Phagocytes and Granulocytes

What are phagocytes and granulocytes derived from?

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

M1macrophages

produce pro-inflammatory cytokines and chemokines, promote adaptive immunity

M2/alternatively activated macrophages

aid in resolving inflammation by anti-inflammatory cytokines, chemokines and growth factors → 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)

macrophages mount response and recruit immune cells via chemokines and cytokines eg granulocyte-macrophage-colony stimulating factor. 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?

adhesion molecules (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

Recognition of invading microbe → ingestion, phagosome formation → phagolysosome formation → microbial killing, residual body forms → elimination/exocytosis

examples of phagocytic receptors

glucan receptor (Dectin-1) and mannose receptors

opsonins

soluble molecules deposited onto foreign surfaces which act as adaptors to bind and activate phagocytic receptors eg antibodies, fibronectins and complement factors

aggregation of Fc gamma receptors

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

ingestion

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

pseudopodium

envelopes pathogen

oxidative killing mechanisms

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

non-oxidative killing mechanisms

lysozymes, hydrolytic enzymes, transferrin and defensins

autophagy (‘self eating’)

delivery of cytoplasmic proteins and organelles to the lysozyme for degradation in response to stress

non-selective autophagy

part of cytoplasm engulfed due to amino acid deprivation

selective autophagy

xenophagy - microbe degradation. Double membrane compartments forms around target bacteria and transported to lysosomes

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

NOX function

transfer electron from NADPH to oxygen → superoxide released into phagolysosome. Superoxide → H2O2 - reacts with superoxide to create hydroxy radicals

chronic granulomatous disease (CGD)

genetic disorder of mutations in genes encoding NOX2 components. Patients hypersensitive to infection as phagocytes can’t kill pathogens due to low oxidative burst

myeloperoxidase

• in neutrophils

• H2O2 + chloride → hypochlorous acidhypochlorite

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

macrophage inactivation and reactive nitrogen species

• inducible NO synthase expression (iNOS)

• iNOS oxidises L-arginine → NO

• NO + superoxide →peroxynitrite

hydrolytic enzymes

• stored in lysozymes

• optimal pH 5.4 to 5.5 maintained by H+ ATPase

• produce antigenic peptides for MHC II loading

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?

inhibit fusion of phagosome with lysosome

How does L. monocytogenes prevent killing by macrophages?

destroy phagosome before it fuses with lysosomes

NETosis

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