E1 - Immunity Flashcards
(25 cards)
what is the process of kinetic proofreading?
ligand binds to TCR -> irreversible, biochemical receptor modifications until signalling-competent state is reached
intermediate states do not relay the signal
if ligand unbinding causes the modifications to be removed, it means only long binding life times -> receptor signalling
can account for TCR discrimination, and fits experimental TCR data better than models w/o proofreading steps
describe the kinetic segregation model
TCR activation is determined by TCR’s residence time in close contacts between T cells and APCs, when their membranes are in close proximity for TCR-pMHC binding
close contacts have no deactivating phosphatases (too large), but retain Lck -> TCRs activate if remain in close contact for sufficiently long time to be fully phosphorylated
ligand binding enhances residence times by preventing TCR diffusion from close contact
TCR elongation prevents TCR signalling: evidence to support KS model
why are the KS and KP models incompatible seemingly?
KP assumes that TCR is reset to initial state by ligand unbinding
KS assumes phosph can occur as long as TCR is in close contact (ligand bound or not), and resetting occurs when it leaves close contact
are not typically formulated in a way to be directly compared: KS requires careful handling of diffusion in close contact, KP tends to ignore it
KS-KP model generated in silico, has some implications for situations e.g. when few non-self antigens are present
what are immunoreceptors ‘abbreviated’ to?
NTRs
non-catalytic tyrosine-phosphorylated receptors
rely on external tyrosine kinases to initiate their intracellular signalling pathways via phosph of tyrosine residues
what is the process of receptor activation?
- pMHC binds TCR
- Lck phosphs multiple ITAMs
- ITAMs serve as docking sites for SH2 domains of ZAP70
- phosph of ZAP70 by Lck increases ITAM affinity + catalytic activity
- phosph + membrane-localised ZAP70 phosphs LAT
what is the process of adaptor activation?
- ZAP-70 phosphs tyrosines on LAT & SLP-76
- these generate docking sites
- leads to actin cytoskeletal rearrangement, cell adhesion via inside out signalling to integrins and TF activation
what TFs are activated in TCR signalling? what enzyme is required for this?
PLCy
1. NFAT
2. NFkB
3. AP-1
why must co-signalling receptors integrate at different places in TCR signalling pathways?
the receptors span diverse families and use diverse signalling mechanisms
the mechanism of co-signalling places constraints on co-localisation requirements, and co-signalling is also regulates at the receptor and ligand expression level
why must T cells discriminate between self and foreign peptides?
because APCs do not
therefore must act as checkpoints, as vast majority of pMHCs are likely to be self
briefly describe generation of an adjusted T cell
- thymocytes express randomly generated TCR
- somatic gene rearrangement can produce >10^15
- TCRs than can bind self pMHC produce +ve selectively signal: ensures TCR function + MHC-restricted
- +vely selected T cells then -vely selected (induced to die) if they bind too strongly to the self pMHC (ensures not overly autoreactive
- T cells leaving thymus have weak self pMHC affinity and hopefully strong foreign pMHC affinity
- ~10^8 distinct TCRs in the periphery
what did Kirberg 1997 find?
T cell survival in the periphery requires TCR signals generated by self pMHC
i.e. survival requires continual ligation of TCR to MHC-encoded molecules
therefore, self pMHC can bind TCR and generate signals
how did they investigate whether T cells are ‘specific’ to a single (or a few) peptide sequences?
mixed lymphocyte reactions
* clonal population of T cells expressing same TCR are mixed with APCs loaded w different concs of peptide
* [Evavold 1991, Lyons 1996, Altan-Bonnet 2005]
* T cell responses could be abolished with single peptide mutations at even high peptide conc
* therefore concluded T cells are specific to A peptide or a very small set
but like… there would need to be a single TCR for each pMHC, biologically unlikely – a single TCR can recognise more than 1 mil peptidess [Woodridge 2012]
what 3 signals are needed for T cell activation?
- pMHC complex
- co-stimulatory molecules (CD80/CD28)
- pro-inflammatory cytokines (IL-12)
who first crystallised the MHC-TCR? what did it show?
Pam Bjorkman 1987
showed that TCR recognises the peptide and the MHC together + T cells stabilise the interact
describe the general process of MHC CI antigen processing
- proteins from obligate intracell pathoens don’t need to be endocytosed: degraded by endogenous pathway (both host and viral)
- MHC CI expressed into the ER , cytosolic proteins enter ER through TAP
- tapasin binds TAP to bring MHC CI into proximity w channels through which peptides enter ER
- tapasin holds binding site open until peptide binds
- then chaperones release, MHC-1-p leaves ER for presentation, transported by golgi to CSM, where presented to CTLs
- or if no peptide binds, MHC I returns to cytoplasm for ERAD destruction
[Cresswell 2005 ]
describe the general process of MHC CII antigen processing
- MHC CII generated in ER + stabilised by binding invariable light chain, then fuses to antigen processing compartment
- light chain clipped to CLIP
- DM/DO bind CLIP-MHC, opens groove to allow peptide exchange and replacement of CLIP by antigen fragment
- MHC CII-peptide now stable, shuttled through vesicles to CSM
what determines conc of antigen required to elicit a T cell response?
affinity of TCR for pMHC (Kd)
what did Pettmann et al 2021 confirm with their meta-analysis?
- TCRs exhibit enhanced discriminiation based on TCR/pMHC affinity
- TCR exhibits 2-fold higher discrimination power vs other receptors
- found evidence for imperfect discrimination: revised murine data
what is T cell cross-reactivity?
where T cells can recognise many peptides (~10^5 or 10^6) presented on MHC bc they can bind TCR with sufficiently high affinity or are expressed at sufficiently high concs
why is there some evidence knocking about that the kinetic proofreading model is insufficient to explain T cell antigen discrimination?
it is based on original murine data that exhibited ‘near-perfect’ discrimination w a = 9, which we now know is inaccurate
what is the methodology of T cell sensitivity expts?
- CD4+ T cells mixed w APCs bearing qdot labelled pMHC
- cytokine captured on T cell surface allowing detection by fluorescent antibodies (preventing TNFa shedding)
- T cells respond to even a single pMHC (high sensitivity)
- likely evolved as copy number of pMHC is low on APCs e.g. 6-40 copies of HIV
why is T cell sensitivity remarkable?
in light of several observations:
* TCR/pMHC lifetime is short (s) [Huppa 2010] and subjected to many molecular forces, but cytokine production requires TCR/pMHC interaction for several hours [Huppa 2003]
what mechanisms improve TCR/pMHC binding?
- microvilli-like protrusions (actin mediated)
- adhesion receptors (LFA-1, CD2)
- coreceptors (CD4/CD8) [Li 2004]
what is the trade off between T cell sensitivity and discrimination?
a longer kinetic proofreading time:
* increases discrimination bc fold-difference between different pMHCs increases
* decreases sensitivity bc probability of signalling decreases for all pMHC
