T-cell 3: Receptor diversity and T-cell development

Question 1

Receptor diversity

Answer 1

• similar to immunoglobulin (antibody) light chain and heavy chain gene rearrangement (VDJ recombination)

TCR rearrangement

• alpha chain (V joins with J) - like the light chain
• beta chains (V, D, and J chains) - like the heavy chain

Diversity:

• -> which V and D and J
• -> which alpha and beta combine

Question 2

TRECs (T-cell receptor excision circles)

Answer 2

• TRECs are segments of DNA that are excised during VDJ rearrangement in T-cells and B-cells
• TRECs remain in the T and B cells, but do not replicate and are diluted by cell division
• measured in newborn screening (detects severe combined immunodeficiency)
• low levels are bad - mean rearrangement is not happening a lot - we expect a lot in babies

Question 3

Development of T-cells

Answer 3

• Progenitor T-cells migrate from the bone marrow to via blood to the thymus
• they mature in the thymus (T-cell = thymus dependent)
• from bone marrow to blood to venules in medulla of the thymus, to cortex of thymus

–> as individual grows older, thymus shrinks
In THYMUS:
- VDJ rearrangement
- Positive selection: t-cells that bind somewhat weakly to MHC
- Negative selection: t-cells that bind too strongly to self-antigens and self MHC will be removed

Migration to peripheral lymphoid organs - waits until it encounters foreign antigens and APCs.
Activated T-cells (differentiate into effector T-cells) - go to sites of infection or will remain in lymphoid tissue where they’re needed to help B-cells make their antibodies

CD3 - surface molecule on T-cells

• early on a T-cell will be double negative (no CD4, no CD8)
• then it displays both CD4 and CD8
• then it commits to having just CD4 or CD8 as it’s co-receptor with the T-cell receptor
• can use CD3 to measure t-cells in the blood (CD3 will tell you levels of all T-cells - as CD4 and CD8 are on and off)

Question 4

Selection of T-cells

Answer 4

DN4 (double negative 4 stage): proliferation of t-cell

Double positive cells move deeper into thymic cortex
Have 3-4 days to be positively selected

Question 5

Positive Selection

Answer 5

TCR must recognize corresponding self-peptide/self-MHC complex to be positively selected (not just the self-peptide) - must bind with some low affinity
At this stage, the t-cell commits to either CD4 or CD8

Thymic cortical epithelial cells: web of cell processes that make close contacts with double positive cells. They express the MHC molecules and self-peptides.

Question 6

Negative Selection

Answer 6

• If the double positive thymocyte’s TCR binds too strongly to MHC/self-peptide complex, it dies by apoptosis
• Antigen presenting cells (dendritic cells and macrophages) are important in negative selection
• -> found in medulla of thymus

Question 7

Affinity hypothesis

Answer 7

How does TCR binding to self-peptide/MHC complex result in 2 opposite outcomes?

1) low affinity binding –> survival
2) high affinity binding –> apoptosis