Block A - lecture 1

Question 1

First stage of t cell activation ?

Answer 1

Presentation and subsequent recognition of specific antigen (from pathogen) by T cell.

Question 2

2nd stage ?

Answer 2

Activation/ early phase initiation of signal transduction cascade happens in the membrane of T cell

Question 3

3rd stage

Answer 3

Activation/ intermediate phase - initiation of signal transduction cascade into the interior of cell cytosol.

Question 4

4th stage ?

Answer 4

activation/ late phase initiation of gene transcription/nuclear events a signal needs to pass from cell membrane to the nucleus

Question 5

5th stage ?

Answer 5

functional responses: effector-cell and end-actions, consequence of T cell activation.

Question 6

what is a native protein?

Answer 6

a protein that is in it’s altered state with no alteration , in it’s original form.

Question 7

what form is the first immunisation in ?

Answer 7

could be a vaccine

Question 8

what occurs in the 2nd response ?

Answer 8

a response is analysed , this is when antibodies are produced in response to the native protein.

Question 9

what is representative to the delayed hypersensitivity response and how is it seen ?

Answer 9

measured by immunising the protein into the limbs of the animal and an inflammatory lump should occur, these are representative of an antibody response. This shows the B cells are activated.

Question 10

when does a change occur in the response ?

Answer 10

native protein is given in the 1st immunisation and in the 2nd immunisation the protein is denatured (the 3D structure is altered as the hydrogen bonding is altered by heating the protein up).

Question 11

what is the change of response ?

Answer 11

no antibody production by the B cells as they couldn’t recognise the protein, they require a non-denatured protein and it needs the 3D structure to be intact. The DTH response is normal and this shows that T cells can still recognise the protein in it’s denatured form. The T cells recognise the protein sequence and not the 3D structure, while B cells recognise the 3D structure and not the protein sequence.

Question 12

when analysing the APC , how are the macrophages fixed ?

Answer 12

fixed sometimes by gamma radiation or formaldehyde (this metabolically kills the cell so it can’t carry out functions but the cells components such as protein and it’s structures are left intact for cytology).

Question 13

when the antigen is added into the fixed cell , they are then incubated with the T cell , what do you look for to occur and what does occur?

Answer 13

you look for T cell proliferation, if division is observed this is by T cells as the macrophage is metabolically inactive. However , there is no T cell proliferation that occurs. An antigen processing cell cannot present if it is just the antigen present

Question 14

In the second experiment , you don’t fix the cells so they are metabolically active , you introduce the antigen and incubation occurs. Why is this done ?

Answer 14

This allows the macrophage time to process the antigen

Question 15

then the T cells are added to the fixed cell , what occurs and what does this show ?

Answer 15

add T cells , T cells will proliferate. This demonstrates that the cells need to process the antigen before proliferation.

Question 16

In the 3rd experiment , the fixed cells can be pre digested with antigen ( peptide) from a pathogen. What occurs after incubation ?

Answer 16

presentation will occur, This again shows the antigen needs to be processed into smaller peptide fragments, as these are as the peptides fit into the MHC molecules on the surface and proliferation occur.presented to the T cells.

Question 17

what are the 2 main pathways for antigen processing pathways ?

Answer 17

cytosolic and endocytic

Question 18

explain cytosolic pathway ?

Answer 18

peptides for loading onto MHC class I and used to process endogenous proteins that are within the cell to begin with. Eventually ends up activating CD8+ T cells ( cytotoxic cells that kill other cells that are our own cells , not a pathogen such as bacteria or a virus). Practically important in viral immunity.

Question 19

explain endocytic pathway ?

Answer 19

peptides for loading onto MHC class II and used to process exogenous proteins which are outside the cell that are deliberately taken into a cell (phagocytosis). They present peptides to CD4+ T cells which are helper cells which can produce signals such as cytokines that activate other cells , they can recruit cells involved in an immune response.

Question 20

explain MHC class I pathway ?

Answer 20

there is an antigen protein ( ribbon) and the normal pathway is to go through the proteosome which converts an untagged protein into peptide fragments. This allows these fragments to be taken into the endoplasmic reticulum which uses ATP an active transport molecule , this takes the peptide fragments from the proteosome and loads then onto the pre formed MHC class one molecules. The endoplasmic reticulum then travels to the Golgi and the MHC class I molecules with the peptides are transported to the surface of the cell. The cell then becomes an antigen presenting cell. This is the cytosolic pathway.

Question 21

what occurs to T cells that can recognise own cells through the peptides ?

Answer 21

they are deleted at birth through apoptosis

Question 22

explain the MHC class II pathway ?

Answer 22

The process brings in dangerous pathogens by phagocytosis to form a vesicle called a phagosome. This becomes fused with the lysosomes found in the cells which are professional cells such as macrophages. This fusion allows the lytic enzymes and proteases to breakdown the pathogen into peptide fragments. The endosome then travels and fuses with the Golgi. The Golgi has budded away from the ER and the ER contains MHC class II molecules , the class II don’t have a free binding region and they contain CLIP to prevent loading until in the Golgi. When the Golgi , lysosome and endosome fuse, the MHC class II has the CLIP removed to allow the peptide fragment from the pathogen to be loaded onto the MHC and shown on the surface of the cell. This is endocytic pathway.

Question 23

why does the APC have the ability to present different sequences ?

Answer 23

this increases the chance of matching with a T cell that recognises the sequence.

Question 24

On an APC there is many MHC displayed on the surface , they have the same structure but what about the variability of the peptide fragment ?

Answer 24

the peptide fragment alters.

Question 25

describe the structure of the Class I MHC ?

Answer 25

It contains 2 polypeptide 
 heterodimer chains , one alpha and beta 2 microgobulin . The alpha chain is composed of 3 domains , alpha 1 ,2 ,3. The alpha one domain bends round and interacts with the secondary beta chain. The alpha 3 chain , is that keeps the MHC class I embedded in the membrane as it produces a chain.

Question 26

Are the proteins produced from MHC class I the same or do they alter ?

Answer 26

always the same

Question 27

where does the peptide fragment sit in the MHC class I ?

Answer 27

between the alpha 1 and 2 chain on the MHC class I molecule

Question 28

describe the structure of MHC class II ?

Answer 28

It has 2 chains , alpha and beta. The 2 chains interact and fit together ( not covalently). The alpha and beta domain each have 2 domains. beta 2 and alpha 2 domains both contain tails that keep the MHC class II embedded in the membrane.

Question 29

where is the peptide fragment binding in the MHC class II ?

Answer 29

between the alpha 1 and beta 1 domains.

Question 30

what is the CD8 on and what does it act as ?

Answer 30

on the T cell , it is the receptor for MHC class I domain alpha 3 which is the ligand

Question 31

what is the CD4 on and what does it act on ?

Answer 31

It is on the T cell and it is a receptor which binds to the ligand MHC class II beta 2 domain.

Question 32

In MHC class I , the peptide is held by what and how does it look ?

Answer 32

held in place by the alpha and beta chains , they are completely surrounding the peptide fragment as it is normally smaller in size compared to MHC class II.

Question 33

what does MHC class II look like in terms of holding the peptide fragment ?

Answer 33

the peptide fragment hangs out the side of the chain as the fragment is bigger in size compared to MHC class I fragment.