T-cell Antigen Recognition and Development - Kane Flashcards
What is a T-cell in general (phenotype/function)?
T-cells generally express CD4 or CD8 and engage with MHC (I and II bound to peptides) via their TCR to initiate immune effector or regulatory function.
Describe some T-cell effector functions.
T-cells recognize foreign peptides presented on MHC and can stimulate B-cell antibody secretion, secrete cytokines to stimulate leukocytes, and directly kill virally infected cells.
What are the subsets of T-cells and what do they recognize?
Alpha/beta T-cells can only bind MHC-peptide antigen. Gamma/delta T-cells have more flexibility being able to recognize lipid or whole protein. There are also Tregs.
What are the subsets of alpha/beta T-cells?
CD4+ (helper) and CD8+ (killer) T-cells.
Which MHC is typically recognized by CD8+ T-cells?
Class I
Which MHC is typically recognized by CD4+ T-cells?
Class II
What is MHC restriction?
This means that a T-cell must both recognize its cognate peptide in the context of MHC.
In the TCR loci, where is the greatest source of diversity?
CDR1,2,3 regions (complementarity determining regions) especially CDR3. This is where N- and P-nucleotide addition can occur.
What is combinatorial D-joining?
Process during recombination where two D segments are joined before J segment is added. Extra diversity.
Does N-region addition occur in Ig’s and TCRs? If so, where?
Yes, in Ig’s this only occurs in the heavy chain whereas all TCR genes do this.
Can T-cells undergo class switching?
No.
Why are CDRs of the TCR chains critical to TCR binding?
They are highly variable regions that are directly binding to MHC-antigen during T-cell recognition.
Where does CD4 and CD8 engage with MHC?
They stabilize TCR interaction with MHC by binding to non-polymorphic regions of MHC.
How are the components of TCR assembled?
They are transcribed and translated separately and assembled in the ER. They configure on the surface of the cell mediated by charge residues in the TM regions. The full complex must be present for surface expression.
Affinity of Ig vs. TCR
Ig’s have a much higher affinity for ligand than TCR.
What if TCR does not recognize (bind) MHC?
No immune response
What if TCR recognizes self-MHC too strongly?
Autoimmunity
Where are early T-cells derived and where do they develop?
T-cell progenitors come from the bond marrow and develop in the thymus.
First step of T-cell development.
VDJ recombination of the Beta chain occurs and is trafficked to the surface to form a pre-TCR.
Second step of T-cell development.
VDJ recombination of the alpha chain and pairing with the beta chain on the surface.
Where are T-cells early in development in the thymus located? What does the thymus epithelium do?
The thymocytes are tightly packed in the cortex of the thymus. Thymic cells present antigen to developing T-cells.
Describe the classic experiment regarding MHC restriction in the thymus.
The key concept is that T-cells will only recognize antigen in the context of MHC.
- Remove MHC heterozygous mouse (a/b) thymus
- Lethal x irradiation to kill bone marrow stem cells.
- Graft in a homozygous b mouse thymus and a/b stem cells – immune reset w/ different thymus.
- Infect with LCMV to illicit CD8+ T-cell response.
- Take CD8 T-cells from spleen and see if they can destroy type A or B cells infected with LCMV.
New T-cells coming from the bone marrow cannot recognize A type MHC because they were trained with a B type thymus. T-cells need to be able to recognize YOUR MHC.
Control, infect A/B mouse and see if its T-cells can destroy both a/b cells.
Describe thymic development of T-cells by tracking CD4/8 co-receptors.
- Progenitors enter the thymus as double negative (DN, no CD4/8).
- After TCR assembly, they divide and express both CD4 and CD8 (DP). If TCR assembly fails or has weak binding, the T-cell undergoes negative selection. High affinity TCR T-cells will also die.
- T-cells come out either CD8 or CD4+
What does the pre-TCR do?
Signals successful B-chain rearrangement and tells cell to not rearrange again (allelic exclusion). Signals to express co-receptors and begin A-rearrangement.
Key transcription factor for CD8+ T-cells
Runx3
Key transcription factor for CD4+ T-cells
ThPOK
In the thymus, what mediates positive selection of developing T-cells?
Thymic epithelium presenting antigen on MHC-I in the cortex. Weak or no binding = death
In the thymus, what mediates negative selection of developing T-cells?
In the thymic medulla, developing T-cells encounter dendritic cells presenting MHC-II. Too strong of binding = death.
What determines which co-receptor a T-cell will express? (New key experiment):
Flip flop mice were generated to express CD8 cells from the CD4 regulatory region and vice versa. So, flip-flop mice expressing CD8 express ThPOK transcription factors. Because CD8 expression is transiently terminated during positive selection, it is the duration of TCR signaling during positive selection.
How are self proteins of specific organs not recognized by T-cells? How is it regulated?
Peptide from different organs can be transiently expressed in the thymus and presented on MHC (i.e. insulin). This is regulated by AIRE, a txn factor that drives expression of tissue-specific antigen in the thymus.
How was APECED discovered?
Mutation in AIRE leads to autoreactive T-cells and autoimmunity.
What are Tregs
Regulatory T-cells that have an immune dampening function
What is the Notch Pathway?
It is a regulatory pathway that regulates cell development including T-cells in the thymus.
How are TCR transgenic mice generated?
Manipulation of allelic exclusion of T-cells. You can force expression of a rearranged TCR a/b, recombination is mostly shut down preventing expression of endogenous TCR. Mice now express one TCR that recognizes one MHC-antigen.
What is a technique to track endogenous antigen-specific T-cells?
Use protein multimers with multiple peptide-MHC complexes to increase the avidity of T-cell binding.
