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B-CELL DIFFERENTIATION IN THE PERIPHERY
Ag
Ag
Ag
Memory B-cell
Activated B-cell
Mature naive B-cell
ISOTYPE
SWITCH
SOMATIC
HYPERMUTATION
BONE
MARROW
3
2
1
4
n
5
Potential B-cell
repertoire
Self recognition
Clonal deletion
Self structure
PERIPHERAL
LYMPHOID
ORGANS
3
2
Available B-cell
repertoire
n
4
Antigen dependent
Clonal division
Antigen – non-self
3
3
3
3
3
3
3
3
3
3
3
3
3
Memory cell
repertoire
3
3
3
3
3
3
3
3
3
Effector cell
repertoire
The molecular genetics of immunoglobulins
•
If the BCR and the soluble antibodies are identical, by what
mechanism switch from one to the other is controlled?
MEMBRANE VS SECRETED IMMUNOGLOBULIN
•
By what mechanism are antibodies with the same specificity but with
different isotypes generated?
ISOTYPE SWITCH
•
How could antibodies increase their affinity in the course of the
immune response?
SOMATIC HYPERMUTATION
MEMBRANE BOUND AND SECRETED
IMMUNOGLOBULIN
The constant region has additional optional exons
Cm
Primary transcript RNA
Each domain of the H
chain is encoded by a
separate exon
Cm1
Cm2
AAAAA
Secretion
coding
sequence
Cm3
Polyadenylation
site (secreted)
pAs
Polyadenylation
site (membrane)
pAm
Cm4
Membrane
coding
sequence
Membrane IgM constant region
DNA
Cm1
Cm2
Cm3
Cm4
Transcription
1° transcript
pAm
Cm1
Cleavage &
polyadenylation at pAm
and RNA splicing
mRNA
Cm2
Cm3
Cm1 Cm2 Cm3 Cm4
Cm4
AAAAA
AAAAA
Membrane coding
sequence encodes
transmembrane region
that retains IgM in the
cell membrane
Protein
Fc
Secreted IgM constant region
DNA
Cm1
Cm2
Cm3
Cm4
Transcription
1° transcript
pAs
Cm1
Cm2
Cm3
Cm4
AAAAA
Cleavage polyadenylation
at pAs and RNA splicing
mRNA
Cm1 Cm2 Cm3 Cm4
Protein
AAAAA
Secretion coding
sequence encodes the
C terminus of soluble,
secreted IgM
Fc
ISOTYPE SWITCH
Antibody isotype switching
Throughout the immune response the specificity of an antibody will
be essentially the same (notwithstanding affinity maturation)
The effector function of antibodies throughout a response needs to
change drastically as the response progresses.
Antibodies are able to retain Variable regions whilst exchanging
Constant regions that contain the structures that interact with cells.
Organisation of the functional human heavy chain C region genes
J regions
Cm
Cd
Cg3
Cg1 Ca1 Cg2
Cg4
Ce
Ca2
embrionális
Embryonal
L1 V1 L2 V2 Ln Vn D1 - 12
DNS
DNA
5'
szomatikus
rekombináció
Somatic
recombination
D–
D-J kapcsolódás
átrendeződött
Rearranged
DNA
DNS
L1
V1
Ln
Vn
Cm Cδ Cg3
CM CD CG3
J1-4
J
CA1Cε2
Ca1
CG1
CE2Cg1
CE1 Ca2
CG
2 CG
4 Cε1
CA2
C
g1
Cg4
D1D2 J1J2-4 Cm
CM Cδ
CD
5'
3'
szomatikus rekombináció
Somatic
recombination
V
-D-J kapcsolódás
CM Cδ
CD
D2J1 J2-4 Cm
3'
L1 V1
5'
Primerprimer
RNA transcript
RNS-átirat
mRNA
mRNS
3’
Transcription
transzkripció
5'
L1 V1D2 J1 J2-4 Cm
CM
Cδ
CD
IgM
Cγ1
IgG
Cγ2
IgG
Cγ3
IgG
Modification Cγ4
IgG
AAAA
transzláció
Translation
L V DJ Cm
C
polipeptid
Ig ISOTYPES
Cµ
3'
Processing
átalakítás
Cm
L1 V1 D2 J1 CM
naszcens
Nascent
polypeptide
módosítás
V
C
V–D–J
NEHÉZL
ÁNC (M
)
Heavy
chain
IgM
Cα
IgA
Cε
IgE
Switch regions
Cm
Sm
Cd
Cg3
Sg3
Cg1
Sg1
Ca1
Sa1
Cg2
Sg2
Cg4
Sg4
Ce
Se
Ca2
Sa2
• Upstream of C regions are repetitive regions of DNA called switch regions.
(The exception is the Cd region that has no switch region).
• The Sm consists of 150 repeats of [(GAGCT)n(GGGGGT)] where n is
between 3 and 7.
• Switching is mechanistically similar in many ways to V(D)J
recombination, but
• All recombination events are productive
• Different recombination signal sequences and enzymes are involved
• Requires antigen stimulation of B cell
• Not a random event, but regulated by external signals such as T cell
derived cytokines
• Isotype switching does not take place in the bone marrow, but occurs after
B cell activation in the peripheral lymphoid organs
Switch recombination
Cm
Cd
Cg3
Cg1
Ca1
Cg2
Cg4
Cd
Ce
Cd
Ca2
Sg3
Cg3
Cg3
Cm
Sg1
Cm
Cg1
V23D5J4
Cg3
V23D5J4
Ca1
V23D5J4
Ca1
V23D5J4
Cg3
V23D5J4
Ca1
V23D5J4
Ca1
IgG3 produced.
Switch from IgM
IgA1 produced.
Switch from IgG3
IgA1 produced.
Switch from IgM
At each recombination constant regions are deleted from the genome
An IgE - secreting B cell will never be able to switch to IgM, IgD, IgG1-4 or IgA1
Model for Class Switch Recombination (CSR)
AID (Activation Induced (citidin) Deaminase C →U, RNA editing enzyme)
UNG excises U → abasic sites, AP-endonuclease/lyase activity → ss nicks
Class switch defects - Hiper IgM syndrome type 2 in humans (autosomal)
•HYPER IgM SYNDROME (Autosomal)
-Intrinsic B cell defect, activation induced deaiminase
(AID) deficiency. Cytidine uridine conversion.
-The enyme is involved in affinity maturation and
Ig. class switch