Transcript B cell Plasma Cell

B 细胞的活化
Lecture Outline
• 1. What is meant by ‘B cell activation’?
• 2. Why do some B cell responses require T cell
help and others not?
• 3. What is ‘T cell help’?
• 4. What are plasma cells?
• 5. How do activated B cells undergo ‘affinity
maturation’?
• 6. What is the mechanism for B cell memory?
Antigen
Antibody 1
Antibody 3
Antibody 2
Antibody formats
VH
scFv
VL
CH1
CL
CH2
Fab
Whole
Antibody
CH3
SS
What is meant by ‘B cell activation’?
• Go -> G1 of cell cycle (increase in size)
• upregulate
–
–
–
–
MHC class II
costimulatory molecules (B7-2)
adhesion molecules (ICAM1)
cytokine receptors (IL-2R)
• migrate to outer T zone
–
•
altered response to chemokines
become receptive to T cell help
–
protected from fas
• enter mitosis if provided with submitogenic doses of
other stimuli (LPS, CD40L, IL-4)
Types of antigen
• T-independent (TI) antigens - Type I
– induce division/differentiation independently of BCR
(polyclonal mitogens)
• LPS, bacterial (CpG) DNA
• T-independent (TI) antigens - Type II
– induce division/differentiation by BCR signaling alone
• bacterial polysaccharides, repeating surface molecules on viruses
• T-dependent (TD) antigens
– activate via BCR but depend on additional signals from helper
T cells to cause division/differentiation
• any antigen containing protein
• Most pathogens contain both T-I and T-D antigens
• Only TD antigens can induce Germinal Center responses
Types of B cell Antigens
T-independent (TI)
T-cell dependent (TD)
T cell
present
Ag
mitogenic
BcR signal
'activation' signal
but not mitogenic
mitogenesis
differentiation
-> most pathogens contain both T-independent and T-dependent antigens
Innate features of pathogens act
as B cell costimulators
• pathogen multivalency
– provides a level of BCR crosslinking optimal for
activation
• many pathogens activate TLRs
– TLR signaling synergizes with BCR signal
• many pathogens activate the complement
cascade and become C3d coated
– complement receptor (CR) crosslinking synergizes
with BCR signal
Antigen-C3d complexes cross-link BCR and CR2CD19 complex - increase sensitivity to antigen
antigen C3d
CR2/CD21
CD19
2+
IgM
[Ca ]
TAPA1
HEL-C3d
HEL
0
lyn
Y
syk
multiple downstream
pathways
0.1
1
10
100
1000
Concentration (nM)
Y
PI3-K
Vav
Dempsey et al. (1996) Science 271, 348
Sensitivity of anti-HEL Ig-transgenic B cells to
HEL and HEL-C3d
T-Independent (type II) Responses
Current Paradigm :
Multivalent Antigen
B cell
B cell
plasma cells
Emerging Model:
B cell
B cell
plasma cells
DCs support Ag-specific plasma cell
differentation
Plasma cell
marker
• 3 day co-cultures
PC-binding
Balazs et al., Immunity 17, 341 (2002)
-> DCs are a source of BAFF
-> Costimulatory effect of DCs partly due to BAFF
T-Dependent Responses
Antigen
DC
T cell
Dendritic Cell (DC)
internalizes antigen (Ag),
processes into peptides,
presents peptides
together with MHC
molecules to T cells
T cell
B cell
B cell binds Ag via surface
Ig, transmits BCR signals
and presents peptides to T
cells, receives
T cell help (growth and
differentiation factors)
plasma cells
Secretes
Antibody (Ab)
B cells are antigen-presenting cells
• BCR cross-linking induces antigen
internalization to endosomes
• antigen is proteolysed to peptides
• peptides associate with MHC class II
• MHC class II-peptide complexes traffic to
surface of B cell\
• B cells present antigen recognized by
their BCR ~105 x more efficiently than
other antigens
B cell antigen presentation and
the concept of linked help
protein
sugar
T
Sugar Specific
B cell
Protein Specific
Antigen internalization, proteolysis
T cell
-> presentation of peptides
Cardinal features of B - T interaction
antigen-binding B cell
antigen-specific
T cell
TCR
MHC
B7.2
CD28
(death) MITOSIS
CD40
IL4R
CD40L
IL4
FasL
FAS
additional cytokines,
costimulatory signals
isotype switching,
proliferation
Plasma cell
?
GC cell
Role of CD40 in B cell activation
CD4 T cell
CD40L (TNF family)
CD40 (TNF-R family)
B cell
TRAF2
TRAF3
• TCR triggering upregulates CD40L on T cell
• CD40 signaling promotes B
cell activation
• CD40L-deficiency = 'hyperIgM syndrome' (no isotype
switching, no Germinal
Centers)
•
increased expression of cell cycle
molecules,
survival molecules, cytokines,
promotes isotype switching
Note: CD40 signaling also important
in DC, Macrophage function
Key components of T cell help
• CD40L triggers CD40 -> synergizes with BCR
signals to promote mitosis; cytokine (e.g. IL4)
signals also contribute
• FasL triggers Fas -> BCR signaling protects from
apoptosis
• T cell derived cytokines influence differentiation,
isotype switching:
–
–
–
–
IL2, IL6 promote differentiation
IL4 -> IgG1, IgE
I FNg -> IgG2a, IgG3
TGFb and IL5 -> IgA
• Many other molecules involved in T-B interactions
– e.g. ICAM1/LFA1, CD30/CD30L, CD27L/CD27,
OX40L/OX40, ...
Altered signaling in anergic B cells
Activated
Anergic
- acutely exposed to
cross-linking antigen
- chronically exposed
to antigen
b 
SH3
SH2
lyn
SH2
lyn
SH2
syk
negative
negative
feedback
syk
feedback
Reduced tyrosine phosphorylation
Elevated basal Ca - but reduced flux
Constitutive erk and NFAT
No induction of NFkB, JNK
antigen- ACTIVATED B cell
antigen-specific
CD4+ T cell
MHC
B7.2
TCR
CD28
(death) MITOSIS
CD40
IL4R
CD40L
IL4
FAS
FasL
antigen- TOLERANT B cell
MHC
antigen-specific
CD4+ T cell
TCR
CD28
DEATH
(mitosis)
CD40
CD40L
IL4R
FAS
FasL
Anergic B cells can respond to
'stronger' antigens
Oligovalent Self-Ag,
Constitutively Exposed
Anergic
B cell
Remains Anergic
-> killed if presents to
antigen specific T cell
Multivalent Foreign-Ag,
Acutely Exposed
Anergic
B cell
Becomes Activated
-> can receive T cell help
-> proliferate and secrete Ab
After appropriate activation the B cell differentiates into an
antibody secreting cell, also known as a Plasma Cell
B cell
Plasma Cell
membrane Ig
secretory Ig
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Production of membrane vs secreted Ig
membrane Ig
CH 1 tm cy 2
splice, use
poly A site 2
secretory Ig
CH 1 tm cy 2
no splice, use
poly A site 1
CH
CH tmcy
AAAA
AAAA
Plasma Cells are antibody secreting cells
Two types:
1. Plasma cells generated early in the primary response
- short-lived (~ few days)
- typically low affinity
- form in T-independent and T-dependent responses
- home to red pulp of spleen, medullary cords of lymph nodes
- IgM but also IgG and other isotypes
2. Plasma cells generated later in the primary response
and that predominate in secondary responses
- arise predominantly from germinal centers (in primary) or from memory
B cells (in secondary)
- long-lived (possibly several months)
- often home to bone marrow, gut, lactating mammary gland
- predominantly isotype switched
Plasma cell differentiation
B
blimp1
XBP1
Plasma cell differentiation
•
•
•
•
Shaffer et al., Immunity. 21:81-93
Blimp1 represses PAX5 and genes associated with B cell identity,
induces increases in a subset of plasma cell genes (Syndecan, XBP1)
and induces conversion of membrane to secretory Ig
XBP1 is a component of the UPR (unfolded protein response)
Increased sIg production ‘stresses’ ER and promotes processing of
XBP1 mRNA into active transcript
XBP1 induces increases in
– Secretory pathway gene expression
– Organelle biogenesis
– Cell size and protein synthesis
B cell migration in the spleen during
differentiation to plasma cells
1 day after antigen
5 days after antigen
F
F
T
RP
B cells Activated B cells
T
4 days
RP
B cells Plasma cells
Plasma Cell Migration Summary
B
ELC
SLC
ELC
SDF1
ELC
SDF1
SDF1
SDF1
SDF1
SDF1
SDF1
SDF1
SDF1
SDF1
blood
SDF1
SDF1
venous
sinsus
blood
Colon
M
blood
blood
MEC
RP
J
SLC
SDF1
SLC
T
ELC
T
ELC
SLC
SDF1
SLC
BLC
BLC
SLC
SLC
Bone Marrow
MEC
B
Small Intestine
BLC
TECK
+Ag
BLC
BLC +Ag
BLC
BLC
IgA Plasmablast
IgA Plasma cell
TECK
SDF1
MZ
Lymph Node
B cell
T cell
Plasmablast
Plasma cell
Spleen
lymph
 SDF1 for splenic red-pulp, LN medullary cords, Bone Marrow
 TECK/CCR9 for small intestine (IgA)
 MEC/CCR10 for large intestine, stomach, salivary glands (IgA)
B cell antibody response - clonal replication enters into a
higher order upon plasma cell differentation
3 days
12 divisions
naive
B cell
1
1 day
differentiation
activated B cells
212 = 4,096
1 day
104 Ab/cell/sec
plasma cells
4,096
bacteria - dividing every ~60 min
5 days = 2120 divisions = 1.3x1036
antibodies
>1012
Ig Heavy chain class (isotype) switching
VDJ
g
m
e

55 kb
T cell help
(cytokines, CD40L)
antigen
IgM+ naive
B cell
IgG
secreting
plasma cell
IgG+
memory
cell
Ig Heavy chain class (isotype) switching
Affinity Maturation
• Affinity maturation occurs in germinal
centers and is the result of somatic
hypermutation of Ig-genes in dividing B
cells followed by selection of high affinity
B cells by antigen displayed by FDCs
• The high affinity B cells emerging in
germinal centers give rise to long-lived
plasma cells and memory B cells
Germinal Centers
Function: to generate B cells that produce antibodies with increased
affinity for the inducing antigen
=> affinity maturation
Germinal Center Reaction:
Activated B cells give rise to Centroblasts
- localize in follicle, undergo rapid cell division and turn on machinery that
causes somatic mutation in V-regions
Centroblasts give rise to Centrocytes
- migrate to the FDC-rich region of the Germinal Center
- survival is dependent on interaction with FDC-bound Ag and presentation of
Ag to T cells
- centrocytes that successfully compete to bind antigen (e.g. by having higher
affinity BCR) and that receive T cell help are selected and may differentiate
into long-lived plasma cells or memory B cells
Germinal Center
Light Zone
- B cells (centrocytes, CC)
compete to bind Ag and
receive T cell help
- selects for cells with
higher affinity BCR
Dark Zone
- activated B cells
(centroblasts, CB)
undergo somatic mutation
of Ig V genes
T FDC T
T
CC CC
T FDC
FDC
CC
CC CC
CC
MØ
CB
CB
CB
CB
CB CB
CB
CB CB
CB CB
T B
memory
B cell
plasma
cell
Somatic mutation of Ig V region in GC B cell
-> mutations are actively induced in the V-regions of the
antibody heavy and light chain genes
V
CH1
Met ... Gly Tyr Val His Arg ...
...Gly, Pro...
...GGC, CCT...
ATG ... GGC TAT GTT
A CAC CGT ...
DNA replication
error
AID dependent
mutator
complex
AID = Activation Induced Deaminase
Somatic mutation of Ig V region in GC B cell
V
CH1
Met ... Gly Tyr Val
Asp His Arg ...
...Gly, Pro...
...GGC, CCT...
ATG ... GGC TAT GTT
A CAC CGT ...
-> now encodes antibody molecule with slightly altered antigen
binding site
-> sometimes, by chance, this site will have an improved ability
to bind the inducing antigen (i.e. a higher affinity)
Mutations are targeted to antigen binding region
of antibody
Affinity maturation improves the ‘fit’ of
the antibody for the inducing antigen
Ag
CDR CDR
1 2 31 2 3
VH
CH1
VL
before
after
Ag
Ag
CL
- increasing the binding affinity
CH2
CH3
CDR = complementarity determining region, also known as the
hypervariable region (part of V domain that binds the antigen)
Affinity maturation and antibody responses
Germinal Center
mantle zone
(CD23+ naive B cells)
GC light zone
(CD23++ FDC
and centrocytes)
GC dark zone
(Ki67 cell cycle antigen+
centroblasts)
T zone
from Liu et al., Immunology Today 13, 17-21 (1992)
Germinal Center
from Hutloff et al., Nature 397, 263-6 (1999)
B cell differentiation in the germinal center
mature
B cell
helper
T cell
bacteria
Centroblast
DARK ZONE
LIGHT
ZONE
-CLASS SWITCH
RECOMBINATION
-SOMATIC
HYPERMUTATION
FDC
centrocyte
Memory
B cell
apoptosis
anergic
Plasma B cell
Memory B cells
• Generated in germinal centers
•
•
•
•
– therefore we only have humoral memory to T-dependent
antigens
Small, recirculating cells
Typically isotype switched (e.g. IgG+ or IgA+)
Typically have higher affinity for the inducing Ag
Longer lived than naïve B cells
– Persistence of memory B cells after an immune response
ensures that we have increased numbers of B cells specific
for the antigen and ready to respond on re-encounter
Antigen-IgG complexes cross-link BCR and
FcgRII - inhibit signaling
antigen
IgG
IgM
FcgRII
I
T
I
Y
M
SHIP
lyn
syk
calcium and lipid dependent
signaling pathways
Features of primary and secondary antibody responses
Related information:
Tumors of B Cells
Tumor
Equivalent normal cell
Location
Chronic lymphocytic leukemia
CD5 B-1 B cell
Blood
Acute lymphoblastic leukemia
Lymphoid progenitor
Pre-B cell leukemia
Pre-B cell
Follicular center cell lymphoma
Burkitt’s lymphoma
Mature B cell
Waldenstrom’s macroglobulinemia
B cell
IgM secreting
Multiple myeloma
Plasma cell secreting
various Ig isotypes
Bone marrow
Blood
Periphery
Bone marrow
Related information:
Effects on Cells of Immune System
B
cell
Proliferation,
Differentiation,
Ig secretion
and selection
NK
T cell
Proliferation,
Differentiation,
Cytokine
production
Activation of cells of
immune system
Lymphocyte
cytokines
LAK
Cytokine production
Macrophage
Recommended Reading
Primary papers:
• Jun, J.E. et al. (2003) Identifying the MAGUK protein Carma-1 as a central
regulator of humoral immune responses and atopy by genome-wide mouse
mutagenesis. Immunity 18:751-62
• Lesley, R. et al. (2004) Reduced competitiveness of autoantigen-engaged B
cells due to increased dependence on BAFF. Immunity. 2004 20:441-53
• Shaffer et al. (2004) XBP1, downstream of Blimp-1, expands the secretory
apparatus and other organelles, and increases protein synthesis in plasma cell
differentiation. Immunity. 21:81-93
Reviews:
• Kurosaki, T. (2002) Regulation of B cell fates by BCR signaling components.
Curr Opin Immunol. 14(3):341-7.
• Mackay, F., Schneider,P., Rennert,P. and Browning, J. (2002) BAFF and APRIL:
A Tutorial on B Cell Survival. Annu. Rev. Immunol.; 10.114
• Kurosaki, T. (1999) Genetic analysis of B cell antigen receptor signaling. Annu.
Rev. Immunol. 17: 555-592
• Healy, J.I., and C.C. Goodnow. 1998. Positive versus negative signaling by
lymphocyte antigen receptors. Annu Rev Immunol 16:645-670.
• Ravetch, J.V. and Bolland, S. (2001) IgG Fc Receptors. Annu. Rev. Immunol.
19, 275
• MacLennan, I.C.M. 1994. Germinal Centers. Annu. Rev. Immunol. 12:117-139
• Thank you!