Host Restriction Factor SAMHD1 Limits Human T Cell Leukemia Virus Type 1 Infection of Monocytes via STING-Mediated Apoptosis

SummaryHuman T cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia and HTLV-1-associated myelopathies. In addition to T cells, HTLV-1 infects cells of the myeloid lineage, which play critical roles in the host innate response to viral infection. Investigating the monocyte depletion observed during HTLV-1 infection, we discovered that primary human monocytes infected with HTLV-1 undergo abortive infection accompanied by apoptosis dependent on SAMHD1, a host restriction factor that hydrolyzes endogenous dNTPs to below the levels required for productive reverse transcription. Reverse transcription intermediates (RTI) produced in the presence of SAMHD1 induced IRF3-mediated antiviral and apoptotic responses. Viral RTIs complexed with the DNA sensor STING to trigger formation of an IRF3-Bax complex leading to apoptosis. This study provides a mechanistic explanation for abortive HTLV-1 infection of monocytes and reports a link between SAMHD1 restriction, HTLV-1 RTI sensing by STING, and initiation of IRF3-Bax driven apoptosis

Systems analysis of a RIG-I agonist inducing broad spectrum inhibition of virus infectivity.

The RIG-I like receptor pathway is stimulated during RNA virus infection by interaction between cytosolic RIG-I and viral RNA structures that contain short hairpin dsRNA and 5' triphosphate (5'ppp) terminal structure. In the present study, an RNA agonist of RIG-I was synthesized in vitro and shown to stimulate RIG-I-dependent antiviral responses at concentrations in the picomolar range. In human lung epithelial A549 cells, 5'pppRNA specifically stimulated multiple parameters of the innate antiviral response, including IRF3, IRF7 and STAT1 activation, and induction of inflammatory and interferon stimulated genes - hallmarks of a fully functional antiviral response. Evaluation of the magnitude and duration of gene expression by transcriptional profiling identified a robust, sustained and diversified antiviral and inflammatory response characterized by enhanced pathogen recognition and interferon (IFN) signaling. Bioinformatics analysis further identified a transcriptional signature uniquely induced by 5'pppRNA, and not by IFNα-2b, that included a constellation of IRF7 and NF-kB target genes capable of mobilizing multiple arms of the innate and adaptive immune response. Treatment of primary PBMCs or lung epithelial A549 cells with 5'pppRNA provided significant protection against a spectrum of RNA and DNA viruses. In C57Bl/6 mice, intravenous administration of 5'pppRNA protected animals from a lethal challenge with H1N1 Influenza, reduced virus titers in mouse lungs and protected animals from virus-induced pneumonia. Strikingly, the RIG-I-specific transcriptional response afforded partial protection from influenza challenge, even in the absence of type I interferon signaling. This systems approach provides transcriptional, biochemical, and in vivo analysis of the antiviral efficacy of 5'pppRNA and highlights the therapeutic potential associated with the use of RIG-I agonists as broad spectrum antiviral agents

Linear Ubiquitination of NEMO Negatively Regulates the Interferon Antiviral Response through Disruption of the MAVS-TRAF3 Complex

SummaryThe RIG-I/Mda5 sensors recognize viral intracellular RNA and trigger host antiviral responses. RIG-I signals through the adaptor protein MAVS, which engages various TRAF family members and results in type I interferon (IFNs) and proinflammatory cytokine production via activation of IRFs and NF-κB, respectively. Both the IRF and NF-κB pathways also require the adaptor protein NEMO. We determined that the RIG-I pathway is differentially regulated by the linear ubiquitin assembly complex (LUBAC), which consists of the E3 ligases HOIL-1L, HOIP, and the accessory protein SHARPIN. LUBAC downregulated virus-mediated IFN induction by targeting NEMO for linear ubiquitination. Linear ubiquitinated NEMO associated with TRAF3 and disrupted the MAVS-TRAF3 complex, which inhibited IFN activation while stimulating NF-κB-dependent signaling. In SHARPIN-deficient MEFs, vesicular stomatitis virus replication was decreased due to increased IFN production. Linear ubiquitination thus switches NEMO from a positive to a negative regulator of RIG-I signaling, resulting in an attenuated IFN response

Linear ubiquitination of NEMO negatively regulates the interferon antiviral response through disruption of the MAVS-TRAF3 complex

The RIG-I/Mda5 sensors recognize RNA virus infection through their ability to bind intracellular viral RNA and trigger the host antiviral response. In the present study, we investigated the role of linear ubiquitin assembly complex (LUBAC) - consisting of the E3 ligases HOIL-1L, HOIP, and the accessory protein SHARPIN- in the differential regulation of the RIG-I pathway. LUBAC downregulated virus-mediated interferon (IFN) induction by targeting NEMO for linear ubiquitination. Linear ubiquitinated NEMO associated with TRAF3 and disrupted the MAVSTRAF3 complex, leading to the inhibition of IFN activation while stimulating NF-κB dependent signaling. In SHARPIN deficient MEFs, vesicular stomatitis virus replication was decreased as a consequence of increased IFN production. Linear ubiquitination thus switches NEMO from a positive to a negative regulator of RIG-I signaling that dissociates the MAVS-TRAF3 complex and contributes to the negative regulation of the IFN antiviral response

Histone Deacetylase Inhibitors Potentiate Vesicular Stomatitis Virus Oncolysis In Prostate Cancer Cells By Modulating NF-KB-Dependent Autophagy

Vesicular stomatitis virus (VSV) is an oncolytic virus that induces cancer cell death through activation of the apoptotic pathway. Intrinsic resistance to oncolysis is found in some cell lines and many primary tumors as a consequence of residual innate immunity to VSV. In resistant-tumor models, VSV oncolytic potential can be reversibly stimulated by combination with epigenetic modulators, such as the histone deacetylase inhibitor vorinostat. Based on this reversible effect of vorinostat, we reasoned that critical host genes involved in oncolysis may likewise be reversibly regulated by vorinostat. A transcriptome analysis in prostate cancer PC3 cells identified a subset of NF-κB target genes reversibly regulated by vorinostat, as well as a group of interferon (IFN)-stimulated genes (ISGs). Consistent with the induction of NF-κB target genes, vorinostat-mediated enhancement of VSV oncolysis increased hyperacetylation of NF-κB RELA/p65. Additional bioinformatics analysis revealed that NF-κB signaling also increased the expression of several autophagy-related genes. Kinetically, autophagy preceded apoptosis, and apoptosis was observed only when cells were treated with both VSV and vorinostat. VSV replication and cell killing were suppressed when NF-κB signaling was inhibited using pharmacological or genetic approaches. Inhibition of autophagy by 3-methyladenine (3-MA) enhanced expression of ISGs, and either 3-MA treatment or genetic ablation of the autophagic marker Atg5 decreased VSV replication and oncolysis. Together, these data demonstrate that vorinostat stimulates NF-κB activity in a reversible manner via modulation of RELA/p65 signaling, leading to induction of autophagy, suppression of the IFN-mediated response, and subsequent enhancement of VSV replication and apoptosis

Histone deacetylase inhibitors potentiate vesicular stomatitis virus oncolysis in prostate cancer cells by modulating NF-κB-dependent autophagy.

Vesicular stomatitis virus (VSV) is an oncolytic virus that induces cancer cell death through activation of the apoptotic pathway. Intrinsic resistance to oncolysis is found in some cell lines and many primary tumors as a consequence of residual innate immunity to VSV. In resistant-tumor models, VSV oncolytic potential can be reversibly stimulated by combination with epigenetic modulators, such as the histone deacetylase inhibitor vorinostat. Based on this reversible effect of vorinostat, we reasoned that critical host genes involved in oncolysis may likewise be reversibly regulated by vorinostat. A transcriptome analysis in prostate cancer PC3 cells identified a subset of NF-κB target genes reversibly regulated by vorinostat, as well as a group of interferon (IFN)-stimulated genes (ISGs). Consistent with the induction of NF-κB target genes, vorinostat-mediated enhancement of VSV oncolysis increased hyperacetylation of NF-κB RELA/p65. Additional bioinformatics analysis revealed that NF-κB signaling also increased the expression of several autophagy-related genes. Kinetically, autophagy preceded apoptosis, and apoptosis was observed only when cells were treated with both VSV and vorinostat. VSV replication and cell killing were suppressed when NF-κB signaling was inhibited using pharmacological or genetic approaches. Inhibition of autophagy by 3-methyladenine (3-MA) enhanced expression of ISGs, and either 3-MA treatment or genetic ablation of the autophagic marker Atg5 decreased VSV replication and oncolysis. Together, these data demonstrate that vorinostat stimulates NF-κB activity in a reversible manner via modulation of RELA/p65 signaling, leading to induction of autophagy, suppression of the IFN-mediated response, and subsequent enhancement of VSV replication and apoptosis

BCL-2 inhibitors sensitize therapy-resistant chronic lymphocytic leukemia cells to VSV oncolysis.

Many primary cancers including chronic lymphocytic leukemia (CLL) are resistant to vesicular stomatitis virus (VSV)-induced oncolysis due to overexpression of the antiapoptotic and antiautophagic members of the B-cell lymphoma-2 (BCL-2) family. In the present study, we investigated the mechanisms of CLL cell death induced as a consequence of VSV infection in the presence of BCL-2 inhibitors, obatoclax, and ABT-737 in primary ex vivo CLL patient samples. Microarray analysis of primary CD19⁺ CD5⁺ CLL cells treated with obatoclax and VSV revealed changes in expression of genes regulating apoptosis, the mechanistic target of rapamycin (mTOR) pathway, and cellular metabolism. A combined therapeutic effect was observed for VSV and BCL-2 inhibitors in cells from untreated patients and from patients unresponsive to standard of care therapy. In addition, combination treatment induced several markers of autophagy--LC3-II accumulation, p62 degradation, and staining of autophagic vacuoles. Inhibition of early stage autophagy using 3-methyladenine (3-MA) led to increased apoptosis in CLL samples. Mechanistically, a combination of BCL-2 inhibitors and VSV disrupted inhibitory interactions of Beclin-1 with BCL-2 and myeloid cell leukemia-1 (MCL-1), thus biasing cells toward autophagy. We propose a mechanism in which changes in cellular metabolism, coupled with pharmacologic disruption of the BCL-2-Beclin-1 interactions, facilitate induction of apoptosis and autophagy to mediate the cytolytic effect of VSV

BCL-2 inhibitors sensitize therapy-resistant chronic lymphocytic leukemia cells to VSV oncolysis.

Many primary cancers including chronic lymphocytic leukemia (CLL) are resistant to vesicular stomatitis virus (VSV)-induced oncolysis due to overexpression of the antiapoptotic and antiautophagic members of the B-cell lymphoma-2 (BCL-2) family. In the present study, we investigated the mechanisms of CLL cell death induced as a consequence of VSV infection in the presence of BCL-2 inhibitors, obatoclax, and ABT-737 in primary ex vivo CLL patient samples. Microarray analysis of primary CD19⁺ CD5⁺ CLL cells treated with obatoclax and VSV revealed changes in expression of genes regulating apoptosis, the mechanistic target of rapamycin (mTOR) pathway, and cellular metabolism. A combined therapeutic effect was observed for VSV and BCL-2 inhibitors in cells from untreated patients and from patients unresponsive to standard of care therapy. In addition, combination treatment induced several markers of autophagy--LC3-II accumulation, p62 degradation, and staining of autophagic vacuoles. Inhibition of early stage autophagy using 3-methyladenine (3-MA) led to increased apoptosis in CLL samples. Mechanistically, a combination of BCL-2 inhibitors and VSV disrupted inhibitory interactions of Beclin-1 with BCL-2 and myeloid cell leukemia-1 (MCL-1), thus biasing cells toward autophagy. We propose a mechanism in which changes in cellular metabolism, coupled with pharmacologic disruption of the BCL-2-Beclin-1 interactions, facilitate induction of apoptosis and autophagy to mediate the cytolytic effect of VSV