INNATE IMMUNE RESPONSE

characteristics of innate immunity (5)

• primitive (spread across species)

• unlearned, instinctive response

• slow response

• does not depend on immune recognition therefore has no long lasting memory

• integrates w adaptive immune response

what is innate immunity composed of (3)

• physical and chemical barriers

• phagocytic cells: neutrophils, macrophages, dendritic cells

• blood proteins: complement, acute phase

what is innate immunity composed of: physical barriers (3)

• skin

• mucus

• cilia

what is innate immunity composed of: chemical barriers (3)

• lysozyme in tears

• low vaginal pH

• HCl in stomach

general exterior defenses

define inflammation

a series of reactions that brings cells and molecules of the immune system to sites of infection or damage

when does an inflammatory response occur

when physical barriers are breached

process of inflammatory response (7)

• Stop bleeding (coagulation)

• Acute inflammation (leukocyte recruitment - macrophages and dendritic cells live in tissues)

• Kill pathogens, neutralise toxins, limit pathogen spread

• Clear pathogens/dead cells

• Proliferation of cells to repair damage

• Remove blood clot – remodel extracellular matrix

• Re-establish normal structure/function of tissue

how innate immune cells sense foreign molecules/ antigens (4)

• in blood: monocytes and neutrophils

• in tissues: macrophages and dendritic cells (initially recognise non-self)

• Pathogen Associated Molecular Patterns (PAMPs): on microbes i.e. non-self

• Pattern Recognition Receptors (PRRs): on self cells (mainly on APCs + neutrophils + monocytes)

how innate immune cells sense foreign molecules/ antigens: C-Type Lectin receptors Type I

• have several carbohydrate recognition domains (CRD)

• DEC205 (CD205): recognise apoptotic and necrotic human cells (clean up mechanism)

• Macrophage Mannose Receptor (MMR/ CD206): recognises terminal mannose, N-acetylglucosamine and fucose residues on glycans attached to proteins on surface of some microorganisms (particularly fungi and bacteria)

ONLY ON NON-SELF

how innate immune cells sense foreign molecules/ antigens: C-Type Lectin receptors

• expressed by macrophages, monocytes, neutrophils and dendritic cells

• bind to non-self carbohydrates in a Ca2+ dependent manner

• receptors have a carbohydrate recognition domain (CRD)

how innate immune cells sense foreign molecules/ antigens: C-Type Lectin receptors Type II

• only have one carbohydrate recognition domain (CRD)

• Dectin-1: binds beta-glucans that are glucose polymers found in the cell walls of fungi

• Dectin-2: binds mannans (mannose-type carbohydrate) mainly on fungi

• Mincle: binds mycobacteria, fungi and damage associated molecular patterns (DAMPs) released by dying human cells

• DC-SIGN: recognition of several viruses e.g. HIV-1, HCV, dengue virus, Ebola and other microbes of the Leishmania and Candida species

• DNGR-1: binds damaged or dead human cells via exposed actin filaments (DAMP)

how innate immune cells sense foreign molecules/ antigens: C-Type Lectin receptors: Type III

• soluble receptor

• Mannose Binding Lectin (MBL): binds to repetitive mannose and/ or N-acetylglucosamine residues on microorganisms, leading to opsonization and activation of the lectin complement pathway

• MBL has a crucial role in innate immunity against yeast/ fungi by enhanced complement activation and enhanced uptake by polymorphonuclear cells

• MBL also interacts w carbohydrates on surface of HIV-1

how innate immune cells sense foreign molecules/ antigens: Scavenger receptors (SRs)

• mainly recognise lipids/ lipoproteins

• expressed by macrophages, dendritic cells

• superfamily of membrane bound receptors

• bind to variety of ligands incl. host proteins and pathogens, particularly bacterial cell wall components (lipids/ lipoproteins) of Gram -ve and Gram +ve bacteria

• also functions in homeostasis where SR binds and internalises lipid containing molecules e.g. modified LDL and oxLDL (oxidised LDL) from plasma

can lead to atherosclerosis if dysregulated; when oxLDL binds to macrophages it can drive atherosclerosis too

how innate immune cells sense foreign molecules/ antigens: Toll-Like receptors

• recognise a variety of PAMPs expressed by microbes as well as damaged cells

• different TLRs bind to Gram +ve and Gram -ve bacteria

• when TLR binds to PAMPs, it is activated and causes intracellular signalling which triggers an inflammatory response

• proinflammatory cytokines are transcribed (NFκB is a transcription factor), signalling cascade then activates macrophages

• e.g. lipoproteins and peptidoglycans are part of Gram +ve bacterial cell walls which will bind w TLR1 and TLR2

• e.g. TLR4 will bind to LPS (lipopolysaccharides) within Gram -ve cell wall

• ssRNA is from viruses

inflammation: complement (C’)

• complements are a group of ≈ 20 serum proteins that need to be ‘activated’ to be functional and are secreted by the liver

activated by an immune response - normally circulates around body in an inactivated form

• C’ have three different activation pathways

• once C’ is activated, it causes several subsequent events to be continually activated i.e. it causes an immunological cascade

inflammation: complement (C’): activation pathways (3)

• classical - activated when Ab binds to microbe

• alternative - activated when C’ binds to microbe

• lectin - activated when mannose binding lectin (MBL) binds to microbe

MBL is C type Lectin receptor Type III - soluble

C3a and C5a; C3b; C5-C9 (all formed during complement activation)

• C3a and C5a are released

• both are chemoattractants which recruit leukocytes to site of inflammation

• C3b gets inserted into surface of microbe - it sticks out and coats microbe (i.e. opsonisation)

• C3b also activates MAC

• C’ can also kill microbes directly via Membrane Attack Complex (MAC)

when C5-C9 combine they form a pore in surface of pathogen, causing contents to spill out which kills organism in process

C3b leading to microbe destruction

• C3b receptor (CD11c/ CD18) is a Pattern Recognition Receptor (PRR) expressed on surface of macrophages/ neutrophils

• C3b receptor recognises and binds to C3b (opsonin) when it is bound to the surface of microbes

• binding of C3b to C3b receptor causes phagocytosis and destruction of the microbe

summary of C’ functions (3)

• C’ can lyse microbes directly via the Membrane Attack Complex (MAC)

• can cause chemotaxis via C3a and C5a (chemoattractants)

• C3b can coat microbes i.e. opsonisation - helps leukocytes attracted to site of inflammation phagocytose pathogen

process of how immune cells travel from bloodstream to the site of infection (12)

• once tissue macrophage senses microbe, inflammatory response occurs - gaps between endothelial cells increase

• proinflammatory cytokine released e.g. TNF alpha

• TNF alpha interacts w surrounding cells, causing them to produce chemokines

• results in chemokine gradient (high levels at infection site)

• TNF alpha activates endothelium which expresses E-Selectin (sticky factors)

• neutrophils (NP) that normally pass by endothelium interacts w E-Selectin via CD15 on cell surface

• NP starts to become sticky and ‘roll’ on E-Selectin i.e. tethering

• as NP rolls, it encounters chemokines attached to endothelium via GAGs

• chemokines bind to chemokine receptor on NP, this activates the NP

• NP binds to adhesion molecules on endothelial cell surface

• chemokine gradient pulls NP in through gap between endothelial cells, NP migrate up gradient to the site of infection

• NP then uses other receptors on its surface to phagocytose non-self organisms

term for migration of leukocytes to site of infection

diapedesis (type of extravasation)

define extravasation

general term for movement of cells out of a blood vessel into surrounding tissue

haematopoiesis and infection

response to infection:

• increased secretion of colony stimulating factors (CSFs)

• increased release of leukocytes from bone marrow - especially neutrophils

during infection neutrophils increased to 11 × 10³ cells/L (normally 3-7 x 10⁹ cells/L)

• lymphocytes are mainly involved in viral infections

ESR

• increased Erythrocyte Sedimentation Rate (ESR) is a test

• check if infection is present or not

• during infection, fibrinogen levels increased which sticks erythrocytes together - this causes them to settle faster

which cells mainly participate in phagocytosis

• macrophages

• dendritic cells

• neutrophils

phagocytosis and antigen presentation

• phagosome binds to other vacuoles (lysosomes) containing substances » phagolysosome

• inside phagolysosome degrade bacterium

• can reprocess other bacterium material like nucleotides

• Major Histocompatibility Complex (MHC) displays the antigen on outside of immune cell - antigen presentation

histological section of macrophage undergoing phagocytosis

• N = nucleus

• B= bacteria

• ≈ 20-30 bacteria inside macrophage

• macrophages will phagocytose until they cannot anymore, then die via apoptosis

do macrophages discriminate between different foreign material?

no - as long as macrophages recognise a non-self antigen it will phagocytose the material

which cells can present antigens

• macrophages

• dendritic cells

• B cells

NEUTROPHILS DO NOT PRESENT ANTIGENS

what process is the link between innate and adaptive immunity

antigen presentation

mechanisms of microbial killing (2)

2 killing pathways present in neutrophils and macrophages:

• oxygen dependent pathway

• oxygen independent pathway

^ after recognising, sensing, extravasating, phagocytosing microbe

mechanisms of microbial killing: oxygen dependent pathway (2)

Reactive Oxygen Species/ Reactive Oxygen Intermediates

• superoxides e.g. O2- are converted to H2O2 which is then converted to •OH (damages DNA)

• nitric oxide (NO) causes vasodilation, increasing extravasation (NO also antimicrobial)

macrophages and neutrophils + ROS production

macrophages and neutrophils produce free radicals during phagocytosis

mechanisms of microbial killing: oxygen independent pathway (3)

• enzymes: lysozymes are present in tears but also secreted into phagosomes - very good at breaking down bacterial cell walls

• proteins: defensins (inserted into membranes and creates pores), TNF alpha

• pH: massively drops inside vacuoles