Cellular Innate Immunity: Neutrophils Flashcards
Phagocytes
Cells that protect the body by ingesting (phagocytosing) harmful foreign particles, bacteria, and dead or dying cells.
Cells of the Immune System
Innate Immune Cells
- Neutrophils
- Macrophages
- Dendritic Cells
- NK Cells
- Eosinophils
- Basophils
Innate immune cells and their function
Neutrophils
- Phagocytosis and intracellular killing
- Inflammation and tissue damage
Innate immune cells and their function
Macrophages
- Phagocytosis and intracelullar killing.
- Extracellular killing of infected or altered self targets.
Innate immune cells and their function
Dendritic Cells
- Tissue repair
- Antigen presentation for specific immune response
Innate immune cells and their function
NK Cells
Killing of virus-infected and altered self targets
Eosinophils
Killing of certain parasites
Basophils
Modulate Immune Response to allergens
Hematopoietic orgini of neutrophils or Polymorphonuclear neutrophils (PMNs) cells
Neutrophil homeostasis and life cycle
Neutrophils/Polymorphonuclear Neutrophils (PMNs) cells
- 6x10^7/minute released into circulation
- Live max 10 hours (1/2 life = 7 hours)
- About 40-60% of white blood cells ar PMNs
- Kill microbia by Oxygen-dependent and independent mechanism
Factors that increase PMN #s
Stress
Injury
Infection
Upregulation of cytokines and inflammatory mediators
Neutrophil response to infection
Circulate for 7 hours
Respond to SOS signals:
* N-formyl methionine-containing peptides
* Clotting system peptides
* Complement products
* Cytokines released by tissues and macrophages
PMN Response:
* Vascular adherence
* Diapedesis
* Chemotaxis
* Activation
* Phagocytosis and killing**
Blood neutrophils
Neutrophil Chemotaxis
- Neutrophils are very motile
- Move by rearranging cytoplasmic microfilaments and microtubule
- Portions that face upstream (leading edge) in chemotactic gradient form structure called lamellipodium
- Cytoplasm is densely packed with microfilaments
- Portions face downstream gradient (trailing edge) form knob-like uropod.
Effectiveness of chemotaxis
Methods to study Neutrophil
Chemotaxis
Effector functions of the mature neutrophil
- Degranulation
- Phagocytosis
- NETosis
Neutrophil killing mechanism
- Intracellular
- Extracellular:
Reactive Oxygen Species (ROS)
Neutrophil extracellular traps (NETs)
Phagocytosis of Microogranisms
Neutrophils can internalize both opsonized and non-opsonized particles
Two main receptors:
* Fc - binds immunoglobulin coated particles
* b2 integrin (C3b receptor) - binds complement coated particles (c3bi)
Phagocytosis-steps
- Attachment via receptors: FcR, complement R, scavenger R, TL-R
- Pseudopod extension
- Phagosome formation
- Granule fusion and phagolysosome formation
NADPH oxidase and respiratory burst in phagocytes
NADPH oxidase:
structure and assembly
- Inactive NADPH oxidase
- Active NADPH oxidase
Phagocyte intracellular killing pathways
Phagocyte Respiratory Burst
O2 + NADPH - dependent MPO - independent reactions
Phagocyte Respiratory Burst
O2 - dependent MPO - dependent reactions
Summary of NADPH oxidase activity
Fusion of specific granules with phagosome membrane (derived from plasma membrane) brings together:
* NADPH oxidase (oxidase NADPH; found in neutrophil plasma membrane)
* Unique cyt b (granule membrane)
* A quinone contribute to transport - electrons.
NADPH oxidase complex (together with a cytochrome b complex) reduce O2 to O2- (superoxide).
Superoxide can be reduced to OH- (hydroxide ion) or dismutated to H2O2 by superoxide dismutase.
O2-, H2O2 and OH- adversely affect cellular structures including membranes and nucleic acids.
Oxygen - Independent Killing
Triggered by binding opsonized bacteria to the plasma membrane of neutrophils
Specific granules fuse first
* Deliver several bactericidal proteins, including AMPs, lysozyme, lactoferrin and other cationic proteins.
Other O2 - independent killing
Microbial killing neutrophil effector mechanism
Neutrophil degranulation
Azurophil granules discharge antimicrobial cationic proteins (AMPs)
Azurophilic degranulation
AMPs are amphipathic and resemble other cationic surface proteins.
AMPs disrupt outer membrane of Gram negative and kill by causing leakage of vital components
* Each of the proteins has unique antimicrobial spectrum, but tend to affect Gram negative and Gram positive.
* AMPs may account for survival of some chronic granulomatous diseases (CGD) patients, who have deficient neutrophil NADPH oxidase function.
Neutrophils/Polymorphonuclear Neutrophils (PMNs) secondary granules contain the pleiotropic AMP cathelicidin LL-37
Neutrophils most abundant cytosolic protein
Calprotectin
- Calprotectin (also known as calgranulin A and B, MRP8/MRP14 and S100A8/A9) is a molecular complex that constitutes up to 45% of the cytosolic protein of neutrophils.
- It is composed of two calcium-binding proteins, MRP8 and MRP14, with respective masses of ~10 and 14 kDa.
Definition
Neutrophils most abundant cytosolic protein
Calprotectin
Both components belong to the S-100 protein family, whose other members are involved in cell cycle progession, cell diferentiation, and cytoskeleton-membrane interactions.
Calprotectin released from PMNs contributes to host defense, by inhibiting the growth of pathogens such as Candida albicans and modulates the inflammatory response by recruiting leukocytes and stimulating cytokine secretion.
Function
Calprotectin increase in serum correlates with aggravated respiratory disease and lethality in COVID19 patients.
Microbial killing neutrophil effector mechanism:
Neutrophil extracellular traps (NETs)
Neutrophil defenses against respiratory pathogens
Microbial Evasion of Oxygen-dependent killing
Microbial production of superoxide dismutase and catalase
Factors secreted out of the phagosome/into the cytosol that inhibit the respiratory burst and block phagocytosis.
Block recruitment of NADPH oxidase subunits
Cause granules to target the plasma membrane vs. the phagosome.
Host microbicidal factors and bacterial defenses
Inflammatory and regulatory subsets of neutrophils
Neutrophils engage in bidirectional cross-tlak with various leukocytes during infection, inflammation and disease
The role of neutrophils in the resolution of inflammation
Neutrophils
Summary
- Neutrophils are the most abundant blood phagocytic cells, they are the first line of defense against microbial infections
- Neutrophils rapidly respond to endogenous and exogenous, (microbial derived), chemoattractants upon inflammation or infection.
- The main effector functions of neutrophils are phagocytosis, oxidative burst, degranulation and extracellular traps (NETs).
- Microbial killing in neutrophils occurs by O2-dependent and O2-independent mechanism.
- Azurofilic degranulation is an O2-independent microbicidal mechanism of neutrophils via granule release of chemical effectors, including AMPs, and other cationic peptides.
- Neutrophild functions are critical for microbial clearance and for resolution of inflammation following infection.
