Phospholipid scramblase 1 regulates Toll-like receptor 9-mediated type I interferon production in plasmacytoid dendritic cells

Machine learning framework develops neutrophil extracellular traps model for clinical outcome and immunotherapy response in lung adenocarcinoma

Neutrophil extracellular traps (NETs) are novel inflammatory cell death in neutrophils. Emerging studies demonstrated NETs contributed to cancer progression and metastases in multiple ways. This study intends to provide a prognostic NETs signature and therapeutic target for lung adenocarcinoma (LUAD) patients. Consensus cluster analysis performed by 38 reported NET-related genes in TCGA-LUAD cohorts. Then, WGCNA network was conducted to investigate characteristics genes in clusters. Seven machine learning algorithms were assessed for training of the model, the optimal model was picked by C-index and 1-, 3-, 5-year ROC value. Then, we constructed a NETs signature to predict the overall survival of LUAD patients. Moreover, multi-omics validation was performed based on NETs signature. Finally, we constructed stable knockdown critical gene LUAD cell lines to verify biological functions of Phospholipid Scramblase 1 (PLSCR1) in vitro and in vivo. Two NETs-related clusters were identified in LUAD patients. Among them, C2 cluster was provided as "hot" tumor phenotype and exhibited a better prognosis. Then, WGCNA network identified 643 characteristic genes in C2 cluster. Then, Coxboost algorithm proved its optimal performance and provided a prognostic NETs signature. Multi-omics revealed that NETs signature was involved in an immunosuppressive microenvironment and predicted immunotherapy efficacy. In vitro and in vivo experiments demonstrated that knockdown of PLSCR1 inhibited tumor growth and EMT ability. Besides, cocultural assay indicated that the knockdown of PLSCR1 impaired the ability of neutrophils to generate NETs. Finally, tissue microarray (TMA) for LUAD patients verified the prognostic value of PLSCR1 expression. In this study, we focus on emerging hot topic NETs in LUAD. We provide a prognostic NETs signature and identify PLSCR1 with multiple roles in LUAD. This work can contribute to risk stratification and screen novel therapeutic targets for LUAD patients.

MiR-103-5p deficiency suppresses lipid accumulation via upregulating PLSCR4 and its host gene PANK3 in goat mammary epithelial cells

Lipids are intimately related to the unique flavor and nutritional values of goat milk. MicroRNAs (miRNA) participate in the regulation of various biological functions, including the synthesis and degradation of lipids. Several studies have shown that miR-103 is involved in the regulation of lipid metabolism, however, the molecular mechanism by which miR-103 regulates lipid metabolism in goat mammary gland is poorly understood. In this study, miR-103 was knocked out in goat mammary epithelial cells (GMECs) by CRISPR/Cas9, and the accumulation of lipid droplets, triglycerides, and cholesterol in the cells was suppressed subsequently. Overexpression or knockdown of miR-103-5p and miR-103-3p in GMECs revealed that it was miR-103-5p that promoted lipid accumulation but not miR-103-3p. In addition, Pantothenate Kinase 3 (PANK3), the host gene of miR-103, and Phospholipid Scramblase 4 (PLSCR4) were identified as the target genes of miR-103-5p by dual fluorescein and miRNA pulldown. Furthermore, we identified that cellular lipid levels were negatively regulated by PANK3 and PLSCR4. Lastly, in miR-103 knockout GMECs, the knockdown of PANK and PLSCR4 rescued the lipid accumulation. These findings suggest that miR-103-5p promotes lipid accumulation by targeting PLSCR4 and the host gene PANK3 in GMECs, providing new insights for the regulation of goat milk lipids via miRNAs.

Phospholipid scramblase 1 is involved in immunogenic cell death and contributes to dendritic cell-based vaccine efficiency to elicit antitumor immune response in vitro

BACKGROUND AIMS

Whole tumor cell lysates (TCLs) obtained from cancer cells previously killed by treatments able to promote immunogenic cell death (ICD) can be efficiently used as a source of tumor-associated antigens for the development of highly efficient dendritic cell (DC)-based vaccines. Herein, the potential role of the interferon (IFN)-inducible protein phospholipid scramblase 1 (PLSCR1) in influencing immunogenic features of dying cancer cells and in enhancing DC-based vaccine efficiency was investigated.

METHODS

PLSCR1 expression was evaluated in different mantle-cell lymphoma (MCL) cell lines following ICD induction by 9-cis-retinoic acid (RA)/IFN-α combination, and commercial kinase inhibitor was used to identify the signaling pathway involved in its upregulation. A Mino cell line ectopically expressing PLSCR1 was generated to investigate the potential involvement of this protein in modulating ICD features. Whole TCLs obtained from Mino overexpressing PLSCR1 were used for DC loading, and loaded DCs were employed for generation of tumor antigen-specific cytotoxic T lymphocytes.

RESULTS

The ICD inducer RA/IFN-α combination promoted PLSCR1 expression through STAT1 activation. PLSCR1 upregulation favored pro-apoptotic effects of RA/IFN-α treatment and enhanced the exposure of calreticulin on cell surface. Moreover, DCs loaded with TCLs obtained from Mino ectopically expressing PLSCR1 elicited in vitro greater T-cell-mediated antitumor responses compared with DCs loaded with TCLs derived from Mino infected with empty vector or the parental cell line. Conversely, PLSCR1 knock-down inhibited the stimulating activity of DCs loaded with RA/IFN-α-treated TCLs to elicit cyclin D1 peptide-specific cytotoxic T lymphocytes.

CONCLUSIONS

Our results indicate that PLSCR1 improved ICD-associated calreticulin exposure induced by RA/IFN-α and was clearly involved in DC-based vaccine efficiency as well, suggesting a potential contribution in the control of pathways associated to DC activation, possibly including those involved in antigen uptake and concomitant antitumor immune response activation.

TLR pathway signaling molecules in burbot (Lota lota): molecular characterization, basal expression, and their response to Poly(I:C)

Burbot (Lota lota), a fish species of economic and ecological significance found across northern hemisphere freshwater ecosystems, was the focus of this study. We characterized 19 Toll-like receptor (TLR) genes in burbot, tracing their expression patterns following pathogen exposure. TLR genes, crucial to the innate immune system, including TLR13-1/2/3, TLR2/2-2/2-3/2-4/2-5, and TLR22a/22b/22c/22d, were discovered to be tandemly repeated, signifying an evolution in the fish's immune system. Notably, different TLR subfamilies displayed tissue-specific expressions, with TLR1 primarily in spleen and head kidney, TLR13 in head kidney, trunk kidney, and heart, TLR22 in trunk kidney and liver, and TLR3 and TLR9 predominantly in spleen and head kidney, but also in trunk kidney. Further, we investigated the response of TLR genes in burbot to pathogen exposure using qRT-PCR. This involved measuring mRNA expressions of identified TLR genes in spleen and liver tissues after injecting Poly(I:C) to simulate a double-stranded RNA viral infection. The results revealed a time and tissue-specific expression pattern. Specifically, LoTLR3 reached peak expression in the spleen 12 h post-injection, declining thereafter, while TLR2 subfamily members only began expressing after 24 h. In the liver, activation of the TLR3-IRF7 and TLR3-IRF3 signaling pathways was noted. Integrating these results with transcriptomic data illuminated the pivotal role of TLR genes in the burbot's immune response. Such findings are vital in shaping future disease prevention and treatment strategies.

Multimodal control of dendritic cell functions by nociceptors

It is known that interactions between nociceptors and dendritic cells (DCs) can modulate immune responses in barrier tissues. However, our understanding of the underlying communication frameworks remains rudimentary. Here, we show that nociceptors control DCs in three molecularly distinct ways. First, nociceptors release the calcitonin gene-related peptide that imparts a distinct transcriptional profile on steady-state DCs characterized by expression of pro-interleukin-1β and other genes implicated in DC sentinel functions. Second, nociceptor activation induces contact-dependent calcium fluxes and membrane depolarization in DCs and enhances their production of proinflammatory cytokines when stimulated. Finally, nociceptor-derived chemokine CCL2 contributes to the orchestration of DC-dependent local inflammation and the induction of adaptive responses against skin-acquired antigens. Thus, the combined actions of nociceptor-derived chemokines, neuropeptides, and electrical activity fine-tune DC responses in barrier tissues.

Phospholipid scramblase 1: a protein with multiple functions via multiple molecular interactors

Phospholipid scramblase 1 (PLSCR1) is the most studied protein of the scramblase family. Originally, it was identified as a membrane protein involved in maintaining plasma membrane asymmetry. However, studies conducted over the past few years have shown the involvement of PLSCR1 in several other cellular pathways. Indeed, PLSCR1 is not only embedded in the plasma membrane but is also expressed in several intracellular compartments where it interacts with a diverse repertoire of effectors, mediators, and regulators contributing to distinct cellular processes. Although most PLSCR1 interactors are thought to be cell-type specific, PLSCR1 often exerts its regulatory functions through shared mechanisms, including the trafficking of different molecules within intracellular vesicles such as endosomes, liposomes, and phagosomes. Intriguingly, besides endogenous proteins, PLSCR1 was also reported to interact with exogenous viral proteins, thereby regulating viral uptake and spread. This review aims to summarize the current knowledge about the multiple roles of PLSCR1 in distinct cellular pathways.

Herpes simplex virus interference with immunity: Focus on dendritic cells

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are highly prevalent in the human population. These viruses cause lifelong infections by establishing latency in neurons and undergo sporadic reactivations that promote recurrent disease and new infections. The success of HSVs in persisting in infected individuals is likely due to their multiple molecular determinants involved in escaping the host antiviral and immune responses. Importantly, HSVs infect and negatively modulate the function of dendritic cells (DCs), key immune cells that are involved in establishing effective and balanced immunity against viruses. Here, we review and discuss several molecular and cellular processes modulated by HSVs in DCs, such as autophagy, apoptosis, and the unfolded protein response. Given the central role of DCs in establishing optimal antiviral immunity, particular emphasis should be given to the outcome of the interactions occurring between HSVs and DCs.

Interleukin-36α inhibits colorectal cancer metastasis by enhancing the infiltration and activity of CD8+ T lymphocytes

Colorectal cancer is one of the deadliest cancers, and the discovery of new diagnostic biomarkers and therapeutic targets is vital. Interleukin-36α (IL-36α) is a proinflammatory factor that can initiate the inflammatory response and promote the systemic T helper-1 (Th1) immune response. In this study, we investigated the immunological role of IL-36α in CRC. We found that IL-36α was downregulated in human CRC tissues. Patients with high IL-36α levels showed better survival and low IL-36α expression was significantly associated with greater tumor distal metastasis and TNM stage. We constructed two cell lines overexpressing IL-36α (CT26-IL-36α and HT29-IL-36α cells). In vitro assays revealed that IL-36α overexpression reduced the proliferation, migration, and invasion of CT26-IL-36α, and HT29-IL-36α cells. Using CT26-vector and CT26-IL-36α tumor mouse model and lung metastasis models, we found that IL-36α overexpression elicited a significant antitumor effect and inhibited lung metastasis in vivo. These inhibitory effects were associated with an increase in the number of CD3+CD8+ T lymphocytes within the tumor tissue as well as increased cytokine production in CD8+ T lymphocytes present in the tumor, spleen, and draining lymph nodes. Furthermore, we revealed that CT26-IL-36α cells enhanced the secretion of CXCL10 and CXCL11 from chemotactic CD8+ T lymphocytes, as compared with CT26-vector cells. Taken together, these results suggest that IL-36α is a promising therapeutic agent for targeting CRC by promoting the activation, proliferation, and tumor infiltration of T lymphocytes.

CD303 (BDCA-2) - a potential novel target for therapy in hematologic malignancies

Plasmacytoid dendritic cells (pDCs) serve as immunoregulatory antigen-presenting cells that play a role in various inflammatory, viral, and malignant conditions. Malignant proliferation of pDCs is implicated in the pathogenesis of certain hematologic cancers, specifically blastic plasmacytoid dendritic cell neoplasm (BPDCN) and acute myelogenous leukemia with clonal expansion of pDC (pDC-AML). In recent years, BPDCN and pDC-AML have been successfully treated with targeted therapy of pDC-specific surface marker, CD123. However, relapsed and refractory BPDCN remains an elusive cancer, with limited therapeutic options. CD303 is another specific surface marker of human pDCs, centrally involved in antigen presentation and immune tolerance. Monoclonal antibodies directed against CD303 have been studied in preclinical models and have achieved disease control in patients with cutaneous lupus erythematosus. We performed a comprehensive review of benign and malignant disorders in which CD303 have been studied, as there may be a potential future CD303-directed therapy for many of these conditions.

Ascorbic acid supports ex vivo generation of plasmacytoid dendritic cells from circulating hematopoietic stem cells

Plasmacytoid dendritic cells (pDCs) constitute a rare type of immune cell with multifaceted functions, but their potential use as a cell-based immunotherapy is challenged by the scarce cell numbers that can be extracted from blood. Here, we systematically investigate culture parameters for generating pDCs from hematopoietic stem and progenitor cells (HSPCs). Using optimized conditions combined with implementation of HSPC pre-expansion, we generate an average of 465 million HSPC-derived pDCs (HSPC-pDCs) starting from 100,000 cord blood-derived HSPCs. Furthermore, we demonstrate that such protocol allows HSPC-pDC generation from whole-blood HSPCs, and these cells display a pDC phenotype and function. Using GMP-compliant medium, we observe a remarkable loss of TLR7/9 responses, which is rescued by ascorbic acid supplementation. Ascorbic acid induces transcriptional signatures associated with pDC-specific innate immune pathways, suggesting an undescribed role of ascorbic acid for pDC functionality. This constitutes the first protocol for generating pDCs from whole blood and lays the foundation for investigating HSPC-pDCs for cell-based immunotherapy.

Type I Interferon Production of Plasmacytoid Dendritic Cells under Control

One of the most powerful and multifaceted cytokines produced by immune cells are type I interferons (IFNs), the basal secretion of which contributes to the maintenance of immune homeostasis, while their activation-induced production is essential to effective immune responses. Although, each cell is capable of producing type I IFNs, plasmacytoid dendritic cells (pDCs) possess a unique ability to rapidly produce large amounts of them. Importantly, type I IFNs have a prominent role in the pathomechanism of various pDC-associated diseases. Deficiency in type I IFN production increases the risk of more severe viral infections and the development of certain allergic reactions, and supports tumor resistance; nevertheless, its overproduction promotes autoimmune reactions. Therefore, the tight regulation of type I IFN responses of pDCs is essential to maintain an adequate level of immune response without causing adverse effects. Here, our goal was to summarize those endogenous factors that can influence the type I IFN responses of pDCs, and thus might serve as possible therapeutic targets in pDC-associated diseases. Furthermore, we briefly discuss the current therapeutic approaches targeting the pDC-type I IFN axis in viral infections, cancer, autoimmunity, and allergy, together with their limitations defined by the Janus-faced nature of pDC-derived type I IFNs.

TLR9 Polymorphisms Might Contribute to the Ethnicity Bias for EBV-Infected Nasopharyngeal Carcinoma

Nasopharyngeal carcinoma (NPC) is a rare malignancy in most parts of the world, but is endemic in some ethnic groups. The association of NPC with the Epstein-Barr virus (EBV) is firmly established; however, the mechanism is still unclear. TLR9 is well known for its essential role in viral pathogen recognition and activation of innate immunity. Here, we report a set of TLR9 polymorphisms in the TIR-2 domain of the TLR9 protein collected from the EBV-infected NPC samples from northeast Indian populations sharing the aforesaid ethnicity. The occurrence of mutations is significantly high in these samples as we found a p value of <0.0001 at a significance level of 0.05. These might play an important role for the lack of function of TLR9 and thus for the higher occurrence of EBV-mediated NPC in such ethnic groups.

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EBV-associated nasopharyngeal carcinoma (NPC) is endemic in Mongoloids

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TLR9 is essential for virus recognition and immunity against cancers

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Polymorphisms are common in TLR9 protein in EBV-infected Mongoloids with NPC

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NPC susceptibility prediction by TLR9 mutation screening in people for prevention

IL-36 Promotes Systemic IFN-I Responses in Severe Forms of Psoriasis

Psoriasis is an immune-mediated skin disorder associated with severe systemic comorbidities. Whereas IL-36 is a key disease driver, the pathogenic role of this cytokine has mainly been investigated in skin. Thus, its effects on systemic immunity and extracutaneous disease manifestations remain poorly understood. To address this issue, we investigated the consequences of excessive IL-36 activity in circulating immune cells. We initially focused our attention on generalized pustular psoriasis (GPP), a clinical variant associated with pervasive upregulation of IL-36 signaling. By undertaking blood and neutrophil RNA sequencing, we demonstrated that affected individuals display a prominent IFN-I signature, which correlates with abnormal IL-36 activity. We then validated the association between IL-36 deregulation and IFN-I over-expression in patients with severe psoriasis vulgaris (PV). We also found that the activation of IFN-I genes was associated with extracutaneous morbidity, in both GPP and PV. Finally, we undertook mechanistic experiments, demonstrating that IL-36 acts directly on plasmacytoid dendritic cells, where it potentiates toll-like receptor (TLR)-9 activation and IFN-α production. This effect was mediated by the upregulation of PLSCR1, a phospholipid scramblase mediating endosomal TLR-9 translocation. These findings identify an IL-36/ IFN-I axis contributing to extracutaneous inflammation in psoriasis.

Interaction of phospholipid scramblase 1 with the Epstein-Barr virus protein BZLF1 represses BZLF1-mediated lytic gene transcription

Human phospholipid scramblase 1 (PLSCR1) is strongly expressed in response to interferon (IFN) treatment and viral infection, and PLSCR1 has been suggested to play an important role in IFN-dependent antiviral responses. In this study, we showed that the basal expression of PLSCR1 was significantly elevated in Epstein-Barr virus (EBV)-infected nasopharyngeal carcinoma (NPC). PLSCR1 was observed to directly interact with the EBV immediate-early transactivator BZLF1 in vitro and in vivo, and this interaction repressed the BZLF1-mediated transactivation of an EBV lytic BMRF1 promoter construct. In addition, PLSCR1 expression decreased the BZLF1-mediated up-regulation of lytic BMRF1 mRNA and protein expression in WT and PLSCR1-knockout EBV-infected NPC cells. Furthermore, we showed that PLSCR1 represses the interaction between BZLF1 and CREB-binding protein (CBP), which enhances the BZLF1-mediated transactivation of EBV lytic promoters. These results reveal for the first time that PLSCR1 specifically interacts with BZLF1 and negatively regulates its transcriptional regulatory activity by preventing the formation of the BZLF1-CBP complex. This interaction may contribute to the establishment of latent EBV infection in EBV-infected NPC cells.

Modulation of Mast Cell Reactivity by Lipids: The Neglected Side of Allergic Diseases

Mast cells (MCs) have long been mainly regarded as effector cells in IgE-associated allergic disorders with potential immunoregulatory roles. Located close to the allergen entry sites in the skin and mucosa, MCs can capture foreign substances such as allergens, toxins, or noxious substances and are exposed to the danger signals produced by epithelial cells. MC reactivity shaped by tissue-specific factors is crucial for allergic responses ranging from local skin reactions to anaphylactic shock. Development of Th2 response leading to allergen-specific IgE production is a prerequisite for MC sensitization and induction of FcεRI-mediated MC degranulation. Up to now, IgE production has been mainly associated with proteins, whereas lipids present in plant pollen grains, mite fecal particles, insect venoms, or food have been largely overlooked regarding their immunostimulatory and immunomodulatory properties. Recent studies, however, have now demonstrated that lipids affect the sensitization process by modulating innate immune responses of epithelial cells, dendritic cells, and NK-T cells and thus crucially contribute to the outcome of sensitization. Whether and how lipids affect also MC effector functions in allergic reactions has not yet been fully clarified. Here, we discuss how lipids can affect MC responses in the context of allergic inflammation. Direct effects of immunomodulatory lipids on MC degranulation, changes in local lipid composition induced by allergens themselves and changes in lipid transport affecting MC reactivity are possible mechanisms by which the function of MC might be modulated.

Time-Resolved Systems Medicine Reveals Viral Infection-Modulating Host Targets

Introduction: Drug-resistant infections are becoming increasingly frequent worldwide, causing hundreds of thousands of deaths annually. This is partly due to the very limited set of protein drug targets known for human-infecting viral genomes. The eleven influenza virus proteins, for instance, exploit host cell factors for replication and suppression of the antiviral immune responses. A systems medicine approach to identify relevant and druggable host factors would dramatically expand therapeutic options. Therapeutic target identification, however, has hitherto relied on static molecular networks, whereas in reality the interactome, in particular during an infection, is subject to constant change. Methods: We developed time-course network enrichment (TiCoNE), an expert-centered approach for discovering temporal response pathways. In the first stage of TiCoNE, time-series expression data is clustered in a human-augmented manner to identify groups of biological entities with coherent temporal responses. Throughout this process, the expert can add, remove, merge, or split temporal patterns. The resulting groups can then be mapped to an interaction network to identify enriched pathways and to analyze cross-talk enrichments and depletions between groups. Finally, temporal response groups of two experiments can be intersected, to identify condition-variant response patterns that represent promising drug-target candidates. Results: We applied TiCoNE to human gene expression data for influenza A virus infection and rhino virus infection, respectively. We then identified coherent temporal response patterns and employed our cross-talk analysis to establish two potential timelines of systems-level host responses for either infection. Next, we compared the two phenotypes and unraveled condition-variant temporal groups interacting on a networks level. The highest-ranking ones we then validated via literature search and wet-lab experiments. This not only confirmed many of our candidates as previously known, but we also identified phospholipid scramblase 1 (encoded by PLSCR1) as a previously not recognized host factor that is essential for influenza A virus infection. Conclusion: With TiCoNE we developed a novel approach for conjointly analyzing molecular networks with time-series expression data and demonstrated its power by identifying temporal drug-targets. We provide proof-of-concept that not only novel targets can be identified using our approach, but also that anti-infective drug target discovery can be enhanced by investigating temporal molecular networks of the host in response to viral infection.

Epigenome-wide association study of peripheral blood mononuclear cells in systemic lupus erythematosus: Identifying DNA methylation signatures associated with interferon-related genes based on ethnicity and SLEDAI

Systemic lupus erythematosus (SLE or lupus) is a heterogeneous autoimmune disease characterized by the involvement of multiple organs and the production of antinuclear antibodies. DNA methylation plays an important role in the pathogenesis of lupus. We have performed an epigenome-wide DNA methylation study in lupus and healthy control (non-lupus) subjects to identify epigenetic patterns in lupus characterized ethnicity and SLE disease activity index (SLEDAI). A total of fifty-seven lupus patients (39 African American (AA) and 18 European American (EA)) and 33 healthy controls (17 AA and 16 EA) were studied. Differential DNA methylation between lupus patients and controls was assessed for approximately 485,000 CpG sites across the genome. We identified 41 differentially methylated sites (associated with 30 genes) between lupus and control s subjects, 85% of which were hypomethylated. Significant hypomethylation of differentially methylated sites was associated with several interferon-related genes, including MX1, IFI44L, PARP9, DT3XL, IFIT1, IFI44, RSAD2, PLSCR1, and IRF7. Several of these associated genes were also hypomethylated in comparisons between AA lupus and AA non-lupus subjects and between lupus patients with SLEDAI>6 and non-lupus subjects. Our analysis of gene expression data through RT-PCR confirmed these findings. Thus, the results indicate epigenetics susceptibility in lupus, which may be associated with SLEDAI score and ethnicity. In addition, our findings support the importance of the Type 1 interferon pathway in lupus pathogenesis.

Interferon priming is essential for human CD34+ cell-derived plasmacytoid dendritic cell maturation and function

Plasmacytoid dendritic cells (pDC) are essential for immune competence. Here we show that pDC precursor differentiated from human CD34+ hematopoietic stem and progenitor cells (HSPC) has low surface expression of pDC markers, and has limited induction of type I interferon (IFN) and IL-6 upon TLR7 and TLR9 agonists treatment; by contrast, cGAS or RIG-I agonists-mediated activation is not altered. Importantly, after priming with type I and II IFN, these precursor pDCs attain a phenotype and functional activity similar to that of peripheral blood-derived pDCs. Data from CRISPR/Cas9-mediated genome editing of HSPCs further show that HSPC-pDCs with genetic modifications can be obtained, and that expression of the IFN-α receptor is essential for the optimal function, but dispensable for the differentiation, of HSPC-pDC percursor. Our results thus demonstrate the biological effects of IFNs for regulating pDC function, and provide the means of generating of gene-modified human pDCs.

STAT3 Cooperates With Phospholipid Scramblase 2 to Suppress Type I Interferon Response

Type I interferon (IFN-I) is a pluripotent cytokine that modulates innate and adaptive immunity. We have previously shown that STAT3 suppresses IFN-I response in a manner dependent on its N-terminal domain (NTD), but independent of its DNA-binding and transactivation ability. Using the yeast two-hybrid system, we have identified phospholipid scramblase 2 (PLSCR2) as a STAT3 NTD-binding partner and a suppressor of IFN-I response. Overexpression of PLSCR2 attenuates ISRE-driven reporter activity, which is further aggravated by co-expression of STAT3. Moreover, PLSCR2 deficiency enhances IFN-I-induced gene expression and antiviral activity without affecting the activation or nuclear translocation of STAT1 and STAT2 or the assembly of ISGF3 complex. Instead, PLSCR2 impedes promoter occupancy by ISGF3, an effect further intensified by the presence of STAT3. Moreover, palmitoylation of PLSCR2 is required for its binding to STAT3 and for this suppressive activity. In addition to STAT3, PLSCR2 also interacts with STAT2, which facilitates the suppressive effect on ISGF3-mediated transcriptional activity. Together, these results define the role of a novel STAT3–PLSCR2 axis in fine-tuning IFN-I response.

Identification of Secreted Proteins Involved in Nonspecific dsRNA-Mediated Lutzomyia longipalpis LL5 Cell Antiviral Response

Hematophagous insects transmit infectious diseases. Sand flies are vectors of leishmaniasis, but can also transmit viruses. We have been studying immune responses of Lutzomyia longipalpis, the main vector of visceral leishmaniasis in the Americas. We identified a non-specific antiviral response in L. longipalpis LL5 embryonic cells when treated with non-specific double-stranded RNAs (dsRNAs). This response is reminiscent of interferon response in mammals. We are investigating putative effectors for this antiviral response. Secreted molecules have been implicated in immune responses, including interferon-related responses. We conducted a mass spectrometry analysis of conditioned medium from LL5 cells 24 and 48 h after dsRNA or mock treatment. We identified 304 proteins. At 24 h, 19 proteins had an abundance equal or greater than 2-fold change, while the levels of 17 proteins were reduced when compared to control cells. At the 48 h time point, these numbers were 33 and 71, respectively. The two most abundant secreted peptides at 24 h in the dsRNA-transfected group were phospholipid scramblase, an interferon-inducible protein that mediates antiviral activity, and forskolin-binding protein (FKBP), a member of the immunophilin family, which mediates the effect of immunosuppressive drugs. The transcription profile of most candidates did not follow the pattern of secreted protein abundance.

Phospholipid scramblase 1 interacts with influenza A virus NP, impairing its nuclear import and thereby suppressing virus replication

Transcription and replication of the influenza A virus (IAV) genome occur in the nucleus of infected cells and are carried out by the viral ribonucleoprotein complex (vRNP). As a major component of the vRNP complex, the viral nucleoprotein (NP) mediates the nuclear import of the vRNP complex via its nuclear localization signals (NLSs). Clearly, an effective way for the host to antagonize IAV infection would be by targeting vRNP nuclear import. Here, we identified phospholipid scramblase 1 (PLSCR1) as a binding partner of NP by using a yeast two-hybrid (Y2H) screen. The interaction between NP and PLSCR1 in mammalian cells was demonstrated by using co-immunoprecipitation and pull-down assays. We found that the stable overexpression of PLSCR1 suppressed the nuclear import of NP, hindered the virus life cycle, and significantly inhibited the replication of various influenza subtypes. In contrast, siRNA knockdown or CRISPR/Cas9 knockout of PLSCR1 increased virus propagation. Further analysis indicated that the inhibitory effect of PLSCR1 on the nuclear import of NP was not caused by affecting the phosphorylation status of NP or by stimulating the interferon (IFN) pathways. Instead, PLSCR1 was found to form a trimeric complex with NP and members of the importin α family, which inhibited the incorporation of importin β, a key mediator of the classical nuclear import pathway, into the complex, thus impairing the nuclear import of NP and suppressing virus replication. Our results demonstrate that PLSCR1 negatively regulates virus replication by interacting with NP in the cytoplasm and preventing its nuclear import.

BAD-LAMP controls TLR9 trafficking and signalling in human plasmacytoid dendritic cells

Toll-like receptors (TLR) are essential components of the innate immune system. Several accessory proteins, such as UNC93B1, are required for transport and activation of nucleic acid sensing Toll-like receptors in endosomes. Here, we show that BAD-LAMP (LAMP5) controls TLR9 trafficking to LAMP1+ late endosomes in human plasmacytoid dendritic cells (pDC), leading to NF-κB activation and TNF production upon DNA detection. An inducible VAMP3+/LAMP2+/LAMP1- endolysosome compartment exists in pDCs from which TLR9 activation triggers type I interferon expression. BAD-LAMP-silencing enhances TLR9 retention in this compartment and consequent downstream signalling events. Conversely, sustained BAD-LAMP expression in pDCs contributes to their lack of type I interferon production after exposure to a TGF-β-positive microenvironment or isolation from human breast tumours. Hence, BAD-LAMP limits interferon expression in pDCs indirectly, by promoting TLR9 sorting to late endosome compartments at steady state and in response to immunomodulatory cues.TLR9 is highly expressed by plasmacytoid dendritic cells and detects nucleic acids, but to discriminate between host and microbial nucleic acids TLR9 is sorted into different endosomal compartments. Here the authors show that BAD-LAMP limits type 1 interferon responses by sorting TLR9 to late endosomal compartments.

The role of human phospholipid scramblases in apoptosis: An overview

Human phospholipid scramblases (hPLSCRs) are a family of four homologous single pass transmembrane proteins (hPLSCR1-4) initially identified as the proteins responsible for Ca²⁺ mediated bidirectional phospholipid translocation in plasma membrane. Though in-vitro assays had provided evidence, the role of hPLSCRs in phospholipid translocation is still debated. Recent reports revealed a new class of proteins, TMEM16 and Xkr8 to exhibit scramblase activity challenging the function of hPLSCRs. Apart from phospholipid scrambling, numerous reports have emphasized the multifunctional roles of hPLSCRs in key cellular processes including tumorigenesis, antiviral defense, protein and DNA interactions, transcriptional regulation and apoptosis. In this review, the role of hPLSCRs in mediating cell death through phosphatidylserine exposure, interaction with death receptors, cardiolipin exposure, heavy metal and radiation induced apoptosis and pathological apoptosis followed by their involvement in cancer cells are discussed. This review aims to connect the multifunctional characteristics of hPLSCRs to their decisive involvement in apoptotic pathways.

Phospholipid scramblase 1 amplifies anaphylactic reactions in vivo

Mast cells are critical actors of hypersensitivity type I (allergic) reactions by the release of vasoactive and proinflammatory mediators following their activation by aggregation of the high-affinity receptor for immunoglobulin E (FcεRI). We have previously identified Phospholipid Scramblase 1 (PLSCR1) as a new molecular intermediate of FcεRI signaling that amplifies degranulation of the rat mast cell line RBL-2H3. Here we characterized primary mast cells from Plscr1-/- mice. The absence of PLSCR1 expression did not impact mast cell differentiation as evidenced by unaltered FcεRI expression, general morphology, amount of histamine stored and expression of FcεRI signal effector molecules. No detectable mast cell deficiency was observed in Plscr1-/- adult mice. In dose-response and time-course experiments, primary cultures of mast cells (bone marrow-derived mast cells and peritoneal cell-derived mast cells) generated from Plscr1-/- mice exhibited a reduced release of β-hexosaminidase upon FcεRI engagement as compared to their wild-type counterparts. In vivo, Plscr1-/- mice were protected in a model of passive systemic anaphylaxis when compared to wild-type mice, which was consistent with an observed decrease in the amounts of histamine released in the serum of Plscr1-/- mice during the reaction. Therefore, PLSCR1 aggravates anaphylactic reactions by increasing FcεRI-dependent mast cell degranulation. PLSCR1 could be a new therapeutic target in allergy.

Phosphatidylserine Exposure Controls Viral Innate Immune Responses by Microglia

Microglia are the intrinsic immune sentinels of the central nervous system. Their activation restricts tissue injury and pathogen spread, but in some settings, including viral infection, this response can contribute to cell death and disease. Identifying mechanisms that control microglial responses is therefore an important objective. Using replication-incompetent adenovirus 5 (Ad5)-based vectors as a model, we investigated the mechanisms through which microglia recognize and respond to viral uptake. Transgenic, immunohistochemical, molecular-genetic, and fluorescence imaging approaches revealed that phosphatidylserine (PtdSer) exposure on the outer leaflet of transduced cells triggers their engulfment by microglia through TAM receptor-dependent mechanisms. We show that inhibition of phospholipid scramblase 1 (PLSCR1) activity reduces intracellular calcium dysregulation, prevents PtdSer externalization, and enables months-long protection of vector-transduced, transgene-expressing cells from microglial phagocytosis. Our study identifies PLSCR1 as a potent target through which the innate immune response to viral vectors, and potentially other stimuli, may be controlled.

Cell-Based Systems Biology Analysis of Human AS03-Adjuvanted H5N1 Avian Influenza Vaccine Responses: A Phase I Randomized Controlled Trial

Background

Vaccine development for influenza A/H5N1 is an important public health priority, but H5N1 vaccines are less immunogenic than seasonal influenza vaccines. Adjuvant System 03 (AS03) markedly enhances immune responses to H5N1 vaccine antigens, but the underlying molecular mechanisms are incompletely understood.

Objective and Methods

We compared the safety (primary endpoint), immunogenicity (secondary), gene expression (tertiary) and cytokine responses (exploratory) between AS03-adjuvanted and unadjuvanted inactivated split-virus H5N1 influenza vaccines. In a double-blinded clinical trial, we randomized twenty adults aged 18–49 to receive two doses of either AS03-adjuvanted (n = 10) or unadjuvanted (n = 10) H5N1 vaccine 28 days apart. We used a systems biology approach to characterize and correlate changes in serum cytokines, antibody titers, and gene expression levels in six immune cell types at 1, 3, 7, and 28 days after the first vaccination.

Results

Both vaccines were well-tolerated. Nine of 10 subjects in the adjuvanted group and 0/10 in the unadjuvanted group exhibited seroprotection (hemagglutination inhibition antibody titer > 1:40) at day 56. Within 24 hours of AS03-adjuvanted vaccination, increased serum levels of IL-6 and IP-10 were noted. Interferon signaling and antigen processing and presentation-related gene responses were induced in dendritic cells, monocytes, and neutrophils. Upregulation of MHC class II antigen presentation-related genes was seen in neutrophils. Three days after AS03-adjuvanted vaccine, upregulation of genes involved in cell cycle and division was detected in NK cells and correlated with serum levels of IP-10. Early upregulation of interferon signaling-related genes was also found to predict seroprotection 56 days after first vaccination.

Conclusions

Using this cell-based systems approach, novel mechanisms of action for AS03-adjuvanted pandemic influenza vaccination were observed.

Trial Registration

ClinicalTrials.gov NCT01573312

Transcriptional Repression of IFN Regulatory Factor 7 by MYC Is Critical for Type I IFN Production in Human Plasmacytoid Dendritic Cells

Type I IFNs are crucial mediators of human innate and adaptive immunity and are massively produced from plasmacytoid dendritic cells (pDCs). IFN regulatory factor (IRF)7 is a critical regulator of type I IFN production when pathogens are detected by TLR 7/9 in pDC. However, hyperactivation of pDC can cause life-threatening autoimmune diseases. To avoid the deleterious effects of aberrant pDC activation, tight regulation of IRF7 is required. Nonetheless, the detailed mechanisms of how IRF7 transcription is regulated in pDC are still elusive. MYC is a well-known highly pleiotropic transcription factor; however, the role of MYC in pDC function is not well defined yet. To identify the role of transcription factor MYC in human pDC, we employed a knockdown technique using human pDC cell line, GEN2.2. When we knocked down MYC in the pDC cell line, production of IFN-stimulated genes was dramatically increased and was further enhanced by the TLR9 agonist CpGB. Interestingly, MYC is shown to be recruited to the IRF7 promoter region through interaction with nuclear receptor corepressor 2/histone deacetylase 3 for its repression. In addition, activation of TLR9-mediated NF-κB and MAPK and nuclear translocation of IRF7 were greatly enhanced by MYC depletion. Pharmaceutical inhibition of MYC recovered IRF7 expression, further confirming the negative role of MYC in the antiviral response by pDC. Therefore, our results identify the novel immunomodulatory role of MYC in human pDC and may add to our understanding of aberrant pDC function in cancer and autoimmune disease.

Grancalcin (GCA) modulates Toll-like receptor 9 (TLR9) mediated signaling through its direct interaction with TLR9

Toll-like receptors (TLRs) are playing important roles in stimulating the innate immune response and intensifying adaptive immune response against invading pathogens. Appropriate regulation of TLR activation is important to maintain a balance between preventing tumor activation and inhibiting autoimmunity. Toll-like receptor 9 (TLR9) senses microbial DNA in the endosomes of plasmacytoid dendritic cells and triggers myeloid differentiation primary response gene 88 (MyD88) dependent nuclear factor kappa B (NF-κB) pathways and type I interferon (IFN) responses. However, mechanisms of how TLR9 signals are mediated and which molecules are involved in controlling TLR9 functions remain poorly understood. Here, we report that penta EF-hand protein grancalcin (GCA) interacts and binds with TLR9 in a yeast two-hybrid system and an overexpression system. Using siRNA-mediated knockdown experiments, we also revealed that GCA positively regulates type I IFN production, cytokine/chemokine production through nuclear localization of interferon regulatory factor 7 (IRF7), NF-κB activation, and mitogen-activated protein kinase (MAPK) activation in plasmacytoid dendritic cells. Our results indicate that heterodimerization of GCA and TLR9 is important for TLR9-mediated downstream signaling and might serve to fine tune processes against viral infection.

Tubby-like protein superfamily member PLSCR3 functions as a negative regulator of adipogenesis in mouse 3T3-L1 preadipocytes by suppressing induction of late differentiation stage transcription factors

Decrease in intracellular amount of phospholipid scramblase 3 (PLSCR3) is accompanied by enhanced unconventional secretion during differentiation of mouse preadipocytic 3T3-L1 cells. Forced overexpression of PLSCR3 in 3T3-L1 cells inhibited adipogenesis by suppressing induction of late stage pro-adipogenic transcription factors.

PLSCR3 (phospholipid scramblase 3, Scr3) belongs to the superfamily of membrane-associated transcription regulators named Tubby-like proteins (TULPs). Physiological phospholipid scrambling activities of PLSCRs in vivo have been skeptically argued, and knowledge of the biological functions of Scr3 is limited. We investigated the expression of Scr3 during differentiation of mouse 3T3-L1 preadipocytes by Western blotting (WB) and by reverse-transcription and real-time quantitative PCR (RT-qPCR). The Scr3 protein decreased during 3T3-L1 differentiation accompanied by a reduction in the mRNA level, and there was a significant increase in the amount of Scr3 protein secreted into the culture medium in the form of extracellular microvesicles (exosomes). On the other hand, Scr3 expression did not significantly decrease, and the secretion of Scr3 in 3T3 Swiss-albino fibroblasts (a parental cell-line of 3T3-L1) was not increased by differentiation treatment. Overexpression of human Scr3 during 3T3-L1 differentiation suppressed triacylglycerol accumulation and inhibited induction of the mRNAs of late stage pro-adipogenic transcription factors [CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ)] and X-box-binding protein 1 (XBP1). Expression of early stage pro-adipogenic transcription factors (C/EBPβ and C/EBPδ) was not significantly affected. These results suggest that Scr3 functions as a negative regulator of adipogenesis in 3T3-L1 cells at a specific differentiation stage and that decrease in the intracellular amount of Scr3 protein caused by reduction in Scr3 mRNA expression and enhanced secretion of Scr3 protein appears to be important for appropriate adipocyte differentiation.

The multifaceted biology of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) are a unique DC subset that specializes in the production of type I interferons (IFNs). pDCs promote antiviral immune responses and have been implicated in the pathogenesis of autoimmune diseases that are characterized by a type I IFN signature. However, pDCs can also induce tolerogenic immune responses. In this Review, we summarize recent progress in the field of pDC biology, focusing on the molecular mechanisms that regulate the development and functions of pDCs, the pathways involved in their sensing of pathogens and endogenous nucleic acids, their functions at mucosal sites, and their roles in infection, autoimmunity and cancer.

SCARB2/LIMP-2 Regulates IFN Production of Plasmacytoid Dendritic Cells by Mediating Endosomal Translocation of TLR9 and Nuclear Translocation of IRF7

Scavenger receptor class B, member 2 (SCARB2) is essential for endosome biogenesis and reorganization and serves as a receptor for both β-glucocerebrosidase and enterovirus 71. However, little is known about its function in innate immune cells. In this study, we show that, among human peripheral blood cells, SCARB2 is most highly expressed in plasmacytoid dendritic cells (pDCs), and its expression is further upregulated by CpG oligodeoxynucleotide stimulation. Knockdown of SCARB2 in pDC cell line GEN2.2 dramatically reduces CpG-induced type I IFN production. Detailed studies reveal that SCARB2 localizes in late endosome/lysosome of pDCs, and knockdown of SCARB2 does not affect CpG oligodeoxynucleotide uptake but results in the retention of TLR9 in the endoplasmic reticulum and an impaired nuclear translocation of IFN regulatory factor 7. The IFN-I production by TLR7 ligand stimulation is also impaired by SCARB2 knockdown. However, SCARB2 is not essential for influenza virus or HSV-induced IFN-I production. These findings suggest that SCARB2 regulates TLR9-dependent IFN-I production of pDCs by mediating endosomal translocation of TLR9 and nuclear translocation of IFN regulatory factor 7.

TRIM35 negatively regulates TLR7- and TLR9-mediated type I interferon production by targeting IRF7

Toll-like receptor 7 (TLR7) and TLR9 sense viral nucleic acids and induce type I IFN production, which must be properly controlled to avoid autoimmune diseases. Here, we report the negative regulation of TLR7/9-mediated type I IFN production by TRIM35. TRIM35 expression is induced by TLR7/9 stimulation and then interacts with IRF7, which is the master regulator of type I IFN response. Furthermore, TRIM35 promotes the K48-linked ubiquitination of IRF7 and induces its degradation via a proteasome-dependent pathway. Therefore, TRIM35 is a negative feedback regulator of TLR7/9-mediated type I IFN production due to its ability to suppress the stability of IRF7.

Induction of PLSCR1 in a STING/IRF3-dependent manner upon vector transfection in ovarian epithelial cells

Toll-like receptors (TLRs) are the primary sensors of the innate immune system that recognize pathogenic nucleic acids including double-stranded plasmid DNA (dsDNA). TLR signaling activates multiple pathways including IRF3 which is involved in transcriptional induction of inflammatory cytokines (i.e. interferons (IFNs)). Phospholipid scramblase 1, PLSCR1, is a highly inducible IFN-regulated gene mediating anti-viral properties of IFNs. Herein, we report a novel finding that dsDNA transfection in T80 immortalized normal ovarian surface epithelial cell line leads to a marked increase in PLSCR1 mRNA and protein. We also noted a comparable response in primary mammary epithelial cells (HMECs). Similar to IFN-2α treated cells, de novo synthesized PLSCR1 was localized predominantly to the plasma membrane. dsDNA transfection, in T80 and HMEC cells, led to activation of MAPK and IRF3. Although inhibition of MAPK (using U0126) did not modulate PLSCR1 mRNA and protein, IRF3 knockdown (using siRNA) significantly ablated the PLSCR1 induction. In prior studies, the activation of IRF3 was shown to be mediated by cGAS-STING pathway. To investigate the contribution of STING to PLSCR1 induction, we utilized siRNA to reduce STING expression and observed that PLSCR1 protein was markedly reduced. In contrast to normal T80/HMECs, the phosphorylation of IRF3 as well as induction of STING and PLSCR1 were absent in ovarian cancer cells (serous, clear cell, and endometrioid) suggesting that the STING/IRF3 pathway may be dysregulated in these cancer cells. However, we also noted induction of different TLR and IFN mRNAs between the T80 and HEY (serous epithelial ovarian carcinoma) cell lines upon dsDNA transfection. Collectively, these results indicate that the STING/IRF3 pathway, activated following dsDNA transfection, contributes to upregulation of PLSCR1 in ovarian epithelial cells.

Roles and regulation of phospholipid scramblases

Phospholipid scramblase activity is involved in the collapse of phospholipid (PL) asymmetry at the plasma membrane leading to externalization of phosphatidylserine. This activity is crucial for initiation of the blood coagulation cascade and for recognition/elimination of apoptotic cells by macrophages. Efforts to identify gene products associated with this activity led to the characterization of PL scramblase (PLSCR) and XKR family members which contribute to phosphatidylserine exposure in response to apoptotic stimuli. Meanwhile, TMEM16 family members were identified to externalize phosphatidylserine in response to elevated calcium in Scott syndrome platelets, which is critical for activation of the coagulation cascade. Herein, we report their mechanisms of gene regulation, molecular functions independent of their scrambling activity, and their potential roles in pathogenic conditions.

A promoter-level mammalian expression atlas

Regulated transcription controls the diversity, developmental pathways and spatial organization of the hundreds of cell types that make up a mammal. Using single-molecule cDNA sequencing, we mapped transcription start sites (TSSs) and their usage in human and mouse primary cells, cell lines and tissues to produce a comprehensive overview of mammalian gene expression across the human body. We find that few genes are truly 'housekeeping', whereas many mammalian promoters are composite entities composed of several closely separated TSSs, with independent cell-type-specific expression profiles. TSSs specific to different cell types evolve at different rates, whereas promoters of broadly expressed genes are the most conserved. Promoter-based expression analysis reveals key transcription factors defining cell states and links them to binding-site motifs. The functions of identified novel transcripts can be predicted by coexpression and sample ontology enrichment analyses. The functional annotation of the mammalian genome 5 (FANTOM5) project provides comprehensive expression profiles and functional annotation of mammalian cell-type-specific transcriptomes with wide applications in biomedical research.

N-terminal proline-rich domain is required for scrambling activity of human phospholipid scramblases

Human phospholipid scramblase 1 (hPLSCR1), a type II integral class membrane protein, is known to mediate bidirectional scrambling of phospholipids in a Ca(2+)-dependent manner. hPLSCR2, a homolog of hPLSCR1 that lacks N-terminal proline-rich domain (PRD), did not show scramblase activity. We attribute this absence of scramblase activity of hPLSCR2 to the lack of N-terminal PRD. Hence to investigate the above hypothesis, we added the PRD of hPLSCR1 to hPLSCR2 (PRD-hPLSCR2) and checked whether scramblase activity was restored. Functional assays showed that the addition of PRD to hPLSCR2 restored scrambling activity, and deletion of PRD in hPLSCR1 (ΔPRD-hPLSCR1) resulted in a lack of activity. These results suggest that PRD is crucial for the function of the protein. The effects of the PRD deletion in hPLSCR1 and the addition of PRD to hPLSCR2 were characterized using various spectroscopic techniques. Our results clearly showed that hPLSCR1 and PRD-hPLSCR2 showed Ca(2+)-dependent aggregation and scrambling activity, whereas hPLSCR2 and ΔPRD-hPLSCR1 did not show aggregation and activity. Thus we conclude that scramblases exhibit Ca(2+)-dependent scrambling activity by aggregation of protein. Our results provide a possible mechanism for phospholipid scrambling mediated by PLSCRs and the importance of PRD in its function and cellular localization.

Phospholipid scramblase-1-induced lipid reorganization regulates compensatory endocytosis in neuroendocrine cells

Calcium-regulated exocytosis in neuroendocrine cells and neurons is accompanied by the redistribution of phosphatidylserine (PS) to the extracellular space, leading to a disruption of plasma membrane asymmetry. How and why outward translocation of PS occurs during secretion are currently unknown. Immunogold labeling on plasma membrane sheets coupled with hierarchical clustering analysis demonstrate that PS translocation occurs at the vicinity of the secretory granule fusion sites. We found that altering the function of the phospholipid scramblase-1 (PLSCR-1) by expressing a PLSCR-1 calcium-insensitive mutant or by using chromaffin cells from PLSCR-1⁻/⁻ mice prevents outward translocation of PS in cells stimulated for exocytosis. Remarkably, whereas transmitter release was not affected, secretory granule membrane recapture after exocytosis was impaired, indicating that PLSCR-1 is required for compensatory endocytosis but not for exocytosis. Our results provide the first evidence for a role of specific lipid reorganization and calcium-dependent PLSCR-1 activity in neuroendocrine compensatory endocytosis.

Interaction of the phospholipid scramblase 1 with HIV-1 Tat results in the repression of Tat-dependent transcription

Human phospholipid scramblase 1 (PLSCR1) is an interferon (IFN)-stimulated gene and possesses an IFN-mediated antiviral function. We show here that PLSCR1 directly interacts with human immunodeficiency virus type-1 (HIV-1) Tat. This interaction occurs both in vitro and in vivo through amino acids 160-250 of PLSCR1. Overexpression of PLSCR1 efficiently represses the Tat-dependent transactivation of the HIV-1 long terminal repeat (LTR) and reduces the nuclear translocation of Tat. In addition, shRNA-mediated suppression of endogenous PLSCR1 expression enhances the levels of gag mRNA in an HIV-1-infected T-cell line. These findings indicate that PLSCR1 negatively regulates the Tat-dependent transactivation of the HIV-1 LTR during HIV-1 infection.

ALG-2-interacting Tubby-like protein superfamily member PLSCR3 is secreted by an exosomal pathway and taken up by recipient cultured cells

PLSCRs (phospholipid scramblases) are palmitoylated membrane-associating proteins. Regardless of the given names, their physiological functions are not clear and thought to be unrelated to phospholipid scrambling activities observed in vitro. Using a previously established cell line of HEK-293 (human embryonic kidney-293) cells constitutively expressing human Scr3 (PLSCR3) that interacts with ALG-2 (apoptosis-linked gene 2) Ca²⁺-dependently, we found that Scr3 was secreted into the culture medium. Secretion of Scr3 was suppressed by 2-BP (2-bromopalmitate, a palmitoylation inhibitor) and by GW4869 (an inhibitor of ceramide synthesis). Secreted Scr3 was recovered in exosomal fractions by sucrose density gradient centrifugation. Palmitoylation sites and the N-terminal Pro-rich region were necessary for efficient secretion, but ABSs (ALG-2-binding sites) were dispensable. Overexpression of GFP (green fluorescent protein)-fused VPS4B^E235Q, a dominant negative mutant of an AAA (ATPase associated with various cellular activities) ATPase with a defect in disassembling ESCRT (endosomal sorting complex required for transport)-III subunits, significantly reduced secretion of Scr3. Immunofluorescence microscopic analyses showed that Scr3 was largely localized to enlarged endosomes induced by overexpression of a GFP-fused constitutive active mutant of Rab5A (GFP–Rab5A^Q79L). Secreted Scr3 was taken up by HeLa cells, suggesting that Scr3 functions as a cell-to-cell transferable modulator carried by exosomes in a paracrine manner.

Regulation of TLR7/9 signaling in plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs), also known as type I interferon (IFN)-producing cells, are specialized immune cells characterized by their extraordinary capabilities of mounting rapid and massive type I IFN response to nucleic acids derived from virus, bacteria or dead cells. PDCs selectively express endosomal Toll-like receptor (TLR) 7 and TLR9, which sense viral RNA and DNA respectively. Following type I IFN and cytokine responses, pDCs differentiate into antigen presenting cells and acquire the ability to regulate T cell-mediated adaptive immunity. The functions of pDCs have been implicated not only in antiviral innate immunity but also in immune tolerance, inflammation and tumor microenvironments. In this review, we will focus on TLR7/9 signaling and their regulation by pDC-specific receptors.