MODULE 4 - Immunology I: Innate and Adaptive Sensing of Infection Flashcards
why is immune protection of a unicellular organism different to immune protection of a mammal?
as complexity of an organism increases so must the demands on the immune system to repel invaders
why can’t you just have a one size fits all protective mechanism?
because there are multiple and diverse threats to our bodies
haematopoetic system (bone marrow system) creates ways to deal with multiple different threats
why is plasticity in T lymphocytes (Th1, Th2, Th17 etc.) good?
it means they can differentiate into different pathways to deal with different types of infection
this means we can deal with multiple types of threats to our body
give some examples of cellular specialisation?
neutrophils - good for bacterial, fungal, yeast infection
mast cells - good for parasite infections and interact with IgE
what are some important innate molecules made in the liver?
complement (tags and destroys microbes)
mannose binding proteins (bind to mannan chains on microbes, some microbes particularly enriched in mannose, some MBPs activate complement)
haptoglobin (scavenges free iron keeping it away from microbes)
C-reactive protein (CRP) binds bacteria cell membranes, is an important clinical indicator (biomarker) of infection as high in infected people
what are pattern recognition receptors (PRR)?
recognise pathogen associated molecular patterns (PAMPs) such as LPS and bacterial DNA (different to ours cause methylated)
recognise damage associated molecular patterns (DAMPs) such as uric acid (gout) and heat shock proteins
what are DAMPs (damage associated molecular patterns)?
DAMPs immediately alert the immune system that something is going on there just from wounding, they are part of the damaged/stressed host response
what are toll-like receptors (TLRs)?
can distinguish between different classes of bacteria (not species, classes)
what does TLR2 bind?
lipoteichoic acid
what does TLR3 bind?
dsDNA
what does TLR4 bind?
lipopolysaccharide (LPS)
what does TLR5 bind?
flagellin
what does TLR9 bind?
bacterial DNA (CpG)
what class of MHC does dendritic cells express?
both MHC class I and MHC class II
what are the main differences between CD8 and CD4 T cells?
one expressing CD4 molecule and one expressing CD8 molecule
CD8 reacts with MHCI
CD4 reacts with MHCII
what assists the binding of a TCR to a specific type of MHC?
coreceptors (e.g. CD4 and CD8)
what does TCR signalling do?
switches LFA-1 to a high affinity state capable of binding ICAM-1 on the dendritic cell
these are two adhesion molecules and this switching to high affinity allows the cells to lock together enabling efficient T cell interaction with dendrictic cell
this is signal 1
what does MHC-TCR interactions being very weak and having a high on and off rate allow for?
it allows serial triggering to occur which is where the TCR can jump on and off the MHC-peptide and jump onto another and multiple TCRs can become activated then (cause it can jump off then MHC-peptide free for another TCR to engage)
so serial triggering allows activation of multiple TCRs from just one MHC-peptide
why is serial triggering beneficial?
it increases the efficiency of antigen presentation, especially for rare MHC/peptide complexes
what is signal 2?
because signal 1 isn’t enough, we also need signal 2 which is costimulation
T cells express surface costimulator molecule called CD28
CD80 only appears on dendritic cell surface if they activated by by pamps or damps. Once they cause expression on surface costimulation between CD80 and CD28 can occur and T cell gets activated
no signal 2 = no T cell activation
what is it that signal 2 (costimulation) allows?
allows additional signalling in the T cell including growth signals, anti-apoptotic signals etc. and of course T cell activation
what does signal 2 ensure and why?
sometime peptides from microbes resemble our own peptides (molecular mimicry) so signal 2 ensures that dendritic cell has actually seen some microbes and been triggered before activating T cell
what does the T helper cell (CD4) do once activated?
go and give cytokines to help B cells make antibodies and activate CD8 cells
what happens when helper T cells activate CD8 T cells?
they become cytotoxic T lymphocytes (CTL)
what else is required for a CD8 T cell to be activated to a CTL?
cross-presentation of exogenous antigen onto MHC-1 by APC
interaction with APC via the MHC-1-peptide:TCR
and then the cytokine ‘help’ from CD4 T cell and boom you’ve got a CTL
what is the reason behind all the steps for CTL activation?
safety guards to prevent autoimmunity from occurring
what are the steps required for T helper cells to activate B cells?
B cell recognises native antigen (unprocessed)
B cell receives cytokine ‘help’ from a CD4 T cell. The B cell was developed in the bone marrow specific for antigen of whatever peptide was presented to T cell
The native antigen that the B cell recognised earlier was swallowed by the B cell and loaded onto MHCII and this interaction with TCR confirms it is the right B cell and activates CD8 T cell making it CTL
where are CD8 T cells/CTLs usually get stimulated?
in the lymph node or spleen and then they fuck off around the body looking for shit to kill cause they are CTLs now
how do CTLs kill infected cells?
perforin - make hole
granzyme - go in hole and trigger apoptosis
what does antibody do once released from plasma cells?
goes off and neutralises toxins or ‘tags’ microbes for destruction
what happens if an antigen specific CTL encounters a virus infected cell without MHCI and why might it be missing MHCI?
no killing of target cell
many viruses have adapted to not present MHCI or down regulated it to where its not relevant, MHCI also often not presented on cancer cells so these cancer cells can be selected for
what are NK cells and what do they do?
natural killer cells which detect low or absent expression of MHCI
if MHCI drops below certain threshold on cell they fucking kill it
they have activating receptor which binds random receptors on cell giving positive signal to kill and inhibitory receptor which binds MHCI giving negative signal to kill (i.e. chills it out)
if positive signal occurs and no negative signal cause inhibitory receptor can’t find any MHCI then NK cell fucks up the cell using perforin/granzyme —> apoptosis of low/no MHCI cell
what is neonatal tolerance?
occurs in utero and in neonates
at neonatal stage thymus produces a wave of Tregs which can hang around for life and dampen the immune system against auto reaction as they secrete immunosuppressive cytokines (IL-10, TGF-beta) via CTLA-4 which is a molecule on T cells
this also means central tolerance is especially active at neonatal stage
where does central tolerance occur?
T cells screened in thymus
B cells screened in bone marrow
both being screened for auto reaction
what is peripheral tolerance?
ultimate aim is to make cells not autoreactive
T cells that are stimulated by antigen in absence of costimulation become anergic or apoptotic
Tregs involved in this process
I have a feeling this occurs to deal with any escapees from bone marrow/thymus screening
how does B cell screening occur (central tolerance)?
immature B cell chilling on bone marrow stromal cell and self-antigen being run past it
if it responds and binds self-antigen it triggers apoptosis, if not it enters the blood
do the innate and adaptive immune systems only recognise pathogens?
no they recognise commensals too
PRR may be localised away from commensals and therefore are only triggered by invasive microbes e.g. commensals which become opportunistic pathogens in immunocompromised
for example TLR-5 (detects flagella) present on certain bottom membranes of gut epithelium
where are relevant PRRs located?
TLRs associated with plasma membrane or endosomal membrane
cytoplasmic receptors: RIG-I and MDA-5 detect dsRNA from microbes well AIM-2 detects dsDNA
NOD-like receptors present in cytoplasm and can form large complexes called inflammasome
what kind of things do PRRs recognise?
things that bacteria have been unable to evolve away from despite the selection pressure e.g. LPS in gram-negs and lipoteichoic acid in gram-pos
how can PRRs distinguish between bacterial DNA and our DNA?
microbe DNA cytosines are under-methylated (CpG)
what are TLRs?
they signal via other adaptor proteins and activate transcription factors changing gene transcription patterns towards pro-inflammatory e.g. cytokines are secreted
TLR signalling important for priming the genes (pro-IL-1 and pro-IL-18) required for the inflammasome
why do TLRs only signal as dimers?
upon ligand (bacterial molecule) binding this induces dimerisation of TLR which causes adaptor proteins from cytoplasm will bind the TLR leading to shuttling of activated transcription factors to nucleus to induce inflammatory gene transcription
what is the difference between PRRs and adaptive immune system receptors (BCRs and TCRs)?
PRRs are broadly expressed i.e. a single immune cell may express multiple types of PRR
One B cell or T cell expresses one type of BCR or TCR (specific)
also unlike BCR and TCR some PRR are present and active in the cytoplasm
what are NOD-like receptors (NLRs)?
nucleotide oligomerization domain
subset of PRRs and are a family of 23 cytosolic proteins activated by intracellular PAMPs or DAMPs
they activate transcription factors (e.g. NFkappaB) to stimulate inflammatory gene transcription
what do nucleic acid sensors such as RIG-I and MDA5 do?
detect short and long dsRNA and induce a type I interferon which is released from infected cell and go activate nearby cells through interferon alpha receptor (IFNAR) which is like an alarm telling them there’s a virus nearby so those cells will shut down protein transcription and translation (so virus can’t exploit) and up regulate MHCI
this is an especially important response to viral infection (cause dsDNA common in viruses)
what do activated RIG-I or MDA5 do?
coalesce on mitochondria with MAVS (common pathway) ( allowing activation of transcription factors such as IRF and NFkappaB which shuttle to nucleus to activate antiviral response (mainly production of type I IFN such IFN-aplha and beta)
what common pathway do RIG-I and MDA5 act via?
MAVS
what is the inflammasome?
intracellular macromolecular complex that senses ‘danger’ to initiate an inflammatory response
required for innate and adaptive immunity and cell death via pyroptosis
exploited in vaccine design to stimulate innate sensing in APCs (in response to alum hence immediate (i.e. innate) arm swelling after vaccine)
important in auto-immune disease such as gout and IBS
what is the inactive form of inflammasome and what induces transcription of these molecules?
TLR-priming induces transcription of pro-IL-1beta and pro-IL-18 (inactive forms of IL-1 and IL-18 which inflammasome acts upon)
this is the priming stage (TLR activated synthesis of pro-IL-1beta and pro-IL-18)
so this question kinda wrong cause they aren’t inactive form of inflammasome but the answer is useful
what does NEK7 do?
helps assemble the NLRP3 inflammasome and assists with detection of cell stress (indicated by things like ROS production and potassium efflux)
what is NLRP3?
the main type of inflammasome we focusing on
contains a PYD domain which recruits something called ASC which can then recruit caspase 1
nod-like receptor p3
what is caspase 1?
the thing that converts the inactive cytokines pro-IL-1beta and pro-IL-18 to active IL-1beta and IL-18
participates in processing of cytokines and also cell death
once recruited you get the big inflammasome structure (see diagram with highlights in L17)
how are IL-1 and IL-18 cytokines secreted?
they lack signal sequences so cannot be secreted through canonical pathway and are instead secreted through gasdermin pathway
what is gasdermin-D
an inactive protein which is cleaved by inflammasome (important role of inflammasome)
cleavage product of gasdermin D will go form a pore which IL-1 and IL-18 secreted through
if there are a few pores the cell survives but too many and the cell dies
so cell survival depends on how much gasdermin-D shuttled to pores or cleaved
