T-cell receptor Complex: TCR + CD3 • and result from differential RNA splicing Immunoreceptor tyrosine-based activation motif
TCR accessory proteins Ig domains
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
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 CD28 – B7 Activation of TF for IL-2 Antagonists
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
Death Signal!!! 2 Pathways