T cell and B cell Activation and Signalling Flashcards
What are the 3 signals that naive T cells need for activation
- TCR/CD3 complex interacting with peptide in either class II (CD4+) or class I (CD8+) MHC
- Cytokine signalling
- Costimulatory signalling
How soon are immediate genes expressed after TCR signalling?
within 30 minutes
What are immediate genes typically? what are 4 examples
Transcription factors
Fos + Jun = AP1
NFAT
NF-kB
How soon are early genes expressed after TCR signalling?
1-6 hours
What are early genes typically? What are some examples?
cytokines
IL-2, IFNy…
How soon are late genes expressed after TCR signalling?
more than 2 days
What are late genes typically? What are some examples?
Adhesion molecules like HLA-DR and VLA-1 to 5
what is CD45?
Cell surface phosphatase that catalyses the dephosphorylation of inhibitory tyrosine residues on p56lck or p59fyn
Engaging a single TCR with peptide results in ___ while engaging as few as 10 results in ___
intracellular calcium release; maximum calcium release
What is the composition of the CD3 coreceptor?
comprised of a zeta homodimer, gamma epsilon heterodimer and epsilon delta heterodimer
Which of the CD3 homodimers has the most ITAMS?
The zeta
Where is p56lck found normally in resting T cells?
sequestered in lipid rafts which keeps it away from the TCR/CD3
Describe the steps of signal transduction (to IP3 and DAG)
- TCR/CD3 is engaged by peptide/MHC
- TCR/CD3 is recruited to the lipid raft along with CD4/CD8 coreceptors and costimulatory ligands
- immunological synapse formed
- p56lck (w/o tyrosine) is associated with the tails of the CD4/CD8 and phosphorylates the ITAMs on the ζ, γ, δ, ε chains of the CD3 complex
- the phosphorylated ITAMs on the ζ chain homodimer form docking sites for ZAP-70
- Zap-70 is activated upon interacting with ITAMs (is a protein TK)
- ZAP-70 phosphorylates membrane associated adaptor proteins like LAT
- Phospholipase Cγ docks to the phosphorylated LAT and is phosphorylated/activated
- Phospholipase Cγ causes hydrolysis of membrane associated phosphoinositol bisphosphate (PIP2)
- This generates inositol 1,4,5-triphosphate (IP3) ad diacylglycerol (DAG)
What is the result of IP3 being formed?
Increase in intracellular calcium levels
What results from elevated intracellular calcium levels?
Calcineurin mediated dephosphorylation of cytosolic NFAT
Where does the dephosphorylated NFAT translocate to? what does it support the transcription of?
the nucleus, cytokines like IL-2 and IL-4
What is the pathway activated by diacylglycerol?
Activated protein kinase C
Where does the activated PKC go? What does it cause
To lipid rafts where it phosphorylates serine and threonine residues, ultimately culminating in the activation of NF-kB
What normally sequesters NF-kB in the cytoplasm?
IkB
What is the inhibitor of IkB? How?
IKK, deactivates it by phosphorylating it
Whats one example of a cytokine induced by NF-kB?
IL-2
What is Ras? when is it activated?
small G protein that is activated by GTP following TCR/CD3 signaling
What does activation of Ras lead to?
activation of the MAPK pathway including MAPK ERK
What does MAPK ERK activate? what does it induce?
TF Elk which induces Fos expression
What happens to Fos after it is produced?
Phosphorylated by ERK and combines with Jun-P to form AP-1
What is AP-1 involved in regulating
IL-2 gene transcription
during TCR signalling, what normally provides the costimulatory signal? which is on which cell?
CD28 on the Th cell interacting with B7 family members on the APC (DC, macrophages, activated B cells)
What are the two main forms of B7?
B7-1 (CD80) and B7-2 (CD86)
What do CD28 and TCR signalling synergize to augment? How?
cytokine production (e.g. IL-2) by enhancing gene transcription and stabilizing cytokine mRNA.
What is also found on activated T cells but acts as a negative co-stimulatory molecule?
CTLA4
What is CTLA-4 structurally similar to?
CD28
What is the function of CTLA-4 and how does it exert this? When is it expressed ?
Expressed within 24 hours of TCR engagement
Outcompetes CD28 for binding with B7 family members
Provides an inhibitory signal via phosphatase activation
What is the proportion CTLA-4 produced?
expression is proportional to the strength of CD28 signaling
What is PD-1? what does it interact with
another negative costimulator that interacts with PD-L1 and PD-L2 to inhibit T cell activation
Describe the steps in B cell signalling?
- mIg is engaged by antigen crosslinking it
- Src kinase phosphorylates ITAM residues on the cytoplasmic tails of Ig-a and Ig-ß
- phosphorylated ITAMs provide docking sites for Syk
- Syk is activated by phosphorylation
- Syk-P activates/phosphorylates adaptor protein BLK
- BLK-P allows for docking and activation of PLCy2 and Brunton’s tyrosine kinase (Btk)
- PLCy2 will cleave PIP2 into IP2 and DAG and along with Btk will activate similar intracellular signalling to T cells
* small G protein pathways are also activated which lead to the same end point
END RESULT: activation of transcription factors such as NF-κB and AP-1
What 3 proteins make up the B cell coreceptor?
CD19, CR2(CD21), TAPA-1
What is the structure of CD19? what does it act as a docking site for?
long cytoplasmic tail that contains docking sites for intracellular molecules such as the protein tyrosine kinase Lyn
What is the role of TK Lyn?
augment signals delivered by the B cell receptor
What is achieved by having BCR co-receptor signalling?
B cell activation with substantially fewer mIg molecules engaged
What does CR2 interact with? why?
C3d (formed by C3b cleavage by factor I). Forms a connection between CR2- C3d- antigen-mIg
What is CD22? is it constitutively or transiently expressed? How does it signal?
constitutively associated with the BCR
following activation of BCR will deliver a negative signal
Does this through the action of SHP-1 tyrosine phosphatase bound to ITIMs
What are anergic T cells?
T cells unable to respond to anergic peptide
What causes T cell anergy?
receiving signals through the TCR/CD3 complex but fail to receive a costimulatory signal
90-95% of peripheral T cells express what kind of TCR?
αβ TCR
What are the γδ TCR population called? where are they normally found ?
double negative T cells
predominantly in the skin, intestinal epithelium, and pulmonary epithelium
What is special about the γδ TCR cells?
they are not restricted in activity by self MHC
What makes γδ (DN) T cells good at being a 1st line of defense against cancer and infections?
ability to respond directly to heat shock antigens
Where are naturally occurring CD4+CD25+ T regulatory cells generated? where do they go?
generated in the thymus and migrate to the periphery to regulate other T cells activity
Naive T cells express low levels of what cell adhesion protein and high levels of which 2 others?
low levels of CD44 and high levels of CD62L (aka L-selectin) and CCR7
What do CD62L and CCR7 allow for?
homing to secondary lymphoid tissues and extravasation from the circulation via high endothelial venules
in the absence of antigenic stimulation, naive T cells will live for?
5-7 weeks
A naïve T cell that recognizes antigen/MHC on a dendritic cell or target cell becomes __ and initiates __
activated; primary response
activated T cells enlarged into a ___ and undergo____ which generates __ cells or __ cells
Blast cell; repeated rounds of cell division; effector; memory
Since memory T cells have less stringent activation requirements than naïve T cells….
they are able to respond more rapidly to antigens presented
what are the two subsets of memory T cells?
Central and effector
What differentiates between central and effector memory T cells?
presence or absence of CCR7
- central memory T cells have it
- effector memory T cells don’t
Where do CMT cells reside and travel between?
Reside in and travel between 2º lymphoid tissues
What is the difference between CMT and EMT cells in terms of longevity and proliferative capacity?
CMTs are longer lived an have greater proliferative capacity
Where do EMT cells reside and travel between
reside and travel between tertiary lymphoid tissues
- are able to rapidly differentiate there
What is the difference between Naive T cells and effector T cells in terms of Cd44, L- selectin and CCR7?
Naive: low CD44, high L selectin and CCr7
Effector: high CD44, low l-selectin and no CCR7
