Lecture 6: B & T Cell Receptors Flashcards

1
Q

How is B and T cell diversity achieved?

A

Through VDJ recombination: the random assembly of key genes that together make up the B or T cell receptor

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2
Q

Why do B cells produce antibodies?

A
  • neutralise toxins and pathogens
  • promote phagocytosis
  • activate compliment
  • mediate immunity through Fc receptors
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3
Q

Why is tight control of B and T cells important?

A

To prevent self-reactivity

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4
Q

What is the B cell process leading to MHC II expression?

A
  • BCR recognises specific peptide sequence in an antigen
  • BCR/antigen complex induces a signalling cascade resulting in B cell activation
  • B cell endocytoses the antigen and processes it into many peptide fragments to be presented on MHC II
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5
Q

How are T cells recruited for B cell help?

A

Help for the B cell can come from any T cell that is specific for any of the presented peptides, where the T cell epitope is physically linked to the B cell epitope

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6
Q

What sort of cells are B cells?

A

Professional APCs

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7
Q

What are CDRs?

A

Complementary determining regions: 3 short segments in the V region with hyper variable amino acid sequences

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8
Q

What are FRs?

A

Framework regions: 4 regions which alternate with CDRs to provide antibody with structure, and are less variable than CDRs

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9
Q

What determines B cell receptor specificity?

A

VDJ genomic rearrangement, where large numbers of VDJ genes allows for massive diversity

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10
Q

How many variable regions are generated per cell, and how does this impact Ig specificity?

A

Only 1 variable region is generated per B cell, so all Igs from a single b cell will have same specificity

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11
Q

What are the large set of variable regions determined by?

A

Junctional diversity and somatic mutations

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12
Q

What is the impact of affinity maturation?

A

Can increase the binding strength of antibodies via somatic mutations in the hyper variable regions, so improved affinity mean better antibodies

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13
Q

Why is class switching used by B cells?

A

Allows them to alter the effector function of antibodies they produce

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14
Q

In mice, what are the 2 light chains?

A

kappa and lambda

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15
Q

In what order are the mice light chains used?

A

Kappa chains are used first as the gene is more complex, so can give rise to higher diversity

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16
Q

Where is the joint between V and J located?

A

CDR3

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17
Q

How does the V and J joint affect intron arrangement in the kappa chain?

A

J genes can be excised depending on where V joins to
eg. If V joins to J5, J1-4 are excised

18
Q

What segments do heavy chains have?

A

V, J, and extra D (diversity) segment

19
Q

What does the D segment do?

A

10-14 segments, which encodes amino acids 95-97 in the CDR3 to further expand the range of epitopes that could be recognised

20
Q

What do MH genes in the heavy chain do?

A

Determine the class of antibody produced

21
Q

What antibodies do naïve B cells produce?

A

IgM and IgM, which can class switch after activation to generate IgG, IgA and IgE

22
Q

In what order do the chains recombine?

A

First: heavy chain > D to J segment, then V to DJ segment
Second: kappa chain
Last: lambda (last resort)

23
Q

How many chances does each chain have to recombine successfully?

A

Heavy chain: once per chromosome, so 2 chances
Light chains: if unsuccessful, rescue attempt can be made if there is a 5’V and 3’J gene left

24
Q

What is an RSS?

A

Recombination signal sequence found at both 5’ and 3’ ends of V, D, and J genes

25
Q

What components do RSSs consist of?

A
  • Heptamer: conserved region of 7 nucleotides
  • Nonamer: conserved region of 9 nucleotides
  • Spacer: less conserved region of 12/23 nucleotides
26
Q

What are RAG genes?

A

Recombination activating genes

27
Q

What is the function of RAG1 and RAG2 genes?

A
  • RAG1 and RAG 2 function together to rearrange V(D)J genes
  • RAG1: binds both RSS and histone H3
  • RAG2: guided by RAG1, is the enzyme with DNA cleavage activity
28
Q

How are RAG1 and RAG2 involved in enzymatic gene rearrangement?

A
  • RAG1 and RAG2 recognise the conserved heptameter/nonamer sequences of the RSS
  • RAG1/2 complex at one site bind to the RAG1/2 complex at a different site, forming a loop of DNA
  • RAG2 proteins cleave the DNA and form hairpin loops
29
Q

What is the process of enzymatic gene rearrangement once RAG1/2 forms hairpin loops?

A
  • DNA repair enzymes Ku70:Ku80 proteins bind the DNA at the ends and stabilise the strand break complexes
  • Signal joints: the signal join is ligated to form a loop, which dilutes as the cell divides
    OR
  • Coding joints: DNA protein kinase and Artemis binds to the closed loops of DNA covering the ends of the 2 sections to be recombined
  • combination of enzymes cleaves the hairpin loops to create raggedy ends
  • TdT adds random nucleotides to create some complementarity between both ends
  • DNA ligase and XRCC4 edit the ends (base excision and repair) to fully bind and ligate the DNA back together
30
Q

What is the purpose of the promoter at the end of every V gene?

A

It allows equal transcription of both genes

31
Q

In what way are enhancers needed?

A

Needed to initiate transcription of the gene, but are only in close proximity after V(D)J recombination

32
Q

What is allelic exclusion?

A

Rearrangement of the second chromosome is suppressed in cases where the first chromosome generates a functional antibody

33
Q

What is the purpose of allelic exclusion?

A

Ensures B and T cells have single specificity, as if the second allele was allowed to recombine then the cells would have dual specificity

34
Q

What processes are used for allelic exclusion

A

Asynchronous DNA replication, different feedback mechanisms, and histone modifications

34
Q

How does asynchronous DNA replication silence recombination?

A

One allele opens upon first, so the second allele has a extra mechanism to keep it closed. This process allows replication of only the gene that is being recombined first.

35
Q

How do feedback mechanisms silence recombination?

A
  • When the first successful rearrangement is created, it will recombine with section of protein which creates a pre-B or T cell receptor.
  • Creates a surrogate light chains that join onto first recombination, which lets new receptor be expressed on the cell surface
  • If it is a functional receptor that has bound properly and has undergone selection, it will send signals into the cell that will halt rearrangement of second gene, so will inhibit the RAG complex so cannot bind or undergo recombination on the second allele
36
Q

How do histone modifications silence recombination?

A

Histone modifications keep the allele more closed and prevent transcription, so silence any further transcription of the second allele and therefore it cannot be opened up, RAG can’t bind and can’t be transcribed

37
Q

What chains do TCRs have?

A
  • an alpha chain, containing V and J sections
  • a beta chain, containing V, D, and J sections
38
Q

Where is the delta locus located?

A

In the middle of the alpha locus

39
Q

Where is the TCR locus located?

A

Within the alpha locus, where the delta locus is within V regions

40
Q
A