The unique ORF8 protein from SARS-CoV-2 binds to human dendritic cells and induces a hyper-inflammatory cytokine storm.

The novel coronavirus pandemic, first reported in December 2019, was caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). SARS-CoV-2 infection leads to a strong immune response and activation of antigen-presenting cells, which can elicit acute respiratory distress syndrome (ARDS) characterized by the rapid onset of widespread inflammation, the so-called cytokine storm. In response to viral infections, monocytes are recruited into the lung and subsequently differentiate into dendritic cells (DCs). DCs are critical players in the development of the acute lung inflammation that causes ARDS. Here we focus on the interaction of a specific SARS-CoV-2 open reading frame protein, ORF8, with DCs. We show that ORF8 binds to DCs, causes a pre-maturation of differentiating DCs, and induces the secretion of multiple proinflammatory cytokines by these cells. In addition, we identified DC-SIGN as a possible interaction partner of ORF8 on DCs. Blockade of ORF8 leads to reduced production of IL-1ß, IL-6, IL-12p70, TNF-α, MCP-1 (also named CCL2), and IL-10 by DCs. Therefore, a neutralizing antibody blocking the ORF8-mediated cytokine and chemokine response could be an improved therapeutical strategy against severe SARS-CoV-2.

COVID-19 Infection Induce miR-371a-3p Upregulation Resulting in Influence on Male Fertility.

In December 2019, the first case of COVID-19 was reported and since then several groups have already published that the virus can be present in the testis. To study the influence of SARS-CoV-2 which cause a dysregulation of the androgen receptor (AR) level, thereby leading to fertility problems and inducing germ cell testicular changes in patients after the infection. Formalin-Fixed-Paraffin-Embedded (FFPE) testicular samples from patients who died with or as a result of COVID-19 (n = 32) with controls (n = 6), inflammatory changes (n = 9), seminoma with/without metastasis (n = 11) compared with healthy biopsy samples (n = 3) were analyzed and compared via qRT-PCR for the expression of miR-371a-3p. An immunohistochemical analysis (IHC) and ELISA were performed in order to highlight the miR-371a-3p targeting the AR. Serum samples of patients with mild or severe COVID-19 symptoms (n = 34) were analyzed for miR-371a-3p expression. In 70% of the analyzed postmortem testicular tissue samples, a significant upregulation of the miR-371a-3p was detected, and 75% of the samples showed a reduced spermatogenesis. In serum samples, the upregulation of the miR-371a-3p was also detectable. The upregulation of the miR-371a-3p is responsible for the downregulation of the AR in SARS-CoV-2-positive patients, resulting in decreased spermatogenesis. Since the dysregulation of the AR is associated with infertility, further studies have to confirm if the identified dysregulation is regressive after a declining infection.

Preincubation of donor tissue with a VEGF cytokine trap promotes subsequent high-risk corneal transplant survival.

AIMS: Pathological neovascularisation of the host bed and the transplant itself is the main risk factor for graft rejection after corneal transplantation. This study aims to prevent this process by preincubation of the corneal donor tissue ex vivo with an antivascular endothelial growth factor (VEGF) cytokine trap blocking additional postsurgical hemangiogenesis and lymphangiogenesis to promote high-risk graft survival. METHODS: The donor tissue was preincubated with a VEGFR1R2 cytokine trap for 24 hours prior to murine high-risk corneal transplantation (human IgG Fc was used as the control). The distribution of VEGFR1R2 Trap in the cornea was investigated by immunohistochemistry. Corneas were excised to quantify the blood vessels (BVs) and lymphatic vessels (LVs) and draining lymph nodes (dLNs) were harvested to analyse the phenotype of dendritic cells (DCs) and T cells at week 1, 2 and 8 post-transplantation. Graft survival was compared between preincubation with VEGFR1R2 Trap and human IgG Fc in high-risk recipients. RESULTS: VEGFR1R2 Trap was present in the graft for at least 2 weeks after surgery and additionally diffused into the corneal recipient. BVs, LVs and macrophages in the whole cornea were significantly decreased 1-week and 2-week post-transplantation (p<0.05). In dLNs the frequency of CD11c+DCs was significantly reduced, whereas CD200R+ regulatory DCs were significantly increased after keratoplasty (p<0.05). Furthermore, long-term high-risk graft survival was significantly improved (p<0.01). CONCLUSIONS: Preincubation of corneal donor tissue with a VEGFR1R2 cytokine trap can significantly promote subsequent high-risk corneal transplant survival and thereby opens new treatment avenues for high-risk corneal transplantation.

Topical VEGF-C/D Inhibition Prevents Lymphatic Vessel Ingrowth into Cornea but Does Not Improve Corneal Graft Survival.

Vascular endothelial growth factor-C/D (VEGF-C/D) regulates lymphangiogenesis. Ingrowth of lymphatic vessels is negatively associated with corneal transplantation success. In this study, we therefore analyzed the effect local blockade of VEGF-C/D has on inflamed corneas. We used the murine model of suture-induced neovascularization and subsequent high-risk corneal transplantation. Mice were treated with a VEGF-C/D trap prior to transplantation. Topical inhibition of VEGF-C/D significantly reduced lymphatic vessel ingrowth, but increased Macrophage numbers in the cornea. Furthermore, corneal transplantation success was not improved by the topical application of the compound. This study demonstrates that local VEGF-C/D inhibition is insufficient to increases corneal transplantation success, likely due to interaction with immune cells.

Immunomodulatory Strategies Targeting Dendritic Cells to Improve Corneal Graft Survival.

Even though the cornea is regarded as an immune-privileged tissue, transplantation always comes with the risk of rejection due to mismatches between donor and recipient. It is common sense that an alternative to corticosteroids as the current gold standard for treatment of corneal transplantation is needed. Since blood and lymphatic vessels have been identified as a severe risk factor for corneal allograft survival, much research has focused on vessel regression or inhibition of hem- and lymphangiogenesis in general. However, lymphatic vessels have been identified as required for the inflammation's resolution. Therefore, targeting other players of corneal engraftment could reveal new therapeutic strategies. The establishment of a tolerogenic microenvironment at the graft site would leave the recipient with the ability to manage pathogenic conditions independent from transplantation. Dendritic cells (DCs) as the central player of the immune system represent a target that allows the induction of tolerogenic mechanisms by many different strategies. These strategies are reviewed in this article with regard to their success in corneal transplantation.

Interferon-Inducible MicroRNA miR-128 Modulates HIV-1 Replication by Targeting TNPO3 mRNA.

The HIV/AIDS pandemic remains an important threat to human health. We have recently demonstrated that a novel microRNA (miR), miR-128, represses retrotransposon long interspaced element 1 (L1) by a dual mechanism, namely, by directly targeting the coding region of the L1 RNA and by repressing a required nuclear import factor (TNPO1). We have further determined that miR-128 represses the expression of all three TNPO proteins (transportins TNPO1, TNPO2, and TNPO3). Here, we establish that miR-128 also influences HIV-1 replication by repressing TNPO3, a factor that regulates HIV-1 nuclear import and viral; replication of TNPO3 is well established to regulate HIV-1 nuclear import and viral replication. Here, we report that type I interferon (IFN)-inducible miR-128 directly targets two sites in the TNPO3 mRNA, significantly downregulating TNPO3 mRNA and protein expression levels. Challenging miR-modulated Jurkat cells or primary CD4+ T-cells with wild-type (WT), replication-competent HIV-1 demonstrated that miR-128 reduces viral replication and delays spreading of infection. Manipulation of miR-128 levels in HIV-1 target cell lines and in primary CD4+ T-cells by overexpression or knockdown showed that reduction of TNPO3 levels by miR-128 significantly affects HIV-1 replication but not murine leukemia virus (MLV) infection and that miR-128 modulation of HIV-1 replication is reduced with TNPO3-independent HIV-1 virus, suggesting that miR-128-indued TNPO3 repression contributes to the inhibition of HIV-1 replication. Finally, we determine that anti-miR-128 partly neutralizes the IFN-mediated block of HIV-1. Thus, we have established a novel role of miR-128 in antiviral defense in human cells, namely inhibiting HIV-1 replication by altering the cellular milieu through targeting factors that include TNPO3.IMPORTANCE HIV-1 is the causative agent of AIDS. During HIV-1 infection, type I interferons (IFNs) are induced, and their effectors limit HIV-1 replication at multiple steps in its life cycle. However, the cellular targets of INFs are still largely unknown. In this study, we identified the interferon-inducible microRNA (miR) miR-128, a novel antiviral mediator that suppresses the expression of the host gene TNPO3, which is known to modulate HIV-1 replication. Notably, we observe that anti-miR-128 partly neutralizes the IFN-mediated block of HIV-1. Elucidation of the mechanisms through which miR-128 impairs HIV-1 replication may provide novel candidates for the development of therapeutic interventions.

Modulation of the IL-6-Signaling Pathway in Liver Cells by miRNAs Targeting gp130, JAK1, and/or STAT3.

Interleukin-6 (IL-6)-type cytokines share the common receptor glycoprotein 130 (gp130), which activates a signaling cascade involving Janus kinases (JAKs) and signal transducer and activator of transcription (STAT) transcription factors. IL-6 and/or its signaling pathway is often deregulated in diseases, such as chronic liver diseases and cancer. Thus, the identification of compounds inhibiting this pathway is of interest for future targeted therapies. We established novel cellular screening systems based on a STAT-responsive reporter gene (Cypridina luciferase). Of a library containing 538 microRNA (miRNA) mimics, several miRNAs affected hyper-IL-6-induced luciferase activities. When focusing on candidate miRNAs specifically targeting 3' UTRs of signaling molecules of this pathway, we identified, e.g., miR-3677-5p as a novel miRNA affecting protein expression of both STAT3 and JAK1, whereas miR-16-1-3p, miR-4473, and miR-520f-3p reduced gp130 surface expression. Interestingly, combination treatment with 2 or 3 miRNAs targeting gp130 or different signaling molecules of the pathway did not increase the inhibitory effects on phospho-STAT3 levels and STAT3 target gene expression compared to treatment with single mimics. Taken together, we identified a set of miRNAs of potential therapeutic value for cancer and inflammatory diseases, which directly target the expression of molecules within the IL-6-signaling pathway and can dampen inflammatory signal transduction.

Cytokine-mediated modulation of the hepatic miRNome: miR-146b-5p is an IL-6-inducible miRNA with multiple targets.

Interleukin-6 (IL-6)-type cytokines play important roles in liver (patho-)biology. For instance, they regulate the acute phase response to inflammatory signals and are involved in hepatocarcinogenesis. Much is known about the regulation of protein-coding genes by cytokines whereas their effects on the miRNome is less well understood. We performed a microarray screen to identify microRNAs (miRNAs) in human hepatocytes which are modulated by IL-6-type cytokines. Using samples of 2 donors, 27 and 68 miRNAs (out of 1,733) were found to be differentially expressed upon stimulation with hyper-IL-6 (HIL-6) for up to 72 h, with an overlap of 15 commonly regulated miRNAs. qPCR validation revealed that miR-146b-5p was also consistently up-regulated in hepatocytes derived from 2 other donors. Interestingly, miR-146b-5p (but not miR-146a-5p) was induced by IL-6-type cytokines (HIL-6 and OSM) in non-transformed liver-derived PH5CH8 and THLE2 cells and in Huh-7 hepatoma cells, but not in HepG2 or Hep3B hepatoma cells. We did not find evidence for a differential regulation of miR-146b-5p expression by promoter methylation, also when analyzing the TCGA data set on liver cancer samples. Inducible overexpression of miR-146b-5p in PH5CH8 cells followed by RNA-Seq analysis revealed effects on multiple mRNAs, including those encoding IRAK1 and TRAF6 crucial for Toll-like receptor signaling. Indeed, LPS-mediated signaling was attenuated upon overexpression of miR-146b-5p, suggesting a regulatory loop to modulate inflammatory signaling in hepatocytes. Further validation experiments suggest DNAJC6, MAGEE1, MPHOSPH6, PPP2R1B, SLC10A3, SNRNP27, and TIMM17B to be novel targets for miR-146b-5p (and miR-146a-5p).

MicroRNA miR-128 represses LINE-1 (L1) retrotransposition by down-regulating the nuclear import factor TNPO1.

Repetitive elements, including LINE-1 (L1), comprise approximately half of the human genome. These elements can potentially destabilize the genome by initiating their own replication and reintegration into new sites (retrotransposition). In somatic cells, transcription of L1 elements is repressed by distinct molecular mechanisms, including DNA methylation and histone modifications, to repress transcription. Under conditions of hypomethylation (e.g. in tumor cells), a window of opportunity for L1 derepression arises, and additional restriction mechanisms become crucial. We recently demonstrated that the microRNA miR-128 represses L1 activity by directly binding to L1 ORF2 RNA. In this study, we tested whether miR-128 can also control L1 activity by repressing cellular proteins important for L1 retrotransposition. We found that miR-128 targets the 3' UTR of nuclear import factor transportin 1 (TNPO1) mRNA. Manipulation of miR-128 and TNPO1 levels demonstrated that induction or depletion of TNPO1 affects L1 retrotransposition and nuclear import of an L1-ribonucleoprotein complex (using L1-encoded ORF1p as a proxy for L1-ribonucleoprotein complexes). Moreover, TNPO1 overexpression partially reversed the repressive effect of miR-128 on L1 retrotransposition. Our study represents the first description of a protein factor involved in nuclear import of the L1 element and demonstrates that miR-128 controls L1 activity in somatic cells through two independent mechanisms: direct binding to L1 RNA and regulation of a cellular factor necessary for L1 nuclear import and retrotransposition.

The Potential of MicroRNAs as Novel Biomarkers for Transplant Rejection.

The control of gene expression by microRNAs (miRNAs, miR) influences many cellular functions, including cellular differentiation, cell proliferation, cell development, and functional regulation of the immune system. Recently, miRNAs have been detected in serum, plasma, and urine and circulating miR profiles have been associated with a variety of diseases. Rejection is one of the major causes of allograft failure and preventing and treating acute rejection are the central task for clinicians working with transplant patients. Invasive biopsies used in monitoring rejection are burdensome and risky to transplant patients. Novel and easily accessible biomarkers of acute rejection could make it possible to detect rejection earlier and make more fine-tuned calibration of immunosuppressive or new target treatment possible. In this review, we discuss whether circulating miRNA can serve as an early noninvasive diagnostic biomarker and an expression fingerprint of allograft rejection and transplant failure. Understanding the regulatory interplay of relevant miRNAs and the rejecting allograft will result in a better understanding of the molecular pathophysiology of alloimmune injury.

Interferons Induce Expression of SAMHD1 in Monocytes through Down-regulation of miR-181a and miR-30a.

SAMHD1 is a phosphohydrolase maintaining cellular dNTP homeostasis but also acts as a critical regulator in innate immune responses due to its antiviral activity and association with autoimmune disease, leading to aberrant activation of interferon. SAMHD1 expression is differentially regulated by interferon in certain primary cells, but the underlying mechanism is not understood. Here, we report a detailed characterization of the promotor region, the 5'- and 3'-untranslated region (UTR) of SAMHD1, and the mechanism responsible for the cell type-dependent up-regulation of SAMHD1 protein by interferon. We demonstrate that induction of SAMHD1 by type I and II interferons depends on 3'-UTR post-transcriptional regulation, whereas the promoter drives basal expression levels. We reveal novel functional target sites for the microRNAs miR-181a, miR-30a, and miR-155 in the SAMHD1 3'-UTR. Furthermore, we demonstrate that down-regulation of endogenous miR-181a and miR-30a levels inversely correlates with SAMHD1 protein up-regulation upon type I and II interferon stimulation in primary human monocytes. These miRNAs are not modulated by interferon in macrophages or dendritic cells, and consequently protein levels of SAMHD1 remain unchanged. These results suggest that SAMHD1 is a non-classical interferon-stimulated gene regulated through cell type-dependent down-regulation of miR-181a and miR-30a in innate sentinel cells.

miR-128 represses L1 retrotransposition by binding directly to L1 RNA.

Long interspersed element 1 (LINE-1 or L1) retrotransposons compose 17% of the human genome. Active L1 elements are capable of replicative transposition (mobilization) and can act as drivers of genetic diversity. However, this mobilization is mutagenic and may be detrimental to the host, and therefore it is under strict control. Somatic cells usually silence L1 activity by DNA methylation of the L1 promoter. In hypomethylated cells, such as cancer cells and induced pluripotent stem cells (iPSCs), a window of opportunity for L1 reactivation emerges, and with it comes an increased risk of genomic instability and tumorigenesis. Here we show that miR-128 represses new retrotransposition events in human cancer cells and iPSCs by binding directly to L1 RNA. Thus, we have identified and characterized a new function of microRNAs: mediating genomic stability by suppressing the mobility of endogenous retrotransposons.

The non-autonomous retrotransposon SVA is trans-mobilized by the human LINE-1 protein machinery.

SINE-VNTR-Alu (SVA) elements are non-autonomous, hominid-specific non-LTR retrotransposons and distinguished by their organization as composite mobile elements. They represent the evolutionarily youngest, currently active family of human non-LTR retrotransposons, and sporadically generate disease-causing insertions. Since preexisting, genomic SVA sequences are characterized by structural hallmarks of Long Interspersed Elements 1 (LINE-1, L1)-mediated retrotransposition, it has been hypothesized for several years that SVA elements are mobilized by the L1 protein machinery in trans. To test this hypothesis, we developed an SVA retrotransposition reporter assay in cell culture using three different human-specific SVA reporter elements. We demonstrate that SVA elements are mobilized in HeLa cells only in the presence of both L1-encoded proteins, ORF1p and ORF2p. SVA trans-mobilization rates exceeded pseudogene formation frequencies by 12- to 300-fold in HeLa-HA cells, indicating that SVA elements represent a preferred substrate for L1 proteins. Acquisition of an AluSp element increased the trans-mobilization frequency of the SVA reporter element by ~25-fold. Deletion of (CCCTCT)(n) repeats and Alu-like region of a canonical SVA reporter element caused significant attenuation of the SVA trans-mobilization rate. SVA de novo insertions were predominantly full-length, occurred preferentially in G+C-rich regions, and displayed all features of L1-mediated retrotransposition which are also observed in preexisting genomic SVA insertions.

SAMHD1-deficient CD14+ cells from individuals with Aicardi-Goutières syndrome are highly susceptible to HIV-1 infection.

Myeloid blood cells are largely resistant to infection with human immunodeficiency virus type 1 (HIV-1). Recently, it was reported that Vpx from HIV-2/SIVsm facilitates infection of these cells by counteracting the host restriction factor SAMHD1. Here, we independently confirmed that Vpx interacts with SAMHD1 and targets it for ubiquitin-mediated degradation. We found that Vpx-mediated SAMHD1 degradation rendered primary monocytes highly susceptible to HIV-1 infection; Vpx with a T17A mutation, defective for SAMHD1 binding and degradation, did not show this activity. Several single nucleotide polymorphisms in the SAMHD1 gene have been associated with Aicardi-Goutières syndrome (AGS), a very rare and severe autoimmune disease. Primary peripheral blood mononuclear cells (PBMC) from AGS patients homozygous for a nonsense mutation in SAMHD1 (R164X) lacked endogenous SAMHD1 expression and support HIV-1 replication in the absence of exogenous activation. Our results indicate that within PBMC from AGS patients, CD14+ cells were the subpopulation susceptible to HIV-1 infection, whereas cells from healthy donors did not support infection. The monocytic lineage of the infected SAMHD1 -/- cells, in conjunction with mostly undetectable levels of cytokines, chemokines and type I interferon measured prior to infection, indicate that aberrant cellular activation is not the cause for the observed phenotype. Taken together, we propose that SAMHD1 protects primary CD14+ monocytes from HIV-1 infection confirming SAMHD1 as a potent lentiviral restriction factor.

PKCδ-induced PU.1 phosphorylation promotes hematopoietic stem cell differentiation to dendritic cells.

Human CD34(+) hematopoietic stem cells (HSCs) exhibit the potential to differentiate into a variety of specialized blood cells. The distinct intracellular mechanisms that control cell fate and lineage commitment of these multipotent cells are not well defined. In this study, we investigate and modulate the signaling processes during HSC differentiation toward myeloid dendritic cells (mDCs). DC differentiation induced by the cytokines Granulocyte macrophage colony-stimulating factor (GM-CSF) and Interleukin-4 (IL-4) led to activation of the Extracellular-signal-regulated kinase (ERK), protein kinase C (PKC), and Janus kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) but not the SAPK/c-Jun NH(2) -terminal kinase and p38 mitogen-activated protein kinase signaling pathways. From the activated signaling pathways the PKC isoform δ was found to phosphorylate the transcription factor PU.1, which is described as one of the key factors for myeloid HSC differentiation. On molecular level, PKCδ regulated PU.1 activity by affecting its transactivation activity, whereas its DNA binding activity remained unaffected. This was accompanied by PKCδ-induced phosphorylation of the PU.1 transactivation domain. Furthermore, treatment with PKC- and ERK1/2-specific signaling inhibitors impaired both HSC differentiation toward mDCs as well as phosphorylation-mediated transactivation activity of PU.1. Taken together, these results provide new insights into the molecular mechanisms promoting the differentiation process of HSCs toward mDCs and introduce the PKC isoform δ as critical mediator.

Mutation of a diacidic motif in SIV-PBj Nef impairs T-cell activation and enteropathic disease.

BACKGROUND: The non-pathogenic course of SIV infection in its natural host is characterized by robust viral replication in the absence of chronic immune activation and T cell proliferation. In contrast, acutely lethal enteropathic SIVsmm strain PBj induces a strong immune activation and causes a severe acute and lethal disease in pig-tailed macaques after cross-species transmission. One important pathogenicity factor of the PBj virus is the PBj-Nef protein, which contains a conserved diacidic motif and, unusually, an immunoreceptor tyrosine-based activation motif (ITAM). RESULTS: Mutation of the diacidic motif in the Nef protein of the SIVsmmPBj abolishes the acute phenotype of this virus. In vitro, wild-type and mutant PBj (PBj-Nef202/203GG) viruses replicated to similar levels in macaque PBMCs, but PBj-Nef202/203GG no longer triggers ERK mitogen-activated protein (MAP) kinase pathway including an alteration of a Nef-associated Raf-1/ERK-2 multiprotein signaling complex. Moreover, stimulation of IL-2 and down-modulation of CD4 and CD28 were impaired in the mutant virus. Pig-tailed macaques infected with PBj-Nef202/203GG did not show enteropathic complications and lethality as observed with wild-type PBj virus, despite efficient replication of both viruses in vivo. Furthermore, PBj-Nef202/203GG infected animals revealed reduced T-cell activation in periphery lymphoid organs and no detectable induction of IL-2 and IL-6. CONCLUSIONS: In sum, we report here that mutation of the diacidic motif in the PBj-Nef protein abolishes disease progression in pig-tailed macaques despite efficient replication. These data suggest that alterations in the ability of a lentivirus to promote T cell activation and proliferation can have a dramatic impact on its pathogenic potential.

Concomitant type I IFN receptor-triggering of T cells and of DC is required to promote maximal modified vaccinia virus Ankara-induced T-cell expansion.

Virus-induced expansion of CD8(+) T cells may be promoted by type I IFN receptor (IFNAR)-triggering of T cells, depending on the pathogen tested. We studied modified vaccinia virus Ankara (MVA), a promising vaccine vector candidate, which was derived from conventional vaccinia virus (VACV) by more than 570 consecutive in vitro passages. In adoptive transfer experiments, we verified that VACV expressing the gp33 epitope of lymphocytic choriomeningitis virus (VACV(gp33)) induced largely IFNAR-independent expansion of gp33-specific T cells. On the contrary, MVA(gp33)-induced T-cell expansion was IFNAR dependent. Interestingly, under the latter conditions, T-cell activation was IFNAR independent, whereas T-cell apoptosis was enhanced in the absence of IFNAR. To address whether MVA-induced T-cell expansion was solely affected by IFNAR-triggering of T cells, expansion of endogenous T cells was studied in conditional mice with a T-cell- or DC-specific IFNAR deletion. Interestingly, both mouse strains showed moderately reduced T-cell expansion, whereas mice with a combined T-cell- and DC-specific IFNAR ablation showed massively reduced T-cell expansion similar to that of IFNAR(-/-) mice. These results are compatible with the model that IFN-inducing viruses such as MVA confer virus-specific CD8(+) T-cell expansion by concomitant IFNAR-triggering of DC and of T cells.

APOBEC3 proteins inhibit human LINE-1 retrotransposition.

The human cytidine deaminase family APOBEC3 represents a novel group of proteins in the field of innate defense mechanisms that has been shown to be active against a variety of retroviruses. Here we examined whether members of the APO-BEC3 family have an impact on retrotransposition of human long interspersed nuclear elements (LINE-1s or L1s). Using a retrotransposition reporter assay in HeLa cells, we demonstrate that in the presence of transiently transfected APOBEC3A, L1 retrotransposition frequency was reduced by up to 85%. Although APOBEC3G and -3H did not influence L1 retrotransposition notably, expression of APOBEC3B, -3C, and -3F inhibited transposition by approximately 75%. Although reverse transcription of L1s occurs in the nucleus and APOBEC3 proteins are believed to act via DNA deamination during reverse transcription, activity against L1 retrotransposition was not correlated with nuclear localization of APOBEC3s. We demonstrate that APOBEC3C and APOBEC3B were endogenously expressed in HeLa cells. Accordingly, down-regulation of APOBEC3C by RNA interference enhanced L1 retrotransposition by approximately 78%. Sequence analyses of de novo L1 retrotransposition events that occurred in the presence of overexpressed APOBEC3 proteins as well as the analyses of pre-existing endogenous L1 elements did not reveal an enhanced rate of G-to-A transitions, pointing to a mechanism independent of DNA deamination. This study presents evidence for a role of host-encoded APOBEC3 proteins in the regulation of L1 retrotransposition.

An inducible T7 RNA polymerase-dependent plasmid system.

RNA interference (RNAi) has become a powerful tool for the specific silencing of gene transcription. Especially the targeting of genes in mammalian cells has been greatly improved by generating plasmid based and viral vector-based systems. This permits expression of short hairpin RNA (shRNA) on a longterm basis. However, an inducible expression of shRNA is required, if the target is essential for cell survival. We developed a doxycycline-inducible two-plasmid system for the expression of a ribozyme-processed shRNA. In contrast to other existing systems, we use the highly specific T7 phage RNA polymerase, which does not interact with cellular factors; therefore, interference with cellular functions is limited. One plasmid is responsible for doxycycline-dependent expression of T7 RNA polymerase and a second plasmid expresses a ribozyme-processed shRNA under the control of a T7 promoter. Our results showed that doxycycline- dependent expression of T7 RNA polymerase was tightly controlled and expression of an shRNA against firefly luciferase inhibited 86% of luciferase activity. In conclusion, our plasmid system provides a very useful tool for analyzing essential gene functions in vitro.

IL-2 induction by simian immunodeficiency virus involves MAP kinase signaling but is independent of calcineurin/NF-AT activity.

The major T cell growth factor interleukin-2 (IL-2) is secreted by activated T cells in response to antigenic stimulation. This requires signal transduction via the CD3/TCR complex and the CD28 coreceptor, leading to activation of mitogen-activated protein kinase (MAPK) and calcineurin/NF-AT signaling pathways. We observed that simian immunodeficiency virus derived from African green monkeys (SIVagm3) is a potent activator of IL-2 gene expression. IL-2 promoter studies in A3.01 T cells demonstrated that SIVagm3 induced an up to 38-fold increased transcriptional activation of the IL-2 promoter. Inhibition of MAPK signaling pathways using inhibitors of MEK, JNK or p38 abolished SIVagm3-induced IL-2 activation in a dose-dependent manner. In contrast, the immunosuppressive drug cyclosporin A (CyA), a classical IL-2 inhibitor that blocks calcineurin activity, had no effect. Consistent with this finding, the nuclear factor of activated T cells (NF-AT), which is activated by calcineurin, was not induced by SIVagm3. Analyzing further transcription factor binding sites located on the IL-2 promoter we found that SIVagm3 did mainly promote transcriptional activation of the CD28/AP-1 and NF-kappaB responsive elements. These DNA elements were also induced by the viral transactivator protein (Tat) and expression of Tat was sufficient to activate IL-2 induction in stimulated cells. Our results show that SIVagm3 is capable of stimulating IL-2 gene expression via molecular mechanisms different from those induced during classical T cell activation.