1. T Cells More functionally diverse than B cells CD4+ Respond to Ag in context of MHC II Provide helper functions TH1/TH2 A subset of CD4+ have regulatory (suppressive) function CD8+ Respond to Ag in context of MHC I Killer cell function Receptor is not Ig Will not react to free hapten Must see cell-associated Agr

2. MHC Restriction Zinkernagel and Doherty, 1974 Nobel Prize, 1996 CTL will only kill cells of the correct (self) haplotype (MHC constellation) Models: Dual Receptor Altered Self One binds Ag One binds MHC Single receptor Sees self + Ag Altered self correct

3. Molecular Characterization of TCR 1983 Used T cell hybridomas Made monoclonal Abs to cell surface molecules Some blocked response of cells to Ag Precipitated 2-chain structure ( ,  ) about 40kd 1984 Used subtractive hybridization Isolated RNA from T cell clones Subtracted with B cell RNA Made probes to look for rearrangement Liver RNA VS T cell RNA Isolated TCR-  gene Subsequently TCR-  found by same method

4. TCRCD3 Complex TCR will not move to the cell Surface without expression of CD3 CD3 is the definitive T cell marker

5. TCR/CD3 SIGNALING COMPLEX

6. The  /  TCR Expressed early in the thymic embryonic development Low levels in adult peripheral blood, LN Found scattered in epithelial linings gut lung reproductive tract 5% peripheral blood T cells 1-3% lymphoid tissues 1% of epidermal cells are  /  TCR+ Demonstrate restricted variability at any organ site Do not re-circulate May be closer to innate cells May recognize Ag in context of epithelial cells Some  /  cells respond to stress or heat-shock proteins May use alternate MHC molecules (IB) Function unclear likely protect epithelial linings against infection

7. Multiple V  gene segments (100) 20-30 V  gene segments Limited V regions in  or  TCR TCR-  has many J regions  locus located in middle of  locus deleted when  rearranges To control  vs  rearrangement:  silencer: if “on” no  rearrangement,  rearranges If “off”  rearranges,  deleted TCR  and  have D regions

9. Initial Steps in T Cell Activation CD4 (or CD8), associated with Lck, bind MHC TCR binds peptide/ MHC CD45 dephosphorylates kinases Lck and Fyn CD3 components phosphorylated Phosphorylation of CD3  chain allows Docking of Zap-70 Lck and Fyn phosphorylate Zap-70

10. Activation of CD3 Triggers phosphorylation Of phospholipase C PLC hydrolyzes PIP2 This produces DAG, IP3 IP3 causes Ca++ release And activation of NF-AT Via dephosphorylation DAG + Ca++ activate PKC PKC phosphoryates I  -B This activates NF-  B Zap-70 initiates Ras pathway Co-stimulation via CD28 Required for Proliferation of naïve T cells. In the absence of co-stimulation, cell Becomes anergized. Activates Rac pathway Activates PI3-K Stabilizes IL-2 RNA Leads to DNA synthesis

12. B7-1 and B7-2 bind CD28 and CTLA-4 CD28 binding activates T cells CTLA-4 binding inhibits cell activation

13. Lipid Rafts Signal transduction molecules associated with structures containing sphingolipids and cholesterol Upon ligation, CD3 moves to raft, concentrating signal Longer adhesion molecules moved away, tighter binding, less wobble CD45 phosphatase moved away, eliminates potential negative signal Results in “immunologic synapse”

14. MHC Tetramers Fluorescent Antigenic Peptide  2M MHC Class I Matrix CD8 MHC Tetramer Staining Non-Immune Immune CD8+ T Cell TCR