CMB2004/L02 Antigen Recognition Systems I Flashcards
How is it possible that the number of possible antibodies is more than the number of genes?
Gene rearrangement
What 3 gene segments encode the H chain and TCRB (V region)?
V, D and J
Which 2 gene segments encode the L chain and TCRa (V region)?
V and J
How are Ig genes found in the genome?
As gene segments
Describe briefly rearrangement of Ig genes.
In B cells, DNA containing Ig gene segments broken and rearranged randomly to form functional Ig genes
What are the 2 loci containing L chain genes?
Lambda and kappa
In which order are the chains made in antibody formation?
Heavy then light
Describe the order of Ig rearrangements during B cell development. (3)
H chain gene segments rearrange (D-J then V-DJ)
Light chain segments rearrange (k segments V-J first)
If k rearrangement is unsuccessful, lambda gene segments rearrange
Which gene segments are involved in light chains?
V, J and C
Which gene segments are involved in heavy chains?
V, D, J and C
At which chromosome is the H chain locus found?
14
At which chromosome is the kappa chain locus found?
2
At which chromosome is the lambda chain locus found?
22
What are recombination signal sequences?
Sequences flanking each of the V, D and J gene segments that guide Ig gene segment rearrangement
What complex of enzymes is involved in gene rearrangement?
V(D)J recombinase
Which 2 genes encode lymphoid-specific components of the recombinase?
RAG-1
RAG-2
(Recombination activating genes)
What is the result of mutations in RAG genes?
Immunodeficiency
How many rearranged H chain genes are expressed in each B cell?
Only one
How many rearranged L chains from one chromosome is expressed by each B cell?
Only one
What does allelic exclusion ensure in B cells?
That each individual B cell produces just one randomly generated BCR/antibody that is different to that of every other B cell
Give 2 mechanisms for generation of antibody diversity.
Multiple gene segments for each chain
Combinatorial diversity
Combinations of heavy and light chains
Junctional diversity
Somatic hypermutation
Explain somatic hypermutation (SHM).
Mutation frequency in antibody (H/L) V genes in orders of magnitude higher than that seen in all other areas of the genome
Occurs in germinal centres (GC) as B cells recognise Ag and proliferate/become activated
Explain the role of enzymes in somatic hypermutation.
Activation-induced deaminase (AID)
Acts on DNA to deaminate cytosine to uracil
Uracil recognised by error-prone DNA repair pathways leading to mutations
How are the membrane and secreted forms of antibodies produced by B cells?
Alternative RNA processing
How does the secreted form of antibody differ from the BCR?
Has an alternative constant region that lacks a transmembrane region
Same antigen specificity (VDJ not altered)
At the heavy chain locus, which segment is closest to the V, D and J gene segments and what does this mean?
C(mew) segment
IgM is the first class expressed by each developing B cell
What segment is next to C(mew) on DNA and what effect can this have?
C(delta)
IgD can be coexpressed with IgM by differential processing of the RNA from the two C region genes
Give the 3 steps to isotype switching.
Looping out
Switch-region recombination
Further rearrangement (potentially)
What is a key difference in class switching between VDJ and B cells?
B cells do not class switch randomly
Where do V region genes undergo SHM?
Specialised areas of secondary lymph organs
