PRINCIPLES OF INNATE IMMUNITY

THE INNATE IMMUNE SYSTEM First line of defense against pathogens Components Complement system Macrophages and neutrophils Defensins Coagulation system Cytokines and inflammatory cytokines Inflammatory response Natural killer cells

THE COMPLEMENT SYSTEM A set of proteins widely distributed throughout body fluids and tissues Proteins act in a cascade of reactions to attack extracellular forms of pathogens Complement activation results in Inflammatory response Pathogens coated with complement Complement coating of pathogens Enhanced engulfment and destruction by phagocytes Direct killing of pathogens 3

PATHWAYS OF COMPLEMENT ACTIVATION Classic pathway Activated by antibody First discovered Alternative pathway Activated by some bacterial cell surfaces Antibody not involved Lectin pathway Activated by mannose binding lectin

THE COMPLEMENT SYSTEM Nomenclature has developed haphazardly Proteins of classic pathway named with capital “C” followed by a numeral (C1, C2, C3…..C9) Cleavage fragments named as parent followed by lower case letter “a” for smaller fragment (C3a) “b” for larger fragment (C3b) Some classic components participate in other 2 pathways

CLASSIC PATHWAY OF COMPLEMENT ACTIVATION C1 binds to Fc region of antibody part of Ab/Ag complex C1 is complex of 3 proteins C1q is binding protein C1r and C1s are proteases C1q binds to Fc region of antibody which activates C1r which activates C1s Most efficient at activating complement IgM, IgG1 and IgG3

CLASSIC PATHWAY OF COMPLEMENT ACTIVATION Activated C1s cleaves C4 to C4a and C4b Activated C1s cleaves C2 to C2a and C2b C4b and C2b form complex covalently bonded to pathogen surface C4b/C2b complex (C3 convertase) cleaves C3 to C3a and C3b

ANTIBODY AND COMPLEMENT ENHANCE PHAGOCYTOSIS Enhanced phagocytosis especially important Streptococcus pneumoniae Haemophilus influenzae Cryptococcus neoformans Macrophages and neutrophils have receptors for Antibody Fc-gamma for Fc region Complement Complement receptor 1 (CR1) for C3b

COMPLEMENT RECEPTORS REMOVE IMMUNE COMPLEXES Soluble antibody/antigen complexes Form after immune response to most infections IC must be removed to prevent precipitation and deposition on endothelial membranes Kidneys Removal of IC Complement binds to IC Erythrocytes bind to complement by CR1

DIRECT KILLING OF PATHOGENS BY COMPLEMENT SYSTEM Terminal complement proteins form “membrane attack complex” Mechanism of attack by classic pathway C3b binds to C3 convertase (C4b,2b) / (C4b,2a) results in C5 convertase (C4b,2b,3b) / (C4b,2a, 3b) C5 binds C3b of C5 convertase C5 cleaved to C5a and C5b C5b initiates assembly of attack membrane components C6 – C9 Deficiency increases susceptibility to Neisseria meningitidis and Neisseria gonorrhoeae

RECOGNITION OF PATHOGENS FOR PHAGOCYTOSIS Mechanism of recognition Toll-like receptors (innate immune receptors) Toll-like receptors Named for ‘Toll’  receptor in fruitfly Polypeptides with horseshoe-shaped structure Recognition by macrophages initiates activation Phagocytosis Secretion of cytokines 24

ACTIVATION OF MACROPHAGES Activated macrophages secret Cytokines Chemokines (chemoattractant cytokines) Inflammatory mediators Cytokines and chemokines Interleukin-1 (IL-1), IL-6, IL-8, IL-12 and TNF-alpha Prostaglandins, leukotrienes, plasminogen activator, platelet-activating factor (PAF) 29

Figure 8-15 30

MIGRATION OF NEUTROPHILS INTO TISSUE (EXTRAVASATION) Rolling adhesion Slowing down leukocytes (margination) Weibel-Palade bodies in vascular endothelial cells secreting P and E selectins Tight binding Interaction between LFA-1 and ICAM-1 Diapedesis Passage between vascular endothelial cells Migration to infection site 31

Figure 8-19 32

Chemokines (Chemoattractant Cytokines) Family of small soluble molecules that stimulate activation and migration of cells Group classification CC Two adjacent cysteine amino acids Chromosome 4 CXC Two separated cysteine amino acids Chromosome 17 33

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BIOLOGICAL ACTIVITY OF IL-1, IL-6 AND TNF-ALPHA Induce hepatocytes to produce acute-phase proteins C-reactive protein (CRP) Mannose binding lectin (MBL) Induce bone marrow to release neutrophils Induce hypothalamus to raise temperature Induce fat and muscle cells to generate heat 37

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DEFENSINS Family of amphipathic antimicrobial peptides 35 to 40 amino acids Mechanism of action Disruption of cell membranes Classification Alpha Neutrophils and Paneth cells Beta Epithelial cells of skin, respiratory tract and UG tract 40

THE INNATE RESPONSE TO VIRAL PATHOGENS Virus infected healthy cells produce Interferon-alpha (IFN-alpha) Interferon-beta (IFN-beta) IFN-alpha and IFN-beta are type 1 interferons Type 1 interferons Inhibit virus replication Activate natural killer (NK) cells Increases expression of MHC-1 molecules 43

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Figure 8-25 45

NATURAL KILLER (NK) CELLS Large granular lymphocytes that circulate in blood Functions Killing infected cells (cytotoxic) Secretion of cytokines Activation by Type 1 interferons Infected cells Stimulates cytotoxic function IL-12 and TNF-alpha Macrophages Stimulates cytokine secretion 46

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NATURAL KILLER (NK) CELLS Activated NK cells release IFN-gamma which activates Macrophages Release IL-12 Positive feedback system for NK and macrophages Differentiate infected from uninfected cells NK cells express receptors for MHC class I molecules Binding of NK cells to MHC class I molecules turn off NK cells NK cells provide innate immunity to intracellular pathogens 48