Delayed cellular maturation and decreased immunoglobulin kappa light chain production in immature B lymphocytes lacking B cell linker protein

Spliceosome component PHD finger 5A is essential for early B lymphopoiesis

The spliceosome, a multi-megadalton ribonucleoprotein complex, is essential for pre-mRNA splicing in the nucleus and ensuring genomic stability. Its precise and dynamic assembly is pivotal for its function. Spliceosome malfunctions can lead to developmental abnormalities and potentially contribute to tumorigenesis. The specific role of the spliceosome in B cell development is poorly understood. Here, we reveal that the spliceosomal U2 snRNP component PHD finger protein 5A (Phf5a) is vital for early B cell development. Loss of Phf5a results in pronounced defects in B cell development, causing an arrest at the transition from pre-pro-B to early pro-B cell stage in the bone marrow of mutant mice. Phf5a-deficient B cells exhibit impaired immunoglobulin heavy (IgH) chain expression due to defective V-to-DJ gene rearrangement. Mechanistically, our findings suggest that Phf5a facilitates IgH gene rearrangement by regulating the activity of recombination-activating gene endonuclease and influencing chromatin interactions at the Igh locus.

The histone H2B ubiquitination regulator Wac is essential for plasma cell differentiation

Naïve B cells become activated and differentiate into antibody-secreting plasma cells (PCs) when encountering antigens. Here, we reveal that the WW domain-containing adapter protein with coiled-coil (Wac), which is important for histone H2B ubiquitination (ubH2B), is essential for PC differentiation. We demonstrate that B cell-specific Wac knockout mice have severely compromised T cell-dependent and -independent antibody responses. PC differentiation is drastically compromised despite undisturbed germinal center B cell response in the mutant mice. We also observe a significant reduction in global ubH2B in Wac-deficient B cells, which is correlated with downregulated expression of some genes critical for cell metabolism. Thus, our findings demonstrate an essential role of Wac-mediated ubH2B in PC differentiation and shed light on the epigenetic mechanisms underlying this process.

The unbalanced p53/SIRT1 axis may impact lymphocyte homeostasis in COVID-19 patients

Background/objectives

A dysregulated inflammatory profile plays an important role in coronavirus disease-2019 (COVID-19) pathogenesis. Moreover, the depletion of lymphocytes is typically associated with an unfavourable disease course. We studied the role and impact of p53 and deacetylase Sirtuin 1 (SIRT1) on lymph-monocyte homeostasis and their possible effect on T and B cell signalling.

Methods

Gene expression analysis and flow cytometry were performed on peripheral blood mononuclear cells (PBMC) of 35 COVID-19 patients and 10 healthy donors (HD). Inflammatory cytokines, the frequency of Annexin+ cells among CD3+ T cells and CD19+ B cell subsets were quantified.

Results

PBMC from COVID-19 patients had a higher p53 expression, and higher concentrations of plasma proinflammatory cytokines (IL1β, TNF-α, IL8, and IL6) than HD. Deacetylase Sirtuin 1 (SIRT1) expression was significantly decreased in COVID-19 patients and was negatively correlated with p53 (p = 0.003 and r = −0.48). A lower expression of IL-7R and B Cell linker (BLNK), key genes for lymphocyte homeostasis and function, was observed in COVID-19 than in HD. The reduction of IgK and IgL chains was seen in lymphopenic COVID-19 patients. A significant increase in both apoptotic B and T cells were observed. Inflammatory cytokines correlated positively with p53 (IL-1β: r = 0.5 and p = 0.05; IL-8: r = 0.5 and p = 0.05) and negatively with SIRT1 (IL1-β: r = −0.5 and p = 0.04; TNF-α: r = −0.4 and p = 0.04).

Conclusions

Collectively, our data indicate that the inflammatory environment, the dysregulated p53/SIRT1 axis and low expression of IL7R and BLNK may impact cell survival, B cell signalling and antibody production in COVID-19 patients. Further studies are required to define the functional impact of low BLNK/IL7R expression during severe acute respiratory syndrome coronavirus-2 infection.

The RNase III enzyme Dicer is essential for germinal center B-cell formation

MicroRNAs (miRNAs) are short noncoding RNAs that regulate gene expression and are important for pre-B and follicular B lymphopoiesis as demonstrated, respectively, by mb-1-Cre- and cd19-Cre-mediated deletion of Dicer, the RNase III enzyme critical for generating mature miRNAs. To explore the role of miRNAs in B-cell terminal differentiation, we use Aicda-Cre to specifically delete Dicer in activated B cells where activation-induced cytidine deaminase is highly expressed. We demonstrate that mutant mice fail to produce high-affinity class-switched antibodies and generate memory B and long-lived plasma cells on immunization with a T cell-dependent antigen. More importantly, germinal center (GC) B-cell formation is drastically compromised in the absence of Dicer, as a result of defects in cell proliferation and survival. Dicer-deficient GC B cells express higher levels of cell cycle inhibitor genes and proapoptotic protein Bim. Ablation of Bim could partially rescue the defect in GC B-cell formation in Dicer-deficient mice. Taken together, our data suggest that Dicer and probably miRNAs are critical for GC B-cell formation during B-cell terminal differentiation.

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.

Redundancy in B Cell Developmental Pathways: c-Cbl Inactivation Rescues Early B Cell Development through a B Cell Linker Protein-Independent Pathway

Deficiency in the adaptor protein B cell linker protein (BLNK) results in a substantial but incomplete block in B cell development, suggesting that alternative pathways exist for B lineage differentiation. Another adaptor protein, c-Cbl, plays a negative regulatory role in several BCR-signaling pathways. We therefore investigated the role of c-Cbl during B cell development and addressed the possibility that redundancies in pathways for B cell differentiation could be further revealed by eliminating negative effects mediated by c-Cbl. Strikingly, c-Cbl inactivation reversed a number of the critical defects in early B cell differentiation that are seen in BLNK-deficient mice. c-Cbl(-/-)BLNK(-/-) mice exhibited normalized down-regulation of pre-BCR and CD43, up-regulation of MHC class II, and augmented L chain rearrangement, resulting in a successful transition from pre-B cells to immature B cells. c-Cbl inactivation also reversed the potentially tumor-predisposing hyperproliferative response of BLNK(-/-) pre-B cells to IL-7. Pre-BCR cross-linking induced enhanced and prolonged tyrosine phosphorylation in c-Cbl(-/-)BLNK(-/-) pre-BCR(+) pre-B cells compared with c-Cbl(+/-)BLNK(-/-) cells, including elevated phosphorylation of Lyn, Syk, Btk, and phospholipase C-gamma2. Our studies suggest that some, but not all, pre-BCR-triggered developmental events can be mediated by BLNK-independent pathways that are negatively regulated by c-Cbl, and further suggest that different events during early B cell development require different strength or duration of pre-BCR signaling.

3BP2 deficiency impairs the response of B cells, but not T cells, to antigen receptor ligation

The adapter protein 3BP2 is expressed in lymphocytes; binds to Syk/ZAP-70, Vav, and phospholipase C-gamma (PLC-gamma); and is thought to be important for interleukin-2 gene transcription in T cells. To define the role of 3BP2 in lymphocyte development and function, we generated 3BP2-deficient mice. T-cell development, proliferation, cytokine secretion, and signaling in response to T-cell receptor (TCR) ligation were all normal in 3BP2(-/-) mice. 3BP2(-/-) mice had increased accumulation of pre-B cells in the bone marrow and a block in the progression of transitional B cells in the spleen from the T1 to the T2 stage, but normal numbers of mature B cells. B-cell proliferation, cell cycle progression, PLC-gamma2 phosphorylation, calcium mobilization, NF-ATp dephosphorylation, and Erk and Jnk activation in response to B-cell receptor (BCR) ligation were all impaired. These results suggest that 3BP2 is important for BCR, but not for TCR signaling.

Regulation of peripheral B cell maturation

Although it is clear that the final phases of B cell maturation occur after newly formed B cells exit the bone marrow, the mechanisms underpinning the maturation, selection, and long-term survival of immature peripheral B cells remain poorly understood. Here, we review recent advances in our understanding of how B cell receptor (BCR)-mediated signaling events integrate with additional environmental cues to promote the selection and differentiation of immature B cells into functionally distinct subpopulations of mature B cells. We pay particular attention to the role of the Baff cytokine family and the Notch receptor-ligand family and their unique roles in promoting B cell survival and differentiation into follicular and marginal zone B cells.

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.

The pre-B-cell receptor: selector of fitting immunoglobulin heavy chains for the B-cell repertoire

In this Opinion article, I address the role of the pre-B-cell receptor (pre-BCR) in the development of antigen-specific B cells in terms of immunoglobulin heavy chain (IgH) variable-region repertoire selection, precursor B-cell differentiation and proliferation, and IgH allelic exclusion. Comparisons with the role of the pre-T-cell receptor (pre-TCR) in T-cell development raise provocative questions. Why do B- and T-cell lineages both use a surrogate chain - the surrogate light chain and the pre-TCR alpha-chain, respectively - as a step to develop their repertoires of antigen-recognizing cells? What are the functions of the pre-BCR and pre-TCR in lymphocyte differentiation and antigen-receptor allelic exclusion? This article, together with the accompanying article by Harald von Boehmer, hopes to answer some of these questions.

Dual Requirement for the Igα Immunoreceptor Tyrosine-Based Activation Motif (ITAM) and a Conserved Non-Igα ITAM Tyrosine in Supporting Igαβ-Mediated B Cell Development

Surface Ig (sIg) expression is a critical checkpoint during avian B cell development. Only cells that express sIg colonize bursal follicles, clonally expand, and undergo Ig diversification by gene conversion. Expression of a heterodimer, in which the extracellular and transmembrane domains of murine CD8alpha or CD8beta are fused to the cytoplasmic domains of chicken Igalpha (chIgalpha) or Igbeta, respectively (murine CD8alpha (mCD8alpha):chIgalpha + mCD8beta:chIgbeta), or an mCD8alpha:chIgalpha homodimer supported bursal B cell development as efficiently as endogenous sIg. In this study we demonstrate that B cell development, in the absence of chIgbeta, requires both the Igalpha ITAM and a conserved non-ITAM Igalpha tyrosine (Y3) that has been associated with binding to B cell linker protein (BLNK). When associated with the cytoplasmic domain of Igbeta, the Igalpha ITAM is not required for the induction of strong calcium mobilization or BLNK phosphorylation, but is still necessary to support B cell development. In contrast, mutation of the Igalpha Y3 severely compromised calcium mobilization when expressed as either a homodimer or a heterodimer with the cytoplasmic domain of Igbeta. However, coexpression of the cytoplasmic domain of Igbeta partially complemented the Igalpha Y3 mutation, rescuing higher levels of BLNK phosphorylation and, more strikingly, supporting B cell development.

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

The editing of B cell Ag receptor (BCR) through successive rearrangements of Ig genes has been considered to be a major mechanism for the central B cell tolerance, which precludes appearance of self-reactive B cells, through studies using anti-self-Ig transgenic/knock-in mouse systems. However, contribution of the receptor editing in the development of the normal B cell repertoire remains unclear. In addition, the signaling pathway directing this event is unknown. In this study, we demonstrate that receptor editing in anti-DNA Ig knock-in mice is impaired in the absence of an adaptor protein BASH (BLNK/SLP-65) that is involved in BCR signaling. Remarkably, the supposed hallmarks of receptor editing such as Iglambda chain expression, recombination sequence rearrangements at Igkappa loci, and presence of in-frame VkappaJkappa joins in the Igkappa loci inactivated by the recombination sequence rearrangements, were all diminished in BASH-deficient mice with unmanipulated Ig loci. BCR ligation-induced Iglambda gene recombination in vitro was also impaired in BASH-deficient B cells. Furthermore, the BASH-deficient mice showed an excessive Ab response to a DNA carrier immunization, suggesting the presence of unedited DNA-reactive B cells in the periphery. These results not only define a signaling pathway required for receptor editing but indicate that the BCR-signaled receptor editing indeed operates in the development of normal B cell repertoire and contributes to establishing the B cell tolerance.

Bruton's tyrosine kinase cooperates with the B cell linker protein SLP-65 as a tumor suppressor in Pre-B cells

Expression of the pre-B cell receptor (pre-BCR) leads to activation of the adaptor molecule SLP-65 and the cytoplasmic kinase Btk. Mice deficient for one of these signaling proteins have an incomplete block in B cell development at the stage of large cycling pre-BCR+CD43+ pre-B cells. Our recent findings of defective SLP-65 expression in approximately 50% of childhood pre-B acute lymphoblastic leukemias and spontaneous pre-B cell lymphoma development in SLP-65-/- mice demonstrate that SLP-65 acts as a tumor suppressor. To investigate cooperation between Btk and SLP-65, we characterized the pre-B cell compartment in single and double mutant mice, and found that the two proteins have a synergistic role in the developmental progression of large cycling into small resting pre-B cells. We show that Btk/SLP-65 double mutant mice have a dramatically increased pre-B cell tumor incidence ( approximately 75% at 16 wk of age), as compared with SLP-65 single deficient mice (<10%). These findings demonstrate that Btk cooperates with SLP-65 as a tumor suppressor in pre-B cells. Furthermore, transgenic low-level expression of a constitutive active form of Btk, the E41K-Y223F mutant, prevented tumor formation in Btk/SLP-65 double mutant mice, indicating that constitutive active Btk can substitute for SLP-65 as a tumor suppressor.

Regulation of B-cell fate by antigen-receptor signals

Recent evidence indicates that B cells are instructed continuously by B-cell receptor (BCR) signals to make crucial cell-fate decisions at several checkpoints during their development. Targeted disruption of BCR signalling components leads to distinct blocks in B-cell maturation, which indicates that key kinases and adaptors fine-tune BCR signalling to direct appropriate cell fates. Recent progress in unravelling the molecular mechanisms of the BCR signalling pathways has helped to clarify how BCR signals regulate the proliferation, survival and apoptosis of developing B cells.