B Cell Tolerance Wendy Davidson Ph.D. May 3, 2011 Contact information: Email:[email protected] Tel:301-402-8399.

B Cell Tolerance

Wendy Davidson Ph.D.

May 3, 2011

Contact information:

Email:[email protected]

Tel:301-402-8399

Factors that contribute to the increased threshold of activation in anergic B cells

HEL and Ars/A1 models:

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

Ferry et.al. 2006. Transplantation 81:308-315

B cell signaling in response to acute and chronic stimulation

Cambier JC, Nature Reviews, 2007

Dual phosphorylation

of ITAMS

Mono phosphorylation of ITAMS

SHIP1 and DOK activated by LYN, modulate signaling

• Chronic BCR signaling required to maintain anergic state

• 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-/-, FcRIIb-/- 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

Cambier JC Nature, 2007

Checkpoint 1Newly formed B cells

Checkpoint 2Transitional B cells

Checkpoint 3GCCentral tolerance Peripheral tolerance

Stages of transitional B cells

BM Spleen

Immature B cells

sIgM +

IgD-

CD93+

CD23-

CD21-

CD24 ++++

Rag1, Rag2 +/-

Transitional B cells

T1 T2 T3 follicular B cells

sIgM ++ ++ + +/++

IgD - ++ ++ ++

CD93 ++ + + -

CD23 - + + +

CD21 - + + +

CD24 ++++ +++ ++ +

Rag1,2 - - - -

Non-dividing, half-life 2-4 daysCD93 = AA4.1

CD24= HSA

Proposed Model of Human B-Cell Development

Differentiation of transitional B cells in the spleen

Immature B cells

T1 T2 Naïve B cell

Autoantigens

Anergic B cell

T3Ag removal and decreased stimulation

threshold

Autoreactive naïve B cell

Induction of anergy

Increasing BAFF-R expression

J. Cambier, Immunity 2006; M. Cancro, Immunol. Rev. 2004

Significant cell loss

Only ~5% of immature B cells produced in BM enter the mature B cell pool.

Mature B cell

Dependence on BAFF for survival

BM SP

Rescue

Death

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 pro-survival 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.

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.

Influence of excess BAFF on the selection of self-reactive B cells

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

Immature B cells

T1 T2 Naïve B cell

Autoantigens

Anergic B cell

T3Ag removal and decreased stimulation

threshold

Autoreactive naïve B cell

Induction of anergy

Increasing BAFF-R expression

J. Cambier, Immunity 2006; M. Cancro, Immunol. Rev. 2004

Significant cell loss

Only ~5% of immature B cells produced in BM enter the mature B cell pool.

Mature B cell

Dependence on BAFF for survival

BM SP

Rescue

Death

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

Ferry et.al. 2006. Transplantation 81:308-315

RESCUE

AUTOIMMUNITY

Autoreactive B cells generated in germinal centers by somatic hypermutation normally are eliminated. Possible role for Fas/FasL.

Lack of T cell help

CHECKPOINT 3: Elimination of autoreactiveB cells generated in GC

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)