Immunogly Chp 7

1. Cortex: The outermost region of the thymus. Contains thymocytes, which are immature T cells undergoing differentiation. It also contains epithelial cells that help support thymocyte development and express proteins that influence T-cell maturation. The cortex is densely packed with immature T cells, which are undergoing positive selection (testing their ability to recognize self-MHC molecules).

2. Medulla: The inner region of the thymus, located beneath the cortex. Contains mature thymocytes, which have passed through the selection processes in the cortex. It also contains specialized epithelial cells, such as medullary thymic epithelial cells (mTECs), which play a role in negative selection (removal of T cells that are self-reactive). The medulla also contains Hassall's corpuscles, which are clusters of degenerating epithelial cells and may play a role in the regulation of T-cell development.

3. Thymic Epithelial Cells: Found throughout the cortex and medulla. These cells provide structural support and produce cytokines and growth factors that are essential for T-cell maturation. In the cortex, these epithelial cells form a network that interacts with thymocytes and helps in the selection process.

4. Dendritic Cells: Found in both the cortex and medulla. They help in presenting antigens to thymocytes, playing a role in the selection process.

5. Macrophages: Present in both the cortex and medulla. They help eliminate apoptotic thymocytes, particularly those that fail selection.

6. Blood-Thymus Barrier: A specialized structure found at the interface between the cortex and the surrounding blood vessels. It serves to protect the developing thymocytes from potential external antigens and ensures that the developing T cells do not prematurely encounter peripheral antigens

The phenotype of lymphocytes, defined by the expression of specific surface markers, is crucial for tracking their development. These markers help identify and differentiate lymphocyte stages, providing insight into immune system function and maturation. In T cells, key stages include the CD4–CD8– (double-negative) phase in the thymus, progressing to CD4+CD8+ (double-positive) and then to single-positive (CD4+ or CD8+) cells after positive and negative selection. Mature T cells express CD45RA+ and CCR7+ for migration to secondary lymphoid organs.

1. DN T Cell Progenitor Stage (CD4–CD8–): T cell progenitors entering the thymus are double-negative (DN), meaning they lack the expression of both CD4 and CD8 co-receptors. These progenitors are CD34+, CD45RA+, and express surface markers like CD44 and CD25. The process of TCR gene rearrangement begins in the DN stages, specifically during DN2 and DN3.

2. Commitment to T Cell Lineage: Notch signaling plays a pivotal role in the commitment of progenitor cells to the T cell lineage. Notch1 activation directs the differentiation towards T cells and initiates the rearrangement of the TCR genes. The GATA-3 transcription factor is also involved in promoting T cell differentiation and the progression of TCR rearrangement.

3. TCR β-Chain Gene Rearrangement (DN2/DN3): The first step in TCR gene rearrangement is the rearrangement of the TCR β-chain (TCRβ). The TCRβ gene rearranges through VDJ recombination (variable, diversity, and joining segments) using the recombination machinery mediated by RAG1 and RAG2 enzymes (Recombination-activating genes). This process occurs in the DN2 and DN3 stages and results in a productive β-chain if the rearrangement is successful.

4. Pre-TCR Expression: Once a functional TCRβ chain is produced, it combines with a pre-Tα chain (a surrogate α-chain) to form a pre-TCR. The expression of the pre-TCR on the surface of DN3 thymocytes triggers signals that promote the cells to progress to the CD4+CD8+ (double-positive, DP) stage.

5. TCR α-Chain Rearrangement (DN4/DP Stage): After the β-chain rearrangement, the TCR α-chain undergoes rearrangement in the DN4 stage and during the transition to the DP stage. Similar to the β-chain, the TCRα chain undergoes VJ recombination and, if successful, combines with the previously formed TCRβ chain to form a complete TCR.

6. Selection and Maturation: Once the complete TCR is formed (TCRαβ), the thymocyte undergoes positive selection, which ensures the TCR can recognize self-MHC molecules. Negative selection eliminates thymocytes that strongly react with self-antigens, ensuring self-tolerance. The surviving thymocytes mature into CD4+ or CD8+ single-positive T cells and leave the thymus to perform their immune functions.

The pre-TCR is a developmental receptor that consists of a rearranged TCR β-chain paired with a pre-Tα chain, playing a role in early thymocyte development and signaling during β-selection. The TCR, on the other hand, is a fully functional receptor composed of rearranged TCR α and β chains, capable of recognizing antigens in the context of MHC molecules. The pre-TCR is a temporary structure during thymocyte development, while the TCR is the mature, antigen-specific receptor that drives immune responses.

Positive selection: Ensures that T cells can recognize MHC molecules and are capable of participating in immune responses.

Negative selection: Eliminates autoreactive T cells, promoting self-tolerance and preventing autoimmune diseases.

Lineage commitment: Directs T cells to become either CD4+ or CD8+, ensuring they specialize in the appropriate immune functions, with CD4+ T cells assisting other immune cells and CD8+ T cells directly eliminating infected or cancerous cells. Together, these processes are essential for the generation of a functional and self-tolerant T cell repertoire that contributes to effective immune defense.

AIRE: Role in negative selection in the thymus is enabling the expression of a diverse array of tissue-specific self-antigens in thymic medullary epithelial cells. This exposure ensures that self-reactive T cells are eliminated, thereby promoting self-tolerance and preventing the development of autoimmune diseases.

Tregs: A specialized subset of CD4+ T cells that are essential for maintaining immune tolerance by suppressing autoreactive immune cells, controlling inflammation, and promoting immune homeostasis. Their ability to regulate immune responses is critical for preventing autoimmune diseases, controlling chronic inflammation, and maintaining tissue integrity. Their function and stability depend on the expression of FOXP3, and their dysfunction can lead to a variety of immune-related diseases.

T Cell Progenitor Migration (Pre-thymic stage)

• Location: Bone marrow and thymus

• Cell Types Involved: Hematopoietic stem cells (HSCs), progenitor cells

• Key Processes:

  • Hematopoietic stem cells in the bone marrow give rise to common lymphoid progenitors (CLPs), which then migrate to the thymus as double-negative (DN) thymocytes (CD4–CD8–).

  • These progenitors are CD34+, and they enter the thymus via the thymic blood vessels.

2. Double-Negative (DN) Stages (Thymus)

• Location: Thymic cortex

• Cell Types Involved: CD4–CD8– (double-negative) thymocytes

• Key Processes:

  • TCR β-chain rearrangement occurs during the DN2 and DN3 stages. These thymocytes are undergoing VDJ recombination to form the TCR β-chain.

  • During DN1, progenitor cells enter the thymus and begin to commit to the T cell lineage under Notch signaling.

  • In DN2, the TCR β-chain starts rearranging. The TCR β-chain then pairs with the surrogate pre-Tα chain to form the pre-TCR in DN3.

  • The expression of the pre-TCR signals thymocytes to progress further in development and ensures successful rearrangement of the TCR β-chain.

  • Checkpoint: Thymocytes that do not successfully rearrange the TCR β-chain are eliminated.

3. Double-Positive (DP) Stage (Thymus)

• Location: Thymic cortex

• Cell Types Involved: CD4+CD8+ (double-positive) thymocytes

• Key Processes:

  • After the successful rearrangement and expression of a functional TCR β-chain with pre-Tα, thymocytes proceed to the double-positive (DP) stage, where they express both CD4 and CD8 co-receptors.

  • The TCR α-chain undergoes rearrangement at this stage, completing the formation of the full TCR (TCR αβ).

  • The thymocytes now undergo positive selection, a process where those thymocytes that can weakly recognize self-MHC molecules (with either CD4 or CD8) survive. This ensures that the T cells can interact with MHC molecules, a requirement for antigen recognition.

4. Positive Selection (Thymus)

• Location: Thymic cortex

• Cell Types Involved: CD4+CD8+ (double-positive) thymocytes

• Key Processes:

  • Positive selection ensures that thymocytes with TCRs that can recognize self-MHC molecules are allowed to survive. Thymocytes that fail to bind to self-MHC undergo apoptosis.

  • The interaction with thymic epithelial cells presenting self-peptide-MHC complexes on their surface promotes the survival of T cells that have moderate affinity for self-MHC.

  • Thymocytes that recognize MHC class I molecules are committed to becoming CD8+ cytotoxic T cells, while those that recognize MHC class II molecules are committed to becoming CD4+ helper T cells.

5. Negative Selection (Thymus)

• Location: Thymic medulla

• Cell Types Involved: Single-positive CD4+ or CD8+ thymocytes

• Key Processes:

  • Negative selection eliminates thymocytes that bind too strongly to self-peptides presented by MHC molecules. This process occurs in the thymic medulla and is crucial for self-tolerance.

  • AIRE (Autoimmune Regulator) plays a key role in presenting a diverse array of tissue-specific antigens to thymocytes.

  • Thymocytes that strongly recognize self-antigens are induced to undergo apoptosis, preventing autoreactive T cells from escaping into the periphery, which could lead to autoimmune diseases.

6. Lineage Commitment (Thymus)

• Location: Thymic cortex and medulla

• Cell Types Involved: Single-positive CD4+ or CD8+ thymocytes

• Key Processes:

  • After positive selection, thymocytes are committed to either the CD4+ helper T cell lineage or the CD8+ cytotoxic T cell lineage, depending on the type of MHC molecule they recognized.

  • This commitment is influenced by transcription factors: ThPOK for CD4+ T cells and Runx3 for CD8+ T cells.

  • These cells are now single-positive (either CD4+ or CD8+) and will mature further to acquire functional competence.

7. Maturation and Exit (Thymus to Periphery)

• Location: Thymic medulla and peripheral blood

• Cell Types Involved: Mature CD4+ or CD8+ T cells

• Key Processes:

  • After negative selection and lineage commitment, thymocytes become mature single-positive (SP) T cells.

  • These mature T cells now express CD45RA and CCR7, which are important for migration to secondary lymphoid organs (such as lymph nodes and spleen) to participate in immune responses.

  • These mature T cells leave the thymus and enter the peripheral circulation, ready to encounter antigens and initiate immune responses.