In vivo ablation of surface immunoglobulin on mature B cells by inducible gene targeting results in rapid cell death

Towards a unifying model for B-cell receptor triggering

Antibodies are exceptionally versatile molecules with remarkable flexibility in their binding properties. Their natural targets range from small-molecule toxins, across viruses of different sizes, to bacteria and large multicellular parasites. The molecular determinants bound by antibodies include proteins, peptides, carbohydrates, nucleic acids, lipids and even synthetic molecules that have never existed in nature. Membrane-anchored antibodies also serve as receptors on the surface of the B cells that produce them. Despite recent structural insights, there is still no unifying molecular mechanism to explain how antibody targets (antigens) trigger the activation of these B-cell receptors (BCRs). After cognate antigen encounter, somatic hypermutation and class-switch recombination allow BCR affinity maturation and immunoglobulin class-specific responses, respectively. This raises the fundamental question of how one receptor activation mechanism can accommodate a plethora of variant receptors and ligands, and how it can ensure that individual B cells remain responsive to antigen after somatic hypermutation and class switching. There is still no definite answer. Here we give a brief historical account of the different models proposed to explain BCR triggering and discuss their merit in the context of the current knowledge of the structure of BCRs, their dynamic membrane distribution, and recent biochemical and cell biological insights.

ATM in immunobiology: From lymphocyte development to cancer immunotherapy

Ataxia Telangiectasia Mutated (ATM) is a protein kinase traditionally known for its role in DNA damage response and cell cycle regulation. However, emerging research has revealed its multifaceted and crucial functions in the immune system. This comprehensive review explores the diverse roles of ATM in immune regulation, from lymphocyte development to its involvement in cancer immunotherapy. The review describes ATM's critical functions in V(D)J recombination and class switch recombination, highlighting its importance in adaptive immunity. It examines ATM's role in innate immunity, particularly in NF-κB signaling and cytokine production. Furthermore, the review analyzes the impact of ATM deficiency on oxidative stress and mitochondrial function in immune cells, providing insights into the immunological defects observed in Ataxia Telangiectasia (A-T). The article explores ATM's significance in maintaining hematopoietic stem cell function and its implications for bone marrow transplantation and gene therapy. Additionally, it addresses ATM's involvement in inflammation and immune senescence, linking DNA damage response to age-related immune decline. Finally, this review highlights the emerging role of ATM in cancer immunotherapy, where its inhibition shows promise in enhancing immune checkpoint blockade therapy. This review synthesizes current knowledge on ATM's functions in the immune system, offering insights into the pathophysiology of ATM-related disorders and potential therapeutic strategies for immune-related conditions and cancer immunotherapy.

Antigen receptor ITAMs provide tonic signaling by acting as guanine nucleotide exchange factors to directly activate R-RAS2

The small GTPase R-RAS2 regulates homeostatic proliferation and survival of T and B lymphocytes and, when present in high amounts, drives the development of B cell chronic lymphocytic leukemia. In normal and leukemic lymphocytes, R-RAS2 constitutively binds to antigen receptors through their immunoreceptor tyrosine-based activation motifs (ITAMs) and promotes tonic activation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway. Here, we examined the molecular mechanisms underlying this direct interaction and its consequences for R-RAS2 activity. R-RAS2 exhibited direct, high-affinity interactions with ITAM peptides derived from B and T cell receptors through a proline-rich sequence in the hypervariable domain of R-RAS2. In resting T and B cells, the presence of antigen receptors at the plasma membrane was sufficient to promote the activation of R-RAS2 and PI3K, and mutations that abolished the interaction of R-RAS2 with ITAMs reduced R-RAS2 signaling. Binding to ITAMs increased GDP-GTP exchange on R-RAS2 through a mechanism distinct from that by which conventional cytosolic guanosine nucleotide exchange factors (GEFs) activate RAS proteins. These results define antigen receptors as noncanonical GEFs involved in the basal activation state of R-RAS2 in lymphocytes. Such a mechanism may underlie the leukemic transformation of B cells that occurs when wild-type R-RAS2 is present in high amounts.

Acalabrutinib alone or in combination with rituximab for follicular lymphoma: An open-label study

Acalabrutinib is a selective, second-generation Bruton tyrosine kinase inhibitor. In this open-label, parallel-group study, patients with relapsed/refractory (R/R) follicular lymphoma (FL) were randomised to either acalabrutinib monotherapy or acalabrutinib plus rituximab. An additional cohort of patients with treatment-naive (TN) FL received only the acalabrutinib-rituximab combination. Acalabrutinib-rituximab was well tolerated and active in R/R and TN FL; in the TN cohort the overall response rate was 92.3% with most remissions lasting over 4 years. Acalabrutinib monotherapy was also well tolerated and active in R/R FL. These results support further study of acalabrutinib alone and in combination with rituximab in FL.

FoxO1 signaling in B cell malignancies and its therapeutic targeting

FoxO transcription factors (FoxO1, FoxO3a, FoxO4, FoxO6) are a highly evolutionary conserved subfamily of the 'forkhead' box proteins. They have traditionally been considered tumor suppressors, but FoxO1 also exhibits oncogenic properties. The complex nature of FoxO1 is illustrated by its various roles in B cell development and differentiation, immunoglobulin gene rearrangement and cell-surface B cell receptor (BCR) structure, DNA damage control, cell cycle regulation, and germinal center reaction. FoxO1 is tightly regulated at a transcriptional (STAT3, HEB, EBF, FoxOs) and post-transcriptional level (Akt, AMPK, CDK2, GSK3, IKKs, JNK, MAPK/Erk, SGK1, miRNA). In B cell malignancies, recurrent FoxO1 activating mutations (S22/T24) and aberrant nuclear export and activity have been described, underscoring the potential of its therapeutic inhibition. Here, we review FoxO1's roles across B cell and myeloid malignancies, namely acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL), follicular lymphoma (FL), diffuse large B cell lymphoma (DLBCL), mantle cell lymphoma (MCL), Burkitt lymphoma (BL), Hodgkin lymphoma (HL), and multiple myeloma (MM). We also discuss preclinical evidence for FoxO1 targeting by currently available inhibitors (AS1708727, AS1842856, cpd10).

Advances in Targeted Therapy: Addressing Resistance to BTK Inhibition in B-Cell Lymphoid Malignancies

B-cell lymphoid malignancies are a heterogeneous group of hematologic cancers, where Bruton's tyrosine kinase (BTK) inhibitors have received FDA approval for several subtypes. The first-in-class covalent BTK inhibitor, Ibrutinib, binds to the C481 amino acid residue to block the BTK enzyme and prevent the downstream signaling. Resistance to covalent BTK inhibitors (BTKi) can occur through mutations at the BTK binding site (C481S) but also other BTK sites and the phospholipase C gamma 2 (PLCγ2) resulting in downstream signaling. To bypass the C481S mutation, non-covalent BTKi, such as Pirtobrutinib, were developed and are active against both wild-type and the C481S mutation. In this review, we discuss the molecular and genetic mechanisms which contribute to acquisition of resistance to covalent and non-covalent BTKi. In addition, we discuss the new emerging class of BTK degraders, which utilize the evolution of proteolysis-targeting chimeras (PROTACs) to degrade the BTK protein and constitute an important avenue of overcoming resistance. The moving landscape of resistance to BTKi and the development of new therapeutic strategies highlight the ongoing advances being made towards the pursuit of a cure for B-cell lymphoid malignancies.

Theories of immune recognition: Is anybody right?

The clonal selection theory (CST) is the centrepiece of the current paradigm used to explain immune recognition and memory. Throughout the past decades, the original CST had been expanded and modified to explain new experimental evidences since its original publication by Burnet. This gave origin to new paradigms that govern experimental immunology nowadays, such as the associative recognition of antigen model and the stranger/danger signal model. However, these new theories also do not fully explain experimental findings such as natural autoimmune immunoglobulins, idiotypic networks, low and high dose tolerance, and dual-receptor T and B cells. To make sense of these empirical data, some authors have been trying to change the paradigm of immune cognition using a systemic approach, analogies with brain processing and concepts from second-order cybernetics. In the present paper, we review the CST and some of the theories/hypotheses derived from it, focusing on immune recognition. We point out their main weaknesses and highlight arguments made by their opponents and believers. We conclude that, until now, none of the proposed theories can fully explain the totality of immune phenomena and that a theory of everything is needed in immunology.

BCR signaling in germinal center B cell selection

When mature B cells are activated by antigens, the selection of these activated B cells takes place particularly during T cell-dependent immune responses in which an improved antibody repertoire is generated through somatic hypermutation in germinal centers (GCs). In this process the importance of antigen presentation by GC B cells, and subsequent T follicular helper (Tfh) cell help in positive selection of GC B cells, has been well appreciated. By contrast, the role of B cell receptor (BCR) signaling per se remains unclear. Strong experimental support for the involvement of BCR signaling in GC B cell selection has now been provided. Interestingly, these studies suggest that several checkpoints operating through the BCR ensure affinity maturation.

Neu5Gc-mediated high-affinity interaction is dispensable for CD22 cis-ligands to regulate B cell signaling

CD22 (also known as Siglec-2) is an inhibitory receptor expressed in B cells. CD22 specifically recognizes α2,6 sialic acid and interacts with α2,6 sialylated membrane proteins expressed on the same cell (cis-ligands) and those derived from outside of the cell (trans-ligands). Previously, CD22 cis-ligands were shown to regulate the activity of CD22, thereby regulating both BCR ligation-induced signaling and low-level "tonic" signaling in the absence of BCR ligation that regulates the survival and differentiation of B cells. Mouse CD22 prefers Neu5Gc to Neu5Ac thereby binding to α2,6-linked Neu5Gc with high affinity. Although human CD22 binds to a distinct α2,6 sialylated glycan with high affinity, expression of high-affinity ligands is regulated in a conserved and stringent manner. However, how high- versus low-affinity CD22 ligands regulate B cells is poorly understood. Here we demonstrate that the interaction of CD22 with the endogenous ligands enhances BCR ligation-induced signaling but reduces tonic signaling in Cmah-/- mouse B cells deficient in Neu5Gc as well as wild-type B cells. Moreover, Cmah-/- B cells do not show alterations in the phenotypes correlated to tonic signaling. These results indicate that low-affinity interaction of the CD22 cis-ligands with CD22 is sufficient for the regulation of B cell signaling, and suggest that expression of high-affinity CD22 ligands might be involved in the regulation of B cells by competing for the binding of CD22 with exogenous trans-ligands of CD22.

Regulation of Foxo1 expression is critical for central B cell tolerance and allelic exclusion

Resolving the molecular mechanisms of central B cell tolerance might unveil strategies that prevent autoimmunity. Here, using a mouse model of central B cell tolerance in which Forkhead box protein O1 (Foxo1) is either deleted or over-expressed in B cells, we show that deleting Foxo1 blocks receptor editing, curtails clonal deletion, and decreases CXCR4 expression, allowing high-avidity autoreactive B cells to emigrate to the periphery whereby they mature but remain anergic and short lived. Conversely, expression of degradation-resistant Foxo1 promotes receptor editing in the absence of self-antigen but leads to allelic inclusion. Foxo1 over-expression also restores tolerance in autoreactive B cells harboring active PI3K, revealing opposing roles of Foxo1 and PI3K in B cell selection. Overall, we show that the transcription factor Foxo1 is a major gatekeeper of central B cell tolerance and that PI3K drives positive selection of immature B cells and establishes allelic exclusion by suppressing Foxo1.

Rare SH2B3 coding variants in lupus patients impair B cell tolerance and predispose to autoimmunity

Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease with a clear genetic component. While most SLE patients carry rare gene variants in lupus risk genes, little is known about their contribution to disease pathogenesis. Amongst them, SH2B3-a negative regulator of cytokine and growth factor receptor signaling-harbors rare coding variants in over 5% of SLE patients. Here, we show that unlike the variant found exclusively in healthy controls, SH2B3 rare variants found in lupus patients are predominantly hypomorphic alleles, failing to suppress IFNGR signaling via JAK2-STAT1. The generation of two mouse lines carrying patients' variants revealed that SH2B3 is important in limiting the number of immature and transitional B cells. Furthermore, hypomorphic SH2B3 was shown to impair the negative selection of immature/transitional self-reactive B cells and accelerate autoimmunity in sensitized mice, at least in part due to increased IL-4R signaling and BAFF-R expression. This work identifies a previously unappreciated role for SH2B3 in human B cell tolerance and lupus risk.

Factors Governing B Cell Recognition of Autoantigen and Function in Type 1 Diabetes

Islet autoantibodies predict type 1 diabetes (T1D) but can be transient in murine and human T1D and are not thought to be directly pathogenic. Rather, these autoantibodies signal B cell activity as antigen-presenting cells (APCs) that present islet autoantigen to diabetogenic T cells to promote T1D pathogenesis. Disrupting B cell APC function prevents T1D in mouse models and has shown promise in clinical trials. Autoantigen-specific B cells thus hold potential as sophisticated T1D biomarkers and therapeutic targets. B cell receptor (BCR) somatic hypermutation is a mechanism by which B cells increase affinity for islet autoantigen. High-affinity B and T cell responses are selected in protective immune responses, but immune tolerance mechanisms are known to censor highly autoreactive clones in autoimmunity, including T1D. Thus, different selection rules often apply to autoimmune disease settings (as opposed to protective host immunity), where different autoantigen affinity ceilings are tolerated based on variations in host genetics and environment. This review will explore what is currently known regarding B cell signaling, selection, and interaction with T cells to promote T1D pathogenesis.

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.

IGH 3'RR recombination uncovers a non-germinal center imprint and c-MYC-dependent IGH rearrangement in unmutated chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is an incurable indolent non-Hodgkin lymphoma characterized by tumor B cells that weakly express a B-cell receptor. The mutational status of the variable region (IGHV) within the immunoglobulin heavy chain (IGH) locus is an important prognosis indicator and raises the question of the CLL cell of origin. Mutated IGHV gene CLL are genetically imprinted by activation-induced cytidine deaminase (AID). AID is also required for IGH rearrangements: class switch recombination and recombination between switch Mu (Sμ) and the 3' regulatory region (3'RR) (Sμ-3'RRrec). The great majority of CLL B cells being unswitched led us to examine IGH rearrangement blockade in CLL. Our results separated CLL into two groups on the basis of Sμ-3'RRrec counts per sample: Sμ-3'RRrecHigh cases (mostly unmutated CLL) and Sμ-3'RRrecLow cases (mostly mutated CLL), but not based on the class switch recombination junction counts. Sμ-3'RRrec appeared to be ongoing in Sμ-3'RRrecHigh CLL cells and comparison of Sμ-3'RRrec junction structural features pointed to different B-cell origins for both groups. In accordance with IGHV mutational status and PIM1 mutation rate, Sμ-3'RRrecHigh CLL harbor a non-germinal center experienced B-cell imprint while Sμ-3'RRrecLow CLL are from AID-experienced B cells from a secondary lymphoid organ. In addition to the proposals already made concerning the CLL cell of origin, our study highlights that analysis of IGH recombinatory activity can identify CLL cases from different origins. Finally, on-going Sμ-3'RRrec in Sμ-3'RRrecHigh cells appeared to presumably be the consequence of high c-MYC expression, as c-MYC overexpression potentiated IGH rearrangements and Sμ-3'RRrec, even in the absence of AID for the latter.

Antibody production relies on the tRNA inosine wobble modification to meet biased codon demand

Antibodies are produced at high rates to provide immunoprotection, which puts pressure on the B cell translational machinery. Here, we identified a pattern of codon usage conserved across antibody genes. One feature thereof is the hyperutilization of codons that lack genome-encoded Watson-Crick transfer RNAs (tRNAs), instead relying on the posttranscriptional tRNA modification inosine (I34), which expands the decoding capacity of specific tRNAs through wobbling. Antibody-secreting cells had increased I34 levels and were more reliant on I34 for protein production than naïve B cells. Furthermore, antibody I34-dependent codon usage may influence B cell passage through regulatory checkpoints. Our work elucidates the interface between the tRNA pool and protein production in the immune system and has implications for the design and selection of antibodies for vaccines and therapeutics.

The ligand interactions of B cell Siglecs are involved in the prevention of autoimmunity to sialylated self-antigens and in the quality control of signaling-competent B cells

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of membrane molecules that recognize sialic acid. Most of them are inhibitory receptors that inhibit immune-cell activation by recognizing sialic acid as a self-motif. Human B cells express CD22 (also known as Siglec-2), Siglec-5, Siglec-6 and Siglec-10 whereas mouse B cells express CD22 and Siglec-G (ortholog of human Siglec-10). Siglecs recognize both sialylated molecules expressed on the same cell (cis-ligands) and those expressed by other cells (trans-ligands). In Guillain-Barré syndrome (GBS), antibody production to gangliosides (which are sialic acid-containing glycolipids) expressed by neurons plays a pathogenic role. A Siglec-10 variant deficient in recognition of gangliosides is genetically associated with GBS, suggesting that Siglec-10 induces self-tolerance to gangliosides by recognizing gangliosides as trans-ligands. Recognition of the BCR as a cis-ligand by Siglec-G and CD22 suppresses BCR signaling in B-1 cells and conventional B cells, respectively. This signal suppression prevents excess expansion of B-1 cells and is involved in the quality control of signaling-competent B cells by setting a threshold for tonic signaling during B cell development. CD22 recognizes other cis-ligands including CD22 and β7 integrin. Interaction of CD22 with other CD22 molecules induces CD22 clustering that suppresses CD22-mediated signal inhibition upon BCR ligation, and interaction with β7 integrin maintains its function in the gut-homing of B cells. Taken together, interactions of B cell Siglecs with multiple trans- and cis-ligands play important roles in B cell homeostasis and immune responses.

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.

Subcutaneous immunisation with zymosan generates mucosal IgA-eliciting memory and protects mice from heterologous influenza virus infection

Immunoglobulin A (IgA) is the most abundant isotype of antibodies and provides a first line of defense at the mucosa against pathogens invading the host. It has been widely accepted that the mucosal IgA response provided by vaccination requires mucosal inoculation, and intranasal inoculation has been proposed for vaccines against influenza virus. Considering the difficulty of intranasal vaccination in infants or elderly people, however, parenteral vaccination that provides the mucosal IgA response is desirable. Here, we demonstrate that subcutaneous immunisation with zymosan, a yeast cell wall constituent known to be recognised by Dectin-1 and TLR2, potentiates the production of antigen-specific IgA antibodies in the sera and airway mucosa upon intranasal antigen challenge. We confirmed that the antigen-specific IgA-secreting cells accumulated in the lung and nasal-associated lymphoid tissues after the antigen challenge. Such an adjuvant effect of zymosan in the primary immunisation for the IgA response depended on Dectin-1 signalling, but not on TLR2. The IgA response to the antigen challenge required both antigen-specific memory B and T cells, and the generation of memory T cells, but not memory B cells, depended on zymosan as an adjuvant. Finally, we demonstrated that subcutaneous inoculation of inactivated influenza virus with zymosan, but not with alum, mostly protected the mice from infection with a lethal dose of a heterologous virus strain. These data suggest that zymosan is a possible adjuvant for parenteral immunisation that generates memory IgA responses to respiratory viruses such as influenza virus.

Recreation of an antigen-driven germinal center in vitro by providing B cells with phagocytic antigen

Successful vaccines rely on activating a functional humoral immune response through the generation of class-switched high affinity immunoglobulins (Igs). The germinal center (GC) reaction is crucial for this process, in which B cells are selected in their search for antigen and T cell help. A major hurdle to understand the mechanisms of B cell:T cell cooperation has been the lack of an antigen-specific in vitro GC system. Here we report the generation of antigen-specific, high-affinity, class-switched Igs in simple 2-cell type cultures of naive B and T cells. B cell antigen uptake by phagocytosis is key to generate these Igs. We have used the method to interrogate if T cells confer directional help to cognate B cells that present antigen and to bystander B cells. We find that bystander B cells do not generate class-switched antibodies due to a defective formation of T-B conjugates and an early conversion into memory B cells.

B cell-intrinsic requirement for WNK1 kinase in antibody responses in mice

Migration and adhesion play critical roles in B cells, regulating recirculation between lymphoid organs, migration within lymphoid tissue, and interaction with CD4+ T cells. However, there is limited knowledge of how B cells integrate chemokine receptor and integrin signaling with B cell activation to generate efficient humoral responses. Here, we show that the WNK1 kinase, a regulator of migration and adhesion, is essential in B cells for T-dependent and -independent antibody responses. We demonstrate that WNK1 transduces signals from the BCR, CXCR5, and CD40, and using intravital imaging, we show that WNK1 regulates migration of naive and activated B cells, and their interactions with T cells. Unexpectedly, we show that WNK1 is required for BCR- and CD40-induced proliferation, acting through the OXSR1 and STK39 kinases, and for efficient B cell-T cell collaboration in vivo. Thus, WNK1 is critical for humoral immune responses, by regulating B cell migration, adhesion, and T cell-dependent activation.

The intestine: A highly dynamic microenvironment for IgA plasma cells

To achieve longevity, IgA plasma cells require a sophisticated anatomical microenvironment that provides cytokines, cell-cell contacts, and nutrients as well as metabolites. The intestinal epithelium harbors cells with distinct functions and represents an important defense line. Anti-microbial peptide-producing paneth cells, mucus-secreting goblet cells and antigen-transporting microfold (M) cells cooperate to build a protective barrier against pathogens. In addition, intestinal epithelial cells are instrumental in the transcytosis of IgA to the gut lumen, and support plasma cell survival by producing the cytokines APRIL and BAFF. Moreover, nutrients are sensed through specialized receptors such as the aryl hydrocarbon receptor (AhR) by both, intestinal epithelial cells and immune cells. However, the intestinal epithelium is highly dynamic with a high cellular turn-over rate and exposure to changing microbiota and nutritional factors. In this review, we discuss the spatial interplay of the intestinal epithelium with plasma cells and its potential contribution to IgA plasma cell generation, homing, and longevity. Moreover, we describe the impact of nutritional AhR ligands on intestinal epithelial cell-IgA plasma cell interaction. Finally, we introduce spatial transcriptomics as a new technology to address open questions in intestinal IgA plasma cell biology.

The immunosenescence-related factor DOCK11 is involved in secondary immune responses of B cells

Background

Memory B cells are an antigen-experienced B-cell population with the ability to rapidly differentiate into antibody-producing cells by recall responses. We recently found that dedicator of cytokinesis 11 (DOCK11) contributes to the expansion of antigen-specific populations among germinal center B cells upon immunization. In comparison, limited information is available on the contribution of DOCK11 to secondary humoral immune responses.

Results

In this study, effects of the DOCK11 deficiency in B cells were examined on secondary immune responses to protein antigen. The lack of DOCK11 in B cells resulted in the impaired induction of antibody-producing cells upon secondary immunization with protein antigen. DOCK11 was dispensable for the recall responses of antigen-experienced B cells, as demonstrated by the comparable induction of antibody-producing cells in mice given transfer of antigen-experienced B cells with no DOCK11 expression. Instead, the lack of DOCK11 in B cells resulted in the impaired secondary immune responses in a B cell-extrinsic manner, which was recovered by the adoptive transfer of cognate T cells.

Conclusions

We addressed that intrinsic and extrinsic effects of DOCK11 expression in B cells may contribute to secondary humoral immune responses in manner of the induction of cognate T-cell help.

Old and New Facts and Speculations on the Role of the B Cell Receptor in the Origin of Chronic Lymphocytic Leukemia

The engagement of the B cell receptor (BcR) on the surface of leukemic cells represents a key event in chronic lymphocytic leukemia (CLL) since it can lead to the maintenance and expansion of the neoplastic clone. This notion was initially suggested by observations of the CLL BcR repertoire and of correlations existing between certain BcR features and the clinical outcomes of single patients. Based on these observations, tyrosine kinase inhibitors (TKIs), which block BcR signaling, have been introduced in therapy with the aim of inhibiting CLL cell clonal expansion and of controlling the disease. Indeed, the impressive results obtained with these compounds provided further proof of the role of BcR in CLL. In this article, the key steps that led to the determination of the role of BcR are reviewed, including the features of the CLL cell repertoire and the fine mechanisms causing BcR engagement and cell signaling. Furthermore, we discuss the biological effects of the engagement, which can lead to cell survival/proliferation or apoptosis depending on certain intrinsic cell characteristics and on signals that the micro-environment can deliver to the leukemic cells. In addition, consideration is given to alternative mechanisms promoting cell proliferation in the absence of BcR signaling, which can explain in part the incomplete effectiveness of TKI therapies. The role of the BcR in determining clonal evolution and disease progression is also described. Finally, we discuss possible models to explain the selection of a special BcR set during leukemogenesis. The BcR may deliver activation signals to the cells, which lead to their uncontrolled growth, with the possible collaboration of other still-undefined events which are capable of deregulating the normal physiological response of B cells to BcR-delivered stimuli.

Aberrant B Cell Signaling in Autoimmune Diseases

Aberrant B cell signaling plays a critical in role in various systemic and organ-specific autoimmune diseases. This is supported by genetic evidence by many functional studies in B cells from patients or specific animal models and by the observed efficacy of small-molecule inhibitors. In this review, we first discuss key signal transduction pathways downstream of the B cell receptor (BCR) that ensure that autoreactive B cells are removed from the repertoire or functionally silenced. We provide an overview of aberrant BCR signaling that is associated with inappropriate B cell repertoire selection and activation or survival of peripheral B cell populations and plasma cells, finally leading to autoantibody formation. Next to BCR signaling, abnormalities in other signal transduction pathways have been implicated in autoimmune disease. These include reduced activity of several phosphates that are downstream of co-inhibitory receptors on B cells and increased levels of BAFF and APRIL, which support survival of B cells and plasma cells. Importantly, pathogenic synergy of the BCR and Toll-like receptors (TLR), which can be activated by endogenous ligands, such as self-nucleic acids, has been shown to enhance autoimmunity. Finally, we will briefly discuss therapeutic strategies for autoimmune disease based on interfering with signal transduction in B cells.

High surface IgM levels associate with shorter response to ibrutinib and BTK bypass in patients with CLL

Chronic lymphocytic leukemia (CLL) cells have variably low surface IgM (sIgM) levels/signaling capacity, influenced by chronic antigen engagement at tissue sites. Within these low levels, CLL with relatively high sIgM (CLLhigh) progresses more rapidly than CLL with low sIgM (CLLlow). During ibrutinib therapy, surviving CLL cells redistribute into the peripheral blood and can recover sIgM expression. Return of CLL cells to tissue may eventually recur, where cells with high sIgM could promote tumor growth. We analyzed time to new treatment (TTNT) following ibrutinib in 70 patients with CLL (median follow-up of 66 months) and correlated it with pretreatment sIgM levels and signaling characteristics. Pretreatment sIgM levels correlated with signaling capacity, as measured by intracellular Ca2+ mobilization (iCa2+), in vitro (r = 0.70; P < .0001). High sIgM levels/signaling strongly correlated with short TTNT (P < .05), and 36% of patients with CLLhigh vs 8% of patients with CLLlow progressed to require a new treatment. In vitro, capacity of ibrutinib to inhibit sIgM-mediated signaling inversely correlated with pretherapy sIgM levels (r = -0.68; P = .01) or iCa2+ (r = -0.71; P = .009). In patients, sIgM-mediated iCa2+ and ERK phosphorylation levels were reduced by ibrutinib therapy but not abolished. The residual signaling capacity downstream of BTK was associated with high expression of sIgM, whereas it was minimal when sIgM expression was low (P < .05). These results suggested that high sIgM levels facilitated CLL cell resistance to ibrutinib in patients. The CLL cells, surviving in the periphery with high sIgM expression, include a dangerous fraction that is able to migrate to tissue and receive proliferative stimuli, which may require targeting by combined approaches.

Bruton tyrosine kinase inhibitors in B-cell lymphoma: beyond the antitumour effect

Targeting B-cell receptor signalling using Bruton tyrosine kinase (BTK) inhibitors (BTKis) has become a highly successful treatment modality for B-cell malignancies, especially for chronic lymphocytic leukaemia. However, long-term administration of BTKis can be complicated by adverse on- and/or off-target effects in particular cell types. BTK is widely expressed in cells of haematopoietic origin, which are pivotal components of the tumour microenvironment. BTKis, thus, show broad immunomodulatory effects on various non-B immune cell subsets by inhibiting specific immune receptors, including T-cell receptor and Toll-like receptors. Furthermore, due to the off-target inhibition of other kinases, such as IL-2-inducible T-cell kinase, epidermal growth factor receptor, and the TEC and SRC family kinases, BTKis have additional distinct effects on T cells, natural killer cells, platelets, cardiomyocytes, and other cell types. Such mechanisms of action might contribute to the exceptionally high clinical efficacy as well as the unique profiles of adverse effects, including infections, bleeding, and atrial fibrillation, observed during BTKi administration. However, the immune defects and related infections caused by BTKis have not received sufficient attention in clinical studies till date. The broad involvement of BTK in immunological pathways provides a rationale to combine BTKis with specific immunotherapies, such as immune checkpoint inhibitor or chimeric antigen receptor-T-cell therapy, for the treatment of relapsed or refractory diseases. This review discusses and summarises the above-mentioned issues as a reference for clinicians and researchers.

Unveiling the B cell receptor structure

Molecular structures provide a road map for understanding and controlling B cell receptor activation.

Cellular Mechanisms Underlying B Cell Abnormalities in Patients With Gain-of-Function Mutations in the PIK3CD Gene

Background

Activated phosphoinositide 3 kinase (PI3K) -delta syndrome (APDS) is an inborn error of immunity with variable clinical phenotype of immunodeficiency and immune dysregulation and caused by gain-of-function mutations in PIK3CD. The hallmark of immune phenotype is increased proportions of transitional B cells and plasmablasts (PB), progressive B cell loss, and elevated levels of serum IgM.

Objective

To explore unique B cell subsets and the pathomechanisms driving B cell dysregulation beyond the transitional B cell stage in APDS.

Methods

Clinical and immunological data was collected from 24 patients with APDS. In five cases, we performed an in-depth analysis of B cell phenotypes and cultured purified naïve B cells to evaluate their survival, activation, Ig gene class switch recombination (CSR), PB differentiation and antibody secretion. We also analyzed PB differentiation capacity of sorted CD27^-IgD^- double-negative B (DNB) cells.

Results

The patients had increased B cell sizes and higher proportions of IgM⁺ DNB cells than healthy controls (HC). Their naïve B cells exhibited increased death, impaired CSR but relatively normal PB differentiation. Upon stimulation, patient’s DNB cells secreted a similar level of IgG but a higher level of IgM than DNB cells from HC. Targeted therapy of PI3K inhibition partially restored B cell phenotypes.

Conclusions

The present study suggests additional mechanistic insight into B cell pathology of APDS: (1) decreased peripheral B cell numbers may be due to the increased death of naïve B cells; (2) larger B cell sizes and expanded DNB population suggest enhanced activation and differentiation of naïve B cells into DNB cells; (3) the impaired CSR yet normal PB differentiation can predominantly generate IgM-secreting cells, resulting in elevated IgM levels.

Sialic acids on B cells are crucial for their survival and provide protection against apoptosis

Sialic acids (Sias) on the B cell membrane are involved in cell migration, in the control of the complement system and, as sialic acid-binding immunoglobulin-like lectin (Siglec) ligands, in the regulation of cellular signaling. We studied the role of sialoglycans on B cells in a mouse model with B cell-specific deletion of cytidine monophosphate sialic acid synthase (CMAS), the enzyme essential for the synthesis of sialoglycans. Surprisingly, these mice showed a severe B cell deficiency in secondary lymphoid organs. Additional depletion of the complement factor C3 rescued the phenotype only marginally, demonstrating a complement-independent mechanism. The B cell survival receptor BAFF receptor was not up-regulated, and levels of activated caspase 3 and processed caspase 8 were high in B cells of Cmas-deficient mice, indicating ongoing apoptosis. Overexpressed Bcl-2 could not rescue this phenotype, pointing to extrinsic apoptosis. These results show that sialoglycans on the B cell surface are crucial for B cell survival by counteracting several death-inducing pathways.

B-cell receptor signaling induces proteasomal degradation of PDCD4 via MEK1/2 and mTORC1 in malignant B cells

B-cell receptor (BCR) signaling plays a major role in the pathogenesis of B-cell malignancies and is an established target for therapy, including in chronic lymphocytic leukemia cells (CLL), the most common B-cell malignancy. We previously demonstrated that activation of BCR signaling in primary CLL cells downregulated expression of PDCD4, an inhibitor of the translational initiation factor eIF4A and a potential tumor suppressor in lymphoma. Regulation of the PDCD4/eIF4A axis appeared to be important for expression of the MYC oncoprotein as MYC mRNA translation was increased following BCR stimulation and MYC protein induction was repressed by pharmacological inhibition of eIF4A. Here we show that MYC expression is also associated with PDCD4 down-regulation in CLL cells in vivo and characterize the signaling pathways that mediate BCR-induced PDCD4 down-regulation in CLL and lymphoma cells. PDCD4 downregulation was mediated by proteasomal degradation as it was inhibited by proteasome inhibitors in both primary CLL cells and B-lymphoma cell lines. In lymphoma cells, PDCD4 degradation was predominantly dependent on signaling via the AKT pathway. By contrast, in CLL cells, both ERK and AKT pathways contributed to PDCD4 down-regulation and dual inhibition using ibrutinib with either MEK1/2 or mTORC1 inhibition was required to fully reverse PDCD4 down-regulation. Consistent with this, dual inhibition of BTK with MEK1/2 or mTORC1 resulted in the strongest inhibition of BCR-induced MYC expression. This study provides important new insight into the regulation of mRNA translation in B-cell malignancies and a rationale for combinations of kinase inhibitors to target translation control and MYC expression.

Stromal Notch ligands foster lymphopenia-driven functional plasticity and homeostatic proliferation of naïve B cells

In lymphopenic environments, secondary lymphoid organs regulate the size of B and T-cell compartments by supporting homeostatic proliferation of mature lymphocytes. The molecular mechanisms underlying these responses and their functional consequences remain incompletely understood. To evaluate homeostasis of the mature B-cell pool during lymphopenia, we turned to an adoptive transfer model of purified follicular B-cells into Rag2-/- mouse recipients. Highly purified follicular B-cells transdifferentiated into marginal zone-like B-cells when transferred into Rag2-/- lymphopenic hosts, but not into wild-type hosts. In lymphopenic spleens, transferred B-cells gradually lost their follicular phenotype and acquired characteristics of marginal zone B-cells, as judged by cell surface phenotype, expression of integrins and chemokine receptors, positioning close to the marginal sinus, and an ability to rapidly generate functional plasma cells. Initiation of follicular to marginal zone B-cell transdifferentiation preceded proliferation. Furthermore, the transdifferentiation process was dependent on Notch2 receptors in B-cells and expression of Delta-like1 Notch ligands by splenic Ccl19-Cre+ fibroblastic stromal cells. Gene expression analysis showed rapid induction of Notch-regulated transcripts followed by upregulated Myc expression and acquisition of broad transcriptional features of marginal zone B-cells. Thus, naïve mature B-cells are endowed with plastic transdifferentiation potential in response to increased stromal Notch ligand availability during lymphopenia.

Preclinical Analysis of Candidate Anti-Human CD79 Therapeutic Antibodies Using a Humanized CD79 Mouse Model

The BCR comprises a membrane-bound Ig that is noncovalently associated with a heterodimer of CD79A and CD79B. While the BCR Ig component functions to sense extracellular Ag, CD79 subunits contain cytoplasmic ITAMs that mediate intracellular propagation of BCR signals critical for B cell development, survival, and Ag-induced activation. CD79 is therefore an attractive target for Ab and chimeric Ag receptor T cell therapies for autoimmunity and B cell neoplasia. Although the mouse is an attractive model for preclinical testing, due to its well-defined immune system, an obstacle is the lack of cross-reactivity of candidate therapeutic anti-human mAbs with mouse CD79. To overcome this problem, we generated knockin mice in which the extracellular Ig-like domains of CD79A and CD79B were replaced with human equivalents. In this study, we describe the generation and characterization of mice expressing chimeric CD79 and report studies that demonstrate their utility in preclinical analysis of anti-human CD79 therapy. We demonstrate that human and mouse CD79 extracellular domains are functionally interchangeable, and that anti-human CD79 lacking Fc region effector function does not cause significant B cell depletion, but induces 1) decreased expression of plasma membrane-associated IgM and IgD, 2) uncoupling of BCR-induced tyrosine phosphorylation and calcium mobilization, and 3) increased expression of PTEN, consistent with the levels observed in anergic B cells. Finally, anti-human CD79 treatment prevents disease development in two mouse models of autoimmunity. We also present evidence that anti-human CD79 treatment may inhibit Ab secretion by terminally differentiated plasmablasts and plasma cells in vitro.

Competition for refueling rather than cyclic reentry initiation evident in germinal centers

Antibody affinity maturation occurs in germinal centers (GCs) through iterative rounds of somatic hypermutation and proliferation in dark zones (DZs) and selection in light zones (LZs). GC B cells exit cell cycle a number of hours before entering LZs; therefore, continued participation in responses requires that they subsequently reenter cell cycle and move back to DZs, a process known as cyclic reentry. Affinity enhancements are thought to arise by B cells having to compete to initiate cyclic reentry each time they enter LZs, with T cell help being a major determinant; however, direct proof is lacking. Using Fucci2 mice, we confirmed an association between B cell receptor affinity and the first step of cyclic reentry, S phase initiation from a resting LZ state. However, neither T cell ablation nor MHCII deletion prevented resting LZ cells from reentering cell cycle, and this late G₁-S transition was also not detectably restricted by competition. In contrast, using BATF induction as exemplar, we found that T cells "refueled" LZ cells in an affinity-dependent manner that was limited by both competition and cells' intrinsic antigen-acquiring abilities. Therefore, cyclic reentry initiation and B cell refueling are independently regulated in GCs, which may contribute to permitting cells of different competencies to be sustained alongside each other and allow T cell support to be provided across a dynamic range commensurate with affinity. We speculate that this less binary selection mechanism could help GCs nurture complex antibody maturation pathways and support the clonal diversity required for countering fast-evolving pathogens.

Differential Requirement of Vav Proteins for Btk-dependent and –Independent Signaling During B Cell Development

Btk and Vav proteins are all components of the signalosome that builds upon B cell receptor (BCR) activation. However, the role of Vav proteins within the signalosome is quite complex and not yet fully understood. Until now, studies of these have focused predominantly on a deficiency of Vav proteins alone or in combination with other Vav protein family members. Since a physical association of Btk with Vav was shown previously, we asked whether these molecules lie in the same or independent signaling pathways. By analyzing Vav1 and Vav3 single knock-out mice and generating double-knock-out animals deficient for either Vav1 or Vav3 and Btk, we observed, in line with previous publications, no severe B cell developmental defects when either Vav1 or Vav3 alone are not expressed. However, a simultaneous deficiency of Btk together with either Vav1 or Vav3 leads to a severe reduction of splenic B cells, which exhibit an immature phenotype. B cell developmental defects of Btk/Vav1-double deficient mice in the periphery were more severe than those observed in Btk-single-deficient animals. Additionally, morphological changes in splenic microarchitecture were observed in double- but also in single-knock-out mutants. These observations were accompanied by reduced BCR-induced Ca²⁺ mobilization, proliferation, germinal center formation and immunoglobulin secretion. Although deletion of Btk alone impaired Ca²⁺ mobilization upon BCR activation, the defect was even more severe when Vav1 or Vav3 were also mutated, indicating that Btk and the Vav proteins act in separate pathways that converge on Ca2+ signaling. In vitro ASC differentiation suggests that both B and T cells contribute to the observed phenotype of a Btk/Vav-double deficiency. Our results show that Vav proteins and Btk are both components of the BCR-activated signalosome but control separate signaling pathways important for B cell development.

Regulation of B-Cell Receptor Signaling and Its Therapeutic Relevance in Aggressive B-Cell Lymphomas

The proliferation and survival signals emanating from the B-cell receptor (BCR) constitute a crucial aspect of mature lymphocyte's life. Dysregulated BCR signaling is considered a potent contributor to tumor survival in different subtypes of B-cell non-Hodgkin lymphomas (B-NHLs). In the last decade, the emergence of BCR-associated kinases as rational therapeutic targets has led to the development and approval of several small molecule inhibitors targeting either Bruton's tyrosine kinase (BTK), spleen tyrosine kinase (SYK), or phosphatidylinositol 3 kinase (PI3K), offering alternative treatment options to standard chemoimmunotherapy, and making some of these drugs valuable assets in the anti-lymphoma armamentarium. Despite their initial effectiveness, these precision medicine strategies are limited by primary resistance in aggressive B-cell lymphoma such as diffuse large B-cell lymphoma (DLBCL) and mantle cell lymphoma (MCL), especially in the case of first generation BTK inhibitors. In these patients, BCR-targeting drugs often fail to produce durable responses, and nearly all cases eventually progress with a dismal outcome, due to secondary resistance. This review will discuss our current understanding of the role of antigen-dependent and antigen-independent BCR signaling in DLBCL and MCL and will cover both approved inhibitors and investigational molecules being evaluated in early preclinical studies. We will discuss how the mechanisms of action of these molecules, and their off/on-target effects can influence their effectiveness and lead to toxicity, and how our actual knowledge supports the development of more specific inhibitors and new, rationally based, combination therapies, for the management of MCL and DLBCL patients.

Unique B-1 cells specific for both N-pyrrolated proteins and DNA evolve with apolipoprotein E deficiency

Lysine N-pyrrolation, a posttranslational modification, which converts lysine residues to N^ε-pyrrole-L-lysine, imparts electronegative properties to proteins, causing them to mimic DNA. Apolipoprotein E (apoE) has been identified as a soluble receptor for pyrrolated proteins (pyrP), and accelerated lysine N-pyrrolation has been observed in apoE-deficient (apoE^−/−) hyperlipidemic mice. However, the impact of pyrP accumulation consequent to apoE deficiency on the innate immune response remains unclear. Here, we investigated B-1a cells known to produce germline-encoded immunoglobulin M (IgM) from mice deficient in apoE and identified a particular cell population that specifically produces IgM antibodies against pyrP and DNA. We demonstrated an expansion of B-1a cells involved in IgM production in the peritoneal cavity of apoE^−/− mice compared with wild-type mice, consistent with a progressive increase of IgM response in the mouse sera. We found that pyrP exhibited preferential binding to B-1a cells and facilitated the production of IgM. B cell receptor analysis of pyrP-specific B-1a cells showed restricted usage of gene segments selected from the germline gene set; most sequences contained high levels of non-templated-nucleotide additions (N-additions) that could contribute to junctional diversity of B cell receptors. Finally, we report that a subset of monoclonal IgM antibodies against pyrP/DNA established from the apoE^−/− mice also contained abundant N-additions. These results suggest that the accumulation of pyrP due to apoE deficiency may influence clonal diversity in the pyrP-specific B cell repertoire. The discovery of these unique B-1a cells for pyrP/DNA provides a key link connecting covalent protein modification, lipoprotein metabolism, and innate immunity.

Public Immunity: Evolutionary Spandrels for Pathway-Amplifying Protective Antibodies

Humoral immunity is seeded by affinity between the B cell receptor (BCR) and cognate antigen. While the BCR is a chimeric display of diverse antigen engagement solutions, we discuss its functional activity as an ‘innate-like’ immune receptor, wherein genetically hardwired antigen complementarity can serve as reproducible templates for pathway-amplifying otherwise immunologically recessive antibody responses. We propose that the capacity for germline reactivity to new antigen emerged as a set of evolutionary spandrels or coupled traits, which can now be exploited by rational vaccine design to focus humoral immunity upon conventionally immune-subdominant antibody targets. Accordingly, we suggest that evolutionary spandrels account for the necessary but unanticipated antigen reactivity of the germline antibody repertoire.

Temporal dynamics of persistent germinal centers and memory B cell differentiation following respiratory virus infection

Following infection or immunization, memory B cells (MBCs) and long-lived plasma cells provide humoral immunity that can last for decades. Most principles of MBC biology have been determined with hapten-protein carrier models or fluorescent protein immunizations. Here, we examine the temporal dynamics of the germinal center (GC) B cell and MBC response following mouse influenza A virus infection. We find that antiviral B cell responses within the lung-draining mediastinal lymph node (mLN) and the spleen are distinct in regard to duration, enrichment for antigen-binding cells, and class switching dynamics. While splenic GCs dissolve after 6 weeks post-infection, mLN hemagglutinin-specific (HA⁺) GCs can persist for 22 weeks. Persistent GCs continuously differentiate MBCs, with “peak” and “late” GCs contributing equal numbers of HA⁺ MBCs to the long-lived compartment. Our findings highlight critical aspects of persistent GC responses and MBC differentiation following respiratory virus infection with direct implications for developing effective vaccination strategies.

B Cell Recognition of Candida albicans Hyphae via TLR 2 Promotes IgG1 and IL-6 Secretion for TH17 Differentiation

Candida albicans is usually a benign member of the human gut microbiota, but can become pathogenic under certain circumstances, for example in an immunocompromised host. The innate immune system, in particular neutrophils and macrophages, constitutes a crucial first line of defense against fungal invasion, however adaptive immunity may provide long term protection and thus allow vaccination of at risk patients. While TH1 and TH17 cells are important for antifungal responses, the role of B cells and antibodies in protection from C. albicans infection is less well defined. In this study, we show that C. albicans hyphae but not yeast, as well as fungal cell wall components, directly activate B cells via MyD88 signaling triggered by Toll- like receptor 2, leading to increased IgG1 production. While Dectin-1 signals and specific recognition by the B cell receptor are dispensable for B cell activation in this system, TLR2/MyD88 signals cooperate with CD40 signals in promoting B cell activation. Importantly, recognition of C. albicans via MyD88 signaling is also essential for induction of IL-6 secretion by B cells, which promotes TH17 polarization in T-B cell coculture experiments. B cells may thus be activated directly by C. albicans in its invasive form, leading to production of antibodies and T cell help for fungal clearance.

Genome edited B cells: a new frontier in immune cell therapies

Cell therapies based on reprogrammed adaptive immune cells have great potential as "living drugs." As first demonstrated clinically for engineered chimeric antigen receptor (CAR) T cells, the ability of such cells to undergo clonal expansion in response to an antigen promotes both self-renewal and self-regulation in vivo. B cells also have the potential to be developed as immune cell therapies, but engineering their specificity and functionality is more challenging than for T cells. In part, this is due to the complexity of the immunoglobulin (Ig) locus, as well as the requirement for regulated expression of both cell surface B cell receptor and secreted antibody isoforms, in order to fully recapitulate the features of natural antibody production. Recent advances in genome editing are now allowing reprogramming of B cells by site-specific engineering of the Ig locus with preformed antibodies. In this review, we discuss the potential of engineered B cells as a cell therapy, the challenges involved in editing the Ig locus and the advances that are making this possible, and envision future directions for this emerging field of immune cell engineering.

MicroRNA regulation of B cell receptor signaling

B lymphocytes play a central role in host immune defense. B cell receptor (BCR) signaling regulates survival, proliferation, and differentiation of B lymphocytes. Signaling through the BCR signalosome is a multi-component cascade that is tightly regulated and is important in the coordination of B cell differentiation and function. At different stages of development, B cells that have BCRs recognizing self are eliminated to prevent autoimmunity. microRNAs (miRNAs) are small single-stranded non-coding RNAs that contribute to post-transcriptional regulation of gene expression and have been shown to orchestrate cell fate decisions through the regulation of lineage-specific transcriptional profiles. Studies have identified miRNAs to be crucial for B cell development in the bone marrow and their subsequent population of the peripheral immune system. In this review, we focus on the role of miRNAs in the regulation of BCR signaling as it pertains to B lymphocyte development and function. In particular, we discuss the most recent studies describing the role of miRNAs in the regulation of both early B cell development and peripheral B cell responses and examine the ways by which miRNAs regulate signal downstream of B cell antigen receptor to prevent aberrant activation and autoimmunity.

Der selektive Nachweis von HLA-spezifischen B-Gedächtniszellen

In der Transplantationsdiagnostik werden die Seren der Patienten auf der Warteliste jedes Quartal auf präexistierende Anti-HLA-Antikörper untersucht. Jedoch kann die Existenz von zirkulierenden peripheren Antikörpern, die von knochenmarkresidierenden Plasmazellen sezerniert wurden, keine zuverlässige Information über das Zellkompartment von B-Gedächtniszellen geben. Die Exposition mit einem inkompatiblen HLA-Antigen kann immer die Aktivierung vorhandener B-Gedächtniszellen auslösen. Die Analyse von B-Gedächtniszellen ermöglicht die Erhebung von belastbaren Daten über den individuellen Immunstatus eines Patienten.

Insertion of atypical glycans into the tumor antigen-binding site identifies DLBCLs with distinct origin and behavior

Glycosylation of the surface immunoglobulin (Ig) variable region is a remarkable follicular lymphoma-associated feature rarely seen in normal B cells. Here, we define a subset of diffuse large B-cell lymphomas (DLBCLs) that acquire N-glycosylation sites selectively in the Ig complementarity-determining regions (CDRs) of the antigen-binding sites. Mass spectrometry and X-ray crystallography demonstrate how the inserted glycans are stalled at oligomannose-type structures because they are buried in the CDR loops. Acquisition of sites occurs in ∼50% of germinal-center B-cell-like DLBCL (GCB-DLBCL), mainly of the genetic EZB subtype, irrespective of IGHV-D-J use. This markedly contrasts with the activated B-cell-like DLBCL Ig, which rarely has sites in the CDR and does not seem to acquire oligomannose-type structures. Acquisition of CDR-located acceptor sites associates with mutations of epigenetic regulators and BCL2 translocations, indicating an origin shared with follicular lymphoma. Within the EZB subtype, these sites are associated with more rapid disease progression and with significant gene set enrichment of the B-cell receptor, PI3K/AKT/MTORC1 pathway, glucose metabolism, and MYC signaling pathways, particularly in the fraction devoid of MYC translocations. The oligomannose-type glycans on the lymphoma cells interact with the candidate lectin dendritic cell-specific intercellular adhesion molecule 3 grabbing non-integrin (DC-SIGN), mediating low-level signals, and lectin-expressing cells form clusters with lymphoma cells. Both clustering and signaling are inhibited by antibodies specifically targeting the DC-SIGN carbohydrate recognition domain. Oligomannosylation of the tumor Ig is a posttranslational modification that readily identifies a distinct GCB-DLBCL category with more aggressive clinical behavior, and it could be a potential precise therapeutic target via antibody-mediated inhibition of the tumor Ig interaction with DC-SIGN-expressing M2-polarized macrophages.

Protein kinase Cδ is essential for the IgG response against T-cell-independent type 2 antigens and commensal bacteria

Antigens (Ags) with multivalent and repetitive structure elicit IgG production in a T-cell-independent manner. However, the mechanisms by which such T-cell-independent type-2 (TI-2) Ags induce IgG responses remain obscure. Here, we report that B-cell receptor (BCR) engagement with a TI-2 Ag but not with a T-cell-dependent (TD) Ag was able to induce the transcription of Aicda encoding activation-induced cytidine deaminase (AID) and efficient class switching to IgG3 upon costimulation with IL-1 or IFN-α in mouse B cells. TI-2 Ags strongly induced the phosphorylation of protein kinase C (PKC)δ and PKCδ mediated the Aicda transcription through the induction of BATF, the key transcriptional regulator of Aicda. In PKCδ-deficient mice, production of IgG was intact against TD Ag but abrogated against typical TI-2 Ags as well as commensal bacteria, and experimental disruption of the gut epithelial barrier resulted in fatal bacteremia. Thus, our results have revealed novel molecular requirements for class switching in the TI-2 response and highlighted its importance in homeostatic commensal-specific IgG production.

When the human body faces a potentially harmful microorganism, the immune system responds by finding and destroying the pathogen. This involves the coordination of several different parts of the immune system. B cells are a type of white blood cell that is responsible for producing antibodies: large proteins that bind to specific targets such as pathogens. B cells often need help from other immune cells known as T cells to complete antibody production.

However, T cells are not required for B cells to produce antibodies against some bacteria. For example, when certain pathogenic bacteria coated with a carbohydrate called a capsule – such as pneumococcus, which causes pneumonia, or salmonella – invade our body, B cells recognize a repetitive structure of the capsule using a B-cell antigen receptor. This recognition allows B cells to produce antibodies independently of T cells. It is unclear how B cells produce antibodies in this situation or what proteins are required for this activity.

To understand this process, Fukao et al. used genetically modified mice and their B cells to study how they produce antibodies independently of T cells. They found that a protein called PKCδ is critical for B cells to produce antibodies, especially of an executive type called IgG, in the T-cell-independent response. PKCδ became active when B cells were stimulated with the repetitive antigen present on the surface of bacteria like salmonella or pneumococcus. Mice that lack PKCδ were unable to produce IgG independently of T cells, leading to fatal infections when bacteria reached the tissues and blood.

Understanding the mechanism behind the T cell-independent B cell response could lead to more effective antibody production, potentially paving the way for new vaccines to prevent fatal diseases caused by pathogenic bacteria.

The resistance mechanisms and treatment strategies of BTK inhibitors in B-cell lymphoma

Bruton's tyrosine kinase inhibitors (BTKi) have revolutionized the treatment of B‐cell lymphoma (BCL). These drugs interfere with the mechanisms underlying malignant B‐cell pathophysiology, allowing better drug response as well as low toxicity. However, these multiple mechanisms also lead to drug resistance, which compromised the treatment outcome and needs to be solved urgently. This review focuses on genomic variations (such as BTK and its downstream PCLG2 mutations as well as Del 8p, 2p+, Del 6q/8p, BIRC3, TRAF2, TRAF3, CARD11, MYD88, and CCND1 mutations) and related pathways (such as PI3K/Akt/mTOR, NF‐κB, MAPK signaling pathways, overexpression of B‐cell lymphoma 6, platelet‐derived growth factor, toll‐like receptors, and microenvironment, cancer stem cells, and exosomes) involved in cancer pathophysiology to discuss the mechanisms underlying resistance to BTKi. We have also reviewed the newly reported drug resistance mechanisms and the proposed potential treatment strategies (the next‐generation BTKi, proteolysis‐targeting chimera‐BTK, XMU‐MP‐3, PI3K‐Akt‐mTOR pathway, MYC or LYN kinase inhibitor, and other small‐molecule targeted drugs) to overcome drug resistance. The findings presented in this review lay a strong foundation for further research in this field.

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.

Critical requirement for BCR, BAFF, and BAFFR in memory B cell survival

Memory B cells (MBCs) are long-lived cells that form a critical part of immunological memory, providing rapid antibody responses to recurring infections. However, very little is known about signals controlling MBC survival. Previous work has shown that antigen is not required for MBC survival, but a requirement for the B cell antigen receptor (BCR) has not been tested. Other studies have shown that, unlike naive B cells, MBCs do not express BAFFR and their survival is independent of BAFF, the ligand for BAFFR. Here, using inducible genetic ablation, we show that survival of MBCs is critically dependent on the BCR and on signaling through the associated CD79A protein. Unexpectedly, we found that MBCs express BAFFR and that their survival requires BAFF and BAFFR; hence, loss of BAFF or BAFFR impairs recall responses. Finally, we show that MBC survival requires IKK2, a kinase that transduces BAFFR signals. Thus, MBC survival is critically dependent on signaling from BCR and BAFFR.

Exploring the pathways to chronic lymphocytic leukemia

In chronic lymphocytic leukemia (CLL), increasing knowledge of the biology of the tumor cells has led to transformative improvements in our capacity to assess and treat patients. The dependence of tumor cells on surface immunoglobulin receptor signaling, survival pathways, and accessory cells within the microenvironment has led to a successful double-barreled attack with designer drugs. Studies have revealed that CLL should be classified based on the mutational status of the expressed IGHV sequences into 2 diseases, either unmutated (U) or mutated (M) CLL, each with a distinctive cellular origin, biology, epigenetics/genetics, and clinical behavior. The origin of U-CLL lies among the natural antibody repertoire, and dominance of IGHV1-69 reveals a superantigenic driver. In both U-CLL and M-CLL, a calibrated stimulation of tumor cells by self-antigens apparently generates a dynamic reiterative cycle as cells, protected from apoptosis, transit between blood and tissue sites. But there are differences in outcome, with the balance between proliferation and anergy favoring anergy in M-CLL. Responses are modulated by an array of microenvironmental interactions. Availability of T-cell help is a likely determinant of cell fate, the dependency on which varies between U-CLL and M-CLL, reflecting the different cells of origin, and affecting clinical behavior. Despite such advances, cell-escape strategies, Richter transformation, and immunosuppression remain as challenges, which only may be met by continued research into the biology of CLL.

Bruton's Tyrosine Kinase Inhibition as an Emerging Therapy in Systemic Autoimmune Disease

Systemic autoimmune disorders are complex heterogeneous chronic diseases involving many different immune cells. A significant proportion of patients respond poorly to therapy. In addition, the high burden of adverse effects caused by "classical" anti-rheumatic or immune modulatory drugs provides a need to develop more specific therapies that are better tolerated. Bruton's tyrosine kinase (BTK) is a crucial signaling protein that directly links B-cell receptor (BCR) signals to B-cell activation, proliferation, and survival. BTK is not only expressed in B cells but also in myeloid cells, and is involved in many different signaling pathways that drive autoimmunity. This makes BTK an interesting therapeutic target in the treatment of autoimmune diseases. The past decade has seen the emergence of first-line BTK small-molecule inhibitors with great efficacy in the treatment of B-cell malignancies, but with unfavorable safety profiles for use in autoimmunity due to off-target effects. The development of second-generation BTK inhibitors with superior BTK specificity has facilitated the investigation of their efficacy in clinical trials with autoimmune patients. In this review, we discuss the role of BTK in key signaling pathways involved in autoimmunity and provide an overview of the different inhibitors that are currently being investigated in clinical trials of systemic autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus, as well as available results from completed trials.