Cell mediated immunity

Defence against intacellular pathogens

1. Cell-mediated Immunity R. C. Gupta M.D. (Biochemistry) Jaipur, India

2. Our defenses against infection s include innate immunity and acquired immunity Innate immunity acts non-specifically against all pathogens RCG Acquired immunity is specific; one cell or molecule acts against one pathogen

3. A cquired immunity comprises humoral immunity and cell-mediated immunity Humoral immunity acts against pathogens that haven’t entered any self-cell RCG Cell-mediated immunity acts against pathogens that have entered some self-cell

4. L ymphocytes play a key role in humoral as well as cell-mediated immunity L ymphocytes are formed from stem cells in bone marrow

5. RCG T lymphocytes (T cells) are processed in thymus B lymphocytes (B cells) are processed in bone marrow itself L ymphocytes differentiate into two types - B lymphocytes and T lymphocytes

6. The cells responsible for c ell-mediated immunity are T lymphocytes RCG B lymphocytes are responsible for humoral immunity

7. T lymphocytes are of four types: Cytotoxic (Killer) T cells Helper T cells Suppressor T cells Memory T cells RCG

8. Each T cells possesses a number of receptors for a particular antigen The T cell receptor (TCR ) is in many ways similar to an antibody It can recognize a specific antigen RCG

9. Each chain ( a and b) of TCR has: Two extra-cellular domains A trans-membrane region A short cytoplasmic tail TCR is a protein made up of an a chain and a b chain joined by a disulphide bond RCG

10. chain b chain T cell T cell receptor (TCR) a EMB-RCG

11. RCG

12. RCG

13. RCG

14. T cell receptor

15. TCR diversity RCG

16. One of the 70 V segments Thousands of different combinations are possible The gene for the variable region of a chain ( V a ) is constructed from: One of the 61 J segments RCG

17. One of the 52 V segments One of the 2 D segments One of the 13 J segments Thousands of different combinations are possible The gene for the variable region of b chain (V b ) is constructed from: RCG

18. When an a chain combines with a b chain, millions of combinations are possible A given T cell has one combination , and is specific for one particular antigen RCG

19. MHC proteins and MHC genes RCG

20. RCG

21. MHC Proteins

22. MHC genes RCG

23. RCG

24. MHC class II genes encode MHC II proteins MHC II proteins are present on macrophages, B cells and follicular dendritic cells Foreign antigens combined with MHC II proteins are recognized by helper T cells RCG

25. RCG

26. MHC proteins

27. While on their way to the cell membrane, MHC proteins pick up peptide fragments formed by degradation of: The fragments are bound firmly and displayed on the surface of the cell Endogenous proteins Exogenous proteins RCG

28. Endogenous peptides bound to MHC proteins are ignored by the immune system If a T cell finds a foreign peptide bound to an MHC protein, cell-mediated immunity comes into operation RCG

29. MHC class I proteins are made up of a large a subunit and a smaller b subunit ( b 2 -microglobulin) The a subunit has three extra-cellular domains ( a 1 , a 2 and a 3 ), a trans-membrane region and a cytoplasmic tail RCG

30. The a 1 and a 2 domains form the peptide binding cleft

31. MHC I protein – View from above ‒‒‒ Peptide

32. MHC I proteins bind fragments of proteins degraded by cytosolic pathway In cytosolic pathway, the proteins are degraded by 26S proteasome RCG

33. MHC class II proteins are made up of almost equal sized a subunit and b subunit Each subunit has two extracellular domains, a trans-membrane region and a cytoplasmic tail The a 1 and b 1 domains form the peptide binding cleft RCG

34. ‒ Peptide MHC II protein – View from above

35. MHC class II proteins bind fragments of proteins degraded by lysosomal pathway These proteins first enter intracellular vesicles called endosomes The endosomes fuse with lysosomes to form endolysosomes RCG

36. RCG

37. RCG

38. MHC Class II Protein Antigen fragment a 2 b 2 b 1 Cell membrane Newly synthesized MHC II proteins bind peptide fragments, and go to the cell membrane MHC II proteins get inserted in the cell membrane, displaying the bound peptide on the surface of the cell RCG

39. Function of cytotoxic T cells RCG

40. Most viruses and many bacteria reside and replicate in cytosol of the infected cells Their proteins are degraded by 26S proteasome Peptide fragment are displayed on the surface of the cell by MHC class I proteins RCG

41. The foreign peptide bound to a self MHC I protein is recognized by a particular cytotoxic T cell The T cell receptor binds to the peptide:MHC I complex RCG

42. RCG

43. The cytoplasmic tail of CD8 is associated with Lck , a cytosolic tyrosine kinase The cytoplasmic tail of T cell receptor is associated with a trans-membrane protein, CD3 complex RCG

44. CD3 complex consists of a g chain , a d chain, two e chains and two x chains C ytoplasmic portions of these chains have immunoreceptor tyrosine-based activation motifs (ITAMs) RCG

45. Phosphorylated ITAMs act as a docking site for ZAP-70 (zeta associated protein of 70 kD ) On binding of T cell receptor and CD8 to MHC I protein and antigen fragment, Lck becomes active Active Lck phosphorylates the ITAMs of CD3 complex RCG

46. ZAP-70 phosphorylates the tyrosine residues of some target proteins in the cytotoxic T cell This results in release of stored granules from the cytotoxic T cell targeted at the infected cell The granules contain perforin , granzyme and granulysin RCG

47. Action of cytotoxic T cell

48. RCG

49. RCG

50. RCG

51. Function of helper T cells Helper T cells share some common features with cytotoxic T cells But they also differ in some respects RCG

52. RCG

53. RCG

54. CD4: Is made up of a single polypeptide chain Has four domains – D 1 , D 2 , D 3 and D 4 RCG

55. D 3 D 4 D 1 D 2 Four domains in CD4 protein

56. RCG

57. Antigen-presenting cells Dendritic cell Macrophage B lymphocyte

58. RCG

59. RCG

60. RCG

61. Action of helper T cell

62. RCG

63. Actions of lymphokines Continued

64. Continued

65. IFN- g activates macrophages and natural killer ( NK ) cells TNF activates macrophages Some lymphokines attract the phagocytic cells to the site of infection Continued

66. Thus, release of lymphokines leads to: Increase in the number of helper T cells Increase in the number of cytotoxic T cells Increase in the number of B cells and plasma cells Increase in the number of antigen-presenting cells Increase in the number of phagocytic cells Activation of phagocytic cells

67. RCG

68. RCG

69. RCG

70. RCG

71. Also known as regulatory T cells Shut down the immune response after the invading organisms are destroyed Play a regulatory role in immune response Possess TCR and CD8 Function of suppressor T cells RCG

72. Suppressor T cells release some lymphokines after an immune response has achieved its goal Suppressor T cells also prevent immune response against self-molecules This signals all other immune system participants to cease their attack RCG

73. Functions of memory T cells Retain memory of antigen after first exposure Spread throughout the lymphoid tissue Are released quickly into circulation if the same antigen enters again Help in mounting a quick immune response RCG

74. Integrated immune response RCG

75. Self-tolerance RCG

76. RCG

77. Newly-formed lymphocytes are processed during foetal life The purpose of this processing is two-fold: To allow only the immunologically competent lymphocytes to mature To eliminate those lymphocytes that can react with self-antigens

78. Developing T cells express the genes for TCR, CD4 and CD8 Some of the TCRs are incapable of recognizing any self-MHC protein Such T cells would be functionally useless, if allowed to mature Processing of T cells RCG

79. Positive selection RCG

80. RCG

81. Negative selection RCG

82. Receptor editing Clonal deletion Anergy Immunological ignorance Developing B cells recognizing self- antigens can have the following fates: Processing of B cells

83. RCG

84. Multivalent antigen Antigen

85. RCG

86. RCG

87. Antigen of low valency Antigen Epitope Epitope

88. RCG

89. Immunologically privileged sites Brain Eye Testes Foetus

90. Self-antigens present in these areas do not come in contact with immune system The immune system remains ignorant of such self-antigens RCG

91. This results in self-tolerance Thus, potentially self-reacting T cells and B cells are: Deleted or Modified or Incapacitated RCG

92. Self-tolerance is sometimes lost, resulting in self-reactivity When the immune system reacts against a self-antigen, it results in an autoimmune disease RCG Autoimmune diseases

93. Some self-molecules have a structural resemblance with foreign antigens These may be treated as foreign antigens by the immune system Autoimmune diseases can occur because: Some self-molecules combine with microbial proteins to form new molecules These molecules are treated as foreign antigens by the immune system

94. RCG

95. Occurs due to impairment of immune system May be inherited (primary) or acquired (secondary) Immunodeficiency RCG

96. Both Cell-mediated immunity or Humoral immunity or Inherited immunodeficiency may affect : RCG

97. Examples of immunodeficiency diseases Type of immunity Example Humoral Agammaglobulinaemia Cell-mediated DiGeorge syndrome Both humoral and cell- mediated Severe combined immuno-deficiency disease RCG

98. RCG

99. RCG

100. RCG

101. Acquired immunodeficiency RCG

102. HIV and AIDS RCG

103. RCG

104. After infection by a retrovirus: Its genomic RNA enters the infected cell Reverse transcriptase also enters the infected cell RCG

105. RCG

106. Structure of HIV RCG

107. RCG

108. RCG

109. RCG

110. gp120 gp41 p6 Integrase Protease Reverse transcriptase p24 p17 p7 Genomic RNA Lipid bilayer

111. Genome of HIV RCG

112. RCG

113. HIV genes

114. RCG

115. RCG

116. RCG

117. RCG

118. The polyprotein encoded by pol gene is cleaved into: Reverse transcriptase Integrase Protease Reverse transcriptase possesses ribonuclease H activity also RCG

119. HIV infection and immunodeficiency HIV is present in infected persons in: Some blood cells Genital secretions An infected person can transmit the virus to other persons RCG

120. HIV can be transmitted from an infected person to an uninfected person through: Sexual contact (vaginal or anal) Transfusion of infected blood Sharing of contaminated needles Mother to foetus transmission in pregnancy RCG

121. RCG

122. HIV and helper T cell gp 120 of HIV binds avidly to CD4 of helper T cells RCG

123. RCG

124. RCG

125. RCG

126. RCG

127. RCG

128. The new virus infects another uninfected helper T cell This cycle is repeated New virus particles are formed and released More and more helper T cells are infected RCG

129. Multiplication of HIV inside helper T cells leads to rupture of cells

130. Viral proteins are cleaved inside the infected helper T cells Viral peptides are picked up by MHC I proteins of the infected cells They are displayed on the surface of the cell Helper T cells displaying viral peptides are destroyed by killer T cells RCG

131. RCG

132. Thank you