Innate Immune Recognition and Effector Mechanisms Flashcards
what are PAMPs?
pathogen associated molecular patterns
relate PAMPs to microbes and host tissues
common to many microbes but absent from host tissues
give 6 examples of PAMPs
gram positive bacteria: peptidoglycans
gram negative bacteria: lipopolysaccharide
acid fast bacteria: glycolipids
bacteria: unmethylated CpG
yeasts: mannan-B-glucan
viruses: dsRNA
in terms of signals in the immune response, what is PAMPs binding to PRRs?
the first signal
give 3 examples of bacterial PAMPs
- bacterial flagellum (flagellin protein)
- cell wall (peptidoglycan)
- capsule (capsule polysaccharides)
what are PRRs?
pattern-recognition receptors
what are the 2 kinds of PRRs?
signaling and phagocytic
what are 3 locations of signaling PRRs?
- cell surface
- cytoplasmic
- endosomal
what are 3 types of signaling PRRs?
- toll-like receptors
- NOD-like receptors
- RIG-1-like receptors
describe toll-like receptors
membranous and recognize a wide variety of ligands
where are TLRs 1, 2, 4, 5, and 6 located? what specifically do they recognize?
located in plasma membrane and recognize bacterial, fungal, and parasite wall components
what does TLR-4 recognize?
lipopolysaccharides
what does TLR-5 recognize?
flaggellin
where are TLRs 3, 7, 8, and 9 located?
located in the endosomal membrane and recognize bacterial/viral RNA and DNA
why is inflammatory bowel disease common in german shepherds?
due to single polymorphisms in TLR4 and TLR5 which leads to a reduced ability to defend against bacteria in the intestine
describe how PRR signaling works (4)
- PRRs bind their PAMPs, leading to a
- signaling cascade, which results in
- activation of a transcription factor (either NF-kB or IRF), which translocates to the nucleus an results in
- activation of transcription of genes
list 4 kinds of genes that are transcribed in response to PRR signaling
- pro-inflammatory cytokines
- chemokines
- adhesion molecules
- co-stimulatory molecules
why don’t normal flora trigger enteritis?
TLR5 specifically recognizes flagellin; TLR5 is present on basolateral surfaces of enterocytes and should only be triggered by pathogenic bacteria
how do genetic defects lead to IBS/IBD?
if there is a genetic defect in TLR5 then it may be activated by normal flora instead of just pathogenic bacteria
what are the 2 NOD-like receptors, where are they located, and what do they do?
NOD1 and NOD2 are located in the cytosol and recognize peptidoglycan of intracellular bacteria
how do NOD-like receptors signal?
through NF-kB
where are RIG-1-like receptors located, what do the recognize, and what do they do?
in the cytosol; recognize dsRNA and ssRNA (since viral does not have a 5’ cap); lead to expression of type I IFNs (anti-viral responses)
how do RLRs signal?
NF-kB
what recognized viruses in endosomes?
TLRs (NLRs and RLRs are both cytosolic)
what do all the signaling PRRs have in common?
activation of NF-kB
where are phagocytic PRRs found? (3)
- neutrophils
- macrophages
- DCs
what do phagocytic PRRs do? (3)
- trigger phagocytosis
- microbe degradation
- antigen processing
what are 3 kinds of phagocytic PRRs?
- complement receptor
- FcyR
- mannose receptor
describe complement receptor phagocytic PRRs
recognize microbes coated with complement components
describe FcyR phagocytic PRRs
recognize microbes coated with Ig (FC portion of antibody)
describe mannose receptor phagocytic PRRs
recognize sugar residues on microbes
in terms of opsonization, what recognizes C3b?
complement receptors 1 and 3 (phagocytic PRRs)
in terms of opsonization, what recognizes IgG?
FcyR (phagocytic PRRs)
what is the purpose of opsonization?
allows for more efficient phagocytosis than through PRRs alone
what are the 4 steps of phagocytosis?
- recognition and attachment
- engulfment
- phagosome-lysosome fusion
- destruction
how is recognition and attachment of phagocytosis accomplished?
cytoskeletal rearrangement
how is destruction accomplished in phagocytosis? (4)
- lysozomal enzymes
- reduced pH
- reactive oxygen species (respiratory burst)
- reactive nitrogen intermediates
describe the membrane and cytoskeletal reorganization and engulfment of phagocytosis (4)
- ligation of PRRs leads to tight adhesion and receptor clustering
- this signals cytoskeletal rearrangement
- this causes involution of plasma membrane with microbe attached
- this leads to the creation of the phagosome
describe the maturation of the phagosome in phagocytosis (4)
- the pH drops from neutral to acidic within the phagosome
- the fusion of the phagosome with granules or lysosomes results in a phagolysosome with hydrolytic enzymes and antimicrobial peptides
- generation of reactive oxygen species (oxidative burst)
what activates the hydrolytic enzymes and antimicrobial peptides of the phagolysosome?
the drop in pH that occurs within the phagolysosome
describe the oxidative burst that generate4s reactive oxygen species within the phagolysosome (6)
- NADOH oxidase is recruited and converts oxygen to superoxide anion (O2-)
- O2- reacts with H+ and H2O to form hydrogen peroxide (H2O2)
- H2O2 can be further converted to hydroxyl radical (OH-)
- neutrophils have myeloperoxidase, which combines H2O2 with Cl-, resulting in a hypochlorite ion (ClO-)
- inducible nitric oxide synthase is upregulated, which converts arginine into nitric oxide (NO)
- NO can combine with O2- to make peroxynitrie (ONOO-)
what is the purpose of the free radicals produced within the phagolysosome?
extremely damaging to lipids, DNA, and protein in the phagosome
where are soluble PRRs located?
floating around
what are 2 kinds of soluble PRRs?
- mannose-binding lectin
- C-reactive protein
describe mannose-binding lectin, a soluble PRR (3)
- produced by the liver then enters plasma
- binds mannose residues on microbes
- activates complement
describe C-reactive protein, a soluble PRR (5)
- major acute phase protein of primates, rabbits, dogs, pigs
- produced by the liver then enters plasma
- binds microbial polysaccharides and glycolipids
- binds phosphocholine associated with bacteria and damaged host cells
- activates complement, enhances phagocytic clearance
what recognizes IgG bound to microbes?
FcyR
what phagosomal enzyme is NOT in macrophages?
myeloperoxidase
what does cell injury release?
releases cytosolic/nuclear protein and metabolites
what does tissue damage release?
extracellular matrix components
what is tricky about what is released from cell or tissue damage?
pathogenic microbes can also cause that cell or tissue damage and release the same thing; so the immune system needs a way to distinguish from normal cell turnover
how does the immune system distinguish an attack by a foreign invader versus normal cell turnover?
DAMPs! damage associated molecular patterns
in terms of stimulating an immune response, what do DAMPs provide?
the second signal (PAMPs binding to PRRs provide the first signal)
what are the 2 types of DAMPs?
intracellular and extracellular
give 5 examples of intracellular DAMPs
- mitochondrial DNA
- HMGB-1
- adenosine or ATP
- uric acid
- heat shock proteins
give 4 examples of extracellular DAMPs
- heparan sulfate
- hyaluronic acid
- fibronectin
- peptides from collagen or elastin
what is the goal of the 1st and 2nd signal in the innate immune response?
activating IL-1b and IL-18
what are 5 results of PRR signaling in the innate immune response?
- phagocyte activation
- cytokines
- chemokines
- adhesion molecules
- lipid mediators
what is the result of phagocyte activation?
upregulation of respiratory burst enzymes, phagocytic receptors, and antigen presenting molecules
what is the function of cytokines (2)
- induce production of more leukocytes
- start inflammatory cascade
what is the role of chemokines
leukocyte recruitment
what is the role of adhesion molecules?
allow leukocytes to stop and migrate from blood to tissues
what is the role of lipid mediators? (2)
- arachidonic acid cascade
- platelet activating factor
PAMPs and DAMPs must work TOGETHER to coordinate what response?
IL-1b production and activation
what response can DAMPs coordinate by themselves?
inflammasome production
summarize the innate immune response
- sentinel cells recognize and respond to PAMPs and DAMPs through engagement of PRRs
- signaling PRRs (TLRs, NLRs, RLRs) ligand binding results in activation of NF-kB or IRF pathways, resulting in transcription of genes involved in inlammation
- phagocytic PRRs trigger engulfment of microbes, leading to phagosomal degradation involving enzymes, low pH, and free radicals
- recognition of both PAMPs and DAMPs triggers formation of the inflammasome, leading to activation of IL-1b and IL-18
- signaling through PRRs leads to cell activation, production of inflammatory cytokines… see question about all that
what are 5 extracellular effector mechanisms of neutrophils?
- proteases
- myeloperoxidase
- defensins
- cathelicidins
- NADPH oxidase
what is the function of the NADPH oxidase extracellular effector mechanism of neutrophils?
allows respiratory burst to occur in extracellular space too
is the formation of neutrophil extracellular traps (NETs) an active or passive process?
active process
when do neutrophils make NETs? (2)
in response to
1. cytokines (IL-8) and
2. PAMPs (LPS)
what are NETs composed of? (5)
- DNA
- histones
- elastase
- defensins
- myeloperoxidase
what is the function of NETs?
microbicidal
give an example of where NETs are seen in vet med
when actinobacillus pleuropneumoniae causes severe pleuropneumonia in pigs; on histology will see well-demarcated areas of necrosis bordered by neutrophils and NETs caused by bacterial leukotoxin
what is the main substance eosinophils contain that make them cytotoxic to helminths?
a major basic protein: eosinophilic granule protein
describe where NK cells come from and how they are unique
have a lymphoid lineage but don’t need prior sensitization and don’t express T or B cell receptors
how do NK cells recognize other cells?
2 receptors
1. activation receptor
2. inhibitory receptor
what do activation receptors on NK cells recognize? (3)
- viral proteins
- altered surface glycoproteins
- antibody-coated cells
what do inhibitory receptors on NK cells recognize?
normal MHC class I, which is expressed on all nucleated cells as a marker of “self” and overrides activation signaling
what happens to NK cells if their inhibitory signal is absent?
they release the lytic granules perforin and granzyme
what is the role of the perforin and granzyme released by NK cells?
perforin: inserts into the membrane of an invading cell, forming a pore
granzyme: enters the pore and triggers apoptosis
what are the effector mechanisms of the innate immune system? (3)
- phagocytosis and degradation
- extracellular granule release
- NK cell targeted cytotoxicity
give what kind of immune challenge each effector mechanism of the innate immune system is good for
- phagocytosis and degradation: extracellular and intracellular infections
- granule release: extracellular and helminth infections
- NK cells: mainly fight viral infections and cancer
what is phagosomal maturation
the process of acidification and fusion with lysosomes, as well as a respiratory burst
describe NETs
formed in an active process and are a microbicidal combination of extruded DNA, histones, and granule components
how do NK cells recognize and kill cells?
through signaling between activating and inhibitory receptors
describe how the innate immune system relates to gout
gout is an inflammatory condition caused by uric acid, which is also a DAMP that leads to activation of the inflammasome and robust IL-1 activation, but treatment that blocks IL-1 makes a patient prone to infection, so specifically blocking NLR and inflammasome formation is a more targeted tx
describe how the innate immune system is related to lupus erythematosus
this disease is caused by B cell autoimmunity to nucleic acids or their binding proteins, leading to phagocytosis of DNA or RNA immune complexes by FcyR and activation or TLRs 7 and 9 in phagosomes and also activation of B cells and DCs to produce autoantibodies and IFNa; can be treated with quinine-containing compounds to prevent endosomal acidification (which would be required for TLR7 and 9 signaling)
how is the innate immune system, specifically PRRs, related to vaccination?
adjuvants are microbial products added to vaccines to trigger TLRs, NLRs, and RLRs to make the vaccine invoke a more robust immune response, increasing vaccine efficacy
how is innate immunity related to cancer treatment?
adjuvant are added to chemotherapy to induce an inflammatory response against cancer cells
when does frustrated phagocytosis occur? what is a result of this?
when the offending antigen is too big or the target is a surface; can result in bystander damage of damage to host cells or extracellular matrix
why is it important that all the components of the innate immune system work together?
MHC I presents peptides to cytotoxic T cells, but some viruses have evolved to downregulate cell expression of MHC I to hide from cytotoxic T cells but never fear! NK cells can recognize this downregulation as abnormal and destroy the virus anyway
