Phagocytes and Granulocytes

What are phagocytes and granulocytes derived from?

myeloid progenitor cells

macrophage distribution

Distributed in most tissues and are sentinel

What happens to bone-derived monocytes during macrophage maturation

They move into blood and respond to migratory and differentiation signals to migrate to tissues

properties of macrophages

terminally differentiated but highly plastic - can change phenotype

M1 macrophages

produce pro-inflammatory cytokines and chemokines, promote adaptive immunity

M2/alternatively activated macrophages

aid in resolving inflammation by anti-inflammatory cytokines eg IL10, chemokines and growth factors eg TGF beta → tissue repair and remodelling

properties of neutrophils

• multilobulated nucleus

• lifespan of 5 to 135 hours

• not found in healthy tissue - require recruitment

neutropenia

• abnormally low neutrophil levels

• susceptible to infection

• common in chemo and AIDS patients

immune response - reaction by phagocytes (outline)

Response mounted via chemokines and cytokines eg GMCSF. Neutrophils released from bone marrow and circulate to infection site

immune response - resolution by phagocytes

monocytes infiltrate tissue, differentiate and remove dead cells for tissue repair

What regulates neutrophil recruitment?

selectins between leukocytes and the inflamed endothelium. Cytokines upregulate selectin presentation

classical neutrophil recruitment cascade

capture → rolling → slow rolling → arrest → transmigration (leakage to local tissues)

outline the process of phagocytosis

ingestion, phagosome → phagolysosome → microbial killing, residual body forms → elimination/exocytosis

examples of phagocytic receptors

glucan receptor (Dectin-1) and mannose receptors

opsonins (eg antibodies, fibronectins, complement)

soluble molecules deposited onto foreign surfaces which bind and activate phagocytic receptors

aggregation of Fc gamma receptors (receptors for antibody)

Integrins promote adhesion to opsonised particle. Triggers inside-out signal which activates integrins via GTPase Rap. Integrins also bind via C3b

what does ingestion involve?

complex signalling pathways causing cytoskeletal remodelling, dynamic membrane movement and fission

pseudopodium

envelopes pathogen

oxidative killing mechanisms

superoxide anions, hydroxy radicals, hypochloride anion, nitric oxide, nitrogen dioxide, nitrous acid and NETosis

non-oxidative killing mechanisms

lysozymes, hydrolytic enzymes, transferrin and defensins

autophagy (‘self eating’)

cytoplasmic proteins and organelles delivered to lysosome in response to stress

non-selective autophagy

part of cytoplasm engulfed due to amino acid deprivation

selective autophagy

Double membrane compartments form around target bacteria for transport to lysosome. Aka xenophagy

respiratory bursts

Non-mitochondrial generation of antimicrobial reactive oxygen species through NADPH oxidase enzyme (NOX) complex

NOX structure

Made up of gp91 and p22 subunits forming cytochrome b558, and several cytosolic components eg Rac1 protein

NADPH oxidase (NOX) function

oxygen reduced → superoxide released into phagolysosome. Superoxide and H2O2 react creating hydroxy radicals

chronic granulomatous disease (CGD)

NOX2 mutation. Patients hypersensitive to infection as phagocytes can’t kill pathogens due to low oxidative burst

myeloperoxidase (neutrophils only)

• H2O2 + chloride → hypochlorous acidhypochlorite

• target chlorination inactivates proteins and enzymes, can interfere with bacterial replication

inducible NO synthase iNOS)

induced in M1 by TLR ligands and inflammatory cytokines eg IFN gamma. Oxidises L-arginine → NO

hydrolytic enzymes

acidic enzymes stored in lysosomes which create peptides for MHC II presentation

Which granules contain antibacterial proteins and proteases as their major constituents?

azurophil, specific and gelatinase

Which granule does NOT contain antibacterial proteins and proteases? What are its major constituents?

Secretory, major constituent = transmembrane receptors eg TNF and IFN-alpha receptors

order granule contents by speed of exocytosis (fastest to slowest)

secretory → gelatinase → specific. Azurophils undergo limited exocytosis

purpose of granule proteases

• degrade ECM proteins

• facilitate immune receptor (in)activation

• pathogen digestion and clearance

How does S. aureus evade detection?

change PAMP structure preventing uptake

How does M. tuberculosis evade killing by phagocytes?

inhibits phagosome acidification via PtkA and CISH to degrade proteasomes. Phosphatase SapM acts on Pi3P to prevent downstream fusion

How does L. monocytogenes prevent killing by macrophages?

destroy phagosome before it fuses with lysosomes

NETosis

ROS dependent formation of neutrophil extracellular traps, weblike structures made of modified chromatin adorned with bactericidal proteins from granules and cytoplasm