T-cell receptor Complex: TCR + CD3

and result fromdifferential RNA splicing

Immunoreceptor tyrosine-based activation motif

Ig domains

TCR accessory proteins

Coreceptor interactions

3D rendering of TCR/MHC I/ Ag

TCR and MHC I

TCR and MHC II

Overview

• Early Development

• T-Cell Repertoire Selection Processes

• Late Development

• Activation

• Differentiation

• Cell Death

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

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

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 β Chain of TCR– CD3 gene is expressed

TCR is now in the Double Positive (DP) State

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.

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.

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.

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.

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.

Unresolved Thymic Issues

• How do thymocytes survive selection processes???

• Avidity Hypothesis - Strength of signal

• Differential-Signaling Hypothesis

• ??????????????????????????????????

Thymocyte Activation

• TCR complex binds MHC ligand and signal is transduced– Cell cycle– Proliferation– differentiation

Costimulatory Signal

Antagonists

CD28 – B7Activation of TF for IL-2

Cell Death!• How does apoptosis occur during

thymocyte selection??

• Specialized Proteases called “Caspases”

• Every cell produces these proteases which are maintained in an inactive form

• Must activate to undergo apoptosis

2 Pathways

Death Signal!!!