Innate immune recognition

Question 1

The innate immune system recognises

Answer 1

1. Highly conserved molecular patterns expressed by large groups of pathogens.
2. Common biologic consequences of infection.

Question 2

PAMPs

Answer 2

Gram positive bacteria: lipoteichoic acid, peptidoglycan, lipoproteins, DNA, flagellin

Gram negative: all the same plus lipopolysaccharide

Virus: coat protein, nucleic acid

Parasite: GPI anchor

Yeast: zymosan

Question 3

Gram+
Gram-

Answer 3

Thick cell wall with no outer envelope

Thin cell wall with outer envelope.

Question 4

DAMPs

Answer 4

Endogenous molecules - signal tissue injury and initiate repair.

Molecules: DNA, RNA, Glucose
Particles: Amyloid-B, Silica, Nanoparticles
Others: UVB, Mutations

Question 5

PRRs

Answer 5

Secreted and circulating receptors.

Receptors on the cell surface and cell membranes.

Receptors inside cells (cytoplasmic).

Question 6

Secreted and circulating PRRs

Answer 6

Antimicrobial peptides secreted into lining fluids (from epithelia and phagocytes).

Pentraxins: proteins like CRP; react with the C-polysaccharide of pneumococci, some antimicrobial actions. Activate complement, promotes phagocytosis.

Lectins and collectins: CHO-containing proteins, bind CHO or lipids on microbe walls. Activate complement, improve phagocytosis.
MBL and Surfactant proteins A and D.

Question 7

Cell associated PRRs

Answer 7

Receptors present on the cell membrane or on organelles within the cytosol.

TLRs main one.

Recognise a broad range of molecular patterns.

Question 8

TLRs and their ligands (examples)

Answer 8

TLR1/2 -> triacyl lipopeptides
TLR2 -> peptidoglycan, hemagglutinin, inositol, mucin, zymosan, SARS-COV-19.
TLR5 -> flagellin

Question 9

TLR and PAMP signalling

Answer 9

Different TLRs activate different signalling cascades (depending what pathogen they bind) and tailor the immune response accordingly.

Question 10

TLR signalling and DAMPs

Answer 10

TLR2,4,6 -> inflammation repair

TLR7, 9 -> autoimmunity + inflammation

Question 11

Other membrane bound PRRs

Answer 11

Mannose receptor (on macrophages) - fungi.

Dectin-1 (on phagocytes) -> beta glucans in fungal walls.

Scavenger receptors (on macrophages) -> wide range of lipid related ligands.

Question 12

RIG-I-like Receptors (RLRs)

Answer 12

Detect viral RNA in the cytoplasm:

Retinoic acid-inducible gene I (RGI-I)
Influenza A and Respiratory Syncytial virus.
short dsRNA, 5’ppp caps

Melanoma differentiated gene 5 (MDA-5) long ds RNA
Rhinovirus

Laboratory of genetics and physiology 2 (LGP2). dsRNA
Positive regulator for other two.

Question 13

NOD-like receptors

Answer 13

Sensing cytoplasmic bacterial pathogens and DAMPs.

Regulates inflammatory and cell death responses.

Nucleotide-binding and oligomerisation domain (NOD=NACHT)

4 subfamilies - based on N-terminal effector domain.

Question 14

NOD1

Answer 14

Activated by meso-aminopimelic acid (Meso-DAP)-containing PGN fragments (gram-) in periplasmic space.

Question 15

NOD2

Answer 15

Muramyl dipeptide (MDP)=breakdown of PG in the cell wall. ~ in all gram negative and positive bacteria.

Dynamically traffic to intracellular membranes upon detection of PG derivatives.

Question 16

NOD signalling

Answer 16

Activation of MAPK and NF-KB

Induction of pro-inflammatory cytokines and chemokines

Activation of antimicrobial functions

DO NOT activate IRF transcription factors (interferon regulatory factors).

Question 17

Inflammatory response to infection

Answer 17

NFkB-MAPK

IL1B - activates vascular endothelium and lymphocytes. Local tissue destruction= increased access to effector cells.
FEVER and production of IL-6.

TNF-A- Increased vascular permeability. IgG increased lymph drainage.
FEVER.

IL-6 - lymphocyte activate and increased AB production.

CXCL8 - recruits neutrophils, basophils and T cells.

IL-12 - activate NK cells induces diff of CD4 into Th1.

Question 18

Acute phase response (actions of IL-1B/IL-6/TNF-alpha)

Answer 18

Liver: acute phase proteins (C-reactive; MBL) -> activation of complement opsonisation.

Bone marrow endothelium: neutrophils -> phagocytosis.

Hypothalamus: increased body T -> decreased replication; increased AG processing, increased spec. immune response.

Fat, muscle: protein and energy mobilisation: allows for increased temp. -> decreased replication; increased AG processing, increased spec. immune response.

Dendritic cells: TNF-alpha stimulation migration to lymph nodes and maturation -> initiates adaptive response.

Question 19

What is the interferon stimulated response element made up of?

Answer 19

STAT1, STAT2, IRF9

Activates macrophages and dendritic cells.

Activate NK to kill viral cells.

Induces chemokines to recruit lymphocytes.

Induces resistance to viral replication (destruct mRNA; inhibits translation)

Increases MHC-I expression and AG presentation (CD8)

Question 20

The damage chain reaction

Answer 20

Harmful stimuli
Pathogen
Injury
Heat
Autoantigens
Tumours
Necrotic cells

High levels of DAMPs - associated with inflammatory and autoimmune disorders as well as atherosclerosis and CA.

Question 21

Cytokine storm

Answer 21

Profound increase in CK, CXCL, and INFs= severe inflammation and tissue damage.

Induced by: genetic makeup of host, persistent pathogen

Role in sepsis.

Question 22

Immunomodulation has two main strategies…

Answer 22

Agonists and antagonists.

Question 23

Agonists (enhance TLR signalling)

Answer 23

Adjuvant effect - promotes protective responses, i.e. vaccines

Immune stimulators.

Modify adaptive immune response: Th and Treg

Side effect: potentially enhance inflammation.

Question 24

Antagonists (inhibit TLR signalling)

Answer 24

Block receptor-ligand binding -> interferes with signalling.

Sepsis syndromes, inflammation, arthritis.

Side-effects: allow pathogen outgrow, mutation.

Question 25

NOD2 mutations

Answer 25

Non functions mutation: Crohn’s disease

Hyperfunctioning mutations: Blau syndrome (granulomatous inflammation of skin, eyes, joints).

Question 26

TLR7 mutation

Answer 26

Novel gain of function: SLE

Question 27

TLR4 mutation

Answer 27

Asp299Gly and Thr399Ile overexpressed in infants <12 months = hospitalised for RSV bronchiolitis.

Fail to translocate TLR4 to cell surface - reduced NFkB signalling.