# Lecture 14: T Cell Development in the Thymus
## Immunology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the stages of T cell development from bone marrow progenitor to mature naive T cell
2. Explain the processes of positive selection and negative selection
3. Describe the role of AIRE in central tolerance
4. Explain the generation of regulatory T cells (Tregs) in the thymus
5. Discuss the consequences of thymic dysfunction and developmental defects
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## Lecture Content
### I. Overview of T Cell Development
T cell progenitors arise from hematopoietic stem cells in the bone marrow and migrate through the bloodstream to the thymus, attracted by the chemokines CCL25, CCL21, and CXCL12. They enter the thymus at the corticomedullary junction and undergo a developmental program that takes approximately three weeks in humans. This process is remarkably selective: roughly 95 to 98 percent of thymocytes die during development because they fail selection, and only about 2 to 5 percent exit the thymus as mature, self-tolerant, MHC-restricted T cells.
### II. Thymic Microenvironment
The thymus provides distinct microenvironments that guide each stage of T cell development. The **cortex** is densely packed with immature, double-positive thymocytes. Cortical thymic epithelial cells (cTECs) express MHC class I and class II at high levels and possess unique proteolytic machinery -- including the thymoproteasome (which contains the beta-5t subunit), cathepsin L, and thymus-specific serine protease (TSSP) -- that generates a distinctive peptide repertoire used during positive selection. Macrophages in the cortex serve as scavengers, clearing the abundant apoptotic thymocytes that die by neglect.
The **medulla** is less dense and contains mature single-positive thymocytes. Medullary thymic epithelial cells (mTECs) are remarkable for their expression of the transcription factors AIRE (autoimmune regulator) and Fezf2, which drive the promiscuous expression of tissue-restricted antigens (TRAs) -- proteins normally found only in specific peripheral tissues, such as insulin, thyroglobulin, and myelin proteins. This ectopic expression allows negative selection to eliminate T cells that would react against organ-specific antigens. Dendritic cells in the medulla capture TRAs from mTECs and participate in negative selection. Hassall's corpuscles, distinctive structures within the medulla, produce TSLP, which promotes DC maturation and supports regulatory T cell generation. The **corticomedullary junction** is the vascular entry point where progenitors immigrate and mature T cells eventually emigrate.
### III. Stages of Thymocyte Development
Thymocyte development proceeds through three major stages defined by expression of the CD4 and CD8 co-receptors. The **double-negative (DN) stage** (CD4-CD8-) comprises approximately 5 percent of thymocytes and is further divided into four substages based on CD44 and CD25 expression. **DN1** cells (CD44+CD25-) are the earliest thymic settlers and retain some multilineage potential; Notch1 signaling at this stage commits them to the T cell lineage. **DN2** cells (CD44+CD25+) show strengthened T cell commitment and begin TCR gene rearrangement, with D-J rearrangement of the TCR-beta locus initiated. **DN3** cells (CD44-CD25+) complete V-DJ rearrangement of TCR-beta, and this is the critical **beta-selection checkpoint**. **DN4** cells (CD44-CD25-) have passed beta-selection and undergo rapid proliferation.
Beta-selection is a pivotal quality control step. A successfully rearranged TCR-beta chain pairs with the invariant pre-T-alpha chain to form the **pre-TCR**, which signals in a ligand-independent fashion, promoting survival, proliferation, allelic exclusion of the second TCR-beta allele, and progression to the double-positive stage. The CD3 signaling components (CD3-gamma, delta, epsilon, and zeta) associate with the pre-TCR to transduce these signals. Cells that fail to produce a functional TCR-beta chain undergo apoptosis.
The **double-positive (DP) stage** (CD4+CD8+) encompasses 80 to 85 percent of thymocytes. At this stage, RAG expression is reactivated and TCR-alpha chain gene rearrangement occurs. Because the TCR-alpha locus lacks D segments, only V-J rearrangement takes place, and notably, there is no allelic exclusion at this locus -- both alleles can rearrange, allowing dual TCR-alpha expression, although one chain usually dominates functionally. The complete alpha-beta TCR is expressed at low levels on the cell surface, and DP cells undergo **positive selection** in the cortex.
The **single-positive (SP) stage** (either CD4+CD8- or CD4-CD8+) represents 10 to 15 percent of thymocytes. These cells reside in the medulla, where they undergo **negative selection** and mature for 4 to 5 days before emigrating to the periphery.
### IV. Positive Selection
Positive selection occurs in the thymic cortex and is mediated by cortical thymic epithelial cells. Its purpose is to ensure that mature T cells can recognize self-MHC molecules, thereby establishing MHC restriction. During this process, DP thymocytes expressing their newly assembled TCR scan peptide-MHC complexes displayed on cTECs. The TCR must bind self-peptide/MHC with **low to moderate affinity** to receive a survival signal. Successful binding rescues the thymocyte from apoptosis by upregulating Bcl-2 and other anti-apoptotic molecules. Thymocytes whose TCR fails to bind or binds too weakly receive no survival signal and die by a process called **death by neglect** -- the fate of the majority of DP cells.
Positive selection also determines **lineage commitment**, the decision of whether a thymocyte becomes a CD4+ or CD8+ T cell. If the TCR recognizes MHC class II, the cell downregulates CD8 and becomes a CD4+ single-positive cell, a process requiring the transcription factor ThPOK. If the TCR recognizes MHC class I, the cell downregulates CD4 and becomes a CD8+ single-positive cell, driven by the transcription factor Runx3. The **kinetic signaling model** proposes that the duration and strength of TCR signaling determine this lineage choice: prolonged signaling, characteristic of MHC class II engagement, favors the CD4 lineage, while interrupted or brief signaling, characteristic of MHC class I engagement, directs cells toward the CD8 lineage.
A diagram illustrating positive selection in the thymic cortex. Three DP thymocytes (CD4+CD8+) are shown interacting with a cortical thymic epithelial cell (cTEC) displaying various peptide-MHC complexes. Thymocyte 1: TCR binds peptide/MHC II with moderate affinity → receives survival signal → downregulates CD8 → becomes CD4+ SP (ThPOK expression indicated). Thymocyte 2: TCR binds peptide/MHC I with moderate affinity → receives survival signal → downregulates CD4 → becomes CD8+ SP (Runx3 expression indicated). Thymocyte 3: TCR fails to bind any peptide/MHC complex → no survival signal → death by neglect (cell shown undergoing apoptosis with condensed nucleus and membrane blebbing, being engulfed by a cortical macrophage). A note indicates that ~90% of DP thymocytes die by neglect.
### V. Negative Selection (Central Tolerance)
Negative selection occurs primarily in the thymic medulla, with some occurring in the cortex as well, and is mediated by mTECs and thymic dendritic cells. Its purpose is to eliminate T cells that react strongly to self-antigens, thereby preventing autoimmunity. Single-positive thymocytes scan peptide-MHC complexes on mTECs and DCs, and those whose TCR binds self-peptide/MHC with **high affinity** are eliminated through clonal deletion. This apoptotic process is mediated by Bim, a pro-apoptotic member of the Bcl-2 family, and by the upregulation of Nur77, an orphan nuclear receptor that promotes cell death. As an alternative to deletion, some high-affinity self-reactive CD4+ cells may be diverted to the regulatory T cell lineage.
The transcription factor **AIRE (Autoimmune Regulator)** is central to this process. Expressed by mTECs, AIRE drives the ectopic expression of tissue-restricted antigens in the thymus -- proteins such as insulin (normally found only in pancreatic beta cells), thyroglobulin (thyroid), myelin basic protein (CNS), and retinal antigens (eye), totaling over 4,000 gene products. This promiscuous gene expression allows the thymus to test developing T cells against a vast catalog of peripheral self-antigens that would otherwise never be encountered in the thymus. Mutations in AIRE cause **autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED, also known as APS-1)**, an autosomal recessive disorder characterized by chronic mucocutaneous candidiasis, hypoparathyroidism, adrenal insufficiency, and multi-organ autoimmunity resulting from the escape of self-reactive T cells. A second transcription factor, **Fezf2**, drives expression of a complementary set of TRAs in an AIRE-independent manner.
### VI. Thymic Treg Development
Not all thymocytes with self-reactive TCRs are deleted. Some CD4+ thymocytes with moderate-to-high affinity for self-peptide/MHC class II -- an affinity that falls between the positive selection threshold and the deletion threshold -- are diverted into the regulatory T cell lineage. This diversion requires TCR signaling of appropriate strength, CD28 co-stimulation, IL-2 signaling through the IL-2 receptor, and upregulation of **FoxP3**, the master transcription factor for Tregs.
Thymus-derived Tregs (tTregs, also called natural Tregs) bear the phenotype CD4+CD25+FoxP3+ and constitutively express CTLA-4, GITR, and CD25 (IL-2R-alpha). They are essential for maintaining peripheral self-tolerance, suppressing effector T cell responses through multiple mechanisms including the secretion of IL-10, TGF-beta, the inhibitory effects of CTLA-4, and consumption of IL-2. TSLP produced by Hassall's corpuscles activates thymic DCs to express CD80/CD86, further promoting Treg differentiation within the thymus.
### VII. Thymic Emigration and Recent Thymic Emigrants
Mature single-positive T cells exit the thymus through blood vessels at the corticomedullary junction. Emigration depends on sphingosine-1-phosphate receptor 1 (S1P1), which allows thymocytes to follow an S1P gradient from the thymus into the blood. Newly exported naive T cells, known as **recent thymic emigrants (RTEs)**, are still undergoing final maturation in the periphery. They can be identified by T cell receptor excision circles (TRECs), which are circular DNA byproducts of TCR gene rearrangement that are not replicated during cell division and thus become diluted with each round of proliferation. TREC levels therefore serve as a measure of thymic output and are used clinically in the newborn screening assay for SCID, where low or absent TRECs indicate severely impaired thymic T cell production.
### VIII. Thymic Involution and Clinical Significance
**Thymic involution** is the progressive decrease in thymic size and function that begins after puberty, accelerated by sex steroids. Over time, thymic tissue is replaced by adipose tissue, and the output of naive T cells declines. This leads to increased reliance on peripheral expansion of existing T cells and contributes to the immunosenescence observed in the elderly.
Several clinical conditions illustrate the importance of a functional thymus. **DiGeorge syndrome (22q11.2 deletion)** results in thymic hypoplasia or aplasia along with cardiac defects, facial abnormalities, and hypoparathyroidism with resultant hypocalcemia. Patients with partial DiGeorge syndrome have variable T cell deficiency, while those with the complete form present with a SCID-like phenotype and require thymic transplantation. **Nude (FOXN1-deficient) mice**, which are athymic and lack T cells, serve as valuable tools in immunology research. **Thymoma**, a tumor of thymic epithelial cells, is notably associated with myasthenia gravis (autoantibodies to the acetylcholine receptor) and other paraneoplastic autoimmune phenomena, reflecting disordered T cell selection within the neoplastic thymus.
A comprehensive overview diagram of T cell development in the thymus. The thymus is shown in cross-section with cortex and medulla clearly labeled. A winding path traces thymocyte development from entry to exit. At the corticomedullary junction: bone marrow progenitors enter (DN1). In the cortex: DN2-DN4 stages occur, with beta-selection at DN3 (pre-TCR formation, failed cells undergo apoptosis). DP thymocytes undergo positive selection on cTECs (death by neglect for ~90%). Surviving cells commit to CD4 or CD8 lineage and migrate to the medulla. In the medulla: SP thymocytes undergo negative selection on mTECs (expressing AIRE and tissue-restricted antigens) and dendritic cells. High-affinity self-reactive cells are deleted; some moderate-affinity CD4+ cells become FoxP3+ Tregs. Mature naive CD4+ and CD8+ T cells exit via blood vessels (S1P1-dependent). Statistics are annotated: ~5% total survival rate from entry to exit.
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