7. Effector T cells

Question 1

Naïve T cells

Answer 1

mature recirculating T cells that have not yet encountered antigen

Question 2

Effector T cells

Answer 2

encountered antigen, proliferated and differentiated into cells that participate in host defence

Question 3

Memory T cells

Answer 3

Encountered antigen, contracted, ready to respond to future infections.

Question 4

Target cells

Answer 4

Cells on which effector T cells act

Question 5

Why do we need cellular response?

Answer 5

Sometimes Antibody is insufficient (humoral immunity)
Some pathogens are intracellular: hidden in cell (TB, malaria, HIV etc)
Organisms evolve to escape antibody recognition: either by changing shape (influenza) or by coating antigen in carbohydrate (HIV) or producing decoy antigens (RSV)

Question 6

How does Cell mediated immunity protect us against intracellular pathogens?

Answer 6

Enables digestion of pathogens/ infected cells: T cells instruct macrophages to digest
Enables better killing: CD4 instruct CD8 cells to respond and kill infected cells

Question 7

Where are dendritic cells located?

Answer 7

In tissues:

surveillance: looking for broadly foreign non self-material

Question 8

What happens when a DC acquires an antigen?

Answer 8

Move to lymph nodes (needs activation by PAMP/ PRR)
In lymph nodes, DC mature and then present antigen on MHC
T cells detect antigen via TCR and mature to effector T cells

Question 9

Describe MHC I peptides

Answer 9

Intracellular

Bind to CD8

Question 10

Describe MHC II peptides

Answer 10

Extracellular

Bind to CD4

Question 11

Why and where do T cells recirculate?

Answer 11

Recirculation increases likelihood of encountering antigen
Recirculate from blood and lymph to lymphoid organs
Enter lymph nodes through specialised areas in post-capillary venules (high-endothelial venules)

Question 12

Describe cell mediated immunity

Answer 12

DC acquires viral antigens and moves to lymph node
Specific T cells move out of circulation and into lymph looking for DC.
Finds specific DC.
T Cell sees MHC peptide complex, causes activation of T Cell (now effector)
Effector T Cell moves to site of infection- sees infected cells as have same MHC peptide complex on cell surface.
Clears infection
Effector pool contracts down to memory pool (most die)

Question 13

What are the 3 signals required from DCs to activate a T cell?

Answer 13

Antigen Recognition: If T cell has not seen antigen for which it is specific for, it will not activate, so DC has to have MHC showing specific antigen
Co-stimulation: Surface-surface interactions, on DC surface, enabling clustering of receptors on T Cell surface
Cytokine Release: soluble interaction, DC release molecules which instruct T Cell to be activated

Question 14

What are the 4 effector functions of CD4+ T Lymphocytes?

Answer 14

Macrophage Activation
B cell Activation
Delayed Type Hypersensitivity
Regulation

Question 15

Which T lymphocyte subset is involved in cell mediated cytotoxicity?

Answer 15

CD8

Question 16

Describe action of CD8+ cells

Answer 16

CD8 Cell scans MHC I checking for self
Virus infected cell has viral proteins- these get ‘chopped up’ and placed in MHC I complex as antigens- flags that it is infected
CD8 T cell recognises MHC I with viral proteins: this activates CD8 cell which kills infected cell

Question 17

How do CD8 cells induce apoptosis in their target cells?

Answer 17

CTL store perforin, granzymes and granulysin in cytotoxic granules
Granules are released after target recognition
Perforin molecules polymerise, form pores that CD8 can inject granzymes to drive apoptosis
Fas ligands on CD8 can induce apoptosis by interacting with Fas on target cell

Question 18

How many cells can CD8 cells kill?

Answer 18

Multiple (unlike neutrophils)

Question 19

How are T helper subsets defined?

Answer 19

By the cytokines they produce and transcription factors they use

Question 20

What are the 5 T helper cell subsets?

Answer 20

Th1
Th2
Th17 
Follicular T helper cells 
Treg

Question 21

What is the role of Th1 cells?

Answer 21

Produce Interferon gamma (cytokine which informs immune system to be antiviral)
Boost intracellular immune response
Communicate with CD8

Question 22

What is the role of Th2 cells?

Answer 22

Produce IL4, IL-5, IL-13 (inform immune response to be anti-multicellular organism)
Boost anti-multicellular organism response
Communicate with Eosinophils

Question 23

What is the role of Follicular helper cells?

Answer 23

Produce IL-21, reside in B cell follicles
Essential for generation of isotype-switched antibodies
Communicate with B Cells

Question 24

What is the role of Th17 cells?

Answer 24

Secrete IL-17 in autoimmune diseases
Communicate with Neutrophils leading to killing of bacteria
Important for control of bacteria

Question 25

What is the role of Treg cells?

Answer 25

Regulate activation or effector functions of other T cells Control response afterwards Natural and induced regulatory T cells Necessary to maintain tolerance to self-antigens

Question 26

Effector function 1 of T helper cells

Answer 26

Inflammatory Th1 effector cells activate macrophages to promote killing of intracellular pathogens

Question 27

Describe the 2nd effector function of T helper cells: Delayed Type Hypersensitivity

Answer 27

Primary role in defence and eradication of intracellular pathogens If source of antigen is not eradicated causes chronic stimulation and granuloma formation If antigen is not a microbe, like in contact hypersensitivity, DTH produces tissue injury without protection

Question 28

What are the 2 phases involved in Delayed Type Hypersensitivity?

Answer 28

Sensitisation: initial exposure to the antigen Effector: delayed response

Question 29

Describe effector function 3 of t helper cells: B cell activation

Answer 29

2 events simultaneously happen DC and B Cells both at site of infection and both take up same pathogen. Both present antigen on MHC II complex T Follicular Helper Cell specific to pathogen on DC recognises and becomes activated Looks for B Cell with same antigen, instructs B Cell to perform effector function (Become Plasma or memory cell)

Question 30

How do memory T cells differ to memory B cells?

Answer 30

T cells don't undergo isotope switching or affinity maturation

Question 31

How do memory T cells differ qualitatively to naive T cells?

Answer 31

Memory T cells have: Less stringent activation (Less licensing) Proliferate faster Express different chemokine receptors

Question 32

Summarise the 4 stages T memory cells undergo

Answer 32

Expose Expand Contract Memory

Question 33

Describe the life of T memory cells

Answer 33

Person becomes infected T cells are exposed and proliferate (antigen specific T cells) Infection is removed Contraction of T cell pool, reserved as T cell memory

Question 34

Where and why are there subsets of T memory cells?

Answer 34

Different types of immune memory cells | Live in site of infection

Question 35

What causes T cell exhaustion?

Answer 35

``` If source of antigen doesn't disappear T cells constantly proliferate Over time (esp. chronic infections) T cells become exhausted and senesce ```

Question 36

T cell exhaustion is very dangerous to the body, but what situations in particular?

Answer 36

Cancer/ HIV where infection is not fully cleared

Question 37

Beneficial actions of effector T cells

Answer 37

Recognition of antigenic peptides and clearance of pathogen

Question 38

Pathological reactions caused by effector T cells

Answer 38

``` Autoimmunity (antigenic peptide derived from self-protein) Transplant rejection (antigenic peptide derived from self-protein of donor) ```

Question 39

When can immune responses have negative outcomes?

Answer 39

When in excess of what is required When in wrong anatomical compartment When targeted against self (autoimmunity) When targeted against benign antigens (allergy)