Bactericidal/permeability-increasing protein instructs dendritic cells to elicit Th22 cell response

Neutrophil-derived bactericidal/permeability-increasing protein (BPI) is known for its bactericidal activity against gram-negative bacteria and neutralization of lipopolysaccharide. Here, we define BPI as a potent activator of murine dendritic cells (DCs). As shown in GM-CSF-cultured, bone-marrow-derived cells (BMDCs), BPI induces a distinct stimulation profile including IL-2, IL-6, and tumor necrosis factor expression. Conventional DCs also respond to BPI, while M-CSF-cultivated or peritoneal lavage macrophages do not. Subsequent to BPI stimulation of BMDCs, CD4+ T cells predominantly secrete IL-22 and, when naive, preferentially differentiate into T helper 22 (Th22) cells. Congruent with the tissue-protective properties of IL-22 and along with impaired IL-22 induction, disease severity is significantly increased during dextran sodium sulfate-induced colitis in BPI-deficient mice. Importantly, physiological diversification of intestinal microbiota fosters BPI-dependent IL-22 induction in CD4+ T cells derived from mesenteric lymph nodes. In conclusion, BPI is a potent activator of DCs and consecutive Th22 cell differentiation with substantial relevance in intestinal homeostasis.

NFAT signaling is indispensable for persistent memory responses of MCMV-specific CD8+ T cells

Cytomegalovirus (CMV) induces a unique T cell response, where antigen-specific populations do not contract, but rather inflate during viral latency. It has been proposed that subclinical episodes of virus reactivation feed the inflation of CMV-specific memory cells by intermittently engaging T cell receptors (TCRs), but evidence of TCR engagement has remained lacking. Nuclear factor of activated T cells (NFAT) is a family of transcription factors, where NFATc1 and NFATc2 signal downstream of TCR in mature T lymphocytes. We show selective impacts of NFATc1 and/or NFATc2 genetic ablations on the long-term inflation of MCMV-specific CD8+ T cell responses despite largely maintained responses to acute infection. NFATc1 ablation elicited robust phenotypes in isolation, but the strongest effects were observed when both NFAT genes were missing. CMV control was impaired only when both NFATs were deleted in CD8+ T cells used in adoptive immunotherapy of immunodeficient mice. Transcriptome analyses revealed that T cell intrinsic NFAT is not necessary for CD8+ T cell priming, but rather for their maturation towards effector-memory and in particular the effector cells, which dominate the pool of inflationary cells.

Delayed and limited administration of the JAKinib tofacitinib mitigates chronic DSS-induced colitis

In inflammatory bowel disease, dysregulated T cells express pro-inflammatory cytokines. Using a chronic azoxymethane (AOM)/dextran sulfate sodium (DSS)-induced colitis model resembling ulcerative colitis, we evaluated whether and when treatment with the Janus kinase (JAK) inhibitor tofacitinib could be curative. Comparing the treatment with two and three cycles of tofacitinib medication in drinking water - intermittently with DSS induction - revealed that two cycles were not only sufficient but also superior over the 3-x regimen. The two cycles of the 2-x protocol paralleled the second and third cycles of the longer protocol. T cells were less able to express interferon gamma (IFN-γ) and the serum levels of IFN-γ, interleukin (IL)-2, IL-6, IL-17, and tumor necrosis factor (TNF) were significantly reduced in sera, while those of IL-10 and IL-22 increased under the 2-x protocol. Likewise, the frequency and effector phenotype of regulatory T cells (Tregs) increased. This was accompanied by normal weight gain, controlled clinical scores, and restored stool consistency. The general and histologic appearance of the colons revealed healing and tissue intactness. Importantly, two phases of tofacitinib medication completely prevented AOM-incited pseudopolyps and the hyper-proliferation of epithelia, which was in contrast to the 3-x regimen. This implies that the initial IBD-induced cytokine expression is not necessarily harmful as long as inflammatory signaling can later be suppressed and that time-restricted treatment allows for anti-inflammatory and tissue-healing cytokine activities.

NFATc1 induction by an intronic enhancer restricts NKT γδ cell formation

In thymus, the ablation of T cell receptor (TCR)-activated transcription factor NFATc1 or its inducible isoforms during the double-negative (DN) stages of thymocyte development leads to a marked increase in γδ thymocytes whereas the development of αβ thymocytes remains mostly unaffected. These γδ thymocytes are characterized by the upregulation of the promyelocytic leukemia zinc-finger factor (PLZF), the "master regulator" of natural killer T (NKT) cell development, and the acquisition of an NKT γδ cell phenotype with higher cell survival rates. The suppressive function of NFATc1 in NKT γδ cell formation critically depends on the remote enhancer E2, which is essential for the inducible expression of NFATc1 directed by its distal promoter P1. Thus, the enhancer deciphers a strong γδ TCR signal into the expression of inducible NFATc1 isoforms resulting in high levels of NFATc1 protein that are essential to control the numbers of NKT γδ cells.

Murine cytomegalovirus reactivation concomitant with acute graft-versus-host disease is controlled by antibodies

Reactivation of human cytomegalovirus (HCMV) from latency is a frequent complication following hematopoietic stem cell transplantation (HSCT). The development of acute graft-versus-host disease (GVHD) is a significant risk factor for HCMV disease. Using a murine GVHD model in animals latently infected with murine CMV (MCMV), we studied preventive and therapeutic interventions in this high-risk scenario of HSCT. Mice latently infected with MCMV experienced reactivated MCMV and developed disseminated MCMV infection concomitant with the manifestations of GVHD. Dissemination was accompanied by accelerated mortality. We demonstrate that MCMV reactivation and dissemination was modulated by MCMV-specific antibodies, thus demonstrating in vivo protective activity of antiviral antibodies. However, the efficacy of serum therapy required repetitive doses of high-titer immune serum secondary to the shortened serum half-life of IgG in animals with GVHD. In a complementary approach, treatment of GVHD by adoptive transfer of donor-derived Tregs facilitated production of MCMV-specific antibodies from newly developing donor-derived B cells. Together, our findings strongly suggest that antibodies play a major role in controlling recurrent MCMV infection that follows GVHD, and they argue for reassessing the potential of antibody treatments as well as therapeutic strategies that enhance de novo antibody development against HCMV.

NFATc1/αA and Blimp-1 Support the Follicular and Effector Phenotype of Tregs

CD4⁺CXCR5⁺Foxp3⁺ T-follicular regulatory (T_FR) cells control the germinal center responses. Like T-follicular helper cells, they express high levels of Nuclear Factor of Activated T-cells c1, predominantly its short isoform NFATc1/αA. Ablation of NFATc1 in Tregs prevents upregulation of CXCR5 and migration of T_FR cells into B-cell follicles. By contrast, constitutive active NFATc1/αA defines the surface density of CXCR5, whose level determines how deep a T_FR migrates into the GC and how effectively it controls antibody production. As one type of effector Treg, T_FR cells express B lymphocyte-induced maturation protein-1 (Blimp-1). Blimp-1 can directly repress Cxcr5 and NFATc1/αA is necessary to overcome this Blimp-1-mediated repression. Interestingly, Blimp-1 even reinforces the recruitment of NFATc1 to Cxcr5 by protein-protein interaction and by those means cooperates with NFATc1 for Cxcr5 transactivation. On the contrary, Blimp-1 is necessary to counterbalance NFATc1/αA and preserve the Treg identity. This is because although NFATc1/αA strengthens the follicular development of Tregs, it bears the inherent risk of causing an ex-Treg phenotype.

T-Cell Metabolism in Graft Versus Host Disease

Allogeneic-hematopoietic stem cell transplantation (allo-HSCT) represents the only curative treatment option for numerous hematological malignancies. Elimination of malignant cells depends on the T-cells' Graft-versus-Tumor (GvT) effect. However, Graft-versus-Host-Disease (GvHD), often co-occurring with GvT, remains an obstacle for therapeutic efficacy. Hence, approaches, which selectively alleviate GvHD without compromising GvT activity, are needed. As already explored for autoimmune and inflammatory disorders, immuno-metabolic interventions pose a promising option to address this unmet challenge. Being embedded in a complex regulatory framework, immunological and metabolic pathways are closely intertwined, which is demonstrated by metabolic reprograming of T-cells upon activation or differentiation. In this review, current knowledge on the immuno-metabolic signature of GvHD-driving T-cells is summarized and approaches to metabolically interfere are outlined. Furthermore, we address the metabolic impact of standard medications for GvHD treatment and prophylaxis, which, in conjunction with the immuno-metabolic profile of alloreactive T-cells, could allow more targeted interventions in the future.

Lack of NFATc1 SUMOylation prevents autoimmunity and alloreactivity

Posttranslational modification with SUMO is known to regulate the activity of transcription factors, but how SUMOylation of individual proteins might influence immunity is largely unexplored. The NFAT transcription factors play an essential role in antigen receptor-mediated gene regulation. SUMOylation of NFATc1 represses IL-2 in vitro, but its role in T cell-mediated immune responses in vivo is unclear. To this end, we generated a novel transgenic mouse in which SUMO modification of NFATc1 is prevented. Avoidance of NFATc1 SUMOylation ameliorated experimental autoimmune encephalomyelitis as well as graft-versus-host disease. Elevated IL-2 production in T cells promoted T reg expansion and suppressed autoreactive or alloreactive immune responses. Mechanistically, increased IL-2 secretion counteracted IL-17 and IFN-γ expression through STAT5 and Blimp-1 induction. Then, Blimp-1 repressed IL-2 itself, as well as the induced, proliferation-associated survival factor Bcl2A1. Collectively, these data demonstrate that prevention of NFATc1 SUMOylation fine-tunes T cell responses toward lasting tolerance. Thus, targeting NFATc1 SUMOylation presents a novel and promising strategy to treat T cell-mediated inflammatory diseases.

Rapid and Efficient Gene Editing for Direct Transplantation of Naive Murine Cas9+ T Cells

Gene editing of primary T cells is a difficult task. However, it is important for research and especially for clinical T-cell transfers. CRISPR/Cas9 is the most powerful gene-editing technique. It has to be applied to cells by either retroviral transduction or electroporation of ribonucleoprotein complexes. Only the latter is possible with resting T cells. Here, we make use of Cas9 transgenic mice and demonstrate nucleofection of pre-stimulated and, importantly, of naive CD3⁺ T cells with guideRNA only. This proved to be rapid and efficient with no need of further selection. In the mixture of Cas9⁺CD3⁺ T cells, CD4⁺ and CD8⁺ conventional as well as regulatory T cells were targeted concurrently. IL-7 supported survival and naivety in vitro, but T cells were also transplantable immediately after nucleofection and elicited their function like unprocessed T cells. Accordingly, metabolic reprogramming reached normal levels within days. In a major mismatch model of GvHD, not only ablation of NFATc1 and/or NFATc2, but also of the NFAT-target gene IRF4 in naïve primary murine Cas9⁺CD3⁺ T cells by gRNA-only nucleofection ameliorated GvHD. However, pre-activated murine T cells could not achieve long-term protection from GvHD upon single NFATc1 or NFATc2 knockout. This emphasizes the necessity of gene-editing and transferring unstimulated human T cells during allogenic hematopoietic stem cell transplantation.

The extreme C-terminus of IRAK2 assures full TRAF6 ubiquitination and optimal TLR signaling

Macrophages are fundamental for initiation, maintenance, and resolution of inflammation. They can be activated by 'Toll-like receptor' (TLR) engagement, which initiates critical pathways to fight infections. 'Interleukin receptor-associated kinase 2' (IRAK2) is part of the membrane-proximal Myddosome formed at IL-1R/TLRs, but utility and regulation of IRAK2 within is not completely understood. In this study, we addressed the importance of the evolutionary conserved extreme C-terminus of IRAK2 in TLR signaling. The last 55 amino acids lack any known functional domain. The C-terminus deletion mutant IRAK2Δ55 was hypofunctional and disabled to conduct TLR4-inducible NF-κB and ERK2 activation. Accordingly, it could neither fully support subsequent CD40 cell surface expression nor IL-6 and nitric oxide release. Interestingly, IRAK2Δ55 was still capable to bind to 'tumor necrosis factor receptor-associated factor 6' (TRAF6), which is requisite to activate TRAF6 as an E3-ubiquitin ligase for further downstream signaling. However, IRAK-dependent auto-ubiquitination of TRAF6 was impaired, when IRAK2Δ55 was bound. Thus, the conserved last 55 amino acids enable IRAK2 to sustain an optimal TLR response. This knowledge might spark ideas how overshooting inflammatory responses could be modified without blocking the entire immune response.

Deep phenotypical characterization of human CD3+ CD56+ T cells by mass cytometry

CD56⁺ T cells are a group of pro-inflammatory CD3⁺ lymphocytes with characteristics of natural killer cells, being involved in antimicrobial immune defense. Here, we performed deep phenotypic profiling of CD3⁺ CD56⁺ cells in peripheral blood of normal human donors and individuals sensitized to birch-pollen or/and house dust mite by high-dimensional mass cytometry combined with manual and computational data analysis. A co-regulation between major conventional T-cell subsets and their respective CD3⁺ CD56⁺ cell counterparts appeared restricted to CD8⁺ , MAIT, and TCRγδ⁺ T-cell compartments. Interestingly, we find a co-regulation of several CD3⁺ CD56⁺ cell subsets in allergic but not in healthy individuals. Moreover, using FlowSOM, we distinguished a variety of CD56⁺ T-cell phenotypes demonstrating a hitherto underestimated heterogeneity among these cells. The novel CD3⁺ CD56⁺ subset description comprises phenotypes superimposed with naive, memory, type 1, 2, and 17 differentiation stages, in part represented by a phenotypical continuum. Frequencies of two out of 19 CD3⁺ CD56⁺ FlowSOM clusters were significantly diminished in allergic individuals, demonstrating less frequent presence of cells with cytolytic, presumably protective, capacity in these donors consistent with defective expansion or their recruitment to the affected tissue. Our results contribute to defining specific cell populations to be targeted during therapy for allergic conditions.

IgG Fc sialylation is regulated during the germinal center reaction following immunization with different adjuvants

BACKGROUND

Effector functions of IgG Abs are regulated by their Fc N-glycosylation pattern. IgG Fc glycans that lack galactose and terminal sialic acid residues correlate with the severity of inflammatory (auto)immune disorders and have also been linked to protection against viral infection and discussed in the context of vaccine-induced protection. In contrast, sialylated IgG Abs have shown immunosuppressive effects.

OBJECTIVE

We sought to investigate IgG glycosylation programming during the germinal center (GC) reaction following immunization of mice with a foreign protein antigen and different adjuvants.

METHODS

Mice were analyzed for GC T-cell, B-cell, and plasma cell responses, as well as for antigen-specific serum IgG subclass titers and Fc glycosylation patterns.

RESULTS

Different adjuvants induce distinct IgG⁺ GC B-cell responses with specific transcriptomes and expression levels of the α2,6-sialyltransferase responsible for IgG sialylation that correspond to distinct serum IgG Fc glycosylation patterns. Low IgG Fc sialylation programming in GC B cells was overall highly dependent on the Foxp3^- follicular helper T (T_FH) cell-inducing cytokine IL-6, here in particular induced by water-in-oil adjuvants and Mycobacterium tuberculosis. Furthermore, low IgG Fc sialylation programming was dependent on adjuvants that induced IL-27 receptor-dependent IFN-γ⁺ T_FH1 cells, IL-6/IL-23-dependent IL-17A⁺ T_FH17 cells, and high ratios of T_FH cells to Foxp3⁺ follicular regulatory T cells. Here, the 2 latter were dependent on M tuberculosis and its cord factor.

CONCLUSION

This study's findings regarding adjuvant-dependent GC responses and IgG glycosylation programming may aid in the development of novel vaccination strategies to induce IgG Abs with both high affinity and defined Fc glycosylation patterns in the GC.

Lymphoid Aggregates in the CNS of Progressive Multiple Sclerosis Patients Lack Regulatory T Cells

In gray matter pathology of multiple sclerosis, neurodegeneration associates with a high degree of meningeal inflammatory activity. Importantly, ectopic lymphoid follicles (eLFs) were identified at the inflamed meninges of patients with progressive multiple sclerosis. Besides T lymphocytes, they comprise B cells and might elicit germinal center (GC)-like reactions. GC reactions are controlled by FOXP3⁺ T-follicular regulatory cells (T_FR), but it is unknown if they participate in autoantibody production in eLFs. Receiving human post-mortem material, gathered from autopsies of progressive multiple sclerosis patients, indeed, distinct inflammatory infiltrates enriched with B cells could be detected in perivascular areas and deep sulci. CD35⁺ cells, parafollicular CD138⁺ plasma cells, and abundant expression of the homing receptor for GCs, CXCR5, on lymphocytes defined some of them as eLFs. However, they resembled GCs only in varying extent, as T cells did not express PD-1, only few cells were positive for the key transcriptional regulator BCL-6 and ongoing proliferation, whereas a substantial number of T cells expressed high NFATc1 like GC-follicular T cells. Then again, predominant cytoplasmic NFATc1 and an enrichment with CD3⁺CD27⁺ memory and CD4⁺CD69⁺ tissue-resident cells implied a chronic state, very much in line with PD-1 and BCL-6 downregulation. Intriguingly, FOXP3⁺ cells were almost absent in the whole brain sections and CD3⁺FOXP3⁺ T_FRs were never found in the lymphoid aggregates. This also points to less controlled humoral immune responses in those lymphoid aggregates possibly enabling the occurrence of CNS-specific autoantibodies in multiple sclerosis patients.

Wilms' tumor 1-associating protein plays an aggressive role in diffuse large B-cell lymphoma and forms a complex with BCL6 via Hsp90

Background

Wilms’ tumor 1-associating protein (WTAP) is a nuclear protein, which is ubiquitously expressed in many tissues. Furthermore, in various types of malignancies WTAP is overexpressed and plays a role as an oncogene. The function of WTAP in diffuse large B-cell lymphoma (DLBCL), however, remains unclear.

Methods

Immunohistochemistry was applied to evaluate the levels of WTAP expression in DLBCL tissues and normal lymphoid tissues. Overexpression and knock-down of WTAP in DLBCL cell lines, verified on mRNA and protein level served to analyze cell proliferation and apoptosis in DLBCL cell lines by flow cytometry. Finally, co-immunoprecipitation (Co-IP), IP, and GST-pull down assessed the interaction of WTAP with Heat shock protein 90 (Hsp90) and B-cell lymphoma 6 (BCL6) as well as determined the extend of its ubiquitinylation.

Results

WTAP protein levels were consistently upregulated in DLBCL tissues. WTAP promoted DLBCL cell proliferation and improved the ability to confront apoptosis, while knockdown of WTAP in DLBCL cell lines allowed a significant higher apoptosis rate after treatment with Etoposide, an anti-tumor drug. The stable expression of WTAP was depended on Hsp90. In line, we demonstrated that WTAP could form a complex with BCL6 via Hsp90 in vivo and in vitro.

Conclusion

WTAP is highly expressed in DLBCL, promoting growth and anti-apoptosis in DLBCL cell lines. WTAP is a client protein of Hsp90 and can appear in a complex with BCL6 and Hsp90 in DLBCL. Down-regulation of WTAP could improve the chemotherapeutic treatments in DLBCL.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0258-6) contains supplementary material, which is available to authorized users.

The feedback loop of LITAF and BCL6 is involved in regulating apoptosis in B cell non-Hodgkin's-lymphoma

Dysregulation of the apoptotic pathway is widely recognized as a key step in lymphomagenesis. Notably, LITAF was initially identified as a p53-inducible gene, subsequently implicated as a tumor suppressor. Our previous study also showed LITAF to be methylated in 89.5% B-NHL samples. Conversely, deregulated expression of BCL6 is a pathogenic event in many lymphomas. Interestingly, our study found an oppositional expression of LITAF and BCL6 in B-NHL. In addition, LITAF was recently identified as a novel target gene of BCL6. Therefore, we sought to explore the feedback loop between LITAF and BCL6 in B-NHL. Here, our data for the first time show that LITAF can repress expression of BCL6 by binding to Region A (-87 to +65) containing a putative LITAF-binding motif (CTCCC) within the BCL6 promoter. Furthermore, the regulation of BCL6 targets ( PRDM1 or c-Myc) by LITAF may be associated with B-cell differentiation. Results also demonstrate that ectopic expression of LITAF induces cell apoptosis, activated by releasing cytochrome c, cleaving PARP and caspase 3 in B-NHL cells whereas knockdown of LITAF robustly protected cells from apoptosis. Interestingly, BCL6, in turn, could reverse cell apoptosis mediated by LITAF. Collectively, our findings provide a novel apoptotic regulatory pathway in which LITAF, as a transcription factor, inhibits the expression of BCL6, which leads to activation of the intrinsic mitochondrial pathway and tumor apoptosis. Our study is expected to provide a possible biomarker as well as a target for clinical therapies to promote tumor cell apoptosis.

Transcription Factor IRF4 Promotes CD8+ T Cell Exhaustion and Limits the Development of Memory-like T Cells during Chronic Infection

During chronic stimulation, CD8⁺ T cells acquire an exhausted phenotype characterized by expression of inhibitory receptors, down-modulation of effector function, and metabolic impairments. T cell exhaustion protects from excessive immunopathology but limits clearance of virus-infected or tumor cells. We transcriptionally profiled antigen-specific T cells from mice infected with lymphocytic choriomeningitis virus strains that cause acute or chronic disease. T cell exhaustion during chronic infection was driven by high amounts of T cell receptor (TCR)-induced transcription factors IRF4, BATF, and NFATc1. These regulators promoted expression of inhibitory receptors, including PD-1, and mediated impaired cellular metabolism. Furthermore, they repressed the expression of TCF1, a transcription factor required for memory T cell differentiation. Reducing IRF4 expression restored the functional and metabolic properties of antigen-specific T cells and promoted memory-like T cell development. These findings indicate that IRF4 functions as a central node in a TCR-responsive transcriptional circuit that establishes and sustains T cell exhaustion during chronic infection.

Store-Operated Ca2+ Entry Controls Clonal Expansion of T Cells through Metabolic Reprogramming

Store-operated Ca2+ entry (SOCE) is the main Ca2+ influx pathway in lymphocytes and is essential for T cell function and adaptive immunity. SOCE is mediated by Ca2+ release-activated Ca2+ (CRAC) channels that are activated by stromal interaction molecule (STIM) 1 and STIM2. SOCE regulates many Ca2+-dependent signaling molecules, including calcineurin, and inhibition of SOCE or calcineurin impairs antigen-dependent T cell proliferation. We here report that SOCE and calcineurin regulate cell cycle entry of quiescent T cells by controlling glycolysis and oxidative phosphorylation. SOCE directs the metabolic reprogramming of naive T cells by regulating the expression of glucose transporters, glycolytic enzymes, and metabolic regulators through the activation of nuclear factor of activated T cells (NFAT) and the PI3K-AKT kinase-mTOR nutrient-sensing pathway. We propose that SOCE controls a critical "metabolic checkpoint" at which T cells assess adequate nutrient supply to support clonal expansion and adaptive immune responses.

Exogenous TNFR2 activation protects from acute GvHD via host T reg cell expansion

Activation of TNFR2 with a novel agonist expands T reg cells in vivo and protects allo-HCT recipients from acute GvHD while sparing antilymphoma and antiinfectious properties of transplanted donor T cells.

Donor CD4⁺Foxp3⁺ regulatory T cells (T reg cells) suppress graft-versus-host disease (GvHD) after allogeneic hematopoietic stem cell transplantation (HCT [allo-HCT]). Current clinical study protocols rely on the ex vivo expansion of donor T reg cells and their infusion in high numbers. In this study, we present a novel strategy for inhibiting GvHD that is based on the in vivo expansion of recipient T reg cells before allo-HCT, exploiting the crucial role of tumor necrosis factor receptor 2 (TNFR2) in T reg cell biology. Expanding radiation-resistant host T reg cells in recipient mice using a mouse TNFR2-selective agonist before allo-HCT significantly prolonged survival and reduced GvHD severity in a TNFR2- and T reg cell–dependent manner. The beneficial effects of transplanted T cells against leukemia cells and infectious pathogens remained unaffected. A corresponding human TNFR2-specific agonist expanded human T reg cells in vitro. These observations indicate the potential of our strategy to protect allo-HCT patients from acute GvHD by expanding T reg cells via selective TNFR2 activation in vivo.

Store-Operated Ca(2+) Entry in Follicular T Cells Controls Humoral Immune Responses and Autoimmunity

T follicular helper (Tfh) cells promote affinity maturation of B cells in germinal centers (GCs), whereas T follicular regulatory (Tfr) cells limit the GC reaction. Store-operated Ca(2+) entry (SOCE) through Ca(2+) release-activated Ca(2+) (CRAC) channels mediated by STIM and ORAI proteins is a fundamental signaling pathway in T lymphocytes. Conditional deletion of Stim1 and Stim2 genes in T cells abolished SOCE and strongly reduced antibody-mediated immune responses following viral infection caused by impaired differentiation and function of Tfh cells. Conversely, aging Stim1Stim2-deficient mice developed humoral autoimmunity with spontaneous autoantibody production due to abolished Tfr cell differentiation in the presence of residual Tfh cells. Mechanistically, SOCE controlled Tfr and Tfh cell differentiation through NFAT-mediated IRF4, BATF, and Bcl-6 transcription-factor expression. SOCE had a dual role in controlling the GC reaction by regulating both Tfh and Tfr cell differentiation, thus enabling protective B cell responses and preventing humoral autoimmunity.

The transcriptional coactivator Bob1 promotes the development of follicular T helper cells via Bcl6

Follicular T helper (Tfh) cells are key regulators of the germinal center reaction and long‐term humoral immunity. Tfh cell differentiation requires the sustained expression of the transcriptional repressor Bcl6; however, its regulation in CD4⁺ T cells is incompletely understood. Here, we report that the transcriptional coactivator Bob1, encoded by the Pou2af1 gene, promotes Bcl6 expression and Tfh cell development. We found that Bob1 together with the octamer transcription factors Oct1/Oct2 can directly bind to and transactivate the Bcl6 and Btla promoters. Mixed bone marrow chimeras revealed that Bob1 is required for the expression of normal levels of Bcl6 and BTLA, thereby controlling the pool size and composition of the Tfh compartment in a T cell‐intrinsic manner. Our data indicate that T cell‐expressed Bob1 is directly involved in Tfh cell differentiation and required for mounting normal T cell‐dependent B‐cell responses.

TCAIM decreases T cell priming capacity of dendritic cells by inhibiting TLR-induced Ca2+ influx and IL-2 production

We previously showed that the T cell activation inhibitor, mitochondrial (Tcaim) is highly expressed in grafts of tolerance-developing transplant recipients and that the encoded protein is localized within mitochondria. In this study, we show that CD11c(+) dendritic cells (DCs), as main producers of TCAIM, downregulate Tcaim expression after LPS stimulation or in vivo alloantigen challenge. LPS-stimulated TCAIM-overexpressing bone marrow-derived DC (BMDCs) have a reduced capacity to induce proliferation of and cytokine expression by cocultured allogeneic T cells; this is not due to diminished upregulation of MHC or costimulatory molecules. Transcriptional profiling also revealed normal LPS-mediated upregulation of the majority of genes involved in TLR signaling. However, TCAIM BMDCs did not induce Il2 mRNA expression upon LPS stimulation in comparison with Control-BMDCs. In addition, TCAIM overexpression abolished LPS-mediated Ca(2+) influx and mitochondrial reactive oxygen species formation. Addition of IL-2 to BMDC-T cell cocultures restored the priming capacity of TCAIM BMDCs for cocultured allogeneic CD8(+) T cells. Furthermore, BMDCs of IL-2-deficient mice showed similarly abolished LPS-induced T cell priming as TCAIM-overexpressing wild type BMDCs. Thus, TCAIM interferes with TLR4 signaling in BMDCs and subsequently impairs their T cell priming capacity, which supports its role for tolerance induction.

Immature dendritic cells convert anergic nonregulatory T cells into Foxp3- IL-10+ regulatory T cells by engaging CD28 and CTLA-4

Anergic T cells can survive for long time periods passively in a hyporesponsive state without obvious active functions. Thus, the immunological reason for their maintenance is unclear. Here, we induced peptide-specific anergy in T cells from mice by coculturing these cells with immature murine dendritic cells (DCs). We found that these anergic, nonsuppressive IL-10(-) Foxp3(-) CTLA-4(+) CD25(low) Egr2(+) T cells could be converted into suppressive IL-10(+) Foxp3(-) CTLA-4(+) CD25(high) Egr2(+) cells resembling type-1 Treg cells (Tr1) when stimulated a second time by immature DCs in vitro. Addition of TGF-β during anergy induction favored Foxp3(+) Treg-cell induction, while TGF-β had little effect when added to the second stimulation. Expression of both CD28 and CTLA-4 molecules on anergic T cells was required to allow their conversion into Tr1-like cells. Suppressor activity was enabled via CD28-mediated CD25 upregulation, acting as an IL-2 sink, together with a CTLA-4-mediated inhibition of NFATc1/α activation to shut down IL-2-mediated proliferation. Together, these data provide evidence and mechanistical insights into how persistent anergic T cells may serve as a resting memory pool for Tr1-like cells.

The mitochondrial protein TCAIM regulates activation of T cells and thereby promotes tolerance induction of allogeneic transplants

Primary T cell activation and effector cell differentiation is required for rejection of allogeneic grafts in naïve recipients. It has become evident, that mitochondria play an important role for T cell activation. Expression of several mitochondrial proteins such as TCAIM (T cell activation inhibitor, mitochondrial) is down-regulated upon T cell receptor triggering. Here we report that TCAIM inhibited spontaneous development of memory and effector T cells. CD4(+) T cells from Tcaim knock-in (KI) mice showed reduced activation, cytokine secretion and proliferation in vitro. Tcaim KI T cells tolerated allogeneic skin grafts upon transfer into Rag-1 KO mice. CD4(+) and CD8(+) T cells from these mice did not infiltrate skin grafts and kept a naïve or central memory phenotype, respectively. They were unable to acquire effector phenotype and functions. TCAIM altered T cell activation-induced mitochondrial distribution and reduced mitochondrial reactive oxygen species (mROS) production. Thus, TCAIM controls T cell activation and promotes tolerance induction probably by regulating TCR-mediated mitochondrial distribution and mROS production.

Follicular regulatory T cells control humoral autoimmunity via NFAT2-regulated CXCR5 expression

T cell–specific NFAT2 deletion results in reduced CXCR5⁺ follicular regulatory T cells, leading to uncontrolled germinal center responses and humoral autoimmunity.

Maturation of high-affinity B lymphocytes is precisely controlled during the germinal center reaction. This is dependent on CD4⁺CXCR5⁺ follicular helper T cells (T_FH) and inhibited by CD4⁺CXCR5⁺Foxp3⁺ follicular regulatory T cells (T_FR). Because NFAT2 was found to be highly expressed and activated in follicular T cells, we addressed its function herein. Unexpectedly, ablation of NFAT2 in T cells caused an augmented GC reaction upon immunization. Consistently, however, T_FR cells were clearly reduced in the follicular T cell population due to impaired homing to B cell follicles. This was T_FR-intrinsic because only in these cells NFAT2 was essential to up-regulate CXCR5. The physiological relevance for humoral (auto-)immunity was corroborated by exacerbated lupuslike disease in the presence of NFAT2-deficient T_FR cells.

Transcriptional control of tolerance associated gene TCAIM is CREB, NFATc1 and C/EBPβ dependent resulting in generation of tolerogenic dendritic cells (P2152)

Cell therapy by tolerogenic dendritic cells has become an attractive therapeutic option in transplantation. We could recently show that high expression of TCAIM, formerly known as TOAG-1, in dendritic cells (DCs) is associated with stable tolerance following transplantation. Here, we investigated transcriptional control of TCAIM in murine bone marrow derived DCs (BMDCs) and human MUTZ-3 derived DCs (M3DCs). We have cloned the human TCAIM promoter to identify regulatory elements by site directed mutagenesis. In addition, we performed luciferase reporter as well as ChIP assays. Furthermore, BMDCs or M3DCs were pre-incubated with cAMP elevating drugs prior to LPS-mediated maturation. Phenotype and tolerogenic potential of DCs were investigated by qPCR, flow cytometry, CBA and MLR. Site-directed mutagenesis, reporter assays and ChIP defined TCAIM core promoter containing three CREB responsive elements. Co-transfections revealed positive regulation of promoter activity by NFATc1 and C/EBPβ. Furthermore, pre-incubation of M3DCs or BMDCs with cAMP elevating drugs leading to CREB activation resulted in high TCAIM expression, inhibited DC maturation, diminished pro-inflammatory cytokine release and induced high frequency of CD4+CD25+FoxP3+ Tregs in DC-T cell co-cultures. These results indicate that transcriptional activation of TCAIM is cAMP/CREB dependent and further amplified by NFATc1 and C/EBPβ, resulting in induction of tolerogenic potential in human M3DCs and murine BMDCs.

NFATc1 induction in peripheral T and B lymphocytes

NFAT transcription factors control the proliferation and survival of peripheral lymphocytes. We have reported previously that the short isoform NFATc1/αA whose generation is induced by immune receptor stimulation supports the proliferation and inhibits the activation-induced cell death of peripheral T and B cells. We will show in this study that in novel bacterial artificial chromosome transgenic mice that express EGFP under the control of entire Nfatc1 locus the Nfatc1/Egfp transgene is expressed as early as in double-negative thymocytes and in nonstimulated peripheral T and B cells. Upon immune receptor stimulation, Nfatc1/Egfp expression is elevated in B, Th1, and Th2 cells, but only weakly in T regulatory, Th9, and Th17 cells in vitro whose generation is affected by TGFβ. In naive lymphocytes, persistent immune receptor signals led to a 3-5 increase in NFATc1/αA RNA levels during primary and secondary stimulation, but a much stronger induction was observed at the protein level. Whereas anti-CD3(+)CD28 stimulation of primary T cells induces both NFATc1/αA and their proliferation and survival, anti-IgM stimulation of B cells induces NFATc1/αA and proliferation, but activation-induced cell death after 3-d incubation in vitro. The anti-IgM-mediated activation-induced cell death induction of B cells in vitro is suppressed by anti-CD40-, LPS-, and CpG-mediated signals. In addition to inducing NF-κB factors, together with anti-IgM, these signals also support the generation of NFATc1/αA. According to these data and the architecture of its promoter region, the Nfatc1 gene resembles a primary response gene whose induction is affected at the posttranscriptional level.

Octamer-dependent transcription in T cells is mediated by NFAT and NF-κB

The transcriptional co-activator BOB.1/OBF.1 was originally identified in B cells and is constitutively expressed throughout B cell development. BOB.1/OBF.1 associates with the transcription factors Oct1 and Oct2, thereby enhancing octamer-dependent transcription. In contrast, in T cells, BOB.1/OBF.1 expression is inducible by treatment of cells with PMA/Ionomycin or by antigen receptor engagement, indicating a marked difference in the regulation of BOB.1/OBF.1 expression in B versus T cells. The molecular mechanisms underlying the differential expression of BOB.1/OBF.1 in T and B cells remain largely unknown. Therefore, the present study focuses on mechanisms controlling the transcriptional regulation of BOB.1/OBF.1 and Oct2 in T cells. We show that both calcineurin- and NF-κB-inhibitors efficiently attenuate the expression of BOB.1/OBF.1 and Oct2 in T cells. In silico analyses of the BOB.1/OBF.1 promoter revealed the presence of previously unappreciated combined NFAT/NF-κB sites. An array of genetic and biochemical analyses illustrates the involvement of the Ca²⁺/calmodulin-dependent phosphatase calcineurin as well as NFAT and NF-κB transcription factors in the transcriptional regulation of octamer-dependent transcription in T cells. Conclusively, impaired expression of BOB.1/OBF.1 and Oct2 and therefore a hampered octamer-dependent transcription may participate in T cell-mediated immunodeficiency caused by the deletion of NFAT or NF-κB transcription factors.

NFATc1/αA: The other Face of NFAT Factors in Lymphocytes

In effector T and B cells immune receptor signals induce within minutes a rise of intracellular Ca++, the activation of the phosphatase calcineurin and the translocation of NFAT transcription factors from cytosol to nucleus. In addition to this first wave of NFAT activation, in a second step the occurrence of NFATc1/αA, a short isoform of NFATc1, is strongly induced. Upon primary stimulation of lymphocytes the induction of NFATc1/αA takes place during the G1 phase of cell cycle. Due to an auto-regulatory feedback circuit high levels of NFATc1/αA are kept constant during persistent immune receptor stimulation. Contrary to NFATc2 and further NFATc proteins which dampen lymphocyte proliferation, induce anergy and enhance activation induced cell death (AICD), NFATc1/αA supports antigen-mediated proliferation and protects lymphocytes against rapid AICD. Whereas high concentrations of NFATc1/αA can also lead to apoptosis, in collaboration with NF-κB-inducing co-stimulatory signals they support the survival of mature lymphocytes in late phases after their activation. However, if dysregulated, NFATc1/αA appears to contribute to lymphoma genesis and - as we assume - to further disorders of the lymphoid system. While the molecular details of NFATc1/αA action and its contribution to lymphoid disorders have to be investigated, NFATc1/αA differs in its generation and function markedly from all the other NFAT proteins which are expressed in lymphoid cells. Therefore, it represents a prime target for causal therapies of immune disorders in future.

Repression of cyclic adenosine monophosphate upregulation disarms and expands human regulatory T cells

The main molecular mechanism of human regulatory T cell (Treg)-mediated suppression has not been elucidated. We show in this study that cAMP represents a key regulator of human Treg function. Repression of cAMP production by inhibition of adenylate cyclase activity or augmentation of cAMP degradation through ectopic expression of a cAMP-degrading phosphodiesterase greatly reduces the suppressive activity of human Treg in vitro and in a humanized mouse model in vivo. Notably, cAMP repression additionally abrogates the anergic state of human Treg, accompanied by nuclear translocation of NFATc1 and induction of its short isoform NFATc1/αA. Treg expanded under cAMP repression, however, do not convert into effector T cells and regain their anergic state and suppressive activity upon proliferation. Together, these findings reveal the cAMP pathway as an attractive target for clinical intervention with Treg function.

Steady state migratory RelB+ langerin+ dermal dendritic cells mediate peripheral induction of antigen-specific CD4+ CD25+ Foxp3+ regulatory T cells

Tolerance to self-antigens expressed in peripheral organs is maintained by CD4(+) CD25(+) Foxp3(+) Treg cells, which are generated as a result of thymic selection or peripheral induction. Here, we demonstrate that steady-state migratory DCs from the skin mediated Treg conversion in draining lymph nodes of mice. These DCs displayed a partially mature MHC II(int) CD86(int) CD40(hi) CCR7(+) phenotype, used endogenous TGF-β for conversion and showed nuclear RelB translocation. Deficiency of the alternative NF-κB signaling pathway (RelB/p52) reduced steady-state migration of DCs. These DCs transported and directly presented soluble OVA provided by s.c. implanted osmotic minipumps, as well as cell-associated epidermal OVA in transgenic K5-mOVA mice to CD4(+) OVA-specific TCR-transgenic OT-II T cells. The langerin(+) dermal DC subset, but not epidermal Langerhans cells, mediated conversion of naive OT-II×RAG-1(-/-) T cells into proliferating CD4(+) CD25(+) Foxp3(+) Tregs. Thus, our data suggest that steady-state migratory RelB(+) TGF-β(+) langerin(+) dermal DCs mediate peripheral Treg conversion in response to epidermal antigen in skin-draining lymph nodes.

NFATc1 affects mouse splenic B cell function by controlling the calcineurin--NFAT signaling network

Mouse B cells lacking NFATc1 exhibit defective proliferation, survival, isotype class switching, cytokine production, and T cell help.

By studying mice in which the Nfatc1 gene was inactivated in bone marrow, spleen, or germinal center B cells, we show that NFATc1 supports the proliferation and suppresses the activation-induced cell death of splenic B cells upon B cell receptor (BCR) stimulation. BCR triggering leads to expression of NFATc1/αA, a short isoform of NFATc1, in splenic B cells. NFATc1 ablation impaired Ig class switch to IgG3 induced by T cell–independent type II antigens, as well as IgG3⁺ plasmablast formation. Mice bearing NFATc1^−/− B cells harbor twofold more interleukin 10–producing B cells. NFATc1^−/− B cells suppress the synthesis of interferon-γ by T cells in vitro, and these mice exhibit a mild clinical course of experimental autoimmune encephalomyelitis. In large part, the defective functions of NFATc1^−/− B cells are caused by decreased BCR-induced Ca²⁺ flux and calcineurin (Cn) activation. By affecting CD22, Rcan1, CnA, and NFATc1/αA expression, NFATc1 controls the Ca²⁺-dependent Cn–NFAT signaling network and, thereby, the fate of splenic B cells upon BCR stimulation.

Sumoylation of the transcription factor NFATc1 leads to its subnuclear relocalization and interleukin-2 repression by histone deacetylase

The family of NFAT (nuclear factor of activated T-cells) transcription factors plays an important role in cytokine gene regulation. In peripheral T-cells NFATc1 and -c2 are predominantly expressed. Because of different promoter and poly(A) site usage as well as alternative splicing events, NFATc1 is synthesized in multiple isoforms. The highly inducible NFATc1/A contains a relatively short C terminus, whereas the longer, constitutively expressed isoform NFATc1/C spans an extra C-terminal peptide of 246 amino acids. Interestingly, this NFATc1/C-specific terminus can be highly sumoylated. Upon sumoylation, NFATc1/C, but not the unsumoylated NFATc1/A, translocates to promyelocytic leukemia nuclear bodies. This leads to interaction with histone deacetylases followed by deacetylation of histones, which in turn induces transcriptionally inactive chromatin. As a consequence, expression of the NFATc1 target gene interleukin-2 is suppressed. These findings demonstrate that the modification by SUMO (small ubiquitin-like modifier) converts NFATc1 from an activator to a site-specific transcriptional repressor, revealing a novel regulatory mechanism for NFATc1 function.

NFAT in lymphocytes: a factor for all events?

A typical immune response to infection by a microbe results in rapid amplification and subsequent differentiation of a few antigen-specific naïve lymphocytes into many effector lymphocytes. Upon antigen exposure, these effector T or B cells rapidly secrete large amounts of either lymphokines (cytokines produced by lymphocytes) or soluble antibodies, respectively. Although the vast majority of these effector cells die after antigen clearance, some cells survive as memory cells and give lifelong protection to the host against a second infection by the same microbe. It has been appreciated for years that memory cells respond more rapidly than do naïve lymphocytes; however, the molecular mechanisms controlling memory cells remain largely unknown. A study now shows that abundance of the transcription factors nuclear factor of activated T cells c1 and c2 (NFATc1 and NFATc2) is much higher in memory (and effector) T cells than in naïve T cells. This suggests that NFATs have an important function in memory T cells but leaves open the questions of which transcription factors control interleukin-2 (IL-2) synthesis in naïve T cells and which mechanisms generate the high abundance of NFAT in memory T cells.

Interferon-β Is a Potent Inducer of Interferon Regulatory Factor-1/2-Dependent IP-10/CXCL10 Expression in Primary Human Endothelial Cells

Most virus-infected cells release interferon-beta (IFN-beta) as a powerful inducer of antiviral defense. Endothelial cells tightly regulate local immune cell recruitment by expression of adhesion molecules and chemokines. Here, we studied the transcriptional regulation of IFN-beta-induced chemokine expression in primary human endothelial cells. IFN-beta moderately increased monocyte chemoattractant protein-1/CCL2 and potently raised IFN-gamma-inducible protein-10/CXCL10 mRNA steady-state levels and protein release, while no effect was detected on various other chemokines. As shown by transient transfections, induction of CXCL10 expression depends on an IFN-stimulated response element (ISRE) within the CXCL10 promoter. A double point mutation of the putative IFN regulatory factor (IRF)-1/2 binding site within this ISRE motif abolished IFN-beta-induced promoter activity. In electrophoretic mobility shift assays, this ISRE motif showed a basal IRF-2 and an IFN-beta-inducible IRF-1 and augmented IRF-2 binding. Furthermore, stimulation with IFN-beta induced a rapid nuclear translocation of signal transducer and activator of transcription 1 (STAT1) and STAT2 and their transient binding to a gamma-activated site within the CCL2 promoter. The kinetics of transient STAT1 binding to this gamma-activated site element correlated with the amount of Y701-phosphorylated nuclear STAT1, while S727-phosphorylated nuclear STAT1 remained stable over 24 h after stimulation. Therefore, IFN-beta potently induces endothelial chemokine expression at the transcriptional level.

SUMOylation Interferes with CCAAT/Enhancer-Binding Protein β-Mediated c-mycRepression, but Not IL-4 Activation in T Cells

The transcription factor C/EBPbeta transactivates the IL-4 gene in murine T lymphocytes and facilitates Th2 cell responses. In this study, we demonstrate that C/EBPbeta also acts as a repressor of T cell proliferation. By binding to the c-myc promoter(s), C/EBPbeta represses c-Myc expression and, therefore, arrests T cells in the G1 phase of the cell cycle. For C/EBPbeta-mediated repression, the integrity of its N-terminal transactivation domain is essential whereas the central regulatory domain is dispensable. This central regulatory domain is sumoylated in vivo which leads to an alteration of the activity of C/EBPbeta. Whereas sumoylation does not affect the C/EBPbeta-mediated activation of the IL-4 gene, it relieves its repressive effect on c-Myc expression and T cell proliferation. Similar to several other transcription factors, sumoylation redistributes nuclear C/EBPbeta and targets it to pericentric heterochromatin. These results suggest an important role of sumoylation in adjusting the finely tuned balance between proliferation and differentiation in peripheral T cells which is controlled by C/EBPbeta.

Subcutaneous interferon-beta injections in patients with multiple sclerosis initiate inflammatory skin reactions by local chemokine induction

Subcutaneous Interferon-beta (IFN-beta) injections for the treatment of multiple sclerosis (MS) frequently cause inflammatory injection site reactions. To study the role of chemokines we obtained skin biopsies from 7 MS patients 24 h after injection. At the IFN-beta but not at the contralateral placebo injection sites, we observed strong IP-10/CXCL10 and moderate MCP-1/CCL2 expression associated with extensive perivascular, highly CXCR3-positive T cell and macrophage infiltrates. Primary human skin cells displayed a comparable pattern of chemokine induction after stimulation with IFN-beta in vitro. IFN-beta may therefore trigger inflammatory skin reactions through local chemokine induction followed by rapid immune cell extravasation.

NFAT and NF-κB factors—the distant relatives

NFAT and NF-kappaB proteins are members of a superfamily of transcription factors whose activity plays a crucial role in the activation, proliferation and apoptosis of lymphocytes. Both types of factors share a number of properties, including similar DNA binding domains and rapid nuclear translocation upon antigenic stimulation. While NF-kappaBs control both innate and adaptive immune responses, NFATs control the adaptive immune system which emerged-in parallel with the appearance of the NFAT family-in jawed fish. However, NFATs and NF-kappaBs differ remarkably in their function. Whereas NFATs support activation-induced cell death (AICD) of T and B cells, NF-kappaB proteins frequently exert a strong anti-apoptotic effect on lymphocytes and other cells. While the anti-apoptotic activity of NF-kappaBs contributes to their oncogenic capacity, the pro-apoptotic activity favors NFATs as tumor suppressors in lymphoid cells.

NF-kappaB-dependent induction of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and Fas/FasL is crucial for efficient influenza virus propagation

Activation of the transcription factor NF-kappaB is a hallmark of infections by viral pathogens including influenza viruses. Because gene expression of many proinflammatory and antiviral cytokines is controlled by this factor, the concept emerged that NF-kappaB and its upstream regulator IkappaB kinase are essential components of the innate antiviral immune response to infectious pathogens. In contrast to this common view we report here that NF-kappaB activity promotes efficient influenza virus production. On a molecular level this is due to NF-kappaB-dependent viral induction of the proapoptotic factors tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and FasL, which enhance virus propagation in an autocrine and paracrine fashion. Thus, NF-kappaB acts both proapoptotically and provirally in the context of an influenza virus infection.

Borna disease virus nucleoprotein interacts with the CDC2-cyclin B1 complex

Transition from G(2) to M phase, a cell cycle checkpoint, is regulated by the Cdc2-cyclin B1 complex. Here, we report that persistent infection with Borna disease virus (BDV), a noncytolytic RNA virus infecting the central nervous system, results in decelerated proliferation of infected host cells due to a delayed G(2)-to-M transition. Persistent BDV-infected rat fibroblast cells showed reduced proliferation compared to uninfected cells. In pull-down assays we observed an interaction of the viral nucleoprotein with the Cdc2-cyclin B1 complex. Transfection of the viral nucleoprotein but not of the phosphoprotein also results in decelerated proliferation. This phenomenon was found in BDV-susceptible primary rat fibroblast cells and also in primary mouse cells, which are not susceptible to BDV infection. This is the first evidence that the noncytolytic Borna disease virus can manipulate host cell functions via interaction of the viral nucleoprotein with mitotic entry regulators. BDV preferentially infects and persists in nondividing neurons. The present report could give an explanation for this selective choice of host cell by BDV.

Protein kinase A regulates GATA-3-dependent activation of IL-5 gene expression in Th2 cells

Treatment of Th cells with compounds that elevate cAMP levels augments Th2-type lymphokine expression, in particular the synthesis of IL-5. Using primary murine CD4(+) T lymphocytes, we show in this study that inhibition of protein kinase A (PKA) activity in Th2 effector cells impairs IL-5 synthesis, whereas the expression of PKA catalytic subunit alpha enhances IL-5 synthesis in Th0 cells. In addition, we observed by coexpression of PKA catalytic subunit and GATA-3 in Th1 cells that the stimulatory effect of PKA is dependent on GATA-3 activity. These data demonstrate that activation of PKA in Th effector cells induces the IL-5 gene expression in a GATA-3-dependent manner.

Autoregulation of NFATc1/A expression facilitates effector T cells to escape from rapid apoptosis

Threshold levels of individual NFAT factors appear to be critical for apoptosis induction in effector T cells. In these cells, the short isoform A of NFATc1 is induced to high levels due to the autoregulation of the NFATc1 promoter P1 by NFATs. P1 is located within a CpG island in front of exon 1, represents a DNase I hypersensitive chromatin site, and harbors several sites for binding of inducible transcription factors, including a tandemly arranged NFAT site. A second promoter, P2, before exon 2, is not controlled by NFATs and directs synthesis of the longer NFATc1/B+C isoforms. Contrary to other NFATs, NFATc1/A is unable to promote apoptosis, suggesting that NFATc1/A enhances effector functions without promoting apoptosis of effector T cells.

Signal-specific and phosphorylation-dependent RelB degradation: a potential mechanism of NF-kappaB control

RelB is an unusual member of the Rel/NF-kappaB family of transcription factors which are involved in oncogenic processes. Due to a relaxed control by the IkappaBs, the cytosolic NF-kappaB inhibitors, RelB is constitutively expressed in the nuclei of lymphoid cells. We show here that RelB is inducibly degraded upon activation of T cells in a fashion similar to the IkappaBs. However, RelB degradation differs from that of IkappaBs since it is not induced by TNFalpha but only by T cell receptor or TPA/ionomycin stimulation. Moreover, RelB degradation occurs in three steps: (i) after stimulation RelB is rapidly phosphorylated at amino acids Thr84 and Ser552 followed by (ii) an N-terminal cut and, finally, (iii) the complete degradation in the proteasomes. Since mutation of the two phosphoacceptor sites to non-acceptor sites abolished RelB phosphorylation in vivo and led to the stabilization of the mutated RelB(DM), site-specific phosphorylation appears to be a necessary prerequisite for RelB degradation. RelB is a crucial regulator of NF-kappaB-dependent gene expression. Thus, the signal-induced degradation of RelB should be an important control mechanism of NF-kappaB activity.

Apoptosis suppression by Raf-1 and MEK1 requires MEK- and phosphatidylinositol 3-kinase-dependent signals

Two Ras effector pathways leading to the activation of Raf-1 and phosphatidylinositol 3-kinase (PI3K) have been implicated in the survival signaling by the interleukin 3 (IL-3) receptor. Analysis of apoptosis suppression by Raf-1 demonstrated the requirement for mitochondrial translocation of the kinase in this process. This could be achieved either by overexpression of the antiapoptotic protein Bcl-2 or by targeting Raf-1 to the mitochondria via fusion to the mitochondrial protein Mas p70. Mitochondrially active Raf-1 is unable to activate extracellular signal-related kinase 1 (ERK1) and ERK2 but suppresses cell death by inactivating the proapoptotic Bcl-2 family member BAD. However, genetic and biochemical data also have suggested a role for the Raf-1 effector module MEK-ERK in apoptosis suppression. We thus tested for MEK requirement in cell survival signaling using the interleukin 3 (IL-3)-dependent cell line 32D. MEK is essential for survival and growth in the presence of IL-3. Upon growth factor withdrawal the expression of constitutively active MEK1 mutants significantly delays the onset of apoptosis, whereas the presence of a dominant negative mutant accelerates cell death. Survival signaling by MEK most likely results from the activation of ERKs since expression of a constitutively active form of ERK2 was as effective in protecting NIH 3T3 fibroblasts against doxorubicin-induced cell death as oncogenic MEK. The survival effect of activated MEK in 32D cells is achieved by both MEK- and PI3K-dependent mechanisms and results in the activation of PI3K and in the phosphorylation of AKT. MEK and PI3K dependence is also observed in 32D cells protected from apoptosis by oncogenic Raf-1. Additionally, we also could extend these findings to the IL-3-dependent pro-B-cell line BaF3, suggesting that recruitment of MEK is a common mechanism for survival signaling by activated Raf. Requirement for the PI3K effector AKT in this process is further demonstrated by the inhibitory effect of a dominant negative AKT mutant on Raf-1-induced cell survival. Moreover, a constitutively active form of AKT synergizes with Raf-1 in apoptosis suppression. In summary these data strongly suggest a Raf effector pathway for cell survival that is mediated by MEK and AKT.

The role of NF-AT transcription factors in T cell activation and differentiation

The family of genuine NF-AT transcription factors consists of four members (NF-AT1 [or NF-ATp], NF-AT2 [or NF-ATc], NF-AT3 and NF-AT4 [or NF-ATx]) which are characterized by a highly conserved DNA binding domain (is designated as Rel similarity domain) and a calcineurin binding domain. The binding of the Ca(2+)-dependent phosphatase calcineurin to this region controls the nuclear import and exit of NF-ATs. This review deals (1) with the structure of NF-AT proteins, (2) the DNA binding of NF-AT factors and their interaction with AP-1, (3) NF-AT target genes, (4) signalling pathways leading to NF-AT activation: the role of protein kinases and calcineurin, (5) the nuclear entry and exit of NF-AT factors, (6) transcriptional transactivation by NF-AT factors, (7) the structure and expression of the chromosomal NF-AT2 gene, and (8) NF-AT factors in Th cell differentiation. The experimental data presented and discussed in the review show that NF-AT factors are major players in the control of T cell activation and differentiation and, in all likelihood, also of the cell cycle and apoptosis of T lymphocytes.

C/EBPbeta enhances IL-4 but impairs IL-2 and IFN-gamma induction in T cells

C/EBP transcription factors have been described to control the activity of the human IL-4 promoter. The C/EBP binding sites within the IL-4 promoter overlap with composite NF-AT and AP-1 binding motifs. We show here that similar binding sites are part of the murine IL-4 promoter. Retroviral overexpression of C/EBPbeta in murine EL-4 thymoma cells led to a strong induction of endogenous IL-4 and a reduction in IL-2 and IFN-gamma expression. Similarily, in primary murine T cells C/EBPbeta induction resulted in an increase in IL-4 levels, whereas in human Jurkat T cells a decrease in IL-2 RNA was detected. Like AP-1, C/EBP factors belong to the large class of basic leucine zipper proteins. However, unlike AP-1, C/EBPbeta does not act in synergy with NF-AT in the induction of the murine IL-4 promoter. Instead, both factors compete in their binding to the P4/Pu-bD site, one of the most important sequence elements of the IL-4 promoter. Whereas NF-AT factors require high levels of free Ca2+ and calcineurin activity for induction, C/EBP induction in T cells is Ca2+/calcineurin independent. These observations suggest that various induction conditions lead to the activation of transcription factors, inducing IL-4 promoter activity at specific developmental stages of T cells.