3 - Innate immune response 2

Role of complement and activation pathways

• Defense against bacterial infection and removal of immune complexes

• Classical - activated by antibodies binding to antigen, C1 complex binds to immune complex

• Lectin - activated by mannose binding lectin (MBL) or ficolin binding to carbohydrates on bacteria

• Alternative - activated by spontaneous C3 hydrolysis or by direct contact of active complement components with pathogen polysaccharides

• All converge at C3, lead to opsonisation, inflammation and formation of MAC complex

Effector functions of complement

• Lysis by MAC complex (pores) formed by C5b-C9

• Opsonisation by C3b, bind complement receptor 1 on neutrophils and macrophages

• Chemotaxis and activation of neutrophils and monocytes by C5a, also enhances cell adhesionm degranulation and activation of respiratory burst

• Inflammation by C3a and C5a which activate mast cells to release histamins, increase vascular permeability, delivers more immune cells and proteins

• Solubilisation and clearence of immune complexes which can be harmful if deposited in vessel walls as they damage surrounding tissue

• Makes it soluble by destabilising the complex, phagocytic clearence occurs as immune complexes coated with C3b bind to CR1 on red blood cells which are phagocytosed in the liver and spleen

Regulation of complement

• Needed to prevent damage to healthy cells, defects in control molecules more common than pathway proteins

• C1 inhibitor inhibits the association between C1r and C1s with C1q to prevent the formation of the C1 complex

• Deficiency associated with hereditary angiodema

• SERine protease inhibitor inhibits serine proteases like MSP 1 and 2 which drive the lectin pathway

• C4 binding protein accelerates decay of C4b and prevents it from binding to C2, also binds to C3b if present in high concentrations

Control of C3 component (central in all pathways)

• Factors H and I which circulate in plasma, C3b binds factor H and is cleaved by factor I, cannot activate cascade but can still act as opsonin

• Sialic acid found in human tissues also promotes binding of factor H to C3b

• Complement receptor 1 on phagocytic cells and erythrocytes facilitates clearing of immune complexes

• CR2 and 3 found on phagocytes and dendritic cells enable phagocytosis

• CD46 acts as a cofactor for cleavage of C3b by factor I, found on many cells throughout the body

• CD55 - degrades C3 convertase (classical and alternative), found on many cells

Control of final steps in complement cascade (MAC formation)

• C5 convertase bound to C5 will decay if not bound to C6 rapidly

• S protein interacts with C7 and makes it hydrophobic so the complex cannot bind to the cell surface

• Clusterin binds to the C5-9 complex and prevents its activation

• CD59 decreases the binding and prevents polymerisation of C9, found on most cells in the body

Acute phase response

• Initiated by infection, physical injury or malignancy

• Causes changes to vascularity, metabolism, temperature, plasma proteins and antimicrobial activity

• Mediated by phagocytes and the adaptive immune response

• Phagocytes release IL1/6 and TNFa

Acute phase reactants

• Serum amyloid protein - recruitment of immune cells

• CRP - opsonin

• Mannose binding lectin - opsonin

• Fibrinogen - procoagulant

• Ferritin - reduces free iron levels which slows microbial growth

• Complement proteins

• Albumin and transferrin are reduced

Cytokines

• Small soluble proteins produced by cells that alter the behaviour of other cells through cell signalling

• Chemokines promote mobilisation of immune cells from one organ to another

• Interferons (IFN-a), chemokines (IL8), TNF (TNF-a), haematoproteins (IL2/4/6)

Effects and features of cytokines

• Initiate immune response, regulate lymphocyte function and activation, regulate cell movement, influence haematopoietic differenciation

• Have a short duration of action and half life

• Locally active e.g. on cells of origin or local microenvironment, TNF-a can act on distant cells

• Multiple effects - TNF-a neutrophil activation and vascular effects

• Overlapping efefcts e.g. TNF-a and IL-1 are proinflammatory

Defensins

• Secreted by epithelial cells, neutrophils and NK cells, may be spontaneous or following pattern recognition

• Bind to microbial cell membranes and form pore like defects which allows the efflux of essential ions and nutrients

Inflammatory response

• Reaction of living tissue to injury or infection charactarised by heat, redness, swelling and pain

• Mediated by histamine, bradykinin and prostaglandins

• Beneficial effects - removes toxins, limits spread of bacteria, facilitates influx of neutrophils, opsonins, complement, mediators and antibodies, increases nutrient supply for cells, initiates immune response and healing process

• Harmful effects - swelling can cause obstruction and impair bloodflow, can cause tissue damage especially if chronic

Initiation of inflammation

• Physical injury like trauma or burns, infection, allergy or autoimmune disease

• Mast cells and macrophages initiate inflammation

• Whatever the cause is leads to release of proinflammatory mediators like prostaglandins, leukotrines and leakage of cell contents

• Physical trauma and complement activation lead to mast cell degranulation and release of histamine

• Macrophages which ingest pathogens and debris become activated and release proinflammatory cytokes (IL1/6, TNF-a) and chemokines

Consequences of inflammation

• Prostaglandins, histamine and complement cause vasodilation and increased permeability, upregulation of selectins on endothelial cells

• Local clotting stops spread of bacteria

• Neutrophils then macrophages and finally lymphocytes migrate to area by chemotaxis (IL8)

• Innate immune system - phagocytosis, killing by complement, induced apoptosis by NK cells

• Adaptive - antibodies neutralise toxins, opsonisation by antibodies for phagocytosis, apoptosis induced by Tc cells

Outcomes of inflammation

• Acute - resolves when stimulus is removed, no tissue injury, remining leucocytes die by apoptosis and are removed by resident macrophages, fibroblasts repair connective tissue

• Chronic - usually due to persistence of infection or allergy following acute inflammation, ongoing tissue damage at the same time as healing and repair, associated with fibrosis and scarring, can result in loss of organ function

Granulomatous inflammation

• Chronic inflammation associated with intense macrophage activation driven by IFN-y producing T cells

• Macrophages differenciate into epitheloid cells with reduced phagocytic capacity

• These epitheloid cells fuse into multinucleated giant cells which then form granulomas

• Associated with mycobacteria treponema pallidium, fungi and some autoimmune disorders like sarcoidosis