Midterm - Complement System PDF

Complement System (Turgeon, Chapter 5)  Learning Objectives Define the complement system Differentiate and classify the complement system pathways Identify the activators and regulators of each complement system pathway Describe the functions and effects of the complement syste...

Complement System (Turgeon, Chapter 5)  Learning Objectives Define the complement system Differentiate and classify the complement system pathways Identify the activators and regulators of each complement system pathway Describe the functions and effects of the complement system to immune response Determine the diseases caused by complement component deficiencies  The Complement System A defensive system consisting of over 30 proteins (activators and regulators) produced by the liver and found in circulating blood serum. Discovered by Jules Bordet in 1896 Inactivated in the lab by heating serum at 56 degrees Celsius for 30 minutes  The Complement System Beta-1 and Beta 2 globulin – fractions of serum proteins that can be separated by electrophoresis. – These globulins include important proteins such as: Complement proteins (especially C3 and C4). Named with a capital “C”, followed by a number (e.g: C3) Then a small letter after the number indicates the size of the protein once it is ACTIVATED → cleaved OR SPLIT INTO FRAGMENTS (e.g: C3a, C3b) – If: a – smAller, Away b – Bigger, Bind Nomenclature is based on order of discovery rather than order of activation sequence (C1, C4, C2, C3)  1. The Classical Pathway The classical pathway is considered to be part of the specific/adaptive immune response because it relies on antibodies to initiate it. Other activators: 1.Antigen - antibody complex 2.Apoptotic cells 3.Certain viruses / gram negative bacteria 4.CRP bound to ligand C1 becomes activated when it binds to the ends of antibodies (at least 2 CH2) More effective complement fixation: IgM > IgG  Recognition Unit C1 complex is the recognition unit of the Classical Pathway C1 recognizes at least 2 CH2 domains C1 has 3 subunits: C1q, C1r, C1s C1q recognizes 2 CH2, activates C1r which will in turn activates C1s Ch. 7  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  C3 Activation Complex C2b and C4b bind together on the surface to form a C3 activation complex/C3 convertase The function of the C3 (largest amt in plasma) activation complex is to activate C3 proteins. – This is done by cleaving C3 into C3a and C3b It is the pivotal point or heart of the cascade wherein all 3 pathways converge or meet. They may have different activators but they all merge into cleaving C3 to C3a and C3b, leading to the formation of the C5 convertase  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 Opsonins  C3a/C4a/C5a C5a also responds to inflammation by C5a disperses away from the bacteria. triggering phagocytes -Binds to mast cells and increases inflammation. -Most powerful chemotactic factor known for leukocytes C3a increases the inflammatory response by binding to mast cells and causing them to release histamine  The C5 Activation Complex The C5 activation complex (C4b2b3b) activates C5 proteins by cleaving them into C5a and C5b. Many C5b proteins are produced by the C5 activation complex. These C5b will then begin to coat the surface of the bacteria.  Building the Membrane Attack Complex (MAC) 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 (anchors in cell surface), which in turn binds to many C9s Together these proteins form a circular complex called the Membrane attack complex (MAC)  Membrane Attack Complex The MAC causes cytolysis The circular membrane attack complex acts as a channel in which cytoplasm can rush out of and Na+ ions plus H20 rushes in The cells inner integrity is compromised and it dies Overview  2. The Alternative Pathway The alternative pathway is part of the non- specific / innate defense because it does not need antibodies to initiate the pathway. Activated by fungal cell wall (zymosan), snake venom, lipopolysaccharide, bacterial polysaccharides, and tumor cells Activation: Activated spontaneously on microbial surfaces without the need for antibodies  Factor B C3b on the surface of a foreign cells binds to another plasma protein called factor B C3b + Factor B = C3bBb The unique things about the alternative pathway is that it bypasses C1, C4, and C2 Factor B structurally resembles C2 in the classical pathway  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.  The C3 activation complex Properdin, also called factor P, binds to the C3bBb complex to stabilize it. C3bBbP makes up the C3 activation complex for the alternative pathway  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  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. The Alternative Complement Pathway  Classic and Alternative Pathways Classic Pathway Alternative Pathway * Specific acquired immunity * Non-specific innate immunity * Initiated by antibody * Bacterial endotoxin, capsule * Interaction of all components * C1, C4, C2 are by-passed * Properdin system not involved * Properdin system is involved  3. Mannose Binding Lectin (MBL) Pathway Lectin – protein present in human host Mannose – sugar not found in humans ; only in microbes Part of the nonspecific, innate immunity MBL is homologous to C1q in structure Mannose binding lectin-associated serine protease (MASP) ; MASP1 and MASP2 Key Activators: MBL binds to mannose residues on microbial surfaces, initiating the pathway through activation of MASP-1 and MASP-2 (MBL-associated serine proteases). C4b2b C4b2b3b  Biological Functions of Complement System 1. Lysis of target cell = MAC 2. Opsonin = C3b 3. Anaphylatoxins = C3a, C4a, C5a 4. Increases vascular permeability = C2a 5. Chemotaxin = C5a (recruits neutrophil) * C5a both anaphylatoxin and chemotaxin  Complement System Regulators 1. C1 Inhibitor (C1 INH) – prevents spontaneous activation classical pathway 2. C4b-Binding Protein (C4b-BP) – prevents C4b and C2b binding 3. Decay Accelerating Factor (DAF) – dissociates C3 convertases of classical, lectin, & alternative 4. Factor I – cleaves C4b into smaller C4c and C4d fragments 5. CD59 – prevents association of C5b678 to C9 6. Factor H – prevents binding of C3b to Factor B, cleaves C3b into C3c and C3d  Diseases / Disorders 1. C1 Inhibitor deficiency – Hereditary Angioedema 2. DAF deficiency – Paroxysmal Nocturnal Hemoglobinuria Night time acidity in blood increases which activates complement but without DAF, tendency for red cell lysis to occur and seen as elevated hemoglobin levels in the urine 3. MAC deficiency – recurrent infections (particularly, Neisseria meningitidis)  Total Complement Activity Total complement activity (CH50, CH100) looks at the integrity of the classical complement pathway (screening test) Based on the capacity of a serum to lyse sheep RBCs coated with anti-sheep Abs – CH50 - one CH50 unit is defined as the volume or dilution of serum that lyses 50% of erythrocytes in the reaction mixture. – AH50 - an analogous test to measure alternate-pathway function. This test is available only in specialized laboratories.  Quiz 1 Next Meeting ☺