Lecture 2 Flashcards

1
Q

Specific/adaptive immunity:

A
  • is induced by exposure to a particular infection
  • shows a high degree of specificity
  • exhibits “memory”
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2
Q

Antibodies

A

Number of different possible antibodies in humans:
>1011
< 30,000 (3 x104) genes in human genome
(approx. 170 human H/L chain genes)

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3
Q

How does the number of antibosies work?

A

How can there be 3 x106 more (1011/3 x104) possible BCR/antibodies than there are genes present in the human genome?

A range of mechanisms allows the generation of an enormous antibody repertoire from relatively few (~200) genes
(and similar (but not identical) mechanisms exist for generation of equally large repertoire of different TCRs)
- very fine tuned

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4
Q

Multiple genes encode a single polypeptide chain:

A

• V and C regions of antibody and TCR polypeptide chains are encoded by separate gene segments that rearrange during lymphocyte differentiation
• H chain and TCR:
V region encoded by three gene segments:
V, D and J (V is the biggest)
• L chain and TCR :
V region encoded by two gene segments: V and J

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5
Q

What are the V & C regions of antibody & TCR polypeptide chains are encoded by?

A

Separate gene segments that rearrange during lymphocyte differentiation

Domains are encoded by very small mini genes

There are bits in-between V & C

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6
Q

What are the bits in-between V & C?

A

D = diversity

J = joining

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7
Q

What is the V region of H chains & TCR-beta encoded by?

A

V
D
J

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8
Q

What is the V region of L chains & TCR-alpha encoded by?

A

V

J

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9
Q

What do V, D & J encode?

A

V region of H chains & TCR-beta

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10
Q

What do V & J encode?

A

V region of L chains & TCR-alpha

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11
Q

Rearrangement of Ig genes in B cells

A

Genes rearrange during B cell development to form a functional gene

The genes in B cells are closer than in every other type of cell in the body

DNA is different in B cells than every other cell

Non-homologous end joining (NHEJ) recombination

B cells break their DNA in a way no other cells do (apart from T cells) & move genes so they are closer

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12
Q

How do B cells produce a functional immunoglobulin?

A

Involves recombination to bring gene segments together

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13
Q

Light chain of an antibody

A

After DNA breaks, a single V & a single J gene segment are joined together to encode the V region of the light chain

The 2 exons come together

DNA is broken randomly to bring 2 of each together randomly

Leads to different BCRs in every cell

B cell has to decide to break its DNA at one locus – either lambda or kappa

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14
Q

What happens once V & J are next to each other for the light chain?

A

It gets transcribed into RNA like a normal gene making the light chain – can then fold up to form a unique antigen binding site

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15
Q

Heavy chain of an antibody

A

Similar mechanism but extra sequence section with the D region

2 breaks needed

A single V, D & J gene fragment are joined together to encode the V region of the heavy chain

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16
Q

Order of breakage in the heavy chain

A

Each B cell first breaks between D & J to bring them together

Then is sequentially broken again & the DJ joins the V

Lots of combinations – every V can go with every D & J

17
Q

Hierarchy of rearrangements in light and heavy chains

A
  • greater variability in H chain as V,D and J
  • first H chain genes (D-J then V-D)
  • then light chain genes: κ first (V-J)
  • if κ rearrangement unsuccessful then λ genes rearrange
18
Q

What forms the binding site?

A

Variable regions of H & L chain

19
Q

What chain has greater variability?

A

Greater variability in H chain as V, D & J

20
Q

What loci are the H, kappa & lambda chains encoded?

A

H chain – chromosome 14

Kappa chain – chromosome 2

Lambda chain – chromosome 22

21
Q

Recombination signal sequences (RSS):

A
  • DNA rearrangement is guided by special sequences flanking the V, D and J regions = recombination signal sequences
  • involves a complex of enzymes = V(D)J recombinase
  • (recombination activating gene)
  • RAG-1 and RAG-2 genes encode lymphoid-specific components of the recombinase
  • mutations in RAG genes result in immunodeficiency
22
Q

Which gene segments encode the variable region of the Ig heavy chain?

A

V, D, J regions

23
Q

Which gene segments encode the variable region of the light chain of an antibody?

A

V region and a J region

24
Q

How are the heavy (H) chain of an antibody and the beta (b) chain of the TCR similar?

A

Both have a V region, D region and a J region

25
When does rearrangements of Ig genes occur?
Genes rearrange during B cell development to form a functional gene
26
What are the 2 types of light chain found in antibodies?
Kappa and lambda
27
Allelic exclusion
In a single B cell only one allele of H chain expressed, similarly for L chain light chain isotype exclusion a single B cell expresses either k or l ,never both These mechanisms ensure that an individual B cell produces only one specificity of antibody
28
Mechanisms for generation of antibody diversity:
1) multiple germline genes 2) combinatorial diversity 3) junctional diversity 4) combinations of heavy and light chains 5) somatic hypermutation
29
multiple germline genes
Multiple VH, Vk and Vl | also, multiple D and J (no D for k and l)
30
combinatorial diversity
Different V, D and J segments recombine to produce different sequences e.g. 40V x 25D x 6J would give 6,000 combinations (note, due to D segments, H chains potentially more diverse than L chains)
31
junctional diversity
``` Includes: imprecise joining (small differences in position of V-D and D-J join) ``` N regions (random addition of nucleotides at junctions of V-D and D-J by terminal transferase)
32
combinations of heavy and light chains
e.g. 106 H and 104 L would give 1010 possible antibodies | (Further) mechanisms for generation of antibody diversity
33
somatic hypermutation
mutation frequency in antibody VH genes orders of magnitude higher than normal spontaneous mutation rate occurs in germinal centres as B cells recognize Ag and proliferate/become activated
34
Somatic hypermutation:
* Involves the enzyme, activation-induced deaminase (AID) * AID acts on DNA to de-aminate cytosine to uracil * uracil is then recognized by error-prone DNA repair pathways leading to mutations
35
Membrane (BCR) vs. secreted antibody:
* following Ag recognition as each B cell differentiates, it will start to secrete its unique BCR as an antibody * the secreted form made by each B cell has an alternative constant region that lacks a transmembrane region. As the original re-arranged VDJ regions are not altered, the secreted antibody has the same antigen specificity as the membrane BCR. * the membrane and secreted forms are produced by alternative RNA processing
36
Antibody classes:
``` IgM - Heavy chain μ IgD - Heavy chain δ IgG - Heavy chain γ IgA - Heavy chain α IgE - Heavy chain ε ```
37
Heavy chain constant region genes and class-switching:
* constant region of each heavy chain is encoded by a different C region gene segment (i.e. Cμ, Cδ, Cγ, Cε, Cα) * four g chain genes correspond to the four IgG subclasses: two α chain genes * As the Cμ is physically the closest to the V, D and J genes at the heavy chain locus this is why IgM is first class/isotype BCR/antibody expressed by each developing B cell
38
Summary
* the BCR/antibody polypeptide in each B cell is encoded by multiple genes segments * these rearrange during B cell development (randomly so each B cell will assembly a different BCR) * the random gene segments used will determined antibody specificity * the joining process introduces further variation
39
After B cells have assembled a random BCR
After leaving bone marrow, V region genes may undergo somatic hypermutation in specialized areas of secondary lymph organs following the binding of Ag. and B cell activation this leads to further BCR specificities being generated activated B cells start to secrete their BCR (antibody) class-switch recombination may then occur resulting from the presence of cytokines