Killing Pathogens: Intra-cellular and Extra-cellular Flashcards
What is the initial inflammatory response?
Barrier break, microbes enter
Microbes/injury activate sentinel cells
Sentinels secrete inflammatory mediators
Increased vascular permeability; fluid and proteins enter tissues
Complement, antibody and anti-microbial proteins kill microbes
Adhesion molecules and chemokines cause leukocyte migration into tissue
Phagocytosis and killing of microbes
What happens after a microbe enters the system?
- Initial inflammatory response
- Dendritic cells activate T cells
- T cells activate, divide and differentiate into effector cell
What are the different Th cell subtypes?
Th1
Th2
Th17
Tfh
Th1 cell properties?
Cytokines:
IFN-gamma
Target:
Macrophages
Immune reactions:
Macrophage activation
Host defence:
Intracellular pathogens
Extracellular bacteria
Immune pathology:
Autoimmunity: chronic inflammation
Th2 cell properties?
Cytokines:
IL-4
IL-5
IL-13
Target cells:
Eosinophils
Immune reactions:
Eosinophils and mast cell activation
Alternative macrophage activation
Host defence:
Helminths
Immune pathology:
Allergy
Th17 cell properties?
Cytokines:
IL-17
IL-22
Targets:
Neutrophils
Immune reaction:
Neutrophil recruitment and activation
Host defence:
Ectracellular bacteria and fungi
Immune pathology:
Autoimmunity- inflammation
Tfh cell properties?
Cytokines:
IL-21
(Also IFN-gamma or IL-4)
Targets:
B cells
Immune reactions:
Antibody production
Host defence:
Extracellular pathogens
Immune pathology:
Autoimmunity (antobodies)
What directs Th cell differentiation pathways?
Different cytokines direct Th cells down different differentiation pathways for different immune responses
How does Th cell differentiation occur?
Signals 1 and 2 from APC activate T cell (recognised Ag + B7 danger signal)
Signal 3 directs T cell effector function (cytokines)
PRRs can tell to some degree what sort of pathogen is present
Certain cytokines released depending on microbe type
What happens once Th cell has differentiated?
Th1/2/17 cells migrate to infection site, aid innate immune response
Tfh cells remain in Lymph node and give permission to B cells to activate
What are “Plasma cells”
Activated antibody producing B cells
What are antibody effector mechanisms?
Ab Ag site binds to microbe/Ag
Fc region interacts with innate cells/molecules to activate effector response
Neutralisation of microbes and toxins (antibodies alone)
Opsonisation and phagocytosis of microbes
Antibody dependent cellular cytotoxicity (NK cells)
Complement activation
Benefit of antibody effector response?
Can give specificity to innate immune response
What defines B cell effector functions?
The class of antibody it produces
Different Ab classes/isotypes interact with different innate molecules/cells so enhance different effector responses
Different isotopes deal with different infection types
What are the properties of different antibody classes?
IgM:
Great complement fixer
Good opsoniser
First antibody made
IgA:
Resistant to stomach acid
Protects mucosal surfaces
Secreted in milk
IgG:
Ok complement fixer
Good opsoniser
Helps NK cells kill (ADCC)
Can cross placenta
IgE:
Defends against parasites l
Causes anaphylactic shock
Causes allergies
Classical pathway of complement system?
Uses adaptive sensing as opposed to innate sensing used in alternative and lectin pathways
Ab (IgM, IgG) identifies pathogen with high specificity
Need an antibody response first so doesn’t work until there’s a lot of An for that specific microbe
3 main effector functions of complement system?
Opsonisation for enhance phagocytosis (C3b)
Stimulates inflammation by recruiting and activating immune cells (C3a, C5a)
Lysing microbes and cells (C6-9)
Complement mediated opsonisation process?
C3b binds microbe
Phagocyte C3b receptor recognises microbe
Phagocytoses and kills microbe
Complement mediated inflammation process?
C3a, C4a, and C5a released during complement activation
Act locally similarly to inflammatory cytokines
-recruit cells to infection site
-can activate cells
Complement helps inflammatory response pull in effector cells
Complement mediated cytolysis process?
Membrane attac complex (MAC) forms on bacterial membrane
Water rushes in, ions rush out, microbe bursts
Can also kill foreign/host cells
Killing of host cells makes it quite damaging
Types of phagocyte?
Macrophage- generalist:
Resides in tissues
Can be recruited from blood monocytes
Talk to T cells
Neutrophils- killers on call:
Specialist killers (can be dangerous)
Kept in blood (50-70% of blood monocytes)
Recruited rapidly to infection site
Short lived to about collateral damage
Dendritic cells- talkers:
Specialised in activating naïve T cells
Don’t kill
Steps of phagocytosis?
Detection of microbe via PRR, complement, or Ab
Membrane invaginates forming phagosome vesicle
Phagosome fuses with lysosome to form a phagolysosome containing toxic molecules to degrade microbes
Chemicals in phagolysosome activate and digest microbe
Phagosome killing?
Vacuoles ATPases pump H+ ions into phagosome - acidify environment
Phagocyte oxidase and it’s cofactor NADPH oxidase initiate respiratory burst
Converts O2 into reactive oxygen species ROS (eg H2O2)
Neutrophils express myeloperoxidase (MPO) that makes HOCl (hypochlorite)
Also produce elastase that degrades bacteria
Macrophages produce Nitric Oxide via inductible Nitric Oxide Synthase (iNOS)
Phagosome also withhold pathogen nutrients (Lactoferrin and other scavenger proteins bind and transport out iron)
Also contain defensins - other directly microbicidal proteins
This makes and contains a v toxic environment for killing
How do Ab increase phagocytosis efficiency?
IgG opsonises microbe
Phagocyte Fc receptors bind Ab Fc region
Fc receptor signals activate phagocyte
Microbe phagocytosed and killed
What are Neutrophil extracellular traps?
NETs
Neutrophil dies via NETosis
Nucleus swells and bursts extruding DNA like a net
DNA has anti microbial molecules attached (defensins, proteases)
Traps and kills bacteria, fungi, and viruses (aggregate into pus)
How do T cells enhance macrophage and neutrophil killing?
Th1 cells release IFN-gamma:
Classical macrophage activation
Th17 cells release IL-17:
[According to diagram] causes epithelial cells to release chemokines, TNF, IL-1, IL-6, CSFs
Causes inflammation and neutrophil recruitment/response
How does neutralisation by antibodies work?
Antibody binds Ag on microbe
Prevents penetration of microbe through epithelial barrier
Prevents binding of microbe and infection of cells
Prevents binding of toxin to receptor
Bind multiple microbes and clumping them together (Ab has 2 binding sites), blocks entry through barrier
Granulocytes?
Mast cells
Basophils
Eosinophils
Neutrophils
Pre-store effector molecules
Ready to fire out when activated
ADCC?
Antibody dependent cell-mediated cytotoxicity:
Ab bind target cell/microbe
Fc portion of Ab binds FcR on innate cell
Cell activated if multiple FcR-Ab interactions occur
Cell fires out granules at target
Destructive process - collateral damage to cells around it
Important for killing of intra- and extracellular microbes
Killing of helminth via ADCC?
Too large to phagocytose :(
IgE coats helminth
Eosinophil binds IgE
Then degranulates releasing toxic compounds
Mast cell degranulation?
IgE coats resting mast cell
Ag binds to multiple IgE and cross links them
Mast cell activation
Weep response:
Liquid flows from tissue into lumen to push helminths off gut wall
Sweep response:
Histamine causes muscle contractions
Push helminth out of gut with faeces after it has been detached by weep
Causes wheeze in allergy
Cytokine production:
Amplifies inflammatory response
How do Th2 cells help with helminth immunity?
Produce specific cytokines
IL-4:
B cells produce antibodies - mast cells and eosinophils need IgE
IL-13 and IL-4:
Weep and sweep responses
IL-5:
Activates eosinophils
Benefits of being intracellular for microbes?
Hidden from effector mechanisms
Ready suppply of resources
(Nutrients, cellular machinery for viruses to hijack)
Hitch ride around host
Different strategies for cell entry?
Phagocytes:
Have mechanisms to survive phagosome
Others escape from phagolysosome into cytoplasm
Nonphagocytic cells (eg epithelial):
Bind cellular receptor and use that to enter
Why infect immune cells?
Macrophages trying to eat you anyway
If you can evade killing mechanisms then makes infection easier
They migrate around host - perfect transporters
Easier to manipulate immune pathways inside immune cell
How are intracellular microbes detected?
PRRs inside cell
Endosomes (eg phagosome):
TLRs
Cytosol:
NOD-like receptors (eg NOD1 and NOD2) recognise bacteria
RIG-Like receptors (eg RIG-1) recognise viral RNA
How do microbes avoid macrophage killing mechanisms?
Legionella pneumophila:
prevent s lysosome fusion with phagosome and ends up in rough-ER like vesicle where it proliferates until it lyses the vesicle and then the cell
How do microbes escape phagosome before lysosome fusion?
Listeria monocytogenes:
can break out of lysosome and then live and replicate in cytosol
Listeriolysin O (LLO) disrupts phagosome membrane
Bacteria then uses host actin to move within and between cells (actin rocket)
Shigella also escapes phagosome
How are macrophages helped with intracellular microbes?
Macrophage needs to be fully activated by IFN-gamma signal from NK cells
IFN-gamma signal can also come from a T cell (CTL or Th1)
Macrophage base MHC class II on surface so displays antigen
Antigen + B7 danger signal causes IFN-gamma release from T cell
What happens if Th1 and Th2 cells are activated at the same time?
Th1 cell releases cytokines which activate macrophages (IFN-gamma and TNF)
Th2 cell releases cytokines that inhibit classical macrophage activation (IL-10, IL-4, IL-3)
Th2 activation can inhibit the activation of macrophages when they’re needed, blocking Th1 action
How does tuberculosis cause immune pathology?
Mycobacterium tuberculosis v good at evading phagocytosis
Leads to chronic infection - continual IFN-gamma production and macrophage activation
Leads to continual release of macrophage killing molecules (eg NO)
Causes immune (? 🦑) pathology
Granulomas form around infected macrophages to wall off infection
What is the anti viral state
Viral infection stimulates production of Type 1 interferons (IFN-alpha, IFN-beta)
Type 1 IFNs induce anti viral state in cell that protects from viral infection
How do type 1 IFNs inhibit viral gene expression?
Part of anti viral state
Block viral transcription and translation
Degradation of viral DNA
Autophagy - cell eats it’s own organelles so virus can’t use them
How do type 1 IFNs cause apoptosis?
Part of anti viral state
Misfolded proteins trigger unfolded protein response causing apoptosis
Alters cells response to TNF-alpha from pro-inflammatory to apoptosis
How do type 1 IFNs promote T cell and NK cell activation?
Sequester lymphocytes in LN
Increase cytotoxicity of CTL and NK cells
Promote Th1 differentiation
Upregulates Class I MHC
NK cells properties?
Target cells infected with bacteria viruses and Protozoa (and tumour cells)
Innate counterparts to CTL
Faster but less precise
Important backup mechanism to deal with intracellular pathogens that evade CTL by inhibiting MHC class I
IFN-gamma from Th1 cells and CTL is important
NK cells source of IFN gamma to activate macrophages
Necrosis?
Uncontrolled cell death (mechanical damage or microbe bursting cell)
Cell ruptures releasing contents
Highly inflammatory
Apoptosis?
Programmed cell death
Fragmentation of DNA, membrane blebbing
Apoptotic cells cleared by phagocytosis
Non-inflammatory
What causes NK cell activation?
NK cells Express range of activating and inhibitory receptors
Inhibitory receptors recognise ligands on healthy cells (eg MHC class 1 which is often downregulated by intracellular pathogens)
Activating receptors recognise markers on infected or injured cells (stress markers)
Activation depends on balance if these signals
Lack of inhibitor (working MHC class I) or presence of activator (presented antigens) signal killing
ADCC and NK cells?
Ab can enhance NK killing of microbe when Ag remains on cell surface (eg microbe leaving it behind as it enters cell, doesn’t include Ag presented by MHC class I and II)
Fc of Ab interacts with FcR on NK - killing of antibody coated cell
How do Th1 cells enhance CD8+ T cell differentiation?
CD4+ helper T cells produce cytokines which stimulate CTL differentiation from CD8+ T cells (IL-2, IFN-gamma)
They also enhance abilities of APCs to stimulate CTL differentiation
Benefits of adaptive CTL over innate NK?
Precisely identify infected cells
More specific than NK cells - more accurate and efficient
MHC class I expressed by all nucleated cells so can identify any cell that becomes infected
How do CTL kill with high precision?
Engage target cell
Form synapse with target where all interactions occur
This allows killing to be precise as it’s done by the synapse
Important as killing mechanisms are dangerous to healthy cells
What are the CTL/NK cell killing mechanisms?
Perforin/granzyme mediated:
Perforin creates pore in infected cells membrane
Granzyme enters via pore, induces apoptosis
Fas/FasL mediated:
Target cells express Fas (eg hepatocytes)
CTL/NK cells express FasL (Fas ligand)
Fas activation signals apoptosis
How are naïve CTL activated if DC is not infected?
All Naïve T cells (including CTL) need activation from DC
If the DC itself isn’t infected then it will only be presenting Ag it picked up from extracellular environment on its MHC class II which CD8+ CTL don’t interact with
In this case the DC can cross present the exogenous antigens on MHC class I instead of II- allowing for activation of naïve CTL
How do viruses evade CTL and NK cells?
Disrupting MHC class I Ag presentation
This is why downregulated MHC1 is detected by NK cells
How do antibodies neutralise intracellular microbes?
Target them during their extracellular stages (eg moving through cells)
Protects from further infection
Effector functions of memory T and B cells?
Effector functions of memory cells are already established
They won’t undergo differentiation when activated for a second time (unlike thei naïve counterparts which have yet to differentiate)
