Transcription factors that regulate memory in humoral responses

Dysregulated translational factors and epigenetic regulations orchestrate in B cells contributing to autoimmune diseases

B cells play a crucial role in antigen presentation, antibody production and pro-/anti-inflammatory cytokine secretion in adaptive immunity. Several translational factors including transcription factors and cytokines participate in the regulation of B cell development, with the cooperation of epigenetic regulations. Autoimmune diseases are generally characterized with autoreactive B cells and high-level pathogenic autoantibodies. The success of B cell depletion therapy in mouse model and clinical trials has proven the role of B cells in pathogenesis of autoimmune diseases. The failure of B cell tolerance in immune checkpoints results in accumulated autoreactive naïve B (B_N) cells with aberrant B cell receptor signaling and dysregulated B cell response, contributing to self-antibody-mediated autoimmune reaction. Dysregulation of translational factors and epigenetic alterations in B cells has been demonstrated to correlate with aberrant B cell compartment in autoimmune diseases, such as systemic lupus erythematosus, rheumatoid arthritis, primary Sjögren's syndrome, multiple sclerosis, diabetes mellitus and pemphigus. This review is intended to summarize the interaction of translational factors and epigenetic regulations that are involved with development and differentiation of B cells, and the mechanism of dysregulation in the pathogenesis of autoimmune diseases.

Defective TLR9-driven STAT3 activation in B cells of patients with CVID

B cell activation by Toll-like receptor 9 (TLR9) ligands is dependent on STAT3 and is important for optimal antibody responses to microbial antigens. B cells from patients with common variable immune deficiency (CVID) have impaired proliferation and differentiation in response to the TLR9 ligand CpG, despite normal levels of TLR9 expression. We demonstrate that CpG-driven STAT3 phosphorylation, but not activation of NFκB and p38, is selectively impaired in B cells from CVID patients. These results suggest that defective STAT3 activation contributes to the defective TLR9 and antibody response of B cells in CVID.

Blimp-1 impairs T cell function via upregulation of TIGIT and PD-1 in patients with acute myeloid leukemia

Background

T cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif (ITIM) domain (TIGIT) and programmed cell death protein 1 (PD-1) are important inhibitory receptors that associate with T cell exhaustion in acute myeloid leukemia (AML). In this study, we aimed to determine the underlying transcriptional mechanisms regulating these inhibitory pathways. Specifically, we investigated the role of transcription factor B lymphocyte-induced maturation protein 1 (Blimp-1) in T cell response and transcriptional regulation of TIGIT and PD-1 in AML.

Methods

Peripheral blood samples collected from patients with AML were used in this study. Blimp-1 expression was examined by flow cytometry. The correlation of Blimp-1 expression to clinical characteristics of AML patients was analyzed. Phenotypic and functional studies of Blimp-1-expressing T cells were performed using flow cytometry-based assays. Luciferase reporter assays and ChIP assays were applied to assess direct binding and transcription activity of Blimp-1. Using siRNA to silence Blimp-1, we further elucidated the regulatory role of Blimp-1 in the TIGIT and PD-1 expression and T cell immune response.

Results

Blimp-1 expression is elevated in T cells from AML patients. Consistent with exhaustion, Blimp-1⁺ T cells upregulate multiple inhibitory receptors including PD-1 and TIGIT. In addition, they are functionally impaired manifested by low cytokine production and decreased cytotoxicity capacity. Importantly, the functional defect is reversed by inhibition of Blimp-1 via siRNA knockdown. Furthermore, Blimp-1 binds to the promoters of PD-1 and TIGIT and positively regulates their expression.

Conclusions

Our study demonstrates an important inhibitory effect of Blimp-1 on T cell response in AML; thus, targeting Blimp-1 and its regulated molecules to improve the immune response may provide effective leukemia therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-017-0486-z) contains supplementary material, which is available to authorized users.

T-bet+ B cells are induced by human viral infections and dominate the HIV gp140 response

Humoral immunity is critical for viral control, but the identity and mechanisms regulating human antiviral B cells are unclear. Here, we characterized human B cells expressing T-bet and analyzed their dynamics during viral infections. T-bet+ B cells demonstrated an activated phenotype, a distinct transcriptional profile, and were enriched for expression of the antiviral immunoglobulin isotypes IgG1 and IgG3. T-bet+ B cells expanded following yellow fever virus and vaccinia virus vaccinations and also during early acute HIV infection. Viremic HIV-infected individuals maintained a large T-bet+ B cell population during chronic infection that was associated with increased serum and cell-associated IgG1 and IgG3 expression. The HIV gp140-specific B cell response was dominated by T-bet-expressing memory B cells, and we observed a concomitant biasing of gp140-specific serum immunoglobulin to the IgG1 isotype. These findings suggest that T-bet induction promotes antiviral immunoglobulin isotype switching and development of a distinct T-bet+ B cell subset that is maintained by viremia and coordinates the HIV Env-specific humoral response.

TLR2, TLR4 and the MyD88 signaling are crucial for the in vivo generation and the longevity of long-lived antibody-secreting cells

This study was undertaken to gain better insights into the role of TLRs and MyD88 in the development and differentiation of memory B cells, especially of ASC, during the Th2 polarized memory response induced by Natterins. Our in vivo findings demonstrated that the anaphylactic IgG1 production is dependent on TLR2 and MyD88 signaling, and that TLR4 acts as adjuvant accelerating the synthesis of high affinity-IgE. Also, TLR4 (MyD88-independent) modulated the migration of innate-like B cells (B1a and B2) out of the peritoneal cavity, and the emigration from the spleen of B1b and B2 cells. TLR4 (MyD88-independent) modulated the emigration from the spleen of Bmem as well as ASC B220^pos. TLR2 triggered to the egress from the peritoneum of Bmem (MyD88-dependent) and ASC B220^pos (MyD88-independent). We showed that TLR4 regulates the degree of expansion of Bmem in the peritoneum (MyD88-dependent) and in BM (MyD88-independent) as well as of ASC B220^neg in the spleen (MyD88-independent). TLR2 regulated the intensity of the expansion of Bmem (MyD88-independent) and ASC B220^pos (MyD88-dependent) in BM. Finally, TLR4 signals sustained the longevity of ASC B220^pos (MyD88-independent) and ASC B220^neg into the peritoneum (MyD88-dependent) and TLR2 MyD88-dependent signaling supported the persistence of B2 cells in BM, Bmem in the spleen and ASC B220^neg in peritoneum and BM. Terminally differentiated ASC B220^neg required the cooperation of both signals through TLR2/TLR4 via MyD88 for longevity in peritoneum, whereas Bmem required only TLR2/MyD88 to stay in spleen, and ASC B220^pos rested in peritoneum dependent on TLR4 signaling. Our data sustain that earlier events on memory B cells differentiation induced in secondary immune response against Natterins, after secondary lymph organs influx and egress, may be the key to determining peripheral localization of innate-like B cells and memory B cells as ASC B220^pos and ASC B220^neg.

The proto-oncogene c-myc regulates antibody secretion and Ig class switch recombination

The immune response involves the generation of Ab-secreting cells and memory B cells through a process called terminal B lymphocyte differentiation. This program requires the transcriptional repressor Blimp-1, which inhibits c-myc expression and terminates proliferation. Although the role of c-Myc in cell proliferation is well characterized, it is not known whether it has other functions in terminal differentiation. In this study, we show that c-Myc not only regulates cell proliferation, but it is also essential for Ab-secreting cell function and differentiation in vivo. c-Myc-deficient B lymphocytes hypersecrete IgM and do not undergo Ig class switch recombination (CSR). CSR has been previously linked to proliferation, and in this study we mechanistically link class switching and proliferation via c-Myc. We observed that c-Myc regulates CSR by transcriptionally activating the B cell-specific factor activation-induced cytidine deaminase. By linking cell proliferation and CSR, c-Myc is thus a critical component for a potent immune response.

Long-term maintenance of polysaccharide-specific antibodies by IgM-secreting cells

Many bacteria-associated polysaccharides induce long-lived Ab responses that protect against pathogenic microorganisms. The maintenance of polysaccharide-specific Ab titers may be due to long-lived plasma cells or ongoing Ag-driven B cell activation due to polysaccharide persistence. BALB/c and V(H)J558.3 transgenic mice respond to α1→3-dextran (DEX) by generating a peak anti-DEX response at 7 d, followed by maintenance of serum Ab levels for up to 150 d. Analysis of the cellular response to DEX identified a population of short-lived, cyclophosphamide-sensitive DEX-specific plasmablasts in the spleen, and a quiescent, cyclophosphamide-resistant DEX-specific Ab-secreting population in the bone marrow. BrdU pulse-chase experiments demonstrated the longevity of the DEX-specific Ab-secreting population in the bone marrow. Splenic DEX-specific plasmablasts were located in the red pulp with persisting DEX-associated CD11c(+) dendritic cells 90 d after immunization, whereas DEX was not detected in the bone marrow after 28 d. Selective depletion of short-lived DEX-specific plasmablasts and memory B1b B cells using cyclophosphamide and anti-CD20 treatment had a minimal impact on the maintenance of serum anti-DEX Abs. Collectively, these findings demonstrate that the maintenance of serum polysaccharide-specific Abs is the result of continuous Ag-driven formation of short-lived plasmablasts in the spleen and a quiescent population of Ab-secreting cells maintained in the bone marrow for a long duration.

Protective antiviral antibody responses in a mouse model of influenza virus infection require TACI

Antiviral Abs, for example those produced in response to influenza virus infection, are critical for virus neutralization and defense against secondary infection. While the half-life of Abs is short, Ab titers can last a lifetime due to a subset of the Ab-secreting cells (ASCs) that is long lived. However, the mechanisms governing ASC longevity are poorly understood. Here, we have identified a critical role for extrinsic cytokine signals in the survival of respiratory tract ASCs in a mouse model of influenza infection. Irradiation of mice at various time points after influenza virus infection markedly diminished numbers of lung ASCs, suggesting that they are short-lived and require extrinsic factors in order to persist. Neutralization of the TNF superfamily cytokines B lymphocyte stimulator (BLyS; also known as BAFF) and a proliferation-inducing ligand (APRIL) reduced numbers of antiviral ASCs in the lungs and bone marrow, whereas ASCs in the spleen and lung-draining lymph node were surprisingly unaffected. Mice deficient in transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), a receptor for BLyS and APRIL, mounted an initial antiviral B cell response similar to that generated in WT mice but failed to sustain protective Ab titers in the airways and serum, leading to increased susceptibility to secondary viral challenge. These studies highlight the importance of TACI signaling for the maintenance of ASCs and protection against influenza virus infection.

Inhibition of terminal differentiation of B cells mediated by CD27 and CD40 involves signaling through JNK

B cells responding to cognate Ag in vivo undergo clonal expansion that is followed by differentiation into Ab-secreting plasma cells or into quiescent restimulable memory. Both these events occur in the germinal center and require that cells exit from proliferation, but the signals that lead to one or the other of these mutually exclusive differentiation pathways have not been definitively characterized. Previous experiments have shown that signals transduced through the TNFRs CD27 and CD40 at the time of B cell stimulation in vitro or in vivo can influence this cell fate decision by inhibiting terminal differentiation and promoting memory. In this study, we show that the PIQED domain of the cytoplasmic tail of murine CD27 and the adapter molecule TNFR-associated factor 2 are involved in this effect. Using pharmacological inhibitors of signaling intermediates, we identify JNK as being necessary and sufficient for the observed inhibition of terminal differentiation. While JNK is involved downstream of CD40, inhibition of the MEK pathway can also partially restore plasma cell generation, indicating that both signaling intermediates may be involved. We also show that inhibition of induction of IFN regulatory factor 4 and B lymphocyte induced maturation protein 1 are downstream events common to both receptors.

Effector and memory CD8+ T cell differentiation: toward a molecular understanding of fate determination

CD8(+) T cells play a key role in protecting the body against invading microorganisms. Their capacity to control infection relies on the development of peripheral effector and memory T cells. Much of our current knowledge has been gained by tracking alterations of the phenotype of CD8(+) T cells but the molecular understanding of the events that underpin the emergence of heterogeneous effector and memory CD8(+) T cells in response to infection has remained limited. This review focuses on the recent progress in our understanding of the molecular wiring of this differentiation process.

Diversity in T cell memory: an embarrassment of riches

The adaptive immune response meets the needs of the organism to generate effector cells capable of controlling pathogens but also leads to production of memory cells, which mediate more effective protection during rechallenge. In this review, we focus on the generation, maintenance, and function of memory T cells, with a special emphasis on the increasing evidence for great diversity among functional memory T cell subsets.

A role for the transcriptional repressor Blimp-1 in CD8(+) T cell exhaustion during chronic viral infection

T cell exhaustion is common during chronic infections and can prevent optimal immunity. Although recent studies have demonstrated the importance of inhibitory receptors and other pathways in T cell exhaustion, the underlying transcriptional mechanisms are unknown. Here, we define a role for the transcription factor Blimp-1 in CD8(+) T cell exhaustion during chronic viral infection. Blimp-1 repressed key aspects of normal memory CD8(+) T cell differentiation and promoted high expression of inhibitory receptors during chronic infection. These cardinal features of CD8(+) T cell exhaustion were corrected by conditionally deleting Blimp-1. Although high expression of Blimp-1 fostered aspects of CD8(+) T cell exhaustion, haploinsufficiency indicated that moderate Blimp-1 expression sustained some effector function during chronic viral infection. Thus, we identify Blimp-1 as a transcriptional regulator of CD8(+) T cell exhaustion during chronic viral infection and propose that Blimp-1 acts as a transcriptional rheostat balancing effector function and T cell exhaustion.

B cell receptor revision diminishes the autoreactive B cell response after antigen activation in mice

Autoreactive B cells are regulated in the BM during development through mechanisms, including editing of the B cell receptor (BCR), clonal deletion, and anergy. Peripheral B cell tolerance is also important for protection from autoimmune damage, although the mechanisms are less well defined. Here we demonstrated, using a mouse model of SLE-like serology, that during an autoimmune response, RAG was reinduced in antigen-activated early memory or preplasma B cells. Expression of RAG was specific to antigen-reactive B cells, required the function of the IL-7 receptor (IL-7R), and contributed to maintenance of humoral tolerance. We also showed that soluble antigen could diminish a non-autoreactive antibody response through induction of BCR revision. These data suggest that tolerance induction operates in B cells at a postactivation checkpoint and that BCR revision helps regulate autoreactivity generated during an ongoing immune response.

Mesenchymal stromal cell-derived CCL2 suppresses plasma cell immunoglobulin production via STAT3 inactivation and PAX5 induction

We demonstrate that the secretome of mesenchymal stromal cells (MSCs) suppresses plasma cell (PC) immunoglobulin (Ig) production, induces plasmablast proliferation, and leads to interleukin-10-mediated blockade in vitro. We found that these effects are the result of MSC-derived CC chemokine ligands CCL2 and CCL7. More specifically, MSCs further processed these CC chemokines by the activity of matrix metalloproteinases (MMPs), leading to the generation of proteolytically processed antagonistic CCL2 variant. Neutralizing CCL2 or inhibiting MMP enzymatic activity abolished the PC-suppressive effect of MSCs. We also observed that MMP-processed CCL2 suppresses signal transducer and activator of transcription 3 (STAT3) activation in PC. As a result, the transcription factor PAX5 is induced, thus explaining the inhibition of Ig synthesis. The absence of inhibitory effects by MSC on the humoral response of CCR2(-/-) mice to xenoantigen suggests that MMP-cleaved CCL2/CCR2 interaction as well as downstream phosphatase activity is necessary for antagonistic effect. We tested syngeneic MSCs in hemophilic B6 mice with predeveloped antihuman factor VIII (hFVIII) antibodies and demonstrated a robust decrease in hFVIII-specific IgG levels. Thus, MSCs may play a role in modulating Ig production by PCs via MMP processing of CCL2 and may represent an appealing cell therapy approach for pathologic humoral responses.

Regulation and functions of Blimp-1 in T and B lymphocytes

B lymphocyte-induced maturation protein-1 (Blimp-1), discovered 16 years ago as a transcriptional repressor of the IFNbeta promoter, plays fundamentally important roles in many cell lineages and in early development. This review focuses on Blimp-1 in lymphocytes. In the B cell lineage, Blimp-1 is required for development of immunoglobulin-secreting cells and for maintenance of long-lived plasma cells (LLPCs). Direct targets of Blimp-1 and the transcriptional cascades Blimp-1 initiates to trigger plasmacytic differentiation are described. Blimp-1 also affects the homeostasis and function of CD4(+), CD8(+), and regulatory CD4(+) T cells, and Blimp-1 levels are highest in antigen-experienced T cells. Blimp-1 attenuates T cell proliferation and survival and modulates differentiation. Roles for Blimp-1 in Th1/Th2 specification, regulatory T cell function, and CD8 differentiation and function are under investigation. Signals that induce Blimp-1 in B cells include Toll-like receptor ligands and cytokines; in T cells, T cell receptors and cytokines induce Blimp-1. In spite of some commonalities, different targets and regulators of Blimp-1 in B and T cells suggest intriguing evolutionary divergence of this regulatory machinery.

Oct2 enhances antibody-secreting cell differentiation through regulation of IL-5 receptor alpha chain expression on activated B cells

Mice lacking a functional gene for the Oct2 transcriptional activator display several developmental and functional deficiencies in the B lymphocyte lineage. These include defective B cell receptor (BCR) and Toll-like receptor 4 signaling, an absence of B-1 and marginal zone populations, and globally reduced levels of serum immunoglobulin (Ig) in naive and immunized animals. Oct2 was originally identified through its ability to bind to regulatory regions in the Ig loci, but genetic evidence has not supported an essential role for Oct2 in the expression of Ig genes. We describe a new Oct2-mediated role in B cells. Oct2 augments the ability of activated B cells to differentiate to antibody-secreting plasma cells (ASCs) under T cell-dependent conditions through direct regulation of the gene encoding the alpha chain of the interleukin (IL) 5 receptor. Ectopic expression of IL-5Ralpha in oct2-deficient B cells largely restores their ability to differentiate to functional ASCs in vitro but does not correct other phenotypic defects in the mutants, such as the maturation and specialization of peripheral B cells, which must therefore rely on distinct Oct2 target genes. IL-5 augments ASC differentiation in vitro, and we show that IL-5 directly activates the plasma cell differentiation program by enhancing blimp1 expression.

Ets-1 regulates plasma cell differentiation by interfering with the activity of the transcription factor Blimp-1

Development of immunoglobulin-secreting plasma cells from B cells is a tightly regulated process controlled by the action of a number of transcription factors. In particular, the transcription factor Blimp-1 is a key positive regulator of plasmacytic differentiation via its ability to suppress expression of genes involved in the mature B cell program. The transcription factor Ets-1 is a negative regulator of plasmacytic differentiation, as indicated by the development of increased numbers of IgM-secreting plasma cells in Ets-1 knock-out mice. We have previously shown that Ets-1-deficient B cells undergo enhanced differentiation into IgM-secreting plasma cells in response to Toll-like receptor 9 (TLR9) signaling. We now explore the mechanism by which Ets-1 limits differentiation downstream of TLR9. Our results indicate that Ets-1 physically interacts with Blimp-1, which leads to a block in Blimp-1 DNA binding activity and a reduction in the ability of Blimp-1 to repress target genes without interfering with Blimp-1 protein levels. In addition, we show that Ets-1 induces the expression of several target genes that are repressed by Blimp-1, including Pax-5. These results reveal a previously unknown mechanism for the control of Blimp-1 activity by Ets-1 and suggest that expression of Ets-1 must be down-regulated before plasmacytic differentiation can occur.

Molecular signature of CD8+ T cell exhaustion during chronic viral infection

Chronic viral infections often result in T cell exhaustion. To determine the molecular signature of exhaustion, we compared the gene-expression profiles of dysfunctional lymphocytic choriomeningitis virus (LCMV)-specific CD8(+) T cells from chronic infection to functional LCMV-specific effector and memory CD8(+) T cells generated after acute infection. These data showed that exhausted CD8(+) T cells: (1) overexpressed several inhibitory receptors, including PD-1, (2) had major changes in T cell receptor and cytokine signaling pathways, (3) displayed altered expression of genes involved in chemotaxis, adhesion, and migration, (4) expressed a distinct set of transcription factors, and (5) had profound metabolic and bioenergetic deficiencies. T cell exhaustion was progressive, and gene-expression profiling indicated that T cell exhaustion and anergy were distinct processes. Thus, functional exhaustion is probably due to both active suppression and passive defects in signaling and metabolism. These results provide a framework for designing rational immunotherapies during chronic infections.

A novel role for the Aurora B kinase in epigenetic marking of silent chromatin in differentiated postmitotic cells

Combinatorial modifications of the core histones have the potential to fine-tune the epigenetic regulation of chromatin states. The Aurora B kinase is responsible for generating the double histone H3 modification tri-methylated K9/phosphorylated S10 (H3K9me3/S10ph), which has been implicated in chromosome condensation during mitosis. In this study, we have identified a novel role for Aurora B in epigenetic marking of silent chromatin during cell differentiation. We find that phosphorylation of H3 S10 by Aurora B generates high levels of the double H3K9me3/S10ph modification in differentiated postmitotic cells and also results in delocalisation of HP1beta away from heterochromatin in terminally differentiated plasma cells. Microarray analysis of the H3K9me3/S10ph modification shows a striking increase in the modification across repressed genes during differentiation of mesenchymal stem cells. Our results provide evidence that the Aurora B kinase has a role in marking silent chromatin independently of the cell cycle and suggest that targeting of Aurora B-mediated phosphorylation of H3 S10 to repressed genes could be a mechanism for epigenetic silencing of gene expression.

TLR agonists selectively promote terminal plasma cell differentiation of B cell subsets specialized in thymus-independent responses

Naive murine B cells are known to proliferate and differentiate in response to LPS or CpG, which bind to TLR4 and TLR9, respectively. However, the naive murine B cell compartment is heterogeneous and comprises four different B cell subsets: B-1a, B-1b, marginal zone (MZ), and follicular (FO) B cells. B-1a, B-1b, and MZ B cells are specialized in the response to thymus-independent Ag, and FO B cells are involved in the response to thymus-dependent Ag. This study was undertaken to compare those four naive B cell subsets for their responses to TLR agonists. Quantitative RT-PCR analysis revealed that expression of TLR transcripts differs quantitatively but not qualitatively from one subset to the other. All TLR agonists, with the exception of flagellin and poly(I:C), stimulate B cell proliferation whatever the subset considered. However, TLR ligation leads to massive differentiation of B-1 and MZ B cells into mature plasma cells (PC) but only marginally promotes PC differentiation of FO B cells. Moreover, TLR stimulation strongly up-regulates expression of Blimp-1 and XBP-1(S), two transcription factors known to be instrumental in PC differentiation, in B-1 and MZ B cells but not in FO B cells. Altogether, our findings suggest that B-1 and MZ B cells are poised to PC differentiation in response to the microbial environment and that TLR agonists can be instrumental in stimulating Ab-mediated innate immune protection during microbial infections.

Cell lineage regulators in B and T cell development

This special issue highlights a pivotal set of regulatory molecules that have emerged as central controllers of cell-type identity in the immune system. Each in its own way has been considered as a kind of 'master' regulator of a particular cell fate choice, but the actual modes of action of these factors vary widely. The comparison among them sheds light on the different ways that an essential regulatory input can affect cellular identity.