Immunology Unit Department of Pathology

1. Complement and Cytokines Immunology Unit Department of Pathology

2. Complement • The complement system consists of about 20 proteins in normal human serum • They are synthesized mainly by the liver • Complement is heat labile i.e. it is inactivated at 56oC for 30 minutes • Immunoglobulins are not inactivated at this temperature

3. Main Functions of Complement • Lysis of cell • Bacteria • Generation of mediators • Participation in inflammation and attract neutrophils • Opsonization • Enhancement of phagocytosis

4. Bacteria in extracellular space + Ab Antibody receptor binding Phagocytosis Effector Mechanisms Against Extracellular Pathogens OPSONISATION OPSONISATION

5. Bacteria in plasma + Ab & COMPLEMENT Opsonisation Complement & Fc receptor Phagocytosis binding Effector Mechanisms Against Extracellular Pathogens COMPLEMENT Activation Lysis

6. Complement Activation • Several complement components are pro-enzymes and required cleavage to form active enzymes • Activation of complement is initiated either by antigen-antibody complexes or by non-immunologic molecules such as endotoxin

7. Complement Activation • Pathways of activation • Classic Pathway • The Lectin Pathway • The Alternative Pathway • Lectin and alternative pathways are activated with the first encounter with bacteria since the antibody required to trigger classic pathway is not present

8. Classic Pathway • Antigen-antibody complexes activate C1 to form protease which cleaves C2 and C4 to form C4b,C2b complex • The Lectin Pathway • Mannan-binding lectin (MBL) binds mannan present on the surface of bacteria and cleaves C2 and C4 to activate classic pathway without the help of an antibody • Alternative Pathway • Cell surface substances such as bacterial lipoploysaccharides (endotoxin), fungal cell walls and viral envelopes can activate complement system directly by cleaving C3 into C3a and C3b

9. Complement Activation Cascade

10. Lytic pathway Generation of C5 convertase leads to the activation of the Lytic pathway

11. C8 C7 Components of the lytic pathway C6 C5 C 9

12. C5a b C3b C4b C2 a Lytic pathwayC5-activation C5

13. C7 C5 b Lytic pathwayassembly of the lytic complex C6

14. C8 C6 C7 C5 b Lytic pathway:insertion of lytic complex into cell membrane C 9 C 9 C 9 C 9 C 9 C 9 C 9 C 9 C 9

15. C9 complex Complement-induced lesions on the membrane of a red blood cell Kuby J et al., Immunology 2003

16. Complement functions • Host benefit: • Opsonization to enhance phagocytosis (C3b) • Phagocyte attraction and activation (C5a and C5,6,7) • Lysis of bacteria and infected cells • Regulation of antibody responses • Clearance of immune complexes • Clearance of apoptotic cells • Host detriment: • Inflammation • Anaphylaxis – mast cell degranulation (C3a, C4a, C5a)

17. Regulation of Complement System • Antigen-antibody reaction is necessary for complement activation. • C1 inhibitor • Human cells are protected from lysis by membrane attack complex by “decay accelerating factor” (DAF) • DAF de-stabilizes C3 and C5 convertase to prevent the formation of membrane attack complex

18. Clinical Syndromes Associated with Deficiencies of Complement Components Hereditary Angioedema C1 Inhibitor Immune complex disease Recurrent bacterial infections Recurrent Neisserial Infections

19. Cytokines

20. Cytokines • Facts • They are low molecular weight proteins • They are involved in immunity and inflammation where they regulate the amplitude and durationof inflammation • They are extremely potent • They are produced transiently (short duration of action)

21. Cytokines • They act with cell surface receptors specific for each cytokine group • Individual cytokines have multiple overlapping cell regulatory actions and interact in the form of a network • Synergistic and antagonistic actions

22. Cytokines : Mode of Action Pleiotropic Redundant Synergistic Antagonistic -

23. General Properties of Cytokines • Cytokines induce their effects in three ways • Autocrine effect: ie, they act on the same cell that produces the cytokine eg, IL-2 • Paracrine effect: that effect other cells in the vicinity eg, IL-7 in the bone marrow act on B cells progenitors • Endocrine effect: they affect many cells systemically eg, IL-1 and TNF- which produce acute-phase response during inflammation

24. General Properties of Cytokines Mode of Action Autocrine Paracrine Endocrine Close proximity Distant cells

25. Working Classification of Cytokines • Cytokines that mediate natural immunity • Interleukin-1 (IL-1), tumor necrosis factor alpha (TNF), interferons and IL-6 • Cytokines that regulate lymphocyte growth, activation and differentiation • IL-2, IL-4, IL-5, IL-12, IL-15 and transforming growth factor- (TGF- )

26. Working Classification of Cytokines • Cytokines that activate inflammatory cells • IFN-, TNF, lymphotoxin (TNF-) and migratory inhibitory factor • Cytokines that affect leukocyte movements also called “chemokines” • IL-8, Macrophage Inflammatory Protein (MIP), Macrophage Chemotactic Protein (MCP) etc.

27. Working Classification of Cytokines • Cytokines that stimulate hematopoiesis • Stimulate the production of blood cells by acting on hematopoietic progenitor cells. • The members of this family are called “colony-stimulating factors” (CSFs) eg, granulocyte-monocyte colony stimulating factor (GM-CSF), granulocyte-colony stimulating factor (G-CSF)

29. Cytokines and Disease

30. Cytokine Related Therapies • Soluble form of IL-1 receptor inhibits Th cell activation – prolongs graft survival in heart transplantation • IL-2 conjugated with toxin diminishes rejection of kidney and heart transplants • Lymphokine activated killer cells in tumor therapy • Antibody to IL-4 reduces IgE production

31. Cytokine Network • Targets

32. Thank you