Lecture 10 - Lymphocyte Development

Where do mature T lymphocytes originate and mature?

Mature T lymphocytes arise from the thymus after bone marrow-derived precursors undergo stringent selection and maturation processes.

What surface receptor is expressed on most T cells?

T lymphocytes express the T Cell Receptor (TCR), and most T cells express the αβTCR on their surface

How do TCRs recognize antigens?

TCRs do not recognize antigens directly; they bind to processed antigenic peptides presented on MHC molecules.

What does "MHC restriction" mean for T cells?

It means T cell receptors only recognize antigens when they are presented on MHC molecule

What are the two types of MHC molecules?

MHC class I and II

Which MHC class pairs with CD4 vs. CD8 T cells?

CD4+ T cells recognize antigens presented on MHC-II, whereas CD8+ T cells bind and respond to antigens presented on MHC-I

CD4 - MHC-II

CD8- MHC-I

What is the protein chain composition of a T Cell Receptor (TCR)?

Each TCR is composed of two protein chains—either an α paired with a β protein (αβ TCR) or a γ paired with a δ protein (γδ TCR).

What two main structural regions are found on each TCR protein chain?

Each α, β, γ, and δ protein contains a variable region and a constant region.

How is the TCR attached to the T cell membrane?

The constant region is anchored via a cytoplasmic tail that extends through the cell membrane into the T-cell cytoplasm.

Which associated proteins mediate signal transduction for all TCRs?

All TCRs are associated with CD3 and zeta (ζ) chains, which function as signal-transducing proteins.

What is the primary function of CD4 and CD8 co-receptors during antigen recognition?

They stabilize the binding between the TCR and the MHC–peptide complex.

How do the physical structures of CD4 and CD8 co-receptors differ?

CD4 consists of a single elongate chain, while CD8 is composed of either a shorter α and β chain or two α chains (the CD8 homodimer)

How do a limited number of genes generate the vast diversity of TCRs needed to recognize millions of foreign antigens?

diversity is created at the molecular level by rearranging the series of genes that encode different areas (pieces) of the complete TCR chains.

What 3 regions does the TCR alpha chain have?

1. variable domain (Vα)

2. joining region (Jα),

3. constant domain (Cα)

How does the domain structure of the TCR beta (β) chain differ from the alpha (α) chain?

The TCR β chain is slightly more complex because it includes a Diversity region (Dβ) in addition to the Variable (Vβ), Joining (Jβ), and Constant (Cβ) domains.

Approximately how many Vα, Jα, Vβ, and Jβ1 gene segments exist?

For the α chain there are several possible Vα genes (Vα1, Vα2…to Vαn) and J region genes (Jα1, Jα2… to Jαn), but only a single constant region gene (Cα).

Approximately how many Vα, Jα, Vβ, and Jβ1 gene segments do humans possess?

Humans have approximately 100 Vα genes, 75 Jα genes, 75 Vβ genes, and 6 Jβ1 genes.

Where are the TCR delta (δ) chain genes located in the genome?

Genes encoding elements of the TCR δ chain are integrated within the α chain gene locus.

How is the TCR delta (δ) chain utilized?

Immature T cells may utilize δ chain genes, but in most species there is a predominance of α chain utilization.

What physical pathway do developing T cells follow within the thymus?

T cells first enter and develop in the cortex of the thymus and progress to the medulla, from where mature cells are released into secondary lymphoid tissues.

Where do mature T cells migrate after being released from the thymic medulla?

They enter the circulation to populate secondary lymphoid tissues.

What percentage of immature T cells die during thymic maturation?

Approximately 98% of immature T cells that enter the thymus do not survive to leave.

By what process do non-surviving immature T cells die in the thymus?They die by apoptosis (programmed cell death).

They die by apoptosis (programmed cell death), then the cellular debris is phagocystosed by macrophages

What are thymocyte progenitors and where do they migrate?

They are precursors of T cells that migrate from the bone marrow to the thymus.

What defines the "double negative" (DN) phase of T cell development?

Progenitor T cells in this phase do not express either CD4 or CD8 surface receptors.

Which enzymes facilitate V(D)J gene segment recombination during TCR chain formation?

RAG-1, RAG-2, and TdT (Terminal deoxynucleotidyl transferase).

What happens if a developing T cell successfully forms its gamma (γ) and delta (δ) chains?

It becomes a γδ T cell, enters the circulation, and plays roles in both innate and adaptive immunity

How is the pre-TCR complex assembled following successful beta (β) chain formation?

The β chain associates with a pre-TCR α (pre-Tα) chain and the CD3 complex.

What characterizes the "double positive" (DP) phase of T cell development?

Pre-TCR formation drives cell proliferation and the expression of both CD4 and CD8 receptors.

• These cells are now ready for alpha-chain rearrangement

How is the complete TCR complex formed following alpha (α) chain rearrangement?

The newly rearranged α chain complexes with the β chain and CD3.

Where does positive selection occur, what percentage of cells survive, and what is its outcome?

It occurs in the thymic cortex. Only about 5% of DP cells survive and commit to becoming single-positive (CD4+ or CD8+) based on MHC specificity.

Where does negative selection occur and what primary function does it serve?

It occurs in the thymic medulla to eliminate T cells that recognize auto-antigens, establishing central tolerance.

Which cells present self-antigens to developing T cells in the thymic cortex?

Thymic epithelial cells, which express surface MHC class I and class II molecules presenting self-peptide fragments and present a wide range of normal tissue antigens.

What type of interaction affinity is required for a T cell to pass positive selection?

A moderate-affinity interaction between the TCR and the peptide–MHC complex.

What is the fate of T cells that successfully pass positive selection?

They survive and move on to the second examination (negative selection in the thymic medulla).

What is "death by neglect" during T cell development?

The apoptotic elimination of immature T cells whose TCRs have no or low affinity for peptide–MHC complexes.

Where does negative selection ("the second exam") take place during T cell maturation?

In the thymus medulla.

Which antigen-presenting cells test immature T cells during negative selection?

Thymic dendritic cells displaying MHC class I or II molecules loaded with self-protein peptides.

What crucial role does the AIRE gene play during negative selection?

AIRE drives the expression of thousands of tissue-restricted self-peptides (such as from the brain, endocrine pancreas, and joints) within the thymus.

What test must a T cell pass to survive negative selection?

Its TCR must prove incapable of binding self-antigens with high affinity (ensuring central tolerance).

What is the fate of potentially autoreactive T cells that bind self-antigens with high affinity?

They fail the test and are instructed to undergo apoptosis (programmed cell death).

What stage of development do T cells reach once they pass negative selection?

They pass the selection process and become mature, naïve T cells.

How long do the combined processes of positive and negative selection take in the thymus?

The selection processes take approximately 3 weeks to complete.

What percentage of double-positive thymocytes successfully graduate to become single-positive T cells?

Only 2% of double-positive thymocytes survive and graduate to become single-positive (CD4+ or CD8+) T cells.

What term is used for newly graduated single-positive T cells that exit the thymus into circulation?

They are called "naïve" T cells.

What are High Endothelial Venules (HEVs) and what role do they play in T cell trafficking?

HEVs are specialized blood vessels in secondary lymphoid organs (such as lymph nodes and Peyer's patches) that express signals allowing naïve T cells expressing homing molecules to cross into the tissue.

What must happen before a naïve T cell can migrate to non-lymphoid peripheral sites?

It must recognize its specific antigen and become activated, which causes it to acquire receptors necessary for entry into peripheral sites (all tissues other than primary and secondary lymphoid organs).

Where do B lymphocytes originate?

Like all haemopoietic and immune cells, B lymphocytes arise from bone marrow stem cells.

What role do stromal cells play in B cell development?

Developing B cells interact with stromal cells, which supply essential cytokines for their maturation.

How does our understanding of B cell maturation compare to T cell development?

B cell maturation is not as well characterized as the development of T cells.

Where does B cell maturation primarily take place?

Some stages occur within the bone marrow, but extramedullary locations are also involved in certain species.

What are two notable extramedullary sites for B cell maturation in specific animal species?

The ileal Peyer's patch in ruminants and the bursa of Fabricius in birds.