T Cell Development - 1

Module 7 pt 1

Lymphocytes derive

from a common progenitor

lymphocyte lineage commitment

is controlled by master transcription factors

Thymocytes undergo

critical selection steps before becoming mature T cells

• Common lymphoid progenitor (CLP)

• Thymic Progenitor Cells (T Cell Commitment- Notch Signalling)

• Double Negative Thymocytes (DN)

  • Checkpoint #1: Beta selection - V/D/J recombination of TCR beta chain

• Double Positive Thymocytes (DP)

  • Checkpoint #2: Positive Selection - TCR interacts with self MHC (MHC restricition)

  • Checkpoint #3: Negative Selection: Removal of TCR clones that recognize self antigen (central tolerance)

Immature thymocytes undergo

sequential maturation stages in the thymic cortex

T cell lineage commitment and β selection happen in

DN thymocytes

DN1

highly proliferative

• CD44 drives homing to thymus

DN2

CD25 promotes survival

• cortical thymic epithelial cells stimulate Notch

• cTec

HSC to E2A/EDF

B cell

HSC to Id2

NK Cells and ILCs

HSC to thymus and Notch

Thymic progenitor cells

CLP to

Thymic progenitor

• double negative thymocytes (DN = CD4-CD8-)

• Checkpoint 1

• Double positive Thymocyte (DP)

  • Positive Selection OR NEGATIVE SELECTION

• Single Positive Thymocyte (CD4+CD8- OR CD4-CD8+)

beta selection

VDJ recombination of TCRbeta chain

Positive Selection

TCR interacts with self MHC (MHC restriction)

Negative selection

Removal of TCR clones that recognize self antigen (central tolerance)

Immature thymocytes undergo

sequential maturation stages in the thymus

• CD44 and DN1

• CDD25 and DN2

• DN3

• DN4

• CD4 + CD8

• leads to mature T-cells in medulla

  • CD4 or CD8

DN3

pre-TCR

• begin recombination of the TCRbeta chain

• express surrogate TCR alpha to form pre-TCR

DN4

express functional TCRbeta chain

• rapidly proliferate

T cell lineage commitment is transcriptionally controlled by NOTCH

Notch —> ADAM10 with S2 cleavage

• then S3 cleavage

• - intracellular notch

if no notch= transcriptional repression

if notch = transcriptional activation

Notch signaling

induces recombination in the TCRbeta using clusters of D and J

• chromosome 7

Successful recombination of a TCRβ chain

promotes thymocyte survival (β selection)

• survival signal to DN3 thymocyte

• from pre-TCR and MHC

β selection tests successful

V/D/J recombination of the TCRβ chain at the DN stage

beta-selection

CHECKPOINT #1

• Pairs with α surrogate chain (pTα) to form preTCR (allows for testing β chain functionality)

• Result: proliferation + expression of CD4 and CD8 (yields DP thymocytes)

• Expression of only 1 β chain per T cell (allelic exclusion)

• preTCR signaling is antigen-independent

Summary 7.1

T cells become lineage committed using master transcription factors

• Expression of Notch commits lymphocyte progenitors to thymic progenitor cells

• Functional Notch signaling requires cTEC help

T cell development occurs in a spatially and temporally restricted fashion

• T cells first mature from DN to DP in the thymic cortex

• β chain recombination occurs during DN stage and uses DJ configuration to make multiple attempts

Summary 7.1 pt. 2

Beta selection acts as a checkpoint for developing thymocytes

• Beta selection uses a surrogate alpha chain and is antigen independent

• Successful selection results in proliferation and allelic exclusion

DP thymocytes undergo

additional selection steps to ensure a functional TCR repertoire

• between double positive and single positive

Checkpoint #2- positive selection

Tests whether V/J recombination of α chain pairs with β chain to produce a TCR that recognizes self MHC (MHC Restriction)

Checkpoint #3 - negative selection

Removal of TCR clones that recognize self antigen; avoid autoimmunity

• Result = Mature CD4+ or CD8+ T cells that are not autoreactive

DP thymocytes engage in successive

recombination in the TCR alpha locus

• initial non productive rearrangement

• subsequent rearrangements bypass nonfunctional VJ gene segment

• multiple rounds of rearrangement may occur to generate a functional alpha chain

Successfully recombined TCRalpha chains

weakly interact with self peptide/MHC

• lead to survival signal

Most TCRs have too low affinity

for self MHC and fail positive selection

• microscopy of mouse hymus has more apoptotic cells than macrophages blue

TCR affinity for self

too low - not positively selected == death by neglect (90-95%)

• intermediate affinity — positively selected —> survive (2-5%)

• affinity too high — negatively selected —> deleted (2-5%)

TCR with intermediate affinity

receive survival signals

• positive selection

Checkpoint #2: Positive selection

• TCR must bind to self antigen-MHC with moderate affinity: confirms ability to recognize

self MHC (MHC restriction)

• Tested using self peptide, but this group of TCRs will include ones that are ideal for recognizing foreign peptide-MHC

• Answers the question: Is this TCR useful?

Affinity too high - strongly autoreactive T cell receptors

removed via negative selection

• deleted (2-5%)

Checkpoint #3: Negative selection

• If TCR binds to self antigen-MHC with strong affinity, the TCR recognizes self peptide-MHC

and is autoreactive

• Negative selection is an essential mechanism of central tolerance

• Answers the question: Is this TCR dangerous?

central tolerance

a critical immune system mechanism that prevents autoimmunity by deleting or neutralizing self-reactive T and B lymphocytes during their development in primary lymphoid organs (thymus for T cells, bone marrow for B cells). It ensures immature lymphocytes recognizing self-antigens are eliminated via apoptosis or receptor editing before reaching the periphery

Successful positive selection also drives

CD8 or CD4 lineage commitment

• CTec has MHC with DP thymocyte

Runx3

CD8 expression, CD4 repression

ThPOK

CD4 expression, CD8 repression

T cell development results in

and MHC-restricted repertoire

TCR recognition of

self-MHC on APC

• no recognition because antigenic mismatch with self MHC

• no recognition because MHC mismatch with nonself MHC

Summary 7.2

⍺ chain recombination occurs in thymocytes with a successful β chain recombination

• Alpha chain recombination can occur successively at the same locus

• Successful recombination results in positive/negative selection

Positive selection ensures thymocytes interact with self MHC

• Intermediate affinity interaction results in survival signal

• The vast majority of thymocytes die by neglect during positive selection

Summary 7.2 (extra)

Negative selection ensures autoreactive thymocytes are removed

• Autoreactive T cells are removed via apoptosis

• Negative selection is essential mechanism to maintain central tolerance

CD4/CD8 lineage commitment results in functional naïve T cells

• Lineage commitment is driven by interaction with MHC and respective transcription factors

• Functional naïve T cells only recognize self MHC (MHC-restricted)