Lecture - Infection and Immunity (Immunity in Meningitis) Flashcards
So with meningitis - what’s the basic way the bacteria starts and all?
They start with nasal pharan and then blood and then to meninges
If have trauma, then can get direct transmission to menin but uncommon route
So like S. pneumonia, explain the process of the exposure, uptake and all in terms of
- serotype
- M cells (what they do)
- AB (main ones secreted, how they get across etc)
- So all the above is the normal process - but how can you get invasive disease?
- Bloodstream
So you have exposure to a new serotype will lead to its colonisation on the surface of the epithelium. This can be like new capsular type or something.
I’m guessing this is what should happen:
- Special cells (M cells) that can take up antigen from luminal surface and present to APC (in the lamina propria) and then start adaptive immune response - have B and T cells waiting to be primed*
- Those T and B cells result in immune response and then B cells produce AB which is secreted cross lumen and the AB are specific to that capsule*
- They will bind to that organism and then prevent it from causing invasion so protect against infection*
- Main AB that are secreted across epithelium are the ones that have the secretor composntent and J chain. IgG is main isotype in blood but other heavy chains like IgM and IgA - J chain joins them together and these can then be recognised by the polymeric immunologbulin receptor and then they get across and protect*
- So the B cells will make an AB that is secreted across the epithelium by a polymeric immunoglobulin receptor (PIgR). ANd these capsule specific AB will protet against colonisation and invasive infection witht hat serotype.*
Invasive disease occurs with colonisation with a novel serotype in nasopharynx
So, what can happen is that we have a new serotype of bacteria that might invade the epithelium before we have a chance to get the protective response going. How does it get through? By reverse transcytosis - same mechanism but going backwards. It will use the polymeric immunoglobin (that the AB use to get across too) and that allows the bacteria to cross in vesicles and enter our body.
It will then invade our bloodstream after it gets into our mucosa. It doesnt matter what the receptors or proteins on the bacteria are - the bacteria is going to hijack normal host proteins to invade.
With this slide, memorise it and tell me how opsonisation helps
Coat surface of bac, phagocytes that have receptors are able to phagocyose easier
More, they have effect by activating complement - greatly enchances opsonisation
And promote inflamation
What are the three mechanisms of complement activation?
- Lectin pathway (microbial(glycans)
- Classical pathway (antibody)
- Alternative pathway (deposition of spontaneously formed C3b)
All three converge and activate a vairety of components and promote inflamation by coating bacteria so oposnisation
And have membrane attack complex
Lectin pathway
- What is it activated by?
- What do they recognise?
- WHat don’t they recognise?
- What does MBL protect against?
- What does FL protect against?
- What are MASP-1 and MASP-2?
- Collectins (eg MBL, ficolin) circulate in blood
- Recognise microbial glycans
- Vetebral glycans
- N meningitidis
- S pneumoniae
- They are MBL-associated serine protease - these proteins have a role in activating cascade
So like, this pathway is activated when the collectins are bound to the surface of the bac.
Lectin pathway is activated by collectins which are in blood and these are recognising the sugars/glycans which are on the surface of the bacteria and these collectins won’t recognise the glycans/sugars which are coating the vertebrate cells so they are like pattern-recognition receptors and are recognising a pathogen-assocated molecular pattern and binding to it.
Imp proteins bc if people have mutations in MBL then bad - more susceptible to eg neirssiria menin
If have mutation in FL then also bad - susceptible to S pnue
Classical pathway
- It is similar to lectin pathway except?
- What three things does C1q bind to?
- C1r and C1s are related to what?
- You had lectin binding domains in the lectin pathway - here, you have C1 complex.
- It binds directly to bacterial cell wall, CRP on the bacteria surface and binds complement-fixing AB binds antigen. Aka, it also mediates AB complement activation. Not only does C1q recognise bacterial cell wall’s CRP, but also recognises AB that are bound to the surface of the bacteria. IgM and some isotypes of IgG are particularly good at activating complement (IgM > IgG3 > IgG1 > IgG2)
- C1r and C1s are the equivalanet of the MASP from previous - they are the enzymatic part. They cleave downstream complement components. They are related to MASP-2
So when the lectin and classical pathway’s molecules (I think collectins and C1 complex/C1q respectively) have bound to the pathogen surface, what happens?? Remember the lectin binds to the sugars/glycans and classical’s C1q will bind to bacterial cell wall, CRP or AB on surface.
The MASP-2 (lectin pathway) or C1s (classical pathway - I think C1r too?) are activated and they will cleave C4 and then C2 to form C4bC2a on the surface which is a classical C3 convertase.
This C3 convertase will convert C3 to C3a (which is released into ECF) and also C3b which will bind to the surface of bacteria
Alternative pathway
So you’ve just learnt about the upstream component of the lectin and classical pathway and saw how both of them lead to C3 convertase being formed. Well even the alternative pathway will lead to C3 convertase being formed. Explain how it works in terms of these workds:
- C3b, lectin classical, spontaenous
- Factor B
- Factor P and definciency
- Factor H, negative regulator
Okay so the alternative pathway is initiated by a C3b molecule on the surface which is either deposited by classical or lectin pathway’s C3 convertase orrrrrr C3b can spontanously form on microbial surfaces from ciruclating C3 in blood.
When you have the formation of C3b, it will bind to another factor B in the blood and you get this complex called C3bBb and this is the alternative pathway’s C3 convertase
Now, this C3bBb is stabalised by factor P (properdin) and if you are decicient in this, you are more susceptible to N. meningitidis infection.
Sometimes, C3b can also spontaneously form on your own cells (not just bac surface) but mammalian cells are coated in facor H and it wont let it activate on our cells. Factor H destabalises C3bBb and is a negative regulator that binds glycans on mammalian cells.
Now we know that all pathways generate a C3 convertase (so all three coalesce into same pathway), which cleaves C3 and leaves C3b bound to the microbial surface and releases C3a. But like, what is the purpose of C3b?
What about C3a (and C5a?)
C3b is recognised by complement receptors on phagocytes and so you’ll get phagocytosis and you’ll destroy pathogen.
C3a and C5a are anaphyotxin and they are released and cause a local pro-inflammatory effect. So, C3a and C5a recruit phagocytic cells to the site of infection and promote inflammation.
-Anaphylotxin is a fancy word for the complement components that promote inflammation
Another thing that happens (three things in total) is that there is the formation of MAC. Completion of the complement cascade leads to the formation of a MAC which distrupts the cell membrane and causes cell lysis
How is C5 convertase generated? What does it cleave and then what do they do?
It is generated by binding of C3b to C3 convertase
- So from alternative pathway: C3b + C3bBb —-> C3b(2)Bb
- From Classical and lectin pathway: C3b + C4b2a —-> C4b2a3b
- These C5 convertase will leave C5 to C5a and C5b
- C5a is released and is an anaphylotoxin
- C5b binds to pathogen surface (like C3b) but it will initiate formation of MAC but there are much smaller amounts of this formed than C3b (which is used for phagocytosis). C5a will recruit other things like C6, C7, C8 and C9 to form the MAC.
How does MAC work? What of S pneu or Neissieria is this formation of MAC cirticla for immunity to?
It destroys membrane integrity, thus you’ll have loss of proton gradient, and this will kill pathogen
Neissaria meningitidis
What’s going on here?
Bound to receptor in membrane and it’s got into the subarachnoid space
To get the signs of menin:
Not much activation of complement in CSF since it is a protected space for bacteria (blood is dangerous) but have macrophages in CSF and they can recognise bac through other patterns so like with toll-like receptor signalling - like collectins,the pentanrix are recognsing highly conserved ish. So PAMPs are ceognised by toll-like recptor 2 and you activate pro-inflam cytokines? And you hav chemokines and recruitment of nutrophils
They activate endothelium so they become stickey and have adhesion moleues and then you can get nutrophils in the subarachnoid space and follow the chemokines etc
So there is rolling adhesion and then tight adhesion by which the leukocytes will get into to where they need to (applies to any issue).
- For each, what is the molecule expressed on the endothelial cell and what on the leukocyte binds to it?
- What type of reaction is each?
Blood flowing aling the edges of the BV, nutrophils (if have activated endothelium and have E selectins upregulation) then stickey and the nutrophils with theur Sialyl will bind specifically to the E selectin - this is a lectin-glycan reaction and they’ll slow down and then another more molecules will come into contact and they’ll start crwaling and then they’ll undergo tight adhesaion (another reaction with integrin) so the LFA-1 will receognise and that wil allow it to stop and get through endothelial layer and then follow the chemokines to get to site of infection
So we have our bacteria sitting in the subarachnoid space, it has activated the macrophages through their pattern recognition receptors, they hv released their inflam cytokines, which have activated endothelium and they’ve secreted chemokines which have lead to receuitment of nutrophils in CSF and you get this slide from peorson with menin:
So you know that host defence mechaimss in the subarachnoid space are inadequate to control infection - why? Tell me in terms of complement and IgG
- No/minimal complement in CSF (even with inflammation) - this is required for efficient phagocytosis in absence of AB
- IgG concentrations are low in normal CSF - this is increased in menin but still low compared with serum
As far as that sldie is concerned,
Purple dots (in the sub arachnoid space) = nutrophils that have been called in to sub arachnoid space to control infection
No or little complement in CSF so bad for efficient phagcytosis - nutrophils have touble since not coated in complement and IgG low there too but if person doesnt have IgG then even worse
Increase in pressure in CSF and all after this
Bad after it gets into brain, bascially
What do the capsules of S, pneumoniae, N. meningitidis and H. influenza mask?
What kind of AB is required for effiicient opsonisation and phagocysis (IgG) and complement deposition (IgM and IgG)
They mask complement and Ab inhibiting opsonisation and phagocytosis
They all basically have a capsule - polysaccarhide and it protects it against opsonisation and any completment that forms on srface is hidden from complement receptirs
Need anti-capsular AB so alternative pathway won’t work against encapsulated organisms
So there is a marginal zone in the spleen and has marginal zone B cells. What can you tell me about them in terms of:
- Produce what AB to polysaccarhside capsule?
- Is polysaccarhside a thymus-independent antigen? What does this mean?
- What can you tell me about the numer of marginal zone B cells in relation to age
- How are follicular B cells different?
Resp against capsule of bacteria
Special B cells in the margial zones - these B cells are capable of producing IgM to polysaccarshide in the absense of T cell help thus thymus-indpenedent
What’s the difference between polysaccaride vaccines and conjugate vaccines and why are they important?
Polysaccharide vaccines vs Conjugate vaccines :
The original vaccines against encapsulated bacteria - the capsule is just sugars, no peptide invovled so can’t stimulate T cell response as well so relying on these marginal zone B cells to get activated to produce AB by differentiating into plasma cells
Neonates and infants don’t have marginal B cells so can’t develop these Ab so these vaccines dont work
Conjugate vaccine = protein been conjugated to this polysaccarhide and that allows you to have a T cell dependent immune response. Not only do you get the B cell receptor recgonsing the polysaccarshide but also this carrier portein that is something unrealted (like tetanus toxoid) to the polysaccahride and the B cells can recgonise the polysaccaridhe but take the tetanus toxioid peptide and present on MHC-II and bc paitent already been vaccinated against tetanus toxoid, they already have memory T cells which are then able to provide help and lead to long lasting AB producing plasma and memory cells
Compare polysaccharide to cojugate in terms of
- immunogenicity in children below 2 years
- B cell response
- T cell response
- Immune memory
- Booster effect
- Long term protection
- Reduction of carriage
- Herd immunity
Conjugate vaccines are able to do all the things poly can’t
Since there is no peptide component, polysacc doesnt lead to a T cell response
Boosting vaccines
- AB levels decline after vaccination
- Memory B and T cells persist but response may be too slow in some infections (-2 to 5 days)
- What do booster vaccines do?
They increase Ab levels so that protective
-In neissiera menin, need the AB present there and then - no time to produce AB so have booster vccines so you ca increase AB in circulation
