Innate Immunity

Question 1

Innate Immunity

Answer 1

• ability of the organism to detect and defend against invading microbes or other foreign life-forms to which it hasn’t been exposed
• found in all domains of life and conserved

Question 2

Detection and effectors

Answer 2

• the immune system needs to be able to detect an infection and its general type as well as trigger effector mechanisms killing that class of infecting organism
• inappropriate effectors do not clear the infection and can damage the host
• adaptive immunity relies on innate detection and effectors: innate immunity instructs adaptive responses and effectors

Question 3

Responses to Infection

Answer 3

1. barriers can prevent infection
2. recognition by preformed nonspecific and broadly specific effectors (complement)
3. recruitment of effector cells : recognition of PAMPs for effector activation/inflammation (early induced response)
4. adaptive response: lymphocytes like B and T cells recognise antigen transported into lymphoid organs.

Question 4

Microbe Detection

Answer 4

• innate immune response is dependent on the ability to recognise microbes
• detection can be direct (via recognition of molecule moieties of pathogens) or indirect (via detection of pathology generated by infection)
• pathology detected by changes in physiology
• PAMPs detected by PRRs

Question 5

Fundamentals of Innate Detection

Answer 5

1. direct detect of molecules present on foreign organisms and absent from host (PAMP)
2. detection of modifications made to host molecules
3. detection of host derived danger signals / signs of aberrant processes
4. detection of missing self molecules (ie. antigen production shut down)

Question 6

PAMP

Answer 6

• pathogen associated molecular moiety
  Want moieties that are:
• conserved across many microbial species (limited number of innate receptors)
• not found in host cells
• fairly abundant so immune response can be efficiently activated

Question 7

Types of Bacterial PAMPs

Answer 7

Gram Positive: lipoteichoic acid, lipoprotein, peptidoglycan
Gram Negative: LPS, porin, peptidoglycan
Mycobacteria: glycolipid, LAM, mycolic acid, peptidoglycan, galactan

Question 8

Fungal PAMPs

Answer 8

• mannan (poly-saccharide)
• B-glucan (poly-saccharide)
• Chitin (N-acetylglu)

Question 9

Innate Immune Receptors

Answer 9

1. phagocytic receptors: expressed by phagocytes and mdediate internalization of detected nonself organisms
2. signalling receptors: detect invaders and alter gene expression in the detecting cell often driving signalling to other cells (activate cytokine expression instructing further cellular activity)

Question 10

Phagocytic Receptors

Answer 10

• bind to PAMPs or antibodies bound to the microbe
• internalised cells put into the lysosome with proteases and hydrolytic enzymes for destruction
• these receptors are a specific kind of pattern recognition receptor : bind to PAMPs to allow recognition of invasive particles by phagocytes allowing the invader to be phagocytosed and killed

Question 11

Signalling Receptors

Answer 11

• activated by microbial moieties to drive the activation of signal transduction pathways resulting in gene expression changes

Question 12

Types of Signalling PRRs

Answer 12

1. lectins: recognise carbohydrate moieties
2. peptidoglycan recognition proteins
3. toll-like receptors: wide microbial recognition
   - expression of these systems is found in the surveillance cells of the innate system
   - eg. dendritic cells, neutrophils, macrophages
   - important for the type of infection to be distinguished for B/T cell activation

Question 13

Toll Like Receptors

Answer 13

• type 1 integral membrane glycoproteins with extracellular domains containing varying numbers of Leucine Rich Repeat motifs and cytoplasmic domains homologous to that of IL-1 receptor (signalling domain)
• homologous structure within the family
• recognises induces conformational changes of TIR domain constitutes the signal to initiate a cascade of cytoplasmic protein interaction
• leads to transcriptional activation of cytokine and chemokine genes

Question 14

Types of TLRs

Answer 14

Cell surface: recognise microbes and cellular invaders

Phagosome/endosome: recognise nucleic acids from viruses

Question 15

Cell Surface TLR Recognition

Answer 15

• specific cytosolic components involved for scaffolding
• intracellular signalling domain recruits transduction proteins like Mal and MyD88, signalling to IRAK1/4 and therefore NF-kB
• promotes transcription of inflammatory cytokines

Question 16

NFkb/Rel

Answer 16

• key transcriptional activator downstream of TLR’s
• conserved family of TF activated by innate immune response
• TLF activation triggers a signalling pathway resulting in cleavage of an inhibitory domain of NFkB
• permits DNA binding domain access to the nucleus where it drives transcription of its target genes
• targets are proinflammatory cytokines (TNFa, IL1)

Question 17

Virus Detection

Answer 17

• viruses produced in host cells from host components and the only non host molecules are specific proteins
• adaptive responses detect viral proteins by binding to antibodies or T cell receptors
• innate responses detect viral proteins by detecting cell damage and unusual nucleic acid signatures (wrong place and shape)

Question 18

Endosomal TLR Recognition

Answer 18

• NFkB pathway to produce inflammatory cytokines
• IRF3 (TF) pathway to produce type 1 interferons
• this is activated mainly by nucleotide ligands
• endosomal detection is tuned for viruses as viral RNA/DNA is exposed in the endosome due to low pH levels

Question 19

IRF3

Answer 19

• second transcription factor of endosomal pathways
• key in engaging the antiviral response in mammals
• endosomal activation triggers signalling pathway resulting in phosphorylation of IRF3
• dimerisation + nuclear localisation to drive target gene transcription
• key target is type 1 interferons

Question 20

TLR-NFkB

Answer 20

• induces proinflammatory cytokines and chemokines
• IL-1 and 6 enhances the immune response and induces protein secretion
• IL-8 is a chemoattractant for neutrophils
• TNF-a enables a systemic endothelial response by inducing vascular changes for cell recruitment

Question 21

TLR-IL-1 Amplificaiton

Answer 21

• TLRs and IL-1R share common signal pathways using IRAK1/4 and TRAF6 signal proteins
• thus a persistent and chronic infection can cause chromic inflammator
• the inflammasome is a signal mechanism that can facilitate or prevent this

Question 22

Inflammasome

Answer 22

• detects intracellular/cytosolic bacterial infection
• animal cells have no cytosolic TLR’s
• cytosolic bacteria are recognised by NLR (NOD and leucine rich repeat) proteins
• NOD: nucleotide binding and oligomerization domain)
• LRR: binds microbial moieties
• NLR’s triggers activation of IL-1B via inflammasome

Question 23

Inflammasome Formation

Answer 23

• primary step of TLR/TNFR/IL-1R receptor activation causing de novo transcription of pro-IL-1B and NLRP3
• activation step forms the assembled NLRP3 inflammasome complex that activates pro-IL-1B
• IL-1B has caspase activity activating pore formation in the cell membrane
• IL-1B can activate IL-1R to permit further amplification

Question 24

Viral Nucleic Acid Detection

Answer 24

• numerous sensors
• for RNA viruses
  1. RIG-I like
  2. DExD helicases: similar to RIG like lacking some domains
  3. others
• signalling pathways then lead to IRF and NFkB TF production of IFN-a/B

Question 25

RIG-I like receptors

Answer 25

- 3 domains: signal, binding of viral RNA, and CTD shutting the system down when not detecting RNA - CTD binds the CARD N terminal (signal) domain when not active - activation leads to similar signal path as TLR - IRF3/NF-kB activation

Question 26

Identifying foreign RNA structures

Answer 26

- foreign RNA recognition depends on presence of long dsRNA structure - respond to absence of 5' RNA modification - RIG-1 recognises mRNA with naked 5' triphosphate or bad cap as foreign

Question 27

TNF

Answer 27

- causes endothelial permeability - allows immune cell entry - leads ultimately to circulation loss - can be dangerous as permeability throughout the body causes a loss of vasculature integrity - organ failure

Question 28

Type 1 Interferon

Answer 28

- antiviral cytokines - transcription targets of IRF3 - activated by mainly nucleotide ligands - IFNa comes from immune cells and IFNB from virus infected non-immune cells - IFNy activity focused on epithlial layers - IFNB induces IFNa production (signals to plasmid dendritic cells for antiviral amplification)

Question 29

IFNa/B

Answer 29

- inhibit proliferation/translation - promot apoptosis - stimulate cytotoxic activity of NK and T cells + other immune cells - stimulate expressed of other surface molecules - repression of anti-apoptotic genes