Resting innate-like B cells leverage sustained Notch2/mTORC1 signaling to achieve rapid and mitosis-independent plasma cell differentiation

PI3Kγ in B cells promotes antibody responses and generation of antibody-secreting cells

The differentiation of naive and memory B cells into antibody-secreting cells (ASCs) is a key feature of adaptive immunity. The requirement for phosphoinositide 3-kinase-delta (PI3Kδ) to support B cell biology has been investigated intensively; however, specific functions of the related phosphoinositide 3-kinase-gamma (PI3Kγ) complex in B lineage cells have not. In the present study, we report that PI3Kγ promotes robust antibody responses induced by T cell-dependent antigens. The inborn error of immunity caused by human deficiency in PI3Kγ results in broad humoral defects, prompting our investigation of roles for this kinase in antibody responses. Using mouse immunization models, we found that PI3Kγ functions cell intrinsically within activated B cells in a kinase activity-dependent manner to transduce signals required for the transcriptional program supporting differentiation of ASCs. Furthermore, ASC fate choice coincides with upregulation of PIK3CG expression and is impaired in the context of PI3Kγ disruption in naive B cells on in vitro CD40-/cytokine-driven activation, in memory B cells on toll-like receptor activation, or in human tonsillar organoids. Taken together, our study uncovers a fundamental role for PI3Kγ in supporting humoral immunity by integrating signals instructing commitment to the ASC fate.

New insights into the ontogeny, diversity, maturation and survival of long-lived plasma cells

Plasma cells are unique immune effectors, capable of producing large amounts of high-affinity antibodies that protect against pathogenic infections. Although most plasma cells have short lifespans, certain conditions or vaccinations can give rise to long-lived plasma cells (LLPCs) that provide individuals with lifelong protection against pathogen exposure. The nature of these LLPCs is poorly understood; however, recent studies have shed new light on the ontogeny, diversity, maturation and survival of these unique cells. Whereas LLPCs had been thought to arise preferentially from germinal centres, novel genetic tools have revealed that they can originate from various stages throughout the humoral response. Furthermore, new single-cell analyses have shown that mouse and human plasma cells are heterogeneous and may undergo further maturation in situ in the bone marrow niche. Finally, plasma cells were previously considered to be sessile cells maintained in fixed survival niches, but new data show that plasma cell subsets can differentially migrate and organize into clusters that may be associated with survival niches. These descriptive findings provide new insights into how cell-intrinsic programmes and extrinsic factors may regulate the longevity of plasma cells in various contexts, which suggest new research avenues for their functional validation.

Marginal Zone B Cells Are Necessary for the Formation of Anti-donor IgG After Allogeneic Sensitization

BACKGROUND

The formation of anti-major histocompatibility complex (MHC) antibodies is a significant barrier for many patients awaiting organ transplantation. Patients with preformed anti-MHC antibodies have limited options for suitable donors, and the formation of donor-specific anti-MHC antibodies after transplantation is a harbinger of graft rejection. Despite the recognized importance of anti-MHC antibodies, the mechanisms responsible for the differentiation of B cells after exposure to allogeneic antigens are poorly understood.

METHODS

To evaluate the differentiation of B cells in response to allogeneic antigen, we used a model of H-2 b C57Bl/6 sensitization with H-2 d antigen. We used a class I MHC tetramer-based approach to identify allogeneic B cells and flow cytometric crossmatch to identify allogeneic IgM and IgG.

RESULTS

We found that although the formation of anti-H-2 d IgG was robust, few class-switched B cells and germinal center B cells were formed. Antigen-specific B cells did not express classical memory B-cell markers after sensitization but had an IgM + CD21 + marginal zone B-cell phenotype. The frequency of marginal zone B cells increased after sensitization. Depletion of marginal zone B cells before sensitization or skin grafting resulted in a significant diminution of anti-H-2 d IgG and fewer germinal center B cells. Adoptive transfer experiments revealed that marginal zone B cells more efficiently differentiated into germinal center B cells and anti-donor IgG-producing cells than follicular B cells.

CONCLUSIONS

These results demonstrate an important role for marginal zone B cells as a reservoir of alloreactive B cells that are activated by allogeneic antigens.

Spleen-targeted delivery systems and strategies for spleen-related diseases

The spleen, body's largest secondary lymphoid organ, is also a vital hematopoietic and immunological organ. It is regarded as one of the most significant organs in humans. As more researchers recognize the functions of the spleen, clinical methods for treating splenic diseases and spleen-targeted drug delivery systems to improve the efficacy of spleen-related therapies have gradually developed. Many modification strategies (size, charge, ligand, protein corona) and hitchhiking strategies (erythrocytes, neutrophils) of nanoparticles (NPs) have shown a significant increase in spleen targeting efficiency. However, most of the targeted drug therapy strategies for the spleen are to enhance or inhibit the immune function of the spleen to achieve therapeutic effects, and there are few studies on spleen-related diseases. In this review, we not only provide a detailed summary of the design rules for spleen-targeted drug delivery systems in recent years, but also introduce common spleen diseases (splenic tumors, splenic injuries, and splenomegaly) with the hopes of generating more ideas for future spleen research.

Type I interferon promotes the fate of Toll-like receptor 9-stimulated follicular B cells to plasma cell differentiation

The activation and differentiation of B cells into plasma cells (PCs) play critical roles in the immune response to infections and autoimmune diseases. Toll-like receptor 9 (TLR9) responds to bacterial and viral DNA containing unmethylated CpG motifs and triggers immune responses in B cells; however, abnormal recognition of self-DNA by TLR9 can cause autoimmune diseases. When stimulated with TLR9 agonists, follicular (FO) B cells, a subset of B cells residing in the FO regions of secondary lymphoid organs, exhibit a propensity for activation but fail to give rise to PCs. The factors that enable the transition of TLR9-activated FO B cells from activation to differentiation into PCs remain unclear. In this study, we show that type I interferon-alpha (IFNα) signaling causes FO B cells activated by CpG stimulation to differentiate into PCs. Although CpG stimulation alone only temporarily increased interferon regulatory factor 4 (IRF4) expression in FO B cells, co-stimulation with both CpG and IFNα enhanced and maintained high IRF4 expression levels, ultimately enabling the cells to differentiate into PCs. Overexpression of IRF4 in FO B cells results in CpG-induced PC transition without IFN signaling. Furthermore, co-stimulation of TLR9 and IFNα receptors significantly enhanced mammalian target of rapamycin (mTOR) signaling, which regulates IRF4 expression and PC generation. These findings suggest that IFNα may play a key role in promoting the fate of PC differentiation in FO B cells activated by TLR9 stimulation.

Two distinct subpopulations of marginal zone B cells exhibit differential antibody-producing capacities and radioresistance

Marginal zone (MZ) B cells, which are splenic innate-like B cells that rapidly secrete antibodies (Abs) against blood-borne pathogens, are composed of heterogeneous subpopulations. Here, we showed that MZ B cells can be divided into two distinct subpopulations according to their CD80 expression levels. CD80high MZ B cells exhibited greater Ab-producing, proliferative, and IL-10-secreting capacities than did CD80low MZ B cells. Notably, CD80high MZ B cells survived 2-Gy whole-body irradiation, whereas CD80low MZ B cells were depleted by irradiation and then repleted with one month after irradiation. Depletion of CD80low MZ B cells led to accelerated development of type II collagen (CII)-induced arthritis upon immunization with bovine CII. CD80high MZ B cells exhibited higher expression of genes involved in proliferation, plasma cell differentiation, and the antioxidant response. CD80high MZ B cells expressed more autoreactive B cell receptors (BCRs) that recognized double-stranded DNA or CII, expressed more immunoglobulin heavy chain sequences with shorter complementarity-determining region 3 sequences, and included more clonotypes with no N-nucleotides or with B-1a BCR sequences than CD80low MZ B cells. Adoptive transfer experiments showed that CD21+CD23+ transitional 2 MZ precursors preferentially generated CD80low MZ B cells and that a proportion of CD80low MZ B cells were converted into CD80high MZ B cells; in contrast, CD80high MZ B cells stably remained CD80high MZ B cells. In summary, MZ B cells can be divided into two subpopulations according to their CD80 expression levels, Ab-producing capacity, radioresistance, and autoreactivity, and these findings may suggest a hierarchical composition of MZ B cells with differential stability and BCR specificity.

Notch2 controls developmental fate choices between germinal center and marginal zone B cells upon immunization

Sustained Notch2 signals induce trans-differentiation of Follicular B (FoB) cells into Marginal Zone B (MZB) cells in mice, but the physiology underlying this differentiation pathway is still elusive. Here, we demonstrate that most B cells receive a basal Notch signal, which is intensified in pre-MZB and MZB cells. Ablation or constitutive activation of Notch2 upon T-cell-dependent immunization reveals an interplay between antigen-induced activation and Notch2 signaling, in which FoB cells that turn off Notch2 signaling enter germinal centers (GC), while high Notch2 signaling leads to generation of MZB cells or to initiation of plasmablast differentiation. Notch2 signaling is dispensable for GC dynamics but appears to be re-induced in some centrocytes to govern expansion of IgG1+ GCB cells. Mathematical modelling suggests that antigen-activated FoB cells make a Notch2 dependent binary fate-decision to differentiate into either GCB or MZB cells. This bifurcation might serve as a mechanism to archive antigen-specific clones into functionally and spatially diverse B cell states to generate robust antibody and memory responses.

CircLZIC regulates ox-LDL-induced HUVEC cell proliferation and apoptosis via Micro-330-5p/NOTCH2 axis in atherosclerosis

Atherosclerosis (AS) is a major chronic non-communicable disease and a primary cause of cardiovascular disease. Recent studies have shown that circRNAs are potential epigenetic factors that regulate vascular endothelial inflammatory responses and AS progression. Therefore, identification of the circRNAs that regulate ox-LDL levels is a critical step to understanding the pathology of AS. Our study is aim to investigate how circLZIC regulates atherosclerosis (AS) via the Micro-330-5p/NOTCH2 regulatory axis. The results showed that CircLZIC and NOTCH2 are highly expressed in human AS clinical samples, while Micro-330-5p is expressed locally. The CCK-8 experiment results showed that circLZIC promotes the proliferation of HUVECS cells. Flow cytometry analysis showed that circLZIC act as an inhibitor of HUVEC cell apoptosis. The expression level of Micro-330-5p can be up-regulated by transfection of small interfering RNA against circLZIC. Further, Starbase predicted that Micro-330-5p could target and regulate NOTCH2. Next, we confirmed that overexpression of Micro-330-5p could significantly reduce the expression of fluorescein using the double Luciferase reporter assay. RIP-qRT-PCR experiment showed that Micro-330-5p and NOTCH2 mRNAs are effectively enriched by ago2 protein. Further, we found that knocking down circLZIC increases the expression of Micro-330-5p and promotes cell apoptosis, while inhibiting the expression of NOTCH2 and cell activity. On the other hand, co-transfection of Micro-330-5p inhibitor decreases Micro-330-5p expression and inhibit cell apoptosis, while increasing NOTCH2 expression and cell activity. In conclusion, CircLZIC regulates HUVEC cell activity by the Micro-330-5p/NOTCH2 signaling pathway, suggesting that circLZIC plays a key role in atherosclerosis development.

The Proteasome Inhibitor Bortezomib Induces p53-Dependent Apoptosis in Activated B Cells

The proteasome inhibitor bortezomib (BTZ) is proposed to deplete activated B cells and plasma cells. However, a complete picture of the mechanisms underlying BTZ-induced apoptosis in B lineage cells remains to be established. In this study, using a direct in vitro approach, we show that deletion of the tumor suppressor and cell cycle regulator p53 rescues recently activated mouse B cells from BTZ-induced apoptosis. Furthermore, BTZ treatment elevated intracellular p53 levels, and p53 deletion constrained apoptosis, as recently stimulated cells first transitioned from the G1 to S phase of the cell cycle. Moreover, combined inhibition of the p53-associated cell cycle regulators and E3 ligases MDM2 and anaphase-promoting complex/cyclosome induced cell death in postdivision B cells. Our results reveal that efficient cell cycle progression of activated B cells requires proteasome-driven inhibition of p53. Consequently, BTZ-mediated interference of proteostasis unleashes a p53-dependent cell cycle-associated death mechanism in recently activated B cells.

B-1a Cells, but Not Marginal Zone B Cells, Are Implicated in the Accumulation of Autoreactive Plasma Cells in Lyn-/- Mice

Mice deficient in Lyn, a tyrosine kinase that limits B cell activation, develop a lupus-like autoimmune disease characterized by the accumulation of splenic plasma cells and the production of autoantibodies. Lyn-/- mice have reduced numbers of marginal zone (MZ) B cells, a B cell subset that is enriched in autoreactivity and prone to plasma cell differentiation. We hypothesized that this is due to unchecked terminal differentiation of this potentially pathogenic B cell subpopulation. However, impairing MZ B cell development in Lyn-/- mice did not reduce plasma cell accumulation or autoantibodies, and preventing plasma cell differentiation did not restore MZ B cell numbers. Instead, Lyn-/- mice accumulated B-1a cells when plasma cell differentiation was impaired. Similar to MZ B cells, B-1a cells tend to be polyreactive or weakly autoreactive and are primed for terminal differentiation. Our results implicate B-1a cells, but not MZ B cells, as contributors to the autoreactive plasma cell pool in Lyn-/- mice.

B cell expression of E3 ubiquitin ligase Cul4b promotes chronic gammaherpesvirus infection in vivo

IMPORTANCE

The human gammaherpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus are etiologic agents of numerous B cell lymphomas. A hallmark of gammaherpesvirus infection is their ability to establish lifelong latency in B cells. However, the specific mechanisms that mediate chronic infection in B cells in vivo remain elusive. Cellular E3 ubiquitin ligases regulate numerous biological processes by catalyzing ubiquitylation and modifying protein location, function, or half-life. Many viruses hijack host ubiquitin ligases to evade antiviral host defense and promote viral fitness. Here, we used the murine gammaherpesvirus 68 in vivo system to demonstrate that the E3 ligase Cul4b is essential for this virus to establish latency in germinal center B cells. These findings highlight an essential role for this E3 ligase in promoting chronic gammaherpesvirus infection in vivo and suggest that targeted inhibition of E3 ligases may provide a novel and effective intervention strategy against gammaherpesvirus-associated diseases.

B Cells from Aged Mice Do Not Have Intrinsic Defects in Affinity Maturation in Response to Immunization

Affinity maturation, the progressive increase in serum Ab affinity after vaccination, is an essential process that contributes to an effective humoral response against vaccines and infections. Germinal centers are key for affinity maturation, because they are where B cells undergo somatic hypermutation of their Ig genes in the dark zone before going through positive selection in the light zone via interactions with T follicular helper cells and follicular dendritic cells. In aged mice, affinity maturation has been shown to be impaired after immunization, but whether B cell-intrinsic factors contribute to this defect remains unclear. In this study, we show that B cells from aged BCR transgenic mice are able to become germinal center B cells, which are capable of receiving positive selection signals to a similar extent as B cells from young adult mice. Consistent with this, aging also does not impact the ability of B cells to undergo somatic hypermutation and acquire affinity-enhancing mutations. By contrast, transfer of B cells from young adult BCR mice into aged recipients resulted in the impaired acquisition of affinity-enhancing mutations, demonstrating that the aged microenvironment causes altered affinity maturation.

B cell diversification in gut-associated lymphoid tissues: From birds to humans

Several species generate their preimmune repertoire in gut-associated lymphoid tissues (GALT), compensating a reduced germline V gene repertoire by post-rearrangement diversification mechanisms (gene conversion and/or somatic hypermutation) in these environments that act as primary lymphoid organs. We summarize here these processes for three different species (chickens, sheep, and rabbits) and further discuss the analogous process that T-independent B cell responses in humans represent: we indeed recently showed that response against bacterial polysaccharides mobilize marginal zone B cells that prediversified against gut antigens. While the initial diversification strategy differs in these two cases, i.e., repertoire formation driven by gut-derived mitotic signals vs. response against gut antigens, the common feature of these two processes is the mobilization of a B cell compartment prediversified in GALT for immune responses against distinct systemic antigens.

AKT activity orchestrates marginal zone B cell development in mice and humans

The signals controlling marginal zone (MZ) and follicular (FO) B cell development remain incompletely understood. Here, we show that AKT orchestrates MZ B cell formation in mice and humans. Genetic models that increase AKT signaling in B cells or abolish its impact on FoxO transcription factors highlight the AKT-FoxO axis as an on-off switch for MZ B cell formation in mice. In humans, splenic immunoglobulin (Ig) D+CD27+ B cells, proposed as an MZ B cell equivalent, display higher AKT signaling than naive IgD+CD27- and memory IgD-CD27+ B cells and develop in an AKT-dependent manner from their precursors in vitro, underlining the conservation of this developmental pathway. Consistently, CD148 is identified as a receptor indicative of the level of AKT signaling in B cells, expressed at a higher level in MZ B cells than FO B cells in mice as well as humans.

The cellular biology of plasma cells: Unmet challenges and opportunities

Plasma cells and the antibodies they secrete are paramount for protection against infection but can also be implicated in diseases including autoantibody-mediated disease and multiple myeloma. Plasma cell terminal differentiation relies on a transcriptional switch and on important morphological changes. The cellular and molecular mechanisms underlying these processes are partly understood and how plasma cells manage to survive for long periods of time while secreting large quantities of antibodies remains unclear. In this review we aim to put in perspective what is known about plasma cell cellular biology to highlight the challenges faced by this field of research but also to illustrate how new opportunities may arise from the study of the fundamental mechanisms sustaining plasma cell survival and function.

Integrin β1 regulates marginal zone B cell differentiation and PI3K signaling

Marginal zone (MZ) B cells represent innate-like B cells that mediate a fast immune response. The adhesion of MZ B cells to the marginal sinus of the spleen is governed by integrins. Here, we address the question of whether β1-integrin has additional functions by analyzing Itgb1fl/flCD21Cre mice in which the β1-integrin gene is deleted in mature B cells. We find that integrin β1-deficient mice have a defect in the differentiation of MZ B cells and plasma cells. We show that integrin β1-deficient transitional B cells, representing the precursors of MZ B cells, have enhanced B cell receptor (BCR) signaling, altered PI3K and Ras/ERK pathways, and an enhanced interaction of integrin-linked kinase (ILK) with the adaptor protein Grb2. Moreover, the MZ B cell defect of integrin β1-deficient mice could, at least in part, be restored by a pharmacological inhibition of the PI3K pathway. Thus, β1-integrin has an unexpected function in the differentiation and function of MZ B cells.

Human anti-smallpox long-lived memory B cells are defined by dynamic interactions in the splenic niche and long-lasting germinal center imprinting

Memory B cells (MBCs) can persist for a lifetime, but the mechanisms that allow their long-term survival remain poorly understood. Here, we isolated and analyzed human splenic smallpox/vaccinia protein B5-specific MBCs in individuals who were vaccinated more than 40 years ago. Only a handful of clones persisted over such an extended period, and they displayed limited intra-clonal diversity with signs of extensive affinity-based selection. These long-lived MBCs appeared enriched in a CD21hiCD20hi IgG+ splenic B cell subset displaying a marginal-zone-like NOTCH/MYC-driven signature, but they did not harbor a unique longevity-associated transcriptional or metabolic profile. Finally, the telomeres of B5-specific, long-lived MBCs were longer than those in patient-paired naive B cells in all the samples analyzed. Overall, these results imply that separate mechanisms such as early telomere elongation, affinity selection during the contraction phase, and access to a specific niche contribute to ensuring the functional longevity of MBCs.

The metabolic plasticity of B cells

The humoral response requires rapid growth, biosynthetic capacity, proliferation and differentiation of B cells. These processes involve profound B-cell phenotypic transitions that are coupled to drastic changes in metabolism so as to meet the extremely different energetic requirements as B cells switch from resting to an activated, highly proliferative state and to plasma or memory cell fates. Thus, B cells execute a multi-step, energetically dynamic process of profound metabolic rewiring from low ATP production to transient and large increments of energy expenditure that depend on high uptake and consumption of glucose and fatty acids. Such metabolic plasticity is under tight transcriptional and post-transcriptional regulation. Alterations in B-cell metabolism driven by genetic mutations or by extrinsic insults impair B-cell functions and differentiation and may underlie the anomalous behavior of pathological B cells. Herein, we review molecular switches that control B-cell metabolism and fuel utilization, as well as the emerging awareness of the impact of dynamic metabolic adaptations of B cells throughout the different phases of the humoral response.

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

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

H3K27me3 Demethylase UTX Restrains Plasma Cell Formation

B cell differentiation is associated with substantial transcriptional, metabolic, and epigenetic remodeling, including redistribution of histone 3 lysine 27 trimethylation (H3K27me3), which is associated with a repressive chromatin state and gene silencing. Although the role of the methyltransferase EZH2 (Enhancer of zeste homolog 2) in B cell fate decisions has been well established, it is not known whether H3K27me3 demethylation is equally important. In this study, we showed that simultaneous genetic deletion of the two H3K27 demethylases UTX and JMJD3 (double-knockout [Utx ^fl/fl Jmjd3 ^fl/fl Cd19 ^cre/+] [dKO]) led to a significant increase in plasma cell (PC) formation after stimulation with the T cell-independent Ags LPS and NP-Ficoll. This phenotype occurred in a UTX-dependent manner as UTX single-knockout mice, but not JMJD3 single-knockout mice, mimicked the dKO. Although UTX- and JMJD3-deficient marginal zone B cells showed increased proliferation, dKO follicular B cells also showed increased PC formation. PCs from dKO mice upregulated genes associated with oxidative phosphorylation and exhibited increased spare respiratory capacity. Mechanistically, deletion of Utx and Jmjd3 resulted in higher levels of H3K27me3 at proapoptotic genes and resulted in reduced apoptosis of dKO PCs in vivo. Furthermore, UTX regulated chromatin accessibility at regions containing ETS and IFN regulatory factor (IRF) transcription factor family motifs, including motifs of known repressors of PC fate. Taken together, these data demonstrate that the H3K27me3 demethylases restrain B cell differentiation.