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Obesity is linked to inflammation and downstream metabolic dysregulation. In this issue of Cell Metabolism, Hägglöf et al. show that iNKT cells enable the accumulation of T-bet + B cells in white adipose tissue, which in turn produce chemokine and antibody mediators that exacerbate the onset and severity of metabolic disease., Competing Interests: Declaration of interests The author declares no competing interests., (Copyright © 2022 Elsevier Inc. All rights reserved.)

The recombination-activating genes (RAG) 1 and 2 are indispensable for diversifying the primary B cell receptor repertoire and pruning self-reactive clones via receptor editing in the bone marrow; however, the impact of RAG1/RAG2 on peripheral tolerance is unknown. Partial RAG deficiency (pRD) manifesting with late-onset immune dysregulation represents an 'experiment of nature' to explore this conundrum. By studying B cell development and subset-specific repertoires in pRD, we demonstrate that reduced RAG activity impinges on peripheral tolerance through the generation of a restricted primary B cell repertoire, persistent antigenic stimulation and an inflammatory milieu with elevated B cell-activating factor. This unique environment gradually provokes profound B cell dysregulation with widespread activation, remarkable extrafollicular maturation and persistence, expansion and somatic diversification of self-reactive clones. Through the model of pRD, we reveal a RAG-dependent 'domino effect' that impacts stringency of tolerance and B cell fate in the periphery., (© 2022. The Author(s).)

Adjuvants are critical for improving the quality and magnitude of adaptive immune responses to vaccination. Lipid nanoparticle (LNP)-encapsulated nucleoside-modified mRNA vaccines have shown great efficacy against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but the mechanism of action of this vaccine platform is not well-characterized. Using influenza virus and SARS-CoV-2 mRNA and protein subunit vaccines, we demonstrated that our LNP formulation has intrinsic adjuvant activity that promotes induction of strong T follicular helper cell, germinal center B cell, long-lived plasma cell, and memory B cell responses that are associated with durable and protective antibodies in mice. Comparative experiments demonstrated that this LNP formulation outperformed a widely used MF59-like adjuvant, AddaVax. The adjuvant activity of the LNP relies on the ionizable lipid component and on IL-6 cytokine induction but not on MyD88- or MAVS-dependent sensing of LNPs. Our study identified LNPs as a versatile adjuvant that enhances the efficacy of traditional and next-generation vaccine platforms., Competing Interests: Declaration of interests In accordance with the University of Pennsylvania policies and procedures and our ethical obligations as researchers, we report that D.W. and N.P. are named on a patent describing the use of nucleoside-modified mRNA in lipid nanoparticles as a vaccine platform. We have disclosed those interests fully to the University of Pennsylvania, and we have in place an approved plan for managing any potential conflicts arising from licensing of our patents. K.K. is an employee of BioNTech. P.J.C.L., B.L.M., and Y.K.T. are employees of Acuitas Therapeutics, a company involved in the development of mRNA-LNP therapeutics. Y.K.T., D.W., and M.G.A. are named on patents that describe lipid nanoparticles for delivery of nucleic acid therapeutics, including mRNA and the use of modified mRNA in lipid nanoparticles as a vaccine platform. The Icahn School of Medicine at Mount Sinai has filed patent applications regarding SARS-CoV-2 and influenza virus vaccines that name F.K. as co-inventor., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Nucleoside-modified mRNA vaccines have gained global attention because of COVID-19. We evaluated a similar vaccine approach for preventing a chronic, latent genital infection rather than an acute respiratory infection. We used animal models to compare an HSV-2 trivalent nucleoside-modified mRNA vaccine with the same antigens prepared as proteins, with an emphasis on antigen-specific memory B cell responses and immune correlates of protection. In guinea pigs, serum neutralizing-antibody titers were higher at 1 month and declined far less by 8 months in mRNA- compared with protein-immunized animals. Both vaccines protected against death and genital lesions when infected 1 month after immunization; however, protection was more durable in the mRNA group compared with the protein group when infected after 8 months, an interval representing greater than 15% of the animal's lifespan. Serum and vaginal neutralizing-antibody titers correlated with protection against infection, as measured by genital lesions and vaginal virus titers 2 days after infection. In mice, the mRNA vaccine generated more antigen-specific memory B cells than the protein vaccine at early times after immunization that persisted for up to 1 year. High neutralizing titers and robust B cell immune memory likely explain the more durable protection by the HSV-2 mRNA vaccine.

Global inactivation of IκB kinase (IKK)-α results in defective lymph node (LN) formation and B cell maturation, and loss of IKK-α-dependent noncanonical NF-κB signaling in stromal organizer and hematopoietic cells is thought to underlie these distinct defects. We previously demonstrated that this pathway is also activated in vascular endothelial cells (ECs). To determine the physiologic function of EC-intrinsic IKK-α, we crossed Ikkα F/F mice with Tie2-cre or Cdh5-cre mice to ablate IKK-α in ECs. Notably, the compound defects of global IKK-α inactivation were recapitulated in Ikkα Tie2 and Ikkα Cdh5 mice, as both lacked all LNs and mature follicular and marginal zone B cell numbers were markedly reduced. However, as Tie2-cre and Cdh5-cre are expressed in all ECs, including blood forming hemogenic ECs, IKK-α was also absent in hematopoietic cells (HC). To determine if loss of HC-intrinsic IKK-α affected LN development, we generated Ikkα Vav mice lacking IKK-α in only the hematopoietic compartment. While mature B cell numbers were significantly reduced in Ikkα Vav mice, LN formation was intact. As lymphatic vessels also arise during development from blood ECs, we generated Ikkα Lyve1 mice lacking IKK-α in lymphatic ECs (LECs) to determine if IKK-α in lymphatic vessels impacts LN development. Strikingly, while mature B cell numbers were normal, LNs were completely absent in Ikkα Lyve1 mice. Thus, our findings reveal that IKK-α in distinct EC-derived compartments is uniquely required to promote B cell homeostasis and LN development, and we establish that LEC-intrinsic IKK-α is absolutely essential for LN formation., Competing Interests: The authors declare no competing interest.

Immunological memory is a composite of lasting antibody titers maintained by plasma cells in conjunction with memory T and B cells. Memory B cells are a critical reservoir for plasma cell generation in the secondary response. Identification of memory B cells requires that they be distinguished from naïve, activated, and germinal center precursors and from plasma cells. Memory B cells are heterogeneous in isotype usage, immunoglobulin mutational content, and phenotypic marker expression. Phenotypic subsets of memory B cells are defined by PD-L2, CD80, and CD73 expression in mice, by CD27 and FCRL4 expression in humans and by T-bet in both mice and humans. These subsets display marked functional heterogeneity, including the ability to rapidly differentiate into plasma cells versus seed germinal centers in the secondary response. Memory B cells are located in the spleen, blood, other lymphoid organs, and barrier tissues, and recent evidence indicates that some memory B cells may be dedicated tissue-resident populations. Open questions about memory B cell longevity, renewal and progenitor-successor relationships with plasma cells are discussed., (© 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Systemic lupus erythematous (SLE) is a female-predominant disease characterized by autoimmune B cells and pathogenic autoantibody production. Individuals with two or more X chromosomes are at increased risk for SLE, suggesting that X-linked genes contribute to the observed sex bias of this disease. To normalize X-linked gene expression between sexes, one X in female cells is randomly selected for transcriptional silencing through X-chromosome inactivation (XCI), resulting in allele-specific enrichment of epigenetic modifications, including histone methylation and the long noncoding RNA XIST/Xist on the inactive X (Xi). As we have previously shown that epigenetic regulation of the Xi in female lymphocytes from mice is unexpectedly dynamic, we used RNA fluorescence in situ hybridization and immunofluorescence to profile epigenetic features of the Xi at the single-cell level in human B cell subsets from pediatric and adult SLE patients and healthy controls. Our data reveal that abnormal XCI maintenance in B cells is a feature of SLE. Using single-cell and bulk-cell RNA sequencing datasets, we found that X-linked immunity genes escape XCI in specific healthy human B cell subsets and that human SLE B cells exhibit aberrant expression of X-linked genes and XIST RNA interactome genes. Our data reveal that mislocalized XIST RNA, coupled with a dramatic reduction in heterochromatic modifications at the Xi in SLE, predispose for aberrant X-linked gene expression from the Xi, thus defining a genetic and epigenetic pathway that affects X-linked gene expression in human SLE B cells and likely contributes to the female bias in SLE., Competing Interests: The authors declare no competing interest.

B cell subsets expressing the transcription factor T-bet are associated with humoral immune responses and autoimmunity. Here, we examined the anatomic distribution, clonal relationships, and functional properties of T-bet + and T-bet - memory B cells (MBCs) in the context of the influenza-specific immune response. In mice, both T-bet - and T-bet + hemagglutinin (HA)-specific B cells arose in germinal centers, acquired memory B cell markers, and persisted indefinitely. Lineage tracing and IgH repertoire analyses revealed minimal interconversion between T-bet - and T-bet + MBCs, and parabionts showed differential tissue residency and recirculation properties. T-bet + MBCs could be subdivided into recirculating T-bet lo MBCs and spleen-resident T-bet hi MBCs. Human MBCs displayed similar features. Conditional gene deletion studies revealed that T-bet expression in B cells was required for nearly all HA stalk-specific IgG2c antibodies and for durable neutralizing titers to influenza. Thus, T-bet expression distinguishes MBC subsets that have profoundly different homing, residency, and functional properties, and mediate distinct aspects of humoral immune memory., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

The age-associated B cell subset has been the focus of increasing interest over the last decade. These cells have a unique cell surface phenotype and transcriptional signature, and they rely on TLR7 or TLR9 signals in the context of Th1 cytokines for their formation and activation. Most are antigen-experienced memory B cells that arise during responses to microbial infections and are key to pathogen clearance and control. Their increasing prevalence with age contributes to several well-established features of immunosenescence, including reduced B cell genesis and damped immune responses. In addition, they are elevated in autoimmune and autoinflammatory diseases, and in these settings they are enriched for characteristic autoantibody specificities. Together, these features identify age-associated B cells as a subset with pivotal roles in immunological health, disease, and aging. Accordingly, a detailed understanding of their origins, functions, and physiology should make them tractable translational targets in each of these settings.

B cell receptor (BCR) engagement induces naive B cells to differentiate and perform critical immune-regulatory functions. Acquisition of functional specificity requires that a cell survive, enter the cell cycle, and proliferate. We establish that quantitatively distinct Ca 2+ signals triggered by variations in the extent of BCR engagement dynamically regulate these transitions by controlling nuclear factor κB (NF-κB), NFAT, and mTORC1 activity. Weak BCR engagement induces apoptosis by failing to activate NF-κB-driven anti-apoptotic gene expression. Stronger signals that trigger more robust Ca 2+ signals promote NF-κB-dependent survival and NFAT-, mTORC1-, and c-Myc-dependent cell-cycle entry and proliferation. Finally, we establish that CD40 or TLR9 costimulation circumvents these Ca 2+ -regulated checkpoints of B cell activation and proliferation. As altered BCR signaling is linked to autoimmunity and B cell malignancies, these results have important implications for understanding the pathogenesis of aberrant B cell activation and differentiation and therapeutic approaches to target these responses., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Although central tolerance mechanisms purge self-reactive B cells during development based on BCR signal strength, mechanisms that block the differentiation of autoreactive effector and memory B cells from mature pools remain poorly understood. Prior observations implicate nucleic acid sensing TLRs in autoimmunity, and more recent findings show that TLR9 is also involved in maintaining peripheral tolerance. Studies of the immunological changes that occur during aging revealed a subset of B cells denoted Age-associated B cells which expands in settings of aging and in autoimmunity. Further studies demonstrated that TLR9 signals poise activated B cells to adopt an Age-associated B cell phenotype, but BCR-delivered TLR9 signals cause programmed cell death that, if circumvented by costimulation, allows continued differentiation to the ABC fate. Together, these observations suggest molecular pattern recognition, rather than BCR epitope specificity per se, is a fundamental mediator of tolerogenic outcomes in the peripheral B cell activation., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Antibodies are key to cutaneous host defense and inflammation. Despite their importance, the mechanisms by which skin antibodies are sustained are poorly described. Here, we identified that, in addition to antibody production in lymphoid tissues, plasma cells reside in healthy mouse and human skin. In naïve mice, IgM was the predominant isotype produced in skin. Skin plasma cells developed independently of T cells and microbiota. Importantly, chronic skin inflammation promoted the massive accumulation of IgM-secreting cells, and cutaneous immunization directed both T cell-dependent and -independent antigen-specific IgM-secreting cells into skin. Unlike their counterparts in lymphoid tissues, cutaneous IgM-secreting cells were completely dependent on survival factors such as a proliferation-inducing ligand or B cell-activating factor, which were constitutively expressed and upregulated during inflammation in skin. Our data support a model in which skin plasma cells supply natural and adaptive IgM to the cutaneous environment, thereby supporting homeostatic skin barrier functions and providing defense against pathogen intrusion. Our results are also of potential relevance for manipulation of cutaneous plasma cells in inflammatory skin diseases or cutaneous plasma cell malignancies., (Copyright © 2019 The Authors. Published by Elsevier Inc. All rights reserved.)

The TNF superfamily ligands BAFF and APRIL interact with three receptors, BAFFR, BCMA, and TACI, to play discrete and crucial roles in regulating B cell selection and homeostasis in mammals. The interactions between these ligands and receptors are both specific and redundant: BAFFR binds BAFF, whereas BCMA and TACI bind to either BAFF or APRIL. In a previous phylogenetic inquiry, we identified and characterized a BAFF -like gene in lampreys, which, with hagfish, are the only extant jawless vertebrates, both of which have B-like and T-like lymphocytes. To gain insight into lymphocyte regulation in jawless vertebrates, in this study we identified two BCMA -like genes in lampreys, BCMAL1 and BCMAL2 , which were found to be preferentially expressed by B-like lymphocytes. In vitro analyses indicated that the lamprey BAFF-like protein can bind to a BCMA-like receptor Ig fusion protein and to both BCMAL1- and BCMAL2-transfected cells. Discriminating regulatory roles for the two BCMA-like molecules are suggested by their differential expression before and after activation of the B-like lymphocytes in lampreys. Our composite results imply that BAFF-based mechanisms for B cell regulation evolved before the divergence of jawed and jawless vertebrates., (Copyright © 2019 by The American Association of Immunologists, Inc.)

The goals of a genital herpes vaccine are to prevent painful genital lesions and reduce or eliminate subclinical infection that risks transmission to partners and newborns. We evaluated a trivalent glycoprotein vaccine containing herpes simplex virus type 2 (HSV-2) entry molecule glycoprotein D (gD2) and two immune evasion molecules: glycoprotein C (gC2), which binds complement C3b, and glycoprotein E (gE2), which blocks immunoglobulin G (IgG) Fc activities. The trivalent vaccine was administered as baculovirus proteins with CpG and alum, or the identical amino acids were expressed using nucleoside-modified mRNA in lipid nanoparticles (LNPs). Both formulations completely prevented genital lesions in mice and guinea pigs. Differences emerged when evaluating subclinical infection. The trivalent protein vaccine prevented dorsal root ganglia infection, and day 2 and 4 vaginal cultures were negative in 23 of 30 (73%) mice compared with 63 of 64 (98%) in the mRNA group ( P = 0.0012). In guinea pigs, 5 of 10 (50%) animals in the trivalent subunit protein group had vaginal shedding of HSV-2 DNA on 19 of 210 (9%) days compared with 2 of 10 (20%) animals in the mRNA group that shed HSV-2 DNA on 5 of 210 (2%) days ( P = 0.0052). The trivalent mRNA vaccine was superior to trivalent proteins in stimulating ELISA IgG antibodies, neutralizing antibodies, antibodies that bind to crucial gD2 epitopes involved in entry and cell-to-cell spread, CD4 + T cell responses, and T follicular helper and germinal center B cell responses. The trivalent nucleoside-modified mRNA-LNP vaccine is a promising candidate for human trials., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The E3 ubiquitin ligase Itch regulates antibody levels and prevents autoimmune disease in humans and mice, yet how Itch regulates B cell fate or function is unknown. We now show that Itch directly limits B cell activity. While Itch-deficient mice displayed normal numbers of preimmune B cell populations, they showed elevated numbers of antigen-experienced B cells. Mixed bone marrow chimeras revealed that Itch acts within B cells to limit naive and, to a greater extent, germinal center (GC) B cell numbers. B cells lacking Itch exhibited increased proliferation, glycolytic capacity, and mTORC1 activation. Moreover, stimulation of these cells in vivo by WT T cells resulted in elevated numbers of GC B cells, PCs, and serum IgG. These results support a novel role for Itch in limiting B cell metabolism and proliferation to suppress antigen-driven B cell responses., (© 2019 Moser et al.)

Impaired classical NF-κB pathway signaling causes reduced antibody responses to T-independent (TI) antigens. We investigated the potential reasons for defective TI responses in mice lacking the atypical inhibitory kappa B (IκB) protein of the NF-κB pathway, IκBNS. Analyses of the plasma cell compartment in vitro and in vivo after challenge with lipopolysaccharide (LPS) showed significant decreases in the frequencies of plasma cells in the absence of IκBNS. In vitro activation of B cells via the B cell receptor or via Toll-like receptor 4 revealed that early activation events were unaffected in IκBNS-deficient B cells, while proliferation was reduced compared to in similarly stimulated wildtype (wt) B cells. IκBNS-deficient B cells also displayed impaired upregulation of the transmembrane activator and calcium modulator cyclophilin ligand interactor (TACI), which is essential for TI responses, and decreased sensitivity to TACI ligands upon stimulation. Furthermore, IκBNS-deficient B cells, in contrast to wt B cells, displayed altered expression of IRF4, Blimp-1 and Pax5 upon LPS-induced differentiation, indicating impaired transcriptional regulation of plasma cell generation., (© 2018 The Authors Immunology & Cell Biology published by John Wiley & Sons Australia, Ltd on behalf of Australasian Society for Immunology Inc.)

T-bet + B cells have emerged as a key component of the humoral immune response in both infections and autoimmune disorders, with many of their phenotypic and functional attributes conserved between mice and humans. They are protective (infections) and pathogenic (autoimmunity), although the associated commonalities and differences remain unclear. Heterogeneity within this pool, in terms of origin, fate and function may underlie these divergent roles. Their significance is context-dependent- they may constitute a persistent effector memory cell pool, or products of recent primary responses. In both cases however, T-bet + cells likely represent antigen-experienced progenitors of antibody-secreting cells with multipotent properties. Given their key contributions to both immunity and disease, T-bet + B cells are an attractive target for vaccination and therapeutic strategies., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

B cells expressing the transcription factor T-bet have emerged as participants in a number of protective and pathogenic immune responses. T-bet + B cells characteristically differentiate in response to combined Toll-like receptor and cytokine signaling, contribute to protective immunity against intracellular pathogens via IgG2 a/c production and antibody-independent mechanisms, and are prone to produce autoantibodies. Despite recent advances, a number of questions remain regarding the basic biology of T-bet + B cells and their functional niche within the immune system. Herein, we review the discovery and defining characteristics of the T-bet + B cell subset in both mice and humans. We further discuss their origins, the basis for their persistence, and their potential fate in vivo. Evidence indicates that T-bet + B cells represent a distinct, germinal center-derived memory population that may serve as an important therapeutic target for the improvement of humoral immunity and prevention of autoimmunity., (© 2019 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

T follicular helper (Tfh) cells are required to develop germinal center (GC) responses and drive immunoglobulin class switch, affinity maturation, and long-term B cell memory. In this study, we characterize a recently developed vaccine platform, nucleoside-modified, purified mRNA encapsulated in lipid nanoparticles (mRNA-LNPs), that induces high levels of Tfh and GC B cells. Intradermal vaccination with nucleoside-modified mRNA-LNPs encoding various viral surface antigens elicited polyfunctional, antigen-specific, CD4 + T cell responses and potent neutralizing antibody responses in mice and nonhuman primates. Importantly, the strong antigen-specific Tfh cell response and high numbers of GC B cells and plasma cells were associated with long-lived and high-affinity neutralizing antibodies and durable protection. Comparative studies demonstrated that nucleoside-modified mRNA-LNP vaccines outperformed adjuvanted protein and inactivated virus vaccines and pathogen infection. The incorporation of noninflammatory, modified nucleosides in the mRNA is required for the production of large amounts of antigen and for robust immune responses., (© 2018 Pardi et al.)

Although the aetiology of systemic lupus erythematosus (SLE) is unclear, dysregulated B cell responses have been implicated. Here we show that an unusual CD11c hi T-bet + B cell subset, with a unique expression profile including chemokine receptors consistent with migration to target tissues, is expanded in SLE patients, present in nephrotic kidney, enriched for autoreactive specificities and correlates with defined clinical manifestations. IL-21 can potently induce CD11c hi T-bet + B cells and promote the differentiation of these cells into Ig-secreting autoreactive plasma cells. While murine studies have identified a role for T-bet-expressing B cells in autoimmunity, this study describes and exemplifies the importance of CD11c hi T-bet + B cells in human SLE.

Transcription factors regulate various developmental and functional aspects of B cells. T-bet is a recently appreciated transcription factor associated with "Age-associated B cells" or ABCs, the development of autoimmunity, and viral infections. T-bet expression is favored by nucleic acid-containing antigens and immune complexes and is regulated by interplay between various cytokines, notably, the TFH cytokines IL-21, IL-4 and IFNγ. Adaptive signals by themselves cannot upregulate T-bet; however, they have a synergistic effect on induction of T-bet by innate receptors. The functional role of T-bet+ B cells is unclear, although it is known that T-bet promotes class switching to IgG2a/c. It is likely T-bet serves dichotomous roles in B cells, promoting pathogenic autoreactive antibodies on one hand but mediating microbial immunity on the other, making it a target of interest in both therapeutic and prophylactic settings., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Mature B cell pools retain a substantial proportion of polyreactive and self-reactive clonotypes, suggesting that activation checkpoints exist to reduce the initiation of autoreactive B cell responses. Here, we have described a relationship among the B cell receptor (BCR), TLR9, and cytokine signals that regulate B cell responses to DNA-containing antigens. In both mouse and human B cells, BCR ligands that deliver a TLR9 agonist induce an initial proliferative burst that is followed by apoptotic death. The latter mechanism involves p38-dependent G1 cell-cycle arrest and subsequent intrinsic mitochondrial apoptosis and is shared by all preimmune murine B cell subsets and CD27- human B cells. Survival or costimulatory signals rescue B cells from this fate, but the outcome varies depending on the signals involved. B lymphocyte stimulator (BLyS) engenders survival and antibody secretion, whereas CD40 costimulation with IL-21 or IFN-γ promotes a T-bet+ B cell phenotype. Finally, in vivo immunization studies revealed that when protein antigens are conjugated with DNA, the humoral immune response is blunted and acquires features associated with T-bet+ B cell differentiation. We propose that this mechanism integrating BCR, TLR9, and cytokine signals provides a peripheral checkpoint for DNA-containing antigens that, if circumvented by survival and differentiative cues, yields B cells with the autoimmune-associated T-bet+ phenotype.

The origin and nature of age-associated B cells (ABCs) in mice are poorly understood. In this article, we show that their emergence required MHC class II and CD40/CD40L interactions. Young donor B cells were adoptively transferred into congenic recipients and allowed to remain for 1 mo in the absence of external Ag. B cells expressing the T-bet transcription factor, a marker for ABCs, were generated after multiple cell divisions from C57BL/6 donors but not from MHC class II- or CD40-deficient donors. Furthermore, old CD154 (CD40L)-deficient mice did not accrue ABCs, confirming that they arise primarily through T-dependent interactions. To determine what Igs ABCs express, we sequenced V H and Vκ rearranged genes from unimmunized 22-mo-old C57BL/6 mice and showed that they had a heterogeneous repertoire, which was comparable to that seen in old follicular and marginal zone B cell subsets. However, in contrast to the follicular and marginal zone cells, ABCs displayed significant somatic hypermutation. The mutation frequency was lower than found in germinal center cells after deliberate immunization, suggesting that ABCs have undergone mild stimulation from endogenous Ags over time. These observations show that quiescent ABCs are Ag-experienced cells that accumulate during T cell-dependent responses to diverse Ags during the life of an individual., (Copyright © 2017 by The American Association of Immunologists, Inc.)

Long-lived plasma cells (PCs) in the bone marrow (BM) are a critical source of antibodies after infection or vaccination, but questions remain about the factors that control PCs. We found that systemic infection alters the BM, greatly reducing PCs and regulatory T (Treg) cells, a population that contributes to immune privilege in the BM. The use of intravital imaging revealed that BM Treg cells display a distinct behavior characterized by sustained co-localization with PCs and CD11c-YFP + cells. Gene expression profiling indicated that BM Treg cells express high levels of Treg effector molecules, and CTLA-4 deletion in these cells resulted in elevated PCs. Furthermore, preservation of Treg cells during systemic infection prevents PC loss, while Treg cell depletion in uninfected mice reduced PC populations. These studies suggest a role for Treg cells in PC biology and provide a potential target for the modulation of PCs during vaccine-induced humoral responses or autoimmunity., (Copyright © 2017 The Authors. Published by Elsevier Inc. All rights reserved.)

T-bet and CD11c expression in B cells is linked with IgG2c isotype switching, virus-specific immune responses, and humoral autoimmunity. However, the activation requisites and regulatory cues governing T-bet and CD11c expression in B cells remain poorly defined. In this article, we reveal a relationship among TLR engagement, IL-4, IL-21, and IFN-γ that regulates T-bet expression in B cells. We find that IL-21 or IFN-γ directly promote T-bet expression in the context of TLR engagement. Further, IL-4 antagonizes T-bet induction. Finally, IL-21, but not IFN-γ, promotes CD11c expression independent of T-bet. Using influenza virus and Heligmosomoides polygyrus infections, we show that these interactions function in vivo to determine whether T-bet(+) and CD11c(+) B cells are formed. These findings suggest that T-bet(+) B cells seen in health and disease share the common initiating features of TLR-driven activation within this circumscribed cytokine milieu., (Copyright © 2016 by The American Association of Immunologists, Inc.)

NF-κB-inducing kinase (NIK) is a key mediator of the noncanonical NF-κB signaling pathway, which is critical for B-cell development and function. Although complete deletion of NIK in mice has been shown to result in defective B cells and impaired secondary lymphoid organogenesis, the consequences of deleting NIK exclusively in B cells have not been determined. In this issue of the European Journal of Immunology, Hahn et al. [Eur. J. Immunol. 2016. 46: 732-741] describe mice in which the NF-κB2 pathway mediator, NIK, is deleted at different points in B-cell lineage differentiation and activation. The results show that the survival of mature peripheral B cells, as well as appropriate kinetics of germinal center reactions, rely on noncanonical NF-κB signaling. These findings confirm and extend prior observations implicating a nonredundant role for NF-κB2 downstream of BAFF signaling via BAFF-R, and prompt assessment of the growing literature regarding the relative roles of BCR and BAFF signals in B-cell homeostasis, as well as the downstream pathways responsible., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A subset of B cells with unique phenotypic and functional features-termed Age-associated B cells (ABCs)-has recently been identified in both mice and humans. These cells are characterized by a T-BET driven transcriptional program, robust responsiveness to TLR7 and TLR9 ligands, and a propensity for IgG2a/c production. Beyond their age-related accumulation, these cells play roles in both normal and pathogenic humoral immune responses regardless of host age. Thus, B cells with the ABC phenotype and transcriptional signature appear during viral, bacterial, and parasitic infections, but also arise during humoral autoimmune disease in both mouse models and humans. These observations suggest that both autoantigens and certain classes of pathogens provide the signals required for ABC differentiation. Herein, we review the discovery and features of ABCs, and propose that they are a memory subset generated by nucleic acid-containing antigens in the context of a promoting inflammatory cytokine milieu., (© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Distinct families of germline-encoded pattern recognition receptors can sense both microbial and endogenous nucleic acids. These DNA and RNA sensors include endosomal TLRs and cytosolic sensors upstream of stimulator of type I IFN genes (STING) and MAVS. The existence of overlapping specificities for both foreign and self nucleic acids suggests that, under optimal conditions, the activity of these receptors is finely tuned to effectively mediate host defense yet constrain pathogenic self-reactivity. This equilibrium becomes disrupted with the loss of either TLR9 or STING. To maintain immune protection, this loss can be counterbalanced by the elevated response of an alternative receptor(s). Unfortunately, this adjustment can lead to an increased risk for the development of systemic autoimmunity, as evidenced by the exacerbated clinical disease manifestations of TLR9-deficient and STING-deficient autoimmune-prone mice. These studies underscore the delicate balance normally maintained by tonic signals that prevent unchecked immune responses to nucleic acids released during infections and cellular duress or death., (Copyright © 2015 by The American Association of Immunologists, Inc.)

The HuR RNA-binding protein posttranscriptionally controls expression of genes involved in cellular survival, proliferation, and differentiation. To determine roles of HuR in B cell development and function, we analyzed mice with B lineage-specific deletion of the HuR gene. These HuRΔ/Δ mice have reduced numbers of immature bone marrow and mature splenic B cells, with only the former rescued by p53 inactivation, indicating that HuR supports B lineage cells through developmental stage-specific mechanisms. Upon in vitro activation, HuRΔ/Δ B cells have a mild proliferation defect and impaired ability to produce mRNAs that encode IgH chains of secreted Abs, but no deficiencies in survival, isotype switching, or expression of germinal center (GC) markers. In contrast, HuRΔ/Δ mice have minimal serum titers of all Ab isotypes, decreased numbers of GC and plasma B cells, and few peritoneal B-1 B cells. Moreover, HuRΔ/Δ mice have severely decreased GCs, T follicular helper cells, and high-affinity Abs after immunization with a T cell-dependent Ag. This failure of HuRΔ/Δ mice to mount a T cell-dependent Ab response contrasts with the ability of HuRΔ/Δ B cells to become GC-like in vitro, indicating that HuR is essential for aspects of B cell activation unique to the in vivo environment. Consistent with this notion, we find in vitro stimulated HuRΔ/Δ B cells exhibit modestly reduced surface expression of costimulatory molecules whose expression is similarly decreased in humans with common variable immunodeficiency. HuRΔ/Δ mice provide a model to identify B cell-intrinsic factors that promote T cell-dependent immune responses in vivo., (Copyright © 2015 by The American Association of Immunologists, Inc.)

A newly discovered B cell subset, age-associated B cells, expresses the transcription factor T-bet, has a unique surface phenotype, and accumulates progressively with age. Moreover, B cells with these general features are associated with viral infections and autoimmunity in both mice and humans. In this article, we review current understanding of the characteristics, origins, and functions of these cells. We also suggest that the protective versus pathogenic actions of these cells reflect appropriate versus aberrant engagement of regulatory mechanisms that control the Ab responses to nucleic acid-containing Ags., (Copyright © 2015 by The American Association of Immunologists, Inc.)

Objective: To determine the development of systemic lupus erythematosus (SLE) in NZM 2328 (NZM) mice deficient in 2 BAFF receptors., Methods: NZM.BR-3(-/-) .BCMA(-/-) , NZM.BR-3(-/-) .TACI(-/-) , and NZM.BCMA(-/-) .TACI(-/-) mice were evaluated on the clinical, pathologic, serologic, and cellular levels. BAFF receptor expression and lymphocyte phenotype were assessed by flow cytometry, IgG-secreting cells by enzyme-linked immunospot assay, B cell responsiveness to BAFF and generation of Treg cells by in vitro culture, serum BAFF and total IgG and IgG autoantibody levels by enzyme-linked immunosorbent assay, renal immunopathology by immunofluorescence and histologic analyses, and clinical disease by assessment of proteinuria and mortality., Results: Renal immunopathology and clinical disease were attenuated in NZM.BR-3(-/-) .BCMA(-/-) and NZM.BR-3(-/-) .TACI(-/-) mice but were accelerated in NZM.BCMA(-/-) .TACI(-/-) mice. Accelerated disease was associated with increases in B cells, IgG-secreting cells, serum autoantibody levels, and T cells (especially CD4+ activated memory cells), whereas attenuated disease was associated with reductions in many of these parameters. Serum BAFF levels were increased in all double-deficient NZM mice. Exogenous BAFF promoted the in vitro survival of B cells from NZM.BCMA(-/-) .TACI(-/-) or NZM wild-type mice but not those from NZM.BR-3(-/-) .BCMA(-/-) or NZM.BR-3(-/-) .TACI(-/-) mice. In vitro generation of Treg cells was reduced in NZM.BCMA(-/-) .TACI(-/-) mice, but not in NZM.BR-3(-/-) .BCMA(-/-) or NZM.BR-3(-/-) .TACI(-/-) mice., Conclusion: Elimination of B lymphocyte stimulator receptor 3 (BR-3) and TACI or BR-3 and BCMA inhibits the development of SLE in NZM mice. Selective targeting of BR-3 plus TACI or BR-3 plus BCMA may be an efficacious therapeutic approach in human SLE., (© 2015, American College of Rheumatology.)

B-lineage cells (B lymphocytes and plasma cells) predominate in the inflammatory infiltrate of human chronic periodontitis. However, their role in disease pathogenesis and the factors responsible for their persistence in chronic lesions are poorly understood. In this regard, two cytokines of the TNF ligand superfamily, a proliferation-inducing ligand (APRIL) and B-lymphocyte stimulator (BLyS), are important for the survival, proliferation, and maturation of B cells. Thus, we hypothesized that APRIL and/or BLyS are upregulated in periodontitis and contribute to induction of periodontal bone loss. This hypothesis was addressed in both human and mouse experimental systems. We show that, relative to healthy controls, the expression of APRIL and BLyS mRNA and protein was upregulated in natural and experimental periodontitis in humans and mice, respectively. The elevated expression of these cytokines correlated with increased numbers of B cells/plasma cells in both species. Moreover, APRIL and BLyS partially colocalized with κ L chain-expressing B-lineage cells at the epithelial-connective tissue interface. Ligature-induced periodontitis resulted in significantly less bone loss in B cell-deficient mice compared with wild-type controls. Ab-mediated neutralization of APRIL or BLyS diminished the number of B cells in the gingival tissue and inhibited bone loss in wild-type, but not in B cell-deficient, mice. In conclusion, B cells and specific cytokines involved in their growth and differentiation contribute to periodontal bone loss. Moreover, APRIL and BLyS have been identified as potential therapeutic targets in periodontitis., (Copyright © 2015 by The American Association of Immunologists, Inc.)

Antibody affinity maturation involves selective survival of high affinity B cells and is thought to require the germinal center (GC) microenvironment. In this issue of Immunity, Di Niro et al. (2015) challenge this view, showing that low affinity B cells initiate Salmonella responses and affinity mature outside of GCs., (Copyright © 2015 Elsevier Inc. All rights reserved.)

The BAFF family of receptors and ligands controls B cell homeostasis and selection. Recent studies reveal distinct sources and roles for systemic versus locally produced BAFF. Moreover, the notion that differential BAFF receptor expression patterns establish independent homeostatic and selective niches has been strengthened. Finally, unique roles for BAFF family members in the regulation of antigen experienced and innate B cell subsets have been revealed. Herein, we overview current knowledge in these areas, emphasizing recent findings that inform these ideas., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Endosomal TLRs play an important role in systemic autoimmune diseases, such as systemic erythematosus lupus, in which DNA- and RNA-associated autoantigens activate autoreactive B cells through TLR9- and TLR7-dependent pathways. Nevertheless, TLR9-deficient autoimmune-prone mice develop more severe clinical disease, whereas TLR7-deficient and TLR7/9-double deficient autoimmune-prone mice develop less severe disease. To determine whether the regulatory activity of TLR9 is B cell intrinsic, we directly compared the functional properties of autoantigen-activated wild-type, TLR9-deficient, and TLR7-deficient B cells in an experimental system in which proliferation depends on BCR/TLR coengagement. In vitro, TLR9-deficient cells are less dependent on survival factors for a sustained proliferative response than are either wild-type or TLR7-deficient cells. The TLR9-deficient cells also preferentially differentiate toward the plasma cell lineage, as indicated by expression of CD138, sustained expression of IRF4, and other molecular markers of plasma cells. In vivo, autoantigen-activated TLR9-deficient cells give rise to greater numbers of autoantibody-producing cells. Our results identify distinct roles for TLR7 and TLR9 in the differentiation of autoreactive B cells that explain the capacity of TLR9 to limit, as well as TLR7 to promote, the clinical features of systemic erythematosus lupus., (Copyright © 2015 by The American Association of Immunologists, Inc.)