Immunology UnitDepartment of Pathology

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

Main Functions of Complement

• Lysis of cell – Bacteria

• Generation of mediators– Participation in inflammation and attract

neutrophils

• Opsonization– Enhancement of phagocytosis

Antibody receptorbinding

Effector Mechanisms Against Extracellular Pathogens

OPSONISATION

OPSONISATION Phagocytosis

Bacteria in extracellular space

Ab

+

Effector Mechanisms Against Extracellular Pathogens

COMPLEMENT Activation

Bacteria in plasma

Ab & COMPLEMENT

+

Phagocytosis

binding

Complement &Fc receptor

Lysis

Opsonisation

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

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

• 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

Complement Activation Cascade

Generation of C5 convertase leads to the activation of the

Lytic pathway

Lytic pathway

Components of the lytic pathway

C6

C9

C8

C7C5

Lytic pathwayC5-activation

C3b C2 aC4b

C5 b

C5a

Lytic pathwayassembly of the lytic complex

C5 b

C6

C7

Lytic pathway:insertion of lytic complex into cell membrane

C5 b

C6

C7C8

C9

C9

C9

C9C9

C9 C

9C9

C9

C9 complex Complement-induced lesions on themembrane of a red blood cell

Kuby J et al., Immunology 2003

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)

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

Hereditary Angioedema

Immune complex disease

Recurrent bacterial infections

Recurrent Neisserial Infections

Clinical Syndromes Associated with Deficiencies of Complement Components

C1 Inhibitor

Cytokines

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)

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

Pleiotropic

Redundant

Synergistic

Antagonistic-

Cytokines : Mode of Action

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

Autocrine

Paracrine

Endocrine

General Properties of Cytokines Mode of Action

Close proximity

Distant cells

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- )

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.

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)

Cytokine ActionsIL-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 cellsPromotes 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

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

Cytokines and Disease

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

Cytokine Network

• Targets

Thank you