Immunology Unit

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Complement and Cytokines
Immunology Unit Department of Pathology
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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

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Main Functions of Complement

• Lysis of cell
• Bacteria
• Generation of mediators
• Participation in inflammation and attract
  neutrophils
• Opsonization
• Enhancement of phagocytosis

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Effector Mechanisms Against Extracellular
Pathogens OPSONISATION
OPSONISATION
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Effector Mechanisms Against Extracellular
Pathogens COMPLEMENT Activation
Lysis
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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

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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

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• 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

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Complement Activation Cascade
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Lytic pathway
Generation of C5 convertase leads to the
activation of the Lytic pathway
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Components of the lytic pathway
C6
C5
C 9
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Lytic pathwayC5-activation
C5
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Lytic pathwayassembly of the lytic complex
C6
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Lytic pathwayinsertion of lytic complex into
cell membrane
C 9
C 9
C 9
C 9
C 9
C 9
C 9
C 9
C 9
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C9 complex
Complement-induced lesions on the membrane of a
red blood cell
Kuby J et al., Immunology 2003
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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)

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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

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

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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

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Cytokines Mode of Action
Pleiotropic
Redundant
Synergistic
Antagonistic
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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

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General Properties of Cytokines Mode of Action
Autocrine Paracrine Endocrine
Close proximity
Distant cells
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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- ?)

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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.

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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)

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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
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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
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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

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Cytokine Network

• Targets

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