New Mansoura University General Microbiology & Immunology Lecture 3B PDF

# New Mansoura University ## Level 2, Semester 3 ## General Microbiology and Immunology ## PMB201 ## Lecture 3B ### Prof. Mona Ibrahem Shaaban Prof. of Microbiology and Immunology, Faculty of Pharmacy, Mansoura University ## Lecture Summary - Classical Pathway (CP) - The Lectin Pathway (LP)...

# New Mansoura University ## Level 2, Semester 3 ## General Microbiology and Immunology ## PMB201 ## Lecture 3B ### Prof. Mona Ibrahem Shaaban Prof. of Microbiology and Immunology, Faculty of Pharmacy, Mansoura University ## Lecture Summary - Classical Pathway (CP) - The Lectin Pathway (LP) - Alternative Pathway (AP) ## 5- The Complement System - Complement system; it consists of more than 34 serum proteins and membranes expressed proteins with important role in the immune response to pathogens. - The complement system is the major humoral nonspecific defense mechanism. - Bridges the innate and adaptive immunity. ## The Complement System ### Function of the Complement System 1. Opsonization; Identifying materials for removal by phagocytic cells 2. Chemotaxic; Phagocyte attraction and activation 3. Formation of membrane attack complex and Cell lysis. ### Pathways of activation of the Complement System: All three pathways activate the central component ‘C3’ 1. Classical pathway: Activated predominantly by Ig-containing complexes. 2. Alternative pathway: Spontaneously activated 3. Lectin pathway: Activated by lectin-carbohydrate interactions. ## I- CLASSICAL PATHWAY The classic pathway is initiated by antigen-antibody complex. ### 1. C1 activation A multi-subunit protein (C1q, C1r and C1s), binds to the Fc region of IgG and IgM antibody molecules that have interacted with antigen. ## Classical Pathway - Image: A microscopic diagram that shows the process of the classical pathway. - A microbe, with antigen attached to it - C1, with subunits C1q, C1r and C1s - C2, C4a and C4b - C3a and C3b - C5a and C5b - C6-C9 - A mast cell - The diagram shows how C1 is activated to cleave C4 and C2, leading to the formation of the C3 convertase (C4b.C2a). - This convertase then cleaves C3 into C3a and C3b. - C3b acts as an opsonin by binding to the cell surface, which enhances phagocytosis by macrophages and neutrophils. - C3a, on the other hand, acts as an anaphylatoxin by binding to basophils and mast cells, causing the release of histamine and anaphylaxis. - The diagram also illustrates the formation of the membrane attack complex (MAC), consisting of C5b, C6, C7, C8, and C9. - The MAC creates pores in the cell membrane, leading to cell lysis. - This is a crucial step in the elimination of pathogens by the complement system. ### 2. C4 and C2 activation (generation of C3 convertase) Activated CIs (a serine protease) cleaves two serum proteins: - C4, it is cleaved into a large fragment C4b, and to a smaller, inactive, fragment of C4a which diffuses away. - C2, it is cleaved into C2a, which binds to C4b, and C2b (smaller inactive). - The resulting complex C4b.C2a is called “C3 convertase” as it catalyzes the cleavage of C3 into C3a and C3b. - C3b binds to the cell surface as an opsonin. Macrophages and neutrophils have receptors for C3b and bind the C3b-coated cell for phagocytosis. - C3a. bind to receptors on basophils and mast cells and release histamine causing anaphylaxis, C3a is called an anaphylatoxin. ### 3- C3 activation and Generation of C5 convertase - Some of the C3b binds to the membrane in association with C4b and C2a. The resulting C4bC2aC3b is a C5 convertase to initiate the assembly of a set ofcomplement proteins that make up the membrane attack complex. ### 4- The resulting complex C5b･6･7･8 triggers the polymerization of 18 molecules of C9 as a tube inserted into the lipid bilayer of the plasma membrane causing cell lyses. ## II- LECTIN PATHWAY - The lectin pathway is activated by carbohydrate recognition molecules that bind to carbohydrate components on microbial surfaces. - Two kinds of recognition molecule have been described, mannan-binding lectin (MBL) and the ficolins or collectin. - Binding activates MASP-1 and MASP-2 (MBL associated serine proteases) that cleave and activate C4 and C2 Cleaved to generate C3 convertase. - Image: Diagram that shows the process of the lectin pathway. - A microbe with carbohydrate containing mannose - Lectin - C2, C4a and C4b - C3a and C3b - C5a and C5b - C6-C9 - A mast cell - - The diagram shows how the lectin pathway is activated by the binding of mannan-binding lectin (MBL) to carbohydrate-containing components on microbial surfaces. - MBL activates MASP-1 and MASP-2, which in turn cleave C4 and C2 to generate the C3 convertase (C4b.C2a). - The C3 convertase then cleaves C3 into C3a and C3b, leading to opsonization and inflammation similar to the classical pathway. ## III-ALTERNATIVE PATHWAY - This pathway, is considered to be a part of the innate immune system. - Initiation of the alternative pathway of complement activation is independent of antibody-antigen interactions. - This pathway is initiated by: Bacterial endotoxin, polysaccharide capsule, bacterial surface components. - Image: A microscopic diagram that shows the process of the Alternative pathway, including the formation and action of the soluble C3 convertase iC3bB. - C3 - B - D - Bactera + D - I C3bB - C3a - C3b - In the Alternative pathway, C3 is hydrolyzed spontaneously in the presence of water to activate C3b. - C3b binds with a protein called "Factor B," - which is then cleaved by another protein called "Factor D," resulting in the formation of a soluble form of C3 convertase called "iC3Bb." - The iC3Bb, then amplifies the pathway by activating more C3. - This process is controlled by the presence of properdin and Factor H, which regulate the formation of the C3 convertase. - The outcome of the alternative pathway is similar to the classical and lectin pathways, including opsonization and cell lysis. - In serum there is low level spontaneous hydrolysis of C3 to produce C3(H2O) which in turn binds to factor B to form a C3(H2O)B complex. - In this complex, factor B is cleaved by factor D releasing a Ba fragment, while Bb remains attached to form complex C3(H2O)Bb which forms a C3 convertase enzyme. - Binding of a molecule called properdin cleaves C3 into C3a and C3b. - Once C3b is generated, it will bind to the surface of pathogens where it can bind to another molecule of factor B and form a new alternative pathway C3 convertase C3bBb. - It starts at C3 then C5, C6, C7, C8, C9. The complement componts. C1, C4, C2 are by-passed. ## 0-4 hours ## ALTERNATIVE PATHWAY Pathogen surface creates local environment conducive to complement activation. ## First to act ## 4-96 hours ## LECTIN PATHWAY Mannose-binding lectin binds to pathogen surface. ## Second to act ## >96 hours ## CLASSICAL PATHWAY C-reactive protein or antibody binds to specific antigen on pathogen surface. ## Third to act ## Complement Activation - Image: A diagram that shows the three pathways of complement activation. ## Function of the C-activation products 1. Activation of C3a, C5a (anaphylatoxin) causes mast cell degranulation, enhanced vascular permeability; anaphylaxis. 2. C3b (opsonin) associated with opsonization; phagocyte activation. 3. C4a (anaphylatoxin), but less potent than C3. 4. C4b (opsonin) associated with opsonization; phagocytosis. 6. C5b6789 attaches to cell membranes - - The diagram shows the activation of the complement system, which is a crucial part of the immune response to pathogens. - The complement system is a cascade of proteins that are activated in a specific order, resulting in several downstream effects: opsonization, inflammation, and cell lysis. - The final step is the formation of the MAC, which is responsible for the lysis of bacterial cells. - *Thao Doan, Fabio Lievano, Susan M. Viselli., Michelle Swanson-Mungerson (2021). Lippincott Illustrated Reviews: Immunology (Lippincott Illustrated Reviews Series, 3rd edn, Wolters Kluwer Health)* ## Thank You