Lecture 4: Innate Responses to Infection: Phagocytes Flashcards
What are macrophages derived from?
derived from the myeloid progenitor and mature continuously from blood monocytes
What is the role of macrophages?
carry out phagocytosis and inflammatory response signalling
important role in adaptive immunity
How do macrophages detect pathogens?
using pattern recognition receptors e.g. bacteria to mannose receptors
What happens during phagocytosis after the pathogen becomes bound to PRRs on the cell surface?
the pathogen is engulfed into the cell by the process of endocytosis into a phagosome, it fuses with a lysosome to form a phagolysosome and the bacterium is completely degraded
What do phagocytes present to the adaptive immune cells?
peptide fragments of broken down pathogens
How do phagocytes break down pathogens?
acidification, toxic oxygen-derived products, toxic nitrogen oxides, antimicrobial peptides, enzymes and competitors
What is respiratory burst?
when phagosome fuses with lysosome, NADPH-dependent oxidases generate toxic oxygen radicals and hydrogen peroxide (H2O2) -> accompanied by transient increase in O2 consumption
Why do activated phagocytes also produce nitric oxide?
NO reacts with oxygen radicals to produce peroxynitrite (ONOO-) -> damages molecules including DNA
Why aren’t our own cells also damaged by respiratory burst?
this process is accompanied by synthesis of enzymes to inactivate the damaging molecules e.g. superoxide dismutase converts superoxide to hydrogen peroxide and catalase converts H2O2 to water and O2
What induces the synthesis of inflammatory cytokines in macorphages?
binding of bacterial components to signalling receptors
What do cytokines produced by macrophages cause?
dilation of local small blood vessels
What are cytokines? How do they act on cells?
small glycoproteins that mediate immune cell communication and act by binding to specific receptors on cells
What happens if there are defects in cytokine production, signalling or regulation?
this can lead to autoimmunity (e.g. inflammatory disease)
How do autocrine, paracrine and endocrine cytokines act?
autocrine: affect the behaviour of the same cell
paracrine: affect the behaviour of a local cell
endocrine: affect the behaviour of a distant cell
What do chemokines regulate?
the movement of cells
What is the role of IL-1B?
activates vascular endothelium
activates lymphocytes
local tissue destruction
increases access of effector cells
What is the role of TNF-a
activates vascular endothelium and increases vascular permeability, which leads to increased entry of IgG, complement, and cells to tissues and increased fluid drainage to lymph nodes
What is the role of IL-6
lymphocyte activation
increased antibody production
What is the role of IL-8
chemotactic factor recruits neutrophils, basophils, and T cells to site of infection
What is the role of IL-12
activates NK cells
induces the differentiation of CD4 T cells into Th1 cells
What are acute phase proteins?
proteins produced by the liver and released into circulation
How does plasma concentration of acute phase proteins differ in the absence of infection and within a day or two of infection?
in the absence of infection plasma concentration is very low and within a day or two of infection plasma concentration rises very quickly
What is the synthesis and release of acute phase proteins stimulated by?
inflammatory cytokines
What are some examples of acute phase proteins?
CRP and mannose-binding lectin
What is the role of CRP?
binds phosphocholine on bacterial surfaces, acting as an opsonin and as a complement activator
What is the role of mannose-binding lectin?
binds to carbohydrates on bacterial surfaces, acting as an opsonin and as a complement activator
What is CRP an indicator of?
inflammation
What are neutrophils derived from and where are they stored?
derived from the myeloid progenitor and stored in bone marrow -> released progressively in steady state
Are macrophages or neutrophils terminally differentiated in bone marrow?
neutrophils
What is the role of neutrophils?
move in large numbers to sites of infection (cytokine stimulation) so that they can carry out phagocytosis and form NETs
Are macrophages or neutrophils short lived?
neutrophils
How do cytokines direct neutrophil migration?
induce higher expression of adhesion molecules and stronger interactions
How do neutrophils migrate?
bind to endothelium and squeeze between cells -> enter infected tissue and make up the first wave of cells that cross the blood vessel wall to enter and inflamed tissue
What allows leukocytes to roll along the vascular endothelial surface?
selectin-mediated adhesion to leukocyte sialyl-Lewis^x is weak
What is the process by which neutrophils enter into the site of infection?
rolling adhesion -> tight binding -> diapedesis -> migration
What are some examples of adhesion molecules?
LFAs: leukocyte functional antigen
ICAMs: intercellular adhesion molecules
How does rolling adhesion occur?
selectin is transported to surface of the endothelium after exposure to inflammatory mediators -> selectins bind to the S-LEx carbohydrate ligand on the neutrophil allowing adherence to endothelium -> reversible reactions allow neutrophils to “roll”, or be forming and breaking new adhesive interactions
How does tight binding occur?
interaction of the LFA-1 on neutrophil and adhesion molecules on endothelium (ICAM-1) -> exposure to chemokine (CXCL8) released by stimulated macrophages promotes conformation change to strengthen the adhesion (strongly induces expression of ICAM-1) -> neutrophil stops rolling and tightly binds to endothelium
How does diapedesis occurs?
neutrophil crosses blood vessel wall by squeezing between cells (extravasation) -> proteases (such as elastase) secreted by neutrophil break down basement membrane
How does migration occur?
neutrophil migrates towards centre of infection following concentration gradient (CXCL8/IL-8 and TNF-a)
How do neutrophils undergo phagocytosis?
neutrophils express receptors for many bacterial and fungal constituents -> neutrophils bind bacteria, engulf them, and destroy them with the toxic contents of the neutrophil granules -> they quickly die after phagocytosis and degradation of the pathogen -> build-up of dead neutrophils at site of infection leads to the formation of pus
What is fMet-Leu-Phe?
bacterial tri-peptide released by bacteria (with formylated Met)
- > chemotactic factor
- > activator of phagocytosis / degradation
How do neutrophils kill microbes apart from phagocytosis?
neutrophils use a novel mechanism of destruction that is directed at extracellular pathogens
How are NETs formed?
neutrophil undergoes cell death (NETosis), the nuclear chromatin is released into extracellular space and forms a fibril matrix know as NETs
How do NETs kill?
capture microorganisms and work to increase pathogen clearance by increasing the efficiency of phagocytosis by neutrophils or macrophages
