Lecture 10 - Development of T Lymphocytes

Lymphopoiesis

Production of new lymphocytes in central lymphoid organs

Where does T-lymphocyte maturation occur

In the thymus

Where do Common Lymphoid Progenitors (CLP) come from

Bone marrow

What is the outer region of the thymus

Cortex

What is the inner region of the thymus

Medulla

Are more immature lymphocytes in the cortex or medulla

Cortex, they move inner as they mature

In the cortex:

Immature thymocytes and scattered macrophages

In the medulla

More mature thymocytes, macrophages, DCs, a few B cells

Why do 98% of T-cell precursors die in the thymus

Pressure for positive selection is strict, makes sure thymocytes are carefully selected, if there is ANY chance they’ll become self-reactive they die

Key players in apoptosis

Macrophages in thymic cortex and medulla

What are successive stages in thymocyte development marked by

Status of TCR genes, changes in TCR expression, changes in cell-surface molecule expression

Where is the double negative region

Subcapsular zone of cortex - outer region

What is in the double-negative phase of T-cell development

ETP (DN1), DN2, DN3, DN4

What is the irreversible commitment stage to becoming a T cell

DN2

Where does the double-positive region occur in T-cell development

Inner cortex

What is in the double-positive region

DP

Where is the single-positive region of T-cell development

Medulla

What is in the single-positive region

CD4+ SP and CD8+ SP

Which stages of T-cell development are alpha beta T cells

DN3, DN4, DP, CD4+ SP, CD8+ SP

What signal commits to T cell lineage

Notch signaling

Where is Notch signaling from

Thymic epithelial cells

What stages is notch signalling required for T cell lineage commitment

Early stages of thymocyte differentiation up to DN3 stage

Who receives the Notch signalling from Thymic Epithelial Cells (TECs)

Early thymic progenitors (ETPs)

During Beta-selection (DN3) why is Notch signaling turned off

Consequence of pre-TCR signaling

What are the earliest T cell precursors called

Double negatives

Have double negatives not yet done

Do not express CD4 or CD8, have not yet rearranged TCR loci

Markers: c-KIT

Receptor for stem cell growth factor

Markers: CD44

Adhesion molecule - homing to thymus

Markers: CD25

Alpha chain of IL2 receptor

Which double negative is ETP

DN1

Which double negative is Pro-T

DN2

Which double negative is small pre-T, pre-TCR, VDJ recombination

DN3

Which double negative is large pre-T

Proliferation

Where does CD3 expression first appear

Between DN2 to DN3

What locus does DN2 begin to rearrange

B chain

In DN3, newly formed B chain combines with what to form Pre-T cell receptor

Pre-Talpha and CD3

If B chain arrangement is productive

Proceed to DN4, CD4 and CD8 expression initiated

Where is alpha chain made

Double positive

Where does CD3 appear and what does it do

Appears at DN → DP transition

Part of TCR complex, responsible for signal transduction when TCR recognizes peptide-MHC

Indicates that pre-TCR or TCR complex is forming and functional

Where is IL-7R expressed and what does it do

Early DN

Provides survival and proliferation signals during early thymocyte growth

Why is IL-7R downregulated in DP

No longer needed once TCR selection begins

When is RAG-1/2 active and what does it do

DN2-DN3 and DP

Encodes enzymes that cut and join V(D)J segments during TCR gene rearrangement

Why is RAG-1/2 turned off after successful rearrangement

To prevent further DNA breaks

What is Pre-TCR the checkpoint for

B-chain checkpoint that drives proliferation and triggers a-chain rearrangement

Allelic exclusion

Once productive rearrangement has occurred at the B-chain locus, events occur to prevent further recombination of the B chain

Why is allelic exlcusion vital

Prevents the thymocyte from producing more than one functional B chain as it continues development

Expression of CD4 and CD8 initiates the rearrangement of

alpha chain locus in DP cells

What happens when alpha chain pair with already functional beta chain

Complete abTCR on the cell surface

At DP stage, TCR and CD3 complex are expressed at low levels to test

Whether receptor can recognize self-MHC molecules presenting self-peptides

What is the selection process

Thymocytes that can moderately recognize self-MHC survival signals vs those that can’t or react too strongly

cTECs (cortical epithelial cells): express MHC and MHC II, role in

Positive selection

mTECs (medullary epithelial cells): present peripheral antigens, role in

Negative selection

How many attemps to produce functional B chain

4

Why do B chains have 4 attemps

2 clusters and 2 alleles

Increases likelihood of producing a productive B-chain before the cell dies

How many attemps at a-chain rearrangement and until when

Multiple, until positive selection forming functional abTCR to recognize self-MHC or death intervenes

Which chains have allelic exclusion

Beta, not alpha

When failed rearrangements occur on alpha chain, where does the cell try again

Each failed rearrangement deletes DNA, the cell can try again downstream

What happens to thymocytes that pass the selection step

Lose either CD4 or CD8 and become single positives, leaving the thymus

Where does positive selection occur

Thymic cortex

What does positive selection do

Keeps only T cells that can bind to self-MHC

Results in MHC restriction

Where does negative selection occur

Both in cortex and medulla

What does negative selection do

Removes T cells whose TCR bind self-peptide:self-MHC too strongly

Results in self-tolerance

What selection occurs for T-cells with no binding or very weak binding

No selection, death by neglect

What is AIRE (autoimmune regulator)

Enables thymic epithelial cells to undergo promiscuous gene expression

Enables presentation of tissue-specific self-antigens from across the body

What does AIRE establish

Central tolerance and prevents autoimmunity

After completing selection in the medulla, what receptor do mature CD4 and CD8 T cells upregulate

S1PR1 which senses sphingosine-1-phosphate (S1P) gradients in blood and lymph

What does S1PR1 do

Guides mature T cells exit, THYMIC EMIGRATION, into circulation