Phagocyte function and macrophages Flashcards
Why is phagocytosis important?
Protects against invading organisms
Processes foreign antigens
Presents parts of foreign antigens
Purges debris and damaged/dying host cells
Which cells are phagocytic?
Macrophage
Neutrophil
DC
Eosinophil
Basophil
Where do phagocytes act?
Sites of infection
Mucosal lymphoid tissue
Lymph nodes
Spleen
Describe the overall process of phagocytosis
Activation of resting phagocytes
Chemotaxis
Attachment and signalling
Phagosome maturation
Killing and antigen presentation
What happens in activation of resting phagocytes?
Inflammatory mediators activate resting phagocytes
Expression of PRRs and ability to recognise and adhere to microbes increases
Production of ATP, lysosomal enzymes and lethal oxidants increases
Metabolic and microbicidal activity increases
What happens in chemotaxis?
Movement towards an increasing concentration of attractant bacterial protein, capsule, cell wall, complement (C5a), chemokine (CXCL-8)
What are the 2 types of attachment?
Unenhanced
Enhanced
How does unenhanced attachment occur?
Via PRR interaction with PAMPs
How does enhanced attachment occur?
Via opsonin recognition receptors interacting with Antibody (Fc) or complement (C5a) fractions previously fixed to microbe components
When does unenhanced attachment occur?
In innate immunity - so is antigen non-specific
Designed to recognise a few highly conserved structures (PAMPs)
Name some unenhanced PAMPs
Bacteria DNA
Glucans from fungal cell walls
Mannose
LPS from gram negative cell wall
Lipoteichoic acids from gram positive cell wall
N-formylmethionine in bacterial proteins
Microbial peptidoglycans
Viral dsRNA
How can PAMPs be hidden/absent?
By pathogen diversity in cell wall structure
Name some unenhanced PRRs
Endocytic PRRs
Signalling PRRs
What are some endocytic PRRs?
Mannose receptors - bind to mannose and fucose on microbial glycoproteins/lipids
Scavenger receptors - bind to other bacterial cell wall components e.g. LPS, peptidoglycan
How do signalling PRRs work?
Binding to bacterial and fungal components
Binding to viral components
What happens if signalling PRRs bind bacterial and fungal components?
Transmits a signal to the nucleus, inducing cytokine production e.g. IL-12, TNF-a, IL-6
This leads to innate immune defences such as inflammation, activation of other phagocytes
What happens if signalling PRRs bind viral components?
Triggers anti-viral IFNs
Describe enhanced attachment
More specific and efficient than unenhanced
Mediated through opsonins
Involves Fc fragments of IgA and IgG
C3b and C5a promote enhanced attachment
What type of attachment do macrophages use?
Both enhanced and unenhanced
What type of attachment do neutrophils use?
Mainly enhanced
How is the phagosome formed?
Microbe attaches to phagocyte
Actin cytoskeleton rearrangement occurs
Forms pseudopods
Membrane remodelling causes phagosome formation
Lysosomes fuse with phagosome and form phagolysosome, allowing for pathogen destruction
What do lysosomes contain?
Lysozymes
Cathepsins
Lactoferrin
Defensins
What do lysozymes do?
Break down proteoglycans in bacterial cell walls
What are cathepsins?
Proteolytic enzymes that degrade enzymes
What does lactoferrin do?
Deprives bacteria of iron
What are defensins?
Small peptides that form channels in lipid bilayers
What are lysosomes a form of?
Oxygen-independent killing
Describe oxygen-dependent killing
O2 forms O2- (superoxide) via NADPH oxidase
H20 + superoxide forms hydrogen peroxide and hydroxyl radicals via dismutase
Arginine + O2 forms NO and citrulline via inducible NO synthase (iNOS)
NO + hydrogen peroxide forms peroxynitrite radicals
All these oxygen-containing radicals go on to kill the phagocytosed bacteria
How does oxidation kill bacteria?
Oxidation of proteins, carbs and DNA destroys function
Oxidation lipids can destabilise cytoplasmic membrane structure
How do microbes evade immune response?
Block phagocytes
Block engulfment
Block phagolysosome killing
Kill phagocytes
What microbe blocks phagocytes?
Strep pneumoniae has a capsule so is able to resist unenhanced attachment by preventing binding of PRRs
Resists C3b opsonisation
What microbe blocks engulfment?
Yernisia depolymerises actin, preventing engulfment
What microbe blocks phagolysosome killing?
Salmonella - more resistant to toxic ROS and defensins
Mycobacterium - blocks phagosome fusing with lysosome
What microbe kills phagocytes?
Staph aureus produces leukocidin (a toxin which damages membranes)
Why do apoptotic cells need to be phagocytosed?
Important for regulation of immune responses
Prevents leakage of cytotoxic or antigenic contents
Rapid, efficient and silent (no inflammation)
Which phagocytes engulf apoptotic cells?
Professional phagocytes - macrophages because they are mobile and can infiltrate tissues
Amateur phagocytes - essentially any cell can take up a neighbouring dying cell; slower kinetics than the professional phagocytes
What are the signals for phagocytosis of apoptotic cells?
Signals attracting phagocytes - soluble and secreted by apoptotic cells
Signals activating phagocytes - expressed on the apoptotic cell surface
Signals preventing phagocytosis - not expressed by dying cells
What are the signals attracting phagocytes?
Chemoattractants:
- Lysophosphatidylcholine (LPC)
- ATP
- Chemokines e.g. CX3CL1
What are signals activating phagocytes?
Phosphatidylserine (PtdSer)
Recognition tethering
Promotes engulfment
Describe PtdSer
Normally expressed on inner leaflet of plasma membrane
In apoptosis PtdSer is translocated to outer part of the lipid bilayer
Involves flipase and scramblase enzyme
Describe host cell housekeeping
RPE (retinal epithelial cells) in the eye remove fragments of photoreceptors that die by apoptosis in response to light - essential for maintaining vision
Microglia in the brain help trim and drive neuronal structuring by ‘nibbling’ them (trogocytosis)
What are signals preventing phagocytosis?
CD47
CD31
Describe CD47
Ubiquitously expressed in human cells and overexpressed in many different tumour cells
Interfere with actin engulfment pathway
Describe CD31
Expressed on endothelial cells, platelets, macrophages, Kupffer cells, granulocytes, lymphocytes, megakaryocytes and osteoclasts
Binding leads to repulsion signals
What happens to CD31 signalling during apoptosis?
It is disabled so the phagocyte is not actively rejected
What are consequences of apoptotic cell removal?
Secretion of pro-healing anti-inflammatory cytokine e.g. IL-10, TGF-b
Presentation of antigens
What does secretion of anti-inflammatory cytokines do?
Reduces inflammation
Promotes wound healing
What does presentation of antigens allow?
Maintenance of self tolerance
Release of unprocessed pathogens/antigens
What are some defects that can occur in apoptotic cell clearance?
Leakage of content from non-cleared apoptotic cells act as inflammatory factors
Lack of suppression of inflammatory and immune responses - no release of anti-inflammatory cytokines
Onset of autoimmune disorders linked to inefficient removal of apoptotic cells - knock-out mice studies SLE
What are some diseases associated with defects in phagocytosis?
Chronic granulomatous disease - missing NAPH oxidase components
Chediak-Higashi syndrome, hyperpigmentation/albinism - delayed phagosome-lysosome fusion, low chemotaxis
Both lead to recurrent infections
What are the macrophage subsets?
M1- inflammatory
M2 - healing
Regulatory/suppressive
How do macrophages get the M1 phenotype?
Monocytes recruited from circulation/BM
Monocytes differentiate into macrophages that can help activate DCs to stimulate Th1/NK cells which release IFN-g
IFN-g promotes macrophages to become M1-phenotype
What does the M1 inflammatory phenotype produce?
NO
ROS
IL-1
TNF
MMPs
Causes inflammatory microbial response
How does the M2 phenotype develop?
Alarmins (IL-25, IL-33, TSLP) stimulate Th2 cells and other cell subsets to release IL-4 and IL-13
Macrophages develop M2 and later regulatory phenotypes which begin to accumulate
What do M2 macrophages do?
Antagonise M1 and promote wound healing
What do regulatory macrophages do?
Stop wound healing when complete
What promotes macrophage proliferation in situ?
Helminth infection
Give an example of helminth infection promoting macrophage proliferation
Nematode worm infection drive high levels of IL-4 through Th2 and other cells types
High IL-4 promotes M2 macrophage proliferation in situ
M2 macrophages kill pathogen
High IL-4 and M2 macrophage presence can also produce a M1/M2 combined phenotype that is proliferative
