L6/7 - Ig Variation and Diversity Flashcards
What is an allotype?
Alleles in lg loci, generally in framework regions that do not impact specificity
What is an idiotype?
Variation in variable regions, up to 10^7 specificities
How is variability different Ab molecules measured?
number of different aa at a given position / frequency of most common at that position
What is somatic diversification?
Rearrangement of lg gene segments
- Rearrangement during B cell maturation (permanently changing DNA)
- Unique genes generate unique idiotype
What is the basic composition of a gene?
- Promoter/enhancer, leader exon, exons/introns, downstream enhancer and a poly A addition signal
What are the differences between types of gene segments between light chain and heavy chain?
Light chain - V J C
Heavy - V D J C
How are λ light chain gene segments organised?
[Vλ1, Vλ2, Vλn]-[Jλ1 Cλ1…Jλ4 Cλ4]
- J1C1, J2C2 - J and C segments alternate
How are κ light chain gene segments organied?
[Vκ1, Vκ2, Vκn]-[Jκ1-5]-[Cκ]
- J segments clustered followed by a single C κ locus
How are heavy chain gene segments organised?
[VH1…Vʜn]-[Dʜ1-25]-[Jʜ1-6]-[Cμ]
- Similar to κ in that each section is segmented
Is arrangement of V (D) and J segments completely random?
- For the most part random, but not completely. V gene segments form rosettes bringing differently V segments to J segments with different proximity skewing randomness
What happens to the gene segments in between the rearranged regions?
Permanently lost. Things upstream and downstream aren’t but aren’t transcribed either
Why does CDR3 have the highest variability?
Normal variability of CDR1 and CDR2, but also coincides with the V-J junction adding to increased variability
How does heavy chain rearrangement differ from light chain?
D - J rearrangement occurs first, followed by V - DJ rearrangement
What corresponds to the increased variability of CDR3 in the heavy chain?
- Greater than light chain
- Coincides with V-D junction as well as D-J junction
What is the recombination signal sequence (RSS)?
Recombination signal sequence found upstream or downstream of various segments.
Heptamer-12/23-Nonamer
What spacer flanks the Vʜ segment?
23
What spacer flanks the Dʜ segment?
12
What spacer flanks the Jʜ segment?
23
How does the κ light chain differ from the λ with respect to the RSS and how do they compare to the heavy chain?
λ chain is similar to heavy chain in that 23 nucleotide spacer flanks the V region whereas it is inversed in the κ light chain
- 12 flanks Vκ, 23 flanks Jκ
What is the 12/23 rule?
For gene segments to recombine, a spacer length of 12 matches with a spacer length of 23
What is the role of Tdt?
Terminal deoxynucleotide transferase
- Inserts random non-templated encoded nucleotides between the junctions following RAG
What is the order of switch regions and isotype genes downstream of the VDJ cassette?
—[[Sμ]]–Cμ(IgM)–C𝛿(IgD)–[[Sγ]]–Cγ(IgG)–[[Sε]]–Cε(IgE)—
Is IgD/IgM selection modulated by DNA or RNA
RNA. Alternate splicing of mRNA
Is transmembrane/secreted Ig modulated by DNA or RNA
RNA. Alternative splicing of mRNA. If read through to membrane anchor sequence, transmembrane protein added and secreted form of Ig
What is affinity maturation?
Increasing affinity strength of antibodies as Ab response progresses. Through somatic hypermutation
What is somatic hypermutation?
Mutations occurring predominantly in the CDR following B-cell proliferation aiming to enhance recognition of specific foreign antigen
What is AID?
Activation Induced Cytidine Deaminase
- Not mediated by RAG
- Required for class switching
- Converts cytosine to uridine resulting in introduction of mutations
