28/03

Question 1

Which is the effect of Notch in the development of B and T cells?

Answer 1

• If notch is active will drive the progenitor into the thymus resulting in CD4 and CD8 naïve T cells
• If notch is not active inside the bone marrow, the progenitor will differentiate in B cells

Question 2

How can a T cell exit from the lymphoid tissue

Answer 2

Thanks to the sphingosine I phosphate receptor (S1PR), it recognize sphingosine I phosphate, a lipid abundant into the blood and less in the tissue

Question 3

Which are the secondary lymphoid tissues?

Answer 3

lymph nodes, tonsils, spleen and mucosa associated tissues (lung, gut and skin).

Question 4

What’s the germinal center?

Answer 4

It’s a darker area in the secondary follicle of the lymph node that contains proliferating B cells

Question 5

What’s the ratio between CD4 and CD8 cells?

Answer 5

In physiological condition there are 2 CD4 every single CD8

Question 6

Which receptors can we find on iNKT cells and why?

Answer 6

They have an invariant TCR able to recognize the lipids presented by CD1 molecules.
There is no CD4 and no CD8

Question 7

Which are the most common populations of B cells?

Answer 7

Follicular B cells: differentiate thanks to T cells, can become memory cells

Marginal zone B cell: produce antibodies with a limited diversity, they don’t become memory cells, instead they continue the proliferation

B1 cells: produce constitutive antibodies with limited diversity that go into circulation

Question 8

Describe the structure of a TCR

Answer 8

Heterodimer made of two chain: α and β (more common), sometimes we can have γ and δ.
EXTRACELLULAR PART:
Each chain (α and β) is made of a constant portion (Cα and Cβ), common to all the lymphocytes, and a variable portion (Vα and Vβ) in which we can recognize 3 hyper-variable portions:
- CDR1 and CDR2: complementary determining regions
- CDR3: bind the antigen
Close to the cell membrane there’s the hinge domain that allow the TCR to be more fluid
TRANSMEMBRANE PART
INTRACELLULAR PART: short tail coupled with CD3

Question 9

What’s the role of CD3 in T cells?

Answer 9

CD3 is a huge molecular complex made of different chains containing a globular domain that interact with the TCR and a long intracellular tail that contains ITAM motif.
Those motifs are typical of some receptors in which tyrosine are present in sequence (with some other conserved amino acids beside). These tyrosines can be phosphorylated and become the dock for other messengers that are necessary for the transduction of the signal.

Question 10

List the major events of TCR recombination

Answer 10

1- recombination of beta chain
2- if everything work, recombination of alfa chain
3- assembly of the receptor

Question 11

Describe the structure of a BCR

Answer 11

heterotetrameric structure mad of two identical heavy chain and two identical light chain. Each chain has a variable portion with 3CDRs and a constant portion.
The intracellular tail is small and in association with Igalfa and Igbeta. they contain the ITAM domain and they’re able to start the signal cascade

Question 12

List how the VDJ recombination happens

Answer 12

1- D and J region are combined forming an episome
2- a V region is attached
3- a splicing eliminate all the segment between the variable portion and the constant

Question 13

Which is the structure of the genes in the chains of the BCR?

Answer 13

Heavy chain: variable region V, diversity segment D and joining segment J, constant part
Light chain: V+J+C

Question 14

Which are the specific features of VDJ recombination?

Answer 14

It’s:
- Tissue specific: it occurs only in lymphoid cells (central lymphoid organs)
- Timely regulated: it occurs only when they are immature, so when the T and B precursor have to generate the fully differentiated T and B cells
- Development stage specific: it occurs only in pro- and pre- T and B cells
- Chronologically regulated: the D-J rearrangement (the first event) occurs before that of V-DJ (the second event)
- Cell cycle phase specific: it occurs only during G0-G1 phase so when they are not proliferating
- Characterized by allelic exclusion: the recombination starts on one allele and if it gives a functional chain, it will stop the recombination on the other allele in order to save energy

Question 15

What’s the role of RAG proteins in BCR formation?

Answer 15

They’re endonuclease able to create a complex that put in close proximity the two segment recognizing the RSS (recombination signal sequence), it create an hairpin and cut the dna

Question 16

Which rule follow the VDJ combination and what does it means?

Answer 16

It follow the 12-23 rule: the only possible combination is between a spacer 12 and a spacer 23

Question 17

Which enzyme are necessary for the VDJ recombination?

Answer 17

RAG1 and 2: hairpin formation
Artemis: endonuclease that open the dna ds asymmetrically
TdT: terminal desossinucleotidil tranferase, a polymerase that add random nucleotides to the template

Question 18

Where is the TdT expressed?

Answer 18

In the primary lymphoid organs, expecially by the the thymocytes in the cortical area of the thymus

Question 19

Which cytokines work as survival and proliferation signals? What happens if they’re not felt?

Answer 19

IL2 and IL7, if they’re not felt we have the Xlinked severe combined immunodeficiency disease (SCID)

Question 20

What’s the double positive stage ?

Answer 20

pre-TCR cells become CD3+ and start to express both CD4 and CD8 (CD4+ CD8+ lymphocytes). This happens in the thymus cortex.

Question 21

Which kind of selection happens to T cells?

Answer 21

3 kind of selection:
- death of the neglected: T cells whose TCR is not able to bind the HLA die
- negative selection: CD4+ CD8+ lymphocytes able to bind HLA-p with too much strength need to be eliminated
- positive selection: T cells with a TCR binding Class I HLA-p become single positive CD3+CD8+ cells, T cells with a TCR binding first Class II HLA-p become single positive CD3+CD4+ cell