1. Cell mediated Immunity
Dr. Iman Hussein Shehata
Professor of Microbiology & Immunology

2. By the end of this lesson the student is expected to :(ILOs)
1- Describe main characters of adaptive immune response.
2- Identify the two mechanisms involved in adaptive immune response
3- Identify the components of cell mediated immune response.
4- Discuss the role of antigen presenting cells in adaptive immunity.
5- List changes occur in thymus during maturation of T lymphocytes.
6- Describe the sequences of T cell activation.
7- List mechanisms involved in down regulation of cell mediated immune response

3. Adaptive immune response
1- Specificity: It has an extraordinary capacity to distinguish between different even closely related microbes and molecules.
2- Ability to remember encountered pathogens. A second infection by a pathogen is met with a more rapid and vigorous immune response

4. Mechanisms of adaptive immune response
A-The cell mediated immune response: Mediated by T lymphocytes. These cells have the ability to recognize infected cells and destroy them B-The humoral immune response: Mediated by B lymphocytes and directed against extracellular bacteria and their toxins.

5. “non-self” or “altered-self” characteristics.
the cell-mediated immune response is prepared to recognize and attack any cell that exhibits
“non-self” or “altered-self” characteristics.

6. intracellular microbes
Cell mediated immunity is important for defense against
- Microbes ingested by phagocytes and live within these cells
- microbes that infect non-phagocytic cells
( e.g. Mycobacterium tuberculosis, Mycobacterium leprae, Brucella, Listeria ,
Viruses (obligate intracellular pathogens)
intracellular fungi and parasites
Cell mediated immunity destroys the intracellular pathogen by killing the host cell harboring it.
intracellular microbes

7. Cell mediated immunity also plays a critical role in
eliminating tumor cells
rejecting cells from allogeneic organ transplants.

8. Cell mediated immune response
Immune cells involved in cell mediated immune response
T Lymphocytes
Antigen presenting cells

9. Antigen presenting cells
Cells carrying Professional antigen presenting cells
MHC class II molecules
*macrophages
*dendritic cells
*B lymphocytes

11. Antigen presenting cells can perform the following:
1-Recognize any microbe via
PAMP receptors
and phagocytose it.

12. Digest the phagocytosed particle into fragments by special enzymes.

13. Carry these fragments on their surface in association with MHC Class II molecules to be recognized by T helper cell (CD4+).

14. Secrete several cytokines as interleukins 1, 6, 8, 18, and TNF α
Secrete several cytokines as interleukins 1, 6, 8, 18, and TNF α. to attract neutrophils and T cells to the site of infection (e.g interleukin 8), and activate T cells (e.g interleukin 1)
Inflammatory response
IL-1, IL-6, IL-8,TNF-α,
IL-18 & other chemokines

15. don’t have any surface receptors
T Lymphocytes
Origin and maturation
arise from stem cells in bone marrow
leave the bone marrow via blood stream in immature form called
thymocytes
don’t have any surface receptors
mature in thymus

16. Outer cortex Inner medulla
Thymus consists of
Outer cortex
Inner medulla
packed with immature T cells
cells pass as they mature

17. Both cortex and medulla are laced with a network of epithelial cells, dendritic cells and macrophages which interact physically with developing thymocytes.

18. During passage through the thymus thymocytes differentiate into T cells carrying the following surface receptors
Either CD4 or CD8 surface molecules
T Cell receptor (TCR)
CD 3

19. As the developing thymocytes begins to express their TCR, they are subjected to two steps education process
*Positive selection process
*Negative selection process

20. Within the thymus, two very important processes called thymic education occur.
Thymocytes bearing antigen receptors that react with self MHC proteins class I & II are selected to survive.
While those that don’t react with self MHC proteins are induced to undergo apoptotic cell death
1- Positive selection

21. Negative selection
T cells with an antigen receptor specific for self-molecules are killed by programmed cell death (apoptosis) in order to achieve tolerance to self antigens, i.e. self tolerance.
Aim:
to remove those cells that would bind to normal self-antigens and cause autoimmunity.

22. T lymphocytes are classified into:
1- CD4 positive cells are called
recognize self- molecule MHC class II.
They make up about 60-65% of circulating T cells
T helper cells

23. 2- CD8 positive cells are termed
recognize self-molecules MHC class I.
They make up about 30-35% of circulating T cells
T cytotoxic cells

24. T Cell receptor (TCR)
Each T cell (CD 4+ or CD 8+) possess T cell receptor specific to one epitope.
T lymphocytes population has receptors with marked diversity allowing the recognition of millions of antigens

25. TCR is composed of two parts:
The polypeptide chains are anchored in the cell membrane and parts of them extend into the cytoplasm
beta polypeptide chain
an alpha polypeptide chain

26. The recognition sites of the antigen extend outside cell membrane and have a terminal variable section complementary to specific antigen fragments.

29. CD 3 is associated with The T Cell receptor forming TCR complex The CD3 proteins are involved in transmitting signals from the outside of the cell to the inside, informing that the antigen receptor is occupied.

30. Mature lymphocytes released from thymus are called.
These lymphocytes have never encountered an antigen
naïve lymphocytes

31. Activation of CD4 T helper lymphocytes (TH):
T helper cell is activated when a T cell receptor recognizes its specific antigen presented by antigen presenting cell on MHC class II molecules.
CD4+ interacts with MHC class II to stabilize the interaction of TCR with its specific antigen

34. Other Co-stimulatory molecules needed for T helper cells activation
1- B7 protein on the antigen presenting cell interacts with CD28 protein on the TH cells.
2- CD40 on APCs with CD40L on activated T cell.
3- Intercellular adhesion molecule-1(ICAM-1) protein on APC or target cell binds to Leukocyte function associated antigen -1(LFA-1) protein on TH cells.
4- IL-1 produced by the macrophage is also necessary for efficient helper T-cell activation

35. Co-stimulatory signals between antigen presenting cell and T helper cell

36. anergy If T-cell receptor interacts with its antigen (epitope)
and the co-stimulatory signal does not occur, a state of unresponsiveness called
occurs.
anergy

37. The naïve cell undergoes the following after stimulation
1- Clonal expansion:
Once activated, the naïve lymphocytes will be stimulated to replicate. Increased number of antigen specific cells will help to cope with the ability of microbes to replicate and expand in numbers.

38. 2- Differentiation into effector cells and memory cells
* Effector cells carry specialized function to mediate rapid elimination of the antigen.
* Memory cells have the ability to rapidly respond to the same antigen if encountered later in life.

39. TH17 TH1 TH2 Effector cells are of 3 major subtypes
which secrete different cytokines.
TH1
TH2
TH17

40. Effector T Cells

41. Effector cells of T helper
are of 2 major subtypes which secrete different cytokines:
TH1 cells
TH2 cells
*secrete cytokines such as IL-4, IL-5, IL-6, IL-10, and IL-13 that signal B cells to divide and make antibodies (Humoral Immune response)
*is involved in helminthic infestations and allergic reactions.
*Secrete cytokines as interleukin (IL)-2 and, interferon (IFN)-γ which promote function of **macrophage,
**cytotoxic T cell
**B lymphocytes

42. Subsets of CD4+ T Cells:
Differentiation of activated CD4+ T cells to TH1 or TH2 depends on the cytokine environment dominant at the site of immune response:
If the environment is dominated by
IL-12 and IFN-γ
it will differentiate into TH1 subset.

43. Activated TH1 cells: secrete cytokines as
interleukin IL-2 and, interferon gamma IFN-γ which promote function of:
Macrophages (activate macrophage to kill intracellular organisms).
B lymphocytes (enhance production of opsonizing antibodies to facilitate phagocytosis by macrophages)
Cytotoxic T cells (CTLs): to kill the infected cell.
TH1 cells to secrete more cytokines

44. TH2 subset
If the environment of the immune response is dominated by *IL-4* the activated CD 4+ cells will differentiate into *TH2* subset
Activated TH2 cells:
secrete cytokines such as IL-4, IL-5, IL-6, IL-10, and IL-13 which is involved mainly in
stimulation of humoral immune responses through B cell activation and differentiation and production of all antibodies especially class switching to IgE .
They also stimulate eosinophils and mast cells.
Stimulation of TH2 cells is involved in helminthic infestations and allergic reactions.

45. proinflammatory cytokines including
TH17
The differentiation of CD4+ T cells to the TH17 subset is stimulated by
proinflammatory cytokines including
IL-6,
IL-1
as well as TGF-β
and IL- 23
A TH17 response is produced in response to some bacteria and fungi.

46. TH17 cells play a major defense mechanism against extracellular bacteria and fungi
and induce inflammatory responses that destroy extracellular bacteria and fungi
and may contribute to several inflammatory diseases.
They stimulate the recruitment of neutrophils to the site of infection
Most of their inflammatory actions are mediated by
IL-17 secreted by TH17 but other cytokines produced by this subset may also contribute.

47. Positive Feedback and Cross Regulation of TH Subsets:
Cytokines produced by each subset positively regulate the subset that produces it and negatively regulate the other subset.
IL-4, IL-10 and IL-13
Interferon- γ
secreted by the expanding TH1 population stimulates dendritic cells or macrophages to produce more **IL-12 **
and inhibits generation of TH2.
secreted by TH2 inhibit macrophages and thus inhibiting generation of TH1cells.

49. Activation of CD8+ T cytotoxic lymphocytes ( Tc or CTLs):
Cells expressing MHC Class I molecules carry any endogenously synthesized proteins as viral encoded proteins in association with MHC class 1 molecule on their surface to be recognized by T cytotoxic cell (CD8+).
All nucleated cells including macrophage and dendritic cells carry MHC class I

50. Interaction of TCR of cytotoxic T cell with antigen is stabilized by interaction of CD8 with MHC class I molecule

51. Other Co-stimulatory molecules needed for activation of Tc cells
1-B7 protein on the APC or target cell must interact with CD28 protein on the Tc cell.
2- CD40 on APCs with CD40L on activated Tc cell.
3- Intercellular adhesion molecule-1(ICAM-1) protein on target cell binds to Leukocyte function associated antigen -1(LFA-1) protein on Tc cells.
4- IL-2 secreted by TH1 cell is needed for T-cytotoxic activation

52. Effector CTLs kill target cells by one of two ways:
1- The perforin pathway
Binding of the CTL to the target cell triggers movement of CTL cytoplasmic granules toward the part of the plasma membrane in contact with the target cell.
These CTL granules fuse with the plasma membrane, releasing molecules called perforin and granzymes into the intercellular space.
Perforins form pores in target cells allowing the granzymes to enter the cell, where they induce programmed cell death (apoptosis)

53. Stimulation of cytotoxic T cell

54. CTL-Killing

55. 2- The CD95 or Fas-FasL pathway:
Activated CTL increases expression of a protein called Fas ligand (FasL; also known as CD95L) on its surface.
FasL can interact with the trans-membrane Fas protein (CD 95) receptor found on the target cell surface causing cell death.

56. Both the perforin and the CD95 pathways stimulate phagocytosis and destruction of the target cell by surrounding macrophages. Thus any infectious agent (e.g., a virus) inside the target cell is also destroyed

59. Stimulation of cytotoxic T cell

60. Memory T cells:
have the ability to respond rapidly and vigorously for many years after the initial exposure to a certain microbe or other foreign material.

61. small amounts of antigen and greater amounts of interleukins
Activation of memory cells (secondary immune response) differs from naïve T cell activation (primary immune response) in the following features:
1- Memory cells are activated by
small amounts of antigen and
require less co-stimulation than do naïve T cell.
2-Activated memory cells produce
greater amounts of interleukins
than naïve T cells when they are first activated

62. Stimulation of T lymphocytes by Superantigens:
Superantigens are
not processed by antigen presenting cells
binds to ß chain of TCR outside the antigen binding groove crosslinking it to class II MHC molecule

63. Binding of antigen by MHC and T cell receptor
Binding of antigen by MHC and T cell receptor. A: the interaction between peptide antigen, MHC, and the T cell receptor .B: the interaction between a superantigen, MHC, and the T cell receptor

64. **IL-2 and IFN-γ** from the T cells IL-1 and TNF-α from macrophages
This interaction allows many different T cells with different antigen specificities to become activated with release of large amounts of
**IL-2 and IFN-γ** from the T cells
IL-1 and TNF-α from macrophages
that may lead to shock.

65. exhausted and refractory to further stimulation “anergic”.
After the proliferation and release of cytokines the affected lymphocytes become
exhausted and refractory to further stimulation “anergic”.

66. Down regulation of cell mediated immune response:
1-Regulatory T cells (T reg):
A subset of CD4+ T cells
suppress immune responses.
5–10% of the CD4-positive cells and are characterized by being CD4+ CD25+.
When they are activated, they secrete
*IL-10
*transforming growth factor ß (TGF-ß).

67. blocks the activation of lymphocytes and phagocytes.
Interleukin 10
down-regulates the expression of TH1 cytokines, expression of MHC class II molecules, and co-stimulatory molecules on macrophages.
TGF-β
blocks the activation of lymphocytes and phagocytes.
N.B. Stimulation of these regulatory cells is antigen specific

68. 2- Cytotoxic T lymphocyte - associated protein-4 (CTLA-4)
After the T cell has been activated either (CD4+ or CD 8+), a different protein called cytotoxic T lymphocyte -associated protein-4 (CTLA-4) appears on the T-cell surface and binds to B7 instead of CD28. The interaction of B7 with CTLA-4 inhibits T-cell activation

69. Summary
Any cell carries MHC class II molecules (e.g. macrophage, dendritic cell and B lymphocytes) can present antigen to T helper cell (CD 4+).
Any nucleated cell carries MHC class I molecule can present antigen to cytotoxic T cell (CD 8+).
T cell precursors migrate to the thymus for their development to acquire the following receptors T cell receptor with marked diversity, CD4 or CD 8 , CD3
The removal of self-reactive cells, a process called negative selection, results in tolerance to our own proteins, i.e., self-tolerance
Positive selection process: In order to be positively-selected, thymocytes will have to interact with self molecules MHC class I and II

70. Summary
Anergy a state of unresponsiveness If the T-cell receptor interacts with its antigen (epitope) and the costimulatory signal does not occur,
Effector function of T helper cells is secreting cytokines which promote function of other cells.
Effector function of CTLs is to kill target cells by one of two ways: the perforin pathway and the CD95 pathway.
Down regulation of activated T cells occur by T regulatory cells and a protein called CTLA-4 (cytotoxic T lymphocyte-associated antigen-4) which appears on the T-cell surface and binds to B7.
Superantigens stimulate T cells with different antigen specificity with release of large amounts of IL-2 and IFN from the T cells and IL-1 and TNF from macrophages that may lead to shock.

71. Thank You