Innate Immunity Flashcards
(42 cards)
What are the essential functions of the innate immune system?
To sense and detect the presence of a pathogen - initially kickstart the immune response
To initiate a rapid response to pathogen, immediately after infection, allows control of the infection at a manageable phase until the adaptive response can determine and initiate a specific response
To eliminate damaged/infected cells and stimulate tissue repair
To alert and direct an appropriate adaptive immune response
What are the key features of the innate immune system?
Distinguishes between danger and homeostasis
Preformed (constitutive - ready to act) or rapidly formed components
Response begins within minutes
Low specificity: the same molecules and cells respond to a range of pathogens
Use pattern recognition receptors on their surface to detect molecules unique to pathogens
Reliant on germline genes that are the same regardless of the pathogen; do not undergo modification to enhance responses
No memory; responds the same at each infection with the same magnitude and same type of response.
What are pathogen-associated molecular patterns (PAMPs)?
- The body initially detects the presence of pathogens by
recognising molecules that are present on pathogens but not on
human cells - These molecules are termed pathogen-associated molecular patterns
(PAMPs)- not present on human cells so once detected by the body’s
innate cells, they know there’s an infection, causing an innate response - Good targets because they differ from molecules found in humans
- They are conserved among classes of microorganisms allowing for
broad recognition, so the same response cells are initiated - no
specificity is required - They have an essential role in the function of the microbe so do not
undergo rapid evolution, hence the conservation
What are examples of PAMPs?
- Lipopolysaccharide (LPS) – Cell wall of G -ve bacteria
- Peptidoglycan – abundant on G +ve bacterial cell walls
- Lipoteichoic acids – found in G +ve bacterial cell walls
- Mannose-rich glycans – Short carbohydrate chains that terminate
with mannose sugar residues - Flagellin – Found in bacterial flagella
- CpG – Unmethylated cytosine-guanine dinucleotides found in bacterial
and viral genomes, in bacteria these are unmethylated (unlike in
mammalian cells, where they are masked to prevent an autoimmune
response) - Single-stranded and double-stranded viral RNA
What are pattern recognition receptors (PRR)?
Human cells recognise PAMPs using a range of pattern recognition receptors
PRRs are:
- highly conserved between all mammals, almost identical between
humans so we will all react the same to an infection
– have broad specificity
– recognise conserved components of pathogens, eg PAMPs
PPRs are on the surface of cells which are likely to come into immediate contact with pathogens, for example epithelial cells
Main forms of PRRs are:
– membrane-bound signallers or endocyclic if pathogens enter cells
– soluble cytoplasmic
– phagocytic/scavenging so only send a signals to the phospholipid
membrane to initiate phagocytosis
– secreted and therefore bind to pathogens in the blood stream, many
bind to one pathogen to make it more obvious to innate response cells
in blood.
What are Toll-like receptors (TLR)?
Toll-like receptors are the best characterised form of PRR,
around 10-11 in humans
Highly conserved; also found in plants and invertebrates
Usually function as homodimers or heterodimers require two (same) molecules or two (different) molecules to bind in order to function.
All are membrane-bound, but their cellular location reflects the PAMPs
that they detect:
– TLRs that recognise extracellular ligands are found on the cell surface
– TLRs that recognise intracellular ligands e.g. Viral RNA that gets
expressed in our cells, are found on endosomes
Where are TLRs distributed on different cell types?
Predominately on innate immune cells that are present in tissues waiting to respond or are recruited into tissues then activated.
Some are on the epithelial surface but not the luminal surface as this is where the microbiome is.
What is the general conserved structure of TLRs?
Generally all have a very similar structure, the extracellular part is made up of a leucine-rich repeating structure that points towards the pathogen and has a complementary PAMP binding site. Then the TLR also has a a transmembrane system that holds the TLR in, then finallly and intramembrane system that contains the Toll-IL-1 receptor (TIR) signal transduction domain.
How do TLRs initiate intracellular signalling?
TLR domains recruit adaptor proteins (eg. Myd88)
The IRAK kinase is activated by autophosphorylation, which then recruits TRAF6. This then activates the kinase TAK1 which moves downstream and activates the IKK complex
IκB normally inhibits NF-κB; it is phosphorylated to release NF-κB. NF-κB translocates to the nucleus to initiate gene transcription
Inflammatory cytokines made that initiates other parts of the immune system:
– IFNα, IFNβ
– IL-1β, IL-6, IL-8 (CXCL8), TNFα
Extra cellular receptors converge on certain pathways giving a lot of cross pathways effects, which means no matter which protein is initiated it causes the same response
What are known defects in TLRs?
Myd88 and IRAK4 deficiencies are autosomal recessive:
- Sufferers are susceptible to invasive bacterial infections from
Streptococcus pneumoniae, Staphylococcus aureus and Pseudomonas
aeruginosa
NEMO deficiencies are X-linked (affect males more than females):
- Associated with skin disease and severe bacterial infections
- Can be detected with the CD62L shedding assay via flow cytometry
- Most people who die from Legionnaire’s disease have a mutation in
TLR5 – unable to recognise flagella of Legionella pneumophila
What are Nod-like receptors (NLRs)?
NOD-like receptors are soluble intracellular (cytoplasmic) PRRs - soluble within the plasma of cells
Contain nucleotide-binding oligomerisation domain (NOD) and leucine-rich repeats at the C-terminus
Essentially seek out intracellular pathogens
4 sub-families:
– NRLA
– NLRB (involved in IL-1 family production)
– NLRC (detect bacterial peptidoglycans)
– NLRP (activate caspase-1 to cleave pro-IL1 into an active secreted
form)
What is the structure of NLRs?
Contain leucine rich repeats like TLRs that are good for recognising pathogens.
NAT domains are activated and become open allowing complexes to form. Don’t activate in isolation but in these complexes they cross each and initiate each other.
Some NLRs have similar signalling roles to TLRs others are associated with formation of the inflammasome
- NFκB induces expression of IL-1β and IL-18 in a pro-(inactive) form
- The inflammasome activates caspases
- Caspases cleave IL-1β and IL-18
What are C-type lectins?
Scavenging and phagocytic, don’t really signal
C-type lectins recognise carbohydrate structures on pathogens
Examples include:
– Dectin-1; binds β glucans
– Mannose receptor (DEC205 or CD206); binds mannosylated ligands,
those bound to a mannose sugar
Often act as phagocytic receptors which once bound to their target they clump around the cell to act as a signal for phagocyte cells; they endocytose the ligand they bind for degradation
What are examples of secreted PRRs?
C-reactive protein (CRP) is released in the acute phase response - wider system response in the innate immune system
CRP has roles in:
– Opsonisation - coating of a pathogen to make it more visible
– Complement activation - both this and above then lead to…
– Phagocytosis
Mannose-binding lectin (MBL) :
– Key role in the Lectin pathway of complement
What is phagocytosis?
Phagocytosis is the key mechanism used to remove free microorganisms in the tissues and in blood
Phagocytic cells: Macrophages, Neutrophils, Dendritic cells, Eosinophils, and B-lymphocytes
Unenhanced attachment - activation of TOL like and NOD like receptors; the recognition of PAMPs by endocytic PRRs on the surface of the phagocytes
Opsonisation (making pathogen more visible to immune system); the attachment of microbes to phagocytes using IgG, the complement proteins C3b and C4b (bind to surface of pathogen due to it’s an ionic structure, if the phagocyte then has a C3b receptor it will then engulf it), and secreted acute phase proteins such as MBL and CRP.
Polymerisation and then depolymerisation of actin filaments sends out cell membrane extensions (pseudopodia - reach actin filaments around and then build a plasma membrane around it) out to surround and engulf the microbe (or necrotic cells / tissue debris)
This endocytic vesicle is called a phagosome.
What occurs during phagosome maturation?
An electron pump brings protons (H+) into the phagosome; this lowers the pH within the phagosome from 7.4 to 4.5 so that the acid hydrolases are activated in order to break down cellular proteins.
Phagosomes contain membranous sacs called lysosomes that contain various digestive enzymes, microbicidal chemicals, and toxic oxygen radicals.
The lysosomes fuse with the phagosomes to produce a phagolysosome so molecules are released to increase degradation.
Microorganisms are killed and digested by lysosomal enzymes
Which cells are activated by PRRs?
Tissue macrophages act as sentinels for initial detection of danger for pathogens via expression of PRRs
Cytokines and chemokines secreted affect endothelial cells lining nearby blood capillaries
What are the vascular effects of pattern recognition?
Cytokines secreted by macrophages and other local cells that express PRRs induce changes in endothelial cell walls of blood capillaries, loosens the contact between the endothelial cells
This allows infiltration of inflammatory cells (neutrophils and monocytes) and plasma proteins to the site of infection this leads to responses such as inflammation
Vascular changes lead to characteristic signs of inflammation:
– Vasodilation (increased vascular diameter)
– Tumor (swelling; due to increased vascular permeability)
– Calor (heat)
– Rubor (redness)
– Dolor (pain)
What are the aims of inflammation?
Soon after initial damage and pathogen encounter, neutrophils (main phagocytic cells) are recruited to the infection site, followed by monocytes
Activated tissue macrophages (resident, and differentiated monocytes) and damaged tissue cells continue to release cytokines and chemokines that perpetuate the inflammatory response - keep immune response going until adaptive response can occur
Purpose of inflammation:
– Deliver effector molecules and cells to the infection site (plasma leaks
into tissue; contains antibodies, complement, antimicrobial peptides,
transferrin)
– Produce barriers that prevent spread of infection (eg. activate
fibrinogen that make fibrin clots that prevent spread of infection to new
areas)
– Promote repair
How does diapedesis of neutrophils occur?
Rather than free flow, the neutrophil circulates in peripheral blood by rolling in contact with the endothelial cells through to the site of inflammation.
They circulate until pass chemoattractants (TNFα, IL-1β) released by macrophages at site of inflammation signal to them, this slows the rolling down in the capillaries, allowing the neutrophil to be activated.
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Slowing occurs because the endothelium is induced to express selectins: expression of P- E and L- selectins allows rolling then tethering of neutrophils via Sialyl Lewis X
If it finds cells that have been activated through the innate response adhesion occurs, where integrins (eg VLA4, LFA-1) are upregulated on the neutrophils and bind to VCAM1 and ICAM1 on endothelium.
TNFα increases vascular permeability and allows diapedesis, or movement of neutrophils across the endothelium (where there are gaps between endothelial cells the neutrophils will squeeze through into the damaged tissue)
Neutrophils move towards the site of infection in response to chemokines and complement components such as C5a and C3a.
What is the role of neutrophils?
Chemotaxis – Phagocytosis – Chemokines – Killing – Digestion
Responds to chemotactic factors released from damaged tissue
Rolls and attaches to the endothelial cell wall:
- protein and carbohydrate interactions allow them to exit the capillary at
the correct site close to infection (selectins/integrins and their ligands)
Become activated by chemotactic factors (eg. CXCL8)
Tightly adheres through the integrin family of proteins
Migrates across the endothelial cell wall
Phagocytose microorganisms so that they are contained within a vesicle or phagosome
Releases granule products and reduced oxygen species
(eg. hydrogen peroxide and superoxide) to kill organisms
How do neutrophils undergo a respiratory burst?
A respiratory burst is a so specialised intracellular killing mechanism.
Neutrophils contain cytoplasmic granules which fuse to the phagosome, (giving a species slightly different to phagolysosme as the granule contents are different)
Signalling induces assembly of membrane-associated NADPH-oxidase:
- 6 diff subunits; one subunit transfers an electron to O2 forming superoxide anions (O2–) in lumen of phagolysosome
- This is converted to H2O2 by superoxide dismutase
- Other chemicals are also produced via conversion of H2O2 (hydroxyl
radical, hypochlorite and hypobromite)
- This disrupts the molecular structures on the surface of the pathogens
This NADPH oxidase reaction results in transient oxygen consumption by the cell = RESPIRATORY BURST
How do neutrophil defects lead to poor bacterial clearance?
Defects in structural components of neutrophils are characterised by persistent chronic infections that are autoimmune disorders, leading to repeated bacterial infections.
Defects can occur in:
- Adhesion to endothelium (No rolling / diapedesis)
- Neutrophil chemotaxis (Can’t get to site of infection)
- Phagocytosis (Can’t engulf bacteria)
- Killing of bacteria (Can’t release granule contents)
What common diseases are associated with neutrophil defects?
Disorder: Leucocyte Adhesion Deficiency Type II Phase: rolling Molecular defect: - Sialyl Lewis X carbohydrate epitope - SLC35C1
Disorder: Leucocyte Adhesion Deficiency Type I
Phase: adhesion
Molecular defect:
- Deficiency in CD18, the β2 integrin subunit, preventing expression of
LFA-1 (cannot heterodimerise with CD11)
- ITGB1
Disorder: Chronic Granulomatous Disease
Phase: oxidative burst
Molecular defect:
- NAPDH Oxidase Subunit; Cannot produce ROS
- Extended lifespan and granuloma formation
- Gp91phox, p67phox, p47phox, p22phox
Disorder: Myeloperoxidase Disease Phase: oxidative burst Molecular defect: - Cannot produce hypochlorous acid (HOCl) - Usually clinically silent - MPO
Disorder: Chediak-Higashi Syndrome
Phase: Granule Release
Molecular defect:
- Large granule size and failure to release contents
- LYST ; Lysosomal trafficking regulator gene
