6. MIMM Complement Lecture.pdf

Register for Slido We will be doing some interactive polling questions! Live polls using Slido: https://www.mcgill.ca/polling/ or sign in via Webex Enter #1771037 1 2 Housekeeping SciLearn today from 3-5pm in 2001 McGill College Lobby Quiz: Location: Still TBD Time: In-class 9:35am -10:25am Date: Ja...

Register for Slido We will be doing some interactive polling questions! Live polls using Slido: https://www.mcgill.ca/polling/ or sign in via Webex Enter #1771037 1 2 Housekeeping SciLearn today from 3-5pm in 2001 McGill College Lobby Quiz: Location: Still TBD Time: In-class 9:35am -10:25am Date: January 29 Stage 1: Individual; 10 questions in 25 min – will start at 9:35 am Stage 2: Groups; 3 questions in 15 min (these questions will be the same to a subset of questions from stage 1) – will start at 10:10 am Registration with Student Accessibility & Achievement for the quiz: https://www.mcgill.ca/access-achieve/ 3 Lecture 5 Innate Immunity (II) MIMM 214 – Introductory Immunology: Elements of Immunity Monday January 15th, 2024 Dr. Jasmin Chahal 4 Innate Module Sections Basics, phagocytosis Complement Cell Migration Pattern recognition receptors & signaling Inflammatory cytokines ILCs and NK cells Review Mucosal Immunity (Ch. 13) Cut or splinter 5 Kuby Immunology Figure 1.7 Complement System 6 Complement system (I) Term refers to a group of soluble proteins that cooperate with both the innate and adaptive immune systems to eliminate pathogens, dying cells and immune complexes from the body (Mostly) Proteases (>30) in blood and other fluids: - Protease: an enzyme that performs proteolysis à can break down proteins - Mostly called by “C” followed by a number - Some called “factor” followed by a capital letter (e.g. factor B) - Most are numbered based on when they were discovered 7 Complement system (II) Complement proteins mostly produced by liver Set off a chain reaction that helps to clear pathogens Key mechanisms of action: - Increasing vascular permeability and chemotaxis (inflammation) - Destroying pathogen cell membranes - Increasing recognition of pathogens and facilitating phagocytosis (opsonization) 8 Opsonization & phagocytosis Opsonization: “the coating of the surface of a pathogen by antibody and/or complement that makes it more easily ingested by phagocytes” Phagocytosis: “Internalization of particular matter by cells by a process of engulfment, in which the cell membrane surrounds the material, eventually forming an intracellular vesicle (phagosome) containing the ingested material” 9 How does complement get activated? Initially, components are inactive pro-proteases Activated in 3 ways - Classical pathway - Alternative pathway - Lectin pathway Acting as a cascade: Proteolytic cleavage generating two fragments: One small: - Identified by the letter “a” after the name (e.g. C5a) - With a specific function One large: - Identified by the letter “b” after the name (e.g. C5b) - With proteolytic activity on a new substrate 10 Complement Nomenclature C4b C2a C3b C4b2a Bb C3bBb Both of these form C3 convertase 11 C3 Convertase C3 C3 C4b2a C3a C3bBb C3b C3a C3b 12 Functional categories of complement proteins Group of pro-proteases (inactive) circulate in the blood until activated They initiate their respective complement reactions Cleave and activate the next member of a complement reaction sequence Proteins are inactive until cleaved by proteases Main outcomes How are these activated? Inhibit formation of MAC on host cells 13 What’s essential to know about complement activation Three pathways can activate complement All three pathways generate C3 convertase (which cleaves C3 à C3a + C3b) Three main outcomes 14 Lectin Pathway Triggered by soluble proteins known as lectins. These are also PRRs (pattern recognition receptors) that circulate in blood - Mannose-binding lectin (or MBL) - Ficolins Expression of lectins increase during infection These PRRs can bind surface of pathogens - Triggers signaling cascade on pathogen surface - C3 convertase is generated (C4b2a) Ø C3 cleaved à C3a and C3b 15 Classical Pathway C1q binds pathogen surface - Can bind pathogen directly - Can bind antibodies that are bound to pathogen surface **This can connect adaptive to innate** Once C1q binds - Triggers signaling cascade on pathogen surface - C3 convertase is generated (C4b2a) ØC3 cleaved à C3a and C3b 16 All pathways converge on C3 Classical and lectin pathways result in generation of C3 convertase (C4b2a) C3 convertase CLEAVES C3 à C3a and C3b C3a: Involved in enhancing inflammation C3b: Involved in Opsonization, and is a C5 convertase à C5a and C5b 17 Alternative Pathway (I) 1. Once C3b has been produced by lectin or classical pathway activation - An amplification loop for C3b formation (depositing more C3b molecules on the pathogen) Requires factor B and protease factor D - C3bBb à C3 convertase à C3a and C3b 18 Alternative Pathway (II) 2. When there is a high concentration of C3, C3 can undergo spontaneous hydrolysis, which also involves factors B and D DON’T NEED TO KNOW THE DETAILS OF THE FIGURE, THIS IS JUST A VISUAL TO UNDERSTAND THE MECHANISM 19 Alternative Pathway (III) The alternative pathway C3 convertase (C3bBb) are very unstable - Stabilized by factor called properdin (factor P) secreted by neutrophils - Properdin can stabilize C3 convertase since it can bind to some microbial surfaces 20 All pathways converge on C3 Three pathways result in activation of a C3 convertase - There are many C3 convertases (ex. C4b2a and C3bBb) C3 convertase CLEAVES C3 à C3a and C3b This has many downstream effects 21 Downstream effects: inflammation (I) Inflammation - Additional signaling results in cleavage of other complement molecules - C3a and C5a recruit phagocytes and promote inflammation If present in large amounts, C3a and C5a à anaphylactic shock 22 Downstream effects: inflammation (II) Complement receptors connect complement-tagged pathogens to effector cells - C3aR/C5aR on granulocytes ØStimulates release of proinflammatory cytokines and granule components from basophils, eosinophils, neutrophils, mast cells 23 Downstream effects: increased phagocytosis (I) Increase phagocytosis - Phagocytes have receptors for C3b - Opsonization of pathogen à more readily taken up by phagocytosis - Note: opsonization can occur via complement deposition and/or antibodies (phagocytes also have receptors for antibodies) Opsonization by complement components and/or antibodies 24 Downstream effects: pathogen lysis Pathogen lysis - Additional complement factors create membrane-attack complex (MAC) à Cell lysis! - C5 and C3 are involved (C5b directly involved and C3b indirectly) - *do not memorize the names/order of events here à important thing is that there is a cascade leading to the formation of MAC à a pore in the surface of pathogen that lyses pathogen! Kuby Immunology Figure 5.10 25 Negative regulation of C activation Complement-regulatory proteins in plasma or cell surfaces prevent complement activation from proceeding under normal/basal conditions: - Prevent appearance of C3 convertase - Promote disappearance of C3 convertase Each pathway/process in the immune system has a regulatory component Example of inhibiting MAC formation via CD59 (Protectin) Kuby Immunology Figure 5.16 26 The Complement proteins to know C3 convertase (C4b2a) and (C3bBb) à C3a and C3b C3b is a C5 convertase à C5a and C5b https://youtu.be/Bk6KOFKJVkk?t=932 Biorender. Ariella Coler-Reilly 27 Summary FA #2 COVERS JUST THE Many soluble proteins in serum COMPLEMENT SYSTEM Exist as pro-proteases Activation triggers cascade à three different ones - Lectin - Classical - Alternative All converge on C3 convertase Three outcomes - MAC – pathogen lysis - Opsonization – phagocytosis - Inflammation – recruitment of other immune cells Complement is negatively regulated Many clinical diseases are associated with problems with complement (next class) 28 References Janeway’s Immunobiology, 9th edition, Murphy K. Garland Science, 2016 Janeway’s Immunobiology, 10th edition, Murphy K., Weaver C., Berg S. Norton, 2022 Kuby Immunology, 8th edition, Owen JA, Punt J, & Stranford SA. W.H. Freeman and Company, 2019 Custom images by Elena YH Lin Ariella Coler-Reilly, Biorender. 29