Cytokines

cytokines, their applications, signalling, and classification

1. CYTOKINES 1

2. INTRODUCTION The development of an effective immune response involves lymphoid cells, inflammatory cells, and hematopoietic cells . The complex interactions among these cells are mediated by a group of proteins collectively designated cytokines to denote their role in cell-to-cell communication. 2

3. Introduction Cytokines are low molecular weight regulatory proteins or glycoproteins secreted by white blood cells and various other cells in the body in response to a number of stimuli. These proteins assist in regulating the development of immune effector cells And some cytokines possess direct effector functions of their own. 3

4. Introduction Many referred as interleukins Secreted by leukocytes and act on other leukocytes IL-1 through IL 29 have been described 4

5. Chemokines Group of low molecular weight cytokines Affect chemotaxis and other aspects of leukocyte behaviour Play important role in inflammatory response 5

6. PROPERTIES Bind to specific receptors on the membrane of target cell Cytokine receptors may be made up from several different chains Cytokines & their fully assembled receptors exhibit very high affinity for each other & deliver intracellular signals Particular cytokine bind to receptors on the membrane Autocrine action Paracrine action Endocrine action 6

7. PROPERTIES 7

8. PROPERTIES 8

9. PROPERTIES 5. Cytokines regulate the intensity & duration of immune response Binding of a given cytokine to responsive target cells generally stimulates increased expression of cytokine receptors and secretion of other cytokines Exhibit attributes of pleiotropy , redundancy, synergy, antagonism and cascade induction Share many properties with hormones 9

10. 10

11. ELISA assay of cytokine 11

12. Cytokines belong to four families Falls in 4 families Hematopoietin family Interferon family Interleukin family Tumor necrosis factor family All have molecular mass less than 30kDa All have similarities and few rarely act alone 12

13. Cytokines belong to four families The amino acid sequences of these family members differ considerably All have high degree of α helical structure and little or no β sheet structure Molecules have similar polypeptide fold, with for α helical regions (A-D) In which the 1 st and 2 nd helices & the 3 rd and 4 th helices run roughly parallel to one another & are connected by loops 13

14. INTERLEUKIN 4 14

15. 15

16. Cytokines have numerous biological functions Although a variety of cells can secrete cytokines, the principal producers are T н cells, dendritic cells, and macrophages Cytokines released from these cell types activate an entire network of interacting cells 16

17. Cytokines have numerous biological functions Among numerous physiological responses that require cytokine involvement are Development of cellular and humoral immune responses Induction of inflammatory response Regulation of hematopoiesis Control of cellular proliferation Differentiation Healing of wounds 17

18. Cytokines have numerous biological functions What keeps cytokines from activating cells in a non specific fashion during the immune response? 18

19. Cytokines have numerous biological functions What keeps cytokines from activating cells in a non specific fashion during the immune response? Specificity is maintained by careful regulation of the expression of cytokine receptors on cells Cytokine receptors are expressed on a cell only after that cell has interacted with antigen, limiting cytokine response to antigen activated lymphocytes 19

20. Cytokines have numerous biological functions Specificity maintained if cytokine secretion occurs only when the cytokine-producing cell interacts directly with target cell, thus ensuring that effective concentrations of the cytokine occur in the vicinity of the intended target. In case of T н cell, a major producer of cytokines, cellular interactions occurs when the T-cell receptor recognizes an antigen-MHC complex on an appropriate antigen-presenting cell, such as a macrophage, dendritic cell, or B lymphocyte. 20

21. Cytokines have numerous biological functions The concentration of cytokines secreted at the junction of these interacting cells reaches high enough local concentration to affect the target APC, but not more distant cells. Half-life of cytokines in the blood stream or other extracellular fluids into which they are secreted is usually very short, ensuring that they act for only a limited period and thus over a short distance. 21

22. Cytokines have numerous biological functions 22

23. Cytokine Receptors Cytokine receptors fall into 5 families Immunoglobulin superfamily receptors Class I cytokine receptor family (also known as hematopoietin receptor family) Class II cytokine receptor family (also known as interferon receptor family) TNF receptor family Chemokine receptor family 23

24. Immunoglobulin superfamily 24

25. Class I cytokine receptor family 25

26. Class II cytokine receptor family 26

27. TNF receptors 27

28. Chemokine receptors 28

29. Cytokine receptors initiate signaling Although some cytokine receptors lie outside the class I and class II families, majority are included within these two families. Class I and class II cytokine receptors lack signaling motifs Unifying model emerged from studies of the molecular events triggered by binding of IFN- γ to its receptor, a member of the Class II family 29

30. Interferon gamma Originally discovered because of its ability to induce cells to block or inhibit the replication of a wide variety of viruses Antiviral activity is a property it shares with IFN- α and IFN- β IFN- γ plays a central role in many immunoregulatory proteins including Regulation of mononuclear phagocytes B cell switching to certain IgG classes Support or inhibition of the development of T н cell subsets 30

31. Cytokine receptors initiate signaling The cytokine receptor is composed of separate subunits Different inactive protein kinases are associated with different subunits of the receptor. Cytokine binding induces the association of the two separate cytokine receptors subunits and activation of the receptor-associated JAKs. 31

32. Cytokine receptors initiate signaling Activated JAKs create docking sites for the STAT transcription factors by phosphorylation of specific tyrosine residues on cytokine receptor subunits. After undergoing JAK-mediated phosphorylation , STAT transcription factors translocate from receptor docking sites at the membrane to the nucleus, where they initiate the transcription of specific genes. 32

33. Cytokine receptors initiate signaling 33

34. Cytokine receptors initiate signaling In addition to IFN- γ , a number of other class I and class II ligands have been shown to cause dimerization of their receptors. An important element of cytokine specificity derives from the exquisite specificity of the match between cytokine and their receptors. Another aspect of cytokine specificity is that each particular cytokine induces transcription of a specific subset of genes in a given cell type; the resulting gene products then mediate the various effects typical of that cytokine. 34

35. Cytokine receptors initiate signaling Specificity is traceable to three factors Particular cytokine receptors start particular JAK-STAT pathways Transcriptional activity of activated STATs is specific because a particular STAT homodimer / heterodimer will only recognize certain sequence motifs & thus can interact only with the promoters of certain genes. Only those target genes whose expression is permitted by a particular cell type can be activated within that variety of cell 35

36. Cytokine receptors initiate signaling i.e., in any given cell type only subset of the potential target genes of a particular STAT may be permitted expression. For eg ., IL-4 induces one set of genes in T cells, another in B cells and third in eosinophils . IL-1 does not signal via the JAK-STAT pathway but utilizes a kinase designated IL-1 receptor-associated kinase , or IRAK. IRAK proteins also utilized by TLRs for signal transduction 36

37. Cytokine antagonists Number of proteins can inhibit cytokine activity Can bind to receptor, fail to activate the cell, OR Can bind directly to cytokine, inhibiting it Enzymatic cleavage of receptors and release of these can bind cytokines in the blood Marker of chronic T cell activation (transplant rejection, AIDS) Viruses have developed strategies Cytokine homologs Soluble cytokine binding proteins Homologs of cytokine receptors Interference with intracellular signaling Interference with cytokine secretion Induction of cytokine inhibitors in the host cell 37

38. Cytokine antagonists Epstein-Barr virus (EBV) produces an IL-10-like molecule that binds to the IL-10 receptor and like cellular IL-10, suppresses T н 1-type cell-mediated responses which are effective against many intracellular parasites such as viruses. Molecules produced by viruses that mimic cytokines allow the virus to manipulate the immune response in ways that aid the survival of the pathogen. 38

39. Cytokine antagonists EBV also produce an inducer of IL-1 Ra, the host antagonist of IL-1. The pox viruses have been shown to encode a soluble TNF-binding protein and a soluble IL-1 binding protein. Since both TNF and IL-1 exhibit a broad spectrum of activities in the inflammatory response, these soluble cytokine-binding proteins may prohibit or diminish the inflammatory effects of the cytokines, thereby conferring on the virus a selective advantage. 39

40. Cytokine antagonists 40

41. Cytokine secretion by T н 1 and T н 2 subsets CD4⁺ T H cells exert most of helper functions through secreted cytokines, which either act on the cells that produce them in an autocrine fashion or modulate the responses of other cells through paracrine pathways. Although CD8⁺ CTLs also secrete cytokines, their array of cytokines generally is more restricted than that of CD4⁺ T н cells. Two CD4⁺ T н cell subpopulations designated T н 1 and T н 2 can be distinguished in vitro by the cytokines they secrete. Both subsets secrete IL-3 and GM-CSF but differ in the other cytokines they produce. 41

42. Cytokine secretion by T н 1 and T н 2 subsets T H 1 and T н 2 cells are characterized by the following functional differences: T H 1 subset is responsible for many cell-mediated functions, such as delayed-type hypersensitivity & activation of T C cells, & for the production of opsonization -promoting IgG antibodies, that is, Ab that bind to the high-affinity Fc receptors of phagocytes and interact with the complement system. Associated with promotion of excessive inflammation & tissue injury. T H 2 subset stimulates eosinophil activation, and differentiation, provides help to B cells, promotes production of large amounts of IgM , IgE , and non-complement activating IgG isotypes Supports allergic reactions 42

43. Cytokine secretion by T н 1 and T н 2 subsets Differences in the cytokine secreted by T н 1 and T н 2 cells determine the different biological functions of these subsets. A defining cytokine of the T н 1 subset, INF- γ , activates macrophages, stimulating these cells to increase microbicidal activity, up-regulate the level of class II MHC, secrete cytokines such as IL-12, which induces T н cells to differentiate into the T н 1 subset. IFN- γ secretion by T н 1 cells also induces antibody class switching to IgG classes that support phagocytosis and fixation of complement. 43

44. Cytokine secretion by T н 1 and T н 2 subsets TNF- β and IFN- γ are cytokines that mediate inflammation, and it is their secretion that accounts for the association of T н 1 cells with inflammatory phenomena such as delayed hypersensitivity. T н 1 cells produce IL-2 and IFN- γ cytokines that promote the differentiation of fully cytotoxic T с cells from CD8+ precursors. This pattern of cytokine production makes the T н 1 subset particularly suited to respond to viral infections and intracellular pathogens. Finally, IFN- γ inhibits the expansion of the T н 2 population. 44

45. Development of T н 1 and T н 2 subsets determination The cytokine environment in which antigen-primed T н cells differentiate determines the subset that develops. IL-4 is essential for the development of a T н 2 response, and IFN- γ , IL-12, and IL-18 all are important in the physiology of the development of T н 1 cells. T н 1 development is also critically dependent on IFN- γ , which induces a number of changes, including the up-regulation of IL-12 production by macrophages and dendritic cells, and the activation of the IL-12 receptor on activated T cells, which it accomplishes by up-regulating expression of the chain of the IL-12 receptor. 45

46. Development of T н 1 and T н 2 subsets determination IL-18, promotes proliferation and IFN- γ production by both developing and fully differentiated T н 1 cells and by NK cells. So a regulatory network of cytokines positively controls the generation of T н 1 cells. 46

47. Development of T н 1 and T н 2 subsets determination 47

48. Development of T н 1 and T н 2 subsets determination The generation of T н 2 cells depends critically on IL-4. Exposing naive helper cells to IL-4 at the beginning of an immune response causes them to differentiate into T н 2 cells. This influence of IL-4 is predominant in directing T н cells to the T н 2 route. 48

49. Cytokine profiles are cross regulated The critical cytokines produced by T н 1 and T н 2 subsets have two characteristic effects on subset development. promote the growth of the subset that produces them inhibit the development and activity of the opposite subset, an effect known as cross-regulation IFN- γ (secreted by the T н 1 subset) preferentially inhibits proliferation of the T н 2 subset, and IL-4 and IL-10 (secreted by the T н 2 subset) down-regulate secretion of IL-12, one of the critical cytokines for T н 1 differentiation, by both macrophages and dendritic cells. 49

50. Cytokine profiles are cross regulated Cross regulation- when antibody production is high, cell-mediated immunity is low, and vice versa. Two transcription factors, T-Bet and GATA-3, are key elements in determining subset commitment and cross-regulation. The expression of T-Bet drives cells to differentiate into T н 1 cells and suppresses their differentiation along the T н 2 pathway. Expression of GATA-3 does the opposite, promoting the development of naive T cells into T н 2 cells while suppressing their differentiation into T н cells. 50

51. Cytokine profiles are cross regulated 51

52. TH1/TH2 balance determines disease outcomes The progression of some diseases may depend on the balance between the T н 1 and T н 2 subsets. In humans, a well-studied example of this phenomenon is leprosy, which is caused by Mycobacterium leprae , an intracellular pathogen that can survive within the phagosomes of macrophages. In tuberculoid leprosy, a cell-mediated immune response forms granulomas , resulting in the destruction of most of the mycobacteria , so that only a few organisms remain in the tissues. Although skin and peripheral nerves are damaged, tuberculoid leprosy progresses slowly and patients usually survive. 52

53. TH1/TH2 balance determines disease outcomes In lepromatous leprosy, the cell-mediated response is depressed and, instead, humoral antibodies are formed, sometimes resulting in hypergammaglobulinemia . The mycobacteria are widely disseminated in macrophages, often reaching numbers as high as 1010 per gram of tissue. Lepromatous leprosy progresses into disseminated infection of the bone and cartilage with extensive nerve damage. 53

54. Cytokine Related Diseases Defects in the complex regulatory networks governing the expression of cytokines and cytokine receptors have been implicated in a number of diseases. Genetic defects in cytokines, their receptors, or the molecules involved in signal transduction following receptor-cytokine interaction lead to immunodeficiencies such as severe combined immunodeficiency (SCID). Other defects in the cytokine network can cause inability to defend against specific families of pathogens. 54

55. Cytokine Related Diseases For eg ., people with a defective receptor for INF- γ are susceptible to mycobacterial infections that rarely occur in the normal population. In addition to the diseases rooted in the genetic defects in cytokine activity, a number of disease conditions result from overexpression or underexpression of the cytokine or cytokine receptors. Therapies aimed at preventing the potential harm caused by cytokine activity 55

56. 1. Septic shock Bacterial infections remain a major cause of septic shock, which may develop a few hours after infection by certain Gram negative bacteria including E.coli , Klebsiella pneumoniae , Pseudomonas aeruginosa , Enterobacter aerogenes , and Neisseria meningitidis . Symptom- drop of blood pressure, fever, diarrhea and wide spread clotting of blood in various organs. 56

57. Septic shock Bacterial septic shock apparently develops because bacterial cell wall endotoxins bind TLRs on dendritic cellls and macrophages, causing them to overproduce IL-1 and TNF- α to levels that cause septic shock. A common feature of sepsis is an overwhelming production of proinflammatory cytokines such as TNF- α and IL-1 β . The cytokine imbalance often causes very abnormal body temperature and respiratory rate and high white blood cell counts, followed by capillary leakage, tissue injury, and lethal organ failure 57

58. Septic shock The increases in TNF- α and IL-1 occur rapidly in early sepsis, so neutralizing these cytokines is most beneficial early in the process. Appr . 24 hrs following onset of sepsis, the levels of TNF- α and IL-1 fall dramatically, and other factors become more important. Cytokines critical in the later stages may include IL-6, MIF, and IL-8. 58

59. 2. Bacterial toxic shock is caused by superantigens A variety of MO produce toxins that act as superantigens . Superantigens bind simultaneously to class II MHC molecule and to the variable domain V β domain of the T-cell receptor, activating a particular V β domain. Because of their unique binding ability, superantigens can activate large numbers of T cells irrespective of their antigenic specificity. 59

60. Bacterial toxic shock is caused by superantigens Bacterial superantigens have been implicated as the causative agent of several diseases, such as bacterial toxic shock and food poisoning. Included among these bacterial superantigens are several enterotoxins , exfoliating toxins, and toxic shock syndrome toxin (TSST) from S aureus and Mycoplasma arthritidis supernatant (MAS). The large no of T cells activated by these superantigens results in excessive production of cytokines. TSST, for eg , shown to induce extremely high levels of TNF- α & IL-1 60

61. 3.Lymphoid and myeloid cancers Abnormalities in the production of cytokines or their receptors have been associated with some types of cancer. For eg , abnormally high levels of IL-6 are secreted by cardiac myxoma cells, myeloma and plastocystoma cells, and cervical and bladder cancer cells. In myeloma & plastocytoma cells, IL-6 appears to operate in an autocrine manner to stimulate cell proliferation. When Mabs to IL-6 are added to invitro cultures of myeloma cells, their growth is inhibited. 61

62. 4. Chaga’s disease Causative agent- Trypanosoma cruzi , characterised by severe immune suppression Evidence that soluble factor produced by T. cruzi leads to reduction in T cell IL-2 (CD25) receptor 62

63. Cytokine-based Therapies Problems with cytokine therapies: Effective dose levels Short half-life Potent biological response modifiers Can cause unpredictable side effects 63

64. Cytokine-based Therapies 64

65. Cytokines in hematopoiesis Many cytokines have been shown to play essential roles in hematopoiesis . During hematopoiesis , cytokines act as developmental signals that direct commitment of progenitor cells into and through particular lineages. Suitable concentrations of a group of cytokines including IL-3, GM-CSF, IL-1 and IL-6 will cause it to enter differentiation pathways that lead to the generation of monocytes , neutrophils and other leukocytes of the myeloid group 65

66. Cytokines in hematopoiesis The participation f leukocytes in immune response often results in their death and removal. Hematopoetic cytokines that stimulate production of neutrophils (G-CSF), myleoid cells (GM-CSF), platelets (IL-11), and RBCs (erythropoietin) have been used in clinical applications, most often as supportive therapy for patients with immunodeficiency resulting from a genetic defect or from cancer chemotherapy. 66

67. Cytokines in hematopoiesis 67

68. Cytokines in hematopoiesis 68

69. Cytokines in hematopoiesis 69

70. Classification Major cytokines include: Lymphokines Interleukins (IL) Monokines Interferons (IFN) colony stimulating factors (CSF) Tumor Necrosis Factors-Alpha and Beta (TNF) 70

71. Lymphokines Lymphokines  include:  Colony-stimulating factors ( csfs ), including GM-CSF.   Interferons  ( ifns ) – IFN γ . Interleukins IL-1 to IL-8, IL-10, IL-13. Macrophage inflammatory protein-1 beta (mip-1 β) . Neuroleukin ( lymphokine product of lectin -stimulated T cells). Osteoclast -activating factor. Platelet-derived growth factor (PDGF). Transforming growth factor beta ( tgf β) . Tumour necrosis factor-alpha ( cachectin ) (TNF α) . Tumour necrosis factor-beta ( tnf β,  lymphotoxin α,  LT). 71

72. Monokines A  monokine  is a type of cytokine produced primarily by  monocytes  and macrophages. Examples include interleukin 1 and tumor necrosis factor-alpha. Other monokines include alpha and beta interferon, and colony stimulating factors. 72

73. Interleukeins They are secreted regulatory proteins produced by lymphocytes,  monocytes  and various other cell types and are released by cells in response to antigenic and non-antigenic stimuli. Consist of IL1 to IL37. IL-1 activates Antigen Presenting Cells and CD4+ lymphocytes ; affects the differentiation of the B-Cells and T-Cells and other immunocompetent cells and takes part in the regulation of production of other cytokines and GM-CSF (Granulocyte-Macrophage Colony-Stimulating Factor). 73

74. Interleukeins IL-2 stimulates the proliferation and activation of B-Cells and T-Cells . IL-4 plays a role in the differentiation of TH2, in allergic responses, and in the switching of antibody types. IL-3 is a potent activator of hemopoietic cells. It stimulates NK-Cells and acts as a synergist with IL-4 during the induction of CD4+ lymphocyte activation process. IL-5 stimulates the production and maturation of eosinophils during inflammation. IL-7 is known as the growth factor of the immature B-Cells and T-Cells . It induces apoptosis of tumor cells and causes differentiation of cells from a subgroup of acute myeloblastic leukemia. 74

75. Interleukeins IL-8 acts as a chemotactic factor that attracts neutrophils , basophils and T-Cells to sites of inflammation. IL-9 stimulates the excretion of IL-2, IL-4, IL-6, IL-11 , and takes part in a stimulation of cytotoxicity of T-killers and NK-Cells, inducing apoptosis. IL-10 acts to repress secretion of pro-inflammatory cytokines . IL-11 is a pro- inflammative factor , which regulates the functions of B-Cells and T-Cells. It also takes part in the induction of various killer cells activities and acts as an autocrine factor for the proliferation of megacaryocytes . 75

76. Interleukeins IL-12 is a critical linker between the innate immunity and adaptive immunity , capable of TH1 (T Helper Type-1) differentiation and IFN-Gamma release by T-Cells and NK cells. IL-13 is very sensitive to monocytes and B-Cells . IL-13 does not act on T-Cells but inhibits the proliferation of leukemic pro-B-Cells. IL-14 is a BCGF (B-Cell Growth Factor) and the hyper production of this interleukin enables the progression of NHL-B (B-cell Type Non Hodgkin's lymphoma). IL-15 is analogous to IL-2 and increases the anti-tumor activities of T-killers and NK-Cells, and the production of cytokines CD4+ lymphocytes. 76

77. Interleukeins IL-17 is principally produced by CD4+ T-Cells , which induces granulopoiesis via GMCSF. It takes part in the regulation of many cytokines and can reinforce the antibody dependant tumor cell destruction. IL-18 acts as a synergist with IL-12 , especially in the induction of IFN-Gamma production and inhibition of angiogenesis. IL-19 is produced mainly by monocytes and is similar to IL-10 in its function. It is stimulated by GM-CSF and regulates the functions of macrophages, and also suppresses the activities of TH1 and TH2. 77

78. Interleukeins IL-21 executes an important role in the regulation of hematopoiesis and immune response. It promotes a high production of T-Cells, fast growth and maturation of NK-Cells and B-Cells population. IL-22 is produced by activated T-Cells in acute inflammation. It is similar to IL-10 in function, but does not prohibit the production of pro-inflammatory cytokines through monocytes . 78

79. Interferons Based on the type of receptor through which they signal, human interferons have been classified into three major types. Interferon type I : All type I IFNs bind to a specific cell surface receptor complex known as the IFN α receptor (IFNAR) that consists of IFNAR1 and IFNAR2 chains. The type I interferons present in humans are IFN-α, IFN-β and IFN-ω. Interferon type II : Binds to IFNGR that consists of IFNGR1 and IFNGR2 chains. In humans this is IFN-γ. Interferon type III : Signal through a receptor complex consisting of IL10R2 (also called CRF2-4) and IFNLR1 (also called CRF2-12). 79

80. Chemokines Chemokines have been classified into four main subfamilies : CXC Chemokines (contain CXL1 to CXL17) CC Chemokines (contain CCL1 to CCL28) CX3C Chemokines (contain CX3CL1) XC Chemokines (contain XCL1 & XCL2) All of these proteins exert their biological effects by interacting with G protein-linked  transmembrane receptors called  chemokine receptors, that are selectively found on the surfaces of their target cells. 80

81. Colony stimulating factors Colony-stimulating factors (CSFs) are secreted glycoproteins  that bind to receptor proteins on the surfaces of  hemotopoietic stem cells, thereby activating intracellular signaling pathways that can cause the cells to proliferate and differentiate into a specific kind of blood cell. The colony-stimulating factors are soluble, in contrast to other, membrane-bound substances of the hematopoietic microenvironment. They transduce by  paracrine , endocrine, or  autocrine  signaling. Colony-stimulating factors include: CSF1  - Macrophage colony-stimulating factor(MCSF) CSF2  - Granulocyte macrophage colony stimulating factors(GMCSF). CSF3  - Granulocyte colony-stimulating factors(GCSF) 81

82. Tumor necrosis factors Tumor necrosis factors (or the TNF family) refer to a group of cytokines that can cause cell death (apoptosis). Nineteen cytokines have been identified as part of the TNF family on the basis of sequence, functional, and structural similarities. They include: Tumor necrosis factor (TNF), formerly known as TNFα or TNF alpha , is the best-known member of this class. TNF is a  monocyte -derived  cytotoxin that has been implicated in tumor regression, septic shock, and  cachexia . Lymphotoxin -alpha, formerly known as  Tumor necrosis factor-beta (TNF-β) , is a cytokine that is inhibited by interleukin 10. Lymphotoxin -alpha  (LT-alpha) and  lymphotoxin -beta  (LT-beta), two related cytokines produced by lymphocytes that are cytotoxic for a wide range of tumor cells in vitro and in vivo. 82

83. Tumor necrosis factors T cell antigen gp39 (CD40L), a cytokine that seems to be important in B-cell development and activation . CD27L , a cytokine that plays a role in T-cell activation . It induces the proliferation of co-stimulated T cells and enhances the generation of cytolytic T cells. CD30L , a cytokine that induces proliferation of T cells . FASL , a cytokine involved in cell death . 4-1BBL , an inducible T cell surface molecule that contributes to T-cell stimulation . OX40L , a cytokine that co-stimulates T cell proliferation and cytokine production . TNF-related apoptosis inducing ligand ( TRAIL ), a cytokine that induces apoptosis . 83

84. 84