Innate killing Mechanism Flashcards
Secretory molecules
enzymes/proteins that are secreted into lumen of tissue (extracellular) into cornea, saliva, lumen of gut or into skin, and control microbe growth. Can be secreted by cells of the innate immune system or epithelial cells.
Lysozyme
secreted in fluids of cornea, saliva and gut (Paneth cells)
– breaks down peptidoglycan, a component of the bacterial cell wall
Phospholipase A2
– breaks down phospholipids in cell membrane of bacteria
Examples of Antimicrobials peptides
Defensins, Cathelicidins, Histatins, Lecticidins
Defensins
(alpha-defensins in Paneth cells of the intestinal tract and beta-defensins in lung and skin) – disturb the cell membrane of bacteria and fungi
Cathelicidins
antimicrobial defensins produced by activated neutrophils and epithelial cells
Histatins
(oral cavity, active against fungi)
Lecticidins
creates a pore in microbial cell membranes)
Complement exists in
fluids and blood
Roles of complement
– capable of direct killing of pathogens through membrane attack complex (MAC)
– also potentiates phagocytosis and the recruitment of other inflammatory mediators
What is complement?
Number of small proteins that are synthesized by the liver, tissue macrophages, blood monocytes, and epithelial cells of the genitourinary system and gastrointestinal tract. These small proteins circulate in the blood as inactive precursors. These circulate in blood and can diffuse into tissues
3 Phases of Alternative Complement Cascade
1) Activation
2) Amplification loop
3) Effector functions
Overview of Alternative Complement
- Spontaneous low level activation
- If deposited onto pathogen = Activation
- If deposited into host cell = Inhibited
Complement cascade membrane attack complex (MAC)
- C5b binds to other components of cascade; c6, c7, c8 and c9
- C9 oligomerises which forms a pore around the membrane forming a channel of consecutive units
- When this occurs on bacterial cell then it has a pore in its membrane, which are visible as rings in the bacterial membrane, if you look at a side view you can see tubes.
- Creating pores and directly killing the pathogens.
How to prevent damage of complement?
- activation of complement is highly regulated
- to prevent complement-mediated damage of normal host cells
- The same regulatory mechanisms exist for many other killing mechanisms
Intracellular Killing Mechanims = phagocytosis is a very
- ancient process: means eating (or kills an invader)
* Example: Amoeba eats a paramecium
Where does phagocytosis mostly occur?
in neutrophils and in monocytes/macrophages/dendritic cells (creates harsh environment for microbes)
Phagosome killing
- The phagolysosome is matured through sequential fusion with endosomes and lysosomes and becomes a very specialized intracellular compartment
- very unpleasant environment for microbes
Phagocytosis - microbes recognised by phagocytic surface receptors (2 ways)
PAMPS - PRR
and Opsonisation
Opsonisation (2 types)
- Microbe coated with complement = binds CR3-CR4
2. Microbe coated with antibody = antibody binds FcReceptors
Recognition via PRR-PAMPS and Opsonisation causes
subsequent signalling which trigger PHAGOCYTOSIS (ingestion process + subsequence maturation of the compartment which is formed in the cell)
Phagosomal Maturation
- When cells first ingested a microbe with extra cell or milia
- Host cell will manipulate the phagosome to make a harsh environment:
- Endosomes and lysosomes contain enzymes and proteins that will fuse with phagosome to change its composition to create a harsh environment.
- RAB5 positive endosomes fuse with nascent phagosome (early phagosome)
- Followed by endosomes that have EEA1 fusing to create intermediate phagosome.
- After EEA1 is in the intermediate phagosome, the phagosome loses its RAB5, signaling RAB7 to start merging with a phagosome and is now known as the late Phagosome.
- Lysosomes deliver hydrolytic enzymes which exist in the fibre lysosome.
- At the same time, the endosomes fuse + they deliver components into the lumen into the interior of the vacuole which is the phagosome
- Lots of molecules in the membrane (transporters) which help create this environment i.e.
o Acidity
Acidity in Phagosomal maturation
= the V-ATPase proton channel will start pumping H+ ions into the phagosome to make it acidic, once the lysosomal enzymes, the acidic hydrolyses fuse then they become activated but they only work in acidic pH
Overview of phagosomal maturation
Overview:
• The phagolysosome is matured through sequential maturation with lysosomes and becomes a very specialized intracellular compartment with:
• Low pH (pH=4.5): mediated by a V-ATPase proton channel
• Accumulation of reactive oxygen species (NADPH oxidase - subunit gp91 or NOX2, and others) oxidative stress
• Accumulation of nitric species (iNOS) nitrosative stress
• Antimicrobial enzymes : cathelicidins, defensins, acid hydrolases, which are secreted into lumen of phagosome.
• Low nutrients: no sugars, no micronutrients (Fe, Cu, Zn, etc), which are pumped out of the phagosome or sequestered (lactoferrin).
• Phagosome maturation to phagolysosome takes 30-60 min.
Phagosome killing and more - phagocytosis mostly occur in
neutrophils and in monocytes/macrophages/dendritic cell. Different phagosomes for each of the cell types
Phagosome killing and more - what are the major differences between cell types
- In dendritic cells, phagosome is adapted to perform antigen presentation: to degrade antigens and load them to bridge adaptive immunity.
- In neutrophils, due to their specific granules phagosome is basic (pH=8 for about 30 min) to facilitate optimal activity of elastase and cathepsin G.
Granules
collections of enzymes which have proteins which don’t necessarily exist in macrophages.
