T Cell Development ll

Question 1

Describe the experiment that allowed researchers to know that TCR affinity is important for determining thymocyte fate

Answer 1

1. research made use of cells from mice expressing transgenic TCR and a knock out of the TAP1 gene
2. The OT-1 transgenic TCR binds H2K MHC-I with Ova peptides
3. the TAP1 deficiency ensures that no endogenous peptides are presented on MHC-I molecules on the cell surface
4. exogenous Ova peptides of varying specificity was added to the extracellular environment so MHC-I can bind Ova to stabilize itself on the cell surface
5. the thymus of the transgenic and knockout mice was removed and grown on a medium where OT-1 peptides of intermediate or high affinity can be incorporated onto the surface of cells
6. T cell development is examined

Question 2

What was the experimental outcome of the OT-1 TCR transgenic and TAP1-deficient mice when exogenous Ova peptides of high and intermediate affinity were added, and a control where no peptide was added?

Answer 2

No peptide: no CD8+ T cells -> death by neglect
Intermediate affinity: positive CD8+ T cell selection -> survival and maturation
High affinity: negative CD8+ T cell selection -> clonal deletion

Question 3

What models are there for how a DP cell becomes a SP cell (lineage commitment)?

Answer 3

1. instructive
2. stochastic
3. others

note: these models are not mutually exclusive

Question 4

Describe the instructive model of lineage commitment

Answer 4

1. DP cells interact with cTECs expressing MHC-I and II + self peptides
2. if TCR/CD8 recognize MHC-1 + self-peptides, cells are instructed to become CD8+ cells and downregulate CD4
3. if TCR/CD4 recognize MHC-II + self-peptides, cells are instructed to become CD4+ cells and downregulate CD8

Question 5

Describe the stochastic model of lineage commitment

Answer 5

1. DP cells randomly down-regualte either CD4 or CD8
2. if CD4 is down-regulated, cells become CD8+ cells IF their TCR/CD8 can bind to MHC-I + self-peptide
3. if CD8 is down-regualted, cells become CD4+ cells IF their TCR/CD8 can bind to MHC-II + self-peptide

Question 6

Where does negative selection occur in the thymus and which cells mediate this?

Answer 6

1. cortex
   - mediated by cTECs and DCs
2. medulla
   - mediated by mTECs and DCs

Question 7

Why are mTECs unusual cells, why are they important in negative selection?

Answer 7

have the ability to express proteins that are usually tissue restricted (e.g. insulin)

this allows T cells to be educated against tissue-specific Ag.s before they leave the thymus

Question 8

What does expression of tissue-restricted Ag require, how does it do this?

Answer 8

Requires the epigenetic regulator AIRE which modified the chromatin

Question 9

What is APECED - what protein is affected, what does this cause, what is the outcome?

Answer 9

Protein = AIRE (mutations in the AIRE gene)

causes = T cells reactive to tissue-restricted self-AGs are not deleted in the thymus

outcome = different autoimmunity phenotypes in different patients

Question 10

What are some examples of different automimmune phenotypes caused by APECED?

Answer 10

1. endocrine autoimmunity: hypoadrenalism, hypoparathyroidism, hypothyroidism
2. type I diabetes
3. autoimmunity in other organs

Question 10

What are some examples of different automimmune phenotypes caused by APECED?

Answer 10

1. endocrine autoimmunity: hypoadrenalism, hypoparathyroidism, hypothyroidism
2. type I diabetes
3. autoimmunity in other organs