T-cells, Regulation & Tolerance From Chapter 8 Sections A and C only

1. T-cells, Regulation & Tolerance From Chapter 8 Sections A and C only -- we’ll only briefly cover TCR genes From Chapter 9 We’ll cover sections A – E selectively Self-Test Questions: Chapter 8 A2: 1 – 3 A3: 2 B1: 2 B2-4: 1 - 3, 4, 5 C: 1, 3, 4 Chapter 9 A1-2: 1, 4 A3iii-iv: 1 – 5, 8 B1-2: 1, 3, 4 B3-4: 2 & 3 C1-3: all C4-5: 1, 3, 5 D: 3, 4 E. skip F: 1, 3 T-cells, Regulation & Tolerance

2. How are TCRs different than Antibodies? -- lower affinity for AG -- only bind to AG + MHC -- smaller spectrum of AG bound -- TCRs not secreted Structure of the T-cell receptor -- member of the Ig superfamily -- 1 V and 1 C domain / peptide α & β or γ & δ chains -- we won’t talk much aboutγδTCRs Receptor affinity comparison TCR 3D models T-cells, Regulation & Tolerance 2

3. How is TCR AG-binding diversity created? Arrangement of gene segments similar to Ab Other mechanisms of diversity P & N additions junctional flexibility But no somatic hypermutation T-cells, Regulation & Tolerance 3

4. T-cell development and activation Chapter 9 AG-independent vs antigen dependent development Role of the thymus Progenitor migration Roles for epithelial cells and DCs -- self tolerance Not responsible for all intermediate stages DN = CD8- CD4- DP = CD8+ CD4+ SP = CD8+ CD4- or CD8- CD4+ T-cells, Regulation & Tolerance 4

5. How are MHC-restricted, self-tolerant T-cells selected? “Central tolerance” Selection steps 1) for functional TCR 2) for MHC-restriction of TCR 3) for Self-tolerance of TCR See animation of T-cell selection Department of Biology, Davidson College T-cells, Regulation & Tolerance 5

6. How does “Positive selection” for MHC-restriction occur? Acts upon DP -- CD4+CD8+ thymocytes Must bind to MHC I or MHC II -- or die through apoptosis! T-cells, Regulation & Tolerance 6

7. How does ‘negative selection’ for self-tolerance occur? Testing for high affinity for Self-antigens -- expression of AIRE (autoimmune regulator gene) -- macrophages and DCs -- some auto-reactive T-cells escape Most T-precursor cells eliminated T-cells, Regulation & Tolerance 7

8. T-cell activation The TCR-CD3 receptor signalling Function of CD3 -- signal transduction Function of CD8 & CD4 -- strengthen binding -- triggers CD3 phosphorylation T-cells, Regulation & Tolerance 8

9. How are Naïve TH & Tc cells activated? TH Activation 1) Binding to MHC+Ag -- triggers expression of CD40L on TH 2) Costimulation from DC -- Cd40/CD40L triggers B7 expression on DC 3) Cytokine stimulation -- B7/CD28 triggers IL2 expression -- autocrine stimulation DC activation = “licensing” -- now capable of activating Tc Tc cell activation -- has more constraints T-cells, Regulation & Tolerance

10. Aberrant TH activation can compound disease symptoms “Superantigens” -- T-cell mitogens Staphylococcus, Streptococcus, HIV, etc. May activate 5% of all T-cells Excess production TNF- α -- “cytokine storm” Staph -- food poisonings -- Toxic Shock syndrome HIV – T-cell depletion EBV – B-cell depletion Sun-burn rash of Toxic-shock syndrome T-cells, Regulation & Tolerance 10

11. How do CTLs Kill? Perforin/Granzyme triggered -- homologous with C9 Fas Pathway Cytokine Triggered All trigger apoptosis through Caspase cascade Garland CTL-killing T-cells, Regulation & Tolerance 11

12. Activated TH cells have specialized functions T-cells, Regulation & Tolerance 12

13. Correct T-cell type is critical to effective immune response Lepromatous Leprosy TH1/TH2 imbalance Mycobacterium leprae Intracellular pathogen of macrophages Tuberculoid form Cell mediated response TH1 Lepromatous form Humoral response TH2 -- lots of Ig, not very helpful disfiguring granulomas form, T-cells, Regulation & Tolerance 13

14. Regulation & Tolerance – Overlapping concepts -- Central vs Peripheral Tolerance 1. Missing signals, e.g., -- no TH help for B-cells or Tc cells Checkpoint Regulation -- during chronic activation 2. CTLA4 (CTL-Assoc. AG – 4) -- upregulated, has higher affinity for B8 than CD28 3. PD-1 & PD-L1/2 (Prog cell Death - 1) -- Cell anergization & apoptosis Tolerogenic Cells 4. Treg cells -- Immunosuppressive cytokines -- natural or induced -- High checkpoint regulators T-cells, Regulation & Tolerance 14

15. Various factors influence peripheral tolerance Dosage History & frequency of exposure Route of exposure Subcutaneous -- immunogenic at low dosages -- desensitization therapy Intravenous -- less so Oral --tolerogenic at high dosages The Microbiome T-cells, Regulation & Tolerance 15

16. Our Microbiota influence Immune system function Over 100 trillion cells • ~90% of the cells in our body • ~3 – 5 lbs of body weight • 1000s of species Discourage pathogens Contribute to metabolism Induce tolerance • Suppress autoimmunity & Inflammatory diseases T-cells, Regulation & Tolerance

17. Some tissues are “Immune privileged” e.g., eye, testis, brain, uterus Immune suppressed Mechanisms of tolerance -- low MHC -- immunosuppressive hormones & cytokines -- immune cell anergization -- Fas-FasL -- Tregs -- etc T-cells, Regulation & Tolerance 17