L10. Humoral and Cell-Mediated Immunity 2

Question 1

Describe the T lymphocyte responses 1) ANTIGEN-INDEPENDENT 2) ANTIGEN-DEPENDENT

Answer 1

1) ANTIGEN-INDEPENDENT (thymus)
>T cell receptor genes undergo rearrangement. After thymic selection, “naïve” T cells expressing T cell receptor and CD4/CD8 are generated.

2) ANTIGEN-DEPENDENT (2ndry lymphoid tissue)
>T cells activated by antigen presenting cells displaying MHCI/MHCII + peptide differentiate into “effector” T helper or T cytotoxic cells. Generate cytokines or directly kill infected host cells.

Question 2

What % of T cells which enter the thymus leave?

Answer 2

Only 5% of T cells that enter the thymus get to leave (shows selection is rigorous)

Question 3

What mediates cell-mediated immunity?

Answer 3

“Effector” T cells (antigen-dependent)

Question 4

Why does production of effector T cells require many signals?

Answer 4

Multiple signals to make sure T cells don’t activate over nothing.

Question 5

What are the 3 signals that “NAÏVE” T Cells recognises in lymphoid tissue and what is the function of each?

Answer 5

1) Recognition of MHC + peptide + co-receptor (CD4/8) → SIGNAL 1, allows T cell to recognise antigen (MHCII + peptide).
>This is the only signal needed once a T cell is already activated.

2) Recognition of CO-STIMULATORY molecule(s) → SIGNAL 2
>Co-stimulatory molecules are additional proteins found on T cell and antigen presenting cell that also have to engage in order for the T cell to become activated.

3) Cytokines convert activated T cells into different subsets → (SIGNAL 3)
>Depends on what cytokines the antigen presenting cell is making
>Relevant to T-helper cells helping them differentiate depending on the needed response (type of infection and signal they receive).

Question 6

What happens if Naïve T cells are exposed to signal 1 before signal 2?

Answer 6

Naïve T cells exposed to Signal 1 in the absence Signal 2 become unresponsive or “tolerised” so wont react react or cause an immune response to that antigen (called T cell anergy).

Question 7

What cells is the co-stimulatory molecule B7 expressed by?

Answer 7

dendritic cells, macrophages, B cells

Question 8

What gene family are co-stimulatory molecules apart of?

Answer 8

Co-stimulatory molecules are a part of the immunoglobulin super family (made of immunoglobulin domains)

Question 9

What is the role of B7?

Answer 9

> Interacts with CD28 (on surface of T cell) inducing expression of IL-2 and IL-2 receptor

> Interaction of B7 with CD28 makes sure cell survives and is not tolerised (example signal 2).

Question 10

What is the role of IL-2 after activation of T cell by B7?

Answer 10

> IL-2 acts in autocrine fashion on CD4+ve T helper cells; also required for CD8+ve cytotoxic T cell activation (CD8+ve cells can produce IL-2 but in a less efficient way, so mainly rely on CD4+ve production)

> IL-2 made by T cell, leaves T cell and binds to the T cell IL-2 receptor stimulating it further of the same T cell or neighbouring CD8+ve cells, this promotes T cell poliferation, survival and differentiation

Question 11

What type of T cell has to be activated first?

Answer 11

CD4+ve cells are needed to be activated before CD8+ve cells, as more robust production of IL-2 after signal 2.

Question 12

What is another name for a Naïve T Cell?

Answer 12

TH0 (H is in subscript)

Question 13

What happens to TH0 (Naïve T Cells) in secondary lymphoid tissue?

Answer 13

In secondary lymphoid tissue gets Signal 3, cytokines (vary due to type of infection e.g. differ depending on receptors activated on antigen presenting cells and cause TH0 to differentiate into 5 main subsets of T Helper cells (all CD4+ve))

Question 14

How does the process of TH0 differentiation ensure that pathogen appropriate immunity is caused?

Answer 14

1. The cytokines produced by antigen presenting cells, but this varies due to which antigen stimulates it (type of infection)
2. As signal 3 is produced due to a specific pathogen, The T cell subsets in turn differ in the types of cytokines they make and their roles in immune responses, making a more fine-tuned immune response against the specific pathogen.

Question 15

What is the main different between the T helper cell types?

Answer 15

The cytokine they make (no marker protein that distinguishes them).

Question 16

What are the 5 types of T helper cell that TH0 (naïve T cell) can differentiate into?

Answer 16

1. TH1
2. TH2
3. TH17
4. TFH
5. TREG

(Remember the T is normal, then the H is subscript or FH and REG, then the number is normal script).

Question 17

What is the function of TH1 cells and what activates them?

Answer 17

1. Activation: TH1 cells are activated by IL-12 and gamma-interferon (signal 3).
2. Function:
   >Produce IL-2 (which acts on T cells via autocrine function and CD8 cells), gamma-interferon, and TNF.
   >Activate macrophages leading to inflammation (cell-mediated immunity).
   Help control intracellular infections and support extracellular defence (IgG1/3), producing a pro inflammatory response.
   >Induce B cells to make opsonizing antibodies (IgG1 and IgG3), complement, and enhance phagocytosis.
   >Crucial for the development of cytotoxic T cells against intracellular infections.

Question 18

What is the most abundant T helper cell in blood?

Answer 18

TH1

Question 19

What type of infection are TH1 cells helpful against and why?

Answer 19

Very important against intracellular pathogens as activate cytotoxic T cells, and also help with extracellular by producing IgG1 and IgG3 for opsonisation.

Question 20

What activates TH2 cells, and what are their functions?

Answer 20

1. Activation: TH2 cells are activated by IL-4 (signal 3).
2. Function:
   >Produce IL-4, IL-5, and IL-13, which activate eosinophils and mast cells.
   >Key players in the defence against helminth infections (large extracellular parasites) and in allergic responses.
   >Il-4 and IL-13 induce B cells to switch from IgM to IgE antibodies, which binds with high affinity to mast cells and when mast cells interact with antigen promotes mast cell degranulation to produce a strong inflammatory response to defend against large extracellular parasites and allergens.

Question 21

What activates TH17 cells, and what are their functions?

Answer 21

1. Activation: TH17 cells are activated by TGF-beta and IL-6 (signal 3).
2. Function:
   >Produce IL-17 and IL-22, which activate epithelial cells at mucosal surfaces and fibroblasts under mucosal surfaces, promoting neutrophils to leave blood and into infected tissues.
   >Important in fungal and extracellular bacterial infections.
   >Role in AUTOIMMUNE disease, like Crohn’s disease

Question 22

What is the role of TFH cells, where are they located, and how do they assist in immune responses?

Answer 22

1. Activation: TFH cells are activated by IL-6and IL-21 (Signal 3).
2. Location:
   >Reside in lymphoid follicles within lymph nodes.
   >The lymph node structure is divided into T cell areas and germinal centres on the outside where B cells proliferate and differentiate.
3. Function:
   >Help naïve B cells differentiate into plasma cells and memory cells. Promote somatic hypermutation (affinity maturation) + class switch.
   >Does this by producing cytokines which switch on AID in B cells promoting affinity maturation and class switching. (via IL-21 and contact between co-stimultaory molecules like CD154 (B cell) and CD40 (T cell) as an example)

Question 23

Why are TFH cells called T follicular helper cells?

Answer 23

As are found in lymphoid follicles inside lymph nodes.

Question 24

Why are TFH cells found in lymphoid follicles?

Answer 24

As require close contact with B cells so must be present near where B cells differentiate (Interactions occur in follicles (yellow part of diagram), most T cells do not enter these follicles as this is where B cells are.)

Question 25

Describe the mechanism of TFH cells activating AID

Answer 25

B cell presents antigen to T cell, co stimulatory molecules such as CD154 (B cell) and CD40 (T Cell) and production of IL-21 all needed to switch on AID.

Question 26

Overall how do T cells get activated and help get rid of specific pathogens from the body (I reckon will be question so learn what each T helper cell does)?

Answer 26

1. Against specific pathogens.
   >TH1 activated by IL-12 and IFN-y from intracellular bacteria or virus infection. Produces IFN-y and IL-2 activating cytotoxic T cells (intra) and producing IgG1 and 3 for opsonization (extra).
   >TH2 activated by IL-4 due to extracellular parasite infection. Produces IL-4,5,13 which activate class switching from IgM to IgE so mast cells can degranulate to defend against parasites.
   >TH17 activated by IL-6,23 and TGF-B due to extracellular bacteria and fungi infection. Produces IL-12 promoting neutrophils to leave blood and into infected tissues.
2. By all pathogens
   >TFH activated by IL-6,21 produced by antigen presenting cells from all infections. Produce IL-21 switching on AID in B cells presenting antigen and will under somatic hypermutation and class switching if necessary.

The top ones usually help with cell mediated responses and the bottom for humoral responses, but is cross over as TH2 causes IgM to switch to IgE to defend against parasites.

Question 27

What does TREG stand for

Answer 27

T regulatory cells (Tregs)

Question 28

Are TREGs still T helper cells?

Answer 28

Yes as are CD4+ve

Question 29

Why are TREGs described as a heterogenous group?

Answer 29

As there are 2 types of TREG

Question 30

What are the 2 types of TREGs, where do they develop, what activates them and what is their role?

Answer 30

1. Natural T REG
   >Some develop in thymus
   >Recognise MHC + non self peptide (normally T cells with this are selected against)
   >Targets dendritic cells, effector T cells
   >Produce IL-10 and TGF-B to downregulate T cell responses (suppression of autoreactive T cells that may have developed) and this is contact dependent.
2. Induced T REG
   >Develop in periphery (2ndary lymphoid tissue especially mucosal lymphoid tissue)
   >Activated by TGF-B (signal 3)
   >Recognise MHC + self-peptide
   >Targets effector T cells
   >Produces cytokines to downregulate cytokine production (downregulate mucosal immunity and inflammatory responses).

Question 31

What are the 2 methods of killing pathogen by cytotoxic T cells?

Answer 31

1. Perforin generates channels in membrane, Proteases (granzymes) from cytotoxic T cell enter target cell via perforin channel
   >Similar but more specific than NK cells method.
2. Fas ligand induces clustering of Fas (“death receptor”) on target cell, also done by NK cells
   >Fas is distributed evenly across membrane of all cells, Cytotoxic T cells have ligand called Fas ligand which binds Fas receptor, when this ligand clusters induces activation of caspase cascade due to clustered Fas receptors causing death by apoptosis pathway. This is because clustering leads to formation of death-inducing signaling complex (DISC). Within this complex, procaspase-8 is cleaved to form active caspase-8 activating the caspase cascade leading to the breakdown of cellular structures and eventual cell death.

Question 32

As well as killing pathogens what is thought to be another role of the Fas pathway?

Answer 32

> Fas pathway may be important in down-regulating immune responses

> CD8+ve cells might kill own immune cells once infection has passed.

Question 33

What are the 3 benefits to killing by cytotoxic T cells?

Answer 33

1. SPECIFIC - only infected cells bearing antigen are killed (less damage to healthy tissue)
2. EFFICIENT - granzymes are pre-formed; a single cytotoxic T cell can kill 100s of infected targets
3. “CLEAN” – enzymes formed during apoptosis degrade viral DNA and destroy non-viral pathogens. Apoptotic cells are taken up by phagocytes

Question 34

As well as killing what else can cytotoxic T cells do?

Answer 34

Can also produce some CYTOKINES e.g. IFN-y and TNF-a

Question 35

What is another word for allergy?

Answer 35

Hypersensitivity

Question 36

What is the most common cause of allergy and what is its name?

Answer 36

ALLERGY (Hypersensitivity) disease following an immune response to innocuous antigen (ALLERGEN). Mostly IgE-mediated a.k.a. Type I Hypersensitivity.

Question 37

Why is allergy increasing in developing countries?

Answer 37

People who move from undeveloped countries to more developed countries have more chance of increased allergies.

Question 38

What the Hygiene hypothesis and what is its limitation?

Answer 38

> Children brought up on farms, or from large families, are less prone to develop allergy. Early, repeated childhood infections may be protective (so immune system develops properly)

> Insufficient exposure to certain types of infection (“dirt”) skews TH1/TH2 balance towards TH2 (so more prone to allergies, as normally more TH1 in blood) and as TH2 causes class switching to IgE causes Type 1 hypersensitivity

> But if this is true, people with more parasitic infections would be at more risk of generating allergies as would have more TH2 in blood and yet there is a negative correlation between helminth infections and allergic disease.

Question 39

What is the “Counter regulation” or “Old friends” hypothesis and what does it epxlain?

Answer 39

> Childhood infection protects against allergy by promoting IL-10 production (Treg ↑, TH1 ↓ and TH2↓)

> Not getting enough T regulatory cells due to insufficient childhood infections, as our immune system evolved along side pathogens so also helps it develop.

> Infection with microbes or larger parasites plays a critical role in driving immunoregulation e.g. promotes formation of Treg, IL-10 that downregulates TH1 and TH2.

> May also explain rise in autoimmune disease (TH1/TH17-driven).

(Need childhood infections to generate sufficient T regulatory cells to regulate TH1, TH2, TH17.)

Question 40

What are a) 95% b) 5% of T cells in our body and what do they express?

Answer 40

a) 95% of T cells express alpha, beta T cell receptors on surface and CD4 or CD8 on surface

B) 5% had gamma, delta chains instead and don’t express CD4 and CD8

Question 41

Where are gamma/delta T cells found and when do they develop?

Answer 41

> Generated earlier in development than αβ T cells

> Often found at mucosal epithelium (in between epithelial cells lining mucosal surfaces) not secondary lymphoid tissue.

Question 42

Do gamma/delta T cells recognise different antigen to alpha/beta T cells?

Answer 42

> Less diverse than alpha/beta, but recognise a broader range of antigens (including lipids, phosphorylated antigens, DAMPs e.g. heat shock proteins) , while alpha/beta recognise a broad range of peptides.

> Do not appear to require processing or presentation by MHCI/II.

Question 43

What are 4 roles of gamma/delta T cells?

Answer 43

1. Can make cytokines
   >e.g. proinflammatory IL-17, IFN-γ, TNF but also cytotoxic.
2. Can act as antigen presenting cells to αβ T cells.
3. Role in various bacterial, viral and parasitic infections (mycobacteria, flu, HIV, malaria) and also in cancer.
   >Number in blood increases rapidly after infection.
   >Not MHC restrictive so may deal with cancer.
4. May “bridge” innate and adaptive immunity (as are independent of MHC)

Question 44

Why is it important to terminate the immune response once the infection is cleared?

Answer 44

Would be wasteful to keep immune response on and would cause damage to the body.

Question 45

What happens when the immune system to terminated and what is left?

Answer 45

> Once an infection is cleared, 99% of activated and effector cells die.

> Left just with memory cells to remember infection.

Question 46

What cells are thought to play a role in down-regulating the immune system?

Answer 46

Tregs, cytotoxic T cells may play a role

Question 47

What is an important molecule that plays a role in inhibiting the immune system, and an example?

Answer 47

Inhibitory “immune checkpoints” are related to molecules expressed on lymphocytes also play a role e.g. CTLA-4 (induced on activated T cells) binds with high avidity to B7 (co-stimulatory molecule).

Question 48

Describe the life cycle of T cell interacting with B7 (co-stimulatory molecule)

Answer 48

Normally engagement of B7 (on naïve T cell) to CD28 would switch on T cell, but T cell has been activated (later stages of immune response) starts to express CTLA-4 instead of CD28, when B7 binds to CTLA-4 it switches off T cell

Question 49

What is another example of inhibitory immune checkpoints, but to do with ITAMs, and how do they work?

Answer 49

> ITAM cause activation when phosphorylated, ITIM when phosphorylated inactivate immune system

> These motifs can be found on cytoplasmic domain of different receptors.

Question 50

Give two examples of where ITIMs work to inactivate the immune system?

Answer 50

1. FcγRIIb – on B lymphocytes
   >When bound to IgE switches off immune system.
2. PD-1 on activated B and T lymphocytes
   >Interacts with PD-ligand, which is widely expressed. Causing downregulation leading to apoptosis of B and T cell (PD = programmed death)

Question 51

What is a common method that cancer evades the immune system?

Answer 51

Cancer cells often subvert immune responses by presenting these types of immune checkpoint molecules (ITIMs- e.g. PD ligand) themselves, so B and T cells which bind will be switched off.

Question 52

What is a method to counter the use of ITIM immune checkpoints by cancer?

Answer 52

Agents that target immune checkpoints may be used in cancer immunotherapy.