T-Cell biochemistry Flashcards
Downsides of antibodies:
There are things that antibodies cannot be used to combat effectively, what are these things?
- Intracellular pathogens
- Bacteria and Viruses
- Tumours
Downsides of antibodies:
As antibodies cannot be used to combat everything, what else does the immune system need?
The T-cell
The T-cell receptor (TCR):
Approximately how many T-Cell receptors are on the surface of the the T-Cell?
10^5
The T-cell receptor (TCR):
What does the TCR consist of?
- 2 polypeptide chains
- Majority - ⍺β T-cells
- Minority - 𝛾δ T-cells
Differences between the TCR and Fab:
C⍺:
Half of the domain, what does that closest to the β chain form?
- Forms a β-sheet
Differences between the TCR and Fab:
C⍺:
What is the other half made up of?
- The other half is made up of loosely packed strand and a short stretch of ⍺-helix
- Held to one of the strands of the β-domain to this helix
Differences between the TCR and Fab:
What are the interactions between C⍺ and Cβ assisted by?
- Carbohydrates
- Carbohydrate on C⍺ forming hydrogen bonds with the amino acids of Cβ.
Differences between the TCR and Fab:
CDR loops: what are they fairly similar to?
Fairly similar between an antibody and the TCR, some displacement
Differences between the TCR and Fab:
What do TCRs have a 4th of? and where is it found?
- TCRs have a 4th hypervariable region
- Away from the antibody binding site
T-cell receptor gene rearrangement:
TCR⍺ is similar to what chain? and why?
- The Immunoglobulin light chain
- V and J segments
T-cell receptor gene rearrangement:
TCRβ is similar to what chain? and why?
- The immunoglobulin heavy chain
- V, D, and J segments
T-cell receptor gene rearrangement:
When do the genes rearrange and where does the process happen?
What is this process similar to?
- The genes rearrange during T-cell development
- In the thymus
- In a process similar to the Germinal Centre Reaction for B-cells
T-cell Receptors don’t bind antigen directly:
Where do antibodies bind antigen?
- Antibodies bind antigen in the circulation
- Whether that be free flowing or on the surface of the pathogen
T-cell Receptors don’t bind antigen directly:
What do T cells bind to?
- T-cell bind to an antigen-protein complex
MHC-I and MHC-II:
What is MHC?
MHC= Major histocompatibility complex
T-cell Receptors don’t bind antigen directly:
What is an antigen-protein complex?
- Short, continuous amino acid sequences from an unfolded protein
-Presented as part of a protein complex on the surface of an antigen presenting cell
MHC-I and MHC-II:
The 2 are closely related in overall structure and function, but what do they differ in?
- Differ in their protein subunits
- 2 paired domains nearest the membrane resemble an immunoglobulin
- 2 domains furthest from the membrane produce a peptide-binding cleft
MHC-I and MHC-II:
What does MHC-I consist of?
- 2 Polypeptide chains
- The ⍺-chain, which makes up 3 domains of the protein and crosses the membrane and forms the entire peptide binding cleft
- Β2-microglobulin associates with the ⍺3 domain
MHC-I and MHC-II:
What does MHC-II consist of?
- 2 polypeptide chains
- Both the ⍺ and the β chain cross the membrane
- The peptide binding cleft is made up of both the ⍺ and the β chain
MHC-I and MHC-II:
What do MHCs need to be able to do in order to stimulate T-Cells?
- MHCs need to be able to bind a large variety of peptides
MHC-I and MHC-II:
When are MHC proteins unstable?
- MHC proteins are unstable when not bound to a peptide
MHC-I and MHC-II:
What length of amino acids to MHC-I and MHC-II bind peptides to?
- MHC-I binds peptides 8-10 amino acids in length
- MHC-II can bind peptides of any length
MHC-I and MHC-II:
MHC-I binds and presents intracellular peptides that reside where?
- In the cytosol
MHC-I and MHC-II:
Where do some pathogenic bacteria and some protozoa reside?
- Reside inside vacuoles once inside the cell and peptides are presented on MHC-II
MHC-I and MHC-II:
Extracellular pathogens go through what process? and where are peptides then presented?
- Extracellular pathogens are phagocytosed
- Peptides are presented on MHC-II
MHC-I and MHC-II:
What is cross presentation?
- Some pathogens will not infect phagocytic antigen presenting cells (such as dendritic cells). eg the epithelium
- So the antigen presenting cell will express MHC-I presenting peptide antigens from the infected cell
T-cell Co-receptors:
What are the 2 T-cell co receptors and what are their functions?
- CD4+:
- T-helper cells
- Recognises MHC-II - CD8+
- Cytotoxic T-cells
- Recognises MHC-I
T-cell Co-receptors:
How do co-receptors work?
- TCR binds directly to the MHC peptide binding cleft
- CD4 and CD8 bind to an invariant site away from the peptide binding cleft
TCR signal transduction:
Why is the CD3 complex needed and what does it consist of?
- Needed as the TCR⍺β heterodimer, along with either CD4 or CD8 is not sufficient to initiate activation of the T-cell.
CD3 complex:
CD3𝛾
CD3δ
CD3ε
TCR signal transduction:
What is signalling initiated by in this process?
- Signalling is initiated by ITAMs in the 𝛾, δ, ε, and ζ chains
- ITAMs = Immunoreceptor Tyrosine-based Activation Motifs
How many ITAMs does each of the following have?
1.CD3𝛾
2. δ
3. ε
4. ζ chain
1.CD3𝛾 =1
2. δ =1
3. ε =1
4. ζ chain =3
TCR signal transduction:
What does each ITAM have?
- Each ITAM has 2 Tyrosine residues
TCR signal transduction:
What does the phosphorylation of the tyrosine residues lead to?
- Phosphorylation leads to the recruitment of Zap70
TCR signal transduction:
Zap70 phosphorylates LAT (Linker for Activated T-cells). What does this then lead to?
- Leads to the recruitment of PI 3-kinase
TCR signal transduction:
After Zap70 phosphorylates LAT, What happens from here?
- From here the T-cell signal branches into distinct modules
- leading to the activation of different transcription factors with different effects
TCR signal transduction:
What are the effects of the following transcription factors?
- NFκB
- AP-1
- NFAT
- NFκB: the master inflammatory transcription factor
- AP-1: differentiation, proliferation, and apoptosis
- NFAT:
- Requires Calcium influx
- Family of 5 proteins differentially expressed in different tissues
- For T-cells, NFAT is important for activation and the production of specific signalling molecules
T-cell maturation:
Where are T-cells derived from? and where does all of the development of them happen?
- Derived from the bone marrow
- All the development happens in the thymus
T-cell maturation:
What are the similarities of T and B cell maturation?
- Gene re-arrangement
- Testing of the new receptor
- Release of the new T-cell or cell death
T-cell maturation:
There is a round of positive and negative selection, what is this dependent on?
Dependent on Notch signalling
T-Cell subsets:
CD4+ T-cell produces different subtypes depending on the signal from other parts of the immune system, What are they? (6 answers)
- Tfh
- Th1
- Th2
- Th17
- Treg
- CD4 CTL? (Unclear if this actually exists yet)
T-Cell subsets:
What is the function of Tfh?
Tfh= B-cell formation
T-Cell subsets:
What is the function of Th1?
Th1:
- Type 1 response
- Autoimmunity
- Intracellular bacteria, protozoa and viruses
T-Cell subsets:
What is the function of Th2?
Th2:
- Type 2 response
- Allergy and asthma
- Extracellular helminths and venoms
T-Cell subsets:
What is the function of Th17?
Th17:
- Type 3 response
- Autoimmunity
- Extracellular bacteria and fungi
T-Cell subsets:
What is the function of Treg?
Treg:
- Immune tolerance
- Immune regulation
T-Cell subsets:
What is the suggested function of CD4 CTL?
CD4 CTL:
- MHC II- dependent killing
CD8 Cytotoxic T-cells:
How do they kill cells?
- By inducing cell death by either:
- Intrinsic pathway of apoptosis
- Extrinsic pathway of apoptosis
CD8 Cytotoxic T-cells:
What do both intrinsic and extrinsic pathways of apoptosis require?
Both require capase enzymes:
- Proteases
- Initiator capases
- Effector capases
CD8 Cytotoxic T-cells:
Explain how initiator and effector capases work.
- Initiator capases:
- Promote apoptosis by cleaving and activating other caspases
- Intrinsic = Caspase 9
- Extrinsic = Caspase 8 and 10 - Effector Caspases:
- Initiate the cellular changes associated with apoptosis
- Both pathways use Caspase 3, 6, and 7
CD8 Cytotoxic T-cells:
Why is cytochrome C important?
Important for electron transport chain
