Chapter 7: The Development of T Lymphocytes

Both T lymphocytes (T cells) and B lymphocytes (B cells) develop from bone marrow stem cells.
During development, both cell types undergo gene rearrangement to produce their antigen receptors.
Key differences in development:
- B cells rearrange their immunoglobulin genes while still in the bone marrow.
- T cells must migrate to the thymus, another primary lymphoid organ, to rearrange their T-cell receptor (TCR) genes.

T-cell development proceeds similarly to B-cell development but results in two distinct lineages:
- α:β T cells
- γ:δ T cells
TCRs do not recognize peptide antigens alone; they recognize them in the context of Major Histocompatibility Complex (MHC) molecules.

The thymus helps ensure that mature T cells will have TCRs that successfully engage with self-MHC molecules.
Positive Selection:
- Immature T cells that interact with self-MHC molecules receive survival signals and continue to mature.
- T cells that do not engage self-MHC molecules undergo apoptosis (die by neglect).
Negative Selection:
- T cells that bind too strongly to self-MHC molecules are induced to undergo apoptosis to prevent autoimmunity.
The result is that mature T cells are:
- Tolerant to self antigens
- Responsive to foreign antigens presented by self-MHC molecules

T cells originate from bone marrow stem cells and emigrate to the thymus for maturation.
The term thymus-dependent lymphocytes was shortened to T lymphocytes or T cells.
Immature T cells (thymocytes) begin expressing cell-surface glycoproteins during development, such as CD4 and CD8, which are crucial for recognizing peptide antigens presented by MHC class II and class I, respectively.

Location: Upper anterior thorax above the heart.
Functions: Dedicated to T-cell development, housing immature thymocytes embedded in thymic stroma (network of epithelial cells).
Distinguishing features:
- Outer cortex (dense with cells)
- Inner medulla (less dense)
Does not receive lymph from other tissues or involve lymphocyte recirculation.

Thymic epithelial cells arise from ectodermal cells of the cortex and endodermal cells of the medulla.
The thymic anlage becomes colonized by progenitor cells from the bone marrow, leading to the development of tymocytes and dendritic cells.
Macrophages in the thymus assist in removing defective thymocytes through phagocytosis.

Progenitor T cells entering the thymus are uncommitted but develop into thymocytes committed to the T-cell lineage after interaction with thymic stromal cells.
Initially express CD34 and other stem cell markers; later develop CD2 and CD5.
Upon beginning TCR gene rearrangement, these cells are called double-negative (DN) thymocytes as they lack CD4 and CD8.

Interleukin-7 (IL-7): Critical for T-cell development, secreted by thymic stromal cells. Its receptor (IL-7R) is expressed on progenitor cells.
Notch1 Signaling:
- Notch1 is a cell-surface receptor that interacts with signals from thymic epithelial cells to keep progenitor cells on track toward T-cell differentiation.
- Essential for suppressing B-cell development pathway decisions.

Both α:β and γ:δ T cells derive from a common double-negative thymocyte precursor.
The competition between genes occurs during rearrangement, leading to commitment based on whether a γ:δ or α:β receptor is formed first.
Double-positive thymocytes express both CD4 and CD8 and have reactivated V-α gene rearrangements.
Cell Fate Decisions:
- Failure to produce functional TCRs leads to apoptosis and clearance by macrophages.

T-cell receptor genes undergo rearrangements that can be productive or non-productive.
Rearrangement for the β-chain involves joining D to J segments, and then V to DJ segments.
Effects of Successful Rearrangement:
- If a γ-δ receptor is formed first, the cell becomes a γ:δ T cell and leaves the thymus without undergoing stringent selection.

A successful β-chain rearrangement leads to the assembly of the pre-TCR (pre-T-cell receptor) complex with surrogate α chain (pTα), signaling for proliferation and CD4/CD8 expression.
This process serves as a checkpoint for T-cell development, ensuring that the β chain is functional.

After successful β-chain rearrangement, the cells stop rearranging and enter the proliferative phase.
Transcription Factor Regulation:
- Transcription factors involved in T-cell development such as Ikaros, GATA-3, and Th-POK help differentiate cells into specific T-cell types (CD4 or CD8).

Both co-receptors allow specific engagement with MHC class I and II molecules, which directs T-cell fate during selection processes.
The expression of both CD4 and CD8 enhances the likelihood of successful antigen recognition.

Positive selection tests the ability of T cells to bind to self-MHC molecules presenting self-peptides, allowing T cells to mature further if successful.
Only a small proportion of double-positive T cells (up to 2%) survive this selection and mature into functional T cells.

A critical process to eliminate self-reactive T cells, preventing autoimmune disease.
Involves negative selection mediated by dendritic cells and macrophages, which leads to apoptosis of autoreactive thymocytes.

The autoimmune regulator AIRE promotes the expression of tissue-specific antigens in the thymus to ensure the negative selection of self-reactive T cells.

T cells exiting the thymus enter secondary lymphoid tissues, where they can recognize antigens and begin effector functions.
Regulatory T cells (Tregs) are a distinct subset of CD4 T cells that suppress immune responses, ensuring self-tolerance.