Ke Peng

Severe fever with thrombocytopenia syndrome, an emerging infectious disease caused by the tick-borne bunyavirus severe fever with thrombocytopenia syndrome virus (SFTSV), is associated with a high clinical fatality rate of 12%–50%. Vascular endothelial injury triggered by SFTSV infection has been linked to severe disease progression and mortality. Currently, a suitable model to investigate the underlying mechanisms of SFTSV-induced vascular damage is lacking. Here, we show that the ex vivo human vascular tissue can be used as a model for studying bunyavirus-triggered endothelial injury. Both SFTSV and Rift Valley fever virus (RVFV) can successfully infect ex vivo human vascular tissue, as evidenced by detection of viral nucleoprotein and viral amplification in the tissue. Histopathological analysis, TUNEL staining, and endothelial injury-associated transcriptional changes revealed irregular venous tissue structure, luminal endothelial disruption, and apoptosis following infection. This model enables evaluation of antiviral compound efficacy, demonstrating that niclosamide or 4′-fluorouridine significantly inhibits SFTSV and RVFV replication in ex vivo vascular tissues. These findings establish the ex vivo human vascular tissue as a valuable tool for investigating bunyavirus-induced vascular pathogenesis and for evaluating antiviral therapeutics. IMPORTANCE: Severe fever with thrombocytopenia syndrome virus and other pathogenic bunyaviruses cause severe human diseases that are frequently associated with vascular dysfunction. However, the mechanisms underlying virus-induced vascular injury remain poorly defined due to the lack of physiologically relevant human tissue models. Here, we establish an ex vivo human vascular tissue model that supports productive bunyavirus infection and enables quantitative assessment of endothelial injury, tissue damage, and antiviral efficacy. Using this model, we demonstrate that bunyavirus infection triggers endothelial disruption and that antiviral compounds can suppress viral replication and alleviate infection-associated tissue injury. This model provides a valuable experimental system for studying virus-vascular interactions and can support antiviral development against emerging bunyaviruses.

Necroptosis is an inflammatory programmed cell death pathway triggered by RIPK3 activation through one of the upstream RHIM-domain-containing proteins including RIPK1, TRIF, and ZBP1. Whether necroptosis can be activated independent of the upstream signaling pathways leading to inflammatory pathogenesis remains ambiguous. Here, we revealed a mechanism in which a viral protein mediates direct RIPK3 activation resulting in severe inflammatory pathogenesis in patients. The nonstructural protein NSs of a pathogenic hemorrhagic virus, SFTSV, interacts with the RIPK3 kinase domain and forms biocondensate to promote RIPK3 autophosphorylation and necroptosis activation in an RHIM-independent manner. In parallel, sequestration of RIPK3 within the NSs–RIPK3 condensate inhibited RIPK3-mediated apoptosis and promoted viral replication. Infection with an SFTSV NSs mutant virus not forming NSs condensate triggered pronounced apoptosis resulting in reduced viral replication and decreased fatality in vivo. Blocking SFTSV-triggered necroptosis through depletion of MLKL or treatment with a RIPK3-kinase inhibitor reduced viral inflammatory pathogenesis and fatality in vivo. In contrast, blocking SFTSV-triggered apoptosis through depletion of RIPK3 resulted in enhanced viral replication and increased fatality in vivo. The virus-triggered necroptosis correlated with severe inflammatory pathogenesis and lethality in virus-infected patients. The NSs–RIPK3 condensate may represent a necroptosis activation mechanism that promotes viral pathogenesis.

Infections of many viruses induce caspase activation to regulate multiple cellular pathways, including programmed cell death, immune signaling and etc. Characterizations of caspase cleavage sites and substrates are important for understanding the regulation mechanisms of caspase activation. Here, we identified and analyzed a novel caspase cleavage motif AEAD, and confirmed its caspase dependent cleavage activity in natural substrate, such as nitric oxide-associated protein 1 (NOA1). Fusing the enhanced green fluorescent protein (EGFP) with the mitochondrial marker protein Tom20 through the AEAD motif peptide localized EGFP to the mitochondria. Upon the activation of caspase triggered by Sendai virus (SeV) or herpes simplex virus type 1 (HSV-1) infection, EGFP diffusely localized to the cell due to the caspase-mediated cleavage, thus allowing visual detection of the virusinduced caspase activation. An AEAD peptide-derived inhibitor Z-AEAD-FMK were developed, which significantly inhibited the activities of caspases-1, -3, -6, -7, -8 and -9, exhibiting a broad caspase inhibition effect. The inhibitor further prevented caspases-mediated cleavage of downstream substrates, including BID, PARP1, LMNA, pro-IL-1(3, pro-IL-18, GSDMD and GSDME, protecting cells from virus-induced apoptotic and pyroptotic cell death. Together, our findings provide a new perspective for the identification of novel caspase cleavage motifs and the development of new caspase inhibitors and anti-inflammatory drugs.

Apoptosis has been recognized as a suicidal host-defense programmed cell death pathway against invading pathogens. However, recent evidences showed that viruses can employ caspases to cleave and inactivate immune signaling molecules to facilitate infection. Whether caspases can promote anti-viral immune responses is currently unknown. Here, we demonstrated that the NSs protein of RVFV triggers activation of apoptotic caspases, which cleave the mitochondrial fission factor DRP1 resulting in mitochondrial elongation. Elongated mitochondria promote MAVS aggregation leading to enhanced anti-viral immunity. Apoptotic caspases, including caspase-3, -6, -7 and -8, cleave DRP1 at the motifs of D500FAD503 and/or AEAD556, suggesting that this cleavage event may occur during infection of different viruses. Indeed, infection of H1N1, SeV and HSV-1 all triggered apoptotic caspases activation to cleave DRP1 promoting anti-viral immune responses. Compared with wild-type DRP1, introduction of caspase-resistant DRP1 strongly attenuated immune responses triggered by virus infection. These results revealed a novel mechanism through which apoptotic caspases promote anti-viral immunity by regulating mitochondrial morphodynamics.

Infection of Rift Valley fever virus (RVFV), a highly pathogenic mosquito-borne zoonotic virus, triggers severe inflammatory pathogenesis but the underlying mechanism of inflammation activation is currently unclear. Here, we report that the non-structural protein NSs of RVFV triggers mitochondrial damage to activate the NLRP3 inflammasome leading to viral pathogenesis in vivo. It is found that the host transcription inhibition effect of NSs causes rapid down-regulation of myeloid cell leukemia-1(MCL-1), a pro-survival member of the Bcl-2 (B-cell lymphoma protein 2) protein family. MCL-1 down-regulation led to BAK activation in the mitochondria, which triggered mtROS production and release of oxidized mitochondrial DNA (ox-mtDNA) into the cytosol. Cytosolic ox-mtDNA binds and activates the NLRP3 inflammasome triggering NLRP3-GSDMD pyroptosis in RVFV infected cells. A NSs mutant virus (RVFV-NSs^RM) that is compromised in inducing transcription inhibition did not trigger MCL-1 down-regulation nor NLRP3-GSDMD pyroptosis. RVFV infection of the Nlrp3^-/- mouse model demonstrated that the RVFV-triggered NLRP3 pyroptosis contributed to RVFV inflammatory pathogenesis and fatal infection in vivo. Infection with the RVFV-NSs^RM mutant virus similarly showed alleviated inflammatory pathogenesis and reduced fatality rate. Taken together, these results revealed a mechanism by which a virulence factor activates the mitochondrial MCL-1-BAK axis through inducing host transcription inhibition to trigger NLRP3-dependent inflammatory pathogenesis.

Increasing evidences suggest that the methyltransferase NSUN2 catalyzes 5-methylcytosine (m5C) modifications on viral RNAs, which are essential for the replication of various viruses. Despite the function of m5C deposition is well characterized, other potential roles of NSUN2 in regulating viral replication remain largely unknown. In this study, the m5C modified residues catalyzed by NSUN2 on enterovirus 71 (EV71) RNAs were mapped. NSUN2, along with m5C modifications, played multiple roles during the EV71 life cycle. Functional m5C modified nucleotides increased the translational efficiency and stability of EV71 RNAs. Additionally, NSUN2 was found to target the viral protein VP1 for binding and promote its stability by inhibiting the ubiquitination. Furthermore, both viral replication and pathogenicity in mice were largely attenuated when functional m5C residues were mutated. Taken together, this study characterizes distinct pathways mediated by NSUN2 in regulating EV71 replication, and highlights the importance of its catalyzed m5C modifications on EV71 RNAs for the viral replication and pathogenicity.

Apoptosis has been recognized as a suicidal host-defense programmed cell death pathway against invading pathogens. However, recent evidences showed that viruses can employ caspases to cleave and inactivate immune signaling molecules to facilitate infection. Whether caspases can instead promote anti-viral immune responses is currently unknown. Here, we demonstrated that the nonstructural protein (NSs) of Rift Valley fever virus (RVFV) triggers activation of apoptotic caspases, which cleave the mitochondrial fission factor dynamin-related protein 1 (DRP1) resulting in mitochondrial elongation. Elongated mitochondria promote mitochondrial antiviral-signaling protein (MAVS) aggregation leading to enhanced anti-viral immunity. Apoptotic caspases, including caspase-3, -6, -7 and -8, cleave DRP1 at the motifs of D ⁵⁰⁰ FAD ⁵⁰³ and/or AEAD ⁵⁵⁶ , suggesting that this cleavage event may occur during infection of different viruses. Indeed, infection of influenza A virus subtype H1N1, Sendai virus (SeV) and herpes simplex virus 1 (HSV-1) all triggered apoptotic caspases activation to cleave DRP1 promoting anti-viral immune responses. Compared with wild-type DRP1, introduction of caspase-resistant DRP1 strongly attenuated immune responses triggered by virus infection. These results revealed a mechanism through which apoptotic caspases promote anti-viral immunity by regulating mitochondrial morphodynamics.