Innate immunity Flashcards
Describe the properties of the the innate immune system
Give an example of why it is so important
- Present (and active) from birth
- Quick to respond
- Non-specific, same response for ANY pathogen
- But still really important
- Example: bacterial infection from a wound on the skin
- One bacterium could become 100 trillion in one day
- In the lab, you’d need 100 litres of fluid to grow this many bacteria
- But as you’ve only got about 5 litres of blood innate immune response keeps everything under control in the early stages of infection
The complement cascade:
The complement components are made by which organ?
- Complement components are proteins made by the liver (as well as by tissue macrophages, blood monocytes, and epithelial cells of the genitourinal tract and gastrointestinal tract) and released into the blood
What is the most abundant component of the complement cascade?
What does it bind to?
Many bacterial surfaces have what groups on display?
Most abundant component: C3
C3 will spontaneously split into two fragments
C3a and C3b (a-chemotactic and b-binding fragment)
C3a is an anaphylatoxin, i.e. a danger signal ((complement peptides, are fragments (C3a, C4a and C5a) that are produced as part of the activation of the complement system.))
C3b is very reactive; binds amino or OH groups
Many bacterial surfaces have amino or OH groups on display, so C3b will bind to bacterial surfaces (this is known as opsonisation) and help macrophages eat the bacteria
Explain the activation of the complement cascade:
How many different proteins work together to destroy invaders and signal to other immune system players?
what are the 3 different pathways?
- Around 20 different proteins that work together to destroy invaders and signal to other immune system players
Three different pathways:
- Classical
First to be discovered
Requires antibody
- Alternative (evolved first)
Antibody-independent
Probably evolved before the classical pathway
- Mannose-binding lectin
Recently discovered
Recognizes carbohydrate instead of protein patterns found on the surface of a large number of pathogenic microorganisms
Explain the classical pathway in detail
- Initiated by the formation of antigen-antibody complexes.Triggered by the C1 complex, composed of one molecule of C1q, two molecules of C1r and two molecules of C1s, or C1qr2s2 (protein subunits).
- C1q binds to antibodies complexed with antigens or when C1q binds directly to the surface of the pathogen; this activates C1r (cataylses the next step of the complement pathway).
- C1r2s2 component now splits C4 and then C2, producing C4a, C4b, C2a, and C2b.
- C4b and C2a bind to form C3-convertase (C4b2a complex), which promotes cleavage of C3 into C3a and C3b.
- C3b later joins with C4b2a to make the C4b2a3b complex, which is C5-convertase (crossover to the alternative pathway).
- The main purpose of this cascade is to provide a danger signal and prepare the site of infection for immune cells to enter
Explain the alternative pathaway
What kind of feedback loop is involved?
- Continuously active at a low level (like a car engine idling)
- Does not rely on pathogen-binding antibodies like the classical pathway
- Starts when C3 convertase cleaves C3 into C3a and C3b
- Initiated when C3b binds to bacterial surface, binding to bacteria needs to happen quickly, otherwise C3b is neutralised
- C3b generated next to a bacterium will bind to the bacterium
- Now C3b is stabilised and the activation cascade can continue
- C3b on the bacterium binds factor B, generating C3bB
- Factor D cleaves Factor B to Bb, giving C3bBb (Bb is the b fragement of factor B in the blood) which binds to the outside of the bacteria and decorates them for macrophages (a process called opsonization)
- C3bBb can cleave C3, making more C3b (positive feedback loop)
- C3bBb can also cleave C5 (another complement component) to make C5a which is chemotactic (brings in neutrophils) and C5b
- C5b combines with C6, C7, (initiation)C8, and C9
- Formation of the membrane attack complex (MAC)
- This is inserted into the cell call of the bacterium, forms a pore that allows water in, so the cell bursts
- Destruction of the pathogen
Describe the Classical pathway – alternative pathway crossover
Formation of C3b is the main crossover point
So if this C3b is formed spontaneously
- And this is going on all the time
- And C3b isn’t specific (binds amino and OH)
- Then…why doesn’t the MAC form in all of OUR cells?
- DAF (decay accelerating factor)
Accelerates breakdown of C3bBb
Stops the positive feedback loop from getting going
- Factor I
Cleaves C3b
This reaction is increased by a protein on human cell surfaces
- CD59 (protectin)
Cell surface protein
Prevents formation of MAC in our cells
(your complement wont destroy your own cells)
Recently discovered third pathway: Lectin activation pathway
- Recently discovered third pathway
- Mannose-binding lectin (MBL) pathway
- Annother way of opsonizing bacteria by recognising surgar rather than proteins
MBL is made in the liver, found at moderate concentrations in blood and tissue
Production increases during infection
Binds to mannose (a sugar) found on yeast, viruses, bacteria, and parasites (opsonize bacteria via sugar recognition)
- Effects mediated by MASP-1 and MASP-2 (similar to the C1 complex of the classical pathway)
- MBL and MASP binds to sugar ►conformational change ► cleavage of C2 and C4 ► feeds into the classical pathway
MASP= Mannan -binding lectin associated serine protease
Function of complements: Opsonisation (decoration)
- C3b can be clipped to form iC3b
- i means inactive - can’t make the MAC
- iC3b remains attached to the invader
- Phagocytes have receptors for iC3b
- Binding of iC3b-opsonised invaders facilitates phagocytosis
- Helps the phagocytes get hold of ‘slimy’ bacteria
Function of complement: Chemoattractant (call for help)
- a fragment that act as chemoattractants that tell macrophages and neutrophils to move to the site of infection and they also increase histamine release-blood vessels dilate
- C3 cleavage generates C3b and C3a
- C3a isn’t wasted, it diffuses away to tell the phagocytes where the problem is
- C5a (from C5 cleavage) does the same
- Activation of phagocytes - more ready to kill
- C3a and C5a are the anaphylatoxins – they cause smooth muscle contraction, histamine release from mast cells, and enhanced vascular permeability
- Dendritic cells respond to C1q by reducing their activation threshold (C1q regulates the threshold of DC activation and thereby prevents hyperactivation of the overall immune response)
Complement – the take home message
- Complement is very important in innate immunity
- Complement is multi-functional
Destroys invaders with MAC
Enhances pathogen destruction by stimulating phagocytes
Signals to other cells that they are needed for the attack
- Complement is FAST
Complement components are all preformed
They circulate as inactive enzymes ready for action
- Complement is CONTROLLED
We have proteins that regulate complement activity to protect us
What are the professional phagocytes?
What are the four families of pattern recognition receptors (PRR)?
- Macrophages and neutrophils
- They’re called professional phagocytes because their job is to eat!
- Carry pattern recognition receptors (PRR) on the cell surface – used to identify conserved patterns on pathogens
- The PRR are divided in four families:
- Toll-like receptors (TLR) recognise bacteria and viruses e.g TLR4 recognises LPS (Lipopolysaccharides) produced by gram negative bacteria so doesn’t recognise a specific strain but instead something common to a number of bacterial strains
- Nucleotide oligomerisation receptors (NLR) recognise toxins and cell stress
- C-type lectin receptors (CLR) recognise fungi and viruses (e.g. can bind to HIV)
- RIG-1 like receptors (RLR) recognise viruses
What is the first of the three states of macrophage alertness?
- lounging around in tissues (resting)
Leave the bone marrow as monocytes and mature into macrophages
under normal conditions they are rubbish collectors, slowly proliferating
sipping to keep tissues free of debris
Low MHC class II expression - can’t talk to T cells
But T cells aren’t interested in rubbish anyway
What is the second of the three stages of macrophage alertness?
Alerted to a breach of defences (activated or primed)
Start phagocytosing bacteria, upregulate MHC class II expression
Now they can present antigen to T cells
Helper T cells and NK cells produce cytokines like Interferon gamma (IFNg) to help prime macrophages
How do neutrophils know where to go to get to the site of infection?
- Neutrophils need to leave the blood and get to a site of infection, but this must be directed
- Neutrophils are inactive in the blood
- They are whizzed along at 1,000 microns per second (pretty fast if you’re only 10 microns long)
- ICAM protein in blood vessels that is a tether for immune cells
- Neutrophils cirulate with SLIG receptor and the SEL ligand for SLIG in inflammed tissue
In the case of a bacterial infection
- Macrophage watchdogs have sounded the alarm
Produce cytokines as warning signals (IL-1 and TNF-a)
- Endothelial cells receive the signals
Surface expression of selectin (SEL) in ~ 6h
SEL is the partner (ligand) for SLIG on neutrophils
- SEL + SLIG = SLOW DOWN signal
- Neutrophils start to roll along the blood vessel
- neutrophils express integrin (INT) and thats what binds to ICAM on the inside of blood vessels
Explain the process of diapedesis (how immune cells move from blood to tissue)
- Rolling neutrophils ‘sniff’ for signs of inflammation
e. g. C5a and LPS (Lipopolysaccharide) - When they find these, the neutrophils express integrin (INT) on their surface
- This needs to be quick, neutrophils are still moving and need to stop now to get out at the right place
Integrin is stored inside the cell ready to get put out on the surface
- Integrin binds to ICAM – very strong binding, neutrophil stops
At the site of infection
- Now the neutrophil has stopped, it needs to get out of the blood vessel (diapedesis) and go trap and kill bacteria
- Neutrophils follow a trail of chemoattractants: C5a, LPS, N-formylmethionine (fMet) peptides
fMet peptides are produced by bacteria, mitochondria and chloroplasts
Macrophages ‘burp’ fMet peptides after eating
- Cytokines (e.g. IL-8) detected en route will further activate the neutrophils
- Neutrophils arrive at the site of infection and are activated
Neutrophil extracellular traps (NETosis)
- Neutrophils kill bacteria by phagocytosis, the release of antibacterial enzymes & oxidative burst
- They also use a special method (NETosis) to trap and kill bacteria
- The contents of the cells are disgorged, forming a sticky net of DNA and enzymes
- This is a good thing during infections, but a bad thing in some autoimmune diseases (rheumatoid arthritis and lupus)
How do natural killer cells know what to kill?
NK obey ‘Kill’ and ‘Don’t Kill’ signals
‘Don’t Kill’ – inhibitory signals involving MHC
‘Kill’ involves interaction between NK proteins and surface molecules revealed after infection
Absence of MHC class 1 inside the cells suggests the virus is hiding from the innate immune system so the adaptive immune system will come and kill it
