Lecture 12 - Generation of T Cells

T Cell Origin and Migration

Hematopoietic stem cell progenitors form precursors in bone marrow, travel to the thymus for maturation, and migrate as naïve T cells to secondary lymphoid tissues like lymph nodes, spleen, and GALT

Evidence for Thymus Function

• DiGeorge syndrome lacks a thymus and mature T cells

• nude mice have a FOXN1 mutation causing a lack of a thymus and T cells, neonatal thymectomy eliminates T cells

• thymus transplantation restores T cells as demonstrated by Jacques Miller

Thymic Involution

The thymus naturally shrinks and fills with fat over time, leading to a progressive age-related decline in the production of new T cells

Thymocyte Developmental Stages

• Defined by surface markers CD3, CD4, and CD8

• passing through double-negative (CD4- CD8-),

• double-positive (CD4+ CD8+),

• single-positive (CD4+ or CD8+) stages

Alpha-Beta vs Gamma-Delta T Cells

• Approximately 95 percent of T cells carry alpha-beta receptors

• gamma-delta T cells are primarily found in mucosal tissues like intestines, remain double-negative, show limited diversity, and act like innate cells

First Checkpoint Pre-TCR Test

• Successful rearrangement of the beta chain allows it to pair with a surrogate alpha chain (pT-alpha) forming a pre-TCR, which signals survival

• halts further beta rearrangement, and triggers cellular proliferation and CD4/CD8 co-receptor expression

Beta Chain Rearrangement Rescue

• Cells can make two rearrangement attempts per homologous chromosome utilizing downstream C1 and C2 regions

• 66 percent of attempts fail primarily due to frameshift mutations creating premature stop codons

Death by Neglect

Thymocytes that fail to produce a functional receptor or signal properly receive no survival signals and undergo programmed cell death in the thymus

Second Checkpoint Alpha Chain Test

Following proliferation, thymocytes rearrange alpha chain V and J segments, express a complete alpha-beta TCR using a new alpha chain, and turn on both CD4 and CD8 to become double-positive cells

Positive Selection Overview

Occurs in the thymic cortex where cortical epithelial cells present self-peptide:MHC complexes to test if the TCR can bind MHC molecules

Positive Selection Outcome

• Moderate or strong binding to MHC class I or II provides a survival signal and dictates lineage commitment

• causing cells to retain either CD8 or CD4 while weak binding results in death by neglect

How do we know positive selection is done by stromal cells?

• In these bone marrow chimeras:

  • Stromal cells express EITHER MHCa (yellow mouse) or MHCb (blue mouse).

  • The immune cells express both MHCa and MHCb.

Negative Selection Overview

Occurs predominantly at mice cortico-medullary junctions as thymocytes travel toward the medulla, eliminating self-reactive cells to prevent autoimmune disease

Negative Selection Mechanism

Mediated primarily by bone marrow-derived immune cells like dendritic cells and macrophages that induce apoptosis in thymocytes binding self-peptide:MHC complexes too tightly

AIRE Transcription Factor

Autoimmune regulator expressed by thymic medullary epithelial cells allowing them to transcribe every protein in the body so developing T cells can be tested against diverse self-antigens

Thymic Selection Statistics

About 98 percent of thymocytes die in the thymus through selection processes, with approximately 10 to the 8th power thymocytes generated daily and only a small fraction entering circulation as mature T cell