Chapter 5
Organization and Expression of Immunoglobulin Genes
Dr. Capers
Kindt • Goldsby • Osborne
Kuby IMMUNOLOGY
Sixth Edition
Chapter 5
Organization and Expression of
Immunoglobulin Genes
Copyright © 2007 by W. H. Freeman and Company

How does antibody diversity arise?

What causes the difference in amino
acid sequences?

How can different heavy chain constant
regions be associated with the same
variable regions?

In germ-line DNA, multiple gene
segments code portions of single
immunoglobulin heavy or light chain
 During B cell maturation and stimulation,
gene segments are shuffled leaving coding
sequence for only 1 functional heavy chain
and light chain
○ Chromosomal DNA in mature B cells is not the
same as germ-line DNA

Dreyer and Bennett – 1965
 2 separate genes encode single immunoglobulin
heavy or light chain
○ 1 for the variable region
 Proposed there are hundreds or thousands of these
○ 1 for the constant region
 Proposed that there are only single copies of limited
classes

Greater complexity was revealed later
 Light chains and heavy chains (separate multi-
gene families) are located on different
chromosomes
DNA rearrangement: produces variable
region
 Later mRNA splicing: produces constant
region


Kappa (κ) and lamda (λ) light chain
segments:
○ L – leader peptide, guides through ER
○V
VJ segment codes for variable region
○J
○ C – constant region

Heavy chain
○L
○V
○D
○J
○C
VDJ segment codes for variable region
Variable-region gene rearrangements

Variable-region gene rearrangements
occur during B-cell maturation in bone
marrow
○ Heavy-chain variable region genes rearrange
first
○ Then light-chain variable region
○ In the end, B cell contains single functional
variable-region DNA sequence
○ Heavy chain rearrangement (“class
switching”) happens after stimulation of B cell
Mechanism of Variable-Region DNA rearrangements

Recombination signal sequences
(RSSs)
○ Between V, D, and J segments
○ Signal for recombination
○ 2 kinds
- 12 base pairs (bp) – 1 turn of DNA
- 23 bp – 2 turns of DNA
- 12 can only join to 23 and vice versa
Mechanism of Variable-Region DNA rearrangements

Catalyzed by enzymes
○ V(D)J recombinase

Proteins mediate V-(D)-J joining
○ RAG-1 and RAG-2
Same transcriptional
orientation

Gene arrangements may be nonproductive
○ Imprecise joining can occur so that
reading frame is not complete
○ Estimated that less than 1/9 of early
pre-B cells progress to maturity

Gene rearrangement video:
Opposite
transcriptional
orientation

http://www.youtube.com/watch?v=
AxIMmNByqtM
Allelic Exclusion

Ensures that the rearranged heavy and
light chain genes from only 1
chromosome are expressed
Generation of Antibody Diversity
Multiple germ-line gene segments
 Combinatorial V-(D)-J joining
 Junctional flexibility
 P-region nucleotide addition
 N-region nucleotide addition
 Somatic hypermutation
 Combinatorial association of light and
heavy chains

○ This is mainly in mice and humans – other studied species differ in
development of diversification
Ab diversity – Multiple gene-line segments
AND combination of those segments
Ab diveristy – junctional flexibility

Random joining of V-(D)-J segments
○ Imprecise joining can result in nonproductive
rearrangements
○ However, imprecise joining can result in new
functional rearrangements
Ab diversity – P-addition and N-addition
Ab diversity – somatic hypermutation
Mutation occurs with much higher
frequency in these genes than in other
genes
 Normally happens in germinal centers in
lymphoid tissue

Class Switching
Isotype switching
 After antigenic stimulation of B cell
 VHDHJH until combines with CH gene
segment
 Activation-induced cytidine deaminase
(AID)

 Somatic hypermutation
 Gene conversion
 CLASS-SWITCH recombination

IL-4 also involved
μ→δ→γ→ε→α
IgM→IgD→IgG→IgE→IgA
Ig Gene Transcripts

Processing of immunoglobulin heavy
chain primary transcript can yield
several different mRNAs
○ Explains how single B cell can have secreted
and membrane bound Ab
Regulation of Ig-Gene Transcription

2 major classes of cis regulatory sequences in DNA regulate
 Promoters – promote RNA transcription in specific
direction
 Enhancers – help activate transcription
 Gene rearrangement brings the promoter and enhancer
closer together, accelerating transcription
Antibody Engineering



Monoclonal Abs used for
many clinical reasons
(anti- tumor Ab, for
instance)
If developed in mice, might
produce immune response
when injected
○ Can be cleared in
which they will not be
efficient
○ Can create allergic
response
Creating chimeric Abs or
humanized Abs are
beneficial