Module 5b + bit of 6

Question 1

What are some features of the LFA-1 and ICAM-1 binding synpase?

Answer 1

LFA-1 and ICAM-1 molecule interaction facilitates T-cell and APC to be close. Stabilises binding of the molecules involved, narrows the space for that binding, reorientation of cytoskeleton & delivery of molecules.

Question 2

What happens to CD8+ T cells when they recognise infected target cells?

Answer 2

Initially bind by LFA-ICAM (non-specific adhesion) and antigen-specific binding. CTL MTOC
makes and release granzymes and perforin into target cell and it undergoes apoptosis. CD8+ T cell detaches and moves onto to kill again.

Question 3

How does Th1 help naive CD8+ T cells become cytotoxic T lymphocytes (CTL)?

Answer 3

Th1 will provide IL-2 to support CD8+ proliferation. Naive CD8+ (CTL-P) needs both antigen on MHC I and help from Th1 for full activation. Memory CTL-P’s can be reactivated with just MHC I and costimulation (B7-CD28) producing their own IL-2

Question 4

How do cytotoxic T cells direct granule release specifically at target cell?

Answer 4

Initial contact by LFA-1 and ICAM-1. Upon antigen recognition, the MTOC reorients towards the target cell. Cytoskeleton and golgi reorient guilding cytotoxic granules to contact site. Granules are released precisely at the synapse.

Question 5

Why is cytokine production tightly regulated?

Answer 5

• They are short lived proteins & mRNA, degrade in minutes to hours.
• Regulated at TFs, RNA stability & protein processing levels.
• Act in high local concentration in microenvironments (lymph nodes).
• Effective at picomolar levels, extremely potent.
• Misregulation can lead to tissue damage due to overactivation.
• Produced as precursors and rapidly secreted when needed.

Question 6

How do cytokines show pleiotropy, redundancy, synergy and antagonism?

Answer 6

Pleiotropy - one cytokine, many effects. Such as IL-4 can go to B-cells, thymocytes and mast cells.
Redundancy - different cytokines cause the same effect. Such as IL-2, Il-4, IL-5 go to B-cells just for proliferation.
Synergy - Cytokines work together IL-4 + IL-5 will class switch IgE.
Antagonism - One cytokine blocks another (IFN-y blocks IL-4-induced IgE switch)

Question 7

What are some cytokines in the Th1 crowd? and their features.

Answer 7

Il-2: stimulates growth for B cells and T cells. Stimulates NK cell growth.
IFN-Y: Inhibits Th2 cell growth, activation of macrophages MHC I and II, activates NK cells.
TNF-B: Inhibits B-cells, kills T-cells induces NO production in macrophages, activates neutrophils

Question 8

What are some cytokines in the Th2 crowd? and their features.

Answer 8

IL-4: activates and growth of B-cells - igG1 and IgE; increased MHC class II induction, T-cell growth, inhibits macrophages activation.
IL-5: IgA synthesis for B cells.
IL-10: Increased MHC class II for B cells, inhibits Th1, inhibits macrophage cytokine release.

Question 9

What are some cytokines in the Th2/Th1 crowd? and their features.

Question 10

How does cytokine receptor signaling via the JAK-STAT pathway lead to gene transcription?

Answer 10

1. Cytokine receptors consist of at least two chains (JAKs). Cytokine binding dimerizes the receptor, bringing together the JAKs which activate each other and phosphorylate the receptor.
2. Transcription factors (STATs) bind to phosphorylated receptors, and in turn are phosphorylated by JAKs.
3. Phosphorylated STATs form dimers and translocate to the nucleus to initiate gene transcription.

Question 11

What is the Th1 and Th2 dichtomy?

Answer 11

Naive Cd4 T cell -> proliferating T cell -> immature effector T cell (Th0) -> Th1 cell which activates macrophages. or Th2 cell which activates B cells to make neutralizing antibody

Question 12

For isotype switching to occur, what cytokine involved and the mechanism?

Answer 12

IL-4, sometimes together with IL-5 produced by Th2. Same B-chain and different a-chains for interactions with different cytokines to bind and induce isotype switching.

Question 13

Autocrine action example?

Answer 13

IL-2 by activated T cells
A CD4⁺ T cell gets activated
It secretes IL-2
IL-2 binds to IL-2 receptors on itself
Promotes its own proliferation
🧠 Think: “I got this — I’m helping myself grow.”

Question 14

Paracrine example?

Answer 14

IFN-γ from Th1 cell to macrophage
A Th1 cell secretes IFN-γ
The cytokine binds to a macrophage nearby
Activates macrophage to increase killing functions
🧠 Think: “Hey bestie, I’m helping you level up.”

Question 15

Cytokine mode of action?

Answer 15

Cytokine receptors consist of two receptors , the cytoplasmic kinases (JAKs). Cytokine binding dimerizes the receptor bringing together the JAKs which activate and phosphorylate each other. STATs come and bind to the JAKs and in turn are phosphorylated by the JAKs. These STATs form dimers and will go translocate to the nucleus and initiate new gene transcription.

Question 16

What are the inducing cytokines of Th1 and Th2?

Answer 16

Th1 - IL-12 and IFNy
Th2 - IL4

Question 17

What are the master transcription factor for Th1 and Th2?

Answer 17

Th1 - T-bet
Th2 - GATA3

Question 18

Do both Th1 and Th2 use the CD40/CD40L second signal?

Answer 18

Yes of course! Th1 secretes IFN-y onto macrophage to activate it. Th2 will secrete IL-4 that participate in activation of this B-cell to produce antibodies.

Question 19

Fas ligand is monomeric and FasL is?

Answer 19

Trimeric

Question 20

Fas ligand induces apoptosis how?

Answer 20

1. Binding of FasL causes trimerization of Fas, which binds death domain-containing adaptor proteins.
2. These adaptor proteins recruit and activate caspase 8, which cleaves caspase 3.
3. This activated caspase 3 cleaves I-CAD, the inhibitor of CAD, which is released into nucleus and cleave DNA.

Question 21

Steps of CTL killing and the two mechansims it can do?

Answer 21

1. CTL binds to virus-infected cell
2. CTL programs target for death inducing DNA fragmentation
3. CTL migrates to new target
4. Target cell death by apoptosis

🔫 Granule exocytosis pathway
➡️ Main and fast-acting method
CTL releases:
Perforin → forms pores in target cell membrane
Granzymes → enter through the pores and trigger apoptosis (via caspase activation)

This is often used for virus-infected cells and tumors

☎️ Fas-FasL (Death receptor pathway)
➡️ Backup or secondary killing mechanism, killing overactivated T-cells

• CTL expresses Fas Ligand (FasL) on its surface
• FasL binds to Fas receptor (CD95) on the target cell
• This activates the caspase cascade inside the target cell → apoptosis
• No granule release needed!

Question 22

Is there cytoplasmic rearrangement in CTL-target cell conjugate?

Answer 22

Yes

Question 23

How do the CTL and target cell form together?

Answer 23

1. CTL finds target cell and interacts in conjugate formation.
2. CTL-target cell now conjugate and cytoplasmic rearrangement sorted in CTL.
3. CTL release granules and exocytosis.
4. dissociation of the CTL and target cell dies

Question 24

What does perforin do?

Answer 24

Polymerizes to form a pore in target membrane

Question 25

What do granzymes do?

Answer 25

They are serine proteases, which activate apoptosis once in the cytoplasm of target cell

Question 26

What signals required for the activation of macrophages?

Answer 26

- IFN-y (directed to target). - CD40L

Question 27

What is the role of CD40/CD40L interaction between a Th1 cell and a dendritic cell in the activation of CD8+ T cells?

Answer 27

1. Dendritic cell presents antigen to naïve CD4+ T cell (CD28/B7), which becomes activated and differentiated into Th1. 2. CD4+ Th1 cell binds to same dendritic cell this time with CD40/CD40L interaction. 3. This CD40/CD40L is the licensing signal basically telling dendritic cell to activate power. DC upregulates B7 (more co-stimulation), 4-1BBL (supports CD8 survival). This APC now licensed to fully activate CD8+ T cells. 4. The licensed DC presents antigen on MHC I to CD8

Question 28

What are the effects of IFN-y?

Answer 28

IFN-y released by Th1 can provide several things for an activated macrophage. - Fusion of lysozomes into phagosomes - production of NO and O2 radicals - B7 molecules, TNF receptors, IL-2 expressed.

Question 29

Activation of B-lymphocytes?

Answer 29

Tfh cells recognise peptide derived from B-cells antigen. The naive B cell and Tfh echange signals which begin the process of B cell activation,

Question 30

How does FcRn protect circulating IgG from degredation & transport it across epithelium?

Answer 30

IgG taken up from blood via fluid-phase endocytosis. In the acidic pH of the endocytic vesicle, IgG binds to FcRn receptors, preventing lysosomal degredation. The FcRn-IgG complex is transported across the cell to the basolateral surface. In the basic extracellular pH, IgG dissociates from FcRn and is released into the tissue.

Question 31

How is the dimeric IgA transported across epithelial cells into mucous secretions?

Answer 31

Dimeric IgA joined by a J chain. It binds to the poly-Ig receptor (pIgR) in basolateral surface of epithelial cells. The IgA-pIGR complex is endocytosed across epithelial cell. At the apical surface, the receptor is cleaved, releasing secretory IgA to a secretory component to protect it from degredation.

Question 32

Which immunoglobin found in plasma?

Answer 32

IgG and IgM

Question 33

Which antibody found in breast milk?

Answer 33

IgA (dimeric). also found in tears, saliva, mucosa etc.

Question 34

Which receptor provides passive immunity to the fetus by crossing placenta?

Answer 34

IgG

Question 35

Where is IgE found and where does it bind to?

Answer 35

Found beneath epithelial surfaces, and binds to mast cells and basophils.

Question 36

What are Fc receptors and their main functions?

Answer 36

Fc receptors bind to the fc (constant) region of immunoglobulins. They are found on the surface of immune cells (macrophages, NK cells, mast cells). The a-chain binds to the Fc portion of the antibody. Fc receptors will do phagocytosis, ADCC, mediator release (histamine from mast cells), enhancement of antigen presentation.

Question 37

What is FcyRI and how does it function?

Answer 37

Has a high affinity receptor for IgG1 and IgG3. Expressed on on myeloid cells. Binds to lower hinge and CH2 domain. IgG3 bound to FcyRI now antigen can bind.

Question 38

How does ADCC work and what cells/receptors are involved?

Answer 38

Antibodies bind to antigens on the surface of target cells. NK cells recognise the Fc region of these bound antibodies via FcyRIII receptors. Crosslinking of FcyRIII triggers NK cell activation. NK cell releases perforin and granzymes, inducing apoptosis in target cell.

Question 39

What is FceRI and where is it found?

Answer 39

It is a high-affinity receptor for IgE. Found on mast cells, basophils and eosinophils. a-chain flowing outside and B chain is crossed along membrane. a-chain binds to Fc portion of IgE.

Question 40

How do mast cells get activated in allergy?

Answer 40

Multivalent allergen must bind to multiple IgE antibodies on a mast cell. This causes cross-linking of FceRI receptors. Triggers degranulation, release of histamine, cytokines and prostaglandins.

Question 41

What triggers the complement system?

Answer 41

A cascade of zymogens part of the innate immune system. Triggered by pattern recognition, pathogen surfaces or dying cells. Casn also be activated by antibody-antigen complexes.

Question 42

Name 3 complement activation pathways

Answer 42

Classical pathway - triggered by antibody-antigen complexes. Lectin pathway - triggered by mannose-lectin binding Alternative pathway - activated directly on pathogen surfaces All converge at C3 convertase formation

Question 43

How does IgM initiate the classical complement pathway?

Answer 43

In circulation, IgM exists in planar form that cant bind complement. Once IgM binds to antigen on pathogen surface, it changes to a staple form expsoing binding sites for C1q. C complex binds to Fc region of surface-bound IgM

Question 44

How does CRP inititae the classical complement pathway?

Answer 44

CRP is acute phase protein. Binds to phosphocholine on pathogen surfaces. Bound CRP acts like IgM by binding to C1q initiating classical pathway. This activates C1 complex (c1q, c1r, c1s) leads to formation of C3 convertase.

Question 45

What happens during amplification phase of complement cascade?

Answer 45

C1 binds to Ab:Ag complexes. Leads to C4b deposition and formation of C3 convertase. C3b is deposited on pathogen surface ->promote opsonisation. C3b can bind factor B forming alternative convertase.

Question 46

What makes up the C1 complex and how does it initiate the classical pathway?

Answer 46

C1q - recognition subunit (binds to Fc region of IgM or IgG, or CRP) C1r or C1s are two serine proteases. C1q binds to Ag;Ab complexes or CRP, binding activates C1q which cleaves and activates C1s. C1 into C3 convertase