Immunology and Serology - 03 Complement System

IMMUNOLOGY AND SEROLOGY THE COMPLEMENT SYSTEM PATHWAYS OF COMPLEMENT ACTIVATION • OUTLINE • • • • • • • • Complement Cascade Activation Pathways of Complement Activation Alternative Pathway Mannose-Binding Lectin Pathway Regulation of the Complement Cascade Complement Deficiency and Associated Diseases Key Facts about the Complement System • • 3 major pathways of complement activation o Classical pathway - activated by Ab bound to Ag o Alternative pathway - activated on microbial cell surfaces in the absence of antibody o Lectin pathway - activated by plasma lectin that binds to mannose residues on microbes Central event in complement activation is proteolysis of C3 to generate biologically active products and subsequent covalent attachment of C3b to microbial cell surfaces or to Ab bound to Ag Four Important Functions COMPLEMENT SYSTEM • Complement system – system of plasma proteins which completes the process initiated by the interaction of antigen and antibody combination. The complement system plays a major part in the inflammatory response directed against foreign antigens. The end product of complement activation is lysis. COMPLEMENT • • • • • • Series of more than 25 serum proteins that interact to enhance host defense reactions. Most are inactive enzyme precursors that are converted to active enzymes in a precise order. It constitute 5% of normal serum protein, heat labile that are present in normal human serum and all tissue fluids except urine and CSF. C3 is the key mediator in all pathways Free floating in the circulation Complement will only be activated once bound w/ antibody Main effects: o Lysis of cells (bacteria, allografts, tumor cells) o Generation of mediators of inflammation (activation of inflammatory response) o Opsonization – enhancement of phagocytosis o Clearance of immune complexes Sequential activation of complement components occurs via one of the three pathways o Classic Pathway o Alternative Pathway o Lectin Pathway Final steps identical in all 3 pathways o Classical – initiated by formation of antigen-antibody complex o Alternative – initiated by foreign cell surface; antibody independent ; part of innate immunity o Lectin – initiated by host proteins binding microbial surface ▪ Presence of mannose in microbial surface • o o o o o o • CASCADE ACTIVATION • • • • • Complement proteins are often designated by an ppercase letter C and are inactive until they are split into products. o Example: C1 When the products are split they become active. The active products are usually designated with a lower case a or b. o Example: C1a and C1b All classes of immunoglobulin do not fix complement only IgM, IgG3, IgG1 and IgG2 fix in the respective order. IgG4, IgA, IgD and IgE do not fix complement. Complement does not bind to free antigen and antibody Overview of the Complement Pathways o Indicating components required for recognition, enzymatically active components, and complexes, major opsonic inflammatory and membranolytic products. • MASP - mannose associated serine protease C3a - anaphylatoxin 3 stages of activation of complement (Classical) Recognition unit - complement recognizes ab-ag rxn Amplification unit - activation unit composed of C4, C2, C3 Membrane attack unit/complex - cause destruction of cell that is composed of C6-C9 o Complement deficiency disease General Functions of Complement Classical Alternative Lectin Immune LPS (bacterial Mannose Complexes capsule) IgA groups on (IgG OR IgM) microbial cell Recognition C1q, C1r,C1s C3, Factor B, MBP, MASPUnit Factor D 1, MASP-2 C3 Convertase C4b2a C3bBb C4b2a C5 Convertase C4b2a3b C3bBb3b C4b2a3b MAC C5b6789 End Result Cell Lysis Activating Subs • • REGULATORS OF THE C3 AND C5 CONVERTASES • ALTERNATIVE PATHWAY • • • • • • • • • Four components: C3, factor B, factor D, properdin o Factor P (properdin) o Factor B - aka C3 proactivator, must be hydrolyzed to Bb & Ba that requires presence of factor D (C3 proactivator convertase) Triggering substances may be pathogens or nonpathogens: o Bacterial cell wall components, fungi, viruses, parasites, immune complexes, RBCs, polymers Activation: proteolysis of C3 C3 tickover C3b, binding site for factor B covalently tethered to surface of microbial or host cell Bound factor B cleaved by factor D (serine preotease) C3 convertase: C3bBb Properdin bind to and stabilize C3bBb complex C5 convertase: C3bBb3b • • • • • • • • • • • • • Lectin is a protein that binds to CHO MBL binds to mannose on many bacterial cells MBL is produced by liver in acute phase inflammatory reactions Once MBL binds to target cell, 2 serine proteases MASP1, MASP-2 bind Acts like C1 MBL, an acute phase protein, binds to mannose residues When MBL complex binds to a pathogen surface, MASP-2 is activated to cleave C4 and C2 The MBL pathway is of importance in innate host defense mechanism in early childhood Activation D-mannose, GlcNAc (microbial polysaccharides) Binding to circulating lectins, eg. Mannose-binding lectin (MBL) or to N-acetylglucosamine recognizing lectins (ficolins) collectin family and structurally resemble C1q MASPs eg. MASP-1, 2, and 3 (tetrameric complex similar to C1r and C1s) MASP-1 cleave C3 MASP-2 cleave C4 and C2 C3 convertase: MASP-1 C5 convertase: C4b2b3b REGULATION OF THE COMPLEMENT CASCADE • Short half-time of o C3b o C3bBb o C5b DAF o glycophosphatidylinositol-linked membrane protein o on endothelial cells and erythrocytes. o If deficiency, Causes paroxysmal nocturnal hemoglobinuria. o unusual feature, acquired mutation in defective gene, inhematopoietic stem cells. factor I o plasma serine protease degrade cell-associated C3b o active only in presence of regulatory proteins o MCP, factor H, C4BP, and CR1 serve as cofactors o cause dissociation of C3b (and C4b)-containing complexes o Generates iC3b, C3d, and C3dg, recognized by receptors on phagocytes and B lymphocytes REGULATION OF FORMATION OF THE MAC • MANNOSE-BINDING LECTIN PATHWAY • • • C1 inhibitor o Inhibits the C1s activity Protein S in Serum o Binds to C5b67 o Inhibits Formation of the Membrane Attack Complex • • • • CD59 o Membrane protein inhibit formation of the MAC o glycophosphatidylinositol-linked protein expressed on many cell types. o incorporate into assembling MACs after the membrane insertion of C5b-8, thereby inhibiting the subsequent addition of C9 molecules o present on normal host cells, where it limits MAC formation, but it is not present on microbes S protein o binding to soluble C5b,6,7 complexes o Prevent insertion into cell membranes near the site where the complement cascade was initiated HRF or CD59 o Bind to C8 o Inhibits C9 binding Factor H o Binds to C3b o Facilitates binding of Factor | ▪ cleaves C3b to inactive iC3b ▪ cleaves C4b to inactive fragments Decay Accelerating Factor COMPLEMENT DEFICIENCY AND ASSOCIATED DISEASES • • • • • • • • • • • • • C1, C2, C3 & C4 disorders - immune and rheumatic C5, C6, C7, C8 disorders- recurrent infection (Neisseria) C1q disorders - combined immunodeficiency states C1r disorder - many infections and lupus like symptoms C1s disorder - Systemic Lupus Erythematosus C4 disorder - Lupus like symptoms C2 disorder - increased susceptibility to infection C3 disorder - sever pyogenic infections C5 disorder - recurrent infection of GIT C6, C7, & C8 disorder - disseminated gonococcal infections and recurrent meningococcal meningitis C9 disorder - not more susceptible to disease than other individual in general populations C1 inhibitors - hereditary angioneurotic edema C3b inactivator factor 1 MUTATION OF THE GENE ENCODING THE MANNOSEBINDING LECTIN • DEFICIENCIES IN COMPLEMENT REGULATORY PROTEIN • GENETIC DEFICIENCIES IN CLASSICAL PATHWAY COMPONENTS • • • • C1q deficiency o Present with early onset SLE , anti-dsDNA less common, more severe disease , less steroid responsive o Strongest known genetic risk factor for lupus C4 deficiency o C4A deficiency , risk factor for SLE (less severity than sufficient complement) early onset SLE , anti-dsDNA less common , o C4B deficiency, with invasive bacterial disease C2 deficiency o most common of the inherited classical complement component deficiencies in Caucasians o most common cause of death: sepsis o most common organisms: S. pneumoniae and H. influenzae C3 deficiency o rarest of the four early component deficiencies o most severe phenotype o Membranoproliferative glomerulonephritis o neutrophil dysfunction (abscesses), humoral deficiencies (sinopulmonary disease), and complement deficiencies (sepsis, meningitis). • • • • Factor B Deficiency o single case has been repoREPORTEDñed ( meningococcemia ) Factor D Deficiency o Neisserial infections are the most common manifestation Properdin Deficiency o X-linked complement deficiency o one of the more common complement deficiencies and occurs largely in Caucasians o one or more episodes of meningococcal disease o high fatality rate in contrast to terminal complement component deficiencies C1 Inhibitor Deficiency o Hereditary angioedema o mildly increased susceptibility to infection increased risk of SLE o Recurrent episodes of angioedema, involvement of airway in absence of anaphylaxis, abdominal episodes, a positive family history, or angioedema arising after trauma o Type I deficiency ,most common (85%) of inherited cases o Type II deficiency normal production ,but dysfunctional protein CD 59 Deficiency and Paroxysmal Nocturnal Hemoglobinuria (PNH) o recurrent episodes of hemoglobinuria due to intravascular hemolysis o Thrombosisoccurs for unknown reasons o aplastic anemia o due to acquired somatic mutations of PIG-A or PIG-M in a clone of bone marrow progenitor cells o These protein products are required for GPI-anchored proteins and C8-binding protein, DAF and CD59 are all GPI-anchored proteins which protect hematopoietic cells from complement-mediated lysis o DAF deficiency does not have a hemolytic phenotype o CD59 is more important KEY FACTS ABOUT THE COMPLEMENT SYSTEM • DEFICIENCIES IN COMPONENTS OF ALTERNATIVE PATHWAY MBL Deficiency therapy o Plasma derived MBL is now being produced by Cooperative Research Centre for Vaccine Technology (CRC-VT) o on phase I safety,tolerability andpharmacokinetics o MBL replacementtherapyused in 3 situations. ▪ where MBL deficiency leads to increased susceptibility of infection, increase resistance to that infection ▪ In acute infection,may enhance the resolution of the disease ▪ Used to alter natural history of chronic disease • • • • • • • • The complement system, a multi-component triggered enzyme cascade, attracts phagocytic cells to microbes which engulf them Complement can be activated by classical and alternative pathways The amount of complement present in the serum cannot be increased by immunization Complement participates in type II and type III hypersensitivity Several serum compliment components are lowered in many autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. They may, be involved in the pathogenesis of auto immune diseases Complement mediates immunological membrane damage C fragments released during cascade reaction help in amplifying the inflammatory response C3 and C4 mediate immune adherence C3 and C6 participate in Coagualtion process