1. Phase 2 of thymic development: selection of T cells that can interact with self MHC and antigen This phase of T cell development consists of two steps: –positive selection (TCR that can interact with self-MHC) –negative selection (eliminate self-reactive cells that are stimulated by MHC + self)

2. The specificity or affinity of positive selection must differ from that of negative selection:

3. Antigen-driven T cell Differentiation in Secondary Lymphoid Organs Mature T cells leave the thymus and migrate to secondary lymphoid tissues (lymph nodes, spleen, mucosa-associated lymphoid tissue), recirculating via the blood and lymph, just like mature B cells do. Mature T cells are longer lived than mature B cells, and can survive for years without antigenic stimulation.

4. Unlike B cells, which have just one type of terminally- differentiated cell (plasma cell), there are various types of effector T cells: –CD8 T cells, which can differentiate into cytotoxic T cells –CD4 T cells, which can become either TH1 or TH2 helper cells.

5. T cells interact with antigen in the T cell-rich areas of peripheral lymphoid tissues. There, they can interact with antigen-presenting cells and be stimulated on encounter with an appropriate antigen, and function as helper T cells, interacting with B cells and other lymphocytes.

6. Ligation of the T cell’s receptor for antigen results in an initial activation signal (first signal), as is true for B cells. Again, as with B cells, this first signal is not sufficient to activate the cell: –second signals (co-stimulatory signals) are necessary for activation –The principal co-stimulatory signal for T cells is delivered via ligation of CD28 by B7 on the APC

7. Ligation of the TCR without co-stimulation results in T cells becoming non-responsive or apoptotic:

8. T cell signaling occurs via the cytoplasmic tails of the molecules that make up the CD3 complex, which is associated with the TCR. These associate with protein tyrosine kinases and initiate intracellular signaling that results in altered gene expression:

10. Encounter with antigen can result in the formation of memory T cells. Continuing re-contact with antigen may be important for the survival of these memory T cells. Small amounts of antigen can persist for a remarkably long time, localized on follicular dendritic cells in secondary lymphoid organs. One significant differences between memory T cells and memory B cells is that the TCR does not undergo isotype switching or affinity maturation by somatic mutation, unlike the BCR.

11. Memory T cells can be defined by a change in the expression of certain surface molecules:

12. Helper T cell activity – TH1 and TH2 responses After mature CD4 T cells are activated by contact with antigen and APC, they are induced to proliferate and differentiate into effector cells – cells that can exert helper activity. Most of the effector function of CD4 T cells is mediated by the secretion of cytokines, and by the expression of various cell surface stimulatory molecules. Activated CD4 T cells can differentiate down two pathways, becoming either type 2 (TH2) helper cells, helper cells for humoral (antibody) responses, or type 1 (TH1) helper cells, which are helper cells for cell-mediated responses:

13. TH1 cells induce cellular and inflammatory responses, activating macrophages, cytotoxic CD8 T cells, and other effector cells

14. Antibody production is enhanced in TH2-dominated responses:

15. Initially, mature naïve CD4 T cells are not committed to being either TH1 or TH2 cells After activation, these cells proliferate and differentiate into TH1 or TH2 cells

16. Factors that are thought to be important in TH1/TH2 differentiation include: –the type of infectious agent and genetics –together, these result in the pattern of cytokines produced by APC and by other cells: IL-12, IFN  - TH1 cells; IL-4 - TH2 cells

17. Single Nucleotide Polymorphisms (SNP) in the promoter region of the human IL10 gene 0-1082-851-592 G / AC / TC / A = high IL10 secretor genotype = low IL10 secretor genotype -592 C/A and A/A alleles were seen to be associated with more rapid progression to AIDS (Shin, et al. PNAS 97:14467, 2000)

18. Therefore, the nature of the initial innate immune response helps determine whether TH1 or TH2 cells are stimulated

19. TH1 and TH2 cells are defined by the pattern of cytokines that they produce - there is no unambiguous phenotypic profile for TH1 or TH2 cells, based on the expression of cell surface markers: –TH1 cells produce various cytokines that are macrophage-activating (IFN , TNF  ) and/or enhance cellular responses (IL-2). –TH2 cells produce various cytokines that are B cell- activating factors (IL-4, IL-5, IL-6, IL-10).

20. TH1 cytokines (IFN , TNF  ) are potent macrophage- activating factors. TH2 cytokines (IL4, IL5, IL10) are potent B cell- stimulatory cytokines. Therefore, TH1 and TH2 cells not only have stimulate different types of immune responses, they also enhance the activation of different sorts of APC:

21. Cytokines produced by TH1 or TH2 cells not only exert the effector functions associated with these helper T cells, they also can act on the opposite type of helper T cell: cross- regulation

22. Both TH1 and TH2 responses play important roles in defense against pathogens. Also, all immune responses contain TH1 and TH2 components However, there can be pathological consequences that result from a TH1 or a TH2-dominated response. In leprosy, a TH2-dominant response is associated with disease progression, and a TH1 response is associated with a better disease outcome.

25. Cytokines CD4 effector functions are mediated by the production and secretion of cytokines. Different types of CD4 cells produce different cytokines, and these cytokines have different effects on their target cells. However, many types of cells, both immune system cells and non-immune system cells, can produce cytokines. Therefore, cytokine production is not restricted to helper T cells, although these cells seem to be specialized to secrete cytokines that are important for inducing and enhancing immune responses.

26. Cytokines are a heterogenous group of protein mediators that play important roles in the generation and maintenance of immune responses. While heterogenous, cytokines do share some features: –low-intermediate molecular weight (10-80 kDa) –interact with high-affinity receptors, specific for each cytokine –glycosylated, secreted proteins –produced during innate and specific adaptive immune responses –mediate and regulate immune and inflammatory responses –produced briefly and locally, in a self-limiting manner

27. Cytokines can act as autocrine, paracrine, or less frequently, as endocrine factors:

28. Cytokines can act on multiple cell types - pleiotropic (many target cell types, as well as many biological activities):

29. Their biological actions often are redundant - several different cytokines may exert the same or similar function:

30. Although heterogenous, cytokines fall into groups of related molecules: cytokine super-families. Cytokine receptors also fall into families of related molecules.

31. Cytokines interact with high-affinity, specific cell surface receptors; cytokine binding results in signal transduction, and ultimately in altered cellular behavior, by inducing a new pattern of gene expression:

32. Although heterogenous, cytokines fall into groups of related molecules: cytokine super-families. The hematopoietin cytokine superfamily is the largest cytokine family, and contains many of the cytokines that play critical roles in immune responses: –IL-2, IL-4, IL-6, IL-10, IL-12, G-CSF –interferons (IFN , IFN , IFN  ) In addition to this, several hematopoietin cytokines (such as erythropoietin) operate primarily outside of the immune system. While the hematopoietins share a similar molecular structure, they clearly have diverse, and some times opposing, biological effects.

33. Another cytokine family that contains several cytokines known to play important roles in immune responses is the tumor necrosis factor (TNF) cytokine family: –CD40-ligand, fas-ligand –TNF  cytokines in this large family typically are ~20kDa have extensive  -pleated sheet structure homotrimers

34. Two other cytokine families that contain important cytokines are the IL-1-like cytokines and the chemokines (chemoattractant cytokines: IL-8 and similar cytokines).

37. IL-2 plays a critical role in T cell growth, following the activation of T cells by exposure to Ag+MHC and co- stimulatory signals. Following T cell activation, T cells begin to produce IL-2, as well as to express a higher-affinity, more responsive, form of the IL-2 receptor complex, resulting in proliferation and differentiation to become effector T cells. IL-2 interacts with the IL-2 receptor complex.

38. The IL-2 receptor can exist in a high-affinity, as well as in lower-affinity, versions. The high-affinity receptor is composed of three polypeptides, the , , and  IL-2 receptor chains. On resting T cells, only the  and  chains are expressed (lower affinity). Activation of T cells results in the expression of the  chain, which when associated with the  and  chains (high affinity form). Following activation, CD4 T cells can go on and produce a wide variety of cytokines, in addition to IL-2.

40. Overproduction of cytokines in response to infection can result in effects that are detrimental to the host. In particular, the overproduction of TNF  in response to infection with gram-negative bacteria can result in a systemic response that can be life-threatening, septic shock.

43. In recent years, several cytokines have become available commercially, for clinical use. EPOGEN®, recombinant human erythropoeitin used to treat anemia – a member of the hematopoietin family INFERGEN®, bioengineered, non-naturally occurring type-I interferon used in the treatment of chronic hepatitis C virus. NEUPOGEN®, a recombinant human granulocyte colony- stimulating factor (G-CSF) used for treatment of severe chronic neutropenia. Unfortunately, the availability of some of these cytokines has resulted in significant problems with abuse.

45. Cytotoxic T cell activity - CD8 cells Stimulation of naïve CD8 T cells by antigen + MHC class I on effective APC results in the induction of effective killer T cells, or cytotoxic T cells (CTL). CD8 cytotoxic T cells can be stimulated in several ways: –dendritic cells acting as APC –CD4 helper cell-activated APC –high concentrations of IL-2 produced by helper T cells

47. The most effective APC for CD8 cells are dendritic cells. Dendritic cells express a wide range of co-stimulatory molecules, such as B7, and can act as effective APC, resulting in the efficient induction of cytotoxic T cells:

48. Activated CTL detect potential targets by binding to cells that express a foreign antigen together with their MHC class I molecule:

49. When a CTL detects a target cell expressing antigen, it kills this cell by releasing: –cytokines that can stimulate apoptosis (TNF , TNF , IFN , fas ligand), –cytotoxins, such as perforin and granzymes, which are directly cytotoxic and apoptosis-inducing molecules

50. CTL store cytotoxins in lytic granules:

51. The interaction of the CTL with the target cell results in selective killing of virus-infected cells, which is induced by programming the infected cell to die by apoptosis. CTL can kill sequentially kill several target cells at the site of viral infection:

52. Innate immune effector mechanisms Innate immune responses are critical in combating infection:

53. There are several forms of innate immunity: natural killer (NK) cells complement system inflammatory cytokine response by macrophages acute-phase response

54. Innate immune effector mechanisms – Natural killer (NK) cells NK cells are another type of cytotoxic cells. NK cells are morphologically large granular lymphocytes (LGL). They do not express the T cell receptor for antigen: are not antigen-specific in their activity. NK cells are sensitive to cells that have lost MHC class I expression, and can kill these cells via cytotoxic mechanisms similar to those exerted by CTL.

55. Some viruses attempt to subvert the immune system by down- regulating MHC class I expression, making these virus-infected cells “invisible” to conventional CD8 CTL NK cells are believed to fill this gap in immunity, by automatically killing such virus infected cells

56. NK cells also can be armed by antibody, and exert an effector function called antibody-dependent cellular cytotoxicity (ADCC) In ADCC, antibody bound to an antigen (tumor cell, virus infected cell) can bind via their FC (non-antigen-binding) portion to NK cells, which express FC receptors that specifically allow binding to antibody.