1. T-Cell Maturation, Activation, and Differentiation

3. T-Cell Repertoire Selection Processes
Overview
Early Development
T-Cell Repertoire Selection Processes
Late Development
Activation
Differentiation
Cell Death

4. Early Development What is required for maturation in the thymus?
Experiment by J.C. Zuniga Pfluker:
Gene knockout on mice
T Cells can develop in the absence of thymic fragments
Notch1 ligand required for T-cell precursor to form

5. Developmental Steps in the Thymus
Cell-Surface Molecules
c-Kit: Receptor for stem cell growith factor
CD44: Adhesion Molecule
CD25: Alpha-Chain of the IL-2 receptor

6. T-Cell Development T-Cell precursor in Double Negative State
DN1 – DN4  changes in cell-surface protein expression
DN2 is a crucial state
RAG expression  gene rearrangements
Determination of Beta Chain of TCR
CD3 gene is expressed

7. TCR is now in the Double Positive (DP) State

8. Thymic Selection Processes
Thymocytes are in the Double Positive State (i.e. express both coreceptors.
Positive and Negative Selection necessary to produce an effective T-Cell repertoire.

10. Positive Selection Results in MHC restriction Mechanism:
Immature thymocytes cluster with MHC molecules on the cortical cells of the thymus
If TCR interacts with MHC  protective signal results that prevents apoptosis.
If TCR does not interact with MHC  no protective signal and apoptosis occurs.
Result? Only reactive thymocytes survive.

12. Negative Selection Ensures self-tolerance
Weeds out High affinity thymocytes
Mechanism:
APC’s bearing MHC’s interact with thymocytes
If avidity is too strong  thymocyte undergoes apoptosis.
Details unknown…
Result? Only self-tolerant thymocytes survive.

14. Positive Selection Experiment
Thymocytes are dependent on the presence of MHC’s in the thymus for survival.
Class I MHC knockout  CD4 is selected for, but CD8 does not survive.
Class II MHC knockout  CD8 is selected for, but CD4 does not survive.

15. Negative Selection Experiment
H-Y peptide expressed form Y Chromosome (only in males)
T-cells against the H-Y peptide are found only in females!!!
Why?
Male cell recognizes H-Y peptide as a self-pathogen  T-cells against H-Y peptide are negatively selected and eliminated.

16. Unresolved Thymic Issues
How do thymocytes survive selection processes???
Avidity Hypothesis - Strength of signal
Differential-Signaling Hypothesis
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18. Thymocyte Activation
TCR complex binds MHC ligand and signal is transduced
Cell cycle
Proliferation
differentiation

21. Costimulatory Signal
CD28 – B7
Activation of TF for IL-2
Antagonists

23. Cell Death! How does apoptosis occur during thymocyte selection??
Specialized Protease called “Caspases”
Every cell produces these proteases which are maintained in an inactive form
Must activate to undergo apoptosis

24. Death Signal!!!
2 Pathways