Innate immunity - innate leukocytes, PRR, MAMP + DAMPs Flashcards

1
Q

Name the innate leukocytes, where are they derived from?

A

HSCs : Myeloid + lymphoid stem cells
- Polymorphonuclear leukocytes (PMN)
- neutrophil, eosinophil, basophil, mast cells
- Macrophage, monocyte, dendritic cells
- NK cells
- all derived from HSCs myeloid lineage except NK cells

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2
Q

Describe the Polymorphonuclear granulocytes (polymorphs or granulocytes) and how they were disovered

A
  • Basophil,
    • found with basic blue dye (→ baso-phil)
  • Eosinophil,
    • red dye called eosin (eosino-phil)
  • Neutrophil
    • neutra-phil
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3
Q

Describe Neutrophils

Abundance, where they are released and found, how long they live, main functions, interactions, special features

A
  • Make up most (60-70%) leukocytes in blood
    • millions made every day and released from bone marrow
  • Released in large numbers from bone marrow
  • Live for < 24hr in blood
    • short lived in blood
  • Life extended on entering tissues in response to chemoattractants
    • chemoattractants like C5a
    • longer lived in tissue
  • Receptors for C3b, IgG, IgA
    • opsonisation
  • Main function is phagocytosis, but also “NETosis”
    • NETosis = when neutrophils are dying they throw out fibers (DNA and histones) to form a “net” which can catch bacteria (contain antimicrobials and e.g. defensins)
    • so even when dying they try kill bacteria
    • neutrophil extracellular trap
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4
Q

Describe Eosinophils

Abundance, where they are found, main function, interactions, special features

A
  • Few in blood (normally), but also beneath mucous surfaces
  • Receptors for C3b, IgG, IgA, (IgE)
    • opsonisation phagocytosis (not very good at phagocytosis)
  • Release toxic proteins, free radicals
    • upon recognition pathogen → release toxic proteins from granules and free radicals (e.g ROS which can bind to components to pathogens to cause damage)
  • Defence against multicellular parasites
    • through releasing these toxic proteins and free radicals onto parasite
  • Role in allergy (especially asthma)
    • if eosinophils activated inappropriately → can cause damage in the lungs
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5
Q

Describe Basophils

A
  • Very few in blood. ~1% of white blood cells
  • Receptors for C3a, C5a, IgE
    • chemoattractants response, and IgE
  • Release heparin and histamine
    • upon stimulation by pathogen
    • don’t really phagocytose
    • release heparin (increase blood flow) and histamine (inflammatory mediator)
  • Defence against parasites, role in allergy
    • induce inflammation, increase blood flow
    • increase in Basophils in allergy suffering people.
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6
Q

Describe mast cells

A
  • Restricted to tissues - protect mucosal surfaces.
    • just under the skin
  • similar properties to basophils
  • Receptors for C3a, C5a, IgE
  • Release histamine etc.
    • from their granules
  • “Sentinel” cells, defence against parasites, role in allergy
    • sentinel as in alert the body
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7
Q

What are monocytes? whats the difference between monocytes and macrophages

A
  • In blood → Monocytes
  • Macrophages found in tissues
  • Upon infectoin → monocytes can/may migrate to site of infection and differentiate into macrophages
  • However! macrophages seed into the tissue very early into development (feotus development) and self renew in the tissues
  • macrophages more
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8
Q

Describe macrophages

A

Macrophages can be derived from monocytes during infection
- Different tissues have different resident macrophages
- e.g. alveolar macrophages (lung), microglia (brain)
- Long-lived
- Act as “sentinel” cells – often the first to detect infections
- “Big eaters” – can phagocytose 100 bacteria/cell
- Receptors for C3b, IgG, IgA
- opsonins
- IgG and IgA help recognition
- Produce pro-inflammatory mediators
- e.g. cytokines
- Can present antigen to T lymphocytes
- important
- antigen-presenting leukocyte → can initiate adaptive immune response

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9
Q

Describe Dendritic cells

A
  • Found in skin and lymphoid tissues
  • Take up foreign material by phagocytosis/micropinocytosis
    • “cell drinking” can drink fluid around cells?
  • Digest foreign material and display fragments on their cell surface
  • Specialised for presenting antigen to T cells
    • efficient at presenting because →
  • Constitutively express high levels of Major Histocompatibility Type II (MHCII) proteins
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10
Q

Describe the importance and process of phagocytosis

A
  • Phagocytosis
    • professional phagocytes” (neutrophils, monocytes, macrophages, dendritic cells)
      • these carry out phagocytosis very effciently
      • many other normal cell types can do phagocytosis like e.g. epithelial cells but not efficiently
    • mainly important in bacterial and fungal infections
  1. Bacterium binds to the phagocyte (recognition)
  2. Phagocyte pseudopods extend and engulf organism
  3. Invagination of phagocyte membrane traps the organism within a phagosome
  4. Phagosome vesicle fused with specialised lysosomes present in the phagocyte
    1. lyosome (primarily contain free radicals)
    2. phagolysosome
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11
Q

What stops phagocytes from phagocytosing self cells

A

NOTE: Healthy “self” cells express a protein (CD47) which is recognised by phagocytes and prevents phagocytosis.

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12
Q

What in phagocytes phagosome/phagolysosome kills the pathogen? i.e. the phagocyte bactericidal agents

A
  • Short-lived, contained in phagosome, produced following “oxidative burst”
  • Acidic pH in phagolysosome, usually bacteriostatic but sometimes bactericidal
  • Free radicals (very reactive, bind to bacterial proteins and DNA and denature it)
    • Toxic oxygen-derived products
      • Hydrogen peroxide H2O2, superoxide O2- etc.
    • Toxic nitrogen oxides
      • e.g. nitric oxide NO
  • Antimicrobial peptides
    • e.g. defensins
  • enzymes
    • e.g.lysozyme and acid hydrolases (
  • Competitors
    • stop bacteria dividing, bacteria need Fe and B12
    • Lactoferring (competitor) binds iron → stop bacterial growth
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13
Q

Describe oxygen-dependent killing and “oxidative burst”

A
  • these are contained in lysosome so not dangerous to our other cells
  • Oxygen-dependent killing
  • Oxidative burst” - transient increase in oxygen consumption following phagocytosis due to activation of a membrane-bound NADPH oxidase.
    • rapid increase (transient increase) in oxygen consumption (caused by activation of membrane-bound NADPH oxidase (enzyme normally inactive → activated upon phagocytosis) → oxygen dependent killing → converts oxygen to superoxide ion O2- (very reactive) → superoxide dismutase converts superoxide to hydrogen peroxide (bleach)
    • in neutrophils myloperoxidase converts hydrogen peroixed to chlorite ions (bleach that you pour into toilet)
    • very reactive free radicals → can bind to bacterial DNA and proteins and denature them
    • NADPH + 2O2 → NADP+ + H+ +2●O2- → H2O2 + Cl- → OH- + HOCl
  • increase in oxygen activated nitric oxide synthetase → causes production of nitric oxide form arginine and oxygen (very reactive)
  • Inducible nitric oxide synthetase in the phagosome membrane can also lead to the production of nitric oxide and other toxic reactive nitrogen species
  • arginine + 2O2 → citrulline + NO
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14
Q

Describe NK cells

A
  • not phagocytic
  • Derived from lymphoid progenitor
    • big nucleus and granules
  • Kill infected host cells
  • Important in viral infections and bacteria that survive inside host cells
  • Recognise “altered self”
    • recognise changes in expression of self Major Histocompatibility Type I (MHCI) proteins
    • all nucleated cells express MHC I proteins
      • self labels
    • virus often interferes with levels of expression of MHC I
    • NK cells can detect the MHC1 expression decrease
  • Important in viral and some intracellular bacterial infections, especially until adaptive immunity is triggered
  • Receptors for IgG
    • allows killing of antibody coated infected host cells
  • Also active against some cancer cells
    • cancer cells often down regulate MHC I expression to evade immune system
    • NK cells can also detect and kill these
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15
Q

How do NK cells kill infected host cells?

A
  • Activated NK cells produce a pore-forming protein, perforin(similar in structure to MAC), which inserts into the membrane of the infected host cell, forming a pore. → contents of granules can enter target cell → granule contents (granzymes) released into the target cell → activate apoptosis pathway → infected host cell die
  • Recently discovered that a granzyme can enter intracellular bacteria, killing them directly.
    • some granzymes can kill intracellular bacteria
  • Polarisation of NK cell granules at the interface between the NK cell and target cell
  • Granule contents (“granzymes”) are released into the target cell, activate apoptosis pathway.
  • Target cells undergo apoptosis.
  • Recently discovered that a granzyme can enter intracellular bacteria, killing them directly.
    • some granzymes can kill intracellular bacteria
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16
Q

How do immune cells recognise pathogens

A

Through PRRs and MAMPs

17
Q

What are Pattern Recognition Receptors (PRRs) and Microbe-associated molecular patterns (MAMPs) and DAMPs

A
  • PRRs recognise MAMPs
  • The innate immune system recognises structures that are relatively invariant:
  • Microbe-associated molecular patterns: MAMPs
    • molecules Shared by many microbes
    • Distinct from “self” molecules
    • Critical for survival/function of pathogens (microbe)
    • Conserved (MAMPs conserved amongst pathogens)
  • Damage-associated molecular patterns: DAMPs
    • alarm signals coming from the bodies own cells → DAMPs
    • generated not only in response to infection but also to injury and heart attack and sometimes cancer
      • host components during stress/injury reflecting cell damage → signals to immune system that the damage needs to be repaired
    • Host components released during injury and cell damage
18
Q

Describe some examples of MAMPs in bacteria, fungi and protozoa

A
  • Bacteria
    • Gram -ve: (thick peptidoglycan)
      • Lipopolysaccharides (LPS)
    • Gram +ve:
      • Lipotechoic acid
    • Flagellins
    • unmethylated CpG in DNA
    • N-formylated proteins
      • n terminus of bacterial proteins have this
  • Fungi
    • Chitin
    • Beta-glucans
  • Viruses
    • dsRNA
  • Protozoa (eukaryotes)
    • GPI-linked proteins (we have these, but in very low amounts), mannose-rich glycans
19
Q

Give examples of DAMPs

A
    • Fragments of extracellular matrix proteins (holds cells together)
      • e.g. fibronectin
      • broken (fragments) of fibronectin
    • Exposed phosphatidylserine
      • component of lipid bilayer, usually found on inside of lipid bilayer, when damaged, flips to outside to say the cell is damaged
      • Phosphatidylserine is normally intracellular but is exposed during apoptosis
    • Mitochondrial components
      • mitochondrial are inside cells, if found outside cells → sign of damage
    • Uric acid
      • can build up if excess purines (e.g. during stress)
      • can lead to gout
      • Uric acid, a marker of kidney damage and oxidative stress. (Deposition of crystals causes gout.)
    • DNA
      • DNA found outside cells detected → signal of damage that needs to be dealt with
20
Q

How many PRRs are there and what are the 4 classes of PRRs

A
  • PRRs Recognise >1000 MAMP/DAMP ligands
    • PRRs inherited in DNA, can’t evolve rapidly, but in humans there are PRRs that can recognise more than 1000 MAMPs/DAMPs
  • Classes:
    • Soluble receptors e.g. Mannose-binding lectin (involved in activating complement - is a PRR)
      • can be found in tissue fluids and blood
    • Membrane receptors
      • Lectin receptors
        • like to bind to carbohydrates on bacteria
      • Chemotactic receptors
      • Toll-like receptors
    • Cytoplasmic receptors (found in cytosol and not on cell membrane → e.g. for virsues and intracellular bacteria)
      • NOD-like receptors (NLR)
  • Membrane and cytoplasmic receptors expressed by immune and non-immune cells
    • e.g. fibroblasts, epithelial cells (because they are usually the first cells a pathogen encounters
21
Q

What happens when a MAMP bind on membrane receptors on macrophages?

A
  • Macrophages express multiple PRR membrane receptors
  • MAMP binding may initiate phagocytosis, chemotaxis or signalling.
    • e.g. macrophage mannose receptor (recognises mannose)
    • e.g. CD14 (recognises LPS)
    • Mannose receptor and CD14 expressed on macrophage membranes
  • CHEMOTACTIC RECEPTORS recognise CHEMOATTRACTANTS
    • e.g. f-met-leu-phe receptor (recognises N-formylated polypeptides, produced by bacteria)
    • bacterial components
    • neutrophils, monocytes and macrophages express f-met-leu-phe receptor which recognise N-formylated polypeptides produced by bacteria → acts as chemoattractant for them. Bacterial infection in tissue → move towards concentration gradient of formylated peptide
  • Toll-like receptors or TLRs:
    • sensors that signal the presence of microbial components.
    • don’t trigger phagocytosis or chemotaxis
    • they signal to the body that there is a danger/infection present → triggers changes in gene expression
22
Q

Describe TLRs

A
  • Cause a change in gene expression i.e. allowing production of cytokines
  • 10 TLRs in humans, each recognising distinct MAMPs
  • examples:
    • TLR1 recognises lipopeptides
    • TLR3 recognises dsRNA
    • TLR4 recognises LPS etc.
  • always membrane associated
    • Can be on the cell-surface or endosomal membranes!!
  • The extracellular domain of TLR3 has a horseshoe shape formed by leucine-rich repeats (recognises MAMP (ligand)).
  • The inner surface has β-sheet structure and forms the ligand binding domain (dsRNA in this case)
  • Usually 2 subunits need to come together: dimerisation triggers signalling
23
Q

Describe what NOD-like receptors are and their function

A

Cytoplasmic receptors

NOD-like receptors (NLR)

  • large group of cytoplasmic receptors that recognise bacterial components e.g. peptidoglycan, flagellin

function:

  • trigger cells to make cytokines, particularly pro-inflammatory cytokines
  • signal expression of pro-inflammatory cytokines
  • trigger assembly of inflammasomes
    • inflammasomes are multisubunit complex that cleaves inactive cytokine precursors into active cytokines
24
Q

Describe the mechanism of action of TLRs, NLRs and inflammaosomes upon MAMP recognition

A
  • Cell recognises MAMP through TLR → induces signalling → changes in gene expression → cytokines produced
  • Many cytokines are initially made in an inactive form (important so they aren’t innapropriately produced) these are called pro-cytokines
    • they need to be cleaved to become activated cytokines
  • Bacterium been taken up by phagocytosis → material from phagolyssome released (MAMPs like LPS) → recognised by NLRs (in cytoplasm), and also signal nucleus to start making different genes? → inflammasome produced
  • Inflammasome protein complex needed to process pro-cytokines by cleaving them
  • Cytokines produced fully
  • NLRs important for bacterial infections
  • Receptors in cytoplasm for dealing with viruses
    • RIG-I-like receptors detect viral RNA produced within host cells
    • induce different response → induce cells to start making inteferons (sub-type of cytokine)
25
Q

What are RIG-I-like receptors?

A

RIG-I-like receptors

  • viral sensors that detect viral RNA produced within host cells
  • signal expression of interferons