module 04 section 03 (TCR activation) Flashcards
following antigen processing, peptides are:
stably bound to the polymorphic aa anchor residues within the MHC binding cleft for antigen presentaion
recall: do T-cells have single or dual specificity?
dual:
- the CDR1/2 of the TCR bind to the polymorphic residues of self-MHC molecules
- CDR3 of the TCR binds to the specific residues of the peptide
with respect to TCR, what is a crucial step of T-cell activation?
the specific binding of TCR to the MHC:peptide complex
what signals does a naive T-cell require to become fully activated?
a pre-signal and 3 extracellular signals
what must occur for the 3 extracellular signals to occur?
the T-cell and APC need to be close in proximity, which involves adhesion of the helper T-cell to the APC
the helper T-cell adheres to the APC through?
the binding of adhesion molecules:
- CD2/LFA-3
- LFA-1/ICAM
what is CD2? where is it found?
cell adhesion molecule found on the surface of T-cells and NK cells
what is LFA-3? where is it found?
cell adhesion molecule expressed on antigen presenting cells, particularily macrophages
what is LFA-1? where is it found?
- cell adhesion molecule found on lymphocytes
- plays key role in emigration (process of leaving the bloodstream and entering tissue)
what is ICAM-1? where is it found?
cell adhesion molecule expressed on antigen presenting cells
what is done to stabilize the adhesion process of T-cells to APCs?
the CD4 coreceptor binds to MHC class II on the APC
what family of molecules does LFA-1 belong to?
integrin
what family of molecules does LFA-3 belong to?
Ig superfamily
recall: what is the pre-signal?
adhesion of helper T-cell to the APC
what is the 1st extracellular signal?
antigen recognition:
- TCR recognizes the peptide:MHC complex
- this low affinity TCR-MHC interaction is stabilized by the adhesion molecules which form a synapse
- the synapse consists of the adhesion molecules moving and encircling the TCR-peptide:MHC complex for ~10 mins
- each peptide:MHC complex can sequentially engage w up to 200 TCRs on a T-cell in a process called serial triggering
what is the 2nd extracellular signal?
co-stimulatory molecules:
involves the binding of (non-specific) costimulatory molecules B7/CD28 or CD22/CD45R
what is B7? where is it found?
- proliferatory membrane protein found on activated antigen presenting cells
- there’s two types: B7-1 and B7-2
what is CD28? where is it found?
protein expressed on T-cells that provides co-stimulatory signals that provide T-cell activation and survival
what is CD22? where is it found?
sugar binding transmembrane protein that binds sialic acid and functions as an inhibitory receptor for B-cell receptor signalling
found on the APCs
what is CD45R? where is it found?
- member of the protein tyrosine phosphatase fam
- expressed on B-cells at all developmental stages and some T-cells
do APCs have similar or differential expression of co-stimulatory signals?
differential
describe the expression of co-stimulatory signals for dendritic cells
consecutive expression of B7
constantly expressed on the cell surface
describe the expression of co-stimulatory signals for macrophages
inducible expression of B7
describe the expression of co-stimulatory signals for B-cells
inducible expression of B7
can co-stimulatory singals be excitatory? inhibitory?
yes for both
what differentiates B7-1 from B7-2?
B7-2 expresses an intracellular phosphate group
the binding of B7-1 to CD28 elicits _____ signals
stimulatory
the binding of B7-2 to CTLA-4 elicits _____ signals
inhibitory
what is the 3rd extracellular signal?
effector cell function:
- involves preformance of the cell effector function
- TCR activation induces helper T-cells to produce cytokines that perform the effector function of the T-cell
- same thing occurs for cytotoxic T-cells
what is the result of successful extracellular signalling?
the promotion of a cascade of intracellular signalling to activate T-cells through the TCR/CD3
what are tyrosine-based activation motifs (ITAMs)?
conserved sequences contained by the cytoplasmic domains of CD3 molecules
what do ITAMs contain?
what role do these play?
ITAMs contain tyrosine residues that are phosphorylated by protein tyrosine kinases
of all of the TCR/CD3 complexes, how many are homodimers and how many are heterodimers?
90% homo (ζ,ζ)
10% hetero (ζ,η)
recall: what are the key players in intracellular singalling of the TCR/CD3 complex? (2)
tyrosine kinases and phosphatase
what is a kinase?
enzyme that catalyses the transfer of a phosphate group from ATP to a specific molecule
what is a phospatase?
enzyme that catalyses the hydrolysis of organic phosphates in a specified environment
what are the specific tyrosine kinases involved in intracellular signalling? (3)
ZAP-70
Fyn
Lck
what is ZAP-70?
protein normally expressed near the surface membrane of T-cells
what is Fyn?
- protein tyrosine kinase
- involved in the regualtion of cell morphology
what is Lck?
- protein tyrosine kinase found in T-cells
- most commonly associated with cytoplasmic tails of CD4 and CD8 co-receptors
what is step 1 of intracellular singalling for TCR activation?
- simultaneous binding of TCR and CD3 to the peptide:MHC complex results in the close proximity of Lck and Fyn on the ITAMs of CD3
- the kinases are unable to phosphorylate the ITAM tyrosine residues due to the presence of inhibitory phosphate grps
what is step 2 of intracellular singalling for TCR activation?
- binding of CD22 to CD45R activates the protein tyrosine phosphatase on the cytoplasmic tail of CD45R
- the phosphatase cleaves the inhibitory phosphate grps on Lck and Fyn
what is step 3 of intracellular singalling for TCR activation?
activated Lck and Fyn kinases phosphorylate the ITAM residues on CD3-ζε
what is step 4 of intracellular singalling for TCR activation?
the phosporylated CD3ζ chain ITAM residues act as docking sites for ZAP-70, allowing it to bind to CD3ζ and become activated
what is step 5 of intracellular singalling for TCR activation?
activated ZAP-70 and fyn kinases phosphorylate phospholipase C-γ1 (PLC-γ1) in the plasma membrane of the T-cell
what is step 6 of intracellular singalling for TCR activation?
acitvated PLC-γ1 cleaves phosphatidylinositol-4-5-biphosphate (PIP2) into inositol triphosphate (IP3) and diacylglycerol (DAG) - which activate transcription factors, NF-AT and NF-kB, respecitvely
what is step 7 of intracellular singalling for TCR activation?
- IP3 diffuses through the cytoplasm and binds to IP3-sensitive Ca channels, releasing intracellular Ca stores
- increased intracellular Ca concentration promotes protein kinase C expression on the plasma membrane
- prot. kinase C expressed on plasma membrane can be activated by DAG
what is step 8 of intracellular singalling for TCR activation?
- the increase of intracellular ca from the activation of IP3 sensitive CA channels also promoted binding of Ca to calmodulin
- calmodulin then activates calcineurin phosphatase which dephosorylates a family of transcription factors, including NF-AT
- NF-AT translocates into the nucelus and binds to the NF enhancer region on the IL-2 gene
what is step 9 of intracellular singalling for TCR activation?
- prot kinase C activated by DAG phosphorylates an inhibitory factor, releasing transcription factor NF-kB
- NF-KB then translocates to the nucleus and dimerizes with another IL-2 enhancing region
recap: what does the TCR signalling cascade ultimately lead to?
the enhanced transcription of the IL-2 gene
the production of IL-2 within the activated T-cell promotes what?
proliferation and differentiation of the T-cell into effector and memory T-cells
