6 T-Lymphocytes And Antigen Recognition

Question 1

Q: Which immune system cells monitor what is happening inside cells?

Answer 1

A: T lymphocytes

Question 2

Q: What main components make the T cell receptor?

Answer 2

A: Has alpha and beta components which have short cytoplasmic tails

A small subset uses gamma and delta chains

There are charged residues in the transmembrane region of the alpha and beta chains that interact with oppositely charged residues in the transmembrane region of CD3 polypeptides

Question 3

Q: What is CD3? Why is it important?

Answer 3

A: CD3 polypeptides are a constant part of the TCR

They are important in the delivery of the signal to the T lymphocyte once the antigen has been recognised.

They have much longer cytoplasmic tails that have motifs containing tyrosine residues

CD3 is important in sending signals to the lymphocyte and it is useful as a marker because it is present on all T lymphocytes

Question 4

Q: What occurs to CD3 when TCR meets its antigen?

Answer 4

A: PHOSPHORYLATION OF TYROSINE in the motifs occur -> This triggers several other chemical cascades

ITAM = Immunoreceptor Tyrosine-based Activation Motif

Question 5

Q: What are the 2 major populations of T cells? What do they do?

Answer 5

A: CD4 - MHC Class II - T helper cells
Secretes cytokines
Recruit effector cells - activate macrophages
Help and activate CTL and B cell responses

CD8 - MHC Class I - Cytotoxic T Lymphocytes
Kill target cells
Induce apoptosis in target cells

Question 6

Q: What are CD4 and CD8?

Answer 6

A: CO-RECEPTORS WHICH BIND TO THE SIDE OF THE MHC MOLECULE and increase the avidity of T cell-target cell interaction and are important in signalling

THEY DO NOT BIND TO THE ANTIGEN PRESENTED BY THE MHC, TCR DOES THAT

Question 7

Q: Do T lymphocytes recognise processed or unprocessed antigens?

Answer 7

A: processed

Question 8

Q: Describe the main 3 T cells subsets- their process of recognition of an antigen.

Answer 8

A: ‘CD8+ T cell’ = CTL = cytotoxic T lymphocyte -> has CD8 co receptor and a TCR = T cell receptor
Viral infected cells have viral proteins which are processed and presented on MHC Class I for recognition by CD8+
TCR binds to the MHC and the CD8 binds to the side of the MHC

‘CD4+ Th1’ = T helper cells 1
Involved in inflammatory responses
Activates the macrophages so they can kill phagocytosed material

‘CD4+ Th2’ = T helper cells 2
Important in helping B cell response
It captures the antigen on the BCR and takes up the antigen
The antigen is processed an fragments are presented on the MHC Class II
Th2 binds and activates the B cell so it starts producing a response

Question 9

Q: Explain what happens to the progenitor cells to form mature T cells- CD4/8+.

Answer 9

A: Progenitor cells move from the bone marrow to the thymus

They begin as immature thymocytes in the cortex

As they develop they move towards the medulla where they become mature thymocytes

When the precursors arrive at the thymus, they have NO TCR, CD4 or CD8 - there are no co-receptors or antigen specific receptors

Rearrangement of the gene segments takes place to form the antigen specific receptor

Then CD4 and CD8 are expressed

There is a selection process to see which kind of MHC molecule the cell can recognise

In relation to the TCR:
Beta chain is rearranged
Alpha chain is rearranged
Proper alpha-beta TCR is formed

If it works thus far - it starts expressing both CD4 and CD8

Final Selection Stage: You see which MHC molecule it recognises and dependent on this it ends up as either CD4+ or CD8+

Question 10

Q: Describe alpha and beta chain rearrangement to form the TCR. How is diversity achieved?

Answer 10

A: germline DNA –recombination–> rearranged DNA -> mRNA -> protein (TCR) = occurs to both alpha and beta and comes together

Similar to how antibody diversity is generated but with different gene segments

Beta chain is rearranged FIRST
Beta chain has VDJ

Alpha chain is rearranged SECOND
Alpha chain has VJ

Diversity is achieved by randomly selecting from the gene segments available

Question 11

Q: What are the 2 checkpoints during T lymphocyte maturation?

Answer 11

A: 1- Pre TCR checkpoint- is the new beta chain functional?
Beta chain rearranges first
If functional it goes on to the next stage
If not, death by apoptosis

2- is the alpha beta TCR functional?/ dangerous?/autoreactive? (against own cells)
Does the TCR recognise self MHC?
If it doesn’t recognise the MHC then it is USELESS
If it binds too tightly to the MHC in the absence of antigen then it is DANGEROUS
If you get something in between it is USEFUL

Question 12

Q: What percentage of thymocytes survive selection?

Answer 12

A: 5

Question 13

Q: Summarise T lymphocytes. What do they recognise? What receptor do they have? Where is the receptor derived? Where are they selected for usefulness? What are the 3 main subsets?

Answer 13

A: Recognise peptides presented on the cell surface by MHC molecules

Have an antigen specific receptor derived by rearrangement of gene segments during development in the thymus.

Are selected for usefulness in the thymus

Three Main Subsets:
CD4+ Th1
CD4+ Th2
CD8+ CTL

Question 14

Q: What is the structure of MHC class I? Draw.

Answer 14

A: brackets below are domains

peptide binding region (alpha 1 and 2)
immunoglobin like region (beta2-microglobin and alpha 3)
transmembrane region
cytoplasmic region

Two Separate Chains: 
Alpha Chain (polymorphic) - Heavy and beta 2-microglobulin (same in everyone) - Light  

The light chain (B2-microglobulin) associates NON-COVALENTLY with the alpha chain

Peptides bind between the alpha-1 and alpha-2 domains

NOTE: B2-microglobulin is NOT TRANSMEMBRANE

Question 15

Q: What family are MHC a part of? Why?

Answer 15

A: Both domains are immunoglobulin like domains so MHC are part of the immunoglobulin superfamily

Question 16

Q: What is the structure of MHC class II? Draw.

Answer 16

A: brackets below are domains

peptide binding region (alpha 1 and beta 1)
immunoglobin like region (alpha 2 and beta 2)
transmembrane region
cytoplasmic region

Similar in structure of MHC Class I

DIFFERENCE: There are two polypeptides of equal size which are BOTH transmembrane

There is an alpha chain and a beta chain which are equal in size

Peptide binds between alpha-1 and beta-1

Question 17

Q: What type of peptide do MHC class I and II present?

Answer 17

A: MHC Class I presents peptides that are 8-10 amino acids long - SHORTER PEPTIDES

MHC Class II presents longer peptides (13+ amino acids) - LONGER PEPTIDES - Ends of the peptide often sticks out of the peptide binding site on the MHC

Question 18

Q: How do peptides bind to MHC so that they can be presented?

Answer 18

A: There are grooves within the MHC molecules where anchor residues from the polypeptide can bind to the MHC via binding pockets

Within the peptide there will be certain positions that are always more or less exactly the same amino acid.

This is known as a BINDING MOTIF - each MHC has a binding motif where certain positions tend to be particular amino acids and the other positions could be anything else.

This means that MHC presents a subset of peptides which have some things in common where characteristics are conserved

Question 19

Q: How do MHC overcome the issue of having low diversity but needing to present a large number of different peptides.

Answer 19

A: They do this by having certain positions on the MHC that are relatively CONSERVED that anchor the peptide into BINDING POCKETS in the MHC.

Question 20

Q: What is the human leukocyte antigen?

Answer 20

A: region on chromosome 6 that codes for the polypeptides that make up the MHC molecules

HLA = human MHC

Three types of Class I = A, B and C
Three types of Class II = DP, DQ and DR

Question 21

Q: In class I MHC, what encodes the heavy chain and what encodes the B2-microglobulin?

Answer 21

A: only the heavy chain that is encoded within HLA - B2-microglobulin is encoded on a different chromosome

Question 22

Q: What does the human MHC contain?

Answer 22

A: doesn’t just contain class I and class II genes, 40% of the genes have immune-related function

Question 23

Q: Describe MHC gene expression.

Answer 23

A: MHC is polygenic - there are several class I and class II loci

It is CODOMINANT - maternal and paternal genes are both expressed

MHC Class I - nearly ALL CELLS - expression varies during infection or by cytokines

MHC Class II - only on PROFESSIONAL APCs - regulated by cytokines

Question 24

Q: Describe MHC polymorphism. What does it result in?

Answer 24

A: Highly polymorphic

MHC Haplotype - group of MHC alleles linked together on a single chromosome

We have two MHC haplotypes because we are diploid - we can have up to SIX different class I and class II molecules  
This variability means that we all have slightly different immune responsiveness to different infections - different susceptibility to different infections 

Some haplotypes are more common than others - they are NOT randomly distributed

We all have different TCR repertoires and different combinations of MHC molecules

Question 25

Q: What is MHC class II linked to?

Answer 25

A: MHC Class II is linked to particular autoimmune diseases

Particular alleles of MHC Class II are linked with susceptibility to autoimmune disease

Question 26

Q: Define exogenous. What presents these type of antigens?

Answer 26

A: captured from outside e.g. via phagocytosis

Exogenous antigens are presented via MHC Class II to CD4 T cells

Question 27

Q: Define endogenous. What presents these type of antigens?

Answer 27

A: synthesised within cells e.g. virus

Endogenous antigens are presented via MHC Class I to CD8 T cells

Question 28

Q: How are antigens presented via MHC Class I?

Answer 28

A: endogenous ones

There are viral proteins in the cytoplasm which are processed by a multisubunit proteolytic system - proteasome

The peptides then move into the ER via TAP (transporter associated with antigen processing)

Newly synthesised MHC Class I moves into the ER (it has a signal sequence) and associates with chaperone proteins which aids its folding

The class I heavy chain binds to the peptide and b2-microglobulin

Once all three are correctly folded, it can go via the golgi to the cell surface for possible recognition by CD8 positive T lymphocytes

Question 29

Q: How are antigens presented via MHC Class II?

Answer 29

A: exogenous ones

Proteins are endocytosed and processed into peptides in endocytic vesicles

The class II molecules get into the ER (as they have signal sequences) -> Once in the ER it associates with INVARIANT CHAIN (invariant because it is always the same - it is not polymorphic)

Once they enter the endocytic pathway, the invariant chain is digested away

You end up with a small peptide fragment derived from the invariant chain bound to the MHC Class II known as the CLIP peptide (Class II Associated Invariant Chain Peptide)

Last Step - swapping the CLIP peptide for the antigenic peptide

The antigenic peptide is then loaded onto MHC Class II and moves to the cell surface for recognition by CD4 T lymphocytes

Question 30

Q: What does the invariant chain do?

Answer 30

A: stops peptides that need to be loaded onto MHC Class I from being loaded onto Class II. It also has a targeting signal sequence so the molecules go to the endocytic pathway via golgi

in relation to how antigens presented via MHC Class II

Question 31

Q: Compare and differentiate between antigen recognition by B and T lymphocytes and by CD4+ and CD8+ T lymphocytes.

Answer 31

A: B- direct recognition of intact, extracellular antigens
T- recognition of processed antigens presented at the surface by MHC molecules (specialised self molecules): class I to CD8+ and class II to CD4+

T cells = PROCESSED antigens