T-cell Antigen Recognition and Development - Kane

Question 1

What is a T-cell in general (phenotype/function)?

Answer 1

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.

Question 2

Describe some T-cell effector functions.

Answer 2

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.

Question 3

What are the subsets of T-cells and what do they recognize?

Answer 3

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.

Question 4

What are the subsets of alpha/beta T-cells?

Answer 4

CD4+ (helper) and CD8+ (killer) T-cells.

Question 5

Which MHC is typically recognized by CD8+ T-cells?

Answer 5

Class I

Question 6

Which MHC is typically recognized by CD4+ T-cells?

Answer 6

Class II

Question 7

What is MHC restriction?

Answer 7

This means that a T-cell must both recognize its cognate peptide in the context of MHC.

Question 8

In the TCR loci, where is the greatest source of diversity?

Answer 8

CDR1,2,3 regions (complementarity determining regions) especially CDR3. This is where N- and P-nucleotide addition can occur.

Question 9

What is combinatorial D-joining?

Answer 9

Process during recombination where two D segments are joined before J segment is added. Extra diversity.

Question 10

Does N-region addition occur in Ig’s and TCRs? If so, where?

Answer 10

Yes, in Ig’s this only occurs in the heavy chain whereas all TCR genes do this.

Question 11

Can T-cells undergo class switching?

Answer 11

No.

Question 12

Why are CDRs of the TCR chains critical to TCR binding?

Answer 12

They are highly variable regions that are directly binding to MHC-antigen during T-cell recognition.

Question 13

Where does CD4 and CD8 engage with MHC?

Answer 13

They stabilize TCR interaction with MHC by binding to non-polymorphic regions of MHC.

Question 14

How are the components of TCR assembled?

Answer 14

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.

Question 15

Affinity of Ig vs. TCR

Answer 15

Ig’s have a much higher affinity for ligand than TCR.

Question 16

What if TCR does not recognize (bind) MHC?

Answer 16

No immune response

Question 17

What if TCR recognizes self-MHC too strongly?

Answer 17

Autoimmunity

Question 18

Where are early T-cells derived and where do they develop?

Answer 18

T-cell progenitors come from the bond marrow and develop in the thymus.

Question 19

First step of T-cell development.

Answer 19

VDJ recombination of the Beta chain occurs and is trafficked to the surface to form a pre-TCR.

Question 20

Second step of T-cell development.

Answer 20

VDJ recombination of the alpha chain and pairing with the beta chain on the surface.

Question 21

Where are T-cells early in development in the thymus located? What does the thymus epithelium do?

Answer 21

The thymocytes are tightly packed in the cortex of the thymus. Thymic cells present antigen to developing T-cells.

Question 22

Describe the classic experiment regarding MHC restriction in the thymus.

Answer 22

The key concept is that T-cells will only recognize antigen in the context of MHC.

1. Remove MHC heterozygous mouse (a/b) thymus
2. Lethal x irradiation to kill bone marrow stem cells.
3. Graft in a homozygous b mouse thymus and a/b stem cells – immune reset w/ different thymus.
4. Infect with LCMV to illicit CD8+ T-cell response.
5. 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.

Question 23

Describe thymic development of T-cells by tracking CD4/8 co-receptors.

Answer 23

1. Progenitors enter the thymus as double negative (DN, no CD4/8).
2. 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.
3. T-cells come out either CD8 or CD4+

Question 24

What does the pre-TCR do?

Answer 24

Signals successful B-chain rearrangement and tells cell to not rearrange again (allelic exclusion). Signals to express co-receptors and begin A-rearrangement.

Question 25

Key transcription factor for CD8+ T-cells

Answer 25

Runx3

Question 26

Key transcription factor for CD4+ T-cells

Answer 26

ThPOK

Question 27

In the thymus, what mediates positive selection of developing T-cells?

Answer 27

Thymic epithelium presenting antigen on MHC-I in the cortex. Weak or no binding = death

Question 28

In the thymus, what mediates negative selection of developing T-cells?

Answer 28

In the thymic medulla, developing T-cells encounter dendritic cells presenting MHC-II. Too strong of binding = death.

Question 29

What determines which co-receptor a T-cell will express? (New key experiment):

Answer 29

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.

Question 30

How are self proteins of specific organs not recognized by T-cells? How is it regulated?

Answer 30

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.

Question 31

How was APECED discovered?

Answer 31

Mutation in AIRE leads to autoreactive T-cells and autoimmunity.

Question 32

What are Tregs

Answer 32

Regulatory T-cells that have an immune dampening function

Question 33

What is the Notch Pathway?

Answer 33

It is a regulatory pathway that regulates cell development including T-cells in the thymus.

Question 34

How are TCR transgenic mice generated?

Answer 34

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.

Question 35

What is a technique to track endogenous antigen-specific T-cells?

Answer 35

Use protein multimers with multiple peptide-MHC complexes to increase the avidity of T-cell binding.