ITAM-mediated tonic signalling through pre-BCR and BCR complexes

N-terminal processing by dipeptidyl peptidase 9: Cut and Go!

Dipeptidyl peptidase 9 (DPP9) is an intracellular amino-dipeptidase with physiological roles in the immune system, DNA repair and mitochondria homeostasis, while its deregulation is linked to cancer progression and immune-associated defects. Through its rare ability to cleave a peptide bond following the imino-acid proline, DPP9 acts as a molecular switch that regulates key proteins, such as the tumor-suppressor BRCA2. In this review we will discuss key concepts underlying the outcomes of protein processing by DPP9, including substrate turn-over by the N-degron pathway. Additionally, we will review non-enzymatic roles and the regulation of DPP9 by discussing the interactome of this protease, which includes SUMO1, Filamin A, NLRP1 and CARD8.

The immunoglobulin M-degrading enzyme of Streptococcus suis (IdeSsuis) leads to long-lasting inhibition of the activation of porcine IgM-secreting B cells

Streptococcus suis (S. suis) is one of the most important porcine pathogens, causing severe pathologies such as meningitis or polyarthritis. It is also a very successful colonizer of mucosal surfaces. The IgM-degrading enzyme of S. suis (IdeSsuis) specifically cleaves porcine IgM, which results in complement evasion. On the basis of our previous finding that IdeSsuis also cleaves the IgM B cell receptor in vitro, we verified IgM B cell receptor cleavage ex vivo in whole regional lymph nodes and investigated the working hypothesis that this IgM B cell receptor cleavage results in a long-lasting impaired B cell function. The number of IgM-secreting cells was determined via ELISpot analysis after porcine peripheral blood mononuclear cells had initially been treated with different recombinant S. suis proteins and subsequently stimulated with interleukin-2 and the toll-like receptor 7/8 ligand R848. Compared with treatment with medium or recombinant muramidase-released protein, treatment with rIdeSsuis but also with a cleavage-deficient variant led to a reduction in the number of IgM-secreting cells as well as the level of secreted IgM. Flow cytometry analysis confirmed that the IgM B cell receptor was cleaved only by rIdeSsuis, and the receptor recovered to pretreatment levels on day 2 after treatment. Flow cytometry analysis of B and T cells incubated with fluorescein-labelled recombinant proteins revealed that different rIdeSsuis variants bind specifically to B cells, most prominently the cleavage-deficient variant. Our results indicate that in vitro interference of rIdeSsuis with the IgM B cell receptor results in long-lasting impaired IgM secretion by B cells after toll-like receptor activation. Further studies are warranted to prove that the modulation of B cell function by IdeSsuis could play a role in vivo.

Spleen tyrosine kinase (SYK): an emerging target for the assemblage of small molecule antitumor agents

INTRODUCTION

Spleen tyrosine kinase (SYK), a nonreceptor tyrosine kinase, has emerged as a vital component in the complex symphony of cancer cell survival and division. SYK activation (constitutive) is documented in various B-cell malignancies, and its inhibition induces programmed cell death. In some instances, it also acts as a tumor suppressor.

AREAS COVERED

Involvement of the SYK in the cancer growth, specifically in the progression of chronic lymphocytic leukemia (CLL), diffuse large B cell lymphomas (DLBCLs), acute myeloid leukemia (AML), and multiple myeloma (MM) is discussed. Therapeutic strategies to target SYK in cancer, including investigational SYK inhibitors, combinations of SYK inhibitors with other drugs targeting therapeutically relevant targets, and recent advancements in constructing new structural assemblages as SYK inhibitors, are also covered.

EXPERT OPINION

The SYK inhibitor field is currently marred by the poor translation rate of SYK inhibitors from preclinical to clinical studies. Also, dose-limited toxicities associated with the applications of SYK inhibitors have been evidenced. Thus, the development of new SYK inhibitory structural templates is in the need of the hour. To accomplish the aforementioned, interdisciplinary teams should incessantly invest efforts to expand the size of the armory of SYK inhibitors.

UBXN3B is crucial for B lymphopoiesis

BACKGROUND

The ubiquitin regulatory X (UBX) domain-containing proteins (UBXNs) are putative adaptors for ubiquitin ligases and valosin-containing protein; however, their in vivo physiological functions remain poorly characterised. We recently showed that UBXN3B is essential for activating innate immunity to DNA viruses and controlling DNA/RNA virus infection. Herein, we investigate its role in adaptive immunity.

METHODS

We evaluated the antibody responses to multiple viruses and pathogenesis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza in tamoxifen-inducible global and constitutive B cell-specific Ubxn3b knockout mice; quantified various immune populations, B lineage progenitors/precursors, B cell receptor (BCR) signalling and apoptosis by flow cytometry, immunoblotting and immunofluorescence microscopy. We also performed bone marrow transfer, single-cell and bulk RNA sequencing.

FINDINGS

Both global and B cell-specific Ubxn3b knockout mice present a marked reduction in small precursor B-II (>60%), immature (>70%) and mature B (>95%) cell numbers. Transfer of wildtype bone marrow to irradiated global Ubxn3b knockouts restores normal B lymphopoiesis, while reverse transplantation does not. The mature B population shrinks rapidly with apoptosis and higher pro and activated caspase-3 protein levels were observed following induction of Ubxn3b knockout. Mechanistically, Ubxn3b deficiency leads to impaired pre-BCR signalling and cell cycle arrest. Ubxn3b knockout mice are highly vulnerable to respiratory viruses, with increased viral loads and prolonged immunopathology in the lung, and reduced production of virus-specific IgM/IgG.

INTERPRETATION

UBXN3B is essential for B lymphopoiesis by maintaining constitutive pre-BCR signalling and cell survival in a cell-intrinsic manner.

FUNDING

United States National Institutes of Health grants, R01AI132526 and R21AI155820.

Targeting the B cell receptor signaling pathway in chronic lymphocytic leukemia

Aberrant signal transduction through the B cell receptor (BCR) plays a critical role in the pathogenesis of chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL). BCR-dependent signaling is necessary for the growth and survival of neoplastic cells, making inhibition of down-stream pathways a logical therapeutic strategy. Indeed, selective inhibitors against Bruton's tyrosine kinase (BTK) and phosphoinositide 3-kinase (PI3K) have been shown to induce high rates of response in CLL and other B cell lymphomas. In particular, the development of BTK inhibitors revolutionized the treatment approach to CLL, demonstrating long-term efficacy. While BTK inhibitors are widely used for multiple lines of treatment, PI3K inhibitors are much less commonly utilized, mainly due to toxicities. CLL remains an incurable disease and effective treatment options after relapse or development of TKI resistance are greatly needed. This review provides an overview of BCR signaling, a summary of the current therapeutic landscape, and a discussion of the ongoing trials targeting BCR-associated kinases.

Progression of Notch signaling regulation of B cells under radiation exposure

With the continuous development of nuclear technology, the radiation exposure caused by radiation therapy is a serious health hazard. It is of great significance to further develop effective radiation countermeasures. B cells easily succumb to irradiation exposure along with immunosuppressive response. The approach to ameliorate radiation-induced B cell damage is rarely studied, implying that the underlying mechanisms of B cell damage after exposure are eager to be revealed. Recent studies suggest that Notch signaling plays an important role in B cell-mediated immune response. Notch signaling is a critical regulator for B cells to maintain immune function. Although accumulating studies reported that Notch signaling contributes to the functionality of hematopoietic stem cells and T cells, its role in B cells is scarcely appreciated. Presently, we discussed the regulation of Notch signaling on B cells under radiation exposure to provide a scientific basis to prevent radiation-induced B cell damage.

Actinin-4 controls survival signaling in B cells by limiting the lateral mobility of B-cell antigen receptors

The structure and dynamics of F-actin networks in the cortical area of B cells control the signal efficiency of B-cell antigen receptors (BCRs). Although antigen-induced signaling has been studied extensively, the role of cortical F-actin in antigen-independent tonic BCR signaling is less well understood. Because these signals are essential for the survival of B cells and are consequently exploited by several B-cell lymphomas, we assessed how the cortical F-actin structure influences tonic BCR signal transduction. We employed genetic variants of a primary cell-like B-cell line that can be rendered quiescent to show that cross-linking of actin filaments by α-actinin-4 (ACTN4), but not ACTN1, is required to preserve the dense architecture of F-actin in the cortical area of B cells. The reduced cortical F-actin density in the absence of ACTN4 resulted in increased lateral BCR diffusion. Surprisingly, this was associated with reduced tonic activation of BCR-proximal effector proteins, extracellular signal-regulated kinase, and pro-survival pathways. Accordingly, ACTN4-deficient B-cell lines and primary human B cells exhibit augmented apoptosis. Hence, our findings reveal that cortical F-actin architecture regulates antigen-independent tonic BCR survival signals in human B cells.

The diacylglycerol kinase ζ inhibitor ASP1570 augments natural killer cell function

The enhancement of T cell and NK cell function is an immunotherapeutic strategy for combating cancer. Antibodies that block inhibitory receptors, such as PD-1 and CTLA4, augment T cell function and have been successful in curing patients with some types of cancer. As an alternative approach to targeting specific inhibitory receptors by antibodies, small molecule drugs that inhibit negative regulators of T cell activation have been sought. One potential pharmacological target is diacylglycerol (DAG) kinase (DGK)ζ, which is an enzyme that acts as a negative regulator of DAG by phosphorylating DAG and converting it into phosphatidic acid. DAG-mediated signaling is critical for T cell activation through its T cell receptor and NK cell activation downstream of a variety of activating receptors. Thus, DGKζ-deficient T cells and NK cells display increased function upon activating receptor engagement. Moreover, treatment with the DGKζ-selective inhibitor ASP1570 augments T cell function. In this study, we sought to test whether the acute inhibition of DGKζ by ASP1570 augments NK cell function. We find that ASP1570 enhances DAG-mediated signaling in immunoreceptor-stimulated NK cells. Accordingly, ASP1570 treatment enhanced IFNγ production and degranulation of immunoreceptor-activated NK cells in vitro and NK cell-mediated tumor clearance in vivo. Thus, ASP1570 enhances both T and NK cell function, which could possibly induce more durable anti-tumor responses for immunotherapy.

Resolving therapy resistance mechanisms in multiple myeloma by multiomics subclone analysis

Intratumor heterogeneity as a clinical challenge becomes most evident after several treatment lines, when multidrug-resistant subclones accumulate. To address this challenge, the characterization of resistance mechanisms at the subclonal level is key to identify common vulnerabilities. In this study, we integrate whole-genome sequencing, single-cell (sc) transcriptomics (scRNA sequencing), and chromatin accessibility (scATAC sequencing) together with mitochondrial DNA mutations to define subclonal architecture and evolution for longitudinal samples from 15 patients with relapsed or refractory multiple myeloma. We assess transcriptomic and epigenomic changes to resolve the multifactorial nature of therapy resistance and relate it to the parallel occurrence of different mechanisms: (1) preexisting epigenetic profiles of subclones associated with survival advantages, (2) converging phenotypic adaptation of genetically distinct subclones, and (3) subclone-specific interactions of myeloma and bone marrow microenvironment cells. Our study showcases how an integrative multiomics analysis can be applied to track and characterize distinct multidrug-resistant subclones over time for the identification of molecular targets against them.

FCRL1 immunoregulation in B cell development and malignancy

Immunotherapeutic targeting of surface regulatory proteins and pharmacologic inhibition of critical signaling pathways has dramatically shifted our approach to the care of individuals with B cell malignancies. This evolution in therapy reflects the central role of the B cell receptor (BCR) signaling complex and its co-receptors in the pathogenesis of B lineage leukemias and lymphomas. Members of the Fc receptor-like gene family (FCRL1-6) encode cell surface receptors with complex tyrosine-based regulation that are preferentially expressed by B cells. Among them, FCRL1 expression peaks on naïve and memory B cells and is unique in terms of its intracellular co-activation potential. Recent studies in human and mouse models indicate that FCRL1 contributes to the formation of the BCR signalosome, modulates B cell signaling, and promotes humoral responses. Progress in understanding its regulatory properties, along with evidence for its over-expression by mature B cell leukemias and lymphomas, collectively imply important yet unmet opportunities for FCRL1 in B cell development and transformation. Here we review recent advances in FCRL1 biology and highlight its emerging significance as a promising biomarker and therapeutic target in B cell lymphoproliferative disorders.

Glycome dynamics in T and B cell development: basic immunological mechanisms and clinical applications

Glycans cover the surfaces of all mammalian cells through a process called glycosylation. Nearly all proteins and receptors that integrate the intricate series of co-stimulatory/inhibitory pathways of the immune system are glycosylated. Growing evidence indicates that the development of the immune system at the origins of T and B cell development is tightly regulated by glycosylation. In this opinion, we hypothesize that the glycome composition of developing T and B cells is developmentally regulated. We discuss how glycans play fundamental roles in lymphocyte development and how glycans early define T and B cell functionality in multiple aspects of adaptive immunity. These advances can provide opportunities for the discovery of novel disease factors and more effective candidate treatments for various conditions.

Phosphoinositide 3-kinase (PI3K) classes: From cell signaling to endocytic recycling and autophagy

Lipid signaling is defined as any biological signaling action in which a lipid messenger binds to a protein target, converting its effects to specific cellular responses. In this complex biological pathway, the family of phosphoinositide 3-kinase (PI3K) represents a pivotal role and affects many aspects of cellular biology from cell survival, proliferation, and migration to endocytosis, intracellular trafficking, metabolism, and autophagy. While yeasts have a single isoform of phosphoinositide 3-kinase (PI3K), mammals possess eight PI3K types divided into three classes. The class I PI3Ks have set the stage to widen research interest in the field of cancer biology. The aberrant activation of class I PI3Ks has been identified in 30-50% of human tumors, and activating mutations in PIK3CA is one of the most frequent oncogenes in human cancer. In addition to indirect participation in cell signaling, class II and III PI3Ks primarily regulate vesicle trafficking. Class III PI3Ks are also responsible for autophagosome formation and autophagy flux. The current review aims to discuss the original data obtained from international research laboratories on the latest discoveries regarding PI3Ks-mediated cell biological processes. Also, we unravel the mechanisms by which pools of the same phosphoinositides (PIs) derived from different PI3K types act differently.

Multi-targeted immunotherapeutics to treat B cell malignancies

The concept of multi-targeted immunotherapeutic systems has propelled the field of cancer immunotherapy into an exciting new era. Multi-effector molecules can be designed to engage with, and alter, the patient's immune system in a plethora of ways. The outcomes can vary from effector cell recruitment and activation upon recognition of a cancer cell, to a multipronged immune checkpoint blockade strategy disallowing evasion of the cancer cells by immune cells, or to direct cancer cell death upon engaging multiple cell surface receptors simultaneously. Here, we review the field of multi-specific immunotherapeutics implemented to treat B cell malignancies. The mechanistically diverse strategies are outlined and discussed; common B cell receptor antigen targeting strategies are outlined and summarized; and the challenges of the field are presented along with optimistic insights for the future.

Tuning charge density of chimeric antigen receptor optimizes tonic signaling and CAR-T cell fitness

Tonic signaling of chimeric antigen receptor (CAR), i.e., the spontaneous CAR activation in the absence of tumor antigen stimulation, is considered to be a pivotal event controlling CAR-T efficacy. However, the molecular mechanism underlying the spontaneous CAR signals remains elusive. Here, we unveil that positively charged patches (PCPs) on the surface of the CAR antigen-binding domain mediate CAR clustering and result in CAR tonic signaling. For CARs with high tonic signaling (e.g., GD2.CAR and CSPG4.CAR), reducing PCPs on CARs or boosting ionic strength in the culture medium during ex vivo CAR-T cell expansion minimizes spontaneous CAR activation and alleviates CAR-T cell exhaustion. In contrast, introducing PCPs into the CAR with weak tonic signaling, such as CD19.CAR, results in improved in vivo persistence and superior antitumor function. These results demonstrate that CAR tonic signaling is induced and maintained by PCP-mediated CAR clustering. Notably, the mutations we generated to alter the PCPs maintain the antigen-binding affinity and specificity of the CAR. Therefore, our findings suggest that the rational tuning of PCPs to optimize tonic signaling and in vivo fitness of CAR-T cells is a promising design strategy for the next-generation CAR.

Integrated Single-Cell (Phospho-)Protein and RNA Detection Uncovers Phenotypic Characteristics and Active Signal Transduction of Human Antibody-Secreting Cells

Single-cell technologies are currently widely applied to obtain a deeper understanding of the phenotype of single-cells in heterogenous mixtures. However, integrated multilayer approaches including simultaneous detection of mRNA, protein expression, and intracellular phospho-proteins are still challenging. Here, we combined an adapted method to in vitro-differentiate peripheral B-cells into antibody-secreting cells (ASCs) (i.e., plasmablasts and plasma cells) with integrated multi-omic single-cell sequencing technologies to detect and quantify immunoglobulin subclass-specific surface markers, transcriptional profiles, and signaling transduction pathway components. Using a common set of surface proteins, we integrated two multimodal datasets to combine mRNA, protein expression, and phospho-protein detection in one integrated dataset. Next, we tested whether ASCs that only seem to differ in its ability to secrete different IgM, IgA, or IgG antibodies exhibit other differences that characterize these different ASCs. Our approach detected differential expression of plasmablast and plasma cell markers, homing receptors, and TNF receptors. In addition, differential sensitivity was observed for the different cytokine stimulations that were applied during in vitro differentiation. For example, IgM ASCs were more sensitive to IL-15, while IgG ASC responded more to IL-6 and IFN addition. Furthermore, tonic BCR activity was detected in IgA and IgM ASCs, while IgG ASC exhibited active BCR-independent SYK activity and NF-κB and mTOR signaling. We confirmed these findings using flow cytometry and small molecules inhibitors, demonstrating the importance of SYK, NF-κB, and mTOR activity for plasmablast/plasma cell differentiation/survival and/or IgG secretion. Taken together, our integrated multi-omics approach allowed high-resolution phenotypic characterization of single cells in a heterogenous sample of in vitro-differentiated human ASCs. Our strategy is expected to further our understanding of human ASCs in healthy and diseased samples and provide a valuable tool to identify novel biomarkers and potential drug targets.

Spatial Stochastic Model of the Pre-B Cell Receptor

Survival and proliferation of immature B lymphocytes requires expression and tonic signaling of the pre-B cell receptor (pre-BCR). This low level, ligand-independent signaling is likely achieved through frequent, but short-lived, homo interactions. Tonic signaling is also central in the pathology of precursor B acute lymphoblastic leukemia (B-ALL). In order to understand how repeated, transient events can lead to sustained signaling and to assess the impact of receptor accumulation induced by the membrane landscape, we developed a spatial stochastic model of receptor aggregation and downstream signaling events. Our rule- and agent-based model builds on previous mature BCR signaling models and incorporates novel parameters derived from single particle tracking of pre-BCR on surfaces of two different B-ALL cell lines, 697 and Nalm6. Live cell tracking of receptors on the two cell lines revealed characteristic differences in their dimer dissociation rates and diffusion coefficients. We report here that these differences affect pre-BCR aggregation and consequent signal initiation events. Receptors on Nalm6 cells, which have a lower off-rate and lower diffusion coefficient, more frequently form higher order oligomers than pre-BCR on 697 cells, resulting in higher levels of downstream phosphorylation in the Nalm6 cell line.

Immunoglobulin M-degrading enzyme of Streptococcus suis (Ide Ssuis ) impairs porcine B cell signaling

Streptococcus suis (S. suis) is an important porcine pathogen, causing severe disease like meningitis and septicemia primarily in piglets. Previous work showed that the IgM-degrading enzyme of S. suis (Ide _Ssuis ) specifically cleaves soluble porcine IgM and is involved in complement evasion. The objective of this study was to investigate Ide _Ssuis cleavage of the IgM B cell receptor and subsequent changes in B cell receptor mediated signaling. Flow cytometry analysis revealed cleavage of the IgM B cell receptor by recombinant (r) Ide _Ssuis _homologue as well as Ide _Ssuis derived from culture supernatants of S. suis serotype 2 on porcine PBMCs and mandibular lymph node cells. Point-mutated rIde _Ssuis _homologue_C195S did not cleave the IgM B cell receptor. After receptor cleavage by rIde _Ssuis _homologue, it took at least 20 h for mandibular lymph node cells to restore the IgM B cell receptor to levels comparable to cells previously treated with rIde _Ssuis _homologue_C195S. B cell receptor mediated signaling after specific stimulation via the F(ab')₂ portion was significantly inhibited by rIde _Ssuis _homologue receptor cleavage in IgM⁺ B cells, but not in IgG⁺ B cells. Within IgM⁺ cells, CD21⁺ B2 cells and CD21^- B1-like cells were equally impaired in their signaling capacity upon rIde _Ssuis _homologue B cell receptor cleavage. In comparison, intracellular B cell receptor independent stimulation with tyrosine phosphatase inhibitor pervanadate increased signaling in all investigated B cell types. In conclusion, this study demonstrates Ide _Ssuis cleavage efficacy on the IgM B cell receptor and its consequences for B cell signaling.

Low-Level Expression of CD138 Marks Naturally Arising Anergic B Cells

Autoreactive B cells are not entirely deleted, but some remain as immunocompetent or anergic B cells. Although the persistence of autoreactive B cells as anergic cells has been shown in transgenic mouse models with the expression of B cell receptor (BCR) reactive to engineered self-antigen, the characterization of naturally occurring anergic B cells is important to identify them and understand their contribution to immune regulation or autoimmune diseases. We report here that a low-level expression of CD138 in the splenic B cells marks naturally arising anergic B cells, not plasma cells. The CD138^int B cells consisted of IgM^lowIgD^high follicular (FO) B cells and transitional 3 B cells in homeostatic conditions. The CD138^int FO B cells showed an anergic gene expression profile shared with that of monoclonal anergic B cells expressing engineered BCRs and the gene expression profile was different from those of plasma cells, age-associated B cells, or germinal center B cells. The anergic state of the CD138^int FO B cells was confirmed by attenuated Ca²⁺ response and failure to upregulate CD69 upon BCR engagement with anti-IgM, anti-IgD, anti-Igκ, or anti-IgG. The BCR repertoire of the CD138^int FO B cells was distinct from that of the CD138^- FO B cells and included some class-switched B cells with low-level somatic mutations. These findings demonstrate the presence of polyclonal anergic B cells in the normal mice that are characterized by low-level expression of CD138, IgM downregulation, reduced Ca²⁺ and CD69 responses upon BCR engagement, and distinct BCR repertoire.

Establishment of Membrane-Bound IgA1-Specific Antibody Possessing Antibody-Dependent Cellular Cytotoxicity Activity

Therapeutic agents targeting B cells, such as rituximab, have been proven to be effective in several diseases. However, since this therapy targets the whole B cell population, there are still concerns about critical adverse events. Therefore, a new type of antibody that can specifically deplete a particular type of B cell may have the potential to become an efficient and safer therapy. Membrane-bound IgA1 (mIgA1) is expressed on soluble IgA1 (sIgA1) producing B cells and/or their precursor, B cells. Although most of the amino acid sequence in the N-terminal regions of the extracellular domains of mIgA1 and sIgA1 is shared, there is a membrane type-specific region, named the membrane-bound immunoglobulin isotype-specific (migis-α) region, at the membrane-proximal region of mIgA1. We hypothesized that the migis-α region would be a suitable antigen for therapeutic antibodies to target mIgA1-expressing B cells without binding to sIgA1, which may cause undesired adverse effects and poor pharmacokinetics (PK). We established two anti-migis-α monoclonal antibodies (mAbs), KM4641 and KM4644, by immunization of the synthetic peptide corresponding to an migis-α region. These mAbs both showed robust binding to mIgA1-expressing transfectant cells. As we expected, neither mAbs bound to sIgA1 and we found that the mAbs recognized different seven to eight amino acid sequences within the migis-α region. Furthermore, the rat-human chimeric type of both mAbs showed antibody-dependent cellular cytotoxicity against mIgA1-expressing transfectant cells. Taken together, this study showed that established mAbs have therapeutic potential in IgA-related diseases, such as IgA nephropathy.

Regulatory Non-Coding RNAs Modulate Transcriptional Activation During B Cell Development

B cells play a significant role in the adaptive immune response by secreting immunoglobulins that can recognize and neutralize foreign antigens. They develop from hematopoietic stem cells, which also give rise to other types of blood cells, such as monocytes, neutrophils, and T cells, wherein specific transcriptional programs define the commitment and subsequent development of these different cell lineages. A number of transcription factors, such as PU.1, E2A, Pax5, and FOXO1, drive B cell development. Mounting evidence demonstrates that non-coding RNAs, such as microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), modulate the expression of these transcription factors directly by binding to the mRNA coding for the transcription factor or indirectly by modifying cellular pathways that promote expression of the transcription factor. Conversely, these transcription factors upregulate expression of some miRNAs and lncRNAs to determine cell fate decisions. These studies underscore the complex gene regulatory networks that control B cell development during hematopoiesis and identify new regulatory RNAs that require additional investigation. In this review, we highlight miRNAs and lncRNAs that modulate the expression and activity of transcriptional regulators of B lymphopoiesis and how they mediate this regulation.

Signaling networks in B cell development and related therapeutic strategies

B cells are essential for Ab production during humoral immune responses. From decades of B cell research, there is now a detailed understanding of B cell subsets, development, functions, and most importantly, signaling pathways. The complicated pathways in B cells and their interactions with each other are stage-dependent, varying with surface marker expression during B cell development. With the increasing understanding of B cell development and signaling pathways, the mechanisms underlying B cell related diseases are being unraveled as well, making it possible to provide more precise and effective treatments. In this review, we describe several essential and recently discovered signaling pathways in B cell development and take a look at newly developed therapeutic strategies targeted at B cell signaling.

Loncastuximab tesirine for treatment of relapsed or refractory diffuse large B cell lymphoma

INTRODUCTION

Approximately, a third of patients with diffuse large B-cell lymphoma (DLBCL) have refractory or relapsed disease after initial treatment. Despite recent regulatory approval of several new agents, including CAR-T cell therapy, polatuzumab vedotin and tafasitamab, there is still a need for additional therapies that expand the therapeutic alternatives and improve outcomes for patients with DLBCL that progresses after first line therapy.

AREAS COVERED

Studies of recently approved agents for relapsed DLBCL are reviewed. The relevance of CD19 as an immunotherapeutic target. The pharmacologic composition of loncastuximab tesirine and its cytotoxic payload, a pyrrolobenzodiazepine dimer. Phase I and phase 2 data for loncastuximab tesirine in non-Hodgkin lymphoma, showing the safety profile of this drug and the emerging efficacy results in DLBCL.

EXPERT OPINION

Loncastuximab tesirine is an antiCD19 antibody drug conjugate with a novel cytotoxic payload. Early studies showed this drug is tolerable, with a safety profile that is different from other antibody drug conjugates approved for lymphoid malignancies. Efficacy data shows activity in different non-Hodgkin lymphoma entities, and a phase 2 study has been completed in DLBCL showing durable responses, including in high-risk subgroups. Loncastuximab tesirine will be an important addition to the treatment alternatives for DLBCL.

Activated B Cells and Plasma Cells Are Resistant to Radiation Therapy

PURPOSE

B cells play a key role in outcomes of cancer patients and responses to checkpoint blockade immunotherapies. However, the effect of radiation therapy on B cell populations is poorly understood. Here we characterize the effects of radiation on the development, survival, and phenotype of physiological B-cell subsets.

METHODS AND MATERIALS

Naïve and immunized tumor bearing and nontumor bearing mice were treated with large-field or focal stereotactic radiation and distinct B-cell subsets of varying developmental stages were analyzed by flow cytometry and real-time reverse transcription polymerase chain reaction.

RESULTS

We first report that focal stereotactic radiation is highly superior to large-field radiation at inducing tumor infiltration of B cells, CD8⁺ T cells, and macrophages. We observed that radiation affects B cell development in the bone marrow, increasing frequencies of early pro-B cells and late pro-B cells while inducing upregulation of programmed cell death protein 1. We then demonstrate that class switched B cells and plasma cells are highly resistant to radiation therapy compared with naïve B cells and upregulate activation markers programmed cell death 1 ligand 2 and major histocompatibility complex class II) after radiation. Mechanistically, radiation upregulates Xbp1 and Bcl6 in plasma cells, conferring radioresistance. Furthermore, using an immunization approach, we demonstrate that radiation enhances activation-induced cytidine deaminase mediated class switching and somatic hypermutation in primed B cells.

CONCLUSIONS

These data demonstrate that stereotactic radiation is superior to large-field radiation at inducing infiltration of immune cells into tumors and that plasma cells and class switched B cells are highly resistant to radiation therapy. These results represent the most comprehensive analysis of the effects of radiation on B cells to date and identify novel mechanisms by which radiation modulates immune cells within the tumor microenvironment.

Metabolic determinants of B-cell selection

B-cells are antibody-producing cells of the adaptive immune system. Approximately 75% of all newly generated B-cells in the bone marrow are autoreactive and express potentially harmful autoantibodies. To prevent autoimmune disease, the immune system has evolved a powerful mechanism to eliminate autoreactive B-cells, termed negative B-cell selection. While designed to remove autoreactive clones during early B-cell development, our laboratory recently discovered that transformed B-cells in leukemia and lymphoma are also subject to negative selection. Indeed, besides the risk of developing autoimmune disease, B-cells are inherently prone to malignant transformation: to produce high-affinity antibodies, B-cells undergo multiple rounds of somatic immunoglobulin gene recombination and hypermutation. Reflecting high frequencies of DNA-breaks, adaptive immune protection by B-cells comes with a dramatically increased risk of development of leukemia and lymphoma. Of note, B-cells exist under conditions of chronic restriction of energy metabolism. Here we discuss how these metabolic gatekeeper functions during B-cell development provide a common mechanism for the removal of autoreactive and premalignant B-cells to safeguard against both autoimmune diseases and B-cell malignancies.

Comparison of SYK Signaling Networks Reveals the Potential Molecular Determinants of Its Tumor-Promoting and Suppressing Functions

Spleen tyrosine kinase (SYK) can behave as an oncogene or a tumor suppressor, depending on the cell and tissue type. As pharmacological SYK inhibitors are currently evaluated in clinical trials, it is important to gain more information on the molecular mechanisms underpinning these opposite roles. To this aim, we reconstructed and compared its signaling networks using phosphoproteomic data from breast cancer and Burkitt lymphoma cell lines where SYK behaves as a tumor suppressor and promoter. Bioinformatic analyses allowed for unveiling the main differences in signaling pathways, network topology and signal propagation from SYK to its potential effectors. In breast cancer cells, the SYK target-enriched signaling pathways included intercellular adhesion and Hippo signaling components that are often linked to tumor suppression. In Burkitt lymphoma cells, the SYK target-enriched signaling pathways included molecules that could play a role in SYK pro-oncogenic function in B-cell lymphomas. Several protein interactions were profoundly rewired in the breast cancer network compared with the Burkitt lymphoma network. These data demonstrate that proteomic profiling combined with mathematical network modeling allows untangling complex pathway interplays and revealing difficult to discern interactions among the SYK pathways that positively and negatively affect tumor formation and progression.

B cell signalling pathways-New targets for precision medicine in chronic lymphocytic leukaemia

The B cell receptor (BCR) is a master regulator of B cells, controlling cellular processes such as proliferation, migration and survival. Cell signalling downstream of the BCR is aberrantly activated in the B cell malignancy chronic lymphocytic leukaemia (CLL), supporting the pathophysiology of the disease. This insight has led to development and approval of small molecule inhibitors that target components of the BCR pathway. These advances have greatly improved the management of CLL, but the disease remains incurable. This may partly be explained by the inter-patient heterogeneity of the disease, also when it comes to treatment responses. Precision medicine is therefore required to optimize treatment and move towards a cure. Here, we discuss how the introduction of BCR signalling inhibitors has facilitated the development of functional in vitro assays to guide clinical treatment decisions on use of the same therapeutic agents in individual patients. The cellular responses to these agents can be analysed in high-throughput assays such as dynamic BH3 profiling, phospho flow experiments and drug sensitivity screens to identify predictive biomarkers. This progress exemplifies the positive synergy between basal and translational research needed to optimize patient care.

N-Glycan Branching Decouples B Cell Innate and Adaptive Immunity to Control Inflammatory Demyelination

B cell depletion potently reduces episodes of inflammatory demyelination in multiple sclerosis (MS), predominantly through loss of innate rather than adaptive immunity. However, molecular mechanisms controlling innate versus adaptive B cell function are poorly understood. N-glycan branching, via interactions with galectins, controls endocytosis and signaling of cell surface receptors to control cell function. Here we report that N-glycan branching in B cells dose dependently reduces pro-inflammatory innate responses by titrating decreases in Toll-like receptor-4 (TLR4) and TLR2 surface expression via endocytosis. In contrast, a minimal level of N-glycan branching maximizes surface retention of the B cell receptor (BCR) and the CD19 co-receptor to promote adaptive immunity. Branched N-glycans inhibit antigen presentation by B cells to reduce T helper cell-17 (TH17)/TH1 differentiation and inflammatory demyelination in mice. Thus, N-glycan branching negatively regulates B cell innate function while promoting/maintaining adaptive immunity via BCR, providing an attractive therapeutic target for MS.

N-Glycan Branching Is Required for Development of Mature B Cells

Galectins have been implicated in inhibiting BCR signaling in mature B cells but promoting pre-BCR signaling during early development. Galectins bind to branched N-glycans attached to cell surface glycoproteins to control the distribution, clustering, endocytosis, and signaling of surface glycoproteins. During T cell development, N-glycan branching is required for positive selection of thymocytes, inhibiting both death by neglect and negative selection via enhanced surface retention of the CD4/CD8 coreceptors and limiting TCR clustering/signaling, respectively. The role of N-glycan branching in B cell development is unknown. In this study, we report that N-glycan branching is absolutely required for development of mature B cells in mice. Elimination of branched N-glycans in developing B cells via targeted deletion of N-acetylglucosaminyl transferase I (Mgat1) markedly reduced cellularity in the bone marrow and/or spleen and inhibited maturation of pre-, immature, and transitional stage 2 B cells. Branching deficiency markedly reduced surface expression of the pre-BCR/BCR coreceptor CD19 and promoted spontaneous death of pre-B cells and immature B cells in vitro. Death was rescued by low-dose pre-BCR/BCR stimulation but exacerbated by high-dose pre-BCR/BCR stimulation as well as antiapoptotic Bcl_xL overexpression in pre-B cells. Branching deficiency also enhanced Nur77 induction, a marker of negative selection. Together, these data suggest that, as in T cells, N-glycan branching promotes positive selection of B cells by augmenting pre-BCR/BCR signaling via CD19 surface retention, whereas limiting negative selection from excessive BCR engagement.

Environmental complex exposure and the risk of influenza-like illness among housewives: A case study in Shanxi Province, China

The association between environmental pollution and risk of influenza-like illness (ILI) among general population has been reported. However, the relationships between the individual pollutants and ILI risk are still under discussion. Our study aimed to explore the associations of the typical environmental polycyclic aromatic hydrocarbons (PAHs) and metal(loid)s with ILI risk among women population. We carried out a cross-sectional study and included a total of 396 housewives in Shanxi Province, China. The information on their general characteristics and ILI frequency was collected by questionnaire. We collected their hair samples and analyzed the concentrations of PAHs and various metal(loid)s. The results indicated that only acenaphthylene concentration of the nine detected PAH congeners in the hair was significantly associated with ILI risk with adjusted odds ratio (AOR) and 95% confidence interval (95% CI) of 0.58 (0.38 - 0.91). Among the concerned 4 toxic metal(loid)s and 15 rare earth elements, only the hair concentration of arsenic had a positive dose-response relationship with ILI risk. In addition, we found that there were negative dose-response associations of the three essential trace elements (i.e. chromium, cobalt, and nickel), and four essential alkaline earth elements (i.e. magnesium, calcium, strontium, barium) with ILI risk. It was concluded that the environmental exposure to certain compounds of housewives may contribute to their ILI development.

An Ancient BCR-like Signaling Promotes ICP Production and Hemocyte Phagocytosis in Oyster

BCR/TCR-based adaptive immune systems arise in the jawed vertebrates, and B cell receptors (BCRs) play an important role in the clonal selection of B cells and their differentiation into antibody-secreting plasma cells. The existence of BCR-like molecule and the activation mechanism of the downstream response are still not clear in invertebrates. In this study, an ancient BCR-like molecule (designated as CgIgR) with an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic tail was identified from the Pacific oyster Crassostrea gigas to investigate its involvement in immune response. CgIgR could bind different bacteria through five extracellular Ig domains and formed dimers. The activated CgIgR recruited CgSyk to promote CgERK phosphorylation. The CgIgR-mediated signaling promoted the production of immunoglobulin domain-containing proteins (CgICP-2 and CgLRRIG-1) through inducing CgH3K4me2. The produced CgICPs eventually facilitated hemocytes to phagocytize and eliminate V. splendidus. This study proposed that there was an ancient BCR-like molecule and BCR-like signaling in molluscs.

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An ancient BCR-like molecule (defined as CgIgR) was identified from C. gigas

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We propose IgR-mediated signaling induces CgERK activity in oyster

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IgR-mediated signaling induced CgH3K4me2 to promote the production of CgICPs

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CgICPs facilitated the hemocytes to phagocytize and eliminate V. splendidus

Immune modulation by chronic exposure to waterpipe smoke and immediate-early gene regulation in murine lungs

OBJECTIVE

We investigated the effects of chronic waterpipe (WP) smoke on pulmonary function and immune response in a murine model using a research-grade WP and the effects of acute exposure on the regulation of immediate-early genes (IEGs).

METHODS

WP smoke was generated using three WP smoke puffing regimens based on the Beirut regimen. WP smoke samples generated under these puffing regimens were quantified for nicotine concentration. Mice were chronically exposed for 6 months followed by assessment of pulmonary function and airway inflammation. Transcriptomic analysis using RNAseq was conducted after acute exposure to characterise the IEG response. These biomarkers were then compared with those generated after exposure to dry smoke (without water added to the WP bowl).

RESULTS

We determined that nicotine composition in WP smoke ranged from 0.4 to 2.5 mg per puffing session. The lung immune response was sensitive to the incremental severity of chronic exposure, with modest decreases in airway inflammatory cells and chemokine levels compared with air-exposed controls. Pulmonary function was unmodified by chronic WP exposure. Acute WP exposure was found to activate the immune response and identified known and novel IEG as potential biomarkers of WP exposure.

CONCLUSION

Chronic exposure to WP smoke leads to immune suppression without significant changes to pulmonary function. Transcriptomic analysis of the lung after acute exposure to WP smoke showed activation of the immune response and revealed IEGs that are common to WP and dry smoke, as well as pools of IEGs unique to each exposure, identifying potential biomarkers specific to WP exposure.

Overcoming Ibrutinib Resistance in Chronic Lymphocytic Leukemia

Ibrutinib is the first Bruton's tyrosine kinase (BTK) inhibitor, which showed significant clinical activity in chronic lymphocytic leukemia (CLL) and small lymphocytic lymphoma (SLL) patients regardless of cytogenetic risk factors. Recent results of phase III clinical trials in treatment-naïve CLL patients shift the importance of the agent to frontline therapy. Nevertheless, beside its clinical efficacy, ibrutinib possesses some off-target activity resulting in ibrutinib-characteristic adverse events including bleeding diathesis and arrhythmias. Furthermore, acquired and primary resistance to the drug have been described. As the use of ibrutinib in clinical practice increases, the problem of resistance is becoming apparent, and new methods of overcoming this clinical problem arise. In this review, we summarize the mechanisms of BTK inhibitors' resistance and discuss the post-ibrutinib treatment options.

Ionic protein-lipid interactions at the plasma membrane regulate the structure and function of immunoreceptors

Adaptive lymphocytes express a panel of immunoreceptors on the cell surface. Phospholipids are the major components of cell membranes, but they have functional roles beyond forming lipid bilayers. In particular, acidic phospholipids forming microdomains in the plasma membrane can ionically interact with proteins via polybasic sequences, which can have functional consequences for the protein. We have shown that negatively charged acidic phospholipids can interact with positively charged juxtamembrane polybasic regions of immunoreceptors, such as TCR-CD3, CD28 and IgG-BCR, to regulate protein structure and function. Furthermore, we pay our attention to protein transmembrane domains. We show that a membrane-snorkeling Lys residue in integrin αLβ2 regulates transmembrane heterodimer formation and integrin adhesion through ionic interplay with acidic phospholipids and calcium ions (Ca²⁺) in T cells, thus providing a new mechanism of integrin activation. Here, we review our recent progress showcasing the importance of both juxtamembrane and intramembrane ionic protein-lipid interactions.

Coordinated roles for glycans in regulating the inhibitory function of CD22 on B cells

CD22 is an inhibitory B cell co-receptor that recognizes sialic acid-containing glycoconjugates as ligands. Interactions with its glycan ligands are key to regulating the ability of CD22 to modulate B cell function, the most widely explored of which is antagonizing B cell receptor (BCR) signaling. Most importantly, interactions of CD22 with ligands on the same cell (cis) control the organization of CD22 on the cell surface, which minimizes co-localization with the BCR. In contrast with the modest ability of CD22 to intrinsically dampen BCR signaling, glycan ligands presented on another cell (trans) along with an antigen drawn CD22 and the BCR together within an immunological synapse, strongly inhibiting BCR signaling. New concepts are emerging for how CD22 controls B cell function, such as changes in glycosylation at different stages of B cell differentiation, specifically on GC B cells. Related to these changes, new players, such galectin-9, have been discovered that regulate cell surface nanoclusters of CD22. Roles of glycan ligands in controlling CD22 are the primary focus of this review as we highlight the ability of CD22 to modulate B cell function.

Fc receptor-like 1 intrinsically recruits c-Abl to enhance B cell activation and function

B cell activation is regulated by the stimulatory or inhibitory co-receptors of B cell receptors (BCRs). Here, we investigated the signaling mechanism of Fc receptor-like 1 (FcRL1), a newly identified BCR co-receptor. FcRL1 was passively recruited into B cell immunological synapses upon BCR engagement in the absence of FcRL1 cross-linking, suggesting that FcRL1 may intrinsically regulate B cell activation and function. BCR cross-linking alone led to the phosphorylation of the intracellular Y281ENV motif of FcRL1 to provide a docking site for c-Abl, an SH2 domain-containing kinase. The FcRL1 and c-Abl signaling module, in turn, potently augmented B cell activation and proliferation. FcRL1-deficient mice exhibited markedly impaired formation of extrafollicular plasmablasts and germinal centers, along with decreased antibody production upon antigen stimulation. These findings reveal a critical BCR signal-enhancing function of FcRL1 through its intrinsic recruitment to B cell immunological synapses and subsequent recruitment of c-Abl upon BCR cross-linking.

Overview of Targeted Drugs for Mature B-Cell Non-hodgkin Lymphomas

The improved knowledge of pathogenetic mechanisms underlying lymphomagenesis and the discovery of the critical role of tumor microenvironments have enabled the design of new drugs against cell targets and pathways. The Food and Drug Administration (FDA) has approved several monoclonal antibodies (mAbs) and small molecule inhibitors (SMIs) for targeted therapy in hematology. This review focuses on the efficacy results of the currently available targeted agents and recaps the main ongoing trials in the setting of mature B-Cell non-Hodgkin lymphomas. The objective is to summarize the different classes of novel agents approved for mature B-cell lymphomas, to describe in synoptic tables the results they achieved and, finally, to draw future scenarios as we glimpse through the ongoing clinical trials. Characteristics and therapeutic efficacy are summarized for the currently approved mAbs [i.e., anti-Cluster of differentiation (CD) mAbs, immune checkpoint inhibitors, chimeric antigen receptor (CAR) T-cell therapy, and bispecific antibodies] as well as for SMIs i.e., inhibitors of B-cell receptor signaling, proteasome, mTOR BCL-2 HDAC pathways. The biological disease profiling of B-cell lymphoma subtypes may foster the discovery of innovative drug strategies for improving survival outcome in lymphoid neoplasms, as well as the trade-offs between efficacy and toxicity. The hope for clinical advantages should carefully be coupled with mindful awareness of the potential pitfalls and the occurrence of uneven, sometimes severe, toxicities.

Ibrutinib for the treatment of chronic lymphocytic leukemia

INTRODUCTION

Chemoimmunotherapy has improved outcomes in chronic lymphocytic leukemia, yet it is not curative, with very high relapse rates, and is associated with a significant risk of toxicities. Moreover, patients with higher-risk genetic abnormalities continue to experience poorer outcomes and lower survival. Recently, novel targeted therapies have been developed to increase efficacy and reduce toxicity. Areas covered: Ibrutinib is an oral irreversible inhibitor of Bruton's tyrosine kinase, a mediator of B-cell receptor signaling, which plays a vital role in various B-cell neoplasms. The drug has been approved for the treatment of several hematological malignancies, including chronic lymphocytic leukemia/small lymphocytic lymphoma, where large trials have shown outcomes never seen before even in high-risk patients. The safety profile appeared furthermore favorable, even in elderly and unfit patients. Expert opinion: Therapy with ibrutinib rarely provides MRD-negative complete remission; an indefinite maintenance is therefore needed, with the risk of developing adverse events (AE) or resistance resulting in treatment interruption or discontinuation. Novel, extremely promising, combination strategies, based on the association of ibrutinib with chemoimmunotherapy, anti-CD20 monoclonal antibody or other targeted agents, are currently being investigated, with the goal of achieving greater depth of remission, especially MRD-negativity, and removing the need for indefinite treatment.

Turning to Computer-aided Drug Design in the Treatment of Diffuse Large B-cell Lymphoma: Has it been Helpful?

INTRODUCTION

Amidst the numerous effective therapeutic options available for the treatment of Diffuse Large B-cell Lymphoma (DLBCL), about 30-40% of patients treated with first-line chemoimmunotherapy still experience a relapse or refractory DLBCL. This has necessitated a continuous search for new therapeutic agents to augment the existing therapeutic arsenal.

METHODS

The dawn of Computer-Aided Drug Design (CADD) in the drug discovery process has accounted for persistency in the application of computational approaches either alone or in combinatorial strategies with experimental methods towards the identification of potential hit compounds with high therapeutic efficacy in abrogating DLBCL.

RESULTS

This review showcases the interventions of structure-based and ligand-based computational approaches which have led to the identification of numerous small molecule inhibitors against implicated targets in DLBCL therapy, even though many of these potential inhibitors are piled-up awaiting further experimental validation and exploration.

CONCLUSION

We conclude that a successful and a conscious amalgamation of CADD and experimental approaches could pave the way for the discovery of the next generation potential leads in DLBCL therapy with improved activities and minimal toxicities.

Active PI3K abrogates central tolerance in high-avidity autoreactive B cells

High-avidity autoreactive B cells are typically removed by central tolerance mechanisms in the bone marrow. Greaves et al. demonstrate that B cell–intrinsic expression of active PI3Kα prevents central tolerance and effectively promotes differentiation and activation of high-avidity autoreactive B cells in the periphery.

Autoreactive B cells that bind self-antigen with high avidity in the bone marrow undergo mechanisms of central tolerance that prevent their entry into the peripheral B cell population. These mechanisms are breached in many autoimmune patients, increasing their risk of B cell–mediated autoimmune diseases. Resolving the molecular pathways that can break central B cell tolerance could therefore provide avenues to diminish autoimmunity. Here, we show that B cell–intrinsic expression of a constitutively active form of PI3K-P110α by high-avidity autoreactive B cells of mice completely abrogates central B cell tolerance and further promotes these cells to escape from the bone marrow, differentiate in peripheral tissue, and undergo activation in response to self-antigen. Upon stimulation with T cell help factors, these B cells secrete antibodies in vitro but remain unable to secrete autoantibodies in vivo. Overall, our data demonstrate that activation of the PI3K pathway leads high-avidity autoreactive B cells to breach central, but not late, stages of peripheral tolerance.

A reevaluation of the spleen tyrosine kinase (SYK) activation mechanism

Spleen tyrosine kinase (SYK) is a signaling node in many immune pathways and comprises two tandem Src homology (SH) 2 domains, an SH2-kinase linker, and a C-terminal tyrosine kinase domain. Two prevalent models of SYK activation exist. The "OR-gate" model contends that SYK can be fully activated by phosphorylation or binding of its SH2 domains to a dual-phosphorylated immune-receptor tyrosine-based activation motif (ppITAM). An alternative model proposes that SYK activation requires ppITAM binding and phosphorylation of the SH2-kinase linker by a SRC family kinase such as LYN proto-oncogene, SRC family tyrosine kinase (LYN). To evaluate these two models, we generated directly comparable unphosphorylated (upSYK) and phosphorylated (pSYK) proteins with or without an N-terminal glutathione S-transferase (GST) tag, resulting in monomeric or obligatory dimeric SYK, respectively. We assessed the ability of a ppITAM peptide and LYN to activate these SYK proteins. The ppITAM peptide strongly activated GST-SYK but was less effective in activating upSYK untagged with GST. LYN alone activated untagged upSYK to a greater extent than did ppITAM, and inclusion of both proteins rapidly and fully activated upSYK. Using immunoblot and phosphoproteomic approaches, we correlated the kinetics and order of site-specific SYK phosphorylation. Our results are consistent with the alternative model, indicating that ppITAM binding primes SYK for rapid LYN-mediated phosphorylation of Tyr-352 and then Tyr-348 of the SH2-kinase linker, which facilitates activation loop phosphorylation and full SYK activation. This gradual activation mechanism may also explain how SYK maintains ligand-independent tonic signaling, important for B-cell development and survival.

Co-engaging CD47 and CD19 with a bispecific antibody abrogates B-cell receptor/CD19 association leading to impaired B-cell proliferation

CD19 is a B cell-specific receptor that regulates the threshold of B cell receptor (BCR)-mediated cell proliferation. A CD47xCD19 bispecific antibody (biAb) was generated to target and deplete B cells via multiple antibody-mediated mechanisms. Interestingly, the biAb, constructed of a CD19 binding arm and a CD47 binding arm, inhibited BCR-mediated B-cell proliferation with an effect even more potent than a CD19 monoclonal antibody (mAb). The inhibitory effect of the biAb was not attributable to CD47 binding because a monovalent or bivalent anti-CD47 mAb had no effect on B cell proliferation. Fluorescence resonance energy transfer analysis demonstrated that co-engaging CD19 and CD47 prevented CD19 clustering and its migration to BCR clusters, while only engaging CD19 (with a mAb) showed no impact on either CD19 clustering or migration. The lack of association between CD19 and the BCR resulted in decreased phosphorylation of CD19 upon BCR activation. Furthermore, the biAb differentially modulated BCR-induced gene expression compared to a CD19 mAb. Taken together, this unexpected role of CD47xCD19 co-ligation in inhibiting B cell proliferation illuminates a novel approach in which two B cell surface molecules can be tethered, to one another in order, which may provide a therapeutic benefit in settings of autoimmunity and B cell malignancies.

The c-Myc/miR17-92/PTEN Axis Tunes PI3K Activity to Control Expression of Recombination Activating Genes in Early B Cell Development

Appropriate PI3K signals generated by the antigen receptor are essential to promote B cell development. Regulation of recombination activating gene (RAG)-1 and RAG-2 expression is one key process that is mediated by PI3K to ensure developmental progression and selection. When PI3K signals are too high or too low, expression of RAGs does not turn off and B cell development is impaired or blocked. Yet, the mechanism which tunes PI3K activity to control RAG expression during B cell development in the bone marrow is unknown. Recently we showed that a c-Myc/miR17-92/PTEN axis regulates PI3K activity for positive and negative selection of immature B cells. Here, we show that the c-Myc/miR17-92/PTEN axis tunes PI3K activity to control the expression of RAGs in proB cells. Using different genetically engineered mouse models we show that impaired function of the c-Myc/miR17-92/PTEN axis alters the PI3K/Akt/Foxo1 pathway to result in dis-regulated expression of RAG and a block in B cell development. Studies using 38c-13 B lymphoma cells, where RAGs are constitutively expressed, suggest that this regulatory effect is mediated post-translationally through Foxo1.

Monitoring and Management of Toxicities of Novel B Cell Signaling Agents

PURPOSE REVIEW

B cell signaling agents, including ibrutinib, idelalisib, and the BCL-2 inhibitor venetoclax have become an integral part of therapy for patients with non-Hodgkin's lymphomas. The toxicity profiles of these medications is distinct from chemoimmunotherapy. Here, we will review the mechanism of action of these drugs, their efficacy, and toxicity management.

RECENT FINDINGS

Ibrutinib use is associated with increased risk of atrial fibrillation and bleeding which can be managed using dose interruptions and modifications. Patients on idelalisib require close clinical and frequent laboratory monitoring, particularly of liver function tests to ensure there are no serious adverse events. Monitoring for infections is important in patients on both idelalisib and ibrutinib. Venetoclax requires close clinical and laboratory monitoring to prevent significant tumor lysis. Targeted B cell receptor therapies each have unique side effect profiles which require careful clinical monitoring. As we continue to use these therapies, optimal management strategies will continue to be elucidated.

O-GlcNAcylation is required for B cell homeostasis and antibody responses

O-linked N-acetylglucosamine (O-GlcNAc) transferase (Ogt) catalyzes O-GlcNAc modification. O-GlcNAcylation is increased after cross-linking of the B-cell receptor (BCR), but the physiological function of this reaction is unknown. Here we show that lack of Ogt in B-cell development not only causes severe defects in the activation of BCR signaling, but also perturbs B-cell homeostasis by enhancing apoptosis of mature B cells, partly as a result of impaired response to B-cell activating factor. O-GlcNAcylation of Lyn at serine 19 is crucial for efficient Lyn activation and Syk interaction in BCR-mediated B-cell activation and expansion. Ogt deficiency in germinal center (GC) B cells also results in enhanced apoptosis of GC B cells and memory B cells in an immune response, consequently causing a reduction of antibody levels. Together, these results demonstrate that B cells rely on O-GlcNAcylation to maintain homeostasis, transduce BCR-mediated activation signals and activate humoral immunity.

Post-translational modification has a variety of regulatory functions for important immune molecules. Here the authors use B-cell specific knockout mice to show how O-GlcNAcylation is required for functional B cell responses and humoral immunity.

Critical Role for Very-Long Chain Sphingolipids in Invariant Natural Killer T Cell Development and Homeostasis

The role of sphingolipids (SLs) in the immune system has come under increasing scrutiny recently due to the emerging contributions that these important membrane components play in regulating a variety of immunological processes. The acyl chain length of SLs appears particularly critical in determining SL function. Here, we show a role for very-long acyl chain SLs (VLC-SLs) in invariant natural killer T (iNKT) cell maturation in the thymus and homeostasis in the liver. Ceramide synthase 2-null mice, which lack VLC-SLs, were susceptible to a hepatotropic strain of lymphocytic choriomeningitis virus, which is due to a reduction in the number of iNKT cells. Bone marrow chimera experiments indicated that hematopoietic-derived VLC-SLs are essential for maturation of iNKT cells in the thymus, whereas parenchymal-derived VLC-SLs are crucial for iNKT cell survival and maintenance in the liver. Our findings suggest a critical role for VLC-SL in iNKT cell physiology.

Probing Transient Release of Membrane-Sequestered Tyrosine-Based Signaling Motif by Solution NMR Spectroscopy

Membrane sequestration of tyrosine-based signaling motifs of antigen receptors effectively restricts the signaling activities in resting lymphocytes. However, low level of basal signaling in resting cells is required for lymphocyte survival and antigen responsiveness, of which the molecular mechanism remains obscure. Here we probe the transient release of the cytoplasmic domain of the membrane-bound IgG heavy chain (mIgG-tail) by hydrogen exchange NMR spectroscopy, illustrating a dynamic molecular basis for its basal signaling activity. To solve the severe resonance overlap problem in the 2D spectra of mIgG-tail, a non-uniformly sampled pseudo-4D hydrogen exchange NMR experiment has been exploited to quantitatively measure site-specific hydrogen exchange rates. Our solution NMR study reveals transient solvent exposure of the ITT signaling motif that can be further enhanced by calcium ion, and provides insight into the mechanism of lymphocyte basal signaling.