Basic Immunology

1. Basic Immunology

2. White Blood Cell Mononuclear Leukocyte - T Cell , B cell, Null (LGL) Granulocyte – PMN Leukocyte NK cell Mono-Macrophage system

4. Immune system Innate Immunity Granulocyte – PMN Leukocyte NK cell Mono-Macrophage system Adaptive immunity APC T and B cell

11. Organization of normal lymphoid system T B NK CENTRAL COMPARTMENT: Reservoir for precursor cells; 'nursing' home for maturation Thymus ?Bone marrow ? Peyer patches ? PERIPHERAL COMPARTMENT: Reservoir for mature cells, ready torespond to antigens Lymph node, Spleen, Mucosal sites

12. Thymus

14. Thymus • Site of T cell differentiation and maturation • 구조: Lobule: Cortex-Medulla

15. Thymus of a newborn

17. Hassall’s corpuscle

18. Thymic epithelium ( cytokeratin immunostain)

19. T lymphocyte (thymocyte) Medium sized immature thymocyte in subcapsular region and cortex Small lymphocytes in medulla B lymphocytes CD2+ CD40+(subset) Asteroid cell : microenvironment of medulla Lymphocytes

20. CD3 (pan-T)

21. CD20 (pan-B)

22. Other cells • Myoid cell: acetylcholine receptor-like material on surface • Macrophage: cortex and medulla • Interdigitating dendritic cell: medulla, HLA-DR+

24. Subcapsular thymocyte Cortical thymocyte Medullary thymocyte Peripheral T cell Prothymocyte Precursor cells Mature T cells TdT CD7 CD2 CD3 Cytoplasmic Surface CD4 CD4, CD8 CD8 CD1

30. A Model for the Regulation of T Cell Fate by Notch and TCR Signals Cell, 1997, 88;6:833-843

31. DiGeorge syndrome In the mid 1960s, Angelo DiGeorge, MD, an endocrinologist A genetic disorder Clinical features • Hypoparathyroidism (underactive parathyroid gland), which results • in hypocalcemia (low blood calcium levels) • Cypoplastic (underdeveloped) thymus or absent thymus, which • results in problems in the immune system • Conotruncal heart defects (e.g., tetralogy of Fallot, interrupted • aortic arch, ventricular septal defects, vascular rings) • Cleft lip and/or palate

32. Treatment Markert, M. L. et al. Transplantation of thymus tissue in complete DiGeorge syndrome. N. Engl. J. Med.341, 1180–1189 (1999). This paper shows that the transplantation of thymi of young children into patients suffering from complete DiGeorge syndrome results in the appearance of mature T cells. This not only describes a much-needed therapeutic intervention for these patients, but also conclusively shows the essential role of the thymus for T-cell development in humans Thymic epitherial cell : Donor origin Thymocyte : recipient origin However, Miller, J. F. A. P. Immunological function of the thymus. Lancet2, 748–749 (1961). The basic principle of thymic dependency of T-cell production had been established in the mouse more than 40 years earlier

33. The thymus is critical for the maturation of bone marrow derived cells into T cells Defect in antigen-receptor gene rearrangement Defect in the thymus

34. How many T cells are alive and leave the thymus • 108 to 2X108 cells in the thymus (in the case of mouse) • About 5X107 new cells are generated each day • 106 to 2X106 cells leave the thymus (~2-4%) • Despite the disparity, the thymus does not continue to grow in size or cell number • ~98% thymocytes die within the thymus (by apoptosis rather than by necrosis) Red: apoptotic cells Blue: macrophage

35. T cell Maturation & Development-1 • Progenitor T cells from sites of hematopoiesis begin to migrate to thymus at about 11 days of gestation in mice and in the eight or ninth week in humans. • T cell maturation similar to B cells involves rearrangements of the germ-line TCR genes and expression of various membrane markers; • Developing T cells in the thymus are known as thymocytes. • Thymocytes proliferate and differentiate into distinct subpopulations of mature T cells ;. • The antigenic diversity of T cells is reduced during maturation in the thymus by a selection process that allows only MHC-restricted and nonself-reactive T cells to mature ; • T cells selection processes include positive and negative selection in the thymus. • Finally functionally distinct mature CD4+ and CD8+ subpopulations that exhibit class II and class I MHC restriction respectively exit thymus.

36. If injected into the peripheral circulation, can even give rise to B cells and NK cells DN CD2 or Thy-1 molecules : the first cell-surface molecules specific for T cells DP SP

37. T cell development take place in the cortex and medullar

38. abTCR gene rearrangement in the thymus Db/Jb rearrangement DN Vb/DJb rearrangement b/pTaCD3 complex: - Triggers pho-and degradation of RAG-2 - Halting b-chain gene rearrangement (allelic exclusion) DP TCR b/aCD3 complex:

39. Schematic representation of the pTa/TCRb complex Pre-T-cell receptor (pre-TCR) signalling

41. Positive and negative selection in the thymus • Positive selection • After completion of TCRα rearrangements, αβ T cells die unless they are rescued by a low-affinity interaction of the TCRαβ heterodimer with self-peptides complexed with MHC antigens that are expressed on thymic epithelial cells. • Selection for Thymocytes with TCR’s capable of binding MHC (MHC restriction) • Negative selection • Thymocytes that express high-affinity receptors for self-peptide–MHC expressed on thymic DCs are deleted in a process that is known as negative selection • Elimination of thymocytes that have TCR’s that have: • high affinity self MHC / bind self-MHC + self peptide(Self tolerance)

42. Selection of mature T cells from thymocytes

43. Where do thymocytes undergo negative selection? This has been controversial

44. Positive and negative selection in the thymus Positive selection : MHC class I & lass II expressed on epithelial cells Negative selection : Macrophage & Dendritic cells

45. T cell development in mouse thymus : Overview Precursor DN DP SP Success rate (survival rate): <5%

46. T cell development in the human

47. Early stages of human T-cell development • The thymus blood-borne precursor cells (originate from bone-marrow stem cells) • Cord-blood progenitor cells •  CD34+cells •  Lack expression of recombination-activating gene 1 (RAG1),and CD1A, • cytoplasmic CD3 , CD2 and CD7 •  A common T/NK-cell progenitor(J. Exp. Med. 180, 569–576 (1994): • The first study to prove that T cells and NK cells are derived from a common precursor) •  Dendritic-cell (DC) precursors; •  Plasmacytoid DCs (PDCs)precursors • Thymic immigrants enter through the junction between the medulla and cortex • T-cell precursors migrate outwards in the cortex and accumulate in the subcapsular zone • The transition of CD34+CD1A- cells to a CD34+CD1A+ stage is strongly • associated with T-cell commitment, because CD34+CD1A+ cells, in contrast to • CD34+CD1A- precursors, have strong T-cell, but little NK-cell and no DC or PDC, • precursor activity

48. Cell-fate determination of lymphoid lineages at early stages • E proteins • a subfamily of basic helix–loop–helix transcription factors • the inhibitor of DNA binding (ID) proteins • ID Proteins • Determine the lineage choice between T cells, B cells and PDCs • NOTCH1 •  A factor that determines the choice between T- and B-cell fate • (Immunity 10, 547–558 (1999).)

49. CD4ISP CD4 CD1a marks commitment to the T cell lineage

50. Positive & negative selection