Lymphocyte Activation & Immune Tolerance

1. Lymphocyte Activation & Immune Tolerance • Pin Ling (凌 斌), Ph.D. ext 5632; lingpin@mail.ncku.edu.tw • References: 1. Abbas, A, K. et.al, Cellular and Molecular Immunology (6th ed., 2007), Chapter 9-11

2. Question What is happened to the immune system if RAG1 & RAG2 are mutated? • Severe Combined Immunodeficiency Disease (SCID) • Lacking functional T & B cells

3. Outline • Activation of T Lymphocytes • B cell Activation & Ab Production • Immune Tolerance • Summary & Question

4. Key points in T cell activation • Activation of CD4 & CD8 T cells 2. Role of Costimulators in T cell activation 3. Signaling Transduction by the TCR/CD3 complex 4. T cell signal “Attenuation”: Roles of inhibitory receptors & Protein degradation

5. Key Concepts in T cell Activation-I 1. Adaptive immune responses are initiated and mediated by T-cell activation (especially CD4 T cells). 2. Naïve T lymphocytes migrate from Thymus to 2o Lymphoid organs, whereby encounter Ag presented by APCs (e.g. DCs) and then become activated. 3. T-cell activation requires Two Signals: - Primary Signal-TCR/CD3 –Ag/MHC complex - Second Signal (Costimulatory)- Other T cell surface molecules (e.g. CD28) interact with ligands from APCs. 4. T cell responses decline after Antigen (Crisis) is eliminated.

6. Activation of Lymphocytes in Lymph Nodes

7. Phases of T cell Responses IL-2 – a T cell autocrine factor

8. Cross presentation (priming) of Ag to CD8 T cells DC has an unique feature: • Allow protein traffic from endosomal vesicles to the cytosol • Ag-MHC-I presentation • Ingest virus-infected or tumor cells => CD8 T cells

9. Role of CD4 T helper cells in the differentiation of CD8 T cells

10. Functions of co-stimulation in T cell activation

11. T-cell Receptor Signaling

12. The T-cell Immunological Synapse The interface between APC cells and T cells: => Prolong interaction => Enhance signaling transduction => Determine the fate of Effector cells

13. IL-2 Expression during TCR Signaling Cyclosporin A, FK506

14. Signal Attenuation by the Inhibitory Receptor (CTLA-4)

15. TCR Signal Termination by Protein Degradation

16. Key Concepts in T cell Activation-II 1. Activated CD4 T helper type -1 (Th1) cells regulate the cell-mediated immunity (CMI) => Macrophages & CD8 T cells. Th2 cells help Humoral immunity => B cell activation 2. Differentiation of naïve CD8 T cells requires Signals: - Ag presentation, Cytokines (eg. IL-12), and Effector CD4 T cells in some cases. • The Best characterized costimulatory pathway: CD28 (T cell)-B7 (APC) interaction.

17. Outline • Activation of T Lymphocytes • B cell Activation & Ab Production • Immune Tolerance • Summary & Question

18. Key points in B cell activation & Ab production • Overview of B cell activation 2. Signaling Transduction by BCR & its Co-receptor 3. Mechanisms of CD4 T helper cell-mediated B cell activation 4. The Germinal Center Reaction: - Isotype switching - Affinity maturation (Somatic Hypermutation)

19. Key Concepts in B cell Activation-I 1. B cell activation mediates Humoral Immune Responses. 2. Activating B cells => Ab responses => protein Ag => Require CD4 T helper cells => These protein Ag => T-dependent (TD) Ag 3. Activating B cells => Ab responses to multivalent Ags w/ repeating epitopes (lipids, polysaccharides) => No CD4 T helper cells => These lipid & polysaccharide Ags => T-independent (TI) Ag 4. Heavy chain isotype switching & Hypermutation are often seen in T cell-dependent humoral immune responses. 5. Primary & 2nd Ab responses differ qualitatively & quantitatively.

20. Activation of Lymphocytes in Lymph Nodes

21. The Phases of B cell Responses

22. Primary vs. Secondary Ab responses

23. B-cell Receptor Signaling

24. B cell Responses by cross-linking BCR

25. The Events of T-B cell Interaction

26. Helper T cell-mediated B cell activation

27. Germinal center reactions in T cell-mediated Ab responses Germinal centers (2nd follicle) 1. T-dep B cell activation 2. Somatic hypermutation 3. Isotype switching 4. Memory B cells

28. Ig Heavy chain isotype switching

29. Ig Heavy chain isotype switching-II 1. AID (Activation- Induced Deaminase) , ApeI endonuclease & UNG (Uracil N Glycosylase) but not RAG. 2. Other DNA repair proteins 3. Recognize Ig exon or Switch region

30. Ig Heavy chain isotype switching-III

31. Somatic Hypermutation in Ig V region AID is required in this process.

32. B cell selection in Germinal Centers Only B cells w/ a high affinity for Ag on follicular DCs (FDCs) => Survive

33. Outline • Activation of T Lymphocytes • B cell Activation & Ab Production • Immune Tolerance • Summary & Question

34. Key Concepts in Immune Tolerance 1. Self-tolerance may be induced in immature self-reactive lymphocytes in primary lymphoid organs (Central tolerance) or mature lymphocytes in peripheral sites (Peripheral tolerance). 2. Central Tolerance => Cell Death (T & B cells) (Negative selection) => Receptor Editing (B cells) => A change in functional abilities (Regulatory T cells) 3. Peripheral Tolerance Mature lymphocytes  self Ags => No response (Anergy) => Cell Death 4. Foreign Ags may be administered in ways that inhibit immune responses by inducing tolerance.

35. Fates of lymphocytes after encounter with antigens

36. Central vs. Peripheral Tolerance

37. Central T cell Tolerance ?

38. T cell tolerance-A transgenic mouse model

39. Mechanisms of T cell Anergy

40. Regulatory T cell-mediated Suppression

41. Self Ag-induced death of peripheral T cells

42. Central & Peripheral B cell Tolerance

43. B cell tolerance -A transgenic mouse model

44. Features of T cell vs.B cell Tolerance

45. Outline • Activation of T Lymphocytes • B cell Activation & Ab Production • Immune Tolerance • Summary & Question

46. SUMMARY 1. T-cell activation requires “Two Signals”: - Primary Signal-TCR/CD3 –Ag/MHC complex - Second Signal (Costimulatory)- Other T cell surface molecules (eg. CD28) interact with ligands from APCs. 2. B cell activation mediates “Humoral Immune Responses”. Isotype switching & Hypermutation are key features in T cell-dep humoral immune responses. 3. Self-tolerance may be induced in immature self-reactive lymphocytes in primary lymphoid organs (Central tolerance) or mature lymphocytes in peripheral sites (Peripheral tolerance). 4. The failure of self tolerance leads to autoimmune diseases.

47. Question Have you learned something from this series of Immunology Lectures? • Yes • More or less • Not Sure • Definitely Not

48. CD21/CR2 complement receptor as a Co-receptor for B cells

49. T-B collaboration & the Hapten-Carrier effect