Naive and memory human B cells have distinct requirements for STAT3 activation to differentiate into antibody-secreting plasma cells

Functional validation of a novel STAT3 'variant of unknown significance' identifies a new case of STAT3 GOF syndrome and reveals broad immune cell defects

INTRODUCTION

Signal transducer and activator of transcription 3 (STAT3) orchestrates crucial immune responses through its pleiotropic functions as a transcription factor. Patients with germline monoallelic dominant negative or hypermorphic STAT3 variants, who present with immunodeficiency and/or immune dysregulation, have revealed the importance of balanced STAT3 signaling in lymphocyte differentiation and function, and immune homeostasis. Here, we report a novel missense variant of unknown significance in the DNA-binding domain of STAT3 in a patient who experienced hypogammaglobulinemia, lymphadenopathy, hepatosplenomegaly, immune thrombocytopenia, eczema, and enteropathy over a 35-year period.

METHODS

In vitro demonstration of prolonged STAT3 activation due to delayed dephosphorylation, and enhanced transcriptional activity, confirmed this to be a novel pathogenic STAT3 gain-of-function variant. Peripheral blood lymphocytes from this patient, and patients with confirmed STAT3 Gain-of-function Syndrome, were collected to investigate mechanisms of disease pathogenesis.

RESULTS

B cell dysregulation was evidenced by a loss of class-switched memory B cells and a significantly expanded CD19hiCD21lo B cell population, likely influenced by a skewed CXCR3+ TFH population. Interestingly, unlike STAT3 dominant negative variants, cytokine secretion by activated peripheral blood STAT3 GOF CD4+ T cells and frequencies of Treg cells were intact, suggesting CD4+ T cell dysregulation likely occurs at sites of disease rather than the periphery.

CONCLUSION

This study provides an in-depth case study in confirming a STAT3 gain-of-function variant and identifies lymphocyte dysregulation in the peripheral blood of patients with STAT3 gain-of-function syndrome. Identifying cellular biomarkers of disease provides a flow cytometric-based screen to guide validation of additional novel STAT3 gain-of-function variants as well as provide insights into putative mechanisms of disease pathogenesis.

IL-21-STAT3 axis negatively regulates LAIR1 expression in B cells

LAIR1 is an inhibitory receptor broadly expressed on human immune cells, including B cells. LAIR1 has been shown to modulate BCR signaling, however, it is still unclear whether its suppressive activity can be a negative regulator for autoreactivity. In this study, we demonstrate the LAIR1 expression profile on human B cells and prove its regulatory function and relationships to B cell autoreactivity. We show that both the frequency and level of LAIR1 expression decreases during B cell differentiation. LAIR1 expressing (LAIR1 + ) switched memory (SWM) B cells have a transcriptional profile less differentiated toward a plasma cell (PC) phenotype, harbor more autoreactive B cells and exhibit less PC differentiation in vitro than the LAIR1 negative (LAIR1 - ) counterpart. These data suggests that LAIR1 functions as a B cell tolerance checkpoint. We confirm previous data showing that patients with systemic lupus erythematosus (SLE) express less LAIR1 on B cells, implying a breakdown of the checkpoint, consistent with the enhanced PC differentiation seen in SLE. We further demonstrate that LAIR1 expression is down-regulated through the IL-21/STAT3 pathway which is known to be upregulated in SLE. These data suggest therapeutic targets that might decrease the aberrant PC differentiation observed in SLE.

Defective germinal center selection results in persistence of self-reactive B cells from the primary to the secondary repertoire in Primary Antiphospholipid Syndrome

Primary antiphospholipid syndrome (PAPS) is a life-threatening clotting disorder mediated by pathogenic autoantibodies. Here we dissect the origin of self-reactive B cells in human PAPS using peripheral blood and bone marrow of patients with triple-positive PAPS via combined single-cell RNA sequencing, B cell receptors (BCR) repertoire profiling, CITEseq analysis and single cell immortalization. We find that antiphospholipid (aPL)-specific B cells are present in the naive compartment, polyreactive, and derived from the natural repertoire. Furthermore, B cells with aPL specificities are not eliminated in patients with PAPS, persist until the memory and long-lived plasma cell stages, likely after defective germinal center selection, while becoming less polyreactive. Lastly, compared with the non-PAPS cells, PAPS B cells exhibit distinct IFN and APRIL signature as well as dysregulated mTORC1 and MYC pathways. Our findings may thus elucidate the survival mechanisms of these autoreactive B cells and suggest potential therapeutic targets for the treatment of PAPS.

Unraveling the unphosphorylated STAT3-unphosphorylated NF-κB pathway in loss of function STAT3 Hyper IgE syndrome

Background

Patients with loss of function signal transducer and activator of transcription 3-related Hyper IgE Syndrome (LOF STAT3 HIES) present with recurrent staphylococcal skin and pulmonary infections along with the elevated serum IgE levels, eczematous rashes, and skeletal and facial abnormalities. Defective STAT3 signaling results in reduced Th17 cells and an impaired IL-17/IL-22 response primarily due to a compromised canonical Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway that involves STAT3 phosphorylation, dimerization, nuclear translocation, and gene transcription. The non-canonical pathway involving unphosphorylated STAT3 and its role in disease pathogenesis, however, is unexplored in HIES.

Objective

This study aims to elucidate the role of unphosphorylated STAT3-unphosphorylated NF-κB (uSTAT3-uNF-κB) activation pathway in LOF STAT3 HIES patients.

Methodology

The mRNA expression of downstream molecules of unphosphorylated STAT3-unphosphorylated NF-κB pathway was studied in five LOF STAT3 HIES patients and transfected STAT3 mutants post-IL-6 stimulation. Immunoprecipitation assays were performed to assess the binding of STAT3 and NF-κB to RANTES promoter.

Results

A reduced expression of the downstream signaling molecules of the uSTAT3-uNF-κB complex pathway, viz., RANTES, STAT3, IL-6, IL-8, ICAM1, IL-8, ZFP36L2, CSF1, MRAS, and SOCS3, in LOF STAT3 HIES patients as well as the different STAT3 mutant plasmids was observed. Immunoprecipitation studies showed a reduced interaction of STAT3 and NF-κB to RANTES in HIES patients.

Conclusion

The reduced expression of downstream signaling molecules, specially RANTES and STAT3, confirmed the impaired uSTAT3-uNF-κB pathway in STAT3 LOF HIES. Decreased levels of RANTES and STAT3 could be a significant component in the disease pathogenesis of Hyper IgE Syndrome.

From bench to bedside: the past, present and future of IL-21 immunotherapy

As immunotherapy gains momentum as a promising approach for treating several types of cancer, IL-21 has emerged as the latest discovery within the γ chain cytokine family, known for its decisive effects on innate and adaptive immunity and immunopathology. Through the modulation of immune cells, IL-21 has demonstrated significant anti-tumor effects in preclinical studies. The potential of IL-21 in cancer treatment has been explored in phase I and II clinical trials, where it has been utilized both as monotherapy and in combination with other drug agents. Further investigation, alongside larger studies, is necessary before final evaluation and application of IL-21 as immunotherapy. This review aims to summarize these pre-clinical and clinical studies and to discuss the possible future directions of IL-21 immunotherapy development. Such a study may be helpful to accelerate the process of clinical application for IL21 immunotherapy.

Dysfunctional mitochondria, disrupted levels of reactive oxygen species, and autophagy in B cells from common variable immunodeficiency patients

Introduction

Common Variable Immunodeficiency (CVID) patients are characterized by hypogammaglobulinemia and poor response to vaccination due to deficient generation of memory and antibody-secreting B cells. B lymphocytes are essential for the development of humoral immune responses, and mitochondrial function, hreactive oxygen species (ROS) production and autophagy are crucial for determining B-cell fate. However, the role of those basic cell functions in the differentiation of human B cells remains poorly investigated.

Methods

We used flow cytometry to evaluate mitochondrial function, ROS production and autophagy processes in human naïve and memory B-cell subpopulations in unstimulated and stimulated PBMCs cultures. We aimed to determine whether any alterations in these processes could impact B-cell fate and contribute to the lack of B-cell differentiation observed in CVID patients.

Results

We described that naïve CD19+CD27- and memory CD19+CD27+ B cells subpopulations from healthy controls differ in terms of their dependence on these processes for their homeostasis, and demonstrated that different stimuli exert a preferential cell type dependent effect. The evaluation of mitochondrial function, ROS production and autophagy in naïve and memory B cells from CVID patients disclosed subpopulation specific alterations. Dysfunctional mitochondria and autophagy were more prominent in unstimulated CVID CD19+CD27- and CD19+CD27+ B cells than in their healthy counterparts. Although naïve CD19+CD27- B cells from CVID patients had higher basal ROS levels than controls, their ROS increase after stimulation was lower, suggesting a disruption in ROS homeostasis. On the other hand, memory CD19+CD27+ B cells from CVID patients had both lower ROS basal levels and a diminished ROS production after stimulation with anti-B cell receptor (BCR) and IL-21.

Conclusion

The failure in ROS cell signalling could impair CVID naïve B cell activation and differentiation to memory B cells. Decreased levels of ROS in CVID memory CD19+CD27+ B cells, which negatively correlate with their in vitro cell death and autophagy, could be detrimental and lead to their previously demonstrated premature death. The final consequence would be the failure to generate a functional B cell compartment in CVID patients.

The trajectory of human B-cell function, immune deficiency, and allergy revealed by inborn errors of immunity

The essential role of B cells is to produce protective immunoglobulins (Ig) that recognize, neutralize, and clear invading pathogens. This results from the integration of signals provided by pathogens or vaccines and the stimulatory microenvironment within sites of immune activation, such as secondary lymphoid tissues, that drive mature B cells to differentiate into memory B cells and antibody (Ab)-secreting plasma cells. In this context, B cells undergo several molecular events including Ig class switching and somatic hypermutation that results in the production of high-affinity Ag-specific Abs of different classes, enabling effective pathogen neutralization and long-lived humoral immunity. However, perturbations to these key signaling pathways underpin immune dyscrasias including immune deficiency and autoimmunity or allergy. Inborn errors of immunity that disrupt critical immune pathways have identified non-redundant requirements for eliciting and maintaining humoral immune memory but concomitantly prevent immune dysregulation. Here, we will discuss our studies on human B cells, and how our investigation of cytokine signaling in B cells have identified fundamental requirements for memory B-cell formation, Ab production as well as regulating Ig class switching in the context of protective versus allergic immune responses.

A Novel Heterozygous Variant in AICDA Impairs Ig Class Switching and Somatic Hypermutation in Human B Cells and is Associated with Autosomal Dominant HIGM2 Syndrome

B cells and their secreted antibodies are fundamental for host-defense against pathogens. The generation of high-affinity class switched antibodies results from both somatic hypermutation (SHM) of the immunoglobulin (Ig) variable region genes of the B-cell receptor and class switch recombination (CSR) which alters the Ig heavy chain constant region. Both of these processes are initiated by the enzyme activation-induced cytidine deaminase (AID), encoded by AICDA. Deleterious variants in AICDA are causal of hyper-IgM syndrome type 2 (HIGM2), a B-cell intrinsic primary immunodeficiency characterised by recurrent infections and low serum IgG and IgA levels. Biallelic variants affecting exons 2, 3 or 4 of AICDA have been identified that impair both CSR and SHM in patients with autosomal recessive HIGM2. Interestingly, B cells from patients with autosomal dominant HIGM2, caused by heterozygous variants (V186X, R190X) located in AICDA exon 5 encoding the nuclear export signal (NES) domain, show abolished CSR but variable SHM. We herein report the immunological and functional phenotype of two related patients presenting with common variable immunodeficiency who were found to have a novel heterozygous variant in AICDA (L189X). This variant led to a truncated AID protein lacking the last 10 amino acids of the NES at the C-terminal domain. Interestingly, patients' B cells carrying the L189X variant exhibited not only greatly impaired CSR but also SHM in vivo, as well as CSR and production of IgG and IgA in vitro. Our findings demonstrate that the NES domain of AID can be essential for SHM, as well as for CSR, thereby refining the correlation between AICDA genotype and SHM phenotype as well as broadening our understanding of the pathophysiology of HIGM disorders.

Intracellular Flow Cytometry ("Phosphoflow") to Assess Signal Transduction in Rare Populations Such As Memory B Cell Subsets and Plasma Cells

Intracellular flow cytometry is a powerful technique that can be used to interrogate signalling in rare cellular populations. The strengths of the technique are that massively parallel readouts can be gained from thousands of single cells simultaneously, and the assay is fast and relatively straightforward. This plate-based protocol enables different doses and different timepoints of stimulation to be assessed and has been optimized for rare B cell populations. Combining this technique with high-dimensional flow cytometry enables multiple signalling proteins to be measured with high confidence.

TGF-β1 reduces the differentiation of porcine IgA-producing plasma cells by inducing IgM+ B cells apoptosis via Bax/Bcl2-Caspase3 pathway

Transforming growth factor β1 (TGF-β1) performs a critical role in maintaining homeostasis of intestinal mucosa regulation and controls the survival, proliferation, and differentiation of many immune cells. In this study, we discovered that the infection of porcine epidemic diarrhea virus (PEDV), a coronavirus, upregulated TGF-β1 expression via activating Tregs. Besides, recombinant porcine TGF-β1 decreased the percentage of CD21+ B cells within the lymphocyte population in vitro. We further found that TGF-β1 reduced the IgA-secreting B cell numbers and also inhibited plasma cell differentiation. Additional investigations revealed that TGF-β1 induced the apoptosis of IgM+ B cells in both peyer's patches (PPs) and peripheral blood (PB) through the activation of the Bax/Bcl2-Caspase3 pathway. Conversely, the application of the TGF-β1 signaling inhibitor SB431542 significantly antagonized the TGF-β1-induced reduction of IgA secretion and B cell apoptosis and restored plasma cell differentiation. Collectively, TGF-β1 plays an important role in regulating the survival and differentiation of porcine IgA-secreting B cells through the classical mitochondrial apoptosis pathway. These findings will facilitate future mucosal vaccine designs that target the regulation of TGF-β1 for the control of enteric pathogens in the pig industry.

Defining the Basal and Immunomodulatory Mediator-Induced Phosphoprotein Signature in Pediatric B Cell Acute Lymphoblastic Leukemia (B-ALL) Diagnostic Samples

It is theorized that dysregulated immune responses to infectious insults contribute to the development of pediatric B-ALL. In this context, our understanding of the immunomodulatory-mediator-induced signaling responses of leukemic blasts in pediatric B-ALL diagnostic samples is rather limited. Hence, in this study, we defined the signaling landscape of leukemic blasts, as well as normal mature B cells and T cells residing in diagnostic samples from 63 pediatric B-ALL patients. These samples were interrogated with a range of immunomodulatory-mediators within 24 h of collection, and phosflow analyses of downstream proximal signaling nodes were performed. Our data reveal evidence of basal hyperphosphorylation across a broad swath of these signaling nodes in leukemic blasts in contrast to normal mature B cells and T cells in the same sample. We also detected similarities in the phosphoprotein signature between blasts and mature B cells in response to IFNγ and IL-2 treatment, but significant divergence in the phosphoprotein signature was observed between blasts and mature B cells in response to IL-4, IL-7, IL-10, IL-21 and CD40 ligand treatment. Our results demonstrate the existence of both symmetry and asymmetry in the phosphoprotein signature between leukemic and non-leukemic cells in pediatric B-ALL diagnostic samples.

Epstein-Barr virus and genetic risk variants as determinants of T-bet+ B cell-driven autoimmune diseases

B cells expressing the transcription factor T-bet are found to have a protective role in viral infections, but are also considered major players in the onset of different types of autoimmune diseases. Currently, the exact mechanisms driving such 'atypical' memory B cells to contribute to protective immunity or autoimmunity are unclear. In addition to general autoimmune-related factors including sex and age, the ways T-bet+ B cells instigate autoimmune diseases may be determined by the close interplay between genetic risk variants and Epstein-Barr virus (EBV). The impact of EBV on T-bet+ B cells likely relies on the type of risk variants associated with each autoimmune disease, which may affect their differentiation, migratory routes and effector function. In this hypothesis-driven review, we discuss the lines of evidence pointing to such genetic and/or EBV-mediated influence on T-bet+ B cells in a range of autoimmune diseases, including systemic lupus erythematosus (SLE) and multiple sclerosis (MS). We provide examples of how genetic risk variants can be linked to certain signaling pathways and are differentially affected by EBV to shape T-bet+ B-cells. Finally, we propose options to improve current treatment of B cell-related autoimmune diseases by more selective targeting of pathways that are critical for pathogenic T-bet+ B-cell formation.

Human PIK3R1 mutations disrupt lymphocyte differentiation to cause activated PI3Kδ syndrome 2

Heterozygous loss-of-function (LOF) mutations in PIK3R1 (encoding phosphatidylinositol 3-kinase [PI3K] regulatory subunits) cause activated PI3Kδ syndrome 2 (APDS2), which has a similar clinical profile to APDS1, caused by heterozygous gain-of-function (GOF) mutations in PIK3CD (encoding the PI3K p110δ catalytic subunit). While several studies have established how PIK3CD GOF leads to immune dysregulation, less is known about how PIK3R1 LOF mutations alter cellular function. By studying a novel CRISPR/Cas9 mouse model and patients' immune cells, we determined how PIK3R1 LOF alters cellular function. We observed some overlap in cellular defects in APDS1 and APDS2, including decreased intrinsic B cell class switching and defective Tfh cell function. However, we also identified unique APDS2 phenotypes including defective expansion and affinity maturation of Pik3r1 LOF B cells following immunization, and decreased survival of Pik3r1 LOF pups. Further, we observed clear differences in the way Pik3r1 LOF and Pik3cd GOF altered signaling. Together these results demonstrate crucial differences between these two genetic etiologies.

Deficiencies and Dysregulation of STAT Pathways That Drive Inborn Errors of Immunity: Lessons from Patients and Mouse Models of Disease

The STAT family proteins provide critical signals for immune cell development, differentiation, and proinflammatory and anti-inflammatory responses. Inborn errors of immunity (IEIs) are caused by single gene defects leading to immune deficiency and/or dysregulation, and they have provided opportunities to identify genes important for regulating the human immune response. Studies of patients with IEIs due to altered STAT signaling, and mouse models of these diseases, have helped to shape current understanding of the mechanisms whereby STAT signaling and protein interactions regulate immunity. Although many STAT signaling pathways are shared, clinical and immune phenotypes in patients with monogenic defects of STAT signaling highlight both redundant and nonredundant pathways. In this review, we provide an overview of the shared and unique signaling pathways used by STATs, phenotypes of IEIs with altered STAT signaling, and recent discoveries that have provided insight into the human immune response and treatment of disease.

The ups and downs of STAT3 function: too much, too little and human immune dysregulation

The STAT3 story has almost 30 years of evolving history. First identified in 1994 as a pro-inflammatory transcription factor, Signal Transducer and Activator of Transcription 3 (STAT3) has continued to be revealed as a quintessential pleiotropic signalling module spanning fields including infectious diseases, autoimmunity, vaccine responses, metabolism, and malignancy. In 2007, germline heterozygous dominant-negative loss-of-function variants in STAT3 were discovered as the most common cause for a triad of eczematoid dermatitis with recurrent skin and pulmonary infections, first described in 1966. This finding established that STAT3 plays a critical non-redundant role in immunity against some pathogens, as well as in the connective tissue, dental and musculoskeletal systems. Several years later, in 2014, heterozygous activating gain of function germline STAT3 variants were found to be causal for cases of early-onset multiorgan autoimmunity, thereby underpinning the notion that STAT3 function needed to be regulated to maintain immune homeostasis. As we and others continue to interrogate biochemical and cellular perturbations due to inborn errors in STAT3, we will review our current understanding of STAT3 function, mechanisms of disease pathogenesis, and future directions in this dynamic field.

Upregulated Expression of the IL-9 Receptor on TRAF3-Deficient B Lymphocytes Confers Ig Isotype Switching Responsiveness to IL-9 in the Presence of Antigen Receptor Engagement and IL-4

The pleiotropic cytokine IL-9 signals to target cells by binding to a heterodimeric receptor consisting of the unique subunit IL-9R and the common subunit γ-chain shared by multiple cytokines of the γ-chain family. In the current study, we found that the expression of IL-9R was strikingly upregulated in mouse naive follicular B cells genetically deficient in TNFR-associated factor 3 (TRAF3), a critical regulator of B cell survival and function. The highly upregulated IL-9R on Traf3-/- follicular B cells conferred responsiveness to IL-9, including IgM production and STAT3 phosphorylation. Interestingly, IL-9 significantly enhanced class switch recombination to IgG1 induced by BCR crosslinking plus IL-4 in Traf3-/- B cells, which was not observed in littermate control B cells. We further demonstrated that blocking the JAK-STAT3 signaling pathway abrogated the enhancing effect of IL-9 on class switch recombination to IgG1 induced by BCR crosslinking plus IL-4 in Traf3-/- B cells. Our study thus revealed, to our knowledge, a novel pathway that TRAF3 suppresses B cell activation and Ig isotype switching by inhibiting IL-9R-JAK-STAT3 signaling. Taken together, our findings provide (to our knowledge) new insights into the TRAF3-IL-9R axis in B cell function and have significant implications for the understanding and treatment of a variety of human diseases involving aberrant B cell activation such as autoimmune disorders.

A cluster of broadly neutralizing IgG against BK polyomavirus in a repertoire dominated by IgM

The BK polyomavirus (BKPyV) is an opportunistic pathogen, which is only pathogenic in immunosuppressed individuals, such as kidney transplant recipients, in whom BKPyV can cause significant morbidity. To identify broadly neutralizing antibodies against this virus, we used fluorescence-labeled BKPyV virus-like particles to sort BKPyV-specific B cells from the PBMC of KTx recipients, then single-cell RNAseq to obtain paired heavy- and light-chain antibody sequences from 2,106 sorted B cells. The BKPyV-specific repertoire was highly diverse in terms of both V-gene usage and clonotype diversity and included most of the IgM B cells, including many with extensive somatic hypermutation. In two patients where sufficient data were available, IgM B cells in the BKPyV-specific dataset had significant differences in V-gene usage compared with IgG B cells from the same patient. CDR3 sequence-based clustering allowed us to identify and characterize three broadly neutralizing "41F17-like" clonotypes that were predominantly IgG, suggesting that some specific BKPyV capsid epitopes are preferentially targeted by IgG.

Cytokine-mediated STAT-dependent pathways underpinning human B-cell differentiation and function

B cells are fundamental to host defence against infectious diseases; indeed, the ability of humans to elicit robust antibody responses following exposure to foreign antigens underpins long-lived humoral immunity and serological memory, as well as the success of most currently administered vaccines. However, B cells also have a dark side - they can cause myriad diseases, including autoimmunity, atopy, allergy and malignancy. Thus, it is critical to understand the molecular requirements for generating effective, high-affinity, specific immune responses following natural infection or vaccination, as well as for constraining B-cell function to mitigate B-cell-mediated immune dyscrasias. In this review, we discuss recent developments that have been derived from the identification and detailed analysis of individuals with inborn errors of immunity that disrupt cytokine signalling, resulting in immune dysregulatory conditions. These studies have defined fundamental cytokine/cytokine receptor/signal transducer and activator of transcription (STAT) signalling pathways that are critical for the generation and maintenance of human memory B-cell and plasma cell subsets during host defence, as well as revealed mechanisms of disease pathogenesis causing immune deficiency, autoimmunity and atopy. More importantly, these studies have identified molecules that could be targeted to either enhance humoral immunity in the settings of infection or vaccination, or attenuate humoral immunity that contributes to antibody-mediated autoimmunity or allergy.

Molecular Profiling of Inflammatory Processes in a Mouse Model of IC/BPS: From the Complete Transcriptome to Major Sex-Related Histological Features of the Urinary Bladder

Animal models are invaluable in the research of the pathophysiology of interstitial cystitis/bladder pain syndrome (IC/BPS), a chronic aseptic urinary bladder disease of unknown etiology that primarily affects women. Here, a mouse model of IC/BPS was induced with multiple low-dose cyclophosphamide (CYP) applications and thoroughly characterized by RNA sequencing, qPCR, Western blot, and immunolabeling to elucidate key inflammatory processes and sex-dependent differences in the bladder inflammatory response. CYP treatment resulted in the upregulation of inflammatory transcripts such as Ccl8, Eda2r, and Vegfd, which are predominantly involved in innate immunity pathways, recapitulating the crucial findings in the bladder transcriptome of IC/BPS patients. The JAK/STAT signaling pathway was analyzed in detail, and the JAK3/STAT3 interaction was found to be most activated in cells of the bladder urothelium and lamina propria. Sex-based data analysis revealed that cell proliferation was more pronounced in male bladders, while innate immunity and tissue remodeling processes were the most distinctive responses of female bladders to CYP treatment. These processes were also reflected in prominent histological changes in the bladder. The study provides an invaluable reference dataset for preclinical research on IC/BPS and an insight into the sex-specific mechanisms involved in the development of IC/BPS pathology, which may explain the more frequent occurrence of this disease in women.

IL-21R signal reprogramming cooperates with CD40 and BCR signals to select and differentiate germinal center B cells

Both B cell receptor (BCR) and CD40 signaling are rewired in germinal center (GC) B cells (GCBCs) to synergistically induce c-MYC and phosphorylated S6 ribosomal protein (p-S6), markers of positive selection. How interleukin-21 (IL-21), a key T follicular helper (TFH)-derived cytokine, affects GCBCs is unclear. Like BCR and CD40 signals, IL-21 receptor (IL-21R) plus CD40 signals also synergize to induce c-MYC and p-S6 in GCBCs. However, IL-21R plus CD40 stimulation differentially affects GCBC fate compared with BCR plus CD40 ligation-engaging unique molecular mechanisms-as revealed by bulk RNA sequencing (RNA-seq), single-cell RNA-seq, and flow cytometry of GCBCs in vitro and in vivo. Whereas both signal pairs induced BLIMP1 in some GCBCs, only the IL-21R/CD40 combination induced IRF4hi/CD138+ cells, indicative of plasma cell differentiation, along with CCR6+/CD38+ memory B cell precursors. These findings reveal a second positive selection pathway in GCBCs, document rewired IL-21R signaling in GCBCs, and link specific TFH- and Ag-derived signals to GCBC differentiation.

JAK inhibitors differentially modulate B cell activation, maturation and function: A comparative analysis of five JAK inhibitors in an in-vitro B cell differentiation model and in patients with rheumatoid arthritis

Background

Janus kinase (JAK) inhibitors have been approved for the treatment of several immune-mediated diseases (IMIDs) including rheumatoid arthritis (RA) and psoriatic arthritis and are in clinical trials for numerous other IMIDs. However, detailed studies investigating the effects of different JAK inhibitors on B cells are missing. Within this study, we therefore aimed to characterize the effect of JAK inhibition on the B cell compartment.

Methods

To this end, we investigated the B cell compartment under JAK inhibition and compared the specific effects of the different JAK inhibitors tofacitinib (pan-JAK), baricitinib (JAK1/2), ruxolitinib (JAK1/2), upadacitinib (JAK1/2) as well as filgotinib (selective JAK1) on in-vitro B cell activation, proliferation, and class switch recombination and involved pathways.

Results

While B cell phenotyping of RA patients showed an increase in marginal zone (MZ) B cells under JAK inhibition, comparison with healthy donors revealed that the relative frequency of MZ B cells was still lower compared to healthy controls. In an in-vitro model of T-cell-independent B cell activation we observed that JAK1/2 and selective JAK1 inhibitor treatment led to a dose-dependent decrease of total B cell numbers. We detected an altered B cell differentiation with a significant increase in MZ-like B cells and an increase in plasmablast differentiation in the first days of culture, most pronounced with the pan-JAK inhibitor tofacitinib, although there was no increase in immunoglobulin secretion in-vitro. Notably, we further observed a profound reduction of switched memory B cell formation, especially with JAK1/2 inhibition. JAK inhibitor treatment led to a dose-dependent reduction of STAT3 expression and phosphorylation as well as STAT3 target gene expression and modulated the secretion of pro- and anti-inflammatory cytokines by B cells.

Conclusion

JAK inhibition has a major effect on B cell activation and differentiation, with differential outcomes between JAK inhibitors hinting towards distinct and unique effects on B cell homeostasis.

Unusual Talaromyces marneffei and Pneumocystis jirovecii coinfection in a child with a STAT1 mutation: A case report and literature review

Talaromyces marneffei and Pneumocystis jirovecii are the common opportunistic pathogens in immunodeficient patients. There have been no reports of T. marneffei and P. jirovecii coinfection in immunodeficient children. Signal transducer and activator of transcription 1 (STAT1) is a key transcription factor in immune responses. STAT1 mutations are predominately associated with chronic mucocutaneous candidiasis and invasive mycosis. We report a 1-year-2-month-old boy diagnosed with severe laryngitis and pneumonia caused by T. marneffei and P. jirovecii coinfection, which was confirmed by smear, culture, polymerase chain reaction and metagenome next-generation sequencing of bronchoalveolar lavage fluid. He has a known STAT1 mutation at amino acid 274 in the coiled-coil domain of STAT1 according to whole exome sequencing. Based on the pathogen results, itraconazole and trimethoprim-sulfamethoxazole were administered. This patient's condition improved, and he was discharged after two weeks of targeted therapy. In the one-year follow-up, the boy remained symptom-free without recurrence.

Oxygen level is a critical regulator of human B cell differentiation and IgG class switch recombination

The generation of high-affinity antibodies requires an efficient germinal center (GC) response. As differentiating B cells cycle between GC dark and light zones they encounter different oxygen pressures (pO2). However, it is essentially unknown if and how variations in pO2 affect B cell differentiation, in particular for humans. Using optimized in vitro cultures together with in-depth assessment of B cell phenotype and signaling pathways, we show that oxygen is a critical regulator of human naive B cell differentiation and class switch recombination. Normoxia promotes differentiation into functional antibody secreting cells, while a population of CD27++ B cells was uniquely generated under hypoxia. Moreover, time-dependent transitions between hypoxic and normoxic pO2 during culture - reminiscent of in vivo GC cyclic re-entry - steer different human B cell differentiation trajectories and IgG class switch recombination. Taken together, we identified multiple mechanisms trough which oxygen pressure governs human B cell differentiation.

Prolactin promotes proliferation of germinal center B cells, formation of plasma cells, and elevated levels of IgG3 anti-dsDNA autoantibodies

Systemic lupus erythematosus (SLE) mainly affects females at reproductive age, which has been associated with hormones, such as prolactin (PRL). Different studies suggest that PRL exacerbates the clinical manifestations of SLE both in patients and in mouse models (e.g., the MRL/lpr strain), increasing the production of autoantibodies, which can be deposited as immune complexes and trigger inflammation and damage to different tissues. The objective of this work was to explore the potential mechanisms by which PRL increases the concentration of self-reactive antibodies in the MRL/lpr SLE model. To this end, we determined the role of PRL on the activation and proliferation of germinal center B cells (B-GCs) and their differentiation into antibody-secreting cells (ASCs). We show that the absolute number and percentage of B-GCs were significantly increased by PRL in vivo or upon in vitro treatment with anti-IgM and anti-CD40 antibodies and PRL. The augmented B-GC numbers correlated with enhanced proliferation, but we did not observe enhanced expression of CD80 and CD86 activation markers or the BCL6 transcription factor, arguing against a more effective differentiation. Nevertheless, we observed enhanced phosphorylation of STAT1, secretion of IL-6, expression of IRF4, numbers of ASCs, and levels of IgG3 antibodies directed against dsDNA. Altogether, these results support the hypothesis that a PRL-mediated expansion of B-GCs yields more self-reactive ASCs, potentially explaining the pathogenic immune complexes that steadily lead to tissue damage during SLE.

STAT5B restrains human B-cell differentiation to maintain humoral immune homeostasis

BACKGROUND

Lymphocyte differentiation is regulated by coordinated actions of cytokines and signaling pathways. IL-21 activates STAT1, STAT3, and STAT5 and is fundamental for the differentiation of human B cells into memory cells and antibody-secreting cells. While STAT1 is largely nonessential and STAT3 is critical for this process, the role of STAT5 is unknown.

OBJECTIVES

This study sought to delineate unique roles of STAT5 in activation and differentiation of human naive and memory B cells.

METHODS

STAT activation was assessed by phospho-flow cytometry cell sorting. Differential gene expression was determined by RNA-sequencing and quantitative PCR. The requirement for STAT5B in B-cell and CD4+ T-cell differentiation was assessed using CRISPR-mediated STAT5B deletion from B-cell lines and investigating primary lymphocytes from individuals with germline STAT5B mutations.

RESULTS

IL-21 activated STAT5 and strongly induced SOCS3 in human naive, but not memory, B cells. Deletion of STAT5B in B-cell lines diminished IL-21-mediated SOCS3 induction. PBMCs from STAT5B-null individuals contained expanded populations of immunoglobulin class-switched B cells, CD21loTbet+ B cells, and follicular T helper cells. IL-21 induced greater differentiation of STAT5B-deficient B cells into plasmablasts in vitro than B cells from healthy donors, correlating with higher expression levels of transcription factors promoting plasma cell formation.

CONCLUSIONS

These findings reveal novel roles for STAT5B in regulating IL-21-induced human B-cell differentiation. This is achieved by inducing SOCS3 to attenuate IL-21 signaling, and BCL6 to repress class switching and plasma cell generation. Thus, STAT5B is critical for restraining IL-21-mediated B-cell differentiation. These findings provide insights into mechanisms underpinning B-cell responses during primary and subsequent antigen encounter and explain autoimmunity and dysfunctional humoral immunity in STAT5B deficiency.

Termination of CD40L co-stimulation promotes human B cell differentiation into antibody-secreting cells

Human naïve B cells are notoriously difficult to differentiate into antibody-secreting cells (ASCs) in vitro while maintaining sufficient cell numbers to evaluate the differentiation process. B cells require T follicular helper (T_FH ) cell-derived signals like CD40L and IL-21 during germinal center (GC) responses to undergo differentiation into ASCs. Cognate interactions between B and T_FH cells are transient; after T_FH contact, B cells cycle between GC light and dark zones where T_FH contact is present and absent, respectively. Here, we elucidated that the efficacy of naïve B cells in ACS differentiation is dramatically enhanced by the release of CD40L stimulation. Multiparameter phospho-flow and transcription factor (TF)-flow cytometry revealed that termination of CD40L stimulation downmodulates NF-κB and STAT3 signaling. Furthermore, the termination of CD40 signaling downmodulates C-MYC, while promoting ASC TFs BLIMP1 and XBP-1s. Reduced levels of C-MYC in the differentiating B cells are later associated with crucial downmodulation of the B cell signature TF PAX5 specifically upon the termination of CD40 signaling, resulting in the differentiation of BLIMP1 high expressing cells into ASCs. The data presented here are the first steps to provide further insights how the transient nature of CD40 signaling is in fact needed for efficient human naïve B cell differentiation to ASCs.

Epigenetic remodeling by vitamin C potentiates plasma cell differentiation

Ascorbate (vitamin C) is an essential micronutrient in humans. The severe chronic deficiency of ascorbate, termed scurvy, has long been associated with increased susceptibility to infections. How ascorbate affects the immune system at the cellular and molecular levels remained unclear. From a micronutrient analysis, we identified ascorbate as a potent enhancer for antibody response by facilitating the IL-21/STAT3-dependent plasma cell differentiation in mouse and human B cells. The effect of ascorbate is unique as other antioxidants failed to promote plasma cell differentiation. Ascorbate is especially critical during early B cell activation by poising the cells to plasma cell lineage without affecting the proximal IL-21/STAT3 signaling and the overall transcriptome. As a cofactor for epigenetic enzymes, ascorbate facilitates TET2/3-mediated DNA modification and demethylation of multiple elements at the Prdm1 locus. DNA demethylation augments STAT3 association at the Prdm1 promoter and a downstream enhancer, thus ensuring efficient gene expression and plasma cell differentiation. The results suggest that an adequate level of ascorbate is required for antibody response and highlight how micronutrients may regulate the activity of epigenetic enzymes to regulate gene expression. Our findings imply that epigenetic enzymes can function as sensors to gauge the availability of metabolites and influence cell fate decisions.

B cell-intrinsic changes with age do not impact antibody-secreting cell formation but delay B cell participation in the germinal centre reaction

Vaccines typically protect against (re)infections by generating pathogen-neutralising antibodies. However, as we age, antibody-secreting cell formation and vaccine-induced antibody titres are reduced. Antibody-secreting plasma cells differentiate from B cells either early post-vaccination through the extrafollicular response or from the germinal centre (GC) reaction, which generates long-lived antibody-secreting cells. As the formation of both the extrafollicular antibody response and the GC requires the interaction of multiple cell types, the impaired antibody response in ageing could be caused by B cell intrinsic or extrinsic factors, or a combination of the two. Here, we show that B cells from older people do not have intrinsic defects in their proliferation and differentiation into antibody-secreting cells in vitro compared to those from the younger donors. However, adoptive transfer of B cells from aged mice to young recipient mice showed that differentiation into extrafollicular plasma cells was favoured at the expense of B cells entering the GC during the early stages of GC formation. In contrast, by the peak of the GC response, GC B cells derived from the donor cells of aged mice had expanded to the same extent as those from the younger donors. This indicates that age-related intrinsic B cell changes delay the GC response but are not responsible for the impaired antibody-secreting response or smaller peak GC response in ageing. Collectively, this study shows that B cells from aged individuals are not intrinsically defective in responding to stimulation and becoming antibody-secreting cells, implicating B cell-extrinsic factors as the primary cause of age-associated impairment in the humoral immunity.

Immunodeficiency, autoimmunity, and increased risk of B cell malignancy in humans with TRAF3 mutations

Tumor necrosis factor receptor-associated factor 3 (TRAF3) is a central regulator of immunity. TRAF3 is often somatically mutated in B cell malignancies, but its role in human immunity is not defined. Here, in five unrelated families, we describe an immune dysregulation syndrome of recurrent bacterial infections, autoimmunity, systemic inflammation, B cell lymphoproliferation, and hypergammaglobulinemia. Affected individuals each had monoallelic mutations in TRAF3 that reduced TRAF3 expression. Immunophenotyping showed that patients' B cells were dysregulated, exhibiting increased nuclear factor-κB 2 activation, elevated mitochondrial respiration, and heightened inflammatory responses. Patients had mild CD4+ T cell lymphopenia, with a reduced proportion of naïve T cells but increased regulatory T cells and circulating T follicular helper cells. Guided by this clinical phenotype, targeted analyses demonstrated that common genetic variants, which also reduce TRAF3 expression, are associated with an increased risk of B cell malignancies, systemic lupus erythematosus, higher immunoglobulin levels, and bacterial infections in the wider population. Reduced TRAF3 conveys disease risks by driving B cell hyperactivity via intrinsic activation of multiple intracellular proinflammatory pathways and increased mitochondrial respiration, with a likely contribution from dysregulated T cell help. Thus, we define monogenic TRAF3 haploinsufficiency syndrome and demonstrate how common TRAF3 variants affect a range of human diseases.

Optimized Protocols for In-Vitro T-Cell-Dependent and T-Cell-Independent Activation for B-Cell Differentiation Studies Using Limited Cells

Background/Methods

For mechanistic studies, in-vitro human B-cell differentiation and generation of plasma cells are invaluable techniques. However, the heterogeneity of both T-cell-dependent (TD) and T-cell-independent (TI) stimuli and the disparity of culture conditions used in existing protocols make the interpretation of results challenging. The aim of the present study was to achieve the most optimal B-cell differentiation conditions using isolated CD19⁺ B cells and peripheral blood mononuclear cell (PBMC) cultures. We addressed multiple seeding densities, different durations of culturing, and various combinations of TD and TI stimuli including B-cell receptor (BCR) triggering. B-cell expansion, proliferation, and differentiation were analyzed after 6 and 9 days by measuring B-cell proliferation and expansion, plasmablast and plasma cell formation, and immunoglobulin (Ig) secretion. In addition, these conditions were extrapolated using cryopreserved cells and differentiation potential was compared.

Results

This study demonstrates improved differentiation efficiency after 9 days of culturing for both B-cells and PBMC cultures using CD40L and IL-21 as TD stimuli and 6 days for CpG and IL-2 as TI stimuli. We arrived at optimized protocols requiring 2,500 and 25,000 B–cells per culture well for the TD and TI assays, respectively. The results of the PBMC cultures were highly comparable to the B-cell cultures, which allows dismissal of additional B-cell isolation steps prior to culturing. In these optimized TD conditions, the addition of anti-BCR showed a little effect on phenotypic B-cell differentiation; however, it interferes with Ig secretion measurements. The addition of IL-4 to the TD stimuli showed significantly lower Ig secretion. The addition of BAFF to optimized TI conditions showed enhanced B-cell differentiation and Ig secretion in B-cell but not in PBMC cultures. With this approach, efficient B-cell differentiation and Ig secretion were accomplished when starting from fresh or cryopreserved samples.

Conclusion

Our methodology demonstrates optimized TD and TI stimulation protocols for more in-depth analysis of B-cell differentiation in primary human B-cell and PBMC cultures while requiring low amounts of B cells, making them ideally suited for future clinical and research studies on B-cell differentiation of patient samples from different cohorts of B-cell-mediated diseases.

A Novel Targeted Amplicon Next-Generation Sequencing Gene Panel for the Diagnosis of Common Variable Immunodeficiency Has a High Diagnostic Yield: Results from the Perth CVID Cohort Study

With the advent of next-generation sequencing (NGS), monogenic forms of common variable immunodeficiency (CVID) have been increasingly described. Our study aimed to identify disease-causing variants in a Western Australian CVID cohort using a novel targeted NGS panel. Targeted amplicon NGS was performed on 22 unrelated subjects who met the formal European Society for Immunodeficiencies-Pan-American Group for Immunodeficiency diagnostic criteria for CVID and had at least one of the following additional criteria: disease onset at age <18 years, autoimmunity, low memory B lymphocytes, family history, and/or history of lymphoproliferation. Candidate variants were assessed by in silico predictions of deleteriousness, comparison to the literature, and classified according to the American College of Medical Genetics and Genomics-Association for Molecular Pathology criteria. All detected genetic variants were verified independently by an external laboratory, and additional functional studies were performed if required. Pathogenic or likely pathogenic variants were detected in 6 of 22 (27%) patients. Monoallelic variants of uncertain significance were also identified in a further 4 of 22 patients (18%). Pathogenic variants, likely pathogenic variants, or variants of uncertain significance were found in TNFRSF13B, TNFRSF13C, ICOS, AICDA, IL21R, NFKB2, and CD40LG, including novel variants and variants with unexpected inheritance pattern. Targeted amplicon NGS is an effective tool to identify monogenic disease-causing variants in CVID, and is comparable or superior to other NGS methods. Moreover, targeted amplicon NGS identified patients who may benefit from targeted therapeutic strategies and had important implications for family members.

CD45RB Glycosylation and Ig Isotype Define Maturation of Functionally Distinct B Cell Subsets in Human Peripheral Blood

Glycosylation of CD45RB (RB+) has recently been identified to mark antigen-experienced B cells, independent of their CD27 expression. By using a novel combination of markers including CD45RB glycosylation, CD27 and IgM/IgD isotype expression we segregated human peripheral blood B cell subsets and investigated their IGHV repertoire and in vitro functionality. We observed distinct maturation stages for CD27-RB+ cells, defined by differential expression of non-switched Ig isotypes. CD27-RB+ cells, which only express IgM, were more matured in terms of Ig gene mutation levels and function as compared to CD27-RB+ cells that express both IgM and IgD or cells that were CD27-RB-. Moreover, CD27-RB+IgM+ cells already showed remarkable rigidity in IgM isotype commitment, different from CD27-RB+IgMD+ and CD27-RB- cells that still demonstrated great plasticity in B cell fate decision. Thus, glycosylation of CD45RB is indicative for antigen-primed B cells, which are, dependent on the Ig isotype, functionally distinct.

Therapeutic Effects of Tofacitinib on Pristane-Induced Murine Lupus

Objectives

This study aims to investigate the effectiveness of tofacitinib, a Janus kinase (JAK) 1/JAK3 inhibitor, in treating murine lupus, and also explore 12 related genes downstream of JAK-signal transducer and activator of transcription (STAT) signaling pathways to find the underlying mechanism.

Materials and methods

This study was conducted between July 2017 and January 2020. Fifty-seven female BALB/c mice (aging 8 to 10 weeks old; weighing 18 to 20 g) were assigned to a saline control (SC) group and a pristane-induced lupus group. The latter included four groups, namely, pristane control (PC), tofacitinib (T), methylprednisolone (MP), and tofacitinib plus methylprednisolone (T+MP). Animal models of lupus were induced with pristane, whereas SC mice were treated with normal saline. From the 22^nd week after induction, each group was given the aforementioned corresponding intervention for 11 weeks. The following variables were tested: serum concentrations of anti-double-stranded deoxyribonucleic acid (anti-dsDNA), interleukin 6 (IL-6), and interferon gamma (IFN-γ); number of regulatory T (Treg) cells; messenger ribonucleic acid levels of forkhead box P3 and 12 related genes downstream of JAK-STAT pathway; and renal impairment.

Results

Red swollen joints and proteinuria were first observed in PC after the 12^th week. After treatment, T, MP, and T+MP showed relieved red swollen joints and splenomegaly, as well as decreased urine protein, anti-dsDNA, IL-6, IFN-γ, Treg cells, pathological scores, and hyperplasia of mesangial matrix in glomeruli compared with PC. The IFN regulatory factor 7 level was higher in T+MP (p0.05) and MP (p>0.05) than in PC after treatment. The expression of suppressor of cytokine signaling (SOCS) 1 was lower in T (p>0.05), T+MP (p0.05) than in PC. The SOCS3 level was higher in T (p>0.05) and T+MP (p0.05) than in PC.

Conclusion

Tofacitinib can ameliorate glomerulonephritis and arthritis in a pristane-induced murine model of lupus. SOCS3 gene may be involved in the therapeutic mechanism of tofacitinib.

Supplying the trip to antibody production-nutrients, signaling, and the programming of cellular metabolism in the mature B lineage

The COVID pandemic has refreshed and expanded recognition of the vital role that sustained antibody (Ab) secretion plays in our immune defenses against microbes and of the importance of vaccines that elicit Ab protection against infection. With this backdrop, it is especially timely to review aspects of the molecular programming that govern how the cells that secrete Abs arise, persist, and meet the challenge of secreting vast amounts of these glycoproteins. Whereas plasmablasts and plasma cells (PCs) are the primary sources of secreted Abs, the process leading to the existence of these cell types starts with naive B lymphocytes that proliferate and differentiate toward several potential fates. At each step, cells reside in specific microenvironments in which they not only receive signals from cytokines and other cell surface receptors but also draw on the interstitium for nutrients. Nutrients in turn influence flux through intermediary metabolism and sensor enzymes that regulate gene transcription, translation, and metabolism. This review will focus on nutrient supply and how sensor mechanisms influence distinct cellular stages that lead to PCs and their adaptations as factories dedicated to Ab secretion. Salient findings of this group and others, sometimes exhibiting differences, will be summarized with regard to the journey to a distinctive metabolic program in PCs.

Metformin and its therapeutic applications in autoimmune inflammatory rheumatic disease

Metformin is a first-line therapeutic agent for type 2 diabetes. Apart from its glucose-lowering effect, metformin is attracting interest regarding possible therapeutic benefits in various other conditions. As metformin regulates cell metabolism, proliferation, growth, and autophagy, it may also modulate immune cell functions. Given that metformin acts on multiple intracellular signaling pathways, including adenosine monophosphate (AMP)-activated protein kinase (AMPK) activation, and that AMPK and its downstream intracellular signaling control the activation and differentiation of T and B cells and inflammatory responses, metformin may exert immunomodulatory and anti- inflammatory effects. The efficacy of metformin has been investigated in preclinical and clinical studies on rheumatoid arthritis, osteoarthritis, systemic lupus erythematosus, Sjögren's syndrome, scleroderma, ankylosing spondylitis, and gout. In this review, we discuss the potential mechanisms through which metformin exerts its therapeutic effects in these diseases, focusing particularly on rheumatoid arthritis and osteoarthritis.

The expansion of human T-bethighCD21low B cells is T cell dependent

Accumulation of human CD21low B cells in peripheral blood is a hallmark of chronic activation of the adaptive immune system in certain infections and autoimmune disorders. The molecular pathways underpinning the development, function, and fate of these CD21low B cells remain incompletely characterized. Here, combined transcriptomic and chromatin accessibility analyses supported a prominent role for the transcription factor T-bet in the transcriptional regulation of these T-bethighCD21low B cells. Investigating essential signals for generating these cells in vitro established that B cell receptor (BCR)/interferon-γ receptor (IFNγR) costimulation induced the highest levels of T-bet expression and enabled their differentiation during cell cultures with Toll-like receptor (TLR) ligand or CD40L/interleukin-21 (IL-21) stimulation. Low proportions of CD21low B cells in peripheral blood from patients with defined inborn errors of immunity (IEI), because of mutations affecting canonical NF-κB, CD40, and IL-21 receptor or IL-12/IFNγ/IFNγ receptor/signal transducer and activator of transcription 1 (STAT1) signaling, substantiated the essential roles of BCR- and certain T cell–derived signals in the in vivo expansion of T-bethighCD21low B cells. Disturbed TLR signaling due to MyD88 or IRAK4 deficiency was not associated with reduced CD21low B cell proportions. The expansion of human T-bethighCD21low B cells correlated with an expansion of circulating T follicular helper 1 (cTfh1) and T peripheral helper (Tph) cells, identifying potential sources of CD40L, IL-21, and IFNγ signals. Thus, we identified important pathways to target autoreactive T-bethighCD21low B cells in human autoimmune conditions, where these cells are linked to pathogenesis and disease progression.

System-Level Scenarios for the Elucidation of T Cell-Mediated Germinal Center B Cell Differentiation

Germinal center (GC) reactions are vital to the correct functioning of the adaptive immune system, through formation of high affinity, class switched antibodies. GCs are transient anatomical structures in secondary lymphoid organs where specific B cells, after recognition of antigen and with T cell help, undergo class switching. Subsequently, B cells cycle between zones of proliferation and somatic hypermutation and zones where renewed antigen acquisition and T cell help allows for selection of high affinity B cells (affinity maturation). Eventually GC B cells first differentiate into long-lived memory B cells (MBC) and finally into plasma cells (PC) that partially migrate to the bone marrow to encapsulate into long-lived survival niches. The regulation of GC reactions is a highly dynamically coordinated process that occurs between various cells and molecules that change in their signals. Here, we present a system-level perspective of T cell-mediated GC B cell differentiation, presenting and discussing the experimental and computational efforts on the regulation of the GCs. We aim to integrate Systems Biology with B cell biology, to advance elucidation of the regulation of high-affinity, class switched antibody formation, thus to shed light on the delicate functioning of the adaptive immune system. Specifically, we: i) review experimental findings of internal and external factors driving various GC dynamics, such as GC initiation, maturation and GCBC fate determination; ii) draw comparisons between experimental observations and mathematical modeling investigations; and iii) discuss and reflect on current strategies of modeling efforts, to elucidate B cell behavior during the GC tract. Finally, perspectives are specifically given on to the areas where a Systems Biology approach may be useful to predict novel GCBC-T cell interaction dynamics.

Mechanistic insights into the role of B cells in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease epitomized by severe inflammation that induces tendon, cartilage, and bone damage over time. Although different types of cells undertake pathogenic functions in RA, the B cell's significant involvement has increasingly been known following the development of rheumatoid factor and it has been re-emphasized in recent years. Therefore, the rheumatoid factors and anti-cyclic citrullinated peptide antibodies are well-known indications of infection and clinical manifestations, and that they can precede the development of illness by several years. The emergence of rituximab a B cell reducing chimeric antidote in 1997 and 1998 transformed B-cell-targeted therapy for inflammatory disorder from a research hypothesis to a functional fact. Ever since, several autoantibody-related conditions were addressed, including the more intriguing indications of effectiveness seen in rheumatoid arthritis patients. Numerous types of B-cell-targeted compounds are currently being researched. From the beginning, one of the primary goals of B-cell therapy was to reinstate some kind of immune tolerance. While B cells have long been recognized as essential autoantibody producers, certain antibody-independent functions and usefulness as a key targeted therapy were not recognized until recently. The knowledge of B cells' diverse physical and pathogenic roles in autoimmune diseases is growing. As a result, the number of successful agents targeting the B cell complex is becoming more ubiquitous. Therefore, in this article, we explore fresh perspectives upon the roles of B cells in arthritis treatment, as well as new evidence regarding the effectiveness of B lymphocytes reduction and the therapeutic outcome of biological markers.

Molecular regulation and dysregulation of T follicular helper cells - learning from inborn errors of immunity

The production of high-affinity antibodies is a key feature of the vertebrate immune system. Antibodies neutralize and clear pathogens, thereby protecting against infectious diseases. However, dysregulated production of antibodies can cause immune pathologies, such as autoimmunity and immune deficiency. Long-lived humoral immunity depends on B-cell help provided by T follicular helper (Tfh) cells, which support the differentiation of antigen (Ag)-specific B cells into memory and plasma cells. Tfh cells are generated from naïve CD4⁺ T cells following the receipt of inputs from various cell surface receptors, and can undergo further differentiation into subsets with specialised effector functions to induce and maintain serological memory. While genetically modified mice have provided great understanding of the requirements for generating Tfh cells, it is critical that requirements for human Tfh cell generation and function are also established. Key insights into the molecular requirements for human Tfh cells have been elucidated from the systematic analysis of humans with monogenic germline variants that cause inborn errors of immunity characterised by impaired humoral immunity following infection or vaccination or immune dysregulation and autoimmunity. In this review we will discuss how studying rare 'experiments of nature' has enabled discovery of non-redundant molecules and pathways necessary for Tfh cell generation, differentiation, regulation and function in humans, and how these findings inform us about basic and clinical immunology.

Minimalistic In Vitro Culture to Drive Human Naive B Cell Differentiation into Antibody-Secreting Cells

High-affinity antibody-secreting cells (ASC) arise from terminal differentiation of B-cells after coordinated interactions with T follicular helper (Tfh) cells in germinal centers (GC). Elucidation of cues promoting human naive B-cells to progress into ASCs is challenging, as this process is notoriously difficult to induce in vitro while maintaining enough cell numbers to investigate the differentiation route(s). Here, we describe a minimalistic in vitro culture system that supports efficient differentiation of human naive B-cells into antibody-secreting cells. Upon initial stimulations, the interplay between level of CD40 costimulation and the Tfh cell-associated cytokines IL-21 and IL-4 determined the magnitude of B-cell expansion, immunoglobulin class-switching and expression of ASC regulator PRDM1. In contrast, the B-cell-specific transcriptional program was maintained, and efficient ASC formation was hampered. Renewed CD40 costimulation and Tfh cytokines exposure induced rapid secondary STAT3 signaling and extensive ASC differentiation, accompanied by repression of B-cell identity factors PAX5, BACH2 and IRF8 and further induction of PRDM1. Our work shows that, like in vivo, renewed CD40L costimulation also induces efficient terminal ASC differentiation after initial B-cell expansion in vitro. This culture system for efficient differentiation of human naive B-cells into ASCs, while also maintaining high cell numbers, may form an important tool in dissecting human naive B-cell differentiation, thereby enabling identification of novel transcriptional regulators and biomarkers for desired and detrimental antibody formation in humans.

STAT3 couples with 14-3-3σ to regulate BCR signaling, B-cell differentiation, and IgE production

BACKGROUND

STAT3 or dedicator of cytokinesis protein 8 (Dock8) loss-of-function (LOF) mutations cause hyper-IgE syndrome. The role of abnormal T-cell function has been extensively investigated; however, the contribution of B-cell-intrinsic dysfunction to elevated IgE levels is unclear.

OBJECTIVE

We sought to determine the underlying molecular mechanism of how STAT3 regulates B-cell receptor (BCR) signaling, B-cell differentiation, and IgE production.

METHODS

We used samples from patients with STAT3 LOF mutation and samples from the STAT3 B-cell-specific knockout (KO) mice Mb1CreStat3flox/flox mice (B-STAT3 KO) to investigate the mechanism of hyper-IgE syndrome.

RESULTS

We found that the peripheral B-cell homeostasis in B-STAT3 KO mice mimicked the phenotype of patients with STAT3 LOF mutation, having decreased levels of follicular and germinal center B cells but increased levels of marginal zone and IgE+ B cells. Furthermore, B-STAT3 KO B cells had reduced BCR signaling following antigenic stimulation owing to reduced BCR clustering and decreased accumulation of Wiskott-Aldrich syndrome protein and F-actin. Excitingly, a central hub protein, 14-3-3σ, which is essential for the increase in IgE production, was enhanced in the B cells of B-STAT3 KO mice and patients with STAT3 LOF mutation. The increase of 14-3-3σ was associated with increased expression of the upstream mediator, microRNA146A. Inhibition of 14-3-3σ with R18 peptide in B-STAT3 KO mice rescued the BCR signaling, follicular, germinal center, and IgE+ B-cell differentiation to the degree seen in wild-type mice.

CONCLUSIONS

Altogether, our study has established a novel regulatory pathway of STAT3-miRNA146A-14-3-3σ to regulate BCR signaling, peripheral B-cell differentiation, and IgE production.

Genomic Spectrum and Phenotypic Heterogeneity of Human IL-21 Receptor Deficiency

Biallelic inactivating mutations in IL21R causes a combined immunodeficiency that is often complicated by cryptosporidium infections. While eight IL-21R-deficient patients have been reported previously, the natural course, immune characteristics of disease, and response to hematopoietic stem cell transplantation (HSCT) remain to be comprehensively examined. In our study, we have collected clinical histories of 13 patients with IL-21R deficiency from eight families across seven centers worldwide, including five novel patients identified by exome or NGS panel sequencing. Eight unique mutations in IL21R were identified in these patients, including two novel mutations. Median age at disease onset was 2.5 years (0.5–7 years). The main clinical manifestations were recurrent bacterial (84.6%), fungal (46.2%), and viral (38.5%) infections; cryptosporidiosis-associated cholangitis (46.2%); and asthma (23.1%). Inflammatory skin diseases (15.3%) and recurrent anaphylaxis (7.9%) constitute novel phenotypes of this combined immunodeficiency. Most patients exhibited hypogammaglobulinemia and reduced proportions of memory B cells, circulating T follicular helper cells, MAIT cells and terminally differentiated NK cells. However, IgE levels were elevated in 50% of IL-21R-deficient patients. Overall survival following HSCT (6 patients, mean follow-up 1.8 year) was 33.3%, with pre-existing organ damage constituting a negative prognostic factor. Mortality of non-transplanted patients (n = 7) was 57.1%. Our detailed analysis of the largest cohort of IL-21R-deficient patients to date provides in-depth clinical, immunological and immunophenotypic features of these patients, thereby establishing critical non-redundant functions of IL-21/IL-21R signaling in lymphocyte differentiation, humoral immunity and host defense against infection, and mechanisms of disease pathogenesis due to IL-21R deficiency. Outcome following HSCT depends on prior chronic infections and organ damage, which should thus be considered as early as possible following molecular diagnosis.

The dangers of déjà vu: memory B cells as the cells of origin of ABC-DLBCLs

Activated B-cell (ABC)-diffuse large B-cell lymphomas (DLBCLs) are clinically aggressive and phenotypically complex malignancies, whose transformation mechanisms remain unclear. Partially differentiated antigen-secreting cells (plasmablasts) have long been regarded as cells-of-origin for these tumors, despite lack of definitive experimental evidence. Recent DLBCL reclassification based on mutational landscapes identified MCD/C5 tumors as specific ABC-DLBCLs with unfavorable clinical outcome, activating mutations in the signaling adaptors MYD88 and CD79B, and immune evasion through mutation of antigen-presenting genes. MCD/C5s manifest prominent extranodal dissemination and similarities with primary extranodal lymphomas (PENLs). In this regard, recent studies on TBL1XR1, a gene recurrently mutated in MCD/C5s and PENLs, suggest that aberrant memory B cells (MBs), and not plasmablasts, are the true cells-of-origin for these tumors. Moreover, transcriptional and phenotypic profiling suggests that MCD/C5s, as a class, represent bona fide MB tumors. Based on emerging findings we propose herein a generalized stepwise model for MCD/C5 and PENLs pathogenesis, whereby acquisition of founder mutations in activated B cells favors the development of aberrant MBs prone to avoid plasmacytic differentiation on recall and undergo systemic dissemination. Cyclic reactivation of these MBs through persistent antigen exposure favors their clonal expansion and accumulation of mutations, which further facilitate their activation. As a result, MB-like clonal precursors become trapped in an oscillatory state of semipermanent activation and phenotypic sway that facilitates ulterior transformation and accounts for the extranodal clinical presentation and biology of these tumors. In addition, we discuss diagnostic and therapeutic implications of a MB cell-of-origin for these lymphomas.

Human PD-1hiCD8+ T Cells Are a Cellular Source of IL-21 in Rheumatoid Arthritis

Background

Rheumatoid arthritis (RA) is a prototypical autoantibody-driven autoimmune disease in which T-B interactions play a critical role. Recent comprehensive analysis suggests that PD-1⁺CD8⁺ T cells as well as two distinct IL-21-producing PD-1⁺CD4⁺ T cell subsets, follicular helper T (Tfh) and peripheral helper T (Tph) cells, are involved in the pathogenesis of RA. Herein, we aimed to clarify a generation mechanism of IL-21-producing CD8⁺ T cells in humans, and to characterize this novel subset in patients with RA.

Methods

CD8⁺ T cells in the peripheral blood (PB) and synovial fluid (SF) of healthy control (HC) and patients with RA were subject to the analysis of IL-21 mRNA and protein. We evaluated the surface marker, cytokine and transcription profiles of IL-21-producing CD8⁺ T cells in HCPB, RAPB and RASF.

Results

IL-21-producing CD8⁺ T cells were enriched in the CD45RA^-(memory) PD-1⁺, especially PD-1^hi subpopulation, and IL-12 and IL-21 synergistically induced IL-21 production by naïve CD8⁺ T cells. Memory PD-1^hiCD8⁺ T cells in HCPB facilitated plasmablast differentiation and IgG production in an IL-21-dependent manner. In addition, PD-1^hiCD8⁺ T cells in RASF and RAPB produced large amounts of IL-21 and were characterized by high levels of CD28, ICOS, CD69, HLA-DR, and CCR2 but not CXCR5. Furthermore, PD-1^hiCD8⁺ T cells expressed high levels of transcripts of MAF and PRDM1, a feature observed in Tph cells.

Conclusions

Identification of IL-21-producing PD-1^hiCD8⁺ T cells expands our knowledge of T cell subsets with B helper functions in RA. Selective targeting of these subsets could pave an avenue for the development of novel treatment strategies for this disease.

IL-21-Deficient T Follicular Helper Cells Support B Cell Responses Through IL-27 in Patients With Chronic Hepatitis B

Chronic Hepatitis B (CHB) affects over 350 million people worldwide. Current treatment does result in reduced complications; however, a cure (development of antibodies to the S antigen) is not achieved, requiring life-long therapy. Humoral responses contribute to viral elimination by secreting neutralizing antibodies; though, effective induction of humoral immunity require CD4T cell differentiation into T follicular helper (T_FH) cells that support B cell response through interleukin-21 (IL-21). In CHB, mechanism of T_FH-B interactions is seldom described. During CHB, T_FH cells are defective in producing IL-21 in response to hepatitis B surface antigen (HBsAg). However, regardless of low IL-21, T_FH cells efficiently support B cell responses by producing interleukin-27 (IL-27), which directs the formation of plasmablasts and plasma cells from memory and naïve B cells by enhancing B lymphocyte-induced maturation protein-1. IL-27 not only improved total antibody production but HBsAg-specific IgG and IgM secretion that are essential for viral clearance. Importantly, IL-27+T_FH cells were significantly associated with HBV DNA reduction. Therefore, these findings imply a novel mechanism of T_FH mediated B cell help in CHB and suggest that IL-27 effectively compensate the function of IL-21 by supporting T_FH-B cell function, required for protective antibody response and may contribute to viral clearance by providing potential target for achieving a functional cure.

Molecular requirements for human lymphopoiesis as defined by inborn errors of immunity

Hematopoietic stem cells (HSCs) are the progenitor cells that give rise to the diverse repertoire of all immune cells. As they differentiate, HSCs yield a series of cell states that undergo gradual commitment to become mature blood cells. Studies of hematopoiesis in murine models have provided critical insights about the lineage relationships among stem cells, progenitors, and mature cells, and these have guided investigations of the molecular basis for these distinct developmental stages. Primary immune deficiencies are caused by inborn errors of immunity that result in immune dysfunction and subsequent susceptibility to severe and recurrent infection(s). Over the last decade there has been a dramatic increase in the number and depth of the molecular, cellular, and clinical characterization of such genetically defined causes of immune dysfunction. Patients harboring inborn errors of immunity thus represent a unique resource to improve our understanding of the multilayered and complex mechanisms underlying lymphocyte development in humans. These breakthrough discoveries not only enable significant advances in the diagnosis of such rare and complex conditions but also provide substantial improvement in the development of personalized treatments. Here, we will discuss the clinical, cellular, and molecular phenotypes, and treatments of selected inborn errors of immunity that impede, either intrinsically or extrinsically, the development of B- or T-cells at different stages.

Flow Cytometric Methods for the Detection of Intracellular Signaling Proteins and Transcription Factors Reveal Heterogeneity in Differentiating Human B Cell Subsets

The flow cytometric detection of intracellular (IC) signaling proteins and transcription factors (TFs) will help to elucidate the regulation of B cell survival, proliferation and differentiation. However, the simultaneous detection of signaling proteins or TFs with membrane markers (MMs) can be challenging, as the required fixation and permeabilization procedures can affect the functionality of conjugated antibodies. Here, a phosphoflow method is presented for the detection of activated NF-κB p65 and phosphorylated STAT1, STAT3, STAT5 and STAT6, together with the B cell differentiation MMs CD19, CD27 and CD38. Additionally, a TF-flow method is presented that allows the detection of the B cell TFs PAX5, c-MYC, BCL6 and AID and antibody-secreting cell (ASC) TFs BLIMP1 and XBP-1s, together with MMs. Applying these methods on in vitro-induced human B cell differentiation cultures showed significantly different steady-state levels, and responses to stimulation, of phosphorylated signaling proteins in CD27-expressing B cell and ASC populations. The TF-flow protocol and Uniform Manifold Approximation and Projection (UMAP) analysis revealed heterogeneity in TF expression within stimulated CD27- or CD38-expressing B cell subsets. The methods presented here allow for the sensitive analysis of STAT, NF-κB p65 signaling and TFs, together with B cell differentiation MMs, at single-cell resolution. This will aid the further investigation of B cell responses in both health and disease.

Integrative transcriptome and chromatin landscape analysis reveals distinct epigenetic regulations in human memory B cells

Memory B cells (MBCs) are long-lived and produce high-affinity, generally, class-switched antibodies. Here, we use a multiparameter approach involving CD27 to segregate naïve B cells (NBC), IgD⁺ unswitched (unsw)MBCs and IgG⁺ or IgA⁺ class-switched (sw)MBCs from humans of different age, sex and race. Conserved antibody variable gene expression indicates that MBCs emerge through unbiased selection from NBCs. Integrative analyses of mRNAs, miRNAs, lncRNAs, chromatin accessibility and cis-regulatory elements uncover a core mRNA-ncRNA transcriptional signature shared by IgG⁺ and IgA⁺ swMBCs and distinct from NBCs, while unswMBCs display a transitional transcriptome. Some swMBC transcriptional signature loci are accessible but not expressed in NBCs. Profiling miRNAs reveals downregulated MIR181, and concomitantly upregulated MIR181 target genes such as RASSF6, TOX, TRERF1, TRPV3 and RORα, in swMBCs. Finally, lncRNAs differentially expressed in swMBCs cluster proximal to the IgH chain locus on chromosome 14. Our findings thus provide new insights into MBC transcriptional programs and epigenetic regulation, opening new investigative avenues on these critical cell elements in human health and disease.

Human memory B cells differentiate from naïve B cells and can express different immunoglobulin (Ig) isotypes resulted from class-switch recombination. Here the authors describe, using transcriptional and epigenetic data from human memory B cells and integrated multi-omics analyses, the differentiation regulation and trajectory of IgG⁺, IgA⁺ and IgD⁺ memory B cells.

Decoding IL-23 Signaling Cascade for New Therapeutic Opportunities

The interleukin 23 (IL-23) is a key pro-inflammatory cytokine in the development of chronic inflammatory diseases, such as psoriasis, inflammatory bowel diseases, multiple sclerosis, or rheumatoid arthritis. The pathological consequences of excessive IL-23 signaling have been linked to its ability to promote the production of inflammatory mediators, such as IL-17, IL-22, granulocyte-macrophage colony-stimulating (GM-CSF), or the tumor necrosis factor (TNFα) by target populations, mainly Th17 and IL-17-secreting TCRγδ cells (Tγδ17). Due to their pivotal role in inflammatory diseases, IL-23 and its downstream effector molecules have emerged as attractive therapeutic targets, leading to the development of neutralizing antibodies against IL-23 and IL-17 that have shown efficacy in different inflammatory diseases. Despite the success of monoclonal antibodies, there are patients that show no response or partial response to these treatments. Thus, effective therapies for inflammatory diseases may require the combination of multiple immune-modulatory drugs to prevent disease progression and to improve quality of life. Alternative strategies aimed at inhibiting intracellular signaling cascades using small molecule inhibitors or interfering peptides have not been fully exploited in the context of IL-23-mediated diseases. In this review, we discuss the current knowledge about proximal signaling events triggered by IL-23 upon binding to its membrane receptor to bring to the spotlight new opportunities for therapeutic intervention in IL-23-mediated pathologies.