Summary of the last lecture

1. Summary of the last lecture • Features of T cell antigen recognition • APC • Ag processed • MHC restrictions • TCR ligand: Peptide + MHC • Basis for Ag presentation by MHC • Genetic basis • Polygeny • Polymorphism • Structural basis • MHC polymorphic binding pockets • Peptide anchor residues • Types of APC • Target cell • Professional APC

2. Student Q1:What cells express MHC molecules: Class I, Class II or both, & why? • Virtually all nucleated cells express MHC Class I • Potentially infectable (e.g. by virus)  as target cells (for Tc) • Thymic stromal cells express both • T cell selection –thymic education • Key immune cells express both • B cells: present Ag to Th to receive help for Ab responses • MQ: first line of defense, once activated  ‘professional’ APC • DC: initiation of immune responses ‘true professional’ APC

3. Types of pathogens & Ags Two classical pathways for Ag processing Ag processing, presentation, & their clinical relevance Cell interactions & co-operation Pathways of Ag processing,Ag presentation & co-stimulations

4. Question:How is a TCR ligand generated?

5. Ag processing & presentation T APC

6. Types of pathogens 2 3 1 cytosol Vesicles Endocytic vesicles ER “endogenous” “exogenous”

7. Types of pathogens or antigens: • Cytosolic - “endogenous” • - viruses, intracellular bacteria • Endocytic/vesicular - “exogenous” • - Intra-vesicular bacteria, parasite etc. • - Extra-cellular pathogens, toxins

8. Two major subsets of T cells whose recognition of antigens are restricted by two different classes of MHC: • CD8+ T cell (cytotoxic T) • - MHC Class I restricted • CD4+ T cell (T helper) • - MHC Class II restricted

9. Ag processing & presentation TH Tc Exogenous Endogenous/Cytosolic I II TAP APC Intro-vesicular TAP: Transporters associated with Ag processing

10. Two classical pathways of Ag processing: • Endogenous (cytosolic) pathway •  “MHC Class I pathway”  Tc cells • Exogenous (endocytic) pathway •  “MHC Class II pathway”  Th cells

11. (TAP) Proteasome & subunit

12. The Class I pathway

13. 1 2

14. Empty Class I molecules are unstable under physiological temperature TAP deficient (RMA-s) Normal Exogenous peptides (370C) 370C 19 - 330C

15. Empty MHC class I molecule come out in the coldBy Ljunggren HG et al. Nature (1990) 346:476-80

16. 3

17. The Class II pathway Ii CLIP

18. Functions of the Invariant Chain (Ii) • Block MHC Class II molecules from binding of peptides derived from endogenous antigens • Direct MHC Class II molecule to cellular vesicles where exogenous peptides are generated

19. CLIP HLA-DM

21. Cytosolic proteins proteasome peptides TAP

22. Ag processing, presentation & their clinical relevance • Pathogen strategies for immune evasion: • prevent TAP function (HSV) • inhibit endosomal acidification (Helicobacter pylori) • retention of MHC molecules (Retroviruses) • Vaccine design: Types of immune responses: • Humoral (B)  endocytic pathway • Cell-mediated (CTL)  cytosolic pathway • MHC deficiencies: • Congenital MHC class II deficiency • Ir gene defects

23. Heterozygotes • Inbred • Homozygotes MHC deficiency A E b a b a A1 B1 A B 1 21 2 A1 B1 A1 B1 A E b a b a B1 B1 A1 A1 B1 B1

24. Student Q2: ‘Self’ MHC restriction?

25. T cells recognize “self” MHC

26. The environment in which the T cells mature determines the MHC restriction of the mature T cell receptor repertoire

27. Cell interactions & co-operations • Cell adhesion molecules • Cytokines and cytokine receptors

28. Question:Chances of the specific T cell:APC interactions ?

29. (1) (3) (2)

30. Organized distribution F + DC (B + FDC) T: T cell area B: B cell area F: B cell follicle GC: germinal centre GC

31. Question How do cells know where to go and act ?

32. Adhesion molecules: SELECTINS - e.g. L-selectin, P-selectin, E-selectin MUCIN-LIKE MOLECULES - (Addressins) e.g. CD34 INTEGRINS - e.g. LFA-1, VLA-4 IMMUNOGLOBULIN SUPERFAMILY - e.g. CD2 (LFA-2), ICAM-1, 2, 3

33. T cell-endothelium interactions Lymph node cortex (3) HEV Endothelium CD34 Glycam 1 ICAMs LFA-1 L-Selectin T T (1) (2) HEV: high endothelial venue

34. T cell:APC interactions

35. Armed effector T cells are guided to sites of infection by newly expressed adhesion molecules

37. Cytokines • Small pharmacologically active products of cells • Nomenclature & classification • Lymphokines: produced by lymphocytes • Interleukins: interleukin 1 - 26 (IL-1 – IL-26) • interferons, TNF etc. • Monokines: produced by monocytic/phagocytes • Chemokines: CXC (IL-8), CC (DC-CK, MDC), CX3C (Fractalkine)

38. Chemokines & Chemokine receptors Chemokine Group Examples Target cells Receptors CXC ELR+IL-8 neutrophils CXCR1, 2 ELR-Mig, IP-10 activated T CXCR3 CC MIP-3b naïve T CCR7 DC-CK1, naïve T ? MDC DC, T, NK CCR4 C & CX3C Fractalkine T, mono., neutr. CX3CR1 6-Cysteine CC 6Ckine T, B, mesangial ? *ELR: Cysteine residues ‘Glu-Leu-Arg’

42. g IL-2Ra (CD25) - Cell activation marker

43. T cell activation and proliferation (high affinity)

44. Summary 1Two types of pathogens/Ags: • Cytosolic - “endogenous” • - viruses, intracellular bacteria • Endocytic/vesicular - “exogenous” • - Intra-vesicular bacteria, parasite etc. • - Extra-cellular pathogens, toxins

45. Summary 2Two classical pathways for Ag processing • “MHC class I pathway”  CD8+ T cells • (Endogenous/cytosolic, TAP-dependent pathway) • “MHC class II pathway”  CD4+ T cells • (Exogenous/endocytic, TAP-independent pathway)

46. Summary 3 • A defect or defects in Ag processing or presentation may result in severe immunological consequences • Pathogens may evade host immune system by interfering with the mechanisms of Ag processing and presentation • Cells of the immune system interact in a complex network • Cell interactions and re-circulation are mediated by adhesion molecules, cytokines and cytokine receptors

47. Summary 4Cytokines - principles of action • Local & systemic effects • Unique receptor for each cytokine • Pleiotropic • Synergistic & autocrine fashion • Complex network – a single cell can secrete, or be susceptible to, more than one cytokine