Complement System
- 1. THE COMPLEMENT SYSTEM BY MUSOMA JESSE
- 2. INTRODUCTION The complement refers to a series of factors occurring in normal serum that are activated characteristically by antigen-antibody interaction, and subsequently mediate a number of biologically significant consequences
- 3. Synthesis and Denaturing Complement proteins are synthesized mainly by the liver. Complement C1 is synthesized in intestinal epithelium. C2,4 is synthesized in macrophages. C5,8 is synthesized in spleen. C3,6,9 is synthesized in liver.
- 4. Denaturing Complement is heat-labile; i.e., it is inactivated by heating serum at 56°C for 30 minutes
- 5. Biological Effects Cytolysis of cells such as bacteria, allografts, and tumor cells; e.g C5, 6, 7, 8, 9. Opsonization, i.e., enhancement of phagocytosis Enhancement of Antibody Production Anaphylaxis e.g C3a and C5a Chemotaxis e.g C5a
- 7. Terms used in complement C- activation C – fixation Haemolytic unit C – deactivation Convertase or Esterase
- 8. Complement Proteins It consists of approximately 20 proteins that are present in normal human (and other animal) serum C1, C2 , C4,C5,C6,C7,C8,C9 Factors ( B, D, H,I and P) MBL, MASP-1 and MASP-2 C1 INH , C4BP,DAF CR1 and Protein S (vitronectin)
- 9. Nomenclature of complement • Over lining • Large fragments • Smaller fragments • Letters ‘a’ and ‘b’ • EXCEPTION C2
- 10. Nomenclature and activation • Complement proteins are often designated by an uppercase letter C and are inactive until they are split into products. – Example: C1 • When the products are split they become active. The active products are usually designated with a lower case a or b. – Example: C1a and C1b
- 11. The complement system • A defensive system consisting of over 30 proteins produced by the liver and found in circulating blood serum. • Complement kills microbes in three different ways – 1. opsonization – 2. inflammation – 3. Cytolysis
- 12. A Cascade system • The complement works as a cascade system. – Cascade is when one reaction triggers another reaction which trigger others and so on. These types of systems can grow exponentially very fast.
- 13. Complement Pathways • The complement pathway can be activated by either of three different pathways. – Classical pathway (specific immune system) – Lectin – alternative (non-specific immune system)
- 14. The Classical Pathway • The classical pathway is considered to be part of the specific immune response because it relies on antibodies to initiate it. • Only IgM and IgG fix complement • C1 becomes activated when it binds to the ends of antibodies
- 15. The building of a C3 activation complex • Once C1 is activated, it activates 2 other complement proteins, C2 and C4 by cutting them in half • C2 is cleaved into C2a and C2b • C4 is cleaved into C4a and C4b • Both C2b and C4b bind together on the surface of the bacteria • C2a and C4a diffuse away
- 16. C3 Activation complex • C2b and C4b bind together on the surface to form a C3 activation complex • The function of the C3 activation complex is to activate C3 proteins. – This is done by cleaving C3 into C3a and C3b
- 17. C3b • Many C3b molecules are produced by the C3 activation complex. • The C3b bind to and coat the surface of the bacteria. • C3b is an opsonin – Opsonins are molecules that bind both to bacteria and phagocytes – Opsonization increases phagocytosis by 1,000 fold. Bacteria Opsonins
- 18. C3a C3a increases the inflammatory response by binding to mast cells and causing them to release histamine
- 19. Building the C5 activation complex • Eventially enough C3b is cleaved that the surface of the bacteria begins to become saturated with it. • C2b and C4b which make up the C3 activation complex has a slight affinity for C3b and C3b binds to them • When C3b binds to C2b and C4b it forms a new complex referred to as the C5 activation complex
- 20. Overview
- 21. The alternative pathway • The alternative pathway is part of the non- specific defense because it does not need antibodies to initiate the pathway. • The alternative pathway is slower than the Classical pathway
- 22. Points to note: Amplification loop Factors involved Generation of C3 convertase and C5 convertase Control of the loop and the factors involved Stabilization of the loop
- 24. Initiation of The Alternative pathway • C3 contains in unstable thioester bond. • This unstable bond makesC3 subject to slow spontaneous hydrolysis to C3b and C3a • The C3b is able to bind to foreign surface antigens. • Mammalian cells contain sialic acid which inactivates C3b
- 25. Factor B • C3b on the surface of a foreign cells binds to another plasma protein called factor B
- 26. Factor D • The binding of C3b to factor B allows a protein enzyme called Factor D to cleave Factor B to Ba and Bb. • Factor Bb remains bound to C3b while Ba and Factor D disperse away.
- 27. The C3 activation complex • Properdin, also called factor P, binds to the C3bBb complex to stabilize it. • C3bBbP make up the C3 activation complex for the alternative pathway
- 28. The C3 activation Complex • The C3 activation complex causes the production of more C3b. • This allows the initial steps of this pathway to be repeated and amplified • 2X106 molecules can be generated in 5 minutes
- 29. C5 activation complex • When an additional C3b binds to the C3 activation complex it converts it into a C5 activation complex. • The C5 activation complex cleaves C5 into C5a and C5b. • C5b begins the production of the MAC.
- 30. Overview
- 31. • The lectin pathway is very similar to the classical pathway. • It is initiated by the binding of mannose-binding lectin (MBL) to bacterial surfaces with mannose-containing polysaccharides (mannans). • Binding of MBL to a pathogen results in the association of two serine proteases, MASP-1 and MASP-2 (MBL- associated serine proteases). • MASP-1 and MASP-2 are similar to C1r and C1s, respectively and MBL is similar to C1q. Lectin pathway
- 32. • Formation of the MBL/MASP-1/MASP-2 tri-molecular complex results in the activation of the MASPs and subsequent cleavage of C4 into C4a and C4b. The C4b fragment binds to the membrane and the C4a fragment is released into the microenvironment. • Activated MASPs also cleave C2 into C2a and C2b. C2a binds to the membrane in association with C4b and C2b is released into the microenvironment. • The resulting C4bC2a complex is a C3 convertase, which cleaves C3 into C3a and C3b. Lectin pathway
- 33. • C3b binds to the membrane in association with C4b and C2a and C3a is released into the microenvironment. The resulting C4bC2aC3b is a C5 convertase. The generation of C5 convertase is the end of the lectin pathway. • The biological activities and the regulatory proteins of the lectin pathway are the same as those of the classical pathway. Lectin pathway
- 34. Over view of Lectin pathway
- 35. The C5 activation complex • The C5 activation complex (C2b, C4b, C3b) activates C5 proteins by cleaving them into C5a and C5b • Many C5b proteins are produced by the C5activation complex. These C5b begin to coat the surface of the bacteria.
- 36. The function of C5a • C5a disperses away from the bacteria. – Binds to mast cells and increases inflammation. – Most powerful chemotactic factor known for leukocytes – Stimulates respiratory burst – It is a potent anaphylatoxin
- 37. Building the Membrane Attack complex • C5b on the surface of bacteria binds to C6 • The binding of C6 to C5b activates C6 so that it can bind to C7 • C7 binds to C8 which in turn binds to many C9’s • Together these proteins form a circular complex called the Membrane attack complex (MAC)
- 38. Membrane Attack complex • The MAC causes Cytolysis. – The circular membrane attack complex acts as a channel in which cytoplasm can rush out of and water rushes in. • The cells inner integrity is compromised and it dies • Control of lysis of normal cells by Protein S (vitronectin)
- 40. Comparisons among the pathways i. Antibody initiation ii. Divalent cation iii. Prokinin generation iv. Anaphylatoxin generation v. C3 convertase and C5 convertase vi. Feeding into the MAC
- 41. Problems: Which of the following is not true about the complement system? (a)It is a set of more than 20 proteins that play a key role in host defense by specifically acting in different ways toward different microorganisms. (b)Its general functions include enhancing phagocytosis by phagocytes, lysing microbes and enveloped viruses directly, and generating peptide fragments that regulate inflammation and immune responses. (c)It is a fast-acting innate host defense that works in a cascade. (d)There are two pathways (classical and alternative), with the former beginning when antibodies bind to microbes which trigger C1, C4, and C2 complement proteins, and the latter activated by contact between complement protein factors B, D, P and polysaccharides at the pathogen surface. (e)The effects of both pathways are the same
- 42. 2. Describe the complement system, including the classical and alternative (properdin) pathways. 3. What are the results of activating the complement cascade?
- 43. C3a and C5a can cause: (a) bacterial lysis (b) vascular permeability (c) phagocytosis of IgE- coated bactreia (d) aggregation of C4 and C2
- 44. Complement fixation refers: a) the ingestion of C3b- coated bacteria by macrophages b)The destruction of complement in serum by heating at 56oC for 30minutes c)The binding of complement components by antigen-antibody complexes d)The interaction of C3b with mast cell
- 45. THANK YOU