Complement Pathways PDF

Complement Pathways UGME Phase 1 – MED 5710 – The Patient Host Defense & Immune Mediated Disorders Deepak Kaushik, Asst. Prof., Immunology and Infectious Diseases [email protected] https://www.mun.ca/medici...

Complement Pathways UGME Phase 1 – MED 5710 – The Patient Host Defense & Immune Mediated Disorders Deepak Kaushik, Asst. Prof., Immunology and Infectious Diseases [email protected] https://www.mun.ca/medicine/faculty-and-staff-resources/faculty-a-z/kaushik-deepak.php 1  Lecture Objectives Demonstrate an understanding of the three 7099 complement pathways Describe the biological role of complement 7560 activation Explain the involvement of complement 7562 receptors in regulation of complement activity Explain the role of complement and 7564 complement receptors in removal of immune complexes 2 Complement-A large group of circulating and cell membrane proteins that form a part of the innate immune system Major functions- Assist or complement the immune system in eliminating pathogens or damaged cells, either through opsonization & phagocytosis or cells lysis (killing of cells) Promote inflammatory and immune responses Heat sensitive component of serum- Jules Bordet and Paul Ehrlich 3  Complement Pathways & Nomenclature Classical Pathway Lectin Pathway Alternative Pathway (AP) Complement components (C) Mannose binding Activation C1 Lectin (MBL) Factors B, D & Amplification C4, C2 C4, C2 Properdin C3 C3 C3 Terminal C5, C6, C7, C8, C9 C5, C6, C7, C8, C9 C5, C6, C7, C8, C9 4  7099 Overview of Complement Pathways The next 3 slides will focus on how Complement is activated in each pathway to form C3 convertase 5  7099 Activation - The Classical Complement Pathway Proteases Binds The ability of Ig to bind & activate C1q, in decreasing IgG order: IgM>IgG3>IgG1=>IgG2 (IgG4 cannot activate complement) C1q has to cross link the Fc of 2 Ig molecules – IgM or IgG, bound to an antigen. Note: soluble antigens bound to IgG4 cannot activate complement Interaction of C1q with Ab-Ag complex activates the proteases C1r & C1s C1s becomes enzymatically active and is called C1s esterase, which cleaves C4 into C4b & C4a and then C2 into C2a & C2b C4b complexes with C2a on the membrane of the Ab-Ag complex to form C3 6 convertase  7099 Activation - the Mannose Binding Lectin (MBL) Pathway MASP = mannose-associated serine protease is similar to C1r and C1s in the classical complement pathway Terminal mannose residues are not found on mammalian cells; they expressed on numerous types of bacteria, some fungi and viruses MBL is structurally similar to C1q; In the circulation it is bound to MASP. When MBL binds to mannose residues on a bacterium it causes MASP-2 to cleave C4 and C2 to form C4bC2a on the surface of the bacterium As in the classical pathway C4bC2a forms C3 convertase 7  7099 Activation - The Alternative Pathway Can be triggered by a variety of foreign substances including: endotoxins, necrotic cells, some types of microorganisms & aggregated Ig molecules Due to hydrolysis of C3, small amounts of “preformed”C3b are present in the serum. C3b, deposited on the surface of a particle, binds Factor B (B), forming C3bB Factor D cleaves B into Ba (released into fluid phase) & Bb which remains associated with C3bBb. C3bBb forms the C3 convertase & is stabilized by properdin 8 7099 Amplification: Generation of C3 & C5 convertases The Alternative Pathway Amplification loop. Normally C3bBb dissociates from the surface, but if bound by Properdin it binds and cleaves large amounts of C3 to C3b, allowing more and more C3b to bind to the surface. 9 Formation of C3 convertase is required for activation of C5 Summary of the previous 3 slides 1. C3 convertase is formed from either C4bC2a (classical & lectin pathways) or C3bBb (alternative pathway). The alternative pathway additionally has an amplification loop whereby Properdin (Factor P) binds and cleaves large amounts of C3 to C3b, allowing more C3b to bind to the surface. 2. C3 convertase binds and cleaves C3 into C3a (goes into the fluid phase) and C3b which is deposited with C4bC2aC3b (classical or MBL pathways) or C3bBb3b (Alternative pathway). This forms C5 convertase. 3. C5 convertase cleaves C5 into C5a (fluid phase) and C5b which binds to the surface. 10  7560 Main Biological Activities of Complement A. Production of OPSONINS, mediating phagocytosis B. Production of ANAPHYLATOXINS, contributing to inflammation C. Direct killing (LYSIS) of microbes- by forming membrane attack complex (MAC) 11 7560A) Production of OPSONINS, mediating phagocytosis Opsonization – attachment of a protein (e.g. IgG) or one of the indicated complement products (e.g. C3b) to the surface of an antigen, increasing the efficiency of phagocytosis Complement receptors: CR1 (CD35)– present on phagocytic cells and many other types including red blood cells CR3 (CD11b)– Monocytes, macrophages, granulocytes, NK cells CR4 (CD11c)– myeloid cells, DC + other cell types 12 B) Production of Anaphylatoxins & Chemotactic Factors C3a, C4a, C5a are anaphylatoxins - the most potent is C5a. An anaphylatoxin binds to a receptor on mast cells and other cells, triggering a reaction similar to an allergic hypersensitivity reaction (more on this in the next lecture) Anaphylatoxin-binding to its cellular receptor triggers: release of histamine & other mediators increases smooth muscle contraction increases vascular permeability & accumulation of fluid into the tissues. C5a is also a chemoattractant - recruits neutrophils to inflamed tissue 13 7099 C) Formation of Membrane Attack Complex (MAC) is Essential for Cell Lysis (Terminal pathway) C6 binds to C5b, then to C7 C7 inserts into the outer membrane of the cells Binding of C8 to C5b67 allows deeper penetration into the cell membrane C5b-C8 acts as a receptor for C9, a perforin like molecule Several other C9 molecules (MAC) bind (poly-C9), which forms a transmembrane channel allowing water to enter the cell, followed by cell lysis Terminal components: C5b, C6, C7, C8, C9 are common to all 3 pathways Clinical relevance - Deficiencies in these components result in inability to form MAC & are associated with recurrent Neisseria infections (e.g. C9 deficiency is associated with meningitis) 14 7099 Summary Slide of the Complement Pathways 15  Complement Pathways are also activated by Acute Phase Proteins during acute inflammation Recall - IL-6 acts on the liver to induce synthesis of acute phase proteins. Relevant to this lecture: Mannan binding lectin & lectin pathway C-reactive protein (CRP) can interact with C1q to initiate the classical pathway in the absence of antibody. CRP also binds to the phosphorylcholine component on liposaccharides in microbial Clinical relevance – C-Reactive Protein is one of the best cell walls, enhancing indicators of inflammation and is a “lab test” used as part phagocytosis. of the diagnostic work-up on a patient suspected to have an inflammatory disease. 16  7560/62/64 Complement Receptors (CR) CR associated with opsonization & phagocytosis & removal of immune complexes (slide 12 and next slide) CR1 (CD35), found on various phagocytic cells & erythrocytes, binds C3b, C4b & iC3b coated particles to enhance phagocytosis and the transport of immune complexes to the liver CR3 (CD11b), found on various phagocytic cells, binds iC3b & stimulates phagocytosis CR4 (CD11c) - similar to CR3 17  7564 The Role of Complement and Complement Receptors in the Removal of Immune Complexes (IC) C3b + IC binds to CR1 on RBC, which transport them to the liver & spleen. Complexes then bind to CR3 & Fc receptors on macrophages which degrade them. Clinical relevance – Failure to clear IC from the circulation results in IC deposition in vessel walls, joints and other tissues. This can cause acute inflammation and immune pathology - covered in the Overview of Hypersensitivities & Case Studies. 18  7560/62/64 Complement Receptors cont’d CR associated with acute inflammation (see slide 13) C5a receptor (CR5a) ü found on numerous cells including: smooth muscle cells, endothelial cells, mast cells, basophils. Binding of C5a to CR5a on mast cells promote an anaphylatoxin-like response & inflammation ü found on phagocytes such as neutrophils, in presence of C5a enhances chemotaxis C3a receptor (CR3a - binds C3a, also an anaphylatoxin) – similar cell distribution & function as CR5a 19  Complement Regulators Group of plasma and cell membrane proteins that prevent excessive activation of complement. Regulation occurs at 3 steps in Complement pathway – Initiation & Activation – Amplification (C3 and C5 convertases) – Membrane attack complex 20  Complement Regulation Regulatory Proteins Found in the Plasma Controls activation & initiation C1 Inhibitor (CI-INH) binds activated C1r and C1s & removes them from C1q. As such Complement pathway can’t be activated Controls amplification C4 binding protein (C4BP)* – Dissociates C3 convertase subunits in the classical pathway – Cofactor for Factor I-mediated cleavage of C4b Factor I * – cleaves C3b and C4b Factor H * – dissociates C3 convertase subunits in the alternative pathway – cofactor for Factor-I mediated cleavage of C3b *FYI only 21 Clinical relevance - deficiency in C1-INH & Angioedema In addition to dysregulation of the initiation of the complement pathway, C1NH deficiency causes a disease called Angioedema A genetic mutation in the C1NH gene results in Hereditary Angioedema (HAE) C1-INH also regulates interconnecting pathways – coagulation, fibrinolytic and Lab tests aiding in diagnosis: kinin. The latter is central to HAE C1-INH – low or absent as the production of large C4 – levels would be expected to be amounts of bradykinin, a potent low since a deficiency in CIHN vasodilator, largely contributes to allows spontaneous activation of C4, to the angioedema resulting in it being consumed. 22 Membrane proteins that regulate complement activation Decay accelerating factor Clinical Relevance: Deficiencies in CD55 (DAF) or CD55 and C59 occur when a somatic mutation occurs in the gene that controls the – found on many cell types production of – Important in regulating glycosylphosphatidylinositol (GPI). CD55 amplification by and CD59 require GPI to be anchored to dissociating C3 convertase the cell membrane. Absence of these subunits. regulatory proteins makes the cells more susceptible to lysis resulting in – Defect in DAF affects all Paroxysmal Nocturnal Hemoglobinuria. three pathways CD59 – found on many cell types – Regulates the membrane attack complex by binding C5b-C8 & blocking C9 binding & formation of MAC. Urine collected at different time points Monoclonal antibodies, Eculizumab (Soliris) or Ravulizumab (Ultomiris)- bind to C5 complement 23 Resource material: Immunology – A Short Course 6ed. by Richard Coico & Geoffrey Sunshine https://www.uptodate.com/contents/regulators-and-receptors-of- the-complement-system#H4 https://www.news-medical.net/life-sciences/Complement- System-Overview.aspx Immunobiology: the immune system in health and disease You may also find this video, explaining the classical and alternative complement pathways, helpful https://www.youtube.com/watch?v=DPNnZE4OtCM 24