T cell development

Question 1

What lineage of cells gives rise to T cells?

From where do they originate?

Answer 1

Pluripotent hematopoietic stem cells in the bone marrow.

Question 2

Where do the precursors of T cells finish their maturation?

Answer 2

As a fetus before birth: in the fetal liver

As an adult later in life: in the THYMUS (just above the heart)

Question 3

How do the HSCs commit to the T cell lineage?

Answer 3

• Interaction with thymic stromal cells → IL-7 production by thymic stromal cells → **IL-7 binding to the IL-7 receptor **on CD34+ progenitor cells
• Notch-1 on the surface of thymocytes interacts with ligands on thymic epithelial cells
• GATA3

Question 4

Stages of T cell development

Answer 4

• HSC/CLP
• Progenitor-T cell (Pro-T; DN)
• Precursor-T cell (Pre-T; DN with Pre-TCRbeta and surrogate alpha)
• Double Positive (DP)
• Single Positive (SP) [aka Immature T cell]
• Mature T cell

Question 5

The transition from Pro-T (DN) stage to the DP stage is mediated by…

Answer 5

• IL-2
• IL-7
• Notch

Question 6

What happens between the Pro-T and Pre-T stages?

Answer 6

• First, proliferation of Pro-T cells
• Then, TCR gene rearrangement!!
  
  • γδ rearrangements “race with” β rearrangement
  • γδ - must undergo two rearrangements total
    
    • for this reason, they usually don’t “win the race”
    • Once they develop, γδ DNs are committed, leave the thymus immediately, and travel to the epithelial surfaces (e.g., gut/intestines)
  • β - only undergoes one reaarangement
    
    • β-chain undergoes VDJ recombination
    • Then, the β-chain translocates to the ER → β-chain’s ability to bind Pre-T-α is tested → “correct” β and Pre-T-α heterodimer is moved to the cell membrane → Pre-T-cell receptors
    • Cell is now called a Pre-T cell and is is DN with a surrogate alpha
  • During both of these rearrangements RAG and TdT are expressed

Question 7

What happens between the Pre-Tα and DP stages?

Answer 7

• First, proliferation (no RAG or TdT expressed)!
  
  • ​So that you have lots of Pre-T cells to try out the newly rearranged α-segment – this helps ⇡ diversity
  • During proliferation, **CD4 **and CD8 begin being expressed, so that the cells are now DP thymocytes
• Then, α-chain rearrangement
  
  • Cells move into the **inner cortex **of the thymus → reactivation of rearrangement machinery → RAG and TdT expression
  • α-chain = light chain; only undergoes VJ recombination
    
    • Each new α-chain is tested for its ability to bind the β chain → assembly of αβ TCRs
  • Since δ gene is within the α gene, even if the Pre-Tα DN cells weren’t already commited to having αβ TCRs, they could not go back to become γδ TCRs since RAG will cut out the δ gene at this point

Question 8

What happens between the DP and SP stage?

Answer 8

• Positive selection!
  
  • Occurs in the thymic cortex
  • Thymic cortical epithelial cells present self-peptides on MHC I and II
  • ​Can the cells bind the MHC molecules?
    
    • If no, then the cells die!
      
      • Additional α-chain rearrangement is permitted if there is no MHC interaction at first, but after 3-4 days w/o binding the cells apoptose
      • Thymic MØs clear the debris
    • If yes, they survive:
      
      • If they bind MHCI → become CD8+ immature T cell
      • If they bind MHCII → become CD4+ immature T cell
      • This is called MHC restriction → SP T cells

Question 9

How does a SP immature T cell become a mature, but naive T cell?

Answer 9

• Negative selection! (aka CENTRAL tolerance)
  
  • Purpose of negative selection is to check for SP T cells that bind so tightly to self antigen that they could trigger an autoimmune response
  • Occurs in the thymic medulla and several cell types present self-peptide MHC to the SP T cells, predominantly
    
    • Bone marrow derived DCs and MØs
    • Thymic DCs and mTECs express AutoImmune REgulator (AIRE), the transcription factor that mediates negative selection
      
      • AIRE helps generate Tissue Reactive Antigens (TRAs) that will be “presented” on the surface of thymic DCs and mTECs
      • A “self-reactive” T cell that binds to these TRAs on the thymic DCs and mTECs will undergo apoptosis
  • Generates mature, but naive T cells
  • Do I bind self antigen presented on the thymic DCs or mTECs?
    • Yes → die!
    • No → Keep → Travel to 2° lymphoid tissues and become activated effector T cells

Question 10

Peripheral Tolerance

Answer 10

Peripheral tolerance develops after T and B cells mature and enter the periphery (secondary lymphoid organs). It involves:

1. Suppression of autoreactive cells by Tregs
2. Generation of lymphocyte anergy when they encounter antigen in the absence of the co-stimulatory signals