Dendritic Cells

Question 1

Role of dendritic cells

Answer 1

• Innate immune cells - but play a role in the start of adaptive immunity
• Sample environment - capture antigens and present them to T cells
• Important in vaccines - inject antigens + signals that activate DCs

Question 2

Properties of immature DCs

Answer 2

• Peripheral tissue - non-secondary lymphoid tissue
• Skin and mucosa - pathogens most concentrated
• Very good at antigen capture - very bad at antigen presentation
• Need innate danger signal to mature - and migrate

Question 3

Properties of mature DCs

Answer 3

• Secondary lymphoid tissue
• Come into contact with naive T cells
• Good at antigen presentation and co-stimulation
• Naive T cell requires: antigen to be presented to them AND co-stimulatory molecules
• Proliferate and mount adaptive immune response

Question 4

Capture and uptake of antigens - 3 endocytic pathways

Answer 4

• Macropinocytosis
• Receptor mediated pathway
• Phagocytosis

Question 5

Macropinocytosis - antigen capture

Answer 5

• Non specific - involves uptake of fluid from surroundings
• Formation of large membrane bound vesicles
• Projections from cell surface - called ruffles - encapsulate material - allows DC to capture molecules dissolved/ suspended in the extracellular fluid
• No need for receptors - capture anything in that fluid
• Constantly sampling environment using macropinocytosis
• Hence uptaking antigens from surroundings

Question 6

Receptor mediated pathway - antigen capture

Answer 6

• DCs express an array of receptors - which capture molecules
• Receptors function in receptor mediated endocytosis and phagocytosis

E.g., C-type lectins, Fc receptors, Scavenger receptors, Receptors for heat shock proteins etc

Question 7

Phagocytosis - antigen capture

Answer 7

• Receptor mediated uptake of large particulate species - e.g., bacteria, gram+ cocci, yeast cells, apoptotic bodies etc

Question 8

Direct signals - to trigger DC maturation

Answer 8

• TLRs- DCs have TLRs that recognise an array of microbial molecules - e.g., TLR2 - gram+ bacterial cell wall compounds
• Recognition causes activation
• Molecules recognised by TLRs are only present when there is infection

Question 9

Indirect signals - DC maturation

Answer 9

• Molecule secretion triggered by pathogens or response to tissue damage - e.g., macrophages
• Tumor necrosis factor-alpha
• Interleukin - 1- beta
• Prostaglandin E2

Question 10

Migration of DCs

Answer 10

• DCs migrate from peripheral tissues - to lymph nodes
• They home into sites where T cell concentration is highest - to present antigen to them
• Danger signal - into afferent lymphatic tissue - into lymph nodes (secondary lymphoid tissue)
• Dendrites - projections on cell surface of DC - allows DCs to interact with multiple T cells at once

Question 11

Receptors involved in migration of DCs - (3 receptors)

Answer 11

1. Langerhans cells - interact with keratinocytes - via E-cadherin - expression of E-cadherin is reduced upon maturation
2. Chemokine receptor 7 - (CCR7) - expression increased upon maturation - enables homing to lymph nodes
3. CCR7 - has two ligands - molecules that are recognised by the chemokine receptor
   - First uses: CCL21 - expressed by endothelial cells of lymphatic vessels - from skin to lymphatic vessels
   - Once DC enters lymph node - DC follows gradient of CCL19 - expressed by stroll cells in T cell zone of lymph nodes - where T cells are concentrated

Question 12

What type of cells do DCs present antigens to?

Answer 12

• CD4+ T cells (MHC Class 2)

- CD8+ T cells (MHC Class 1)

Question 13

How is antigen presentation regulated?

Answer 13

• By maturation of DCs

- Enhanced when DCs become mature - much better at antigen presentation

Question 14

Properties of MHC Class 2 molecules

Answer 14

• Synthsized in ER
• Alpha and Beta chain
• Have invariant chain - important as it blocks the MHC class 2 binding site in ER + signals MHC class to be packaged into MIIC (endocytic compartment)

Question 15

Antigen presentation - MHC class 2

Answer 15

1. MHC2 molecules are synthesised in ER and enter the MIIC and the invariant chain is chopped up by proteases - opens up peptide binding groove of MHC2 molecules
2. Antigens have been taken up by cell- are taken into MIIC and chopped up (antigen proteolysis) and then bind to MHC2 molecules
3. These are then displayed on the cell surface to T cells

Question 16

Function of the invariant chain?

Answer 16

It blocks the MHC class 2 binding site in ER + signals MHC class to be packaged into MIIC (endocytic compartment)

• Means MHC2 doesn’t bind to free peptides in the ER that are for MHC1 molecules - and only binds when inside the MIIC

Question 17

How is antigen presentation switched off in immature DCs?

Answer 17

• Low activity of vacuolar proton pump = high pH = low protease activity - so inefficient at chopping up invariant chain
• High levels of cystatin C - prevent cleavage of invariant chain to cathepsin S - (which also helps chop up invariant chain)
• Cell surface MHC2 molecules are rapidly endocytosed

Question 18

How is antigen presentation turned on in mature DCs?

Answer 18

• High activity of vacuolar proton pump = low pH = higher protease activity = more efficient chopping of invariant chain
• Low levels of cystitis C - allows cleavage of cathepsin S = more efficient chopping of invariant chain
• Limited endocytosis of MHC2 molecules from surface of cell
• Increase in synthesis of MHC2 molecules by cell

Question 19

Classical MHC1 pathway?

Answer 19

1. Antigens in cell are processed by proteasome into peptides and translocated to the TAP on ER
2. Peptides bind with folded MHC1 molecules
3. Peptide loaded MHC1 molecules are transported to Golgi and then to the surface

Question 20

Expression of MHC1 molecules in immature/mature DCs

Answer 20

• MHC1 pathway is not switched off in immature DCs
• But there is a higher level of expression in mature DCs
• Because there is constant protease activity in the proteasome of the cell

Question 21

What is cross presentation and why is it important for DCs?

Answer 21

• Is presenting antigens from outside of the cell on MHC1 molecules - exogenous antigens - intersect with MHC1 pathway
• DCs still need to present antigens - adaptive immune response against intracellular pathogens that don’t infect DCs
• E.g., measles don’t infect DCs but DCs still present antigens on MHC1 molecules - very important in antiviral immunity

Question 22

What co-stimulatory molecules are help T cell activation?

Answer 22

• B7.1 and B7.2 - transduced by CD28 (on T cell)
• Mature DCs express B7.1 and B7.2 on cell surface - recognised by CD28
• Combination of antigen + CD4/CD8 and B7.1/B7.2 + CD28 to trigger the activation of T cells

Question 23

What happens after T cell activation?

Answer 23

• Proliferation of effector cells