Download B Cell Tolerance

B Cell Tolerance
Wendy Davidson Ph.D.
May 3, 2011
Contact information:
Email:[email protected]
Tel:301-402-8399
What is B cell tolerance?
• The adaptive immune system has evolved to generate Ab against
any foreign protein.
• This unlimited repertoire will include autoreactive specificities with the
potential to cause disease. 50-75% of B cells produced in BM are
autoreactive.
• B cell tolerance is the process by which autoreactive B cells are
silenced.
• Self tolerance is achieved by a series of checkpoints that target
autoreactive B cells at different stages of development.
• Central (BM) and peripheral tolerance.
• B cells responsive to foreign Ag can be rendered tolerant by varying
the dose of Ag. High vs low zone tolerance. IgE-mediated allergy.
Ig transgenic mice are useful models for studying
mechanisms of B cell selection
• Most common model: Anti-hen egg lyzozyme (HEL) transgenic
mice crossed with mice where HEL is expressed as an endogenous
soluble self antigen (sHEL) or a membrane bound self antigen
(mHEL).
• Other models: Anti-ssDNA, -dsDNA, -rheumatoid factor, Ars/A1, Smith, -RBC and -insulin.
• Models used to demonstrate mechanisms of B cell repertoire
selection in the BM and periphery (deletion, receptor editing and
anergy).
Single Cell B Cell cloning
Uses
• BCR repertoire analysis- general and subset
specific
• B cell development
• B cell selection
• B cell response to infection or vaccines
(clonality, affinity, protection, vaccine design)
• Identification of Abs with specific properties
(e.g. Flu vaccines)
• BCR repertoire analysis in disease (e.g. SLE
before and after treatment)
Comparison of anergic B cell phenotypes in BCR
transgenic mice
Cambier JC 2007
Stages of B cell development and repertoire editing
 Ag independent 
Checkpoint 1
Checkpoint 2
Newly formed B
cells
Transitional B cells
Checkpoint 3
GC
Central tolerance
Peripheral tolerance
Cambier JC Nature, 2007
Sequence of B cell selection and maturation
CHECKPOINT 1 (Central tolerance): Possible fates of
newly formed immature autoreactive B cells in BM
Self antigens expressed in BM:
• Receptor editing
• Deletion
• Anergy
Self antigens not expressed in BM:
• Escape (Ignorance)
Possible Fates of Immature B Cells in the BM
Death by
Bimdependent
pathway
e.g. mHEL
e.g. sHEL
e.g. anti-IgG
(RF)
CHECKPOINT 1: Receptor editing
Major mechanism for eliminating autoreactive
specificities from the developing B cell repertoire
• 4-8x107 immature B cells generated/day and a high proportion
(~75%) of these are autoreactive.
• By single cell PCR analyses, ~2/3 of immature B cells in BM
have secondary rearrangements of IgL genes, exhibit Ig gene
rearrangements and continue to express RAG 1 and 2.
• Many of these cells do not make it out of the BM, since only
~1/2 of immature and mature B cells in the periphery show
evidence of receptor editing.
• Receptor editing occurs predominantly in the BM.
(F. Melchers 2006. Immunity:25:864)
CHECKPOINT 1: Ig Receptor Editing in BM
Developmental
arrest associated
with inhibition of
expression of
homing receptors
and BAFF-R and
persistent
expression of
RAG-1,-2.
Bim-dependent
apoptosis
Anergy in immature BM cells
• Anergy is a potentially reversible state of tolerance
(unresponsiveness) that develops when immature autoreactive
BM B cells bind abundant low avidity or soluble Ag that does not
produce a sufficiently strong signal to induce receptor editing or
deletion.
• Anergy is associated with developmental arrest, increased
threshold for activation by Ag, altered migration and shortened
lifespan in the periphery.
• Anergy is plastic and mechanistically diverse.
• Spectrum of anergic phenotypes - influenced by Ag reactivity.
Properties of anergic peripheral B cells in Ig
transgenic mice
• Migrate to the spleen and reside in the extra-follicular T cell zones or red pulp
and are generally excluded from the follicles.
• Retain Ag-binding capacity but are Ag unresponsive.
• Have high basal intracellular Ca++ levels, positive signaling via the BCR is
blocked.
• Short half life (2-3 days) and an increased dependence on BAFF for survival
compared to functional B cells.
• Can be rescued if BAFF is present in excess and competing non-Tg B cells
are absent or reduced in number.
• Maintenance of anergy is dependent on continuous binding of Ag and
inhibitory BCR signaling.
• Anergy can occur in immature and mature B cells and is reversible.
• Escape from anergy can result in autoimmunity.
Anergic B cells have diverse phenotypes in BCR Tg mice
Cambier JC, Nature Reviews, 2007
Factors that contribute to the increased threshold
of activation in anergic B cells
HEL and Ars/A1 models:
Ferry et.al. 2006. Transplantation 81:308-315
1.
Continuous signaling via self
Ag/BCR
2.
Decreasd surface IgM
3.
Chronic ERK and NFAT signaling
4.
Increased intracellular Ca++
5.
Constitutive activation of SHIP-1
and DOK-1
6.
Inhibition of activation of Syk and
Akt survival pathway
B cell signaling in response to acute and chronic
stimulation
Mono phosphorylation of ITAMS
Dual
phosphorylation
of ITAMS
SHIP1 and DOK activated
by LYN, modulate signaling
• Chronic BCR signaling required to maintain anergic state
Cambier JC, Nature Reviews, 2007
• Anergy reversible by removal of Ag
• Most relevant to MD4XMD5 and Ars/A1 Tg models
Anergic B cells can be rescued from death if sufficient
BAFF is available
Fas/FasL and CD40/CD40L interactions
also may contribute to the death of
anergic B cells
Ferry et.al. 2006.
Transplantation 81:308-315
Other mechanisms for rescuing autoreactive anergic B
cells
•  signaling threshold, negative signaling
CD22-/-, SHIP1-/-, FcRIIb-/- mice
•  exposure to auto Ag
Transfer autoreactive Tg B cells to auto Ag-free environment
• Cross anti-DNA and anti-Sm Tg mice to autoimmune MRL-lpr
or B6-lpr mice
• Reduce the affinity of the BCR
• Defects in cell death pathways (Fas, Bim)
Stages of B cell development and repertoire editing:
Checkpoint 2
Checkpoint 1
Checkpoint 2
Newly formed B
cells
Transitional B cells
Central tolerance
Checkpoint 3
Peripheral tolerance
GC
Cambier JC Nature, 2007
Stages of transitional B cells
BM
Spleen
Transitional B cells
Immature B
cells
T1
T2
T3
follicular B cells
++
++
+
+/++
-
++
++
++
sIgM +
sIgM
IgD-
IgD
CD93+
CD93
++
+
+
-
CD23-
CD23
-
+
+
+
CD21-
CD21
-
+
+
+
CD24 ++++
CD24
++++
+++
++
+
Rag1, Rag2 +/-
Rag1,2
-
-
-
-
CD93 = AA4.1
Non-dividing, half-life 2-4 days
CD24= HSA
Proposed Model of Human B-Cell Development
Differentiation of transitional B cells in the spleen
Dependence on BAFF for survival
BM
Increasing BAFF-R expression
SP
T1
Immature B
cells
T2
Naïve B cell
Autoantigens
Mature B cell
Autoreactive naïve B cell
Rescue
Induction of
anergy
T3
Ag removal and decreased stimulation
threshold
Anergic B cell
Significant cell loss
Death
Only ~5% of immature B cells
produced in BM enter the
mature B cell pool.
J. Cambier, Immunity 2006; M. Cancro, Immunol. Rev. 2004
Properties of BAFF
• BAFF (BLyS, TALL-1, THANK, zTNF4) and APRIL, a related cytokine,
are members of the TNF super family.
• BAFF interacts with three receptors BAFF-R (BR3), BCMA and TACI
expressed predominantly on B lineage cells.
• APRIL binds to BCMA and TACI only.
• BAFF is produced predominantly by myeloid cells and acts on
transitional, naive and mature B cells.
• BAFF is essential for normal B cell development and survival in the
periphery.
• BAFF-deficient mice are deficient in B2 cells and MZ B cells but have
normal numbers of B1 cells.
• BAFF Tg mice have excess mature B cells, especially MZ B cells, and
develop systemic autoimmunity and B cell lymphomas with age.
BAFF governs successful transitional B cell
differentiation by enhancing survival
• Interactions between BAFF and BAFF-R are essential for
maturation of transitional B cells.
• BAFF-R-deficient mice have severely impeded transitional B cell
differentiation and mature B cells with significantly shortened
lifespans. TACI and BCMA KO mice exhibit normal transitional B
cell differentiation.
• BAFF-R expression increases as T1 cells differentiate into T2
cells.
• Competition for available BAFF dictates the lifespan of anergic B
cells and resting transitional, naïve and mature B cells.
Mechanisms of BAFF-induced survival of B cells
• BAFF supports the survival of transitional and mature B cells
without inducing proliferation.
• BAFF contributes to B cell survival by inducing the expression of prosurvival members of the Bcl-2 family and interfering with the nuclear
translocation of pro-apoptotic PKC to the nucleus.
• BCR ligation upregulates expression of BAFF-R, but not TACI or
BCMA, on late transitional and mature B cells.
• Both BCR- and BAFF-R mediated signals appear to be essential for
repertoire selection and survival of mature B cells.
Influence of excess BAFF on the selection of selfreactive B cells
Models 1 and 2: Excess BAFF does not rescue cells deleted early in
development.
Models 3 and 4: Responsiveness to excess BAFF corresponds to a
maturational change in T2 cells involving expression of BAFF-R.
Thien et. al. 2004. Immunity 20:785-798.
Anergic B Cells are Susceptible to Fas-mediated Death
Differentiation of transitional B cells in the spleen
Dependence on BAFF for survival
BM
Increasing BAFF-R expression
SP
T1
Immature B
cells
T2
Naïve B cell
Autoantigens
Mature B cell
Autoreactive naïve B cell
Rescue
Induction of
anergy
T3
Ag removal and decreased stimulation
threshold
Anergic B cell
Significant cell loss
Death
Only ~5% of immature B cells
produced in BM enter the
mature B cell pool.
J. Cambier, Immunity 2006; M. Cancro, Immunol. Rev. 2004
Evidence that anergic B cells and T3 cells
have similar properties
• Both have a similar surface phenotype (CD93+ CD23+ IgMlo, CD24inter,
IgDhi) and lifespan.
• Non-autoreactive BCR transgenic mice have few T3 cells suggesting
that this population may not represent a developmental stage between
T2 and naïve B cells.
• Maintenance of the T3 phenotype requires continuous antigen exposure
• T1 and T2 B cells are only found in the blood and spleen. Anergic B
cells with the phenotype of T3 cells are detectable in spleen, LN and
blood.
• Anergic B cells and T3 cells have similar abnormalities in BCR signaling
and have constitutively activated ERK and DOK-1.
• T3 cells from normal mice are enriched for autoreactive specificities and
have a similar gene expression profile to HEL Tg anergic B cells.
Potential dangers of having large numbers of anergized B
cells produced during B cell selection.
• The T3/anergic B cell population in normal mice ranges from 1-5x106
cells/spleen and ~50% of these are replaced every 4 days (Merrell et.al.
2006.Immunity 25:953-962).
• Previous estimates suggest that ~5x106 new B cells arrive in the spleen
every 4 days (Rolink et.al. 1998. EJI 28:3738-3748).
• Therefore, a significant proportion (upto ~50%) of the B cells entering
the spleen every four days from BM may be anergic or become anergic
(Merrell et.al. 2006.Immunity 25:953-962).
• Since anergy is potentially reversible, having large numbers of anergic
autoreactive B cells in the periphery poses a significant risk of
autoimmunity. However, under normal circumstances, this danger is
likely kept in check by the short half life of anergic B cells. (Merrell et.al.
2006.Immunity 25:953-962).
Possible mechanisms for rescue and
activation of autoreactive anergic B cells
1.
Increased availability of BAFF
2.
Removal of self Ag
3.
High avidity Ag stimulation + T cell
help
4.
TLR signals
5.
Defects in Fas, Bim signaling
6.
Altered signaling threshold
AUTOIMMUNITY
Ferry et.al. 2006. Transplantation 81:308-315
RESCUE
CHECKPOINT 3:
Elimination of autoreactive
B cells generated in GC
Autoreactive B cells
generated in germinal
centers by somatic
hypermutation normally
are eliminated.
Possible role for
Fas/FasL.
Lack of T
cell help
Defects in the induction and control of B cell
tolerance lead to systemic autoimmunity
Topics for lecture 2:
• Sources of autoreactive B cells (incompletely tolerized
cells, ignorant B cells)
• Sites and mechanisms of activation of autoreactive B
cells (antigens, TLRs, extrafollicular responses)
• Contributions of autoreactive B cells to disease
(effectors and APC)