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
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
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
Figure 4-4Human Ig Loci
TCR Alpha and Beta Loci
TCR Delta and Gamma Loci
IMMUNOGLOBULIN GENES UNDERGO TWO DNA REARRANGEMENTS
1. V(D)J Recombination: both light and heavy
chains
2. Class switch recombination: heavy
chains only
Figure 4-2DNA Rearrangement RemovesSequences Between V, D and J Segments
RNA Splicing Removes Sequences Between J and C Segments
DNA
RNA
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.
Figure 4-5
Recombination Signal Sequences (RSSs)Flank Rearranging Gene Segments
7bp 9bp12/23 bpRSS=heptamer, spacer and nonamer
Figure 4-7
Initiation ofV(D)Jrearrangement :RAG-dependentcleavage
Resolutionof cleavageproducts:DNA non-homologousend-joiningmachinery
#1:Lymphocyte-Specific
#2:Constitutive
Figure 4-8
Addition of Bases at V-D-J Junctions:An Important Source of Diversity
Figure 7-18
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
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
Heavy chain isotypes are generated by a second DNA rearrangement:
CLASS SWITCH RECOMBINATION (CSR)
Figure 4-18
Figure 4-17
Figure 4-19mRNASplicing
DNA rearrangement: CSR
Figure 4-20IgM and IgD Are Generated from a Single PrimaryTranscript by DIFFERENTIAL mRNA POLY A/SPLICING
mRNAsencoding bothmembrane andsecreted formsof mu heavychain are gen-erated from asingle primarytranscript bydifferential splicing and polyadenylation
Membrane vs. Secreted Mu
Figure 4-21
CSR Involves DNA Deletion and Loss
“Germline” (I region) Transcripts Are Necessary For Isotype Switch Recombination
Figure 9-7
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
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
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