2.6 Antimicrobial mechanisms of phagocytes

Question 1

Acidification

Answer 1

pH = ~3.5 - 4.0

bacteriostatic or bactericidal

Question 2

Toxic oxygen - derived products

Answer 2

• > Superoxide O2-
• > hydrogen peroxide H2O2
• > singlet oxygen 1 O2-
• > hydroxyl radical -OH
• > hypohalite OCl-

Question 3

Toxic nitrogen oxides

Answer 3

Nitric oxide NO

Question 4

Antimicrobial peptides

Answer 4

Macrophage products:

• > Cathelicidin
• > macrophage elastase-derived peptide

Neutrophil product:

• > alpha defensins (HNP 1 - 4)
• > beta defensins HBD4
• > cathelicidin
• > azurocidin
• > bacterial permeability inducing protein (BPI)
• > lactoferricin

Question 5

Enzymes

Answer 5

Lysozyme:
Digests cell walls of some Gram-positive bacteria

Acid hydrolases (e.g. elastase + other proteases):
Break down ingested microbes

Question 6

Competitors

Answer 6

Neutrophil products:

• > Lactoferrin (sequesters Fe2+)
• > Vitamin B12 - binding protein

Question 7

PAMPs - Pathogen associated molecular patterns

Answer 7

• Conserved (unlikely to mutate)
• Clearly distinguished from self
• Common to many infectious agents

Multiple repeating carbohydrates
	- C-type lectins
Bacterial cell components
	- peptidoglycan, MDP
Viral components
	- ds RNA, ss DNA

Question 8

Pagocyte recognition molecule

Answer 8

Pattern recognition receptors:

• Scavenger receptors
• C-type lectin receptors
• Toll-like receptors (TLRs)
• NOD-like receptors (NLRs)

PRRs are BOTH cell surface & intracellular

Question 9

Toll Like Receptors

Answer 9

diacyl lipopeptides
-> TLR 2 / 6

triacyl lipopeptides
-> TLR 2 / 1 -> convex surfaces have bincing sites for lipid chains of triachyl lipopeptides

flagellin
-> TLR 5

LPS

• > TLR 4
• > MD 2

Endosome:

• > ds RNA (TLR 3) -> in endosome binds ds RNA + signals via TRIF to induce IFN gene expression
• > ss RNA (TLR 7) -> in endosome binds ss RNA + signals via MyD88 to induce IFN gene expression
• > CpG DNA (TLR 9)

Binding of each TLR to the same lipopeptide induces dimerization, bringing their cytoplasmic TIR domains into close proximity

TIR = TOL - IL - 1 Recceptor

=> Analogous downstream signalling pathways
=> transcriptional activation

• > Cytoplasmic NOD proteins reside in the cytoplasm in an active form
• > binding of bacteria ligands to NOD proteins induces recruitment of RIPK2, activating TAK1, leading to NFkB activation

Question 10

Toll Like Receptor - Signalling

Answer 10

PRR binding PAMP causes:

• > Activation of transcription factors
  e. g. NFκB

-> Induction of pro-inflammatory cytokines e.g TNFα and IL-1β

• > Induction of anti-viral cytokines
  e. g. IFNα and IFNβ

quickly generates innate response