Antibody Genetics I Flashcards
Where are B cells derived from in adult mammals?
Bone marrow
Where do B cells originally develop?
In foetal liver at 8-9 weeks gestation
What cells line the bone endosteum?
Progenitor cells
What do stromal reticular cells do?
Produce IL-7 to sustain B cell differentiation
Support B cell development
How many progeny can be produced by each progenitor?
64
What happen to the progeny?
They migrate to the centre of spongy bone
Adventitial reticular cells aid egress to the venous sinusoid then out into circulation
What percentage of B cells do not make it out of the bone?
75%
What happens to the B cells that do not leave the bone?
Apoptosis or phagocytosis by macrophages
What allows survival of B cells after selection?
Successful rearrangement of Ig gene (BCR)
How are autoreactive B cells removed?
By negative selection
What was Ehrlich’s side-chain theory?
That each B cell had multiple specificities (side chains)
What are the mechanisms of diversity?
Somatic recombination and somatic mutation
What does somatic recombination allow?
Joining of one segment of the gene to another
What does somatic mutation allow?
Sloppy joining of the gene segments
What happens when there is somatic recombination and mutation together?
Generate heavy and light chains of Ig
Pairing of unique heavy and light chains add diversity
What is the order of diversity generation?
First heavy chain rearranged
If successful then kappa light chain rearranges
If no kappa chain then lambda chain rearranges
What chromosome are the VDJ heavy chain segments on?
Chromosome 14
How is the functional heavy chain VDJ region formed?
Recombination of any one V with any one D and any one J
What does each V region code for?
A single peptide which directs the polypeptide to the RER, golgi and the out the cell
What happens first in heavy chain rearrangement?
Joining of Jh to Dh
What does the VDJ region constitute?
Highly diverse 3rd hypervariable region (CDR3)
What happens if the DJ rearrangement is productive?
DhJh now signals rearrangement to a Vh gene segment
What does the VDJ recombine to?
C mu
What happens in light chain rearrangement?
Kappa light chain locus on chromosome 2
No D regions so VJ joining, forming primary RNA transcript
Additional J regions spliced out to form mRNA
Remaining J region spliced out to form kappa chain
What happens if both kappa gene rearrangements are unsuccessful?
Moves to lambda chain locus on chrom 22
What happens in lambda rearrangement?
VJ joining
Intron between VJ and C lambda is cleaved
How is extra diversity created?
Imprecise joining
What enzymes are involved in recombination?
RAG-1 and 2
What creates N-region junctional diversity?
TdT
What do recombination signal sequences do?
Flank all V, D and J segments
What are the recombination signal sequences?
CACAGTG heptamer downstream 3’ of Vh, VL and Dh
Followed by a spacer of 12 or 23 non-conserved bases
Then a ACAAAAACC nonamer
Upstream 5’ of Jh, Dh and JL is a corresponding nonamer, spacer and heptamer sequence
What is the 12/23 rule?
Only 12 will combine with 23 and vice versa
No 12 will combine with another 12, no 23 will combine with another 23 - ensures correct recombinations occur
What is the mechanism of recombination?
Portions of the gene made available to recombination machinery
2 selected coding segments and their RSSs are brought together forming chromosomal loop
Double strand breaks generated at RSS sequence junctions by RAG1/RAG2 complex
Creates a hairpin end on coding region
Opening of hairpin by artemis
Addition or removal of bases by TdT to add extra diversity
Coding ends are rejoined by number of factors including DNA ligase
What is combinational diversity?
Random pairing of VH and VL chains
Not all pairings will form functional receptors
Describe junctional diversity
Asymmetric breakage of hairpin loop so one DNA loop is longer than the other
Shorter strand extended with complimentary nucleotides called P nucleotides
TdT adds N nucleotides
This adds non-germline sequences
These junctional sequences form CDR3
What do pro B cells express?
CD19 and CD10
What do pre B cells express?
Mu chain
TdT
RAG
CD19
CD10
What do immature B cells express?
Mu chain
Light chain
RAG
CD19
IgM
What do mature B cells express?
Mu chain
Delta chain
Light chain
CD19
IgM
IgD
What is an idiotype?
The region that is specific for an antibody
How does an immature B cell respond to an antigen?
Negative selection
Receptor editing
How do mature B cells respond to antigens?
Activation, proliferation, differentiation
What happens if idiotypes go wrong?
SCID
What does SCID affect?
Mainly T cells but some forms affect both T and B
How does autoimmunity occur?
During B cell maturation in BM, allows non-functional BCR to be deleted or replaced
How can IgM be autoreactive?
If IgM switches to IgG = pathogenic, e.g. rheumatoid arthritis (RA)
What is the function of RAG1/2?
Production of T and B cells
What happens if there are mutations in RAG1/2?
No production of T and B cells
