Lecture 11: Generation of diversity Flashcards Preview

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1
Q

What is the estimated number of antibody molecules that an individual is born with?

A

10^11

Antibody repertoire

More base pairs than in entire genome, how is this so? - lecture explains

2
Q

What is clonal selection theory?

A

Each and every b lymphocyte produced from bone marrow has only one antibody specificity

3
Q

What does the clonal selection theory say about antigen generation?

A
  • antibody repertoire is generated randomly before birth with no antigen knowledge.
  • specifications are selected by antigenic stimulation
  • activated clones expand and produce antibody producing plasma cells or memory cells
4
Q

What else may the clonal selection theory apply to?

A

T cells with at TcR.

5
Q

Describe clonal selection theory

A

Naive B cells

  • clonal expansion
    • affinity maturation
  • plasma cell (AB) or memory cell
6
Q

In broad terms describe B cell origins

A

Immunoglobulin gene locus

Arise in bone marrow and mature in the spleen

7
Q

Describe in broad terms T cell development

A

T cell receptor gene locus

Arise in bone marrow and mature in the thymus

8
Q

What are the seven key elements in adaptive immunity and memory?

(In B cell development)

A
  1. Repertoire Production
  2. Antigen stimulation
  3. Clonal expansion
  4. Somatic hyper mutation
  5. Affinity maturation
  6. Class switching and antibody production
  7. Memory
9
Q

Describe what is meant be repertoire production in B cell development?

A

Many B cells with randomly arranged Ig gene locus are produced in the bone marrow.

This is random (stochastic)

IgM is the default B cell surface antigen receptor

10
Q

Describe what is meant by antigen stimulation in B cell development;

A

Small number of B cells in lymphoid tissue encounter antigen.

Naive B cells have weak affinity toward antigen, respond and proliferate (antigen presentation is essential for this step)

Each has a unique antigen specificity

11
Q

Describe what is meant by clonal expansion in B cell development;

A

Proliferating B cells expand rapidly and form a germinal centre in a follicle.

Antigen drives this activation, expansion and later maturation

12
Q

Describe what is meant by somatic hyper mutation in B cell development;

A

During B cell development, errors occur called somatic hypermaturation, some of these will improve affinity toward and antigen.

13
Q

Describe what is meant by B cell maturation in B cell development;

A

Higher affinity B cell clones expand at a greater rate because they require less antigenic stimulation. Thus overtaking their slower growing predecessors

I.e morehigher affinity B cells are produced

14
Q

Describe what is meant by class switching and antibody production in B cell development

A

Highest affinity clones undergo a change which induces class switching from default IgM to IgG.

Majority because plasma cells producing massive amounts of IgG

A small number of IgM become memory cells. Essential for adaptive immunity

15
Q

What happens to the Ig gene loci during B cell development?

A

Undergoes somatic recombination ( same as TcR)

16
Q

Describe the Ig gene loci

A

It is segmented into germline V, D and J exons

17
Q

What happens to the germ line segments (V,D,J) during B cell development?

A

Germ line segments in the heavy and light chain loci rearrange to produce an entirely new gene.

This is completely random.

Recombination events are not inherited.

18
Q

What cells does recombination occur in!

A

Only t and B cells because of their recombinant enzymes (RAG1 & RAG2)

19
Q

What is recombination controlled by?

A

The order and process of rearrangement is strictly controlled by two important rules

20
Q

Describe the hypervariable regions and the germ line structures;

A

Ig and TcR have three hypervariable regions;

CDR/HV1 = v segment
CDR/HV2 = v segment
CDR/HV3 = VDJ segment ( junction diversity)
21
Q

What are five mechanisms that generate diversity?

A
  1. Random H-L pairing
  2. Segmentation of H and L gene locus
  3. Somatic recombination
  4. N region diversity
  5. Somatic hyper mutation
22
Q

What does random HL pairing cause?

A

Random H L chain pairing provides a degree of diversity (of antibodies)

23
Q

How does H and L loci segmentation create AB diversity?

A

Gene duplication of individual V,d,j,c segments creates a large repertoire of different segments.

24
Q

How does somatic recombination contribute to antigen diversity?

A

Somatic recombination of VJDC segments to create new coding genes.

D joins V

V joints D

J joints C

25
Q

Describe what n region diversity is and how it contributes to antigen diversity

A

N region diversity results from the imprecise joint between V,D,J germ line gene segments when they combine.

Thus allowing greater diversity as the possible alterations are limitless

26
Q

What creates the junction all diversity of the n region?

A
  • d segment can be read in all three reading frames and sometimes in reverse.
  • imprecise joining leaves p nucleotides that would otherwise be removed
  • n nucleotides are added or removed to the ends of D or V segments (TgT)
27
Q

Describe how somatic hypermutation contributes to antibody diversity

A

Continued mutations at hot spots within CD-R regions of Ig gene during clonal expansion.

Key for affinity maturation

28
Q

What gene does somatic hypermaturation not occur in?

A

TcR genes

29
Q

Describe the temporal process of Ig heavy chain rearrangement (look at book page 88)

A

In the genome, somatic recombination occurs twice. Firstly it reorganises genome so that d and j segments join. Then this occurs again that d and v join.

VDJ (now one segment, no other random DNA between them) joins the c region (constant) via mRNA splicing not recombination.

C(m) region is typical at this stage. During affinity maturation isotope switching can occur and c regions can be switched.

30
Q

Describe the temporal process of Ig light chain rearrangement

A

No D segments!

Somatic rearrangement v and j.

mRNA splicing joins VJ and C(kappa) (

31
Q

What is essential for recombination?

A

RAG 1 & 2. These are recombination genes and are tightly regulated to ensure that one cell produces one antibody

32
Q

Describe the RAG origin.

A

RAg 1 & 2 originally transposing inter grated with PRR gene,

Transposable elements genes relocated to different part of genome but left recognition sequence behind, (Rs) could operate in trans

Trans recombination events - gene duplication and segmentation

33
Q

What recognises the RS sequence left behind?

A

RAG 1 and 2

34
Q

What is the key to recombination events?

A

The preserved signal sequence at the flank

3’ end of all v segments

5’ and 3’ of d segments

5’ end of j segments

With 12 or 23 base pair intervening sequence (2 or 1 helix turns)

35
Q

What is recombination regulated by?

A

12/23 base pair joining rule.

I.e 12/12 and 23/23 are not allowed.

36
Q

Considering the 12/23 rule, what happens in segment binding?

A
  1. D regions can be joined in both orientations (by j) as 12bp either end, j is 23
  2. H chains only allow v-d or d-j. But no v-j as j and v both 23 bp for RS
  3. L chains allow v j binding as no d segment. J(l) unlike j(h) has 12 bo at 5 end instead
37
Q

What are the three joining mechanisms of recombination?

A
  1. Loop out deletion(I,e v-3’ joins 5’-D and intervening DNA is excised)
  2. Inversion. V and j brought into parallel and same result, but intervening DNA ends up 5’ end of bind, not excised
  3. Sister chromatid exchange of loci
38
Q

What Ig class has four sub classes?

A

IgG

39
Q

Describe how the initial splicing of mRNA creates and IgM molecule

A

VDJ is spliced (joined) to c(m) region segment. (Default) though IgD can occur too

40
Q

How does c class switching occur?

A

Following antigenic stimulation signals are received by the B cell which initiates c region class switching.

Exact mechanism of how VDJ is spliced or moved to C(g) is unknown but CD40 is required…

41
Q

What is allelic exclusion?

A

It’s believed that one allele is methylated preventing its rearrangement.

This occurs if Ig (heavy) and TcR beta. (Not IgL or TcR a)

Thus preventing two IgG molecules.

42
Q

How does affinity maturation occur?

A

Results from somatic hyper mutation of h and l chains post B cell selection.

Mutations in variable regions

As antigen depletes in germinal centre, higher affinity B cells are selected (boosting)

A very high affinity antibody is obtained eventually and this is the memory cell.