Phagocytes Flashcards
What are the main types of professional phagocytic cells?
Neutrophils
Macrophages
Dendritic cells
How do neutrophils, macrophages, and dendritic cells differ in their functions?
Neutrophils - pathogen clearance and killing
Macrophages - sentinel cells, regulate inflammation and remove apoptotic bodies
Dendritic cells - present antigens to cells for adaptive immunity
What is the primary function of phagocytic cells in immunity?
Eat pathogens
How do neutrophils contribute to immune responses during infection (3)?
Phagocytosis: Engulfing and digesting pathogens like bacteria and fungi.
Antimicrobial Release: Releasing reactive oxygen species, antimicrobial peptides, and enzymes to kill pathogens.
NET Formation: Forming neutrophil extracellular traps (NETs) that capture and kill pathogens outside the cell.
Why are neutrophils referred to as being in the “right place at the right time”?
Quickly recruited to site of infection by macrophages releasing cytokines
What are the four main types of neutrophil granules, and what molecules do they contain?
Primary - directly bactericidal enzymes e.g. myeloperoxidase, defensins etc…
Secondary - 2 bactericidal (cathelcidin, lysozyme), 1 bacteriostatic (lactoferrin), 1 promoting neutrophil migration to site of infection (collagenase)
Tertiary - fewer bactericidal (lysozyme) and bacteriostatic (NGAL), mainly promoting neutrophil migration to site of infection (MMP-9, MMP-25)
Secretory vesicles - mainly contain cell surface receptors used for recognition and internalisation of pathogens (CR3, CD14, CD16)
What happens inside the neutrophil phagosome/ What are the key steps of receptor-mediated phagocytosis in neutrophils ?
- Opsonisation: serum proteins including complement and ig opsonise the surface of microbes
- Receptor mediated phagocytosis: neutrophils express numerous cell surface receptors, including those for complement opsonins and ig. Engagement of these receptors initiates phagocytosis
- Phagosome maturation: end of phagocytosis, beginning of pathogen killing.
- Critical events include:
§ Fusion of granules with phagosome and delivery of granule contents
§ Assembly of NADPH oxidase (subunits self assemble) - Generation of primary ROS: generation of HOCl via MPO and secondary ROS
How do neutrophils kill bacteria through oxidative mechanisms?
Oxidative mechanisms by producing reactive oxygen species (ROS)
E.g. superoxide radicals, hydrogen peroxide (H2O2), and hypochlorous acid (HOCl)
They damage bacterial membranes, proteins, and DNA, ultimately leading to the death of the pathogen
What role does the NADPH oxidase play in oxidative killing?
Generates the ROS
What is the role of phosphorylation of p47phox in NADPH oxidase activation?
RESTING: structurally and functionally constrained state within the cytoplasm
PRIMING: priming agents such as TNF-a and GM-CSF activate MAP-kinase which phosphorylate p47phox, Pin1 can now interact with p47phox inducing a conformational shift
ACTIVATION: When neutrophils are activated by bacterial ligands the minor conformational change to p47phox permits multiple phosphorylation events by PKC, causes a major structural shift and initiates the assembly of NADPH oxidase
How do priming agents like TNF-α and GM-CSF enhance NADPH oxidase activity?
Exposure to ‘priming agents’ (e.g. TNF-a, GM-CSF) enhances subsequent neutrophil responses to bacterial stimuli (e.g. fMLF)
Why does neutrophil oxygen consumption increase tenfold during phagocytosis?
To fuel the respiratory burst, a process that generates reactive oxygen species (ROS) essential for killing engulfed pathogens.
What are the principle mechanisms of anti-bacterial action?
Damage to lipids and proteins in bacterial cell membrane
Loss of membrane polarity»_space;> inability to use chemostatic processes to drive respiration/electron transport and translocation events
Inhibition of genomic DNA binding activity - the prokaryotic DNA ‘replisome’ is anchored to the cell membrane
What is the action of proteases in non-oxidative bacterial killing?
Proteases e.g. lysozyme and also ‘serine-proteases’
KEY FEATURES: stored in granules in complex with proteoglycans (maintain the proteases in an inactive state)
MOA: proteases become active due to changes in phagosomal pH. They target bacterial proteins in cell membrane and act in synergy with other anti-bacterial proteins and ROS
How do proteases limit damage to host tissues?
Containment within the phagosome (limited exocytosis)
Presence of numerous rapid and irreversible protease inhibitors in human tissues and blood
What is the action of iron sequestration in non-oxidative bacterial killing?
E.g. NGAL (neutrophil gelatinase-associated lipocalin), Lactoferrin
MOA:
- iron is fundamental to a range of biological processes in bacteria and humans
- e.g. ETC, DNA replication and free radical scavenging
- Free iron is sequestered by proteins in the body to prevent bacterial growth and iron induced free radical formation
- Bacteria expresses ‘sidephores’ to scavenge iron from their host; NGAL binds to and sequesters these ‘sidephores’
- Lactoferrin sequesters and free iron the bacteria may use. It also has direct anti-bacterial action through interaction with the bacterial membrane
What is the action of anti-bacterial proteins in non-oxidative bacterial killing?
E.g. Defensins, cathelcidins
MOA:
- Bind to negatively charged bacterial membranes/lipids (they are positively charged)
- Disrupt biofilms
- Form pores in bacterial membranes; disrupt integrity and polarity of membrane
- Directly interfere with function of bacterial proteins
- Increasing recognition of immunomodulatory roles; signalling to other immune cells
What are neutrophil extracellular traps (NETs), and how do they function in bacterial control?
Complex of DNA, histones and anti-microbial proteins extruded from dying neutrophils -NET-osis
Capable of trapping bacteria
Prevents spread of infection
Promotes phagocytosis
May also kill directly via proteases and anti-microbial proteins
What is myeloperoxidase, and how does it contribute to oxidative bacterial killing?
Enzyme found in neutrophil granules
Catalyses the production of hypochlorous acid (HOCl) from hydrogen peroxide and chloride ions
Contributes to oxidative bacterial killing by damaging microbial cell structures
