Promotes Plasma Cell Differentiation Research Articles

Endogenous complement-activating IgM is not required for primary antibody responses but promotes plasma cell differentiation and secondary antibody responses to a large particulate antigen in mice.

Lack of complement factor C1q of the classical pathway results in severely impaired primary antibody responses. This is a paradox because antibodies, especially IgM, are the most efficient activators of the classical pathway and very little specific IgM will be present at priming. A possible explanation would be that natural IgM, binding with low affinity to the antigen, may suffice to activate complement. In support of this, mice lacking secretory IgM have an impaired antibody response, which can be rescued by transfer of non-immune IgM. Moreover, passive administration of specific IgM together with antigen enhances the antibody response in a complement-dependent fashion. To test the idea, we have used a knock-in mouse strain (Cμ13) carrying a point mutation in the IgM heavy chain, rendering the IgM unable to activate complement. Mutant mice backcrossed to BALB/c or C57BL/6 background were primed and boosted with a low dose of sheep red blood cells. Confirming earlier data, no impairment in early, primary IgM- or IgG-responses were seen in either of the Cμ13 strains. However, in one of the mutant strains, late primary IgG responses were impaired. A more pronounced effect was observed after boost, when the IgG response, the number of germinal center B cells and antibody secreting cells as well as the opsonization of antigen were impaired in mutant mice. We conclude that complement activation by natural IgM cannot explain the role of C1q in primary antibody responses, but that endogenous, specific, wildtype IgM generated after immunization feedback-enhances the response to a booster dose of antigen. Importantly, this mechanism can only partially explain the role of complement in the generation of antibody responses because the IgG response was much lower in C3- or complement receptor 1 and 2-deficient mice than in Cμ13 mice.

The miR-17∼92 miRNAs promote plasma cell differentiation by suppressing SOCS3-mediated NIK degradation

The miR-17∼92 family microRNAs (miRNAs) play a key role in germinal center (GC) reaction through promoting T follicular helper (TFH) cell differentiation. It remains unclear whether they also have intrinsic functions in B cell differentiation and function. Here we show that mice with B cell-specific deletion of the miR-17∼92 family exhibit impaired GC reaction, plasma cell differentiation, and antibody production in response to protein antigen immunization and chronic viral infection. Employing CRISPR-mediated functional screening, we identify Socs3 as a key functional target of miR-17∼92 in regulating plasma cell differentiation. Mechanistically, SOCS3, whose expression is elevated in miR-17∼92 family-deficient B cells, interacts with NIK and promotes its ubiquitination and degradation, thereby impairing NF-κB signaling and plasma cell differentiation. This moderate increase in SOCS3 expression has little effect on IL-21-STAT3 signaling. Our study demonstrates differential sensitivity of two key signaling pathways to alterations in the protein level of an miRNA target gene.

Pathologic and molecular insights in nodal T-follicular helper cell lymphomas.

T-follicular helper (TFH) cells are one of the T-cell subsets with a critical role in the regulation of germinal center (GC) reactions. TFH cells contribute to the positive selection of GC B-cells and promote plasma cell differentiation and antibody production. TFH cells express a unique phenotype characterized by PD-1hi, ICOShi, CD40Lhi, CD95hi, CTLAhi, CCR7lo, and CXCR5hi . Three main subtypes of nodal TFH lymphomas have been described: 1) angioimmunoblastic-type, 2) follicular-type, and 3) not otherwise specified (NOS). The diagnosis of these neoplasms can be challenging, and it is rendered based on a combination of clinical, laboratory, histopathologic, immunophenotypic, and molecular findings. The markers most frequently used to identify a TFH immunophenotype in paraffin-embedded tissue sections include PD-1, CXCL13, CXCR5, ICOS, BCL6, and CD10. These neoplasms feature a characteristic and similar, but not identical, mutational landscape with mutations in epigenetic modifiers (TET2, DNMT3A, IDH2), RHOA, and T-cell receptor signaling genes. Here, we briefly review the biology of TFH cells and present a summary of the current pathologic, molecular, and genetic features of nodal lymphomas. We want to highlight the importance of performing a consistent panel of TFH immunostains and mutational studies in TCLs to identify TFH lymphomas.

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.

Epigenetic Remodeling by Vitamin C Potentiates the Differentiation of Mouse and Human Plasma Cells

Abstract Ascorbate (vitamin C) is an essential micronutrient in humans. The chronic severe 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. Here, from a micronutrient screen, 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 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. Consistent with its role as a cofactor for epigenetic enzymes, ascorbate potentiates plasma cell differentiation by remodeling the epigenome via TET (Ten Eleven Translocation), the enzymes responsible for DNA demethylation by oxidizing 5-methylcytosines into 5-hydroxymethylcytosine (5hmC). Genome-wide 5hmC profiling identified ascorbate responsive elements (EAR) at the Prdm1 locus, including a distal element with a STAT3 motif overlapped with a CpG that was methylated and modified by TET in the presence of ascorbate. The results suggest that an adequate level of VC is required for antibody response and highlight how micronutrients 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. This research was funded by LJI/KKR young investigator fund; NIH National Cancer Institute K22 (K22CA241290); startup funds from the Department of Microbial Infection and Immunity and from the Pelotonia Institute of Immuno-oncology at the Ohio State University.

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

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

CD8 follicular T cells provide helper and cytolytic function in the B cell follicule during antibody-mediated autoimmunity

Abstract CXCR5+ CD8 T cells arise during situations of chronic antigen and inflammation, including chronic viral infection, cancer, and antibody-mediated autoimmunity. In some conditions CXCR5 expression facilitates CD8 T cell homing to B cell follicles, while some studies find persistent CCR7 expression prevents follicle entry. Despite robust research within this fairly new field variance in tissue localization, expression profiles, inflammatory conditions and developmental stages limit comparisons between existing CXCR5+CD8 T cell studies. We demonstrate that CXCR5+PD-1+ CD8 T follicular (Tfc) cells comprise a functionally non-redundant subset of CXCR5+ CD8 T cells in systemic autoimmune disease. CD8 Tfc develop in the absence of functional Tregs and expand during lymphoproliferation. CD8 Tfc acquire the capacity to produce helper cytokines including co-expression of IL-21 and IL-4, and maintain capacity for cytolytic function expresing granzyme B and TNFα, suggesting the potential for functional mechanisms that diverge from CD4 Tfh cells. IL-4+IL-21+ CD8 Tfc localization in the follicle and germinal center suggest that CD8 Tfc may promote B cell activities in either location during autoimmune disease. B cell activities influenced by CD8 Tfc cells are not fully defined such as regulating somatic mutation, memory and plasma cell differentiation, and germinal center movement. However CD8 T cells promote plasma cell differentiation and antibody class switch, but do not alter germinal center B cell numbers. CD8 Tfc position coupled with cytokine production in the germinal center and follicle, and induction of activation-induced cytidine deaminase by B cells suggest a dominant helper-like role with cytolytic potential.

Ascorbic Acid Promotes Plasma Cell Differentiation through Enhancing TET2/3-Mediated DNA Demethylation

Plasma cells provide high-affinity antibodies against invading pathogens. Although transcriptional and epigenetic mechanisms have been extensively studied for plasma cell differentiation, how these mechanisms respond to environmental cues remains largely unexplored. In this study, we show that ascorbic acid (vitamin C), an essential nutrient, is able to promote plasma cell differentiation and humoral immune response by enhancing TET2/3-mediated DNA demethylation. Ascorbic acid treatment during B cell activation has persistent effects on later plasma cell differentiation by predisposing germinal center B cells toward the plasma cell lineage. Conversely, ascorbic acid deficiency invivo blocks plasma cell differentiation and attenuates the humoral immune response following antigen immunization. We further demonstrate that such effects of ascorbic acid on plasma cell differentiation require DNA methylcytosine oxidases TET2 and TET3. Our study thus reveals a previously uncharacterized link between essential nutrients and epigenetic regulation of plasma cell differentiation and humoral immune response.

BLIMP-1 is insufficient to induce antibody secretion in the absence of IRF4 in DT40 cells.

Differentiation of B cells into antibody-secreting cells (ASCs), plasmablasts and plasma cells is regulated by a network of transcription factors. Within this network, factors including PAX5 and BCL6 prevent ASC differentiation and maintain the B cell phenotype. In contrast, BLIMP-1 and high IRF4 expression promote plasma cell differentiation. BLIMP-1 is thought to induce immunoglobulin secretion, whereas IRF4 is needed for the survival of ASCs. The role of IRF4 in the regulation of antibody secretion has remained controversial. To study the role of IRF4 in the regulation of antibody secretion, we have created a double knockout (DKO) DT40 B cell line deficient in both IRF4 and BCL6. Although BCL6-deficient DT40 B cell line had upregulated BLIMP-1 expression and secreted antibodies, the DKO cell line did not. Even enforced BLIMP-1 expression in DKO cells or IRF4-deficient cells could not induce IgM secretion while in WT DT40 cells, it could. However, enforced IRF4 expression in DKO cells induced strong IgM secretion. Our findings support a model where IRF4 expression in addition to BLIMP-1 expression is required to induce robust antibody secretion.

MTORC1 in B cells regulates antibody responses and promotes mitochondrial and metabolic fitness

Abstract B lymphocytes migrate among different micro-anatomic sites for diversification, selection, and eventual differentiation into antibody-secreting plasma cells. Emerging evidence supports the premise that aspects of the nutrient milieu vary within lymphoid micro-environments. However, the role of B cell-intrinsic metabolic programs in regulating B cell differentiation and antibody response quality remain unclear. We now show that the amino acid-sensing mTOR complex 1 (mTORC1) is essential for induction of Bcl6 and IRF4, key transcriptional regulators of germinal center and plasma cell fates. mTORC1 also enhances B cell proliferation upon exposure to antigen, increases the rate of somatic hyper-mutation, and is essential for generating high-affinity class switched antibodies. We also find that AMP-activated kinase (AMPK), an intracellular energy sensor, promotes plasma cell differentiation and antibody production. Collectively, these findings suggest mechanisms by which mTORC1 activity is critical for the germinal center reaction and producing class-switched, high-affinity antibodies, and indicate that AMPK can regulate the development and functional properties of terminally differentiated plasma cells. Supported by NIH R01 AI113292, HL106812, and NCI T32CA009592-29

Hedgehog and retinoid signaling alters multiple myeloma microenvironment and generates bortezomib resistance.

Interactions between multiple myeloma (MM) cells and the BM microenvironment play a critical role in bortezomib (BTZ) resistance. However, the mechanisms involved in these interactions are not completely understood. We previously showed that expression of CYP26 in BM stromal cells maintains a retinoic acid-low (RA-low) microenvironment that prevents the differentiation of normal and malignant hematopoietic cells. Since a low secretory B cell phenotype is associated with BTZ resistance in MM and retinoid signaling promotes plasma cell differentiation and Ig production, we investigated whether stromal expression of the cytochrome P450 monooxygenase CYP26 modulates BTZ sensitivity in the BM niche. CYP26-mediated inactivation of RA within the BM microenvironment prevented plasma cell differentiation and promoted a B cell-like, BTZ-resistant phenotype in human MM cells that were cocultured on BM stroma. Moreover, paracrine Hedgehog secretion by MM cells upregulated stromal CYP26 and further reinforced a protective microenvironment. These results suggest that crosstalk between Hedgehog and retinoid signaling modulates BTZ sensitivity in the BM niche. Targeting these pathological interactions holds promise for eliminating minimal residual disease in MM.

Bone Marrow Niche - Multiple Myeloma Cross-Talk Generates Bortezomib Resistance

Bortezomib (BTZ), a proteasome inhibitor that induces accumulation of misfolded proteins and apoptosis in highly secretory cells, has improved survival in multiple myeloma (MM) but fails to achieve cure. It has been postulated that a population of MM cells, phenotypically similar to B-cells, are intrinsically resistant to BTZ, and contribute to minimal residual disease (MRD) and relapse (Matsui et al., 2008).Bone marrow (BM) stroma expresses CYP26 enzymes and creates a retinoic acid (RA)-low environment that prevents differentiation of normal and malignant cells (Meng et al., 2015). Since RA promotes plasma cell differentiation, Ig secretion (Ertesvag et al., 2007) and thus, ER-stress (Xu et al., 2007), we tested if the BM niche induces BTZ resistance by promoting a B-cell program in MM. Consistent with this, MM cells (H929 and MM1S) cultured under low RA conditions (co-cultured with BM stroma or treated with AGN194310, a pan-RA receptor inhibitor) up regulated B-cell markers (BCL6), and down regulated genes associated with plasma cell differentiation and BTZ sensitivity (XBP1s, Blimp, CHOP) (Fig 1A). In addition, inhibition of stromal CYP26 via R115866 reversed these gene expression changes in stroma co-culture conditions (Fig 1A).To verify whether a low-RA environment promotes BTZ resistance, we incubated MM cells with BM mesenchymal cells for 5 days. Then, MM cells were separated from stroma, and treated with BTZ. Incubation with BM stroma induced BTZ resistance of MM cells, which was completely overcome by CYP26 inhibition or by the CYP26 resistant retinoid IRX5183 (Fig 1B). Moreover, MM cells pretreated with AGN for 5 days were also resistant to BTZ (Fig 1B). Interestingly, this BTZ resistant phenotype was preserved for up to 48h upon removal of MM cells from stromal co-culture (Fig 1C).Recent studies suggest that malignant cells remodel their microenvironments to build a more protective niche (Schepers et al., 2013). Consistent with this, we found that stromal CYP26A1 was highly upregulated after co-culture with MM cells. Supernatant from MM cells had similar effects, implicating a soluble factor on this interaction. Since secretion of SHH was implicated in chemotherapy resistance in MM (Liu et al., 2014), we tested if MM-induced CYP26 up-regulation was dependent on SHH signaling. As hypothesized, treatment with the Smo antagonist cyclopamine, or the use of SMO knockout (KO) stromal cells, overcame CYP26A1 up regulation by MM cells (Fig 1D)To test if cell extrinsic SHH signaling in stromal niche contributes to BTZ resistance by modulating RA pathway in MM cells, we modeled MM-BM stroma interactions in a xenograft setting. Each mouse carried two subcutaneous tumors consisting of Luciferase expressing MM1S cells and SmoFl/Fl BM stroma cells, transduced with either a control vector (WT stroma) or Cre-recombinase (Smo KO stroma) (Fig E). Mice were treated with IRX5183, BTZ, or the combination. Consistent with our in vitro data, tumors with Smo KO stroma showed a significant response to BTZ, while those with WT stroma were refractory, as determined by exponential increase in bioluminescence. However, the combination of IRX+BTZ resulted in a dramatic and equivalent response regardless of the phenotype of the stromal compartment (Fig 2A, 2B). Moreover, tumors with Smo KO stroma, or treated with IRX, showed decrease expression of B-cell markers, and up-regulation of genes associated with plasma cell differentiation and BTZ sensitivity.In conclusion, we show that the BM niche promotes a B-cell program in MM, characterized by a low ER-stress and BTZ resistance. This niche-induced program is maintained even after displacement from the BM, which may have important implications for the use of mobilization-sensitization strategies. Finally, we propose the existence of a bi-directional crosstalk between MM cells and their respective niches. In our model, MM cells via secretion of SHH modify BM stroma to create RA-low environments, induce BTZ resistance and contribute to MRD. [Display omitted] [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Mechanisms of autoantibody production in systemic lupus erythematosus.

Autoantibodies against a panoply of self-antigens are seen in systemic lupus erythematosus, but only a few (anti-Sm/RNP, anti-Ro/La, anti-dsDNA) are common. The common lupus autoantigens are nucleic acid complexes and levels of autoantibodies can be extraordinarily high. We explore why that is the case. Lupus is associated with impaired central or peripheral B-cell tolerance and increased circulating autoreactive B cells. However, terminal differentiation is necessary for autoantibody production. Nucleic acid components of the major lupus autoantigens are immunostimulatory ligands for toll-like receptor (TLR)7 or TLR9 that promote plasma cell differentiation. We show that the levels of autoantibodies against the U1A protein (part of a ribonucleoprotein) are markedly higher than autoantibodies against other antigens, including dsDNA and the non-nucleic acid-associated autoantigens insulin and thyroglobulin. In addition to driving autoantibody production, TLR7 engagement is likely to contribute to the pathogenesis of inflammatory disease in lupus.

MiR-148a promotes plasma cell differentiation and targets the germinal center transcription factors Mitf and Bach2.

Bcells undergo affinity maturation and class switch recombination of their immunoglobulin receptors during a germinal center (GC) reaction, before they differentiate into long-lived antibody-secreting plasma cells (PCs). Transcription factors such as Bach2 and Mitf are essential during this process, as they delay premature differentiation of GC Bcells by repressing Blimp-1 and IRF4, two transcription factors required for terminal PC differentiation. Therefore, Bach2 and Mitf expression must be attenuated in activated Bcells to allow terminal PC differentiation, but the precise mechanism remains enigmatic. Here, we provide evidence that miR-148a, a small noncoding microRNA, fosters PC differentiation and survival. Next-generation sequencing revealed that miR-148a is the most abundant microRNA in primary human and murine PCs, and its expression is upregulated in activated murine Bcells and coincides with Blimp-1 synthesis. miR-148a targets Bach2, Mitf and proapoptotic factors such as PTEN and Bim. When prematurely expressed, miR-148a promotes the differentiation and survival of plasmablasts and reduces frequencies of IgG1(+) cells in primary B-cell cultures. In summary, we propose that miR-148a is a new player in the regulatory network controlling terminal PC differentiation and could, therefore, be a therapeutic target for interfering with PC differentiation and survival.

MTOR activation promotes plasma cell differentiation and bypasses XBP-1 for immunoglobulin secretion.

Plasma cells (PCs) are responsible for the secretion of antibodies. The development of fully functional PCs relies on the activation of the inositol-requiring enzyme 1/X-box binding protein 1 (IRE1/XBP-1) arm of the unfolded protein response (UPR). XBP-1-deficient PCs secrete antibodies poorly and exhibit distensions of the endoplasmic reticulum (ER). The kinase mammalian target of rapamycin (mTOR) promotes anabolic activities and is negatively regulated by the tuberous sclerosis complex (TSC). Deletion of TSC1 renders mTOR hyperactive. To explore the relationship between mTOR and the UPR in PC development and function, mice with conditional deletions of XBP-1 and/or TSC1 in their B cell lineage were generated. Deletion of TSC1 enhanced Ig synthesis and promoted differentiation into PCs independently of XBP-1, as evidenced by comparison of TSC1/XBP-1 double-knockout (DKO) PCs to XBP-1 knockout (KO) PCs. The typical morphological abnormalities of the ER in XBP-1 KO PCs were alleviated in the DKO PCs. Expression profiling identified the glycoprotein Ly6C as an mTOR target. Ly6C expression contributed to the enhanced Ig secretion from DKO PCs. Our data reveal a functional overlap between mTOR and the UPR in promoting PC development. In addition to the classical mTOR role in promoting protein synthesis, the mechanism entails transcription regulation of accessory molecules, such as Ly6C.

Adaptor protein DOK3 promotes plasma cell differentiation by regulating the expression of programmed cell death 1 ligands.

The adaptor Downstream-of-Kinase (DOK) 3 functions as a negative regulator and attenuates B-cell receptor-mediated calcium signaling. Although DOK3 is dispensable for early B-cell development, its role in plasma cell (PC) differentiation is unknown. Here, we show that Dok3(-/-) mice have increased populations of T follicular-helper (Tfh) and germinal center (GC) B cells upon immunization with a T-cell-dependent antigen. However, interestingly, they generate significantly fewer PCs. Bone marrow reconstitution experiments show that the PC defect is B-cell intrinsic and due to the inability of Dok3(-/-) B cells to sustain programmed cell death 1 (PD-1) ligand 1 (PDL1) and up-regulate PD-1 ligand 2 (PDL2) expressions that are critical for PC differentiation. Overexpression of PDL2 rectifies the PC differentiation defect in Dok3(-/-) B cells. We further demonstrate that calcium signaling suppresses the transcription of PD-1 ligands. Abrogation of calcium signaling in B cells by deleting BTK or PLCγ2 or inhibiting calcineurin with cyclosporine A leads to increased expression of PD-1 ligands. Thus, our study reveals DOK3 as a nonredundant regulator of PC differentiation by up-regulating PD-1 ligand expression through the attenuation of calcium signaling.

Distinct roles for TLR7 and TLR9 in regulation of B cell signal strength and differentiation (LYM6P.766)

Abstract BCR signal strength plays a key role in the fate decision of B cells. Strong BCR signals promote plasma cell differentiation and weaker BCR signals drive germinal center B cell fate. Signals through toll-like receptors (TLRs) also influence B cell differentiation, but it is unclear how the BCR and TLR signaling cascades interface to modulate B cell responses and differentiation. To address this question, we used a BCR sdTg mouse model, AM14, in which all mature B cells have a naïve follicular phenotype. The AM14 BCR recognizes IgG2a-bound immune complexes (ICs) and AM14 B cell activation depends on TLR7 and/or TLR9. ICs formed by the anti-nucleosome antibody PL2-3 can activate AM14 B cells through either TLR7 or TLR9, allowing us to dissect signaling pathways unique to each TLR. We find that activation of AM14 TLR7-deficient B cells leads to sustained activation of the NF-κB pathway and p38 phosphorylation, subsequently leading to cell death. In contrast, TLR9-deficient AM14 B cells activated with ICs are unable to sustain the initial NF-κB and p38 signaling response. Intriguingly, these TLR9-deficient AM14 B cells differentiate into short-lived plasmablasts as characterized by increased levels of plasma cell associated proteins such as IRF-4, Blimp-1 and CD138. Plasma cell development is not observed in IC activated TLR7-deficient AM14 B cells. Our results suggest divergent roles for TLR7 and TLR9 in B cell activation and plasma cell differentiation.

The BTB-ZF transcription factor Zbtb20 is driven by Irf4 to promote plasma cell differentiation and longevity

The transcriptional network regulating antibody-secreting cell (ASC) differentiation has been extensively studied, but our current understanding is limited. The mechanisms of action of known "master" regulators are still unclear, while the participation of new factors is being revealed. Here, we identify Zbtb20, a Bcl6 homologue, as a novel regulator of late B cell development. Within the B cell lineage, Zbtb20 is specifically expressed in B1 and germinal center B cells and peaks in long-lived bone marrow (BM) ASCs. Unlike Bcl6, an inhibitor of ASC differentiation, ectopic Zbtb20 expression in primary B cells facilitates terminal B cell differentiation to ASCs. In plasma cell lines, Zbtb20 induces cell survival and blocks cell cycle progression. Immunized Zbtb20-deficient mice exhibit curtailed humoral responses and accelerated loss of antigen-specific plasma cells, specifically from the BM pool. Strikingly, Zbtb20 induction does not require Blimp1 but depends directly on Irf4, acting at a newly identified Zbtb20 promoter in ASCs. These results identify Zbtb20 as an important player in late B cell differentiation and provide new insights into this complex process.

Update in juvenile myositis.

Purpose of reviewThis update on childhood idiopathic inflammatory myopathies (IIMs) reviews recent progress in the field of translational science and clinical research over the past 12–18 months.Recent findingsSeveral new studies, including results from the international genome-wide association study, point to abnormalities of the adaptive immune system in childhood IIMs. Circulating T-follicular helper cells promote plasma cell differentiation and have been found in high levels in juvenile dermatomyositis (JDM), which may account the frequency of autoantibodies seen in this disease. One of the latest to be identified in JDM targets the protein NXP-2 and is associated with an increased risk of calcinosis in young patients. The first randomized controlled clinical trial in refractory adult and childhood IIMs was reported this year. B-cell depletion with the anti-CD20 antibody, rituximab, failed to achieve its primary end point, but patients with JDM did show good improvement in disease activity. A new international definition of disease remission in JDM has been agreed, which will aid disease assessment in future therapeutic trials.SummaryThe challenges of studying a rare disease such as JDM have been overcome by several collaborative studies and have led to significant progress in understanding the cause, treatment and prognosis of childhood IIMs.

IL-21 and CD40L Synergistically Promote Plasma Cell Differentiation through Upregulation of Blimp-1 in Human B Cells

After undergoing Ig somatic hypermutation and Ag selection, germinal center (GC) B cells terminally differentiate into either memory or plasma cells (PCs). It is known that the CD40L and IL-21/STAT3 signaling pathways play critical roles in this process, yet it is unclear how the B cell transcription program interprets and integrates these two types of T cell-derived signals. In this study, we characterized the role of STAT3 in the GC-associated PC differentiation using purified human tonsillar GC B cells and a GC B cell-like cell line. When primary GC B cells were cultured under PC differentiation condition, STAT3 inhibition by AG490 prevented the transition from GC centrocytes to preplasmablast, suggesting that STAT3 is required for the initiation of PC development. In a GC B cell-like human B cell line, although IL-21 alone can induce low-level Blimp-1 expression, maximum Blimp-1 upregulation and optimal PC differentiation required both IL-21 and CD40L. CD40L, although having no effect on Blimp-1 as a single agent, greatly augmented the amplitude and duration of IL-21-triggered Jak-STAT3 signaling. In the human PRDM1 locus, CD40L treatment enhanced the ability of STAT3 to upregulate Blimp-1 by removing BCL6, a potent inhibitor of Blimp-1 expression, from a shared BCL6/STAT3 site in intron 3. Thus, IL-21 and CD40L collaborate through at least two distinct mechanisms to synergistically promote Blimp-1 activation and PC differentiation.