Interferon-induced human MxA GTPase blocks nuclear import of Thogoto virus nucleocapsids

Interferon-induced human MxA protein belongs to the dynamin superfamily of large GTPases. It exhibits antiviral activity against a variety of RNA viruses, including Thogoto virus, an influenza virus-like orthomyxovirus transmitted by ticks. Here, we report that MxA blocks the transport of Thogoto virus nucleocapsids into the nucleus, thereby preventing transcription of the viral genome. This interaction can be abolished by a mAb that neutralizes the antiviral activity of MxA. Our results reveal an antiviral mechanism whereby an interferon-induced protein traps the incoming virus and interferes with proper transport of the viral genome to its ultimate target compartment within the infected cell.

Human MxA protein inhibits tick-borne Thogoto virus but not Dhori virus

Thogoto and Dhori viruses are tick-borne orthomyxoviruses infecting humans and livestock in Africa, Asia, and Europe. Here, we show that human MxA protein is an efficient inhibitor of Thogoto virus but is inactive against Dhori virus. When expressed in the cytoplasm of stably transfected cell lines, MxA protein interfered with the accumulation of Thogoto viral RNA and proteins. Likewise, MxA(R645), a mutant MxA protein known to be active against influenza virus but inactive against vesicular stomatitis virus, was equally efficient in blocking Thogoto virus growth. Hence, a common antiviral mechanism that is distinct from the antiviral action against vesicular stomatitis virus may operate against both influenza virus and Thogoto virus. When moved to the nucleus with the help of a foreign nuclear transport signal, MxA(R645) remained active against Thogoto virus, indicating that a nuclear step of virus replication was inhibited. In contrast, Dhori virus was not affected by wild-type or mutant MxA protein, indicating substantial differences between these two tick-transmitted orthomyxoviruses. Human MxB protein had no antiviral activity against either virus.

Temporal and spatial resolution of type I and III interferon responses in vivo

Although the action of interferons (IFNs) has been extensively studied in vitro, limited information is available on the spatial and temporal activation pattern of IFN-induced genes in vivo. We created BAC transgenic mice expressing firefly luciferase under transcriptional control of the Mx2 gene promoter. Expression of the reporter with regard to onset and kinetics of induction parallels that of Mx2 and is thus a hallmark for the host response. Substantial constitutive expression of the reporter gene was observed in the liver and most other tissues of transgenic mice, whereas this expression was strongly reduced in animals lacking functional type I IFN receptors. As expected, the reporter gene was induced not only in response to type I (alpha and beta) and type III (lambda) IFNs but also in response to a variety of IFN inducers such as double-stranded RNA, lipopolysaccharide (LPS), and viruses. In vivo IFN subtypes show clear differences with respect to their kinetics of action and to their spatial activation pattern: while the type I IFN response was strong in liver, spleen, and kidney, type III IFN reactivity was most prominent in organs with mucosal surfaces. Infection of reporter mice with virus strains that differ in their pathogenicity shows that the IFN response is significantly altered in the strength of IFN action at sites which are not primarily infected as well as by the onset and duration of gene induction.

Tick-borne thogoto virus infection in mice is inhibited by the orthomyxovirus resistance gene product Mx1

We show that tick-transmitted Thogoto virus is sensitive to interferon-induced nuclear Mx1 protein, which is known for its specific antiviral action against orthomyxoviruses. Influenza virus-susceptible BALB/c mice (lacking a functional Mx1 gene) developed severe disease symptoms and died within days after intracerebral or intraperitoneal infection with a lethal challenge dose of Thogoto virus. In contrast, Mx1-positive congenic, influenza virus-resistant BALB.A2G-Mx1 mice remained healthy and survived. Likewise, A2G, congenic B6.A2G-Mx1 and CBA.T9-Mx1 mice (derived from influenza virus-resistant wild mice) as well as Mx1-transgenic 979 mice proved to be resistant. Peritoneal macrophages and interferon-treated embryo cells from resistant mice exhibited the same resistance phenotype in vitro. Moreover, stable lines of transfected mouse 3T3 cells that constitutively express Mx1 protein showed increased resistance to Thogoto virus infection. We conclude that an Mx1-sensitive step has been conserved during evolution of orthomyxoviruses and suggest that the Mx1 gene in rodents may serve to combat infections by influenza virus-like arboviruses.

A monomeric GTPase-negative MxA mutant with antiviral activity

MxA is a large, interferon-induced GTPase with antiviral activity against RNA viruses. It forms large oligomers, but whether oligomerization and GTPase activity are important for antiviral function is not known. The mutant protein MxA(L612K) carries a lysine-for-leucine substitution at position 612 and fails to form oligomers. Here we show that monomeric MxA(L612K) lacks detectable GTPase activity but is capable of inhibiting Thogoto virus in transiently transfected Vero cells or in a Thogoto virus minireplicon system. Likewise, MxA(L612K) inhibited vesicular stomatitis virus multiplication. These findings indicate that MxA monomers are antivirally active and suggest that GTP hydrolysis may not be required for antiviral activity. MxA(L612K) is rapidly degraded in cells, whereas wild-type MxA is stable. We propose that high-molecular-weight MxA oligomers represent a stable intracellular pool from which active MxA monomers are recruited.

Influenza C and D Viruses Package Eight Organized Ribonucleoprotein Complexes

Influenza A and B viruses have eight-segmented, single-stranded, negative-sense RNA genomes, whereas influenza C and D viruses have seven-segmented genomes. Each genomic RNA segment exists in the form of a ribonucleoprotein complex (RNP) in association with nucleoproteins and an RNA-dependent RNA polymerase in virions. Influenza D virus was recently isolated from swine and cattle, but its morphology is not fully studied. Here, we examined the morphological characteristics of D/bovine/Yamagata/10710/2016 (D/Yamagata) and C/Ann Arbor/50 (C/AA), focusing on RNPs packaged within the virions. By scanning transmission electron microscopic tomography, we found that more than 70% of D/Yamagata and C/AA virions packaged eight RNPs arranged in the "1+7" pattern as observed in influenza A and B viruses, even though type C and D virus genomes are segmented into only seven segments. These results imply that influenza viruses generally package eight RNPs arranged in the "1+7" pattern regardless of the number of RNA segments in their genome.IMPORTANCE The genomes of influenza A and B viruses are segmented into eight segments of negative-sense RNA, and those of influenza C and D viruses are segmented into seven segments. For progeny virions to be infectious, each virion needs to package all of their genomic segments. Several studies support the conclusion that influenza A and B viruses selectively package eight distinct genomic RNA segments; however, the packaging of influenza C and D viruses, which possess seven segmented genomes, is less understood. By using electron microscopy, we showed that influenza C and D viruses package eight RNA segments just as influenza A and B viruses do. These results suggest that influenza viruses prefer to package eight RNA segments within virions independent of the number of genome segments.

Determining the Replication Kinetics and Cellular Tropism of Influenza D Virus on Primary Well-Differentiated Human Airway Epithelial Cells

Influenza viruses are notorious pathogens that frequently cross the species barrier with often severe consequences for both animal and human health. In 2011, a novel member of the Orthomyxoviridae family, Influenza D virus (IDV), was identified in the respiratory tract of swine. Epidemiological surveys revealed that IDV is distributed worldwide among livestock and that IDV-directed antibodies are detected in humans with occupational exposure to livestock. To identify the transmission capability of IDV to humans, we determined the viral replication kinetics and cell tropism using an in vitro respiratory epithelium model of humans. The inoculation of IDV revealed efficient replication kinetics and apical progeny virus release at different body temperatures. Intriguingly, the replication characteristics of IDV revealed higher replication kinetics compared to Influenza C virus, despite sharing the cell tropism preference for ciliated cells. Collectively, these results might indicate why IDV-directed antibodies are detected among humans with occupational exposure to livestock.

Mx1 but not MxA confers resistance against tick-borne Dhori virus in mice

The interferon-induced nuclear Mx1 protein is responsible for innate resistance of mice to influenza virus. It has been unclear why mice are equipped with a powerful and specific defense mechanism against influenza viruses for which they are not natural hosts. Here, we show that Dhori virus, an influenza-like virus transmitted by ticks and known to infect small mammals, is sensitive to the Mx1 resistance mechanism. Influenza virus-susceptible BALB/c and C57BL/6 mice (lacking a functional Mx1 gene) developed severe disease symptoms and died within a few days after intraperitoneal infection with a lethal dose of Dhori virus. In contrast, Mx1(+)-congenic, influenza virus-resistant BALB.A2G-Mx1 and B6.A2G-Mx1 mice remained healthy and survived. The Mx1 resistance phenotype was expressed in cultured peritoneal macrophages and interferon-treated embryonic cells obtained from these mice. Moreover, stable lines of transfected mouse 3T3 cells constitutively expressing Mx1 protein were protected from Dhori virus infection. The MxA protein of human cells shows a high degree of sequence similarity to Mx1 but, unlike Mx1, inhibits a broad range of RNA viruses. Transgenic mice that permanently express the human MxA protein in various organs became resistant to infection with Thogoto virus but remained fully susceptible to Dhori virus. These in vivo results show that DHO virus is unique in being resistant to human MxA but susceptible to mouse Mx1 protein. They further indicate that the Mx1 system functions as a potent defense mechanism against tick-borne influenza-like viruses in mice.

The first decade of research advances in influenza D virus

From its initial isolation in the USA in 2011 to the present, influenza D virus (IDV) has been detected in cattle and swine populations worldwide. IDV has exceptional thermal and acid stability and a broad host range. The virus utilizes cattle as its natural reservoir and amplification host with periodic spillover to other mammalian species, including swine. IDV infection can cause mild to moderate respiratory illnesses in cattle and has been implicated as a contributor to bovine respiratory disease (BRD) complex, which is the most common and costly disease affecting the cattle industry. Bovine and swine IDV outbreaks continue to increase globally, and there is increasing evidence indicating that IDV may have the potential to infect humans. This review discusses recent advances in IDV biology and epidemiology, and summarizes our current understanding of IDV pathogenesis and zoonotic potential.

Amblyomma variegatum (Acari: Ixodidae): mechanism and control of arbovirus secretion in tick saliva

Saliva is considered to be the conduit by which pathogens are transmitted from blood-sucking arthropod vectors to their vertebrate hosts, but supporting evidence for this is fragmentary. To determine if Thogoto (THO) virus, a tick-borne member of the influenza virus family, is transmitted via tick saliva, and whether virus replication is a prerequisite for such transmission, two experimental conditions were compared: (1) "biological transmission" and (2) "mechanical transmission." In (1), THO virus was allowed to infect and replicate in a natural vector, Amblyomma variegatum: virus was detected in saliva collected from 3/22 (14%) ticks. In (2), virus was inoculated directly into the hemocoel with the drug used to induce salivation and saliva was collected immediately to preclude the possibility of virus replication: virus was detected in saliva collected from 31/170 (18%) ticks. The results demonstrate that THO virus is secreted in tick saliva and that virus can pass from the hemolymph to the salivary glands independently of viral replication within the tick. The comparatively low numbers of ticks that yielded virus-positive saliva samples together with the results from assays of serial saliva samples suggested that virus secretion may not be a continuous process during salivation. Ticks in which THO virus had established an infection showed an impaired secretory response compared with uninfected ticks and ticks used for mechanical transmission.

Thogoto and Dhori virus replication is blocked by inhibitors of cellular polymerase II activity but does not cause shutoff of host cell protein synthesis

Tick-transmitted Thogoto and Dhori viruses share structural and genetic properties with the influenza viruses. Here, we compare different steps of their replication cycle in mammalian cells in comparison with influenza A virus. Viral antigens of both viruses accumulated in the nuclei of infected cells, suggesting a nuclear phase of viral replication. Furthermore, as observed with influenza viruses, transcription of Thogoto and Dhori viruses was inhibited by alpha-amanitin and actinomycin D, suggesting a dependence of viral transcription on cellular RNA polymerase II activity. In contrast to influenza viruses, Thogoto and Dhori virus infection did not lead to down-regulation of cellular protein synthesis indicating marked differences regarding the fate of infected cells.

Comparison of Pathogenicity and Transmissibility of Influenza B and D Viruses in Pigs

Influenza viruses are important pathogens causing respiratory disease in humans and animals. In contrast to influenza A virus (IAV) that can infect a wide range of animal species, other influenza viruses, including influenza B virus (IBV), influenza C virus (ICV), and influenza D virus (IDV) have a limited host range. Swine can be infected with all four different genera of influenza viruses. IAV infection of pigs causes the well-known swine influenza that poses significant threats to human and animal health. However, influenza virus infection of pigs with IBV, ICV, and IDV are not well-characterized. Herein, we compared pathogenicity of IBV and IDV using intratracheal and intranasal infection of pigs, which are IAV seropositive, and commingled naïve pigs with the infected animals to determine their transmissibility. Both viruses caused fever and some lung lesions, replicated in the lungs of infected pigs, but only IDV transmitted to the contact animals. Although IBV and IDV displayed differing levels of replication in the respiratory tract of infected pigs, no significant differences in pathogenicity of both viruses were observed. These results indicate that both IBV and IDV can replicate, and are pathogenic in pigs.

Formation of virus-like particles from cloned cDNAs of Thogoto virus

Thogoto virus (THOV) is the type species of tick-transmitted orthomyxoviruses. Here, we describe the generation of virus-like particles (VLP) of THOV from cloned cDNAs. To synthesize the six structural proteins of THOV in mammalian cells, we used T7-controlled expression plasmids and a recombinant vaccinia virus producing T7 RNA polymerase. A minireplicon encoding a reporter gene flanked by THOV promoter sequences was expressed by the cellular RNA polymerase I. The recombinant proteins were functional in encapsidation, amplification and transcription of the minireplicon RNA. Furthermore, the artificial nucleocapsids were packaged into THO-VLPs that transferred the minireplicon to indicator cells. This system should be helpful in generating recombinant THOV entirely from cloned cDNAs.

Mx1-based resistance to thogoto virus in A2G mice is bypassed in tick-mediated virus delivery

The interferon-induced mouse Mx1 protein has intrinsic antiviral activity against orthomyxoviruses, including Thogoto virus. Thus, Mx1(+) A2G mice are apparently resistant to infection following needle- or tick-borne virus challenge. However, tick-borne challenge and, to a lesser degree, injection of virus mixed with tick salivary gland extract resulted in virus transmission to uninfected ticks feeding on the A2G mice. The data indicate that immunomodulatory components in tick saliva can overcome a natural antiviral mechanism.

Rhipicephalus appendiculatus (Acari: Ixodidae): dynamics of Thogoto virus infection in female ticks during feeding on guinea pigs

Engorged nymphs (Rhipicephalus appendiculatus) were inoculated parenterally with Thogoto (THO) virus (approximately 1 microl per nymph; 10(6)-10(7) PFU/ml). The adult females which resulted were used as the source of infected ticks for this study. Hemolymph, salivary glands, synganglion, gut, ovary, and Malpighian tubules were collected on each day of the blood meal and titrated for THO virus by plaque assay. The percent of tissues infected with virus was 16% or less on the day of attachment. Percent infection rose for all tissues throughout 6-7 days of feeding, reaching 40-100% infection during the rapid phase of engorgement. For the first 4 days of feeding, virus titer in the synganglion was higher than in salivary glands (means of 6.4-34.7 PFU/synganglion and 1.6-8.8 PFU/salivary gland pair). From days 5-7, virus titer was generally higher in the salivary gland than the synganglion (means of 422, 408, and 817 PFU/gland pair and means of 62, 811, and 9 PFU/synganglion). However, because a salivary gland pair is much heavier than a synganglion, the virus concentration in the synganglion was much higher than in the salivary gland during the slow phase of feeding. During the rapid phase of feeding, the difference in virus titer between the synganglion and salivary gland reduced. This difference between the early and late stages of feeding may explain why a previous study [J. Gen. Virol. 70 (1989) 1093], using immunofluorescence and immuno-gold labelling, failed to detect virus in the salivary gland early in feeding. These data provide evidence to explain that R. appendiculatus can transmit THO virus within 24h of attachment, an important epidemiological finding.

Intra-stadial tick-borne Thogoto virus (Orthomyxoviridae) transmission: accelerated arbovirus transmission triggered by host death

Haematophagous insect vectors of arthropod-borne viruses (arboviruses) feed repeatedly. Consequently, they can transmit arboviruses to more than one host during the same developmental stage (intra-stadial transmission). By contrast, ixodid ticks generally feed only once at each parasitic stage (larva, nymph, and adult) and hence they have only one opportunity for tickborne virus transmission per stadium (inter-stadial transmission). Under natural conditions, tick-infested hosts may die (from disease or other causes) before the ticks have completed their long period of engorgement. A laboratory model was used to investigate the consequences of premature host death on tick-borne virus transmission. We report intra-stadial transmission of Thogoto virus by the nymphal, male, and female ticks of Rhipicephalus appendiculatus. Tick-borne Thogoto virus infection caused viraemia and death of hamsters before the nymphal and adult ticks feeding on them had completed engorgement. The resulting partially fed ticks were allowed to continue engorgement on new, uninfected hosts (interrupted feeding). During feeding on the subsequent hosts, they transmitted the virus intra-stadially to susceptible hosts (hamsters), and to uninfected co-feeding ticks on non-susceptible hosts (guinea-pigs). Intra-stadial transmission, mediated by interrupted feeding, may help explain outbreaks of rapid and fatal tick-borne viral diseases, and the epidemiology as well as evolution of virulence, in a susceptible host population. Additionally, intra-stadial transmission provides an increased risk of tick-borne pathogen transmission to humans and domestic animals during slaughter and game hunting.

Functional comparison of the two gene products of Thogoto virus segment 6

The sixth genomic segment of Thogoto virus (THOV) encodes two proteins, the viral matrix protein (M) and an accessory protein with an interferon (IFN)-antagonistic function named ML. M and ML are shown in this study to be structural components of the virion. Using an in vivo system based on the reconstitution of functional THOV ribonucleoprotein complexes from cloned cDNAs, it was demonstrated that M has an inhibitory effect on the viral RNA-dependent RNA polymerase (RdRP) and is essential for the formation of virus-like particles (VLPs). The functional domain responsible for the regulation of RdRP activity resides within the C-terminal half of M, while full-length M protein is required for VLP formation. The ML protein cannot complement M with respect to either RdRP downregulation or particle formation, although it is identical to M apart from a 38 aa extension at the C terminus. In contrast, ML, but not M, is able to prevent the induction of IFN-beta by double-stranded RNA. This function is contained within the C-terminal half of ML. These data suggest major structural differences between M and ML that could explain the different activities of the two proteins.

Essential Role of Interferon Response in Containing Human Pathogenic Bourbon Virus

Bourbon virus (BRBV) is a recently discovered tick-transmitted viral pathogen that is prevalent in the Midwest and southern United States. Since 2014, zoonotic BRBV infections have been verified in several human cases of severe febrile illness, occasionally with fatal outcomes, indicating a possible public health threat. We analyzed the pathology of BRBV infection in mice and found a high sensitivity of the virus to the host interferon system. Infected standard laboratory mice did not show clinical signs or virus replication. However, in mice carrying defects in the type I and type II interferon system, the virus grew to high titers and caused severe pathology. In cell culture, BRBV was blocked by antiviral agents like ribavirin and favipiravir (T705). Our data suggest that persons having severe BRBV infection might have a deficiency in their innate immunity and could benefit from an already approved antiviral treatment.

Rescue of synthetic RNAs into thogoto and influenza A virus particles using core proteins purified from Thogoto virus

The ribonucleoprotein (RNP) complexes of Thogoto virus (THOV), a tick-borne orthomyxovirus, have been purified from detergent-lysed virions. The purified RNPs were then disrupted by centrifugation through a CsCl-glycerol gradient to obtain fractions highly enriched in nucleoprotein (NP) and virtually devoid of viral genomic RNA. When these NP-enriched fractions were incubated with a synthetic THOV-like RNA, and the mixtures were transfected into THOV-infected cells, the synthetic RNA was expressed and packaged into THOV particles. Similarly, hybrid mixtures containing purified THOV NP and influenza A virus synthetic RNAs (either a model CAT RNA or a gene encoding the viral neuraminidase), were prepared and transfected into influenza A virus-infected cells. The synthetic CAT RNA, was shown to be expressed and packaged into virus particles, and the neuraminidase gene was rescued into influenza virions. These data are discussed in terms of the similarities observed between THOV and influenza A virus and the potential application of the THOV purified proteins for rescuing synthetic genes into infectious viruses.

Salivary fluid secretion in the ixodid tick Rhipicephalus appendiculatus is inhibited by Thogoto virus infection

Adult Rhipicephatus appendiculatus ticks, infected with Thogoto (THO) virus or control, were fed on guinea pigs and removed at intervals throughout the feeding cycle. Salivary fluid secretion was measured by an in vitro technique. The salivary glands of infected, partially-fed ticks secreted fluid in vitro at about 75% the rate of controls, but the difference between infected and controls among engorged ticks was not statistically significant. Basal and DA-stimulated levels of cyclic AMP (cAMP) were determined in isolated glands and were significantly affected by THO virus infection. The differences in secretory rate among control and infected ticks could not be explained in terms of altered cAMP levels. Haemolymph volume was measured by a tracer-dilution technique using 3H-inulin. The mean haemolymph volume for both THO-infected and control groups was between 23-24% body weight throughout the feeding cycle, indicating that infection by this arbovirus did not influence salivary fluid secretion via altered haemolymph volume. The mechanism by which THO virus affects secretory activity of its tick vector remains unknown.

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

Infections with emerging and re-emerging arboviruses are of increasing concern for global health. Tick-transmitted RNA viruses of the genus Thogotovirus in the Orthomyxoviridae family have considerable zoonotic potential, as indicated by the recent emergence of Bourbon virus in the USA. To successfully infect humans, arboviruses have to escape the restrictive power of the interferon defense system. This is exemplified by the high sensitivity of thogotoviruses to the antiviral action of the interferon-induced myxovirus resistance protein A (MxA) that inhibits the polymerase activity of incoming viral ribonucleoprotein complexes. Acquiring resistance to human MxA would be expected to enhance the zoonotic potential of these pathogens. Therefore, we screened a panel of 10 different thogotovirus isolates obtained from various parts of the world for their sensitivity to MxA. A single isolate from Nigeria, Jos virus, showed resistance to the antiviral action of MxA in cell culture and in MxA-transgenic mice, whereas the prototypic Sicilian isolate SiAr126 was fully MxA-sensitive. Further analysis identified two amino acid substitutions (G327R and R328V) in the viral nucleoprotein as determinants for MxA resistance. Importantly, when introduced into SiAr126, the R328V mutation resulted in complete MxA escape of the recombinant virus, without causing any viral fitness loss. The escape mutation abolished viral nucleoprotein recognition by MxA and allowed unhindered viral growth in MxA-expressing cells and in MxA-transgenic mice. These findings demonstrate that thogotoviruses can overcome the species barrier by escaping MxA restriction and reveal that these tick-transmitted viruses may have a greater zoonotic potential than previously suspected.

Thogotovirus infections are known to cause isolated human fatalities, yet the zoonotic potential of these tick-transmitted pathogens is still largely unexplored. In the present study, we examined if these viruses are able to escape the interferon-induced human MxA, thereby overcoming the human innate antiviral defense. Mx proteins constitute a class of interferon-induced antiviral effector molecules that efficiently block the intracellular replication of many viruses. Here, we studied the MxA sensitivity of various thogotovirus isolates and identified two amino acid residues in the viral nucleoprotein that caused resistance to MxA. One of these exchanges was sufficient to enable an otherwise MxA-sensitive thogotovirus to fully escape MxA restriction without causing any fitness loss. Our study explores the interplay of thogotoviruses with the innate antiviral host defense and sheds light on their zoonotic potential.

Isolation and development of bovine primary respiratory cells as model to study influenza D virus infection

Influenza D virus (IDV) is a novel type of influenza virus that infects and causes respiratory illness in bovines. Lack of host-specific in vitro model that can recapitulate morphology and physiology of in vivo airway epithelial cells has impeded the study of IDV infection. Here, we established and characterized bovine primary respiratory epithelial cells from nasal turbinate, soft palate, and trachea of the same calf. All three cell types showed characteristics peculiar of epithelial cells, polarized into apical-basolateral membrane, and formed tight junctions. Furthermore, these cells expressed both α-2,3- and α-2,6-linked sialic acids with α-2,3 linkage being more abundant. IDV strains replicated to high titers in these cells, while influenza A and B viruses exhibited moderate to low titers, with influenza C virus replication not detected. These findings suggest that bovine primary airway epithelial cells can be utilized to model infection biology and pathophysiology of IDV and other respiratory pathogens.

Establishment of a Reverse Genetic System from a Bovine Derived Influenza D Virus Isolate

The ruminant-associated influenza D virus (IDV) has a broad host tropism and was shown to have zoonotic potential. To identify and characterize molecular viral determinants influencing the host spectrum of IDV, a reverse genetic system is required. For this, we first performed 5′ and 3′ rapid amplification of cDNA ends (RACE) of all seven genomic segments, followed by assessment of the 5′ and 3′ NCR activity prior to constructing the viral genomic segments of a contemporary Swiss bovine IDV isolate (D/CN286) into the bidirectional pHW2000 vector. The bidirectional plasmids were transfected in HRT-18G cells followed by viral rescue on the same cell type. Analysis of the segment specific 5′ and 3′ non-coding regions (NCR) highlighted that the terminal 3′ end of all segments harbours an uracil instead of a cytosine nucleotide, similar to other influenza viruses. Subsequent analysis on the functionality of the 5′ and 3′ NCR in a minireplicon assay revealed that these sequences were functional and that the variable sequence length of the 5′ and 3′ NCR influences reporter gene expression. Thereafter, we evaluated the replication efficiency of the reverse genetic clone on conventional cell lines of human, swine and bovine origin, as well as by using an in vitro model recapitulating the natural replication site of IDV in bovine and swine. This revealed that the reverse genetic clone D/CN286 replicates efficiently in all cell culture models. Combined, these results demonstrate the successful establishment of a reverse genetic system from a contemporary bovine IDV isolate that can be used for future identification and characterization of viral determinants influencing the broad host tropism of IDV.

Segment-specific promoter activity for RNA synthesis in the genome of Oz virus, genus Thogotovirus

Oz virus (OZV), a tick-borne, six-segmented negative-strand RNA virus in the genus Thogotovirus, caused a fatal human infection in Japan in 2023. To study viral RNA synthesis, we developed an OZV minigenome assay using mammalian cells. This revealed variations in promoter activities among the six genome segments. The "distal duplex," a double-stranded RNA structure beginning at the 11th nucleotide on the 5' end and the 10th on the 3' end, was found in all segments. A factor affecting promoter activity was the base pairing between the 12th nucleotide at the 5' end and the 11th at the 3' end, forming either G:C or A:U pairs. Disruption of this pairing caused a significant loss of promoter activity, emphasizing the importance of the distal duplex with at least six consecutive base pairs. Comparative analysis of genome terminal sequences suggests similar structural variations in the promoters of other species in Thogotovirus.

ANP32 proteins from ticks and vertebrates are key host factors for replication of Bourbon virus across species

Bourbon virus (BRBV) is a tick-borne virus in the genus Thogotovirus in the Orthomyxoviridae family. BRBV was initially identified as the presumptive causative agent of a fatal human infection in 2014 and has since been identified in ticks in the Midwest, Northeast, and Southern United States, with occasional spillovers into humans. However, little is known about how virus-host interactions impact their large host range. Here, we show that BRBV polymerase activity in human cells is completely dependent on cellular ANP32 proteins. BRBV polymerase activity was completely lost in cells lacking ANP32A and ANP32B, resulting in failed infections. BRBV polymerase activity was restored in the presence of ANP32 proteins from diverse hosts. Dhori virus and Thogoto virus, other related Thogotovirus members, retained high activity in the absence of ANP32 proteins, showing reduced dependence on these host factors. Interaction studies revealed that the BRBV polymerase trimer binds human ANP32A or ANP32B. Genetic analysis revealed that tick vectors for BRBV encode a single ANP32 locus corresponding to ANP32A. Tick ANP32A produces multiple protein variants through alternative splicing and start-site selection, all of which enhance polymerase activity for Thogotoviruses. Unexpectedly, the BRBV polymerase was highly sensitive to changes at the N-terminus of ANP32, while it was insensitive to changes in the body of ANP32 that restrict the activity of influenza virus polymerases. Thus, ANP32A is a deeply conserved pro-viral cofactor, and Thogotoviruses show remarkable plasticity utilizing ANP32 homologs from different hosts separated by almost 1 billion years of evolution.IMPORTANCEViral polymerases rely on cellular cofactors to support efficient transcription of viral genes and replication of the viral genome. The RNA-dependent RNA polymerase of influenza virus, an orthomyxovirus, requires the cellular ANP32A or ANP32B proteins for genome replication. However, little is known about whether ANP32 proteins are required by other orthomyxovirus family members, like the tick-borne thogotoviruses. We show that thogotoviruses use ANP32 proteins from diverse hosts to enhance polymerase activity, including that encoded by the single ANP32A gene found in ticks. However, thogotovirus polymerase showed varying levels of dependence on ANP32 proteins, with some polymerases functioning at near full activity even in the absence of ANP32 proteins. Thus, ANP32 proteins are deeply conserved viral cofactors, with each virus displaying distinct patterns of ANP32 usage and requirements for function.