Selection of T cell receptor repertoire and CD4/CD8 lineage commitment

Question 1

Describe in greater detail the journey T cells follow during development

Question 2

Identify the role of the thymus as an organ that supports T cell development

Answer 2

1) Site of T cell maturation

2) T cell receptor diversity

• Via V(D)J recombination, a form of somatic recombination that randomly assembles different variable (V), diversity (D), and joining (J) gene segments to form the TCR gene, aided by enzymes like RAG1 and RAG2

3) Positive selection

• In the cortex of the thymus, cortical thymic epithelial cells present self-MHC molecules

4) Negative selection

5) Production of thymic stromal cells for a supportive microenvironment

• cortical and medullary thymic epithelial cells, dendritic cells and macrophages

Question 3

How do T cells that have randomly produced a TCR capable of binding to self-MHC molecules (self-restriction), positively selected

Answer 3

Positive selection occurs in the thymic cortex and is facilitated by cortical thymic epithelial cells (cTECs), the process goes:

1) Binding affinity: The DP thymocytes bear a TCR that’s been generated through a random genetic recombination process (V(D)J recombination). To pass positive selection, its TCR must be capable of binding to self-MHC molecules (MHC class I or II) presented by the cTECs with a certain moderate affinity. If a TCR cannot bind to MHC molecules at all, the thymocyte undergoes apoptosis (by caspase enzymes that cause chromatin disintegration)

2) Signal Transmission: Upon successful binding of the TCR to the self-MHC molecule, signal transduction pathways within the thymocyte are activated. This activation promotes the survival and maturation of the thymocyte (ERK and calcineurin signalling)

3) Lineage Commitment: The affinity of the TCR-MHC interaction also determines the lineage of the thymocyte. Thymocytes with TCRs that bind to MHC class I molecules will down-regulate CD4 and develop into CD8 single-positive (SP) T cells, or cytotoxic T cells. On the other hand, those with TCRs binding to MHC class II molecules will down-regulate CD8 and develop into CD4 SP T cells, or helper T cells

Question 4

How do T cells that have produced a TCR which binds to self-antigens strongly, removed

Answer 4

1) Recognition of self-antigen: In the thymic cortex and medulla, these developing T cells (thymocytes) encounter a wide array of self-antigens presented by antigen-presenting cells (APCs), including thymic epithelial cells and dendritic cells. If a TCR binds with high affinity to these self-antigens, it signals the T cell that the TCR is auto-reactive

2) Signal Initiation: High-affinity binding to self-antigens induces a strong signal through the TCR, the signal strength differs from positive selection, helping the cell distinguish between beneficial self-MHC and detrimental self-antigen T cells

3) Induction of apoptosis: The high-affinity signal triggers apoptosis, and involves a range of intracellular pathways, including the activation of pro-apoptotic proteins (Bim) and inhibition of survival signals

4) Removal of autoreactive T cells: The apoptotic T cells are subsequently phagocytosed and removed by macrophages in the thymus

5) Regulatory T Cells: Some autoreactive T cells that recognize self-antigens with an intermediate affinity are spared from negative selection. Instead, they differentiate into regulatory T cells (Tregs) under the influence of AIRE. Tregs are a specialised T cell subset that suppresses immune responses and maintains self-tolerance

Question 5

What is the relevance of the autoimmune regulator (AIRE) gene

Answer 5

A gene crucial for preventing autoimmune diseases, primarily expressed in the thymus, particularly within the mTECs. AIRE works by regulating the expression of a wide variety of tissue-specific antigens (TSAs) within the thymus:

• Promoting Tissue-Specific Antigen Expression: Tissue-specific antigens are proteins that are usually expressed only in specific peripheral tissues. However, under the regulation of AIRE, these antigens are aberrantly transcribed and translated in the thymus by mTECs
• Negative Selection: The tissue-specific antigens expressed in the thymus are presented to developing T cells (thymocytes) during their maturation process. If a thymocyte’s T cell receptor (TCR) binds to these self-antigens with too high affinity, it indicates that the T cell is potentially autoreactive and could target the body’s own cells. This triggers negative selection, where the autoreactive T cell undergoes apoptosis and is removed from the T cell repertoire
• Development of Regulatory T Cells: Some T cells with intermediate affinity to self-antigens are not destroyed; instead, they differentiate into regulatory T cells (Tregs) under the influence of AIRE. Tregs are a subtype of T cells that play a crucial role in suppressing immune responses and maintaining self-tolerance.