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B Cell Tolerance

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  1. B Cell Tolerance Wendy Davidson Ph.D. May 3, 2011 Contact information: Email:davidsonw@niaid.nih.gov Tel:301-402-8399

  2. 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.

  3. 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).

  4. 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)

  5. Comparison of anergic B cell phenotypes in BCR transgenic mice Cambier JC 2007

  6. Stages of B cell development and repertoire editing  Ag independent  Checkpoint 1 Newly formed B cells Checkpoint 2 Transitional B cells Checkpoint 3 GC Central tolerance Peripheral tolerance Cambier JC Nature, 2007

  7. Sequence of B cell selection and maturation

  8. 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)

  9. Possible Fates of Immature B Cells in the BM Death by Bim-dependent pathway e.g. anti-IgG (RF) e.g. mHEL e.g. sHEL

  10. 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)

  11. 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

  12. 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.

  13. 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.

  14. Anergic B cells have diverse phenotypes in BCR Tg mice Cambier JC, Nature Reviews, 2007

  15. Factors that contribute to the increased threshold of activation in anergic B cells HEL and Ars/A1 models: Continuous signaling via self Ag/BCR Decreasd surface IgM Chronic ERK and NFAT signaling Increased intracellular Ca++ Constitutive activation of SHIP-1 and DOK-1 Inhibition of activation of Syk and Akt survival pathway Ferry et.al. 2006. Transplantation 81:308-315

  16. 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 • Anergy reversible by removal of Ag • Most relevant to MD4XMD5 and Ars/A1 Tg models Cambier JC, Nature Reviews, 2007

  17. 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

  18. 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)

  19. Stages of B cell development and repertoire editing: Checkpoint 2 Checkpoint 1 Newly formed B cells Checkpoint 2 Transitional B cells Checkpoint 3 GC Central tolerance Peripheral tolerance Cambier JC Nature, 2007

  20. Stages of transitional B cells BM Spleen Transitional B cells T1 T2 T3 follicular B cells sIgM ++ ++ + +/++ IgD - ++ ++ ++ CD93 ++ + + - CD23 - + + + CD21 - + + + CD24 ++++ +++ ++ + Rag1,2 - - - - Immature B cells sIgM + IgD- CD93+ CD23- CD21- CD24 ++++ Rag1, Rag2 +/- CD93 = AA4.1 CD24= HSA Non-dividing, half-life 2-4 days

  21. Proposed Model of Human B-Cell Development

  22. Differentiation of transitional B cells in the spleen Dependence on BAFF for survival BM SP Increasing BAFF-R expression T1 T2 Naïve B cell Mature B cell Immature B cells Autoantigens Autoreactive naïve B cell Rescue Induction of anergy Ag removal and decreased stimulation threshold T3 Anergic B cell Death Only ~5% of immature B cells produced in BM enter the mature B cell pool. Significant cell loss J. Cambier, Immunity 2006; M. Cancro, Immunol. Rev. 2004

  23. 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.

  24. 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.

  25. 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.

  26. Influence of excess BAFF on the selection of self-reactive 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.

  27. Anergic B Cells are Susceptible to Fas-mediated Death

  28. Differentiation of transitional B cells in the spleen Dependence on BAFF for survival BM SP Increasing BAFF-R expression T1 T2 Naïve B cell Mature B cell Immature B cells Autoantigens Autoreactive naïve B cell Rescue Induction of anergy Ag removal and decreased stimulation threshold T3 Anergic B cell Death Only ~5% of immature B cells produced in BM enter the mature B cell pool. Significant cell loss J. Cambier, Immunity 2006; M. Cancro, Immunol. Rev. 2004

  29. 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.

  30. Potential dangers of having large numbers of anergized B cells produced during B cell selection. • TheT3/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).

  31. Possible mechanisms for rescue and activation of autoreactive anergic B cells Increased availability of BAFF Removal of self Ag High avidity Ag stimulation + T cell help TLR signals Defects in Fas, Bim signaling Altered signaling threshold RESCUE AUTOIMMUNITY Ferry et.al. 2006. Transplantation 81:308-315

  32. 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

  33. 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)