b and t cell rec. genes handout 2012
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B- and T-cell receptor
genes
Dept of Cell and Molecular Biology!
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Immunoglobulin Variable regions
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RNA-DNAheteroduplex
analysis ofan immuno-globulin lightchain gene
Mouse H and L chain loci!
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Variability generating mechanisms inimmunoglobulin and TCR genes!
! Germline diversity; a large number of different V, D and J segmentswhich differ in nucleotide sequence.
! Combinatorial diversity; the different V, D, and J segments can freelycombine, resulting in a large number of possible combinations(VxDxJ).
! Junctional diversity; variation in the position of the cleavage site at thejunctions between V and J segments in the light chain and the V and Dand D and J in the heavy chain (approximately +3 to -3 nt).
! P-region diversity; difference in where the ligated loops are cleavedgenerates additional diversity at the V-J, V-D and D-J joining sites.
! N-region divesity; addition or removal of up to approximately 10 bp ateither side of the D segment in the IgH and TCR! and " genes, and at
a lesser extent at V- J junctions. Generated by the enzyme TdT(Terminal doeoxy nuclotide Transferase).
! Somatic mutations; random exchange of single nucleotides (one at atime) in and around the rearranged V region. Only found in the Iggenes and not in the TCR genes.
! Free combination of light and heavy chain polypeptides (TCR # and !or TCR $ and" polypeptides)
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Variability in immunoglobulinand TCR genes!
! Total potential diversity withjunctionaldiversity;
Immunoglobulins 10 11.
TCR #! 10 16 TCR $" 10 18
! Total potential diversity with somaticmutations ;Immunoglobulins 1015-16
Transcriptional activation of the %-light chain locus!
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Transcriptional activation of the heavy andlight chain locus!
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Differences between T cell recptorsand immunoglobulins during
somatic rearrangements!
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Switch Regions
G rich, simple repetitive sequences1-10 kb in length.
IgE (&) switch region approx. 1kb.IgG1 ($1) switch region approx 10 kb.
Short 5-7 nt repeats like, GAGCT, GGGGT, GGGCT
S , S & and S # are closely related with 20,40 and 80 bp repeat units respectively.
S $ the most divergent with 49-52 bp repeat units
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!1I-exon !4!3!2
Promoter
AAAAAA
Transcriptional activation ofun-rearranged locus
Germline Transcripts (GL) are essential for switchrecombination. Activated by IL-4 and CD40-CD40L interaction.GL transcripts stimulate recombination not only by openingof chromatin. The specific transcript or splicing is ofimportance, 10-100 fold higher switch frequency with theI exon (than irrelevant transcription)
Switch region (S&)!
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Membrane and secretory forms !
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B cell developmentEarly events!
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Surface markers on peripheral B cells