Elements of the Immune System: Complement S. Barbour 03.31.09

Hallmarks of Complement Sequential Activation Amplification Regulation

Overview of Complement Activation (adaptive immunity) Activation (innate immunity) Activation (innate immunity) Classical Pathway Alternative Pathway Lectin Pathway Antibody binds to specific antigen on pathogen surface Mannose-binding protein binds pathogen surface Pathogen surface creates environment conducive to complement activation a series of enzyme (protease) reactions Complement Activation Formation of C3 and C5 convertases Inflammatory response Membrane Attack Pathway Cytolysis of some pathogens effecter function effecter function Activation of B cells Opsonization & phagocytosis of some pathogens Clearance of immune complexes effecter function effecter function effecter function

Complement Nomenclature 1. CX (X = 1-9); example: C5 2. Factor X (alternative pathway); example: Factor B (B) 3. complement fragments (usually a and b); example: C3 → C3a + C3b Note: the “b” fragment remains surface associated; “a” fragment is soluble Note: “i” denotes “inactive” fragments that do not support complement activation. However, these fragments can have other biological functions. (eg. iC3b or C3bi)

Complement Activation

Activation of the Classical Pathway IgG (IgG1, IgG2, IgG3) IgM C3 C3 convertase C3 C3 convertase Y Y Y Y C3b Y Y C1 C3b C1 C3b Y C3b Antigen (bacterial or yeast cell surface) covalent bond with surface covalent bond with surface -Note: only antigen-antibodies complexes can activate the classical pathway! Antibody alone cannot -Classical pathway results in formation of a C3 convertase that generates C3b -C3b forms a covalent bond with the surface of the pathogen and is part of C5 convertase

Activation of the Alternative Pathway (Sources of C3b) Soluble C3 convertase C3 C3 tickover C3b C3 C3 convertase C3b C3b C3b Y Y C1 covalent bond with surface covalent bond with surface “activating surface” (bacterial or yeast cell surface) -the alternative pathway is initiated by C3b binding to a bacterial or yeast cell surface -C3b preferentially interacts with bacterial or yeast cells; host cells are spared -this is a primitive distinction of self versus non-self

(bacterial or yeast cell surface) Activation of the Alternative Pathway (C3 Convertase) Soluble C3 convertase C3 C3 tickover C3b C3b C3b C3 C3 convertase C3 convertase covalent bond with surface C3b “activating surface” (bacterial or yeast cell surface) covalent bond with surface -alternative pathway results in the formation of a C3 convertase that generates C3b -C3b forms a covalent bond with the surface of the pathogen and is part of C5 convertase

Activation of the Lectin Pathway C3b mannan binding lectin (MBL) C3 C3 convertase C3b mannose sugars MBL covalent bond with surface “activating surface” (bacterial or yeast cell surface) -mannan binding lectin (MBL) recognizes mannose sugars on microbial cells -host mannose is hidden and is not accessible to MBL -lectin pathway results in the formation of a C3 convertase that generates C3b -C3b forms a covalent bond with the surface of the pathogen and is part of C5 convertase

Regulation of Complement

Importance of C3 Activation of Classical, Lectin, and Alternative pathways (alternative pathway is constantly activated in serum!) Fragments have biological activity Opsonization / Phagocytosis B cell activation Inflammation Most abundant complement protein in serum ( > 1 mg/ml)

P C3 Convertases C3 C3a + C3b Classical / Lectin Pathway Alternative Pathway receptor subunit receptor subunit C2b C4b Bb C3b catalytic subunit catalytic subunit P covalent association with pathogen surface Covalent association ensures that C3 convertases remain localized to the pathogens that activate complement! Regulation: Dissociation of the subunits or proteolysis of receptor subunit by Factor I

Regulation of C3 Convertases Dissociation Proteolysis (Factor I cofactor) Protein Classical/Lectin Alternative C4b C3b Factor H - + C4bp DAF MCP CR1 In general, regulatory proteins are expressed on host cells, but not on pathogens. Therefore, host cells are spared from complement attack.

Effecter Functions of Complement

The Membrane Attack Pathway MAC The MAC is especially important for the immune response against Neisseria spp. Membrane proteins (CD59, HRF) prevent MAC formation on host cells.

Opsonization / Phagocytosis C3b or iC3b opsonized bacteria CR1, CR3, or CR4

Inflammation anaphylotoxins C3a, C4a---increased vascular permeability C5a—chemoattraction C3a, C4a----activation

Clearance of Immune Complexes CR1

Activation of B Cells or C3dg -microbial cell expresses antigens recognized by B cell receptor -microbial cell is coated with C3d or C3dg, recognized by CR2 -simultaneous binding to B cell receptor and CR2 results in more efficient B cell activation

Complement: Disease Correlations

Disease Correlations -deficiency in classical pathway components is associated with accumulation of immune complexes, autoimmune diseases. -deficiency in terminal components (components of MAC) results in increased susceptibility to Neisseria infections.

Disease Correlations -C1 inhibitor (C1INH) blocks the activation of C1, the complement component that interacts with antibody in the classical pathway. Deficiency results in uncontrolled production of anaphylotoxins -DAF, HRF, and CD59 are lipid (GPI) linked proteins. Deficiency in production of lipid anchors results in loss of these regulatory proteins, complement activation on host cells

Deficiencies in Complement Receptors CR3, CR4 CR3, CR4 are integrins Loss of these proteins compromises functions of phagocytes (LAD) Result is increased susceptibility to infection CR1 Most CR1 is expressed by erythrocytes Loss of this protein is associated with accumulation of immune complexes (similar to deficiencies in classical pathway components)

Overview of Complement Activation (adaptive immunity) Activation (innate immunity) Activation (innate immunity) Classical Pathway Alternative Pathway Lectin Pathway Antibody binds to specific antigen on pathogen surface Mannose-binding protein binds pathogen surface Pathogen surface creates environment conducive to complement activation a series of enzyme (protease) reactions Complement Activation Formation of C3 and C5 convertases Inflammatory response Membrane Attack Pathway Cytolysis of some pathogens effecter function effecter function Activation of B cells Opsonization & phagocytosis of some pathogens Clearance of immune complexes effecter function effecter function effecter function