Medi 4318 Introduction to host defenses Ahmad Sh. Silmi Rm 326 Admin. Building

1. Medi 4318 Introduction to host defenses Ahmad Sh. Silmi Rm 326 Admin. Building asilmi@iugaza.edu.ps Office hours: Every week Sun &Tues 11:00 to 12:00.

2. What are host defenses? What are the reasons for host defenses? • Defense against infections. • Defense against toxins. • Defense against internal dangers • (cancer, bad pregnancy). • Regulating defenses: autoimmunity • and pregnancy.

3. What are the range of mechanisms called host defenses? Innate defenses: A broad range of mechanisms that don’t require selection of cells to secrete or kill specific things (usually ready instantly or within a few hours). Adaptive immunity: Selection of cells that secrete or kill. Requires DNA rearrangements and takes about a week. Have persistent memory.

4. Why Differentiate between the Innate and Acquired Immunity ? Innate Immunity Characteristics: Universal Rapid Lacks memory Non specific but ... Acquired Immunity Characteristics: Not universal ‘Slow’ to develop Possesses memory Specific but…. ‘Plays to the tune of the Innate immune system’

5. Both innate and immune defense cells are generated in bone marrow (or at least start there). * Mostly adaptive immune cells. * Mostly innate defense cells *,* *,* * * * * * * * * * * Kuby Fig 2.2

6. Adaptive immunity is all about generating functional lymphocytes (B and T). Generation of cellular diversity (bone marrow and thymus) Selection of cells specific to a given antigen (lymph node, spleen, and mucosal tissue) Clonal expansion of these cells (lymph node, spleen and mucosal tissue) Function of mature cells (entire body)

7. A B lymphocyte has a single kind of BCR which binds only one type of antigen. Antigen B cell receptor (BCR) B lymphocyte

8. A T lymphocyte has a single kind of TCR that recognizes only one kind of antigen. Antigen Antigen presenting cell (APC) MHC Antigen (a bit of it) T cell receptor T lymphocyte

9. Adaptive immunity is all about generating functional lymphocytes. Generation of cellular diversity (bone marrow and thymus) Selection of cells specific to a given antigen (lymph node, spleen and mucosal tissue) Clonal expansion of these cells (lymph node, spleen and mucosal tissue) Function of mature cells (entire body)

10. How and where is cellular diversity generated? I. B lymphocytes. Bone marrow is where B progenitor cells become mature B cells and migrate to lymph nodes and spleen looking for antigen. Bone marrow is the primary lymphoid organ for B cells

11. Pre-T cells (no TCR) migrate from bone marrow to the thymus where they rearrange DNA. How and where is cellular diversity generated? II. T lymphocytes: Within the thymus the pre-T cells mature into T cells by rearranging their TCR DNA and generate about 107 cells (each with one copy of unique TCR DNA) per day, then exit looking for that antigen. This process is antigen-independent. Thymus is the primary lymphoid organ for T cells.

12. A question for votes: Where and how is B lymphocyte diversity generated? • In its primary lymphoid tissue, the thymus. • By generating new DNA sequences in every B cell. • In bone marrow by rearranging existing DNA. • In spleen by eliminating cDNA.

13. Adaptive immunity is all about generating functional lymphocytes. Generation of cellular diversity (bone marrow and thymus) Selection of cells specific to a given antigen (lymph node, spleen and mucosal tissue) Clonal expansion of these cells (lymph node, spleen and mucosal tissue) Function of mature cells (entire body)

14. The lymph nodes and spleen are secondary lymphoid organs Naïve T cells (from thymus through blood) Naïve B cells (from bone marrow through blood) If Antigen not found, the naïve B and T cells re- circulate and die. lymph nodes or spleen If Antigen is found, the naïve B and T cells survive and clonally expand in the secondary lymphoid organs. Antigen (either through lymph ducts or blood)

15. Adaptive immunity is all about generating functional lymphocytes. Generation of cellular diversity (bone marrow and thymus) Selection of cells specific to a given antigen (lymph node, spleen, and mucosal tissue) Clonal expansion of these cells (lymph node, spleen and mucosal tissue) Function of mature cells (entire body)

16. A B cell has a single kind of BCR which binds only one type of antigen and can clonally expand and mature into plasma cells that will secrete the BCR (Ig) which will bind that antigen. immunoglobulin B lymphocyte BCR clonal expansion B memory cells Plasma cell

17. Clonal expansion of T Lymphocytes A single T lymphocyte Many T lymphocytes TCR Antigen preCTL MHC Antigen (a bit of it) TCR T cell receptor Clonal Expansion T helper cell T lymphocyte

18. Adaptive immunity is all about generating functional lymphocytes. Generation of cellular diversity (bone marrow and thymus) Selection of cells specific to a given antigen (lymph node, spleen, and mucosal tissue) Clonal expansion of these cells (lymph node, spleen and mucosal tissue) Functional mature cells (entire body)

19. Functional B lymphocytes BCR immunoglobulin rapid clonal expansion Memory B lymphocytes Both plasma cells and memory B cells are present in all the Ig subclasses: IgM, many kinds of IgG, IgA, IgE. Plasma cells

20. TCR TCR Functional T lymphocytes TCR When mature, these cells leave the secondary lymphoid organs, enter the blood steam and home to where they find their antigen. preCTL Clonal expansion also produces memory cells of both types. These memory cells recirculate and can be reactivated by exposure to antigen in secondary lymphoid tissue. T helper cells. TDTH cells T memory cells (both types)

21. A question for voting: What does the term clonal expansion mean? • A stem cell is triggered to proliferate to form • a clone of progeny cells. • A naïve B cell meets its antigen and proliferates • to produce lots of progeny cells. • A T memory cell meets its antigen on an APC in • a secondary lymphoid organ and proliferates. • All of the above. • None of the above.

22. Adaptive immunity is all about generating functional lymphocytes. Generation of cellular diversity (bone marrow and thymus) Selection of cells specific to a given antigen (lymph node, spleen and mucosal tissue) Clonal expansion of these cells (lymph node, spleen and mucosal tissue) Functional mature cells (entire body)

23. In the generation of diversity, why don’t lymphocytes attack our own cells? Because of tolerance: central and peripheral. Central tolerance involves elimination of almost all self-reactive lymphocytes at the site of production: the primary lymphoid organs of bone marrow and thymus. Peripheral tolerance involves suppressing immune responses to antigens not eliminated in central tolerance.

24. How cells are gotten rid of in central tolerance. By apoptosis, which is non- inflammatory.

25. Before B lymphocytes can migrate out of bone marrow, all those cells bearing a BCR that reacts with a ‘self’ antigen bind to bone marrow stromal cells and die. Apoptosis

26. T lymphocyte central tolerance. T cells come in two flavors: CD4 and CD8 positive. Both are generated randomly in the thymus by recognition of a bit of antigen presented on a particular MHC molecule in the context of either CD4 or CD8 binding.

27. Major histocompatibility complex antigens (MHC) • MHC are cell surface molecules on APC that present • the bit of antigen to T lymphocytes. • There are two kinds of MHC: class I and class II. • Both molecules are fairly polymorphic ( different • choices in the population) and are heritable. • MHC class I has a one chain binding site while MHC • class II has two chains.

28. T cells must be activated by specific antigen presented by MHC class II (to CD4 cells) and MHC class I (to CD8 cells)

29. MHC class I presents internal antigen and MHC class II external antigen.

30. A chant for APC: Internal proteins are processed and presented by MHC I to CD8 positive T lymphocytes. External proteins are processed and presented by MHC II to CD4 positive T lymphocytes.

31. External pathogens are complexed by Ig molecules produced by B lymphocytes. Internal pathogens are killed (along with the host cell) by CD8 positive CTL. What do the CD4 positive T lymphocytes do?

32. The role of mature CD4 positive T lymphocytes • They provide ‘help’ in the form of cytokines in an antigen- • specific manner to developing B cells that allow class-switching • and memory cell formation. • They provide ‘help’ in the form of cytokines in an antigen-specific • manner to developing CD8 positive T cells that greatly enhances • the response. • Mature CD4 positive T cells migrate to tissues and help clear • macrophages of infections (the so-called DTH response).

33. So what does CD4 and CD8 have to do with the process of central tolerance in developing T cells? This is called positive selection.

34. Negative selection occurs next and is similar to the B cell process. Cortex Medula 3 3. The CD4/CD8 positive thymocytes (double positive) loose CD4 or CD8 to become CD4 and CD8 T cells and migrate to the medula. 4 4. In the medula, single positive T cells can bind to self peptide antigens presented on MHC class I or II on thymic dendritic cells. If they do, they die.

35. Adaptive immunity is all about generating functional lymphocytes. Generation of cellular diversity (bone marrow and thymus) Selection of cells specific to a given antigen (lymph node spleen and mucosal tissue) Clonal expansion of these cells (lymph node, spleen and mucosal tissue) Functional mature cells (entire body)

36. How does antigen get to lymphocytes?. In three ways: Through lymphatic ducts that lead to lymph nodes. Through blood which goes to spleen. Directly to lymphoid tissue through mucosal tissue (GALT)

37. MALT directly faces its antigen: an example is the gut.

38. The direct interface with outside is provided in the gut by specialized M cells, which take up whole antigen and feed it directly to macrophages, T and B cells inside it.

39. Secondary lymphoid tissues are where lymphocytes meets antigen which comes from lymph ducts (and blood) that connect to lymph nodes (or spleen).

40. The spleen is very much like the lymph node only it does not have lymph node connection: only blood.

41. What form of antigen enters the secondary lymphoid tissue? Free antigen which is necessary for survival and proliferation of B lymphocytes Antigen inside a dendritic cell, a super APC, necessary for T lymphocyte survival and proliferation.

42. What are dendritic cells? A kind of antigen presenting cell.

43. There are several kinds of dendritic cells, but only one that brings antigen into the lymph node.

44. Activation of specific immune response To regional lymph node Dendritic cells stationed beneath skin (a sentinal cell) differentiate and migrate when encountering a dangerous antigen.

45. Adaptive immunity is all about generating functional lymphocytes. Generation of cellular diversity (bone marrow and thymus) Selection of cells specific to a given antigen (lymph node spleen and mucosal tissue) Clonal expansion of these cells (lymph node, spleen and mucosal tissue) Functional mature cells (entire body)

46. Now we have free antigen, antigen being processed by dendritic cells and B and T lymphocytes present in secondary lymphoid tissue, what happens? Free antigen binds to its BCR on B lymphocytes and stimulates limited clonal proliferation and differentiation. Dendritic cells present antigen to CD4 positive T lymphocytes and stimulate clonal proliferation and differentiation. Dendritic cells present antigen to CD8 positive T lymphocytes and activate the cells for clonal expansion and differentiation. The first and third stimulations occur much better with ‘T cell help’ (from antigen-specific, mature CD4 positive helper cells).

47. The life cycle of a B lymphocyte.

48. Immune memory: a much larger and faster response the second time around. (IgM, IgG, IgA, IgE) (mostly IgM) These secondary effects are all due to CD4 T cell help.

49. Antigen driven clonal expansion and maturation without T cell help. APC soluble IgM BCR clonal expansion maturation B lymphocyte Plasma cell

50. The B lymphocyte as an APC T helper cell Antigen-specific BCR Antigen processing into peptides MHC peptide Help comes in the form of cytokines, soluble and cell- associated, produced by the T helper cell.