1. Complement and Cytokines Department of Pathology
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. Effector Mechanisms Against Extracellular Pathogens
OPSONISATION
Bacteria in extracellular space Ab +
Antibody receptor
binding
Phagocytosis
OPSONISATION

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

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 The Lectin Pathway Alternative 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. Generation of C5 convertase leads to the activation of the
Lytic pathway
Generation of C5 convertase leads to the activation of the
Lytic pathway

11. Components of the lytic pathway9

12. Lytic pathway C5-activationb
C5a C5
C3b
C4b C2 a

13. Lytic pathway assembly of the lytic complex

14. Lytic pathway: insertion of lytic complex into cell membrane9 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: Host detriment:
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 duration of 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)

28. Cytokine Actions IL-1 Activates T cells to produce IL-2 IL-2
Stimulates both helper and cytotoxic T cells
IL-4 and IL-5
They promote growth and differentiation of B cells respectively
IL-6
Stimulates B cell differentiation, induces fever
IL-8
Attracts neutrophils
IL-10
Inhibits the development of Th-1 by decreasing production of IF
IL-12
Promotes the development of Th-1 cells
IL-13
Mediates allergic inflammation in asthma
Transforming Growth Fctor beta (TGF-)
Anti-cytokine – inhibits growth and activities of T cells
Promotes synthesis of collagen (wound healing)
Chemokines
Attract neutrophils and macrophages
Tumor Necrosis Factor (TNF)
Promotes neutrophil phagocytosis and killing, mediates extravascular migration of inflammatory cells
Interferones (INF)
Block viral replication, Class switching of IgGs

29. Cytokines and Disease Disease Cytokines Bacterial Septic Shock
High TNF serum levels
Lymphoid and Myeloid Cancers
High levels of IL-6
T cell leukemia is associated with HTLV-1 retrovirus
Low levels of IL-2

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