Transcript The primary -> secondary immune response

Adaptive Immunity
• Central objective:
• Protect against foreign invaders
• Create memory of invasion to prevent
recurrent infection
• Response must be highly specific to
the invader
Ig Isotypes: Classes and Subclasses
Ig isotypes are
defined by the Cterminal portion of
the molecule
There are 5 classes
of Ig and there are 4
sub-classes of IgG.
Isotype class
determines effector
function.
Ig Isotypes
Ig Isotypes Localize to Different
Anatomical Sites in the Body
Ig Isotypes Localize to Different
Anatomical Sites in the Body
Ig Isotypes Have Different Effector
Functions
When Do Isotypes
Matter?
The primary -> secondary
immune response:
•Increase Ig titer
•Switch from IgM to IgG
•Increase Ag binding affinity
When Do Isotypes Matter?
Primary ->Secondary Immune Response Transition
Slide 2
Primary/Secondary Immune
Response
Events associated with
an immune response
over time:
Increase of IgM titer
Increase of IgM titer is
specific to the
immunizing Ag
Slide 2
Isotypes Switch: IgM->IgX
Three Types of Switch:
RNA level
•IgM->IgD
• Membrane->Secreted
DNA level
•Class Switch Recombination
IgM and IgD Are Coexpressed in
Mature B Cells
Differentiation to the
mature B cell stage
is marked by the coexpression of IgM
and IgD on the
plasma membrane
A Shift of IgM to IgM/IgD
Expression in Mature B cells
Mechanism:
Alternative RNA
polyadenylation and
splicing
In Mature B Cells Ig is Located on
the Mb and Functions as the BCR
Location of Ig
on the cell
surface is
transmb and
requires the
presence of the
mb exon
In Plasma Cells Ig is Secreted
Secretion of Ig
from the cell
requires the
presence of the
secretion exon
Secreted Ig Promotes Critical Functions
In mature B
cells Ig is
located on the
plasma mb.
Ig is secreted
from terminally
differentiated
plasma cells
Secreted IgM and IgA are Multimerized
by J Chain
In mature B
cells Ig is
located on the
plasma mb.
Ig is secreted
from terminally
differentiated
plasma cells
Isotypes Switch: IgM->IgX
Three Types:
RNA level
•IgM->IgD
• Membrane->Secreted
DNA level
•Class Switch Recombination
C Regions Are Arrayed in the
IgH Locus
VDJ
E
S C
S3 C3
S1 C1
S2b C2b
Looping Out and Deletion Model of SR
A.
V DJ
S
E
C
C
S3
C3
S1
C1
 mRNA
3 ge r m line t r a ns c r ipt
 mRNA
B.
C
C
S
S3
C3
V DJ
C.
V DJ
E
S S3
C3
3 mRNA
S1
S1
C1
C1
+
C
C
S S3
Germline Transcripts are Required for
SR
VDJ
E
S C
S3 C3
I3 S3
S1 C1
I2b S2b C2b
C3
LPS:
S2b C2b
mRNA
mRNA
sterile transcript
P I1 S1 C1
LPS + IL4:
mRNA
When Do Isotypes
Matter?
The primary -> secondary
immune response:
•Increase Ig titer
•Switch from IgM to IgG
•Increase Ag binding affinity
Affinity Maturation and Ig
Isotype Switch
In a primary immune response, affinity
of Ig for its Ag is usually not high
enough to immediately clear the
pathogen from the system. However,
affinity is increased by somatic
hypermutation (SHM).
SHM alters V genes at the DNA level
Primary/Secondary Immune
Response
Protein level
Slide 2
Acquisition of
mutations in the
V region of the
VH and VL
proteins:
nonrandom
distribution
Primary/Secondary Immune Response
Protein level
The position of
the newly
acquired
mutations is in
the antigen
binding
pocket.
Slide 2
Somatic Hypermutation and Affinity
Maturation
In SHM, individual nucleotides in VJ or
VDJ units are mutated, thus adjusting the
specificity of and potentially increasing
the affinity for Ag.
Somatic Hypermutation and Affinity
Maturation cont...
Mutations translate into a change in the
Ag binding pocket of the Ab. Some
mutations are deleterious while others
are neutral or advantageous for Ab:Ag
fit. Some deleterious mutations generate
autoreactive Ig.
Question: are mutations random or directed at
the level of the genes?
Somatic Hypermutation and Affinity
Maturation
DNA level
Somatic Hypermutation and Affinity
Maturation cont...
Selection of high affinity B cell clones leads to
affinity maturation of the immune response.
B cells expressing high affinity BCR are
favored by clonal selection. This occurs when
Ab:Ag complexes are highly compatible and
possess a “good fit”. The high affinity Ab:Ag
interaction transduces a strong signal to the B
cell to proliferate.
Clonal
Selection: Type I
Clonal selection occurs
following VDJ in the BM to
delete B cells expressing
autoreactive BCR.
Clonal
Selection: Type II
Mutations which increase
affinity of Ab for Ag will
create a BCR which promotes
strong proliferation. Selection
also occurs in the periphery
following SHM on the GC to
delete newly autoreactive
BCR and to preclude the
escape of self-reactive B
cells.
SHM Occurs in
the Germinal
Center B Cells
The Surprising Link between CSR,
SHM and Gene Conversion
Activation induced deaminase (AID) is
responsible for CSR and SHM.
It is the only B cell specific gene product
required for both these genetic alterations.
The Surprising Link Between
CSR, SHM and Gene Conversion
How does AID work?
AID has homology to the APOBEC family
of proteins which are involved in RNA
editing
AID is also a cytidine deaminase and
converts dC ->uracil
Uracil is mutagenic and triggers the BER
pathway
AID Deaminates dC and Triggers the BER
Pathway
AID
G
C
C
G
AID
G
U
U
G
UNG
G
G
APE
5’
3’
G
G
3’
5’
DSBs
5’
Tpol
G
5’
3’
3’
G
5’
3’
G
3’
G
5’
What happens when AID is
mutated?
What happens when UNG is
mutated?
What happens when DSB repair
proteins are mutated?
Hyper-IgM Syndrome: No
Secondary Isotypes
Four Types: Cause????
Mutations in:
•CD40 (T cell); CD40L (B cell)
•AID
•UNG
•Subgroup with unknown cause
CSR Requires DNA Repair
Proteins
DNA Repair Proteins Involved in CSR:
DNA-PKcs, Ku70, Ku80,
H2AX, NBSl, ATM
MMR (Msh2, Msh6,
Pms2, Mlhl)
The End
U=T During Replication
BCR and TCR Break the Rules
of Mendelian Genetics
Monospecific Ag receptors:
Only one H chain and one L
chain is expressed per B/T cell
whereas each cell has 2 alleles
for H and L chains