Immune recognition Flashcards
Early observations in TCR signalling
- If inhibit try phoshatates, induce TCR phosphorylation and signalling without pMHC
- TCR is small compared with other surface molecules
=> kinetic segragation model (1990s/2000s)
Predictions of kinetic segregation model
- Tyr phosphatases segregate from TCR on ligand engagement
- Truncation of phosphatase enzymes inhibits their segregation and thus inhibits TCR triggering
- Elongation of pMHC decreases segregation of phosphatases
Varma et al 2006
Overlayed fluorescence of CD45 and TCR distribution at immunological synapse between T cell and artificial APC
Other NTR triggering mechanisms proposed
- Induced proximity/ aggregation
- Low surface density of pMHC and T cells only need a couple of pMHC to trigger makes this unlikely- how can a single TCR-pMHC induce TCR aggregation?
- Unlikely to be TCR/co-receptor heterodimerisation because not all TCR signalling requires co-receptor - Conformational change -> reveals tyr residue
- Allosteric? structural studies have not found conserved allosteric changes associated with binding
- Mechanical mechanism? Evidence of CD3 conformational change with binding but unclear how mechanical force would transfer. Also, if mechanical mechanism, accessory receptor-ligand interactions (CD2, CD58) would reduce force on TCR, but these interactions enhance signalling.
Kohler et al 2010
Studies NKCs and target cells. Introduced ligands for activator and inhibitory receptors into target cells -> affects cell lysis.
Elongation of inhibitory ligand decreased efficacy of inhibition.
When have longer activator ligands, shorter inhibitory ligands are less effective.
If same size, ligands colocalise => proximal membrane signal integration (allows localisation of abnormal cells and response polarisation)
Yokosuka et al 2012
Optimal PD1 inhibition of TCR requires matched size and colocalisation.
Authors varied size of PD1 and measured IL2 produced.
Long forms of PD1 lost colocalisation and inhibition; shortening restores.
Signal integration TCR and CD28
Can integrate in nucleus so do not need colocalisation
Arase et al 2002
Paired activator and inhibitory NTRs result from gene duplication, possibly due to host-pathogen arms race.
Authors studied mice (have Ly49Rs on NKCs rather than KIRs)
- inhibitory R Ly49I binds MHC-I
- murine CMV expresses decoy receptor m157 which engages the inhibitory receptor (so MHC-I can be down-regulated in infected cells without consequence, escaping NKCs and T cells)
- some mouse strains express a paired activator version of the inhibitory receptor (Ly49H) -> binds decoy receptor so activates NKC vs virally infected cells
Yang et al 2016
APCs only express a few copies of any given foreign pMHC.
Authors found 8-46 specific pMHCs in in vitro HIV infection
~10^12 T cells in periphery with ~10^8 unique TCRs
3 key studies in 90s
[Evavold 1991] [Hogquist 1994] [Lyons 1996]
T cells from mouse spleen -> mix with APCs loaded with different peptide concentrations (mixed lymphocyte reaction).
Good T cell response to wt Ag but abolished with single AA mutation
=> T cells very specific
Don Mason 1998
Theoretical calculations assuming ~10^8 unique TCRs can produce effective immunity.
Recognise peptide 8-14AA length, with 20 proteinogenic AAs.
Predicts ~10^5 to 10^6 pMHC per TCR
Ignatowicz 1996, 1997
Mice achieve comprehensive T cell repertoire with selection by a single peptide.
Their T cells respond to different, unrelated peptides.
Wooldridge et al 2012
Decamer CPLs. 9.36 x 10^12 peptides used to quantify degeneracy of patient derived CD8 TCR.
Subsampled likely peptide candidates from defamer CPLs, then incubate with T cells and C1R-A2 cells.
ELISA for MIP1beta.
Used mathematical algorithm weighting peptides based on circumstances of T cell activation.
~500 peptides within factor of 2 of optimal agonist
~60,000 within factor of 10
~1.3x10^6 within factor of 100
Functional consequences of TCR degeneracy
- solution to providing comprehensive immunity whilst conserving resources
- polyclonal responses
- molecular mimicry -> autoreactivity
T cell discrimination models
- Occupancy model
- assumes activation determined by number of bound TCRs, immediate signalling. pMHC potency depends on Kd. Most R-L interactions in body. - KPR model
- activation based on number of TCRs bound for sufficient duration
- due to proofreading time between binding and signalling (biochemical steps e.g. ITAM phosphorylation, binding and phosphorylation of ZAP70)
- unbinding reverses biochemical steps
- amplifies differences between Koff of different ligands => higher discrimination