T-cell Maturation Flashcards
Compare antibody: antigen binding to t-cellR: antigen binding.
Antibodies: bind epitopes on proteins, carbs or lipids on surface of bacteria, viruses, parasites T-cells. Can bind more than one antigen (2-5 (IgM pentamer))
T-cells: bind peptide antigens from pathogen proteins presented on MHC. TCR binds 1 antigen.
both have 3 CDR domains on V region= hypervariable domains for Ag specificity.
Why don’t T-cell Receptors under somatic hypermutaiton or switching?
T-cell receptors are **ONLY RECEPTORS. **Antibodies are both recognition and effector molecules.
T-Cell receptor classes
Alpha-beta
gamma-delta
T- cell receptor strucure (alpha-beta t-cell)
- TCR alpha chain- variable domain (ag spec) and constant domain
- TCR beta chain- variable domain (Ag spec) and constant domain
- Both chains have transmembrane region with cytoplasmic tail
*
Regions on different TCR chains
β/γ= V, D, J, C regions (like heavy chain of Ab)
α/δ= V, J, C regions ( like light chain of Ab)
Remember: α:β T cells (~90%)>>> γ:δ T-cells
What transmits the signals into the T-cell when an antigen binds the TCR?
CD3 chains. (epsilon: delta), (epsilon: gamma) dimers. Maybe more about this later?
TCR development? Where does this happen?
THYMUS
- Alpha/ Delta: VJ recombination then splicing to make VJC
- beta/gamma: DJ recombination ==> VDJ recombination then splicing.
Pretty similar to antibodies. Uses RSS (recombination signal sequence 12/23bp spacers) and RAG complex
RAG complex (review!)
RAG cleaves recombination signal sequences to make DNA hairpin which is then cleaved. TdT randomly adds nucleotides (increased diversity) between the DJ segments.
TCR complex
TCR complex= TCR + CD3 Complex + 2 zeta chains
CD3 complex= CDepsilon (x2), CDgamma, CDdelta, z
T-Cell development (from bone marrow–> TCR rearrangment)
- CD34+ cell ==> thymus (T-cell progenitor, no t-cell markers/receptors)
- +++IL-7 ==> CD2+, CD8-, CD4- = double negative thymocyte
- Notch1 (on thymocyte) binds receptor on thymic epithelium==> Notch1 intracellular domain removes inhibition of transcription in nucleus
- beta, gamma, delta rearrangement begins (it’s a race!)
T-cell differentiation REQUIRES
**IL-7 and Notch-1. **
Notch1 is equivalent to Pax-5 in B-cell development
How does the t-cell receptor decide what subunits it has?
beta, gamma and delta all compete. If delta and gamma win (successfully recombine first), then TCR is gamma/delta without any CD4/CD8.
If beta recombines first, recombination stops. Proliferation of cell and formation of DP thymocyte (CD4+, CD8+). Then alpha competes with gamma delta. If gamma/delta win, it’s gamma/delta. If alpha wins delta is cut out (via recomb process)= committed to alpha/beta. (it’s double positive)
Alpha chain rearrangement
- VJ joining= removal of delta chain locus
- Lots of changce to make productive rearrangement
- Productive rearrangment in one chromosome DOES NOT inhibit rearrangemnt of alpha chain in the other chromosome
Beta chain rearrangment
- D+J, DJ + V, DJV + C
- 2 chances because there are only 2 C domains
- Successful beta chain rearrangement INHIBITS rearrangement of beta chain in other chromosome
- When successful, Beta chain goes to the ER and if it successfully binds pTalpha (alpha chain surrogate), then it gets CD3 complex and moves to membrane
- Most gamma/delta Tcells don’t express?
- What is the function of gamma/delta t-cells?
- CD4 or CD8
- antibacteria/viral/tumor functions–recognize unusual antigens:
- E4-HMB-PP= found in pathogenic bacteria (mycobacteria, malaria parasites)
- transformed host cell MHC1b
- HSP (heat shock protein), superantigens
Does not require antigen presentation/MHC to do it’s job.
CD4 vs CD8 cell functions
- CD4+ binds MHCII and regulates (activation or repression) macrophges, b-cells, etc
- CD8+ bind MHCI cells infected with intracellular pathogens and kills them
CD8+ structure
alpha-beta heterodimer
CD4+ structure
single chain made of D4,D3,D2,D1 subunits
Are CD4+ more common, or CD8+?
CD4>>CD8
Double positive thymocyte…Now what?
- Positive selection in the cortex of the thymus
- Negative selection in the medulla
Positive selection
- TCR that are double positive need to recognize MHC I or MHC II (on thymic epithelial cells) within 3-4 days or they die (apoptosis)
- alpha chain rearrangement can occur during this time to enhance chances of MHC binding
- Once TCR binds MHC successfully, RAG is degraded (ie no more recombination)
T-cells: can they have more than 1 type of TCR?
Technically, yes. Since alpha chain recombination doesn’t inhibit other alpha chain recombination, it’s possible. But the chances of even making it through the thymus are 1-2%, so it’s unlikely that the cell will be able to find antigens to BOTH recepots. So, usually T-cells are considered to have 1 TCR that binds 1 antigen
negative T-cell selection
After positive selection, the SP (single positive) t-cell moves into the medulla of the thymus. Interaction with dendritic cells and macrophages presenting self-antigen==>apoptososis.
2% of cells survive all this shit.
Lots of MHC molecules means that?
- “arithmetic” increased in number of cells surviving positive selection (xN)
- “geometrical” increase in number of cells failing at negative selection (xN2)
- So, I guess the magic number is 13.
Things needed to increased diversity of t-cell receptors?
TdT and RAG1/2.
FoxP3
“master regulator” of T-Reg cells.
According to Wiki:
down regulation= increased tumors
All t-cells are positive for?
**TCR, CD2, CD3, CD28 **
(TCR + CD3= TCR complex)
