Innate Immunity 1: soluble effectors Flashcards
What is innate immunity?
•1st line of defence against infection
- Present at birth and passed down genetically (you are born with it, its passed down geneticallY, ALTHOUGH YOU CAN HAVE MINOR CHANGES).
- Occurs within minutes of pathogen recognition.
- in the graph, when you encounter a pathogen, you will get this growth that occurs and at this point the innate immune system will limited the growth of the infection along with the adaptive immune system.
- if you are lacking an innate immune system, you don’t get any blocking of the growth of the microorganism whereas if you only have the innate immune system and not the adaptive, you may be able to resolve some infections.
- we need the innate immune response to trigger the adaptive immune response.
Innate immune system is a rapid response
Characteristics of innate immunity
- The innate immune system is evolutionary conserved, and germline encoded (transcribed directly from the genome).
- Unlike the adaptive immune response, it is non-specific instead recognizing pathogen associated molecular patterns (PAMPS) which can be found across many pathogens.
Innate immune memory (trained immunity)
Conventional adaptive immune memory is present only in vertebrates, whereas innate immune memory is an ancient property of host defence present in plants, invertebrates and vertebrates.
-we now believe that there is a short term memory that can be formed in innate immune system called Innate immune memory or trained immunity.
plants and vertebrates don’t have adaptive immunity. so it is their innate immunity that they use to defend against pathogens.
- when you get a pathogenic infection and your immune system responds to it, you can get modifications of histones, DNA, methylation and also the production of coding RNA’s that can all affect coding expression.
- so by changing the gene expression, this can be for maybe a few hours or days or might even be months, you can then have a different response with the same pathogen. this might be to boost or suppress the immune response.
there are a lot of articles giving more information about this topic.
Innate barriers to infection
1. Physical barriers eg
- Skin
- Respiratory tract
- Gastrointestinal tract
if you breach these barriers then you come on to the soluble barriers.,
2. soluble
- Complement
- Defensins
- Collectins
if these molecules are unable to get rid of the pathogen, then you will trigger the induced innate immune response.
3. Induced
-Innate immune cells will recognise the pathogens using Pattern recognition
Receptors (PRRs) and they are gonna induce an inflammatory response to produce cytokines, chemokines, complement proteins, interferons etc.
Anatomical barriers
- Skin has a dense layer of dead keratinocytes that act as a physical barrier.
- In the respiratory tract mucus traps micro-organisms (importance demonstrated by Cystic fibrosis). In the lower airways there are collectins in the surfactant that can activate complement.
- In the gut, people unable to secrete sufficient gastric acid have a high risk of salmonella infection.
- In the airway and lungs, sneezing and coughing help to expel mucus, whilst macrophages in the alveoli of the lungs can ingest pathogens.
- The intestine is colonised with ‘good’ bacteria that form your microbiome.
Tissue Damage
what happens when we breach our anatomical barriers is eg when you have like a tissue damage, when you cut yourself.
- there is a pathogen introduced, they will come across the soluble innate immune molecule which will hopefully eliminate the pathogen.
- but also within the tissue, we have tissue-resident cells like macrophages, neutrophils and other phagocytes which can engulf those bacteria.
- if they get triggered they will released more of the soluble effector molecules but they re also gonna receive cytokines and chemokines which will attract more cytokines and chemokines to the site of the infection.
Soluble innate immune molecules
1. Enzymes such as Lysozyme
Disrupt bacterial cell walls; found in many fluids in the body eg blood and tears
2. Antimicrobial peptides
- found all over the body
- they canDisrupt microbial membranes
3. Collectins, ficolins and pentraxins
-Bind to pathogens targeting them for phagocytosis and activate complement.
4. Complement components
A group of different molecules that worm together to lead to the lysing and optimisation of bacteria and targeting them for phagocytosis.
- so they Lyse bacteria, opsonise bacteria and induce inflammation
Lysozyme disrupts peptidoglycan
Lysozyme is secreted by phagocytes and paneth cells from the small intestine.
- Lysozyme disrupt the membranes of pathogens. the Lysozyme which is secreted by cells like phagocytes or paneth cells in the gut can disrupt peptidoglycan chains that are found on the surface of bacteria.
- they are very effective against gram-positive bacteria but not so effective against gram-negative bacteria
- this is because gram-negative bacteria have an additional outer membrane covered in lipopolysaccharide (LPS). this masks the peptidoglycan so that the lysozyme cannot get to it.
- in the gram-positive bacteria where it is exposed, lysozyme will come along and cleave the alternating sugars that name up the peptidoglycan and cleave the alternating sugars that make up the peptidoglycan and by doing this it’s gonna expose the lipid bilayer and then along come other enzyme-like phospholipase A2, which can then disrupt the phospholipids.
Antimicrobial peptides
Antimicrobial peptides Cover epithelial surfaces, and also found in fluids like saliva.
They are Constitutively secreted which means that they are being made all the time by neutrophils, epithelial cells and paneth cells in the crypts of the small intestine
•Kill bacteria in minutes, by disrupting the membrane. the slowest one is about 90 minutes to kill bacteria. they don’t just kill bacteria, they are also effective against fungi as well as viruses like the flu, herpes.
- the way they work against virus is that they are very good at inhibiting DNA and RNA synthesis but when it comes to disrupting membranes, they disrupt those of bacteria but not our membranes.
- they are quite small (less than 100 amino acids in length)
There 3 of them.
1. Histatins
Produced in the oral cavity. Active against pathogenic fungi, e.g. Candida albican
2. Cathelicidins
LL-37 is the only type known. its a broad-spectrum antimicrobial activity against both Gram-negative and Gram-positive bacteria
-its got its name because its a very short amino acid with 37 chain that starts with 2 Leucines.
3. Defensins
Two classes – α, β defensins
- •Defensins cover our epithelial surfaces and are present in the vernix caseosa and in the skin of the healthy new born.
- these are particularly important in the newborn where the newborn is covered in the vernix caseosa which is like a membrane layer that is full of these defensins. they are all in the skin of the newborn. when you are born, you will come in contact with pathogens for the first time, i.e in the vagina.
these innate pathogens are gonna help defend you against those first encounter.
Defensins
•Quite small, they are about 35-40 amino acid amphipathic peptides which means they have both hydrophilic and hydrophobic regions on their cell surface.
- so one end is positively charged and this is gonna be attracted to the cell membrane and then the hydrophobic region is gonna insert itself into the membrane.
- •Disulphide bonds stabilise the structure to have a positively charged region separated from a hydrophobic region
- lots of defensins inserts themselves into the membrane of the pathogen and disrupts it and that causes a pore to form, this happens for microbial membranes but not our membranes.
- Disrupt microbial membranes but not that of the host (how they discriminate is not known).
- when the pores form, you are gonna get sugar and ions moving out of their cell and that is gonna allow water to go into the cell and that’s gonna cause lysis of the bacteria.
Collectins, ficolins and pentraxins
This is another type of soluble antimicrobial.
Collectins have globular lectin-like heads that bind bacterial cell surface sugars. we don’t trigger collectin to attach our own cell surface sugar because we have Sialic acid which can hide mannose antigens on host cells.
Lectins are a type of protein that can bind to sugar. They’re sometimes referred to as antinutrients.
- Collectins are c-type lectins with a collagenous region -9 types defined to date. The tail forms a coiled coil.
- Most are soluble but some are found on cell surface such as DC-sign and Dectin-1.
- Found in surfactant (surfactant protein A and D) and serum (MBL)
- Sialic acid can “hide” mannose antigens on the surface of host cells or bacteria from mannose-binding lectin. This prevents activation of complement.
Ficolins (have a Fibrinogen like domain) which recognise acylated compounds (COCH3) such as n-acetylglucosamine, a monosaccharide found in bacterial cell walls.
Pentraxins are cyclic multimeric proteins in the plasma. an example of pentraxins is CRP. C-reactive protein (CRP) is used as a clinical measure of inflammation – CRP binds to phosphocholine on bacterial surfaces but may also bind to infected host cell. they are a pentimer . we have 5 different subunits coming together to form a stricture.
*each of the above molecules can induce an innate immune response by activating complement.
Actions of Collectins, ficolins and pentraxins
- Soluble pattern recognition receptors
- Act as opsonins that bind to pathogens and infected cells targeting them for phagocytosis
- Activate complement through the classical pathway/lectin pathway
- both Mannose-binding lectin (MBL), ficolins and C1q will trigger the complement component that becomes activated and this can lead to lots of different events such as an increase in inflammation but also the lysis of a pathogen or maybe through making pores in it or it can trigger other parts of the inflammatory response, even phagocytosis.
Complement pathways
-There are 3 different compliment pathways.
- The classical complement pathway (can be activated by pentraxin CRP and also antigen/antibody complexes)
- The Lectin pathway (activated by the collection and ficolins pathway)
- Alternative pathway (activated by pathogen surfaces)
*On activation of these different pathways, you trigger a series of events that converge on C3 convertase and then that leads to a downstream event that will induce inflammation i.e by producing phagocytosis, other inflammatory events or even lysis of an infected cell or bacteria.
Complement System
- Series of over 30 proteins that constantly circulate in blood and fluids that bathe the body tissues.
- When they detect the presence of foreign material, they initiate a cascade of reactions that amplify the signal
- When activated, cooperate with other host defense systems to generate inflammation and rapidly remove the pathogen
- Most made by the liver but also produced by monocytes, macrophages and epithelial cells of the intestine and urinary tract (places where you are likely to come in contact with pathogens).
-They are being produced all the time but can also be released at high levels if the cells are activated by a pathogen.
Complement components
- Circulate as a pro-form (inactive) in the blood because complement is a powerful inducer of inflammation so you wouldn’t want it just being triggered inappropriately.
- Numbered in the order they were discovered, not in the order they are activated.
- Some have proteolytic enzymatic activity
- On activation they split into a small and large fragments triggering an amplification cascade
in the image. C3 is the whole protein, its been cleaved into C3A and C3b. C3a is the active form.
•Normally ‘a’ is the small fragment - except c2a. •. It was originally given this name because it was considered the active component and the name stuck.
Complement cascade
You don’t need to learn this.
All the arrows show all the different cleavages that occur. in the big circles, you can see the different pathways and this is how you start the cascade.
it doesn’t matter where you start, you are gonna end up converging on C3.
each of these molecules are gonna cleave to make something called C3 convertase.
they are gonna split c3 unto its small and large fragments and then c3b can come together with either of these C3 convertases to then make a c5 convertase which splits C5. this can then come together with a terminal complement component to form a membrane attack complex.
Effects mediated by complement components
The membrane attack complex created by the complement pathway will be able to form pores in the membrane of infected cells and lyse them.
the other complement component can go on and do opsonisation which is where you can label a pathogen so that it is recognised by phagocytes and this will trigger phagocytosis when they bind to complement receptors on the cell.
some of these molecules can also bind to cells and lead to extravasation where they move from the blood into the tissue to help with an infection.
they can also bind to complement receptors or things like mast cells to cause degranulation.
-one important role of the complement system is the removal of antigen/antibody immune complexes. some of the complement systems can bind to the antigen/antibody immune complexes and transport them to the liver where they can come across phagocytes that can then break them down and remove those immune complexes.
Classical pathway
This was the first one that was discovered.
- This can be activated by CRP
- it’s also well known to bind to antigen-antibody complexes
- this is gonna be triggered by the molecule C1 (a protein) when C1 becomes activated.
- C1 is a complex of 3 proteins (C1Q, C1R, C1S). yoi can see them together in the image. c1 and c1r and c1s and in between c1q and it forms a bunch.
- c1q dominates the structure
Classical pathway: Activation
C1 is gonna need to bind to either CRP or antigen-antibody complex to trigger its activation.
C1 need to bind to 2 FC domains to become activated
- IGM is the most effective activating complenet because it has 5 FC domains on 1 IGM molecule.
- one Igm molecule can activate the complement cascade whereas if you wanna activate an IGg, you are gonna need about a thousand of this molecule to get 2 that are close enough together to bind c1 at the same time.
extra note;
- C1 binding an immune complex allows for a conformational change that exposes the C1q binding site (it unwaraps it).
- One IgM molecule bound to antigen can activate complement, whereas around1000 IgG molecules may be required to get 2 close enough together to both bind to C1 .
- The classical pathway is not activated by IgG4.
- C1q can also be activated by binding to CRP attached to necrotic cells
Serum IgM
you have IgM floating around in the serum all the time but you won’t want it to be activating complement unless it was bound to an antigen-antibody complex.
-when in the blood , serum IgM cannot ordinarily activate complement because it has a planer formation. so its got this flat structure, it that conformation, it cannot bind to c1q. but what will happen is that when it binds to a crp or antigen-antibody complex, it changes shape to something like a crab-like structure, this then reveals the binding sites for c1q for complement activation to occur.
Classical pathway: Amplification
- •Binding C1q with the causes a conformational change in C1r
- •C1s is cleaved and can activate C2 and C4 splitting into their large and small fragments.
- The 2 large fragments will then come together to make the c3 convertase
- C3 convertase (C4b2a) can then activate C3 by splitting it into its large and small fragments but at this point we get lots of c3 molecules being converted , so we get a massive amplification of the cascade.
- once c3 has been split , the large fragments (c3a) will then come together with C2a C4b, when it combines with them, it make a c5 convertase, so it can go off to cleave c5.
Lectin pathway
- This is antibody independent and is activated by ficolins and collectins particularly mannose binding lectin (MBL)
-MBL will bind to the sugars, carbohydrates and glycoproteins on bacteria and some viruses, it can then activate.
-• Similar downstream mechanism to the classical pathway
-MBL when activated will form a complex with MASP-1 and MASP-2 (these are serine proteases) which means they have an enzymatic activity to then cleave C2 and C4.
- this then leads to the C3 convertase (C2a and C4b) will come together.
- they can then cleave lots of molecules of C3, getting that amplification of the cascade and the c3b will come together with c3 convertase to make c5 convertase.
extra note;
•MBL is a member of the collectin family that is structurally similar to C1q.
.•The lectin pathway is also activated by Ficolins that have a fibrinogen like domain (3 different ones circulate in the blood) that can recognize acetylated compounds.
- Human cells not targeted as sugars covered by sialic acid residues.
- MASP1 and 2 structurally and behaviourally similar to c1r and c1s.
People deficient in MBL or MASP2 have recurrent respiratory infections in childhood before the adaptive defences develop
Alternative pathway
- •Involves four serum proteins: C3, complement factor B (circulates in the blood), factor D (secreted by adipocytes), and properdin (released by monocytes and neutrophils).
- •Most cells have complement inhibitors on their surface – pathogens don’t!
- In the body, C3 can spontaneously hydrolyse into c3a and c3b. but it will not be around for very long.
- what will happen is that c3b (the large one) can bind to the cell membrane of pathogens.
- when it binds to the cell membrane and comes across factor B, it can become stabilised and that now makes it susceptible to cleavage by another molecule called factor D.
- Factor D is gonna cleave factor B and leave you with 3BB, remember the large fragment of C3 and now the large fragment of factor B. this an alternative form of the C3 convertase.
- this can go on to cleave c3 and then remember the large fragment that can come together to make a c5 convertase.
- this does not tend to be around for very long
- it only has a half-life of about 5 minutes but to extend this, if it binds to properdin, it can then extend the half-life for about half an hour
- this is because properdin is gonna protect it from proteases that are found in serum that can then break down the complex
- properdin can be released from cells like monocytes and macrophages
- if tissue-resident cells detect that there is an infection, they will be releasing properdin which can also help stabilise the complex and really amplify the production of C3.
- that is gonna go on then to join the membrane attack complex.
Complement pathways
Terminal complement components
At this stage, we’ve got the C5 convertase, this all the same now regardless of whichever platform you started from.
-The c5 gets cleaved into the large and small fragments, the c5b will come together with C6, 7, 8, and 9. and it makes this large complex which we called the membrane attack complex.
Membrane attack complex
The membrane attack complex forms a ring-like structure which forms a pore that can insert itself into the membrane of a pathogen and that would then allow the diffusion of small molecules like ions and sugars to come out of the cell and then water moves in causing cell lysis.
this does not tend to haooen to our cells because we’ve got inhibitors on our cell surface that prevents this from happening. prevent the MAC attack complex from being inserted.
this insertion will cause the bacteria cell to swell and lyse.
Complement inhibitors
complement is important and it is a big inducer of inflammation. its very important that we have inhibitors in our body to keep it under control.
-we have various different soluble as well as membrane-bound inhibitors that can helo to keep complement under control but in some people, if you have a deficiency, this can lead to unwanted inflammation when you haven’t necessarily got an infection.
1. we have soluble molecules that prevents the activation of C1 and that prevents C1 from splitting C2 and C4.
2. you’ve got some on the cell surface that prevents C3 when it gets activated spontaneously in the alternative pathway from binding to our cell surface
3. you also have membrane-bound inhibitors that prevent the insertion of the membrane attack complex
Hereditary Angioedema
- caused by C1 inhibitor deficiency
-This affects the classical complement cascade
-Classical complement cascade easily activated but can be treated with an injection of C1 inhibitor
- The smallest thing can trigger a large amount of inflammation
- after being given the c11 inhibitors injection, within a few hours, he looked normal again.
Complement deficiency
•Patients deficient of components of the complement pathway experience recurrent infections.
•MBL deficiency causes serious pyogenic infections ([pore forming infection) in neonates and children
•C3 deficiency is the most severe leading to successive severe infections. this is because all the 3 pathways converge at c3.
•Patients deficient of C8 are prone to infection with Neisseria meningitis
Complement deficiency in SLE (Systemic lupus erythematosus)
•90% of people deficient for C4 develop the autoimmune disease systemic lupus erythematosus (SLE)
-This disease is associated with the build-up of autoantibodies
- C4 deficiency means less C3b (C4b2a is C3 convertase)
- C3b bound to immune complexes and can also bind binds to a CR1 receptor on erythrocytes which transports them to phagocytes in the liver and spleen.
- Phagocytes recognise the immune complexes via their Fc receptors and engulf them
-In lupus, these patients have a large amount of autoantibodies which bind to their own DNA and RNA and they can trigger other parts of the innate immune system (toll liek receptors) to drive the production of pro-inflammatory cytokines generating inflammation.
