Complement activation occurs via a cascade of enzymatic reactions in which one component activates the next The complement system proceeds in distinct phases in the elimination of pathogens Many of the complement proteins Complement pathways are are proteases - they triggered by proteins that act as PRRs to detect the presence of successively cleave and pathogens activate each other These proteases are synthesized as inactive The detection activates an initial pro-enzymes, or zymogen, triggering a cascade of proteolysis – other zymogens zymogens are activated sequentially – Zymogens become amplifying the signal as the enzymatically active only cascade progresses after proteolytic cleavage, usually by Activation of three distinct another complement Fig. 2.13 effector pathways protein Effector pathways of the complement system Complement in pathogen Complement in pathogen recognition \ destruction Complement in inflammation Self vs. non-self: the complement system labels Some components of the complement system are foreign molecules and pathogens as non-self to capable of directly damaging and destroying allow detection by the immune system pathogens Complement fixation – activation of the Insertion of complement components into the complement proteins promotes the attachment pathogen membrane and formation of pores of specific proteins to the pathogen surface that that in the membrane disrupt the osmotic then serves as an opsonin for recognition by balance across the membrane and promote phagocytic cells – phagocytosis pathogen death – membrane attack complex Nomenclature for complement proteins Designated by letter C (because first proteins discovered belong to the classical pathway) followed by a number Inactive complement proteins (aka zymogens) have a simple number designation. For example, C1 and C2 named in the order of discovery – C1, C4, C2, C3, C5, C6, C7, C8, and C9 (the classical pathway) products of cleavage reactions designated by adding a lowercase letter as a suffix o e.g., C3 cleaves to produce C3a (smaller protein fragment) and C3b (larger fragment) o Exception #1: C2 produce C2a (larger fragment) o Exception #2: C1q, C1r and C1s are not cleavage products of C1 but distinct proteins that together compose C1 proteins of the alternative pathway: factor B and factor D o the factor B cleaves into Ba (smaller fragment) and Bb (larger fragment) Key components of the complement system Complement Protein Function Complement Pathway C1 Protease involved in cleaving C2 and C4 Classical C2 Cleaved by either C1or MASP-2 to form C2b fragment of classical C3convertase Lectin Classical C3 Cleaved to form C3a (anaphylatoxin) and C3b (opsonin); also functions in forming alternative C3 Alternative convertase Lectin Classical C4 Cleaved by either C1or MASP-2 to form C4b fragment of classical C3convertase Lectin Classical C5 Cleaved by C3convertase to form C5a (anaphylatoxin) and C5b (initiates formation of the membrane- Alternative attack complex by interacting with C6 and C7) Lectin Classical C6 Interacts with C5b and C7to initiate formation of the membrane-attack complex Alternative Lectin Classical C7 Interacts with C5b and C6 to initiate formation of the membrane-attack complex Alternative Lectin Classical C8 Binds C5b67 to serve as a docking site for C9 Alternative Lectin Classical C9 Binds C8; forms transmembrane pores in pathogen surfaces Alternative Lectin Classical Features of microbial surfaces recognized by the complement system The lectin pathway o initiated by soluble carbohydrate-binding proteins – mannose-binding lectin (MBL) and ficolins o bind to particular carbohydrate structures on microbial surfaces o MASPs (MBL-associated serine proteases) trigger the cleavage of complement routines and activation of the pathway The classical pathway o initiated when C1 (complement component 1; comprised of C1q, C1r and C1s) either recognizes a microbial surface directly or bind to antibodies already bound to a pathogen The alternative pathway o initiated by spontaneous hydrolysis and activation of C3, which can bind directly to microbial surface  Fig. 2.15 The three pathways converge Generated from the enzymatic at the central and most activity of C3 convertase – important step in complement cleaves C3 into C3a and C3b activation – C3 Cleavage of C3 leads directly or indirectly to all the effector activities of the complement system Fig. 2.15 Complement component C3 The most important factor of the complement system Cleaves into C3a (an anaphylatoxin) and C3b (an opsonin) Molecules capable of activating an inflammatory response by triggering degranulation of cells capable of inducing inflammation Covalently attaches (complement fixation) to the pathogen surface. Marks the pathogen for destruction (acts as an opsonin) – renders pathogen more susceptible to phagocytosis The breakdown occurs through the reaction of enzyme C3 convertase C3 convertase activates C3 for covalent bonding to microbial surfaces by cleaving it into C3a and C3b and exposing a highly reactive thioester bond in C3b Fig. 2.16 TED: thioester-containing domain C3 and C5 convertases of the complement pathways C5 convertase of the alternative pathway consists of two C3b Fig. 2.17 subunits and one Bb subunit Complement activation pathways The lectin pathway Mannose-binding lectin and ficolins form complexes with serine proteases and recognize particular carbohydrates on microbial surfaces. MBL primarily exists as trimers and tetramers, with each MBL molecule consisting of two to six trimers. MBL binds with high avidity to repetitive carbohydrate structures on microbes due to its carbohydrate-recognition domains targeting mannose, fucose, and GlcNAc, but it avoids binding to sialic acid residues found on host cells. MBL in plasma binds to five proteins: three MBL-associated serine proteases, MASP-1, MASP-2, MASP-3 and two nonenzymatic proteins, MAp19 and MAp44. Fig. 2.19 The actions of the C3 convertase result in the binding of large numbers of C3b molecules to the pathogen surface Fig. 2.20 The first protein in the classical pathway of complement activation is C1, which is a complex of C1q, C1r, and C1s Fig. 2.21 Fig. 2.22 The lectin and classical pathways C2b C2a C2a C4bC2a (classical C3 convertase)  When and how is the alternative pathway activated? The production of iC3 is accelerated in the First to act presence of a pathogen Relies on soluble proteins, including C3 Factor D - protease of the complement system that functions to activate factor B through proteolytic cleavage Factor B – protease zymogen that functions in the complement cascade; cleavage of factor B by factor D causes its activation and ability to function as a subunit of C3 convertase C3 convertase – protease produced Alternative C3 convertase – C3 convertase during complement activation that is produced when the opsonin C3b binds to capable of cleaving the complement factor B and bound factor B is cleaved by factor protein C3 into C3a and C3b D to produce C3bBb at the pathogen surface The alternative pathway is an amplification loop for C3b formation that is accelerated by properdin in the presence of pathogens Properdin – plasma protein that enhances the activity of the alternative C3 convertase to aid in the complement activation and fixation (aka factor P) Bacterial surfaces do not express complement-regulatory proteins and favor the binding of properdin, which stabilizes the C3bBb convertase. This convertase activity is the equivalent of C4b2a of the lectin and classical pathways. C3bBb then cleaves many more molecules of C3, coating the pathogen surface with bound C3b. The alternative pathway of complement activation can amplify the classical or the lectin pathway by forming an alternative C3 convertase and depositing more C3b molecules on the pathogen. Fig. 2.25 The cleavage products of C3b are recognized by different complement receptors Fig. 2.32 iC3b is a ligand for CR3 and CR4. C3dg is a ligand for CR2 Factor I – serine protease that inactivates C3b through its cleavage into a smaller fragment known as iC3b, which cannot function as a component of C3 convertase Complement fixation tags pathogens for phagocytosis Cell-surface receptors bind to complement proteins fixed on the surface of a pathogen, allowing for recognition and phagocytosis Macrophages express CR1 – binds to C3b – enhanced recognition and phagocytosis Recall from the previous lecture… Regulating complement activation Plasma proteins Factor H – plasma protein that enhances the cleavage of C3b into iC3b by Factor I. Binds to cell membranes by interacting with sialic acid, a common carbohydrate on the surface of eukaryotic cells Some bacteria, S. aureus and S. pyogenes, have evolved a defensive mechanism by incorporating sialic acid on their surfaces, thus inhibiting complement activation Regulating complement activation Membrane proteins To ensure that C3b fixation on host cell surfaces does not result in targeting these cells for destruction, human cells express membrane proteins that inhibit complement DAF (decay-accelerating factor) – Breakdown the alternative C3 convertase MCP (membrane cofactor protein) – binds to C3b and enhances its cleavage to inactive iC3b by factor I Membrane-Attack Complex (MAC) A complex of five complement proteins C5, C6, C7, C8, and C9 that work in concert to form holes in bacterial and eukaryotic membranes C5b functions as an initiating factor in the formation of the MAC Formation and action of C5 convertase Formation of the MAC Regulating the MAC Functions of anaphylatoxins C3a and C5a Local inflammatory response Fig. 2.33 What happens when the complement system malfunctions? Abnormal clearance of bacteria and fungi Hypersensitivity responses and autoimmune disorders (lupus) – due to improper clearance of soluble immune complexes Increased infection rate due to malfunction of MAC Checkpoint questions Which complement protein is most important in directly targeting the pathogens for destruction? What are the cleavage products of that (from above) protein, and what does each cleavage product do to facilitate pathogen elimination? What are the three pathways of complement activation? During the alternative pathway of complement activation, two C3 convertases are formed. Which proteins make up these convertases? Which complement protein is responsible for forming pores in pathogen membrane within the MAC? Which acute-phase proteins are the two initiating opsonins of the lectin and classical pathways? Which complement cleavage products constitute the classical C3 convertase? What are some common complications associated with a deficiency in the complement system? Assigned Reading and InQuizitive Readings Chapter 2 Sections 2-5, 2-6, 2-7, 2-8, 2-9, 2-10, 2-12, 2-13, 2-14, 2-15 InQuizitive 20240917 Readings