The induced response to infection (2) Flashcards
What triggers the lectin pathway of complement activation?
Mannose-binding lectin
What pathway does C-reactive protein trigger?
The classical pathway
What triggers interferon response?
Internal detection of viral infection
What do plasmacytoid dendritic cells do?
They are factories for making large quantities of type I interferons
What are the main lymphocytes involved in innate immunity?
NK cells
Where are NK cells distributed?
2 populations of NK cells are differently distributed in blood and tissues
Where is NK cell cytotoxicity activated?
At sites of virus infection
What happens between NK cells and macrophages at site of infection?
They activate each other
What do CRP and Mannose-binding Lectin do?
They bind do structural features of bacteria and opsonise the bacteria thus activating complement and facilitating phagocytosis and direct lysis of bacteria by terminal complement components.
How does Mannose-binding lectin circulate in plasma?
As a complex of 2 serine protease zymogens - MASP1 and MASP2
What do MBL, MASP1 and MASP2 do?
Together MBL, MASP1, and MASP2 trigger the complement cascade
What is MBL?
Mannose-Binding Lectin which is a C-type lectin and acute phase protein
What does MBL bind?
Mannose-containing carbohydrates of bacteria, fungi, protozoa, and viruses
What does the complex between MBL, MASP1+2 look like?
A bunch of flowers
Each flower has 3 identical polypeptides
Stalks are rigid triple helices like collagen, with a single bend
Each flower comprises 3 carbohydrate binding domains
What are MASPs?
MBL Associated Serine Proteases
How does MBL, MASP1 and MASP2 initiate complement activation?
The complex cleaves C4 into C4a (anaphylatoxin) and C4b can bind to pathogen surface alongside MBL.
C2 gets cleaved by MBL forming C2a which attaches to C4b. The resulting protein is C4b2a.
C4b2a is a C3 convertase which allows C3b opsonisation of pathogen surface.
The C4b2a is called an Classical C3 convertase.
What is C4a like compared to C3a and C5a?
It is much weaker
How does CRP initiate the classical pathway of complement activation?
CRP bound to bacterium can also interact with C1 which is the first component of classical pathway activation.
CRP bound to bacterium interacts with C1 resulting in cleavage of C4 and opsonisation of bacterial surface with C4b.
Also cleaves C2 leading to formation of classical C3 convertase C4bC2a
Which of the 3 pathways for complement activation is fastest?
The alternative pathway
What does C1 complement component look like?
Consists of C1q, C1r, and C1s subunits.
C1q consists of six identical subunits each with a site for binding of Fc region of IgM or IgG. [CRP and antibodies can trigger the classical pathway as a result of this]
Extended amino-terminal stalk regions that interact with each other and with proteases C1r and C1s
How do human cells detect viral infections?
Human cells have sensor proteins in the cytoplasm that can detect viral nucleic acids and initiate defensive response.
What is the key cytokine in viral infections?
Type 1 Interferons
What do interferons do?
They interfere with viral replication
How are viral nucleic acids detected in the cell?
Via RIG-1-like receptors (RLRs)
How do RLRs detect viral presence in the cell?
They have a recognition domain that is similar to RNA helicase
2 CARD domains that interact with MAVS (mitochondrial antiviral signalling) protein
What do RLRs do after binding to viral RNA?
Signalling occurs through transcription factors IRF3 and NFkappaB
What binds to the RLR receptor?
Viral replication in cytoplasm produces uncapped RNA with 5’-triphosphate
How does receptor binding occur with RLR receptors?
Viral replication produces an uncapped RNA with 5’-triphosphate
RLR is suspended in the cyoplasm and binds to the RNA at the triphosphate end.
Binding induces RLR to associate with MAVS and dimerization occurs leading to cascade of signalling.
Dimerization initiates signalling pathways that activate IRF3 and NFkappaB
IRF3 causes sythesis and secretion of type 1 interferons and NFkappaB causes production of inflammatory cytokines
What does IRD3 do?
Causes production of interferon type 1
Where can IFN-beta act?
Autocrine
Paracrine
What are the 2 key types of interferons that are produced by virus infected cells?
IFN-alpha
IFN-beta
What do interferons do?
Induce resistance to viral replication in all cells
Increase expression of ligands for receptors on NK cells
Activate NK cells to kill virus-infected cells
Which cells can produce IFNs?
All cells can produce type 1 IFNs
plasmacytoid dendritic cells can produce 1000 fold more IFN than any other cell.
What kind of cells do pDCs resemble?
They have features of lymphocytes (mostly plasma cells) and myeloid DCs
Where are pDCs located?
Present in blood (<1% of leukocytes) and lymphoid tissues but not other tissues
How do pDCs detect viral infections?
TLR7 and TLR9 and after activation 60% of transcription is devoted to production of IFN (within 6 hours of TLR activation)
Which receptors do pDCs have?
TLR7 and TLR9
What are the 3 types of lymphocytes?
Long acting T cells
Long acting B cells
Early acting NK cells
What are NK cells?
They are large and active lymphocytes that are speedily induced to respond to infection, cancer, or stress
What are the functions of NK cells?
2 distinct functions:
Kill virally infected cells and prevent viral spread
Maintain inflammation in infected issue and secrete cytokines
Do NK cells only work on intracellular pathogens?
No they can act on both intracellular and extracellular infections
What do NK cells typically do to viral titer?
They keep it from growing. However, T cells are necessary to clear infections.
Where are NK cells present?
In most, maybe all, tissue
What percentage of lymphocytes in healthy people are NK cells?
5 - 25%
What are the subpopulations of NK cells?
CD56dim cells comprise >90% of blood NKs, have larger capacity for killing
CD56bright cells predominate in tissues
What are uNKs?
Specialized population of NK cells in uterus
What do uNKs do and what affects their numbers?
Number fluctuate with menstrual cycle and are essential for forming placenta.
They are poor at killing but they produce growth factors and cytokines.
Why are NKs strictly regulated?
NKs circulate at speed through body armed for killing.
Decision for NK cells to kill depends on a sum of interactions of multiple NK cell receptors
The default state for NK interaction with target cells is active inhibition
As a result, number of NK cells must remain low to prevent issues with overkilling of self cells
How do NKs detect infected cells?
NK interaction with cell is by default to kill and interaction with receptor inhibits killing.
If cell is healthy, the NK cell moves on.
If infected, NK cells adhere more tightly (via CR3 and LFA-1) further activating it and concluding with cytotoxic killing and apoptosis.
How many NK cells in blood in absence of infection?
> 100mil
What causes proliferation of NK cells and differentiation into cytotoxic cells?
Infected epithelial cells respond by secreting type I IFN which binds to receptors on epithelial cells, inducing IFN response and also to receptors on NK cells.
How do NK cells inhibit viral replication?
Effector NK cells kill virus-infected cell by inducing apoptosis thus inhibiting spread
What is the process of recruiting NK cells?
Viral infection triggers IFN I production.
IFN I triggers production of more NK cells.
Type I interferon drives the differentiation of NK cells into cytotoxic effector cells.
Effector NK cells kill virus-infected cells inducing apoptosis
Which receptors do NK cells express that give an interferon response?
TLR3
TLR7
TLR8
How does TLR7 trigger a response?
It binds to ssRNA (single stranded)
Signals through MyD88 to phosphorylate and activate IRF7
IFN-alpha and IFN-beta are produced
How does TLR3 trigger a response?
It binds dsRNA and signals through TRIF which is structurally similar to MyD88
TRIF activates serine-threonine kinases IkappaKepsilon and TBK1 leading to IRF3 activation
IRF3 activation leads to production of IFN-beta but not IFN-alpha
What does TLR7 result in?
INF-alpha AND INF-beta
What does TLR3 result in?
INF-beta only
What results from interaction between macrophage and NK cells?
Proliferation and differentiation into effector cells secreting interferon-gamma (IFN-gamma) which further activates the macrophage
How does NK and macrophage interact?
Activated macrophage (by viral infection) secretes inflammatory cytokines that recruit and stimulate NK cells
NK cell and macrophage form a conjugate pair with a synapse in which IL-12 and IL-15 activate the NK cell
NK cells proliferate and differentiate into efector NK cells secreting IFN-gamma
IFn-gamma binds to its receptor on macrophages and activates them to increase phagocytosis and secretion of inflammatory cytokines
What cytokine is secreted by NKs?
IFN-gamma which is a type 2 IFN
What are the type 1 interferons studied so far?
INF-alpha and INF-beta
What else do NK cells interact with?
Dendritic cells
What does the interaction between NK cells and dendritic cells result in?
Dendritic cells bind NK cell and stimulate it to proliferate and differentiate via IL-15. This results in production of effector NK cells that secrete cytokines and kill virus-infected cells.
If there are way more NK cells than dendritic cells the NK cells can kill the dendritic cells (negative feedback holy shit) thus suppressing dendritic cell function
When NK cells are scarce and are outnumbered by dendritic cells they drive dendritic cells to mature into the form that initiates adaptive immunity. The result is that the innate immune system is activating the adaptive immune system due to failure of innate system to terminate infection.
