01242013 - 1 - Innate Response

Front 1

Steps in innate immune response

Back 1

- Recognition
- Containment
- Amplification
- Elimination
- Control

Front 2

Macrophage recognition of bacterial infection

Back 2

- Macrophages (effector cells) - normally hang out in CT
- Have various phagocytic receptors
1) Dectin-1 = glucans-fungal walls
2) Mannose receptor = thought to recognize sugar residues on pathogens
- Actually kind of 2nd receptor - signalled through after macrophage activation
- Enhances macrophage activity - engulfing bacteria, sending out cytokine signals
3) Scavenger/lipid receptors = recognize bacterial cell walls
- Good at sensing lipids in general - take up cholesterol -> foam cells -> atherosclerosis
4) Complement system = CR3 recognizes "marked" bacteria
5) Toll-like receptors = Most important!
- At least 11 types - capable of recognizing virtually any pathogen
- Receptors are on surface AND internally (for viruses, intracellular bacteria, etc.)
- Initiate release of cytokines

Front 3

TLR signalling mechanism

Back 3

- Bacterial lipopolysaccharides (LPS), peptidoglycan, etc. = bind to surface receptor
- TLR forms complex with LPS-bound receptor - TLR has transmembrane domain
- Inside cell - Myd88 associates with intracellular domain of TLR
- TLR interacts with IRAK4, others -> Activation of IκB kinase (IKK)
- IKK phosphorylates IκB-NFκB complex - degradation of IκB
- NFκB -> nucleus -> activates transcription of cytokines
- Cytokines translated in cytoplasm -> packaged/secreted via ER
*** NFκB = huge drug target -> shut down autoimmune/excessive inflammatory responses

Front 4

Inflammatory cytokines of macrophages

Back 4

- CXCL8 = recruits neutrophils & basophils to infection
- IL-12 - activates NK cells (good for viral infected cells)
- TNF-α = activates endothelium - more permeable -> increased entry of complement & cells to CT
- Also increases drainage to lymph vessels
- Fever
- IL-1 = Activates vascular endothelium and production of IL-6
- Fever
- IL-6 = Induces Fever

Front 5

TNF-α details

Back 5

- Important for inflammatory response BUT very damaging to tissues!
- Increases vascular permeability
- Increased rolling adhesion = integrin expression, diapedesis
- Activates vascular endothelial cells - release of platelet-activating factor (PAF)
- Localized clots form - prevent spread of infection

Front 6

Septic shock

Back 6

Spread of infection from CT -> blood
- Activation of macrophages -> TNF-α activity all over body!
- Leaky capillaries, clots forming = bad news...
- Disseminated intravascular coagulation (DIC) - systemic clotting factor depleted
- Leaky vessels cause drop in BP -> lack of ciruclation to capillaries
- Vital organ failure if progresses too far!

Front 7

Acute Phase Response

Back 7

- Inflammatory cytokines (IL-1, IL-6, TNF-α) have systemic effects
- Pyrogens - increase body temperature
- IL-1 -> hypothalamus
- Others -> mm, fat
- IL-6 = travels to liver - activates transcription of acute phase proteins
- Hours after initial recognition

Front 8

Acute phase response proteins

Back 8

Produced in liver in response to IL-6 - Most important 3 are:
- Mannose-binding lectin = binds carbos on pathogen
- Activates lectin pathway of complement system
- C-reactive protein = Binds bacterial cell walls (specifically Phosphocholine of LPS)
- Activates classical complement system
- Fibrinogen - allows clotting - containment of local infection

Front 9

Fever purpose

Back 9

Bacteria typically prefer ~35 degrees
- Increased temps slow down reproduction mechanisms

Front 10

Neutrophil reserves

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- Cytokines induce release of neutrophils/band cells (almost mature forms) from marrow
- Follow cytokine gradient to infection - enter CT

Front 11

Neutrophil overview

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Usually 40-60% of total WBC's
- Normally excluded from tissues - only enter when cytokines signal endothelium
- First wave of innate defense - Very short lives
- Have integrins on surface -> recognize endothelial selectins -> pass through activated tissue into CT

Front 12

Extravasation mechanism

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1) Rolling adhesion
- Sialyl-Lewis Carbos on neutrophil interact with P- and E- selectins of endothelial cells
- P and E not normally present at surface
- P already made, migrates to surface in response to cytokines
- E expression seen hours after TNF-α exposure
- Interaction causes rolling to slow a bit
2) Tight binding
- Integrins LFA-1 and CR3 interact with ICAM-1 and -2
- ICAM-1 induced by TNF
- LFA and CR3 conformations modded by CXCL8
- Neutrophil comes to stop
3) Diapedesis - Move through BM!
- Neutrophil receptor recognizes CD31 at endothelial cell connections
4) Migration - to site of infection - following CXCL8 gradient

Front 13

Neutrophil phagocytosis

Back 13

- Use many of same receptors as macrophages (same lineage)
- Carbo receptors, complement receptors, TLR's
1) Bacteria phagocytosed
2) Phagosome fuses with specific and azurophilic granules
3) pH rises - antimicrobial response kills bacteria
4) Phagosome fuses with lysosomes - pH drops - completely degrades bacteria
5) Neutrophil dies via apoptosis -> phagocytized by macrophage

Front 14

Killing of bacteria

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- Lysozyme, lactoferrin, proteases chew away at bacterial wall, etc.
- ROS do most of the dirty work
- NADPH oxidase - built into wall of phagosome
- Converts O2 -> O2-
- Superoxide dismutase - converts O2- -> H2O2
- Fe, other enzymes make hydroxide, chlorites, etc.