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LIGHT CHAIN
HEAVY CHAIN
Polypeptide
Gene Segments
Variable Constant
J CV
Variable
V
o s aC n t nt
D J CH1 CH2 CH3
Polypeptide
Gene Segments
Light Chain POLYPEPTIDE
Light Chain GENE SEGMENTS
H.C.POLYPEPTIDE
H.C. GENE SEGMENTS
Ig Polypeptides Are Encoded by Multiple Gene Segments
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About 100V gene segments
4 J GeneSegments
1 C GeneSegment
A Prototype Ig Gene: Murine Kappa
Multiple V gene segments, distant from J and C
A few J gene segments
One C gene segment
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Murine Ig Heavy Chain Gene Organization
C C C3 C1 C2b C2a C C
~ 120 V Gene Segments ~20 Ds 4 Js 8 Constant Gene Segments
L VH
C1 C2 C3 C4/S CM
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Figure 4-4Human Ig Loci
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TCR Alpha and Beta Loci
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TCR Delta and Gamma Loci
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IMMUNOGLOBULIN GENES UNDERGO TWO DNA REARRANGEMENTS
1. V(D)J Recombination: both light and heavy
chains
2. Class switch recombination: heavy
chains only
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Figure 4-2DNA Rearrangement RemovesSequences Between V, D and J Segments
RNA Splicing Removes Sequences Between J and C Segments
DNA
RNA
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Figure 4-6V(D)J recombinationinvolves DELETION of DNA betweenV, D and J codingsegments.
The deleted DNA is LOST from the cellbecause it is notreplicated.
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Figure 4-5
Recombination Signal Sequences (RSSs)Flank Rearranging Gene Segments
7bp 9bp12/23 bpRSS=heptamer, spacer and nonamer
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Figure 4-7
Initiation ofV(D)Jrearrangement :RAG-dependentcleavage
Resolutionof cleavageproducts:DNA non-homologousend-joiningmachinery
#1:Lymphocyte-Specific
#2:Constitutive
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Figure 4-8
Addition of Bases at V-D-J Junctions:An Important Source of Diversity
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CONSEQUENCES OF V(D)J RECOMBINATION
1. Combinatorial diversity: # of possible combinationsis the product of the # of recombining segments
i.e. for mouse h.c.: 120x20x4=104
2. Junctional diversity at CDR3Deletion of bases at junctionsN region additions at junctionsP region additions at junctions
3. Activates transcription of the rearranged geneJuxtaposition of intronic enhancers with V regionpromoters.
4. Allows receptor editing to alter potentially self-reactive antibodies
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LIGHT CHAIN
HEAVY CHAIN
Polypeptide
Gene Segments
Variable Constant
J CV
Variable
V
o s aC n t nt
D J CH1 CH2 CH3
Polypeptide
Gene Segments
Light Chain POLYPEPTIDE
Light Chain GENE SEGMENTS
H.C.POLYPEPTIDE
H.C. GENE SEGMENTS
Ig Polypeptides Are Encoded by Multiple Gene Segments
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Heavy chain isotypes are generated by a second DNA rearrangement:
CLASS SWITCH RECOMBINATION (CSR)
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Figure 4-19mRNASplicing
DNA rearrangement: CSR
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Figure 4-20IgM and IgD Are Generated from a Single PrimaryTranscript by DIFFERENTIAL mRNA POLY A/SPLICING
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mRNAsencoding bothmembrane andsecreted formsof mu heavychain are gen-erated from asingle primarytranscript bydifferential splicing and polyadenylation
Membrane vs. Secreted Mu
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Figure 4-21
CSR Involves DNA Deletion and Loss
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“Germline” (I region) Transcripts Are Necessary For Isotype Switch Recombination
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T cell secretes cytokines
Specific I region transcription
Isotype switch recombination to specific CH gene segment
TGF
ISCRNA
Cut and joinS and SDNA
VDJC mRNA
IgA
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V(D)J Recombination CSR
Join in exon Join in intron
RAGs required RAGs Not required AID is required
Repair enzymes Repair enzymes
Generates diversity Changes isotypeAg specificity Ag elimination
Regulated by T cell signalsRandom
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1. Humans with mutations in gene products required forV(D)J recombination are immunodeficient:
RAG Various SCIDs, including Omenn’s syndrome
Artemis Radio-sensitive SCID
Ligase IV SCID with developmental deficiency
2. Humans with mutations affecting CSR have hyper IgM
AID mutations and other mutations