Impaired receptor editing in the primary B cell repertoire of BASH-deficient mice

Kidins220 regulates the development of B cells bearing the λ light chain

The ratio between κ and λ light chain (LC)-expressing B cells varies considerably between species. We recently identified Kinase D-interacting substrate of 220 kDa (Kidins220) as an interaction partner of the BCR. In vivo ablation of Kidins220 in B cells resulted in a marked reduction of λLC-expressing B cells. Kidins220 knockout B cells fail to open and recombine the genes of the Igl locus, even in genetic scenarios where the Igk genes cannot be rearranged or where the κLC confers autoreactivity. Igk gene recombination and expression in Kidins220-deficient B cells is normal. Kidins220 regulates the development of λLC B cells by enhancing the survival of developing B cells and thereby extending the time-window in which the Igl locus opens and the genes are rearranged and transcribed. Further, our data suggest that Kidins220 guarantees optimal pre-BCR and BCR signaling to induce Igl locus opening and gene recombination during B cell development and receptor editing.

IgM to IgG Class Switching Is a Necessary Step for Pemphigus Phenotype Induction in Desmoglein 3-Specific B Cell Receptor Knock-in Mouse

Pemphigus vulgaris is an autoimmune blistering disease caused by IgG targeting desmoglein 3 (Dsg3), an adhesion molecule of keratinocytes. Anti-Dsg3 IgG production is prevented in healthy individuals, but it is unclear how Dsg3-specific B cells are regulated. To clarify the immunological condition regulating Dsg3-specific B cells, a pathogenic anti-Dsg3 Ig (AK23) knock-in mouse was generated. AK23 knock-in B cells developed normally without undergoing deletion or acquiring an anergic phenotype in vivo. The knock-in B cells showed Ca²⁺ influx upon IgM cross-linking and differentiated into AK23-IgG⁺ B cells after LPS and IL-4 stimulation in vitro that induced a pemphigus phenotype after adoptive transfer into Rag2 ^-/- mice. However, the knock-in mouse itself produced AK23-IgM but little IgG without blisters in vivo. Dsg3 immunization and skin inflammation caused AK23-IgG production and a pemphigus phenotype in vivo. Furthermore, Fcgr2b deficiency or haploinsufficiency spontaneously induced AK23-IgG production and a pemphigus phenotype with poor survival rates in AK23 knock-in mice. To assess Fcgr2b involvement in Ig class-switch efficiency, postswitch transcripts of B cells were quantified and significantly higher in Fcgr2b ^-/- and Fcgr2b ^+/- mice than wild-type mice in a gene dose-dependent manner. Finally, RNA sequencing revealed reduced expression of FCGR2B and FcγRIIB-related genes in patient B cells. These results indicated that Dsg3-specific B cells do not spontaneously perform pathogenic class switching in vivo, and pemphigus phenotype induction was prevented under normal conditions. Attenuated FcγRIIB signaling is also one of the drivers for pathogenic class switching and is consistent with immunological features identified from clinical samples. This study unveiled a characteristic immune state silencing autoreactive B cells in mice.

C/EBPβ induces B-cell acute lymphoblastic leukemia and cooperates with BLNK mutations

BLNK (BASH/SLP‐65) encodes an adaptor protein that plays an important role in B‐cell receptor (BCR) signaling. Loss‐of‐function mutations in this gene are observed in human pre‐B acute lymphoblastic leukemia (ALL), and a subset of Blnk knock‐out (KO) mice develop pre‐B‐ALL. To understand the molecular mechanism of the Blnk mutation‐associated pre‐B‐ALL development, retroviral tagging was applied to KO mice using the Moloney murine leukemia virus (MoMLV). The Blnk mutation that significantly accelerated the onset of MoMLV‐induced leukemia and increased the incidence of pre‐B‐ALL Cebpb was identified as a frequent site of retroviral integration, suggesting that its upregulation cooperates with Blnk mutations. Transgenic expression of the liver‐enriched activator protein (LAP) isoform of Cebpb reduced the number of mature B‐lymphocytes in the bone marrow and inhibited differentiation at the pre‐BI stage. Furthermore, LAP expression significantly accelerated leukemogenesis in Blnk KO mice and alone acted as a B‐cell oncogene. Furthermore, an inverse relationship between BLNK and C/EBPβ expression was also noted in human pre‐B‐ALL cases, and the high level of CEBPB expression was associated with short survival periods in patients with BLNK‐downregulated pre‐B‐ALL. These results indicate the association between the C/EBPβ transcriptional network and BCR signaling in pre‐B‐ALL development and leukemogenesis. This study gives insight into ALL progression and suggests that the BCR/C/EBPβ pathway can be a therapeutic target.

Mechanisms of central tolerance for B cells

Immune tolerance hinders the potentially destructive responses of lymphocytes to host tissues. Tolerance is regulated at the stage of immature B cell development (central tolerance) by clonal deletion, involving apoptosis, and by receptor editing, which reprogrammes the specificity of B cells through secondary recombination of antibody genes. Recent mechanistic studies have begun to elucidate how these divergent mechanisms are controlled. Single-cell antibody cloning has revealed defects of B cell central tolerance in human autoimmune diseases and in several human immunodeficiency diseases caused by single gene mutations, which indicates the relevance of B cell tolerance to disease and suggests possible genetic pathways that regulate tolerance.

Intra-Golgi formation of IgM-glycosaminoglycan complexes promotes Ig deposition

Immune complexes arise from interactions between secreted Ab and Ags in the surrounding milieu. However, it is not known whether intracellular Ag-Ab interactions also contribute to the formation of extracellular immune complexes. In this study, we report that certain murine B cell hybridomas accumulate intracellular IgM and release large, spherical IgM complexes. The complexes (termed "spherons") reach 2 μm in diameter, detach from the cell surface, and settle out of solution. The spherons contain IgM multimers that incorporate the J chain and resist degradation by endoglycosidase H, arguing for IgM passage through the Golgi. Treatment of cells with inhibitors of proteoglycan synthesis, or incubation of spherons with chondroitinase ABC, degrades spherons, indicating that spheron formation and growth depend on interactions between IgM and glycosaminoglycans. This inference is supported by direct binding of IgM to heparin and hyaluronic acid. We conclude that, as a consequence of IgM binding to glycosaminoglycans, multivalent IgM-glycan complexes form in transit of IgM to the cell surface. Intra-Golgi formation of immune complexes could represent a new pathogenic mechanism for immune complex deposition disorders.

B cell receptor editing in tolerance and autoimmunity

Receptor editing is the process of ongoing antibody gene rearrangement in a lymphocyte that already has a functional antigen receptor. The expression of a functional antigen receptor will normally terminate further rearrangement (allelic exclusion). However, lymphocytes with autoreactive receptors have a chance at escaping negative regulation by "editing" the specificities of their receptors with additional antibody gene rearrangements. As such, editing complicates the Clonal Selection Hypothesis because edited cells are not simply endowed for life with a single, invariant antigen receptor. Furthermore, if the initial immunoglobulin gene is not inactivated during the editing process, allelic exclusion is violated and the B cell can exhibit two specificities. Here, we describe the discovery of editing, the pathways of receptor editing at the heavy (H) and light (L) chain loci, and current evidence regarding how and where editing happens and what effects it has on the antibody repertoire.

Role of PI3K in the generation and survival of B cells

Engagement of the B-cell antigen receptor (BCR) or its precursor, the pre-BCR, induces a cascade of biochemical reactions that regulate the differentiation, selection, survival, and activation of B cells. This cascade is initiated by receptor-associated tyrosine kinases that activate multiple downstream signaling pathways. Since it is required for metabolism, cell growth, development, and survival, the activation of phosphoinositide 3-kinase (PI3K)-dependent pathways represents a crucial event of BCR/pre-BCR signaling. The phosphorylated substrates of the PI3K promote specific recruitment of selected signaling proteins to the plasma membrane, where important signaling complexes are formed to mediate the above-mentioned biological processes. Here, we review the principles of PI3K signaling and highlight the role of an important PI3K-driven module in VDJ recombination of immunoglobulin (Ig) genes during early B-cell development as compared with class switch recombination of Ig genes in mature B cells after activation by specific antigens. Furthermore, we discuss the role of PI3K in the survival of mature B cells, which is strictly dependent on BCR expression and basal BCR signaling.

Tolerance checkpoints in B-cell development: Johnny B good

B-cell development up to the immature B-cell stage takes place in the bone marrow, while final maturation into mature B cells occurs in the spleen. During differentiation, the precursor and immature B cells have to pass several checkpoints, including those in which they are censored for being auto-reactive, and therefore being potentially dangerous. Numerous studies have shown that the immature B-cell stage in the bone marrow and the transitional B-cell stages in the spleen comprise distinct checkpoints at which auto-reactivity is censored. Recently, evidence has been provided that the large pre-BII stage in the bone marrow, at which the pre-BCR is expressed, is yet another B-cell tolerance checkpoint. Here, we review these findings and speculate on directions for possible further experimentation.

Developmental acquisition of the Lyn-CD22-SHP-1 inhibitory pathway promotes B cell tolerance

To better understand whether autoimmunity in Lyn-deficient mice arises from compromised central or peripheral B cell tolerance, we examined BCR signaling properties of wild-type and Lyn-deficient B cells at different stages of development. Wild-type mature follicular B cells were less sensitive to BCR stimulation than were immature transitional stage 1 B cells with regard to BCR-induced calcium elevation and ERK MAPK activation. In the absence of Lyn, mature B cell signaling was greatly enhanced, whereas immature B cell signaling was minimally affected. Correspondingly, Lyn deficiency substantially enhanced the sensitivity of mature B cells to activation via the BCR, but minimally affected events associated with tolerance induction at the immature stage. The effects of CD22 deficiency on BCR signaling were very similar in B cells at different stages of maturation. These results indicate that the Lyn-CD22-Src homology region 2 domain-containing phosphatase-1 inhibitory pathway largely becomes operational as B cell mature, and sets a threshold for activation that appears to be critical for the maintenance of tolerance in the B cell compartment.

Regulation of B-cell proliferation and differentiation by pre-B-cell receptor signalling

The pre-B-cell receptor (pre-BCR) is expressed following the productive recombination of the immunoglobulin heavy chain gene. Signals through the pre-BCR are required for initiating diverse processes in pre-B cells, including proliferation and recombination of the light chain gene, which eventually lead to the differentiation of pre-B cells to immature B cells. However, the molecular mechanisms by which the pre-BCR promotes these processes remain largely unresolved. Recent findings suggest that forkhead box O (FOXO) transcription factors connect pre-BCR signalling to the activation of the recombination machinery. In this Review, we discuss how FOXO transcription factors are regulated by the pre-BCR to allow the progression of the cell cycle and the recombination of the light chain gene.

Signaling pathways regulating RAG expression in B lymphocytes

Development of B-cell lymphopoiesis is dependent on the presence of recombination activating genes RAG1 and RAG2 enzymes. They control the rearrangements of immunoglobulin variable, diversity and joining region segments, and allow progression of the cellular maturation. RAG1 and RAG2 are successively up- and down-regulated at each B-cell stage to progressively generate a B-cell receptor for which unforeseeable antigenic specificity results from a stochastic process. Therefore, in autoreactive immature B cells, new round of RAG re-expression can be observed to eliminate self-reactivity. In some circumstances, RAG up-regulation can also be found in peripheral mature B lymphocytes, specifically in autoimmune diseases. It is therefore of utmost importance to unravel signaling pathways that trigger RAG induction in normal and pathological conditions. Therapeutic modulation of cytokines or intracellular contacts involved in RAG expression might restrict the development of inappropriate autoimmune repertoire.

SLP-65 regulates immunoglobulin light chain gene recombination through the PI(3)K-PKB-Foxo pathway

Although the essential role of the adaptor protein SLP-65 in pre-B cell differentiation is established, the molecular mechanism underlying its function is poorly understood. In this study, we uncover a link between SLP-65-dependent signaling and the phosphoinositide-3-OH kinase (PI(3)K)-protein kinase B (PKB)-Foxo pathway. We show that the forkhead box transcription factor Foxo3a promotes light chain rearrangement in pre-B cells. Our data suggest that PKB suppresses light chain recombination by phosphorylating Foxo proteins, whereas reconstitution of SLP-65 function counteracts PKB activation and promotes Foxo3a and Foxo1 activity in pre-B cells. Together, these data illuminate a molecular function of SLP-65 and identify a key role for Foxo proteins in the regulation of light chain recombination, receptor editing and B cell selection.

A role for interferon regulatory factor 4 in receptor editing

Receptor editing is the primary means through which B cells revise antigen receptors and maintain central tolerance. Previous studies have demonstrated that interferon regulatory factor 4 (IRF-4) and IRF-8 promote immunoglobulin light-chain rearrangement and transcription at the pre-B stage. Here, the roles of IRF-4 and -8 in receptor editing were analyzed. Our results show that secondary rearrangement was impaired in IRF-4 but not IRF-8 mutant mice, suggesting that receptor editing is defective in the absence of IRF-4. The role of IRF-4 in receptor editing was further examined in B-cell-receptor (BCR) transgenic mice. Our results show that secondary rearrangement triggered by membrane-bound antigen was defective in the IRF-4-deficient mice. Our results further reveal that the defect in secondary rearrangement is more severe at the immunoglobulin lambda locus than at the kappa locus, indicating that IRF-4 is more critical for the lambda rearrangement. We provide evidence demonstrating that the expression of IRF-4 in immature B cells is rapidly induced by self-antigen and that the reconstitution of IRF-4 expression in the IRF-4 mutant immature B cells promotes secondary rearrangement. Thus, our studies identify IRF-4 as a nuclear effector of a BCR signaling pathway that promotes secondary rearrangement at the immature B-cell stage.

Phospholipase Cgamma2 contributes to light-chain gene activation and receptor editing

Phospholipase Cgamma2 (PLCgamma2) is critical for pre-B-cell receptor (pre-BCR) and BCR signaling. Current studies discovered that PLCgamma2-deficient mice had reduced immunoglobulin lambda (Iglambda) light-chain usage throughout B-cell maturation stages, including transitional type 1 (T1), transitional type 2 (T2), and mature follicular B cells. The reduction of Iglambda rearrangement by PLCgamma2 deficiency was not due to specifically increased apoptosis or decreased proliferation of mutant Iglambda+ B cells, as lack of PLCgamma2 exerted a similar effect on apoptosis and proliferation of both Iglambda+ and Igkappa+ B cells. Moreover, PLCgamma2-deficient IgHEL transgenic B cells exhibited an impairment of antigen-induced receptor editing among both the endogenous lambda and kappa loci in vitro and in vivo. Importantly, PLCgamma2 deficiency impaired BCR-induced expression of IRF-4 and IRF-8, the two transcription factors critical for lambda and kappa light-chain rearrangements. Taken together, these data demonstrate that the PLCgamma2 signaling pathway plays a role in activation of light-chain loci and contributes to receptor editing.

Basal B cell receptor-directed phosphatidylinositol 3-kinase signaling turns off RAGs and promotes B cell-positive selection

PI3K plays key roles in cell growth, differentiation, and survival by generating the second messenger phosphatidylinositol-(3,4,5)-trisphosphate (PIP3). PIP3 activates numerous enzymes, in part by recruiting them from the cytosol to the plasma membrane. We find that in immature B lymphocytes carrying a nonautoreactive Ag receptor, PI3K signaling suppresses RAG expression and promotes developmental progression. Inhibitors of PI3K signaling abrogate this positive selection. Furthermore, immature primary B cells from mice lacking the p85alpha regulatory subunit of PI3K suppress poorly RAG expression, undergo an exaggerated receptor editing response, and, as in BCR-ligated cells, fail to progress into the G1 phase of cell cycle. Moreover, immature B cells carrying an innocuous receptor have sustained elevation of PIP3 levels and activation of the downstream effectors phospholipase C (PLC)gamma2, Akt, and Bruton's tyrosine kinase. Of these, PLCgamma2 appears to play the most significant role in down-regulating RAG expression. It therefore appears that when the BCR of an immature B cell is ligated, PIP3 levels are reduced, PLCgamma2 activation is diminished, and receptor editing is promoted by sustained RAG expression. Taken together, our results provide evidence that PI3K signaling is an important cue required for fostering development of B cells carrying a useful BCR.

Central and Peripheral RAG Protein Re-expression: Underestimate Mechanisms of Tolerance?

The generation of developing B cells in the bone marrow is regulated by recombination activating genes RAG1 and RAG2 proteins. They contribute to the synthesis of functional antibodies (Abs) that can present self-reactivities following V(D)J (V, variable; D, diversity and J, joining) recombination. The emergence of autoreactive B cells is prevented by deletion through apoptosis, by stimulation blockade through anergy, or by synthesis of a new B-cell receptor through receptor edition. In the periphery, somatic hypermutation during the course of germinal centre (GC) responses can lead to the appearance of autoreactive and low-affinity Ab-producing B cells. Apoptotic deletion and receptor revision regulate these autoreactive and inappropriate B cells. Moreover, the presence of RAG-positive B cells outside GCs suggest that still uncharacterized regulation checkpoint, associated with secondary V(D)J recombination, also contribute to the regulation of autoreactivities. Failure in central and/or peripheral tolerance mechanisms associated with RAG expression could contribute to the terminal differentiation of autoreactive B cells leading to autoimmune states.

Bruton's tyrosine kinase and SLP-65 regulate pre-B cell differentiation and the induction of Ig light chain gene rearrangement

Bruton's tyrosine kinase (Btk) and the adapter protein SLP-65 (Src homology 2 domain-containing leukocyte-specific phosphoprotein of 65 kDa) transmit precursor BCR (pre-BCR) signals that are essential for efficient developmental progression of large cycling into small resting pre-B cells. We show that Btk- and SLP-65-deficient pre-B cells have a specific defect in Ig lambda L chain germline transcription. In Btk/SLP-65 double-deficient pre-B cells, both kappa and lambda germline transcripts are severely reduced. Although these observations point to an important role for Btk and SLP-65 in the initiation of L chain gene rearrangement, the possibility remained that these signaling molecules are only required for termination of pre-B cell proliferation or for pre-B cell survival, whereby differentiation and L chain rearrangement is subsequently initiated in a Btk/SLP-65-independent fashion. Because transgenic expression of the antiapoptotic protein Bcl-2 did not rescue the developmental arrest of Btk/SLP-65 double-deficient pre-B cells, we conclude that defective L chain opening in Btk/SLP-65-deficient small resting pre-B cells is not due to their reduced survival. Next, we analyzed transgenic mice expressing the constitutively active Btk mutant E41K. The expression of E41K-Btk in Ig H chain-negative pro-B cells induced 1) surface marker changes that signify cellular differentiation, including down-regulation of surrogate L chain and up-regulation of CD2, CD25, and MHC class II; and 2) premature rearrangement and expression of kappa and lambda light chains. These findings demonstrate that Btk and SLP-65 transmit signals that induce cellular maturation and Ig L chain rearrangement independently of their role in termination of pre-B cell expansion.

Receptor editing for better or for worse

Receptor editing has emerged from its original identification as a minor secondary mechanism of B cell tolerance to be considered as a dominant mechanism by which autoreactive immature B cells are rendered tolerant. Clonal deletion, previously regarded as the major mechanism of central B cell tolerance, has been shown by recent studies to operate secondarily and only when receptor editing is unable to provide a non-autoreactive specificity. Receptor editing has also been shown to operate during the development of wild-type B lymphocytes, and ongoing investigations demonstrate the influence of particular signaling molecules in the induction and/or inhibition of receptor editing. Together, these studies begin to map the signaling pathways that regulate receptor editing in autoreactive and non-autoreactive immature B cells.

Involvement of SLP-65 and Btk in tumor suppression and malignant transformation of pre-B cells

Signals from the precursor-B cell receptor (pre-BCR) are essential for selection and clonal expansion of pre-B cells that have performed productive immunoglobulin heavy chain V(D)J recombination. In the mouse, the downstream signaling molecules SLP-65 and Btk cooperate to limit proliferation and induce differentiation of pre-B cells, thereby acting as tumor suppressors to prevent pre-B cell leukemia. In contrast, recent observations in human BCR-ABL1(+) pre-B lymphoblastic leukemia cells demonstrate that Btk is constitutively phosphorylated and activated by the BCR-ABL1 fusion protein. As a result, activated Btk transmits survival signals that are essential for the transforming activity of oncogenic Abl tyrosine kinase.