Test 2 - Chapter 7 - T-Cell Development

Positive selection 1) What two things does positive selection determine?

1) 1. Positive selection determines which co-receptor is expressed: CD4 or CD8 2. Positive selection determines which MHC molecule a T-cell is 'restricted' to. ▪ i.e., it is restricted to recognizing the MHC molecule that selected it.

T cell development: α chain somatic recombination in pre-T cells 1) How many α-chain gene copies does everyone have? 2) How many attempts at somatic recombination can be made on the α-chain?

1) Everyone has 2 α-chain gene copies 2) Several attempts at somatic recombination can be made on each gene copy

Each β chain gene has 2 attempts at a successful recombination 1) How many β-chain gene copies does everyone have? 2) Each of the two constant region exons has its own associated...? (a) Therefore, each gene copy gets...? (b) Therefore, each cell gets...?

1) Everyone has 2 β-chain gene copies (one from the mother and one from the father) 2) D and J segments (a) 2 attempts to produce a functional β-chain (b) 4 total attempts to produce a functional β chain

T cell precursors originate in the bone marrow and develop into T-cells in the thymus 1) Bone marrow is responsible for the...? 2) What 3 things occur in the Thymus? 3) What makes up peripheral circulation? 4) What two things happen in peripheral circulation?

1) Generation of T cell precursors 2) ▪ Production of functional T-cell receptor ▪ Positive selection ▪ Negative selection (central tolerance) 3) blood and secondary lymphoid tissues 4) ▪ Negative selection (peripheral tolerance) ▪ Activation if receptor binds antigen

Negative selection (Central Tolerance) 1) What happens when T-cells bind TOO strongly to self peptide? 2) What types of cells participate in negative selection?

1) MHC complex are induced to undergo apoptosis. Or some become regulatory T cells (Tregs). 2) Dendritic cells, macrophages, and specialized medulla thymic epithelial cells participate in negative selection

Positive selection 1) T cells are positively selected for...? 2) How are self peptides presented to thymocytes? (a) What signals T-cells to survive? (b) What signals a T-cell to continue with receptor editing? (c) What signals a T-cell to undergo apoptosis?

1) T cells are positively selected for their ability to bind a self-MHC molecule (recall everyone has up to 12 different MHC molecules for antigen presentation) 2) Self peptides are presented to thymocytes on MHC class I and II by cortical epithelial cells. (a) T-cells that bind self-peptide/MHC complex are signaled to survive (b) T-cells that don't bind self-peptide/MHC complex continue with receptor editing (c) T-cells unsuccessful after receptor editing undergo apoptosis

Negative Selection (Peripheral tolerance) 1) What occurs for Negative selection in peripheral circulation? 2) 2 major mechanisms?

1) Tolerance to self-antigens continues to develop in the periphery (referred to as peripheral tolerance) 2) ▪ T-cells that recognize self-antigen in the absence of inflammation become anergic. ▪ Regulatory T-cells (Tregs): recognize self-antigen/MHC complex and suppress naïve T-cells that recognize an antigen presented by the same dendritic cell.

MHC Restriction 1) The T-cell receptor is specific for...? 2) Therefore, what must there be for a hematopoietic stem cell transplant transplantation?

1) both peptide and MHC allele 2) Therefore, there must be some degree of match between donor and recipient MHC alleles for a hematopoietic stem cell transplant transplantation.

Pre-T-cell tests β chains for functionality by forming a pre-T-cell receptor 1) Rearranged β chains are tested for their ability to...? 2) If the first gene copy does not produce a functional β-chain after both attempts, what happens? 3) What happens if the second gene copy fails to produce a functional β-chain?

1) form a pre-T-cell receptor 2) the second gene copy undergoes somatic recombination 3)

Somatic recombination in double negative thymocytes 1) Somatic recombination of the γ, δ and β chains commences...? 2) How does somatic recombination occur for each gene? 3) If γ and δ rearrange before β then...? 4) If β chain rearranges before both γ and δ then...? 5) What path is it more likely to take?

1) simultaneously 2) Somatic recombination occurs one chromosome at a time for each gene 3) cell becomes a committed γ:δ T-cell 4) cell pauses somatic recombination of γ and δ (and will re-commence later) 5) β chain rearrangement

Rearranged α-chains are tested for their ability to form a T-cell receptor 1) What happens if a functional T-cell receptor is formed? 2) What happens if a functional T-cell receptor is not formed?

1) somatic recombination ceases 2) somatic recombination continues using remaining gene segments

Somatic recombination of the α chain eliminates the δ chain 1) An α chain rearrangement eliminates...? (a) Therefore, what is more likely to be the outcome of rearrangement?

1) the δ chain gene segments (a) Therefore, it is more likely that a functional α:β receptor will be the outcome rather than a γ:δ receptor

Comparison of α:β and γ:δ T cells 1) Receptor repertoire? 2) What kind of antigens do they recognize? 3) Where do they circuclate? 4) What percentage of T-cells do they make up? 5) How do they interact with MHC molecules? 6) How much time do they take to respond?

1) α:β - Infinite receptor repertoire γ:δ - Restricted receptor repertoire 2) α:β - Recognize primarily microbial antigens γ:δ - Recognize antigens associated with cell stress 3) α:β - Circulate in blood and secondary lymphoid tissues γ:δ - Circulate in tissues 4) α:β - Approx. 90 - 95% of T cells γ:δ - Approx. 5 -10% of T cells 5) α:β - Recognize antigen on MHC molecules γ:δ - Do not interact with MHC molecules 6) α:β - Take time (~5 days) to become activated γ:δ - Respond quickly (often described as innate-like)

Each β chain gene has _____ attempts at a successful recombination

2

What is MHC restriction?

A T cell is restricted to recognizing its peptide only when the peptide is presented on the specific MHC molecule that positively selected the T cell.

Thermocytes become either....?

A committed CD4 T-cell or a committed CD8 T-cell.

When does the α:β T cell become committed to the α:β lineage

After a somatic recombination has been completed on the alpha chain. The delta chain gene is located in between the V and J gene segments on the α chain, so an α chain somatic recombination deletes the delta gene. Each alpha chain gets multiple attempts to produce a successful somatic recombination, so most thymocytes will produce a functional α-chain.

Which cell receptors recognize antigen presented on MHC molecules?

Alpha-beta T-cells

Which cells undergo positive selection?

Alpha-beta T-cells

Which cell receptors recognize surface antigens?

B-cells

Which cells undergo class switching?

B-cells

Which cells undergo somatic hypermutation?

B-cells

Which cells undergo negative selection?

B-cells and T-cells

Which cells undergo somatic recombination and junctional diversity to produce the receptor?

B-cells and T-cells

Production of T cells in the thymus decreases with age When is the thymus most active in producing T-cells?

Before puberty

Where do regulatory T cells arise and how do they participate in negative selection (removal of self-reactive T cells) after T-cells are released into peripheral circulation?

During negative selection, some of the T cells that are strongly self-reactive (bind strongly to the self-peptide:MHC complex) become anti-inflammatory T regulatory (Treg) cells instead of undergoing apoptosis. The Tregs circulate in the periphery and recognize the same self-antigen:MHC complex when presented by a dendritic cell. The Treg will suppress the activation of any other T-cells that are being presented with antigen by the same dendritic cell (because if the dendritic cell is presenting a self-peptide to the Treg then it may also be presenting a self-peptide to the other T-cells too).

What determines whether a T cell will express the CD8 or the CD4 co-receptor?

During positive selection, if the T cell is positively selected by an MHC class I molecule then it will become a CD8 T cell. If it is positively selected by an MHC class II molecule then it will become a CD4 T cell.

In addition to regulatory-T-cells, what is another mechanism of negative selection in peripheral circulation?

If a T-cell receptor binds to its antigen in the absence of inflammation, then the T-cell becomes anergic (permanently unresponsive) rather than activated. Inflammation will be present at an infection site where pathogen antigens are present, but inflammation should not be present in the absence of infection. Therefore, if a T-cell receptor binds its antigen in the absence of inflammation, it is likely a self-antigen. (We will see next class, that this occurs because dendritic cells do not express the B7 costimulatory molecule unless they become activated by inflammation or PAMPs).

Why does the T cell undergo many cell divisions after re-arranging the β chain?

If the cell gets a productive re-arrangement of the β chain, the T cell undergoes many cell divisions to produce many thymocytes that have the same β chain. Each thymocyte then independently undergoes somatic recombination of the α chain, so each one will have a different receptor formed from the same β chain with a different α chain. This increases the efficiency of T cell production by producing many different T-cells from only one β-chain somatic recombination.

What is negative selection? What criteria is used to negatively select T cells?

In negative selection, T cells with a receptor that binds self-antigen are eliminated as we do not want T cells that will produce an immune response against our own tissues. The T cells are again presented with self-antigens on MHC molecules. T cells that bind the selfpeptide:MHC complex strongly are negatively selected because they likely recognize both the MHC molecule AND the self-peptide. They undergo apoptosis. T cells that bind the selfpeptide:MHC complex moderately are allowed to be released from the thymus because these T cells likely recognize the MHC molecule but NOT the self-peptide.

What is positive selection? What criteria is used to positively select T cells?

Positive selection is the selection for T cells that have a receptor that can bind to a self MHC molecule. T cells are presented with self-peptides presented on MHC molecules (recall each individual has up to 12 different MHC molecules for presenting antigen- 6 class I and 6 class II). If the T cell receptor binds moderately/strongly to the peptide:MHC complex then the T cell will be positively selected. If the T cell receptor binds weakly or not at all to the peptide:MHC complex then the T cell will not be positively selected because it is not capable of recognizing MHC molecules. T-cells that fail to be positively selected can undergo receptor editing of the α chain; they try further somatic recombination using remaining V and J gene segments to try to get a receptor that recognizes MHC molecules. T cells that fail to be positively selected after receptor editing undergo apoptosis.

How do we ensure allelic exclusion of the β chain?

Somatic recombination takes place one gene copy at a time. It takes place on the first β chain gene, which gets 2 chances at a successful rearrangement. If successful, the second β chain is not produced. If both re-arrangements on the first gene copy are unsuccessful then somatic recombination will be attempted on the second β chain gene. Allelic exclusion ensures that only one type of T cell receptor β chain is produced by any T cell.

Where do T-cell precursors originate and develop into T-cells?

T cell precursors originate in the bone marrow and develop into T-cells in the thymus

Where does T-cell development take place?

T cells precursors originate from the common lymphoid progenitor (gives rise to B cells, T cells and NK cells), which arises from the hematopoietic stem cell in the bone marrow. Early T-cell progenitors then leave the bone marrow and enter the thymus. T cell development takes place in the thymus.

Which cell receptors recognize degraded peptides?

T-cell receptors bind peptide/MHC complex

What happens if a T-cell binds its antigen in secondary lymphoid tissue?

The T-cell can become activated to produce an effector CD8 T-cell or an effector CD4 T-cell

After the thymocyte has undergone cell divisions, which chains re-arrange next? During this re-arrangement, what determines if the cell will become a γ: δ T cell or an α:β T cell? If it becomes an α:β T cell, is it committed to the α:β lineage or can it still become a γ: δ T cell?

The γ and δ chains commence re-arrangement, and the α chains begin re-arrangement. If the α chains complete a rearrangement before both the γ and δ chain complete a successful rearrangement then the cell will become an α:β T cell (re-arrangement of the alpha chain deletes the delta gene segments). If the γ and δ chains both re-arrange successfully before the α chain completes a re-arrangement then the cell will become a γ: δ T cell. If the cell becomes an α:β T cell then it is committed to that lineage and it can no longer become a γ:δ T cell.

Which genes undergo somatic recombination first? At this stage, what determines if the thymocyte will become a γ, δ T cell or if the thymocyte will progress down the α:β T cell lineage? If it progresses down the α:β T cell lineage, is it committed to the α:β lineage or can it still become a γ, δ T cell?

The γ, δ and β chains re-arrange first. If the γ and δ chains both re-arrange successfully before the β chain then the cell becomes a γ, δ T cell. If the β chain rearranges successfully before both the γ and δ chain then the cell will progress down the α:β T cell lineage. If it progresses down the α:β lineage it is still not committed to that lineage as there will still be another chance to become a γ:δ T cell.

T-cells encounter antigen to become...?

activated in secondary lymphoid tissues

Production of T cells in the thymus decreases with...?

age

everyone has up to 12 different MHC molecules for...?

antigen presentation

α chains are tested for functionality by....?

forming a T-cell receptor

Pre-T-cell tests β chains for functionality by...?

forming a pre-T-cell receptor

Somatic recombination of the α chain eliminates...?

the δ chain

What are the two types of T cells that can arise during T-cell development?

α:β T cells and γ:δ T cells (alpha:beta T cells and gamma:delta T cells)