T Cell Activation Flashcards
Describe the 3 signals required to activate naive Th cell
- TCR + CD4 binds MHC II + Ag
- Co-stimulatory signal: CD28 (on T cell) binds to CD80/CD86 on APC—without signal 2, Th becomes anergic and eventually undergo apoptosis
-
Signal to proliferate and differentiate:
- induced by IL-2 binding to IL-2 receptor on Th cell
- AND cytokines released by APC induces Th differentiation to Th1 or Th2
What happens if the naive Th doesn’t receive the co-stimulators signal ?
Th cell will become anergic and eventually undergo apoptosis
Why can’t the TCR itself generate a signal upon binding to the peptide + MHC ?
- TCR has no cytoplasmic tail
- must complex with proteins called CD3 whose tails have ITAM for signalling
- TCR has low affinity for peptide + MHC; other proteins needed to stabilize connection
What are accessory molecules on T helper cells ? Provide 2 examples.
Ligand to receptor pairs of cell surface molecules on T cell and APC
Ie. CD4 (T helper) and MHC II (APC)
CD28 ( T helper) and CD80/86 (APC)
3 functions of accessory molecules
- Adhesion:
- for multiple TCR to bind MCH
- stabilizes connection long enough for threshold - Amplification:
- enhances first signal to reach threshold for activation - Co-signalling:
- additional signalling pathway that is necessary
Provide 2 examples of accessory molecules that amplify the first signal for T cell activation
- CD8 (T cytotoxic) and MHC I
- CD4 (T helper) and MHC II (APC
NOTE: bolded are called co-receptors; NOT co-signalling
Provide an example of accessory molecules that amplify the second signal for T cell activation
- CD28 (T cell) and CD80/86 (APC)
NOTE: called the co-stimulatory signal
Provide an example of accessory molecules that facilitate adhesion for T cell activation
LFA-1 (T cell) and ICAM-1 (APC)
NOTE:
LFA-1 = leukocyte function antigen 1
ICAM-1 = intracellular adhesion molecule 1
Differentiate kinases vs phosphatase
Kinase: enzymes that add a phosphate
Phosphatase: enzymes that remove phosphate
Why are kinases and phosphatases advantageous in signalling cascades ? (2)
- Rapid; since no new protein synthesis is required for signal
- Reversible active state
Provide an example of how the presence or absence of a phosphate group changes enzyme activity
- Lck activity is blocked by a phosphate group
- a phosphatase can remove phosphate to restore Lck activity
WIP Provide an example of how the addition of a phosphate group can modify protein-protein interactions
- addition of phosphate to tyrosines on ITAM
- An SH2 interacts specifically with phosphorylated tyrosine residues
- SH2 domain of Protein B can now interact with the
tyrosine phosphorylated Protein A. This type of interaction is important for two reasons: (1) If
Protein B is another kinase which is normally free in the cytoplasm, it can recognize the
phosphorylated Protein A (which is also called an adaptor protein) in the cell membrane and be
brought to the cellular membrane where its substrate is located, to carry out the next step in the
signalling cascade, i.e. phosphorylation of Protein C
Protein A =LAT
SH2 = Lck
SH2 domains on ZAP-70 interact with phosphorylated tyrosines on TCR?
Outline the signalling cascade for when: TCR encounters peptide + MHC
- TCR binds peptide + MHC
- Co-receptors (CD8 or CD4) bind MHC = brings inactive Lck from cytoplasm to substrate (CD3)
- CD45 (phosphatase) removes phosphate on Lck
- Lck auto-phosphorylates itself at another tyrosine site = active
- Activated Lck (kinase) phosphorylates ITAM on CD3 = brings inactive ZAP-70 from cytoplasm to P-ITAM
- Lck phosphorylates ZAP-70 = active
- Activated ZAP-70 (kinase) phosphorylates LAT (membrane protein) = brings PLCy1 from cytoplasm to substrate
- ZAP-70 phosphorylates PLCy1 = active
- Activated PLCy1 cleaves PIP2 = IP3 + DAG
- IP3 binds to receptors on ER = opened channels increase intracellular [Ca2+] for other enzymes
- DAG activates PKC for downstream phosphorylation
How is Lck kept inactive when there is no TCR engagement ?
- Csk keeps Lck phosphorylated on regulatory site (tyrosines) until TCR binds peptide + MHC
- SH2 domains recognize phosphorylated tyrosines within Lck = keeps Lck inactive
What is LAT ?
- an adaptor protein located on the cell membrane
- not active, but further amplifies T cell signalling
- phosphorylated by ZAP-70
- brings PLCy1 from cytoplasm to its substrate
Describe signal 2 for T cell activation
Co-stimulatory signal: CD28 (T cell) binds to CD80/86 (APC)
- ensures survival of cell; without signal 2, cell becomes unresponsive and undergoes apoptosis
T or F: anergic T cells are non-responsive even when co-stimulation becomes available
TRUE: anergic T cells cannot be saved even if subsequent co-stimulation occurs after the CD28 to CD80/86 interaction has already been blocked
T or F: anergic T cells are non-responsive even when co-stimulation becomes available
TRUE: anergic T cells cannot be saved even if subsequent co-stimulation occurs after the CD28 to CD80/86 interaction has already been blocked
Purpose of Signal 3 in T cell activation (2)
- IL-2 (APC) to IL-2 receptor (T cell) binding = T cell proliferation in cell cycle
- other cytokines = differentiation to Th1 ad Th2 response
Describe differentiation pathway for T helper cells
- activated by cytokines from signal 3
Th1:
- intracellular pathogens induce cell-mediated response (viruses)
Ie. IL-12 from dendritic cells
Th2:
- extracellular parasites induce humoral response
- supports B cells to produce antibodies
Ie. IL-4 from dendritic cells
If a). TCR on Tc only recognize endogenous antigen on MHC I and b). naive T cells can only be activated with a second signal from APCs…
How are naive CD8+ T cells activated to become T cytotoxic lymphocytes in the absence of APCs ? (2)
Cross-presentation by dendritic cells:
A). dendritic cells first “licensed” by already activated T helper cell:
- dendritic cell presents antigen on MHC II
- CD40 (dendritic cell) binds to CD40L (T helper) = induces cross-presentation
B). Pattern Recognition Receptors or TLRs on dendritic cell:
- interact with extracellular pathogen
= Exogenous antigens originally presented on MHC II enter the endogenous pathway where they can be loaded onto MHC I = activate T cytotoxic cells
How is the T cell response terminated ? (3)
- tyrosine kinases have phosphatase counterparts that reverse kinase activation events
- CTLA-4 is up-regulated on activated T cells (absent on naive T) = recruits phosphatases
- also interferes with CD28 binding to CD80/86 - PD-1 (T cell) is a co-inhibitor receptor that binds PD-L1 or PD-L2 (APCs or tumor cells) = anergy and apoptosis
- Once antigen levels decrease, Bcl-XL levels decrease = Fas receptors on T cells trimerizes and binds Fas ligand
- activated T cells induce apoptosis on each other (naive T cells don’t express Fas or Fas ligand)
