Neutrophils Flashcards
What are the main leukocytes in the blood and in what proportions?
Neutrophils- 40-75% (major cell type) Eosinophils- 1-6% Basophils- <1% Monocytes- 2-10% Lymphocytes- 20-50%
(All of the -phils are granulocytes)
Describe neutrophils.
60 x 10^6/minute released into circulation.
1/2 life of 7 hours in blood- Very short lifespan- May be survival factors that allow them to live longer in tissues.
Most abundant of the leukocytes.
Marrow produces ~100 billion per day for an average adult.
25 billion are in the circulating blood (~5000 per ul).
55% of marrow weight dedicated to neutrophil production.
Crucial for host defence to bacteria and fungi-Viral infections don’t seem to be affected much.
Takes a lot of energy and effort for body to make these cells.
What do PMNs do and what are factors that increase their numbers?
Control release and production of neutrophils.
Stress, injury, infection, increase in cytokines.
How is neutrophil production regulated in steady state and inflammation?
Steady state (G-CSF)- happens all the time, no obvious infection. Increase in C/EBP-a, decrease in proliferation, increase in granulocyte diff. Inflammation (cytokines/PAMPs)- decrease in C/EBP-a, increase in C/EBP-B, Increase in proliferation, indreace in granulocyte diff. Rapid expansion of granulocyte numbers to combat infection.
(C/EBP-a is an imprtant transcription factor that promotes differentiation/proliferation of neutrophils. C/EBP-B allows precursors to proliferate at the same time as differentiation-increases neutrophil production.).
How are neutrophils stored and transported?
Large reserves of neutrophils are stored in the bone marrow and are released when needed to fight infection.
Travel to and enter infected tissue, where they engulf and kill bacteria. The neutrophils die in the tissue and are engulfed and degraded by macrophages.
Near blood vessels in mature form.
Released slowly in normal conditions.
In infection, there is a signal to release many at once.
Important that clearance pathway is controlled carefully- in dead neutrophils are not cleared, toxic mediators can cause damage.
What do tissue macrophages and infiltrating neutrophils do?
They kill microbes.
Bacteria triggers cells to produce cytokines.
There are molecular changes in blood vessels that allow neutrophils to contact endothelial cells, can attach and roll. They will stop, then enter tissues in response to a signal to kill infection.
What signal is secreted by macrophages and what does it do to neutrophils?
Interleukin-1 is secreted by macrophages- this is a potent signal for neutrophils for macrophages to accumulate at infection site.
How are neutrophils recruited to infection sites?
Endothelial cells respond by increasing expression of selectin adhesion molecules. These interact with glycoproteins in neutrophil. Weak interaction- cells keep moving, but get slowed down. Chemokines- on matrix proteins of endothelial cells.
Chemokines interacts with Neutrophils- integrins become active by conformational change- strong adhesions- allows stop.
Paracellular migration- Neutrophil moves between cells into tissue.
Chemotax down gradient.
Move through in a jerky fashion- typical rolling.
Can also use vacuole to move through cell.
What are the molecular players in neutrophil recruitment?
Rolling : Selectins + Selectin counterreceptors.
Firm adhesion : Activated integrins + Ig-like counterreceptors.
Diapedesis (crawling through) : Integrins + endothelial cell adhesion molecules.
Chemotaxis : Integrins + chemokines/chemokine receptors.
Counterreceptors are the glycoproteins.
Whole thing is a 3-step paradigm.
What do selectins do?
Mediate rolling of leukocytes on endothelium.
3 types of selectin- 2 expressed on endothelial cells- E and P selectin.
L selectin less important for neutrophil recruitment.
ESL1, PSGL1- rods are carbohydrates- mediate weak interactions.
Selectins all recognise a particular group of carbohydrates- the fucose part of S-Lex.
Selectin-mediated adhesion is weak and allows neutrophil to roll along the vascular endothelial surface.
What happens in paracellular migration?
Pores are formed between endothelial cells- made by neutrophil as it squeezes through.
What happens in transcellular migration?
Neutrophil moves through the cell. Pore forms through the cell rather than junctions.
Describe beta-2 integrins.
αβ heterodimers.
Different α chain, common β chain.
CD11a (LFA-1), CD11b (Mac-1), CD11c (this not expressed on neutrophils).
Most important on neutrophils are CD11a and CD11b as major adhesion receptors.
Ligands include ICAM-1, Fibrinogen, denatured proteins, C3bi.
Undergo conformational change to high affinity via chemokine ‘inside-out’ signalling.
Leads to stable, high affinity binding to ligands on other cells.
Critical interaction to stop rolling.
Describe chemotaxis.
Chemotaxis is directional migration of leukocytes.
Essential for recruitment to sites of inflammation/infection.
Mediated via chemotactic factors and chemokines.
Bind to G-protein coupled receptors (GPCRs) on neutrophils.
Results in polarisation and directed movement.
Chemotactic factors include:
- PAF, platelet activating factor.
- FMLP – N-formylated peptides from bacteria and mitochondria.
- C5a, C5 breakdown product, anaphylotoxin.
- Results in polarisation and directed movement.
Chemokines include:
-MIP-1, IL-8.
How does chemokine signalling work?
Chemokine, chemokine receptor and membrane associated G protein form a complex.
Chemokine binds to receptor- receptor has high affinity for chemokine.
GTP activates compex by replacing GDP and activaing the G protein.
The complex dissociates to give two parts of the G protein that initiates pathways of signal transduction.
