Lecture 11: Integration: Role of Antigen Presenting Cells

Question 1

What is an antigen presenting cell?

Answer 1

a cell that presents antigens

Question 2

Which cells present antigens?

Answer 2

those that express MHC because MHC molecules can only be expressed on the plasma membrane if they are bound to peptide, so “expressing MHC” and “presenting antigen” is the same thing

Question 3

What happens when there are no foreign proteins?

Answer 3

cells are continually presenting their MHC I peptides derived from their own proteins; they are all APC of self but in normal conditions there are no CD8 T cells that can recognise self and nothing happens

Question 4

What happens when foreign proteins are present e.g. virus? (MHC I)

Answer 4

infected cells start presenting viral peptides along with self peptides and some CD8 T cells will be able to recognise MHC molecules bound to the foreign peptides

Question 5

What does an infected cell act as?

Answer 5

acts as “the APC” of the viral antigen and the CD8 T cell kills it

Question 6

What is a non-professional APC?

Answer 6

do not express MHC II and can only “communicate” with CD8 T cells

Question 7

What is a professional APC?

Answer 7

express MHC II (and also MHC I) so they can “communicate” with CD4 T cells
they can capture material from the extracellular medium

Question 8

What are examples of APCs?

Answer 8

macrophages, B cells and dendritic cells

Question 9

Which professional APCs undergo macropinocytosis?

Answer 9

only dendritic cells

macrophages are inducible

Question 10

Which professional APCs undergo phagocytosis?

Answer 10

dendritic cells and macrophages

Question 11

Which professional APCs undergo receptor-mediated endocytosis?

Answer 11

dendritic cells (mannose, Fc, etc), macrophages (mannose, Fc, etc) and B cells (surface Ig only)

Question 12

What do macrophages use MHC II antigen presentation for?

Answer 12

to “ask for permission” from CD4 T cells to kill phagocytosed material and to promote inflammation in tissues

Question 13

What happens when there is no infection and macrophages use MHC II antigen presentation?

Answer 13

material in endosomes (self, whether endogenous or exogenous) is presented but since there are no CD4 T cells that can recognise it, nothing happens

Question 14

What happens when there is infection and macrophages use MHC II antigen presentation?

Answer 14

some CD4 T cells recognise the foreign peptides and deliver signals to the macrophage to kill the phagocytosed material
(e.g. bacterium) by increasing formation of toxic compounds in the phagosome and release pro-inflammatory compounds

Question 15

What do B cells use MHC II antigen presentation for?

Answer 15

to “ask for permission” from CD4 T cells to secrete Ab

Question 16

What happens when there is no infection and B cells use MHC II antigen presentation?

Answer 16

the sIg has nothing to bind to
material in endosomes (self, whether endogenous or exogenous) is presented, but since there are no CD4 T cells that can recognise it, nothing happens

Question 17

What happens when there is infection and B cells use MHC II antigen presentation?

Answer 17

the pathogen is bound by the sIg, internalised and presented

a CD4 T cell recognises the antigen and signals the B cell to secrete Ab

Question 18

What must happen in order for CD8 T cells to kill targets or for CD4 T cells to “give permission” to macrophages or B cells?

Answer 18

they must be “given permission” by dendritic cells

Question 19

Why do we have dendritic cells?

Answer 19

each T cell cannot contact every other cell of the body
immune responses are dangerous and there have to be several layers of control so that a full-blown response only happens after passing several checks
DCs are in charge of the initial and most important check: activation of naive CD4 and CD8 T lymphocytes

Question 20

How do T lymphocytes search for pathogens?

Answer 20

they cannot scan the whole body and instead recirculate between the blood and the secondary lymphoid organs (spleen and lymph nodes)

Question 21

What do DCs provide a “communication link” between? How do they do this?

Answer 21

the periphery and the lymph nodes
they capture pathogens, cells etc in tissues, migrate to secondary lymphoid organs and present the captured material on their MHC molecules

Question 22

What happens in DCs when there is no infection?

Answer 22

present self antigens

nothing happens

Question 23

What happens in DCs when there is infection?

Answer 23

DCs present pathogen antigens

Question 24

Immature DCs that have not seen pathogen are dedicated to…

Answer 24

sample their environment by endocytosis and detect pathogens or tissue damage

Question 25

Activated dendritic cells that have detected pathogens are dedicated to provide information to T cells by…

Answer 25

presenting antigens and informing about type and location of pathogen

Question 26

How do DCs detect pathogens?

Answer 26

express pathogen associated molecular pattern (PAMP) receptors

Question 27

What is cross-presentation?

Answer 27

the ability of dendritic cells to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells (cytotoxic T cells)

Question 28

How do dendritic cells present endocytosed antigens via MHC class I?

Answer 28

they can transfer antigens from endosomes to the cytosol to access the processing machinery normally used for presentation of endogenous proteins i.e. cross-presentation