Mx1 in Hematopoietic Cells Protects against Thogoto Virus Infection

This study examined the contribution of the interferon-induced restriction factor Mx1 in hematopoietic versus nonhematopoietic compartments during viral infection, with a particular focus on Thogoto virus (THOV). Using reciprocal bone marrow chimera mice carrying functional or nonfunctional Mx1 alleles, the authors compared outcomes following infection with influenza A virus (IAV) and THOV. While resistance to IAV infection depended exclusively on Mx1 expression in nonhematopoietic (stromal) cells, THOV infection revealed a distinct requirement for Mx1 in bone marrow-derived cells. Mice lacking stromal Mx1 but reconstituted with Mx1-competent bone marrow showed delayed morbidity, reduced liver pathology, and significantly lower viral dissemination to spleen, lung, and serum compared with mice receiving Mx1-deficient bone marrow. Histopathology, serum ALT/AST measurements, viral titration, and immunohistochemical analyses demonstrated that Mx1 expression in CD45⁺ hematopoietic cells, likely myeloid cells, restricted THOV replication and delayed liver necrosis, although it did not fully prevent hepatic infection in the absence of stromal Mx1. Collectively, the results establish that, unlike influenza A virus, effective control of THOV infection critically involves Mx1 expression in hematopoietic cells, highlighting virus- and cell type–specific roles of intrinsic antiviral immunity.
(TMR)

In Vivo Conditions Enable IFNAR-Independent Type I Interferon Production by Peritoneal CD11b1 Cells upon Thogoto Virus Infection

This study investigated how type I interferon (IFN-I) can be induced independently of IFNAR-mediated feedback during Thogoto virus infection. Wild-type and innate immune-deficient mice (IFNAR⁻/⁻, MyD88⁻/⁻, TRIF⁻/⁻, MyD88⁻/⁻TRIF⁻/⁻, and MAVS⁻/⁻) were infected intraperitoneally in vivo, and IFN-β production was assessed in serum, peritoneal exudates, and organs by ELISA. To define cellular sources and signaling requirements, bone marrow-derived myeloid and plasmacytoid dendritic cells, as well as peritoneal exudate cells, were analyzed using in vitro infection, ex vivo culture, flow cytometry, reporter mice, MACS enrichment, and quantitative RT-PCR. THOV infection induced robust systemic IFN-β responses in vivo even in IFNAR-deficient mice, whereas comparable in vitro infections failed to do so, demonstrating a strict dependence on in vivo conditions. IFN production localized primarily to the peritoneal cavity and later the spleen. Using replication-incompetent but transcriptionally active THOV-derived virus-like particles, the authors showed that IFNAR-independent IFN induction required viral polymerase activity but not productive viral replication, as UV-inactivated particles were inactive. Genetic ablation revealed that IFN induction depended on MAVS-mediated RIG-I-like helicase signaling and was independent of TLR pathways. Reporter mouse analyses, supported by flow cytometry and transcript quantification, identified CD11b⁺ F4/80⁺ peritoneal myeloid cells as the dominant source of IFN-I. Collectively, the results define an alternative, tissue-restricted pathway enabling strong IFN-I production in vivo without IFNAR amplification.
(TMR)

Thogoto Virus Infection Induces Sustained Type I Interferon Responses That Depend on RIG-I-Like Helicase Signaling of Conventional Dendritic Cells

This study investigated the mechanisms underlying type I interferon (IFN) induction during Thogoto virus (THOV) infection using a recombinant virus lacking the IFN-antagonistic ML protein [THOV(ΔML)]. Mice, including wild-type and innate immune signaling–deficient strains (IFNAR⁻/⁻, MyD88⁻/⁻TRIF⁻/⁻, and IPS-1⁻/⁻), were infected in vivo, and serum IFN levels, viral loads, and survival were assessed. In parallel, bone marrow–derived myeloid dendritic cells (BM-mDC) and plasmacytoid dendritic cells (BM-pDC) were infected in vitro to identify cellular sources of IFN. THOV(ΔML) infection induced unusually high and sustained systemic IFN-α/β responses lasting up to 72 h, even in the absence of IFNAR signaling. Genetic ablation studies showed that early IFN induction partially involved Toll-like receptor pathways, whereas sustained IFN production strictly depended on the RIG-I–like helicase adaptor IPS-1. Viral replication was required for IFN induction, as UV-inactivated virus failed to elicit responses. Unexpectedly, BM-mDC, but not BM-pDC, were the dominant producers of type I IFN, and this response was IPS-1 dependent. Collectively, the results identify myeloid dendritic cells and IPS-1–mediated sensing as central drivers of sustained type I IFN responses during THOV infection.
(TMR)

Virus envelope glycoprotein targeting bispecific T cell engager protects mice from lethal severe fever with thrombocytopenia virus infection

Severe fever with thrombocytopenia syndrome virus (SFTSV) causes a highly fatal disease with no approved antiviral therapy. This study first analyzed peripheral T cell responses in SFTS patients and found that fatal cases showed marked T cell depletion, reduced cytotoxicity, and severe exhaustion, particularly in CD4⁺ T cells. These findings suggest that impaired T cell immunity contributes to disease severity. To restore antiviral T cell function, they developed a bispecific T cell engager (BiTE) targeting CD3 and the SFTSV envelope glycoprotein Gn. In vitro assays demonstrated that the 3A5 BiTE efficiently redirected human T cells to eliminate SFTSV-infected cells and significantly reduced viral replication. Blocking IFN-γ largely abolished this antiviral effect, whereas inhibition of granzyme B or perforin had minimal impact, indicating an IFN-γ–dependent mechanism. In a lethal mouse model, 3A5 BiTE treatment markedly improved survival and reduced viral loads in serum and organs. BiTE therapy also altered immune cell composition, increasing functional T cells while reducing inflammatory neutrophils and regulatory T cells. These results demonstrate that virus-specific BiTEs can revitalize T cell immunity and provide effective protection against lethal SFTSV infection.
(MN)

Tick-transmitted thogotovirus gains high virulence by a single MxA escape mutation in the viral nucleoprotein

Tick-borne thogotoviruses are highly sensitive to the human interferon-induced restriction factor myxovirus resistance protein A (MxA), which limits their zoonotic potential. Here, Fuchs et al. systematically evaluated the MxA sensitivity of ten globally distributed thogotovirus isolates using cell culture, viral minigenome assays, and mouse infection models. While all THOV-like viruses were potently inhibited by MxA, the Nigerian isolate Jos virus (JOSV) showed complete resistance in MxA-expressing cells and in MxA-transgenic mice. Polymerase reconstitution assays identified the viral nucleoprotein (NP) as the determinant of MxA sensitivity. Chimeric NP analysis and targeted mutagenesis mapped MxA escape to two adjacent NP residues (G327 and R328). Introduction of a single substitution, R328V, into the otherwise MxA-sensitive SiAr126 strain fully abolished MxA-mediated restriction without compromising viral replication or fitness. Recombinant SiAr126-NP(R328V) replicated efficiently in MxA-expressing cells and caused lethal infection in MxA-transgenic mice, whereas wild-type virus was completely restricted. Mechanistically, co-immunoprecipitation and imaging studies demonstrated that the R328V mutation disrupts NP–MxA interaction, permitting nuclear import of viral ribonucleoproteins. Collectively, these data provide direct experimental evidence that a single amino acid change enables thogotoviruses to evade human innate immunity, substantially increasing their virulence and zoonotic potential
(TMR)

Thrombocytopenia in Severe Fever with Thrombocytopenia Syndrome Due to Platelets with Altered Function Undergoing Cell Death Pathways

Severe Fever with Thrombocytopenia Syndrome (SFTS) is an important tick borne viral disease in East Asia. An important clinical manifestation of SFTS is thrombocytopenia. To understand the mechanism by which SFTS virus causes thrombocytopenia, platelets from surviving and fatal human cases, as well as from a C57/BL6 mice model were analyzed. Platelet alterations were assessed through RNA transcriptome profiling, ELISA, flow cytometry and quantitative RT-PCR. In humans transcriptome analysis showed that platelets upregulated genes related to type 1 interferon signaling and neutrophil activation. These changes were more significant in survivors than fatal cases. Immunoassays demonstrated increased levels of platelet activation marker CD62P and platelet–neutrophil aggregates. This occur because platelets activate neutrophils to form extracellular traps which capture the virus but result in massive platelets destruction. Platelets underwent multiple death pathways, most notably pyroptosis. In contrast while mice had thrombocytopenia there was no detection of platelet activation or viral proteins in platelets. Instead of widespread death, mice platelets triggered adaptive immunity which led to clearance of viremia through antibodies. 
(SWM)

Interferon and TLR genes, but not endogenous bornavirus-like elements, limit BoDV1 replication after intracerebral infection

Borna disease virus 1 (BoDV1) is a neurotropic RNA virus, but the host factors limiting its replication in the brain are not fully understood. Endogenous bornavirus-like elements (EBLNs) have been proposed to suppress BoDV1 through piRNA-mediated mechanisms. This study aimed to evaluate the relative roles of interferon signaling, Toll-like receptor pathways, and EBLN-derived piRNAs in controlling BoDV1 replication in vivo. Neonatal mice were intracerebrally infected with a GFP-expressing recombinant BoDV1. Viral replication was analyzed in wild-type mice and knockout mice lacking the interferon-γ receptor (Ifngr1), Toll-like receptor 7 (Tlr7), or all piRNA-producing EBLNs. Viral RNA levels and infected brain cells were assessed by RT-qPCR and histological analyses. BoDV1 replication was significantly increased in Ifngr1- and Tlr7-deficient mice, indicating that interferon-γ signaling and TLR7-dependent innate immunity restrict viral growth in the brain. In contrast, deletion of EBLN-derived piRNAs had no effect on viral replication. These results demonstrate that classical immune pathways, rather than endogenous viral elements, are the primary factors limiting BoDV1 replication after intracerebral infection.
(MN)