2. T Cell Receptor (TCR) & MHC Complexes-Antigen Presentation 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 5-7 2. Male D., J. Brostoff, D. B Roth, and I. Roitt Immunology (7th ed., 2006), Chapter 5 & 7

3. Question What mechanisms to achieve the generation of Ab diversity? Ans: 1. VDJ gene recombination - => V regions => Ag binding 2. Somatic hypermutation - 3. Isotype switching => Fc portion => Effector function

4. Outline Structures & Features of T-cell antigen receptor (TCR)Structures & Features of T-cell antigen receptor (TCR) Structures & Features of Major Histocompatibility Complex (MHC) Antigen Presentation to T cells Summary & Question

5. Antibody, TCR & MHC

6. Key Concepts in T-cell Receptor (TCR) 1. T-cell antigen receptor (TCR) is similar to the F(ab) of Ab but only located on the surface of T cells => No secreted form => Two major types:  TCR and  TCR 2. TCR functions to recognize Ag peptide and then to activate T cells => Adaptive immunity 3. Ag recognition by  TCR requires Ag presented by Major Histocompatibility Complex (MHC). => consider both Ag peptide & MHC => Cell-Cell interaction 4. The Ag-binding site region of the TCR is formed by the V  and V  regions.

7. Similarities & Differences between T-cell Receptor (TCR) and Ab

8. The Structure of T-cell Receptor (TCR)

9. T-cell Receptor (TCR) vs. Ab

10. Binding of a TCR to a peptide- MHC complex Front Side

11. TCR & Accessory Molecules Accessory Molecules -Help T cells in response to a specific Ag 1. CD3 (  chains)  associates w/ TCR => intracellular signaling transduction 2. CD4/CD8: CD4  MHC-II CD8  MHC-I 3. CD28: a co-stimulatory receptor 4. Integrin: Adhesion & co-stimulation

12. The TCR/CD3 Complex TCR/CD3 Complex: 1. TCR  dimer + multiple CD3 dimers  - CD3  dimer - CD3  dimer - CD3  dimer 2. The ITAM motif on CD3 => Signaling Transduction

13. TCR/CD3 Complex vs BCR/Ig  Complex

14. Interactions of Accessory molecules between T cells & APCs

15. Outline Structures & Features of T-cell antigen receptor (TCR) Structures & Features of Major Histocompatibility Complex (MHC)Structures & Features of Major Histocompatibility Complex (MHC) Antigen presentation to T cells Summary & Question

16. Key Concepts in Major Histocompatibility Complex (MHC) 1. Ag presentation to TCR is mediated by Two classes of MHC molecules. - Class-I MHC => peptides from cytosolic (intracellular) proteins => CD8 T cells - Class-II MHC => peptides from intracellular (exogenous) proteins from phagocytosis => CD4 T cells 2. In humans, the MHC is also called as the HLA (Human Leukocyte Antigen). 3. MHC genes are the most polymorphic genes present in the genome and co-dominantly expressed in each individual. 4. MHC molecules express on the cellular surfaces of only in presence of Ag-peptides. Class-I => all nucleated cells Class-II => APCs (DC, Macrophages & B cells)

17. TCR-Ag w/ Histocompatibility Complex (MHC)

18. The Discovery of Major Histocompatibility Complex (MHC) Histocompatibility genes: George Snell in 1940s => tumors or tissues => same strain, OK => foreign strains, No

19. For their work leading to the discoveries of MHC (mouse) and HLA (human). - Immune recognition => The foundation of adaptive immunity - Transplantation

20. Schematic maps of human & mouse MHC loci

21. Class-I vs. Class-II MHC molecule

22. Structure of Class-I MHC molecule  domains are polymorphic and form the peptide-binding cleft.  domain is conserved among all MHC class-I, and folds into an Ig domain for CD8 binding.

23. Structure of Class-II MHC molecule  domains are polymorphic and form the peptide-binding cleft.  domain is conserved among all MHC class-II, and folds into an Ig domain for CD4 binding.

24. Polymorphic residues of MHC molecules In Class-I, polymorphic residues => the peptide-binding cleft formed by  domains In Class-II, polymorphic residues => the peptide-binding cleft formed by  domains In this case HLA-DR, polymorphism is on  domain

25. MHC genes are highly Polymorphic

26. Expression of MHC alleles is co-dominant

27. Polymorphism & Polygeny both contribute to the MHC diversity

28. Gene conversion creates new alleles in MHC genes => Copying sequences from one MHC gene to another

29. Gene recombination creates new alleles in MHC genes

30. MHC expression on cells-I

31. MHC expression on cells-II Expression of MHC molecules is increased by cytokines produced during innate & adaptive immune cells, e.g. IFN

32. Features of Peptide-MHC interactions 1. MHC molecules show a broad spectrum for peptide binding, in contrast to the fine specificity of Ag recognition by Ab. 2. Peptide-MHC interactions are non-covalent and mediated by residues both in the peptides and in the clefts of MHC molecules. 3. Each MHC molecule binds one peptide at a time but can bind many different peptides. 4. MHC molecules DO NOT discriminate between “Foreign Peptides” & “Self Peptides”.

33. Outline Structures & Features of T-cell antigen receptor (TCR) Structures & Features of Major Histocompatibility Complex (MHC) Antigen presentation to T cellsAntigen presentation to T cells Summary & Question

34. Key Concepts in Ag presentation between APCs & T cells 1. Most T cells recognize only peptides, whereas B cells can recognize peptides, lipids, nucleic acids,….etc. NK-T cells can recognize lipids. 2. T cells only recognize peptides displayed by MHC molecules on Ag-presenting cells (APCs). 3. APCs are responsible for capturing and displaying different Ags to T cells. 4. APCs serve two key functions for T cell activation: 1 st function => process protein Ags to small peptides => form & present the peptide-MHC complex to T cells 2nd function => provide 2nd co-stimulatory signals, e.g. Cytokines & Surface Molecules

35. Features of Ag recognition by T cells

36. T cells require APCs to respond to a specific Ag

37. Functions of different APCs

38. Features of different APCs

39. Dendritic cells – The Most effective APCs 1. Located at common sites of entry of microbes 2. Express receptors to capture microbes. 3. Migrate preferentially to T- cell zones in LNs. 4. Mature DCs express high levels of costimulators => T-cell activation.

40. Overview of Dendritic cells in Ag capture & presentation

41. MHC Restriction of cytotoxic T cells

42. For their work leading to the discoveries regarding the specificity of cell mediated immunity =>MHC-restriction for T cell recognition => Adaptive immunity

43. Class I vs Class II MHC pathway

44. The Class II MHC pathway of Ag Presentation

45. The Class I MHC pathway of Ag Presentation

46. Ag Presentation to different T cell subsets

47. SUMMARY 1. TCR functions to recognize Ag peptides presented the MHC complexes => Ag peptide specificity => MHC restriction 2. Two classes of MHC molecules. - Class-I MHC => peptides from cytosolic (intracellular) proteins => CD8 T cells - Class-II MHC => peptides from intracellular (exogenous) proteins from phagocytosis => CD4 T cells 3. APCs serve two key functions for T cell activation: 1 st function => process & present Ag peptides w/MHC to T cells 2nd function => provide 2nd co-stimulatory signals, ex. cytokines & surface molecules

48. Questions What is the Bare Lymphocyte Syndrome? What is the advantage of MHC Polymorphism? Is that good if MHC is as diverse as Ig or TCR?