Elements of the Immune System: Complement

1. Elements of the Immune System: Complement S. Barbour 03.31.09

2. Hallmarks of Complement • Sequential Activation • Amplification • Regulation

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

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

5. Complement Activation

6. C3 convertase C3 convertase C1 C1 C3b C3b C3 C3 C3b C3b Activation of the Classical Pathway IgG (IgG1, IgG2, IgG3) IgM Y Y Y Y Y Y Y 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

7. C3 convertase Soluble C3 convertase C1 C3b C3b C3 C3 C3b C3b Activation of the Alternative Pathway(Sources of C3b) C3 tickover Y Y 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

8. C3 convertase C3 convertase Soluble C3 convertase C3b C3b C3 C3 C3b C3b Activation of the Alternative Pathway(C3 Convertase) C3 tickover covalent bond with surface “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

9. C3 convertase C3b C3 C3b Activation of the Lectin Pathway mannan binding lectin (MBL) 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

10. Regulation of Complement

11. 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)

12. C3 C3a + C3b C2b C4b Bb C3b C3 Convertases Classical / Lectin Pathway Alternative Pathway receptor subunit receptor subunit 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

13. Regulation of C3 Convertases Dissociation Proteolysis (Factor I cofactor) In general, regulatory proteins are expressed on host cells, but not on pathogens. Therefore, host cells are spared from complement attack.

14. Effecter Functions of Complement

15. 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.

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

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

18. Clearance of Immune Complexes CR1

19. 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

20. Complement:Disease Correlations

21. 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.

22. 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

23. 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)

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