Immunology II Flashcards
Innate immunity:
Mechanisms of innate immunity discriminate very effectively between host cells and pathogens
Innate immune defenses exist in all individuals and act within minutes-hours after an encounter with infectious agent
Only when innate defenses are overwhelmed/bypassed/evaded is an adaptive immune response required.
Chemical barriers:
Lysozyme:
-Present in secretions (mucus, tears, milk, saliva)
-Uses hydrolysis to break apart the peptidoglycan wall => lysis of bacterial cell wall
Antimicrobial peptide-defensins:
-Small, heterogeneous, cationic peptides
-Kill gram-negative & gram-positive bacteria, some enveloped viruses, fungi
Multiple antimicrobial effects:
-Destabilize membranes and pore formation in bacterial cell walls
-Proteolytic degradation of bacterial proteins
-Inhibit viral binding and entry
-Inhibit virus particle assembly
Defensins-prototypical AMPs:
Defensins can act as a chemical barrier when they are secreted by epithelial cells in a variety of mucosal surfaces.
Defensins and other AMPs (ex: cathelicidins) are also stored in neutrophil granules and can be released within tissues in response to inflammation:
-can kill microbes extracellularly => released when neutrophils die during inflammation
-Can kill microbes intracellularly after a cell (ex: neutrophil) phagocytoses a pathogen
Just like many molecules and cells of the immune system, defensins perform a number of roles- not just a chemical barrier.
Phagocytosis and phagocytes:
One of the first lines of defense if microbes do invade tissue.
Engulf and destroy microorganisms, especially bacteria.
Key role in innate immunity as they can recognize, ingest and destroy many pathogens without aid of an adaptive immune response:
-Phagocytosis can also occur after an antibody has bound to an antigen- the antibody can act as a signal that triggers efficient phagocytosis
Macrophages and neutrophils are the major phagocytes in the body.
Phagocytic cells:
-Monocytes & macrophages
-Neutrophils
Monocytes & macrophages:
-Pro-monocytes (BM) => monocyte (blood) => macrophage/macrophage-like cells (tissues)
-Long-lived cells resident within the tissues
Neutrophils:
-Derived from hematopoietic precursors in the BM
-Non-dividing, short-lived cell type in blood (dominant WBC)
Pattern recognition:
-Evolutionarily conserved mechanism for recognizing common, conserved ‘signs’ of microbial infection, physiological stress, or other damage
-Recognition is immediate, does not require prior recognition, and activates several arms of the innate (and adaptive) immune response
-Responses are elicited via the engagement of pattern recognition receptors (PRRs) found on phagocytes, in response to:
-pathogen associated molecular patterns (PAMPs)
-Danger associated molecular patterns (DAMPs)
Pattern recognition receptors:
Examples of PRRs:
-Toll-like receptors
-Nod-like receptors
-Lectins
Elicit responses such as:
-Phagocytosis
-Cytokine secretion
1st step of phagocytosis:
1) patter recognition receptor (PRR) binds to a microbe or bit of debris, OR an opsonin created by another cell binds to the microbe.
-A microbe
-A bit of debris
-An opsonin
Opsonin:
a soluble, secreted PRR that enhances the effectiveness of phagocytosis.
Coats a microbe, the phagocyte has receptors for parts of that opsonin.
2nd step of phagocytosis:
The microbe is engulfed-the PRR receptors signal the cell membrane to approach, coat and then surround the sites where the receptor is bound:
-forms a phagosome
-Mediated by intracellular signaling events and actin polymerization: PI3 kinase seems to be important here
3rd & 4th step of phagocytosis:
Microbe killing: phagosomes fuse with lysosomes as well as (in neutrophils) primary and secondary granules.
-phagosomes have many molecules that are effective at cellular killing-a little more later
-Major groups include:
-reactive oxygen species
-“pore”-forming proteins or peptides
-Hydrolytic enzymes
-pH changes- ex: acidic environment of the lysozome
5th step of phagocytosis:
The microbe remnants are either digested and used, or can be excreted from the phagocyte.
Microbe killing:
After the microbe has been phagocytosed, the phagosome will dock with a lysosome and/or neutrophil granules.
-Lysosomes can pretty much break down anything (acid hydrolases)
-The low pH of a lysosome is also unpleasant for many bacteria
NADPH complex:
-becomes associated with the membrane of the phagolysosome
-Uses a large amount of oxygen (respiratory burst)
-If a particle is too large to phagocytose, macrophages will surround it and place their NADPH oxidases close to it to try to kill it.
Macrophages in particular are also capable of killing cells by inducing the synthesis of nitric oxide as high concentrations.
Neutrophils have a multitude of pore-forming molecules within their granules-these granules will fuse with the phagosome.
Neutrophil granules:
Defensins are very rich in cysteine:
-form voltage-dependent pores in bacteria that are permeable to water
-Cause lysis
Cathepsin: a type of protease
Cathelicidins: another pore-forming molecule
-causes lysis, multitude of different structures
Lysosome: a glycoside hydrolase
-Doesn’t require an acidic pH
-Found in a variety of glandular secretions
-Great at killing gram + (ve) bacteria
Lactoferrin: iron-binding protein that interferes with iron metabolism in microbes.
When ________ are in an environment with many bacteria (they’re “surrounded”) they can lyse and release their DNA into the ECF.
Neutrophils:
-Known as NET- a neutrophil extracellular trap
-NETs are “sticky”: most bacteria are trapped in the chromatin
-Histones are toxic to many bacteria
-The granule contents will remain close to the NETs and help with killing bacteria, even after the neutrophil itself is dead.
How do phagocytes recognize that something is a “target” for phagocytosis?
What’s an opsonin and how are they involved in the phagocytosis process?
Toll-like receptors:
Family of 10 cell membrane receptors with variability specificity for a range of pathogens
Ligands can include LPs, dsRNA, ssRNA, DNA, Flagellin
Nod-like receptors:
-Family of intracellular receptors found in the cytoplasm that detect products derived from the intracellular degradation of phagocytosed pathogens (ex: components of bacterial cell wall)
-Also recognize DAMPs associated with cellular stress
-Activates expression of inflammatory cytokines
Step 1 of acute inflammation:
A. alteration of vascular caliber-vasodilation:
-leads to increased blood flow at the capillary bed due to arteriolar dilation, dilation of precapillary sphincters
-Nitric oxide and histamine
-A variety of prostaglandins (PGI2, PGE2, PGD2)
-Platelet activating factor (at low concentrations-higher concentrations cause vasoconstriction)
-Complement: C5a and C3a stimulate histamine release
At low concentrations, nitric oxide is a potent vasodilator (why Viagra is a profitable drug)
At high concentrations, it’s capable of destroying both microbes and host cells since it’s a free radical:
-higher concentrations produced by an inducible nitric oxide synthase in macrophages.
Vasodilation:
Arterioles and pre-capillary sphincters dilate leading to vastly increased blood flow in inflamed tissue
Vasodilation and fluid loss (due to increased permeability) lead to slower blood flow
-known as vascular congestion
-This helps with margination of leukocytes
Info you need to know about this system:
-prostaglandins and leukotrienes are produced when PLA2 generates arachidonic acid from membrane phospholipids
-Different types of cyclooxygenases produce different types of prostaglandins from arachidonic acid:
-Important prosaglandins: PGE2, PGD2, and PGI2 cause vasodilation and increase vascular permeability, important acute infammatory mediators
-Different types of 5-lipoxygenase produce different types of leukotrienes from arachidonic acid that seem particularly important in lung tissue:
-LTB4: important chemotactic agent
-Other LTs: increased vascular permeability and smooth muscle constriction (think asthma)
-Lipoxins are generated from arachidonic acid by 12-lipoxygenase: they decrease inflammation
Vascular permeability:
Increased vascular permeability is due to contraction of endothelial cells:
-Occurs mainly in venules
-Often short-lived
Another mechanism is endothelial damage:
-Can be caused by trauma, burns, microbial damage
-Can also be caused by leukocyte-mediated damage to the endothelium (often longer-lived)
Increased transcytosis can also result in leakage of plasma components into the interstitial space.
Transcytosis:
Active, vesicle-mediated transport across the capillary endothelial cell
Large molecules can move across the endothelium via:
-Pinocytosis (caveolin pathway)
-Receptor-mediated endocytosis
Mechanisms of increased vascular permeability:
2 major chemokine families:
CXC:
-CXC chemokines attract neutrophils, are angiogenic, and are very similar in structure
-The “x” indicates the location of a disulphide bond
CC:
-act on/attract a wide variety of other leukocytes
Steps 1 & 2 of emigration and activation of leukocytes:
a) Margination: leukocytes migrate towards vessel wall
b) Rolling: formation & dissociation of adhesion bonds between leukocytes and endothelial cells
Steps 3, 4, & 5 of emigration and activation of leukocytes:
c) stable/tight adhesion: formation of tight/stable adhesion bonds between leukocytes and endothelial cells
d) diapedesis/transmigration: leukocyte migrates through endothelium
e) Chemotaxis of leukocytes to sites of injury or infection
