T-cell receptor Complex: TCR + CD3  and  result from differential 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 – B7 Activation 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!!!