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

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.

G-CSF drives autoinflammation in APLAID

Missense mutations in PLCG2 can cause autoinflammation with phospholipase C gamma 2-associated antibody deficiency and immune dysregulation (APLAID). Here, we generated a mouse model carrying an APLAID mutation (p.Ser707Tyr) and found that inflammatory infiltrates in the skin and lungs were only partially ameliorated by removing inflammasome function via the deletion of caspase-1. Also, deleting interleukin-6 or tumor necrosis factor did not fully prevent APLAID mutant mice from autoinflammation. Overall, these findings are in accordance with the poor response individuals with APLAID have to treatments that block interleukin-1, JAK1/2 or tumor necrosis factor. Cytokine analysis revealed increased granulocyte colony-stimulating factor (G-CSF) levels as the most distinct feature in mice and individuals with APLAID. Remarkably, treatment with a G-CSF antibody completely reversed established disease in APLAID mice. Furthermore, excessive myelopoiesis was normalized and lymphocyte numbers rebounded. APLAID mice were also fully rescued by bone marrow transplantation from healthy donors, associated with reduced G-CSF production, predominantly from non-hematopoietic cells. In summary, we identify APLAID as a G-CSF-driven autoinflammatory disease, for which targeted therapy is feasible.

The role of PLCγ2 in immunological disorders, cancer, and neurodegeneration

Phosphatidylinositol-specific phospholipase Cγ2 (PLCγ2) is a critical signaling molecule activated downstream from a variety of cell surface receptors that contain an intracellular immunoreceptor tyrosine-based activation motif. These receptors recruit kinases such as Syk, BTK, and BLNK to phosphorylate and activate PLCγ2, which then generates 1D-myo-inositol 1,4,5-trisphosphate and diacylglycerol. These well-known second messengers are required for diverse membrane functionality including cellular proliferation, endocytosis, and calcium flux. As a result, PLCγ2 dysfunction is associated with a variety of diseases including cancer, neurodegeneration, and immune disorders. The diverse pathologies associated with PLCγ2 are exemplified by distinct genetic variants. Inherited mutations at this locus cause PLCγ2-associated antibody deficiency and immune dysregulation, in some cases with autoinflammation. Acquired mutations at this locus, which often arise as a result of BTK inhibition to treat chronic lymphocytic leukemia, result in constitutive downstream signaling and lymphocyte proliferation. Finally, a third group of PLCγ2 variants actually has a protective effect in a variety of neurodegenerative disorders, presumably by increased uptake and degradation of deleterious neurological aggregates. Therefore, manipulating PLCγ2 activity either up or down could have therapeutic benefit; however, we require a better understanding of the signaling pathways propagated by these variants before such clinical utility can be realized. Here, we review the signaling roles of PLCγ2 in hematopoietic cells to help understand the effect of mutations driving immune disorders and cancer and extrapolate from this to roles which may relate to protection against neurodegeneration.

PLCγ-dependent mTOR signalling controls IL-7-mediated early B cell development

The precise molecular mechanism underlying the regulation of early B cell lymphopoiesis is unclear. The PLCγ signaling pathway is critical for antigen receptor-mediated lymphocyte activation, but its function in cytokine signaling is unknown. Here we show that PLCγ1/PLCγ2 double deficiency in mice blocks early B cell development at the pre-pro-B cell stage and renders B cell progenitors unresponsive to IL-7. PLCγ pathway inhibition blocks IL-7-induced activation of mTOR, but not Stat5. The PLCγ pathway activates mTOR through the DAG/PKC signaling branch, independent of the conventional Akt/TSC/Rheb signaling axis. Inhibition of PLCγ/PKC-induced mTOR activation impairs IL-7-mediated B cell development. PLCγ1/PLCγ2 double-deficient B cell progenitors have reduced expression of genes related to B cell lineage, IL-7 signaling, and cell cycle. Thus, IL-7 receptor controls early B lymphopoiesis through activation of mTOR via PLCγ/DAG/PKC signaling, not via Akt/Rheb signaling.

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.

Kidins220/ARMS binds to the B cell antigen receptor and regulates B cell development and activation

Fiala et al. report that Kidins220/ARMS is a novel interactor of the B cell antigen receptor (BCR) and its deletion impairs B cell development and B cell functioning.

B cell antigen receptor (BCR) signaling is critical for B cell development and activation. Using mass spectrometry, we identified a protein kinase D–interacting substrate of 220 kD (Kidins220)/ankyrin repeat–rich membrane-spanning protein (ARMS) as a novel interaction partner of resting and stimulated BCR. Upon BCR stimulation, the interaction increases in a Src kinase–independent manner. By knocking down Kidins220 in a B cell line and generating a conditional B cell–specific Kidins220 knockout (B-KO) mouse strain, we show that Kidins220 couples the BCR to PLCγ2, Ca²⁺, and extracellular signal-regulated kinase (Erk) signaling. Consequently, BCR-mediated B cell activation was reduced in vitro and in vivo upon Kidins220 deletion. Furthermore, B cell development was impaired at stages where pre-BCR or BCR signaling is required. Most strikingly, λ light chain–positive B cells were reduced sixfold in the B-KO mice, genetically placing Kidins220 in the PLCγ2 pathway. Thus, our data indicate that Kidins220 positively regulates pre-BCR and BCR functioning.

PLAID: a Syndrome of Complex Patterns of Disease and Unique Phenotypes

PLCG2 associated antibody deficiency and immune dysregulation (PLAID) is a complex dominantly inherited disease characterized almost universally by cold urticaria, and variably by recurrent bacterial infection, autoimmunty and skin granuloma formation. Several striking phenotypes can emerge from this disease, and the pathophysiology leads to a complex mix of loss and gain of function in cellular signaling. This review discusses the key phenotypic characteristics and pathophysiologic observations seen in PLAID, and contrasts PLAID to several related disorders in order to best contextualize this fascinating disease.

VprBP Is Required for Efficient Editing and Selection of Igκ+ B Cells, but Is Dispensable for Igλ+ and Marginal Zone B Cell Maturation and Selection

B cell development past the pro-B cell stage in mice requires the Cul4-Roc1-DDB1 E3 ubiquitin ligase substrate recognition subunit VprBP. Enforced Bcl2 expression overcomes defects in distal VH-DJH and secondary Vκ-Jκ rearrangement associated with VprBP insufficiency in B cells and substantially rescues maturation of marginal zone and Igλ(+) B cells, but not Igκ(+) B cells. In this background, expression of a site-directed Igκ L chain transgene increases Igκ(+) B cell frequency, suggesting VprBP does not regulate L chain expression from a productively rearranged Igk allele. In site-directed anti-dsDNA H chain transgenic mice, loss of VprBP function in B cells impairs selection of Igκ editor L chains typically arising through secondary Igk rearrangement, but not selection of Igλ editor L chains. Both H and L chain site-directed transgenic mice show increased B cell anergy when VprBP is inactivated in B cells. Taken together, these data argue that VprBP is required for the efficient receptor editing and selection of Igκ(+) B cells, but is largely dispensable for Igλ(+) B cell development and selection, and that VprBP is necessary to rescue autoreactive B cells from anergy induction.

Rac-mediated Stimulation of Phospholipase Cγ2 Amplifies B Cell Receptor-induced Calcium Signaling

The Rho GTPase Rac is crucially involved in controlling multiple B cell functions, including those regulated by the B cell receptor (BCR) through increased cytosolic Ca(2+). The underlying molecular mechanisms and their relevance to the functions of intact B cells have thus far remained unknown. We have previously shown that the activity of phospholipase Cγ2 (PLCγ2), a key constituent of the BCR signalosome, is stimulated by activated Rac through direct protein-protein interaction. Here, we use a Rac-resistant mutant of PLCγ2 to functionally reconstitute cultured PLCγ2-deficient DT40 B cells and to examine the effects of the Rac-PLCγ2 interaction on BCR-mediated changes of intracellular Ca(2+) and regulation of Ca(2+)-regulated and nuclear-factor-of-activated-T-cell-regulated gene transcription at the level of single, intact B cells. The results show that the functional Rac-PLCγ2 interaction causes marked increases in the following: (i) sensitivity of B cells to BCR ligation; (ii) BCR-mediated Ca(2+) release from intracellular stores; (iii) Ca(2+) entry from the extracellular compartment; and (iv) nuclear translocation of the Ca(2+)-regulated nuclear factor of activated T cells. Hence, Rac-mediated stimulation of PLCγ2 activity serves to amplify B cell receptor-induced Ca(2+) signaling.

Balancing Proliferation with Igκ Recombination during B-lymphopoiesis

The essential events of B-cell development are the stochastic and sequential rearrangement of immunoglobulin heavy (Igμ) and then light chain (Igκ followed by Igλ) loci. The counterpoint to recombination is proliferation, which both maintains populations of pro-B cells undergoing Igμ recombination and expands the pool of pre-B cells expressing the Igμ protein available for subsequent Igκ recombination. Proliferation and recombination must be segregated into distinct and mutually exclusive developmental stages. Failure to do so risks aberrant gene translocation and leukemic transformation. Recent studies have demonstrated that proliferation and recombination are each affected by different and antagonistic receptors. The IL-7 receptor drives proliferation while the pre-B-cell antigen receptor, which contains Igμ and surrogate light chain, enhances Igκ accessibility and recombination. Remarkably, the principal downstream proliferative effectors of the IL-7R, STAT5 and cyclin D3, directly repress Igκ accessibility through very divergent yet complementary mechanisms. Conversely, the pre-B-cell receptor represses cyclin D3 leading to cell cycle exit and enhanced Igκ accessibility. These studies reveal how cell fate decisions can be directed and reinforced at each developmental transition by single receptors. Furthermore, they identify novel mechanisms of Igκ repression that have implications for gene regulation in general.

Orchestrating B cell lymphopoiesis through interplay of IL-7 receptor and pre-B cell receptor signalling

The development of B cells is dependent on the sequential DNA rearrangement of immunoglobulin loci that encode subunits of the B cell receptor. The pathway navigates a crucial checkpoint that ensures expression of a signalling-competent immunoglobulin heavy chain before commitment to rearrangement and expression of an immunoglobulin light chain. The checkpoint segregates proliferation of pre-B cells from immunoglobulin light chain recombination and their differentiation into B cells. Recent advances have revealed the molecular circuitry that controls two rival signalling systems, namely the interleukin-7 (IL-7) receptor and the pre-B cell receptor, to ensure that proliferation and immunoglobulin recombination are mutually exclusive, thereby maintaining genomic integrity during B cell development.

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.

Ras orchestrates exit from the cell cycle and light-chain recombination during early B cell development

Signals through the pre-B cell antigen receptor (pre-BCR) and interleukin 7 receptor (IL-7R) coordinate pre-B cell population expansion with subsequent recombination of the locus encoding immunoglobulin kappa-chain (Igk). Although many 'downstream' effectors of each receptor are known, how they integrate to mediate development has remained unclear. Here we report that pre-BCR-mediated activation of the Ras-MEK-Erk signaling pathway silenced transcription of Ccnd3 (encoding cyclin D3) and coordinated exit from the cell cycle with induction of the transcription factor E2A and the initiation of Igk recombination. IL-7R-mediated activation of the transcription factor STAT5 opposed this pathway by promoting Ccnd3 expression and concomitantly inhibiting Igk transcription by binding to the Igk intronic enhancer and preventing E2A recruitment. Our data show how pre-BCR signaling poises pre-B cells to undergo differentiation after escape from IL-7R signaling.

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.