Tick-borne viruses structurally similar to Orthomyxoviruses

Cryo-EM structures of Thogoto virus polymerase reveal unique RNA transcription and replication mechanisms among orthomyxoviruses

Influenza viruses and thogotoviruses account for most recognized orthomyxoviruses. Thogotoviruses, exemplified by Thogoto virus (THOV), are capable of infecting humans using ticks as vectors. THOV transcribes mRNA without the extraneous 5' end sequences derived from cap-snatching in influenza virus mRNA. Here, we report cryo-EM structures to characterize THOV polymerase RNA synthesis initiation and elongation. The structures demonstrate that THOV RNA transcription and replication are able to start with short dinucleotide primers and that the polymerase cap-snatching machinery is likely non-functional. Triggered by RNA synthesis, asymmetric THOV polymerase dimers can form without the involvement of host factors. We confirm that, distinctive from influenza viruses, THOV-polymerase RNA synthesis is weakly dependent of the host factors ANP32A/B/E in human cells. This study demonstrates varied mechanisms in RNA synthesis and host factor utilization among orthomyxoviruses, providing insights into the mechanisms behind thogotoviruses' broad-infectivity range.

Emerging tickborne viruses vectored by Amblyomma americanum (Ixodida: Ixodidae): Heartland and Bourbon viruses

Heartland (HRTV) and Bourbon (BRBV) viruses are newly identified tick-borne viruses, isolated from serious clinical cases in 2009 and 2014, respectively. Both viruses originated in the lower Midwest United States near the border of Missouri and Kansas, cause similar disease manifestations, and are presumably vectored by the same tick species, Amblyomma americanum Linnaeus (Ixodida: Ixodidae). In this article, we provide a current review of HRTV and BRBV, including the virology, epidemiology, and ecology of the viruses with an emphasis on the tick vector. We touch on current challenges of vector control and surveillance, and we discuss future directions in the study of these emergent pathogens.

Bourbon virus, a newly discovered zoonotic thogotovirus

The recent discovery of Bourbon virus (BRBV) put a new focus on the genus of thogotoviruses as zoonotic, tick-transmitted pathogens within the orthomyxovirus family. Since 2014, BRBV has been linked to several human cases in the Midwest United States with severe acute febrile illness and a history of tick bites. The detection of the virus in the Lone Star tick, Amblyomma americanum, and a high sero-prevalence in wild animals suggest widespread circulation of BRBV. Phylogenetic analysis of the viral RNA genome classified BRBV into the subgroup of Dhori-like thogotoviruses. Strikingly, BRBV is apathogenic in mice, contrasting not only with the fatal disease in affected patients but also with the severe disease in mice caused by other members of the thogotovirus genus. To gain insights into this intriguing discrepancy, we will review the molecular biology and pathology of BRBV and its unique position within the thogotovirus genus. Lastly, we will discuss the zoonotic threat posed by this newly discovered pathogen.

The Antiviral Activity of Equine Mx1 against Thogoto Virus Is Determined by the Molecular Structure of Its Viral Specificity Region

Mammalian myxovirus resistance (Mx) proteins are interferon-induced, large dynamin-like GTPases with a broad antiviral spectrum. Here, we analyzed the antiviral activity of selected mammalian Mx1 proteins against Thogoto virus (THOV). Of those, equine Mx1 (eqMx1) showed antiviral activity comparable to that of the human MX1 gene product, designated huMxA, whereas most Mx1 proteins were antivirally inactive. We previously demonstrated that the flexible loop L4 protruding from the stalk domain of huMxA, and especially the phenylalanine at position 561 (F561), determines its antiviral specificity against THOV (P. S. Mitchell, C. Patzina, M. Emerman, O. Haller, et al., Cell Host Microbe 12:598-604, 2012, https://doi.org/10.1016/j.chom.2012.09.005). However, despite the similar antiviral activity against THOV, the loop L4 sequence of eqMx1 substantially differs from the one of huMxA. Mutational analysis of eqMx1 L4 identified a tryptophan (W562) and the adjacent glycine (G563) as critical antiviral determinants against THOV, whereas the neighboring residues could be exchanged for nonpolar alanines without affecting the antiviral activity. Further mutational analyses revealed that a single bulky residue at position 562 and the adjacent tiny residue G563 were sufficient for antiviral activity. Moreover, this minimal set of L4 amino acids transferred anti-THOV activity to the otherwise inactive bovine Mx1 (boMx1) protein. Taken together, our data suggest a fairly simple architecture of the antiviral loop L4 that could serve as a mutational hot spot in an evolutionary arms race between Mx-escaping viral variants and their hosts. IMPORTANCE Most mammals encode two paralogs of the interferon-induced Mx proteins: Mx1, with antiviral activity largely against RNA viruses, like orthomyxoviruses and bunyaviruses; and Mx2, which is antivirally active against HIV-1 and herpesviruses. The human Mx1 protein, also called huMxA, is the best-characterized example of mammalian Mx1 proteins and was recently shown to prevent zoonotic virus transmissions. To evaluate the antiviral activity of other mammalian Mx1 proteins, we used Thogoto virus, a tick-transmitted orthomyxovirus, which is efficiently blocked by huMxA. Interestingly, we detected antiviral activity only with equine Mx1 (eqMx1) but not with other nonprimate Mx1 proteins. Detailed functional analysis of eqMx1 identified amino acid residues in the unstructured loop L4 of the stalk domain critical for antiviral activity. The structural insights of the present study explain the unique position of eqMx1 antiviral activity within the collection of nonhuman mammalian Mx1 proteins.

Comparative study of ten thogotovirus isolates and their distinct in vivo characteristics

Thogotoviruses are tick-borne arboviruses that comprise a unique genus within the Orthomyxoviridae family. Infections with thogotoviruses primarily cause disease in livestock with occasional reports of human infections suggesting a zoonotic potential. In the past, multiple genetically distinct thogotoviruses were isolated mostly from collected ticks. However, many aspects regarding their phylogenetic relationships, morphological characteristics and virulence in mammals remain unclear. For the present comparative study, we used a collection of ten different thogotovirus isolates from different geographic areas. Next generation sequencing and subsequent phylogenetic analyses revealed a distinct separation of these viruses into two major clades - the Thogoto-like and Dhori-like viruses. Electron microscopy demonstrated a heterogeneous morphology with spherical and filamentous particles being present in virus preparations. To study their pathogenicity, we analyzed the viruses in a small animal model system. In intraperitoneally infected C57BL/6 mice, all isolates showed a tropism for liver, lung and spleen. Importantly, we did not observe horizontal transmission to uninfected, highly susceptible contact mice. The isolates enormously differed in their capacity to induce disease, ranging from subclinical to fatal outcomes. In vivo multi-step passaging experiments of two low-pathogenic isolates showed no increased virulence and sequence analyses of the passaged viruses indicated a high stability of the viral genomes after ten mouse passages. In summary, our analysis demonstrates the broad genetic and phenotypic variability within the thogotovirus genus. Moreover, thogotoviruses are well adapted to mammals but their horizontal transmission seems to depend on ticks as their vectors. Importance Since their discovery over sixty years ago, fifteen genetically distinct members of the thogotovirus genus have been isolated. These arboviruses belong to the Orthomyxovirus family and share many features with influenza viruses. However, numerous of these isolates have not been characterized in depth. In the present study, we comparatively analyzed a collection of ten different thogotovirus isolates to answer basic questions about their phylogenetic relationships, morphology and pathogenicity in mice. Our results highlight shared and unique characteristics of this diverse genus. Taken together, these observations provide a framework for the phylogenic classification and phenotypic characterization of newly identified thogotovirus isolates that could potentially cause severe human infections as exemplified by the recently reported, fatal Bourbon virus cases in the United States.

Eight years' advances on Bourbon virus, a tick-born Thogotovirus of the Orthomyxovirus family

Bourbon virus (BRBV) was first isolated from a blood sample collected from a male patient living in Bourbon county, Kansas, during the spring of 2014. The patient later died due to complications associated with multiorgan failure. Currently, several BRBV infection-caused deaths have been reported in the United States, and misdiagnosed cases are often undercounted. BRBV is a member of the genus Thogotovirus of the Orthomyxoviridae family, and is transmitted through the Lone Star tick, Amblyomma Americanum, in North America. Currently, there are no specific antivirals or vaccinations available to treat or prevent BRBV infection. Several small molecular compounds have been identified to effectively inhibit BRBV infection of in vitro cell cultures at a single- or sub-micromolar level. Favipiravir, an RNA-dependent RNA polymerase inhibitor, prevented the death of Type I interferon receptor knockout mice infected with BRBV infection.

Novel quaranjavirus and other viral sequences identified from ticks parasitizing hunted wildlife in Trinidad and Tobago

Hunters are at a higher risk for exposure to zoonotic pathogens due to their close interactions with wildlife and arthropod vectors. In this study, high throughput sequencing was used to explore the viromes of two tick species, Amblyomma dissimile and Haemaphysalis juxtakochi, removed from hunted wildlife in Trinidad and Tobago. We identified sequences from 3 new viral species, from the viral families Orthomyxoviridae, Chuviridae and Tetraviridae in A. dissimile.

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.

Electron Microscopy in Discovery of Novel and Emerging Viruses from the Collection of the World Reference Center for Emerging Viruses and Arboviruses (WRCEVA)

Since the beginning of modern virology in the 1950s, transmission electron microscopy (TEM) has been an important and widely used technique for discovery, identification and characterization of new viruses. Using TEM, viruses can be differentiated by their ultrastructure: shape, size, intracellular location and for some viruses, by the ultrastructural cytopathic effects and/or specific structures forming in the host cell during their replication. Ultrastructural characteristics are usually sufficient for the identification of a virus to the family level. In this review, we summarize 25 years of experience in identification of novel viruses from the collection of the World Reference Center for Emerging Viruses and Arboviruses (WRCEVA).

Characterization of a novel thogotovirus isolated from Amblyomma testudinarium ticks in Ehime, Japan: A significant phylogenetic relationship to Bourbon virus

The genus Thogotovirus, as represented by Thogoto virus and Dhori virus, comprises a group of arthropod-borne viruses, most members of which are transmitted by ticks. Here we report the genetic and biological characterization of a new thogotovirus, designated Oz virus (OZV), isolated from the hard tick Amblyomma testudinarium in Ehime, Japan. OZV efficiently replicated and induced a cytopathic effect in Vero cells, from which enveloped pleomorphic virus particles were formed by budding. OZV could also replicate in BHK-21 and DH82 cells and caused high mortality in suckling mice after intracerebral inoculation. Phylogenetic analyses of six viral proteins indicated that OZV is clustered with Dhori and related viruses, and is most closely related in glycoprotein (GP) and matrix protein (M) sequences to Bourbon virus, a human-pathogenic thogotovirus discovered recently in the United States. Our findings emphasize the need for understanding the geographic distribution and ecology of OZV and related viruses and for reevaluation of the medical and public health importance of thogotoviruses.

More pronounced salt dependence and higher reactivity for platination of the hairpin r(CGCGUUGUUCGCG) compared with d(CGCGTTGTTCGCG)

The DNA interference pathways exhibited by cisplatin and related anticancer active metal complexes have been extensively studied. Much less is known to what extent RNA interaction pathways may operate in parallel, and perhaps contribute to both antineoplastic activity and toxicity. The present study was designed with the aim of comparing the reactivity of two model systems comprising RNA and DNA hairpins, r(CGCGUUGUUCGCG) and d(CGCGTTGTTCGCG), towards a series of platinum(II) complexes. Three platinum complexes were used as metallation reagents; cis-[PtCl(NH3)2(OH2)]+ (1), cis-[PtCl(NH3)(c-C6H11NH2)(OH2)]+ (2), and trans-[PtCl(NH3)(quinoline)(OH2)]+ (3). The reaction kinetics were studied at pH 6.0, 25 degrees C, and 1.0 mM < or = I < or = 500 mM. For both types of nucleic acid targets, compound 3 was found to react about 1 order of magnitude more rapidly than compounds 1 and 2. Further, all platinum compounds exhibited a more pronounced salt dependence for the interaction with r(CGCGUUGUUCGCG). Chemical and enzymatic cleavage studies revealed similar interaction patterns with r(CGCGUUGUUCGCG) after long exposure times to 1 and 2. A substantial decrease of cleavage intensity was found at residues G4 and G7, indicative of bifunctional adduct formation. Circular dichroism studies showed that platinum adduct formation leads to a structural change of the ribonucleic acid. Thermal denaturation studies revealed platination to cause a decrease of the RNA melting temperatures by 5-10 degrees C. Our observations therefore suggest that RNA is a kinetically competitive target to DNA. Furthermore, platination causes destabilization of RNA structural elements, which may lead to deleterious intracellular effects on biologically relevant RNA targets.

Novel gene product of Thogoto virus segment 6 codes for an interferon antagonist

Thogoto virus (THOV) is a tick-transmitted orthomyxovirus with a genome of six negative-stranded RNA segments. The sixth segment encodes two different transcripts: a spliced transcript that is translated into the matrix protein (M) and an unspliced transcript. Here, we report that the unspliced transcript encodes an elongated form of M named ML. A THOV isolate deficient in ML expression was an efficient interferon inducer, whereas ML-expressing wild-type strains were poor interferon inducers. These results were confirmed with recombinant THOVs rescued from cDNAs. Expression of ML efficiently suppressed activation of the beta interferon promoter by double-stranded RNA. These results indicate that ML is an accessory protein that functions as a potent interferon antagonist by blocking transcriptional activation of alpha/beta interferons.

Rescue of recombinant Thogoto virus from cloned cDNA

Thogoto virus (THOV) is a tick-transmitted orthomyxovirus with a genome consisting of six negative-stranded RNA segments. To rescue a recombinant THOV, the viral structural proteins were produced from expression plasmids by means of a vaccinia virus expressing the T7 RNA polymerase. Genomic virus RNAs (vRNAs) were generated from plasmids under the control of the RNA polymerase I promoter. Using this system, we could efficiently recover recombinant THOV following transfection of 12 plasmids into 293T cells. To verify the recombinant nature of the rescued virus, specific genetic tags were introduced into two vRNA segments. The availability of this efficient reverse genetics system will allow us to address hitherto-unanswered questions regarding the biology of THOV by manipulating viral genes in the context of infectious virus.

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.

Thogoto virus matrix protein is encoded by a spliced mRNA

Thogoto virus (THOV) is a tick-transmitted orthomyxovirus with a segmented, negative-stranded RNA genome. In this study, we investigated the coding strategy of RNA segment 6 and found that it contains 956 nucleotides and codes for the matrix (M) protein. The full-length cDNA contains a single, long reading frame that lacks a stop codon but has coding capacity for a putative 35-kDa protein. In contrast, the M protein of THOV has an apparent molecular mass of 29 kDa as assessed by polyacrylamide gel electrophoresis. Therefore, we investigated the possibility of posttranscriptional processing of segment 6 transcripts by reverse transcription-PCR and identified a spliced mRNA that contains a stop codon and is translated into the 29-kDa M protein. Interestingly, the nontemplated UGA stop codon is generated by the splicing event itself. Thus, the unusual M coding strategy of THOV resembles that of Influenza C virus.

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.

The putative polymerase sequence of infectious salmon anemia virus suggests a new genus within the Orthomyxoviridae

The infectious salmon anemia virus (ISAV) is an orthomyxovirus-like virus infecting teleosts. The disease caused by this virus has had major economic consequences for the Atlantic salmon farming industry in Norway, Canada, and Scotland. In this work, we report the cloning and sequencing of an ISAV-specific cDNA comprising 2,245 bp with an open reading frame coding for a predicted protein with a calculated molecular weight of 80.5 kDa. The putative protein sequence shows the core polymerase motifs characteristic of all viral RNA-dependent RNA polymerases. Comparison of the conserved motifs with the corresponding regions of other segmented negative-stranded RNA viruses shows a closer relationship with members of the Orthomyxoviridae than with viruses in other families. The putative ISAV polymerase protein (PB1) has a length of 708 amino acids, a charge of +22 at neutral pH, and a pI of 9.9, which are consistent with the properties of the PB1 proteins of other members of the family. Calculations of the distances between the different PB1 proteins indicate that the ISAV is distantly related to the other members of the family but more closely related to the influenza viruses than to the Thogoto viruses. Based on these and previously published results, we propose that the ISAV comprises a new, fifth genus in the Orthomyxoviridae.

In vivo reconstitution of active Thogoto virus polymerase: assays for the compatibility with other orthomyxovirus core proteins and template RNAs

Tick-borne Thogoto virus (THOV), the prototype of a new genus in the Orthomyxoviridae family, contains six single-stranded RNA segments of negative polarity. Four of them encode gene products that correspond to the influenza virus PB1, PB2, PA and NP core proteins. Here we describe an in vivo system in which the expression of a THOV model RNA is driven by THOV core proteins synthesized from cloned cDNAs. Our results demonstrated the biological activity of our cloned genes and showed that the three polymerase subunits and the NP are required for gene expression. For comparison, we also used the in vivo reconstituted systems of the influenza A and B viruses. None of the polymerase or NP proteins was active in a heterologous orthomyxovirus core, indicating a high specificity in core assembly and/or function. Interestingly, the THOV polymerase did not recognize the influenza A virus promoter and vice versa.

An endonuclease switching mechanism in the virion RNA and cRNA promoters of Thogoto orthomyxovirus

An in vitro assay was developed to investigate endonuclease activity of Thogoto virus, a tick-borne orthomyxovirus. Endonuclease activity relied on an interaction between the 3' and 5' termini of virion RNA (vRNA) and not those of cRNA. Evidence was obtained that cap structures are cleaved directly from cap donors and that cleavage does not occur after pyrimidines. A 5' hook structure, present in the vRNA promoter but not the cRNA promoter, was introduced into cRNA promoter mutants. These mutants stimulated endonuclease activity, although at levels slightly lower than that of vRNA. The ability of the cRNA promoter to stimulate endonuclease activity when mutated to contain a 5' hook structure indicates that this structure constitutes a switching mechanism for endonuclease activity between the vRNA and cRNA promoters.

Striking conformational similarities between the transcription promoters of Thogoto and influenza A viruses: evidence for intrastrand base pairing in the 5' promoter arm

In the accompanying report, we describe an in vitro polymerase assay based on reconstituted Thogoto virus (THOV) cores which provided evidence of a double-stranded vRNA promoter consisting of both the 3' and 5' sequences of vRNA (M. B. Leahy, J. T. Dessens, and P. A. Nuttall, J. Virol. 71:8347-8351, 1997). This system was used to investigate further the THOV vRNA promoter structure by using short, synthetic vRNA promoters. The results obtained show that interstrand base pairing between residues 10 and 11 of the 3' promoter arm with residues 11 and 12 of the 5' promoter arm, respectively, is important for promoter activity. In addition, intrastrand base pairing between residues 2 and 3 with residues 9 and 8 of the 5' promoter arm, respectively, was shown to be involved in promoter activity, while no evidence of intrastrand base pairing between residues 2 and 9 of the 3' promoter arm was obtained. These observations are consistent with a hook-like structure in the 5' promoter arm of the THOV promoter. The THOV cores were able to transcribe an influenza A virus (FLUA) vRNA-like promoter, as well as hybrid THOV-FLUA promoters. Hence, the THOV and FLUA vRNA promoters appear to be both structurally and functionally similar.

In vitro polymerase activity of Thogoto virus: evidence for a unique cap-snatching mechanism in a tick-borne orthomyxovirus

The tick-borne Thogoto virus (THOV) is the type species of a new genus in the family Orthomyxoviridae. Its genome comprises six segments of single-stranded, negative-sense RNA. Each segment possesses conserved regions of semicomplementary nucleotides at the 3' and 5' termini which strongly resemble those of influenza virus. An in vitro polymerase assay based on reconstituted THOV viral cores was developed, and activity was shown to rely on an interaction between the conserved 3'- and 5'-terminal sequences and to be primer dependent. Addition of globin mRNA primed transcription, catalyzing the addition of an extra nucleotide to the transcripts, corresponding to the 5'-terminal m7G cap residue. Priming with various cap analogs suggested that THOV transcription is initiated preferentially with m7GpppAm and involves base pairing. This is the first experimental evidence of endonuclease activity in THOV as part of a unique cap-snatching mechanism.

Genomic characterization of the virus causing infectious salmon anemia in Atlantic salmon (Salmo salar L.): an orthomyxo-like virus in a teleost

The genome of infectious salmon anemia virus (ISAV), which infects farmed Atlantic salmon (Salmo salar L.), is characterized here. The virus has an RNA genome, as shown by using specific DNA virus metabolic inhibitors and radioactive in vivo labeling of ISAV nucleic acid. Electrophoresis of [14C]uridine-labeled ISAV RNA revealed that the ISAV genome is segmented. The genome consists of eight segments that range from 1.0 to 2.3 kb, with a total molecular size of approximately 14.5 kb. One ISAV-specific molecular clone, corresponding to the smallest genome segment, was obtained by cDNA cloning of mRNA from an ISAV-infected cell culture. This clone gave a positive hybridization signal on Northern blots of pelleted ISAV. Pretreatment of the ISAV pellet with RNase A resulted in the disappearance of the positive hybridization signal, demonstrating that the genome is single stranded. Reverse transcriptase PCR with primers corresponding to sequences from the molecular clone and target RNA from ISAV-infected and noninfected fish tissues gave specific positive reactions. Alignments of the nucleotide sequence of the molecular clone did not reveal significant homology with any other available sequence in databases. However, the data presented here, together with morphological and replicational properties previously described, indicate that ISAV has a strong resemblance to members of the Orthomyxoviridae family. This is the first thoroughly characterized orthomyxo-like virus isolated from a teleost.

The fourth genus in the Orthomyxoviridae: sequence analyses of two Thogoto virus polymerase proteins and comparison with influenza viruses

The tick-borne Thogoto virus (THOV) is the type species of a newly recognized fourth genus, Thogotovirus, in the family Orthomyxoviridae. Because of the distant relationship of THOV with the influenza viruses, determination of its genomic information can potentially be used to identify important domains in influenza virus proteins. We have determined the complete nucleotide sequence of the second longest RNA segment of THOV. The molecule comprises 2212 nucleotides with a single large open reading frame encoding a protein of 710 amino acids, estimated Mr 81,284. The protein shares 77% amino acid similarity with the PB1-like protein of Dhori virus, a related tick-borne virus, and 50-53% with the PB1 polymerase proteins of influenza virus A, B and C. All the motifs characteristic of RNA-dependent polymerases were identified including the SSDD motif common to all RNA-dependent RNA polymerases, indicating that the THOV protein is functionally analogous to the influenza virus PB1 proteins and involved in chain elongation. We also report the corrected sequence of the third longest RNA segment of THOV, encoding a protein which shares 44-47% amino acid similarity with the PA-like polymerase proteins of influenza virus A, B and C. The biological significance of conserved domains in these orthomyxovirid proteins is discussed.

The 5' ends of Thogoto virus (Orthomyxoviridae) mRNAs are homogeneous in both length and sequence

Thogoto (THO) virus is a tick-borne member of the Orthomyxoviridae whose genome consists of six segments of linear, negative sense, single-stranded RNA. To gain insight into the mechanism by which viral mRNA transcripts are initiated, poly(A)+ RNA isolated from THO virus-infected cells was characterized by (i) primer extension experiments, (ii) immunoprecipitation studies with an anticap monoclonal antibody, (iii) direct sequencing analysis of the isolated RNA, and (iv) cloning and sequencing of individual mRNA molecules. The results indicated that THO virus mRNAs are capped and homogeneous in both length and sequence at their 5' end. These findings contrast with the situation found in all other segmented, negative sense or ambisense, single-stranded RNA viruses so far analyzed in which the 5' ends of viral mRNAs are heterogeneous in length and sequence. These results are discussed in terms of the mechanism used by THO virus to initiate mRNA synthesis.

Nucleoprotein viral RNA and mRNA of Thogoto virus: a novel "cap-stealing" mechanism in tick-borne orthomyxoviruses?

Tick-borne Thogoto virus (THOV) represents the prototype virus of a new genus in the Orthomyxoviridae family. Its genome consists of six segments of negative-sense, single-stranded RNA. We have cloned and sequenced the fifth genomic segment, which codes for the viral nucleoprotein (NP). The deduced amino acid sequence shows 43% similarity to the NP of Dhori virus, a related tick-transmitted orthomyxovirus, and about 14% sequence similarity to those of the influenza viruses. To reveal the mechanism by which THOV initiates mRNA synthesis, we characterized the 5' ends of the NP mRNAs. Transcripts were recognized by a cap-specific monoclonal antibody, indicating that THOV mRNAs are capped. Surprisingly, no large heterogeneous extensions were found at the 5' end, as would have been expected if THOV were using a classical "cap-stealing" mechanism. We therefore propose that THOV is stealing only the cap structure with one or two additional nucleotides from cellular mRNAs to generate appropriate primers for initiation of viral mRNA transcription.

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.

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.

New aspects of influenza viruses

Influenza virus infections continue to cause substantial morbidity and mortality with a worldwide social and economic impact. The past five years have seen dramatic advances in our understanding of viral replication, evolution, and antigenic variation. Genetic analyses have clarified relationships between human and animal influenza virus strains, demonstrating the potential for the appearance of new pandemic reassortants as hemagglutinin and neuraminidase genes are exchanged in an intermediate host. Clinical trials of candidate live attenuated influenza virus vaccines have shown the cold-adapted reassortants to be a promising alternative to the currently available inactivated virus preparations. Modern molecular techniques have allowed serious consideration of new approaches to the development of antiviral agents and vaccines as the functions of the viral genes and proteins are further elucidated. The development of techniques whereby the genes of influenza viruses can be specifically altered to investigate those functions will undoubtedly accelerate the pace at which our knowledge expands.

Evolutionary relatedness of the predicted gene product of RNA segment 2 of the tick-borne Dhori virus and the PB1 polymerase gene of influenza viruses

The complete nucleotide sequence of the second largest RNA segment of Dhori/India/1313/61 virus was determined and the deduced amino acid sequence was compared with the polymerase (P) proteins of influenza A, B, and C viruses. RNA segment 2 (2224 nucleotides) of Dhori virus contains a single long open reading frame that can encode a 716-amino amid polypeptide (81.3 kDa). The predicted polypeptide shares between 27 and 31% sequence identities with the PB1 polypeptides of influenza A, B, and C viruses. Among the regions most highly conserved are the sequences around the Asp-Asp motif common to many RNA polymerases. In spite of the high level of sequence identity between the Dhori RNA segment 2 gene product and the influenza A, B, and C virus PB1 proteins the amino acid composition of the Dhori protein indicates an acidic charge feature at pH 7.0 in contrast to the basic nature of the PB1 proteins of the influenza viruses. We suggest that the Dhori PB1-like protein be designated the Pα protein of this virus.

Non-viraemic transmission of Thogoto virus: vector efficiency of Rhipicephalus appendiculatus and Amblyomma variegatum

Previous studies have demonstrated that Thogoto virus is transmitted from infected to uninfected ticks when co-feeding on uninfected guinea-pigs, even though the guinea-pigs do not develop a detectable viraemia. Furthermore, tick to tick transmission is potentiated by factors associated with the salivary glands of ticks (saliva activated transmission). The vector efficiency of 2 ixodid tick species, Rhipicephalus appendiculatus and Amblyomma variegatum, for Thogoto virus was assessed using this model. The number of uninfected recipient ticks that acquired Thogoto virus when co-feeding with virus-infected ticks (donors) on uninfected guinea-pigs was determined. When nymphs of either tick species were employed as donors, there was no significant difference in the number of infected recipient nymphs. In contrast, a significant difference in the vector efficiency of adults ticks was observed: 77.0% of recipient ticks which co-fed with R. appendiculatus donor adults acquired Thogoto virus compared to 44.7% of recipient ticks which co-fed with A. variegatum donors. No significant difference in susceptibility to Thogoto virus infection was observed between recipient ticks of the 2 species. Thus, adults of R. appendiculatus are more efficient than A. variegatum in mediating non-viraemic transmission.

Influenza: status and prospects for its prevention, therapy, and control

This article describes current knowledge of the molecular properties of orthomyxoviruses, their epidemiology, and approaches to the control of influenza. The host's response to infection, approaches to prevent infection through vaccination, and the use of antiviral agents to treat or prevent this important infection are covered.

Nucleotide sequence of the tick-borne, orthomyxo-like Dhori/Indian/1313/61 virus envelope gene

The complete nucleotide sequence of the fourth largest segment of single-stranded RNA of the tick-borne, orthomyxo-like Dhori/Indian/1313/61 virus was determined by using cloned cDNA derived from infected cell mRNA. The fourth RNA contains 1586 nucleotides and can code for a protein of 521 amino acids with a molecular weight of 58,675 Da. The predicted polypeptide possesses an amino-terminal hydrophobic region that may function as a signal sequence to initiate translocation across the endoplasmic reticulum membrane and a carboxyl-terminal hydrophobic region that could serve as a stop transfer sequence for anchoring this protein in the membrane. The envelope protein of Dhori virus does not display significant amino acid sequence homology with any of the envelope proteins (hemagglutinin or neuraminidase) of the influenza virus family members (or any other virus group) suggesting that the Dhori envelope protein is unique and may at least in part account for its divergent biological properties with other orthomyxoviruses.

The effect of virus-immune hosts on Thogoto virus infection of the tick, Rhipicephalus appendiculatus

Thogoto (THO) virus infections of Rhipicephalus appendiculatus ticks were examined using tick hosts immune to the virus. In the first set of experiments, ticks were infected by feeding on viraemic hamsters. Inter-stadial infection of THO virus was not affected when ticks ingested a virus-immune bloodmeal but there was an effect on persistence of the virus. The incidence of intra-stadial infection was reduced by at least 40% when nymphs partially fed on viraemic hamsters and completed their bloodmeal on a virus-immune guinea pig. When the reverse situation was examined--feeding on a virus-immune host and then a viraemic host--no difference was observed in the number of ticks infected. In the second set of experiments, uninfected ticks acquired virus by co-feeding with infected ticks on apparently non-viraemic guinea pigs. Non-viraemic transmission of the viruses was inhibited when the guinea pigs were immune to either the Sicilian (SiAr 126) or prototype (IIA) isolates of THO virus. The laboratory data indicate that virus-immune hosts may have a significant effect on the role played by ticks in the epidemiology of tick-borne viruses.

Complete nucleotide sequence of the tick-borne, orthomyxo-like Dhori/Indian/1313/61 virus nucleoprotein gene

The complete nucleotide sequence of the fifth largest segment of single-stranded RNA of the tick-borne, orthomyxo-like Dhori/Ind/1313/61 virus was determined by using cloned cDNA derived from infected cell mRNA and dideoxynucleotide sequencing of viral RNA. The fifth RNA contains 1479 nucleotides and can code for a protein of 477 amino acids with a molecular weight of 53,679 Da. The RNA 5 protein of the Dhori/Ind/1313/61 virus possesses five short regions (16-26 amino acids) which share a high degree (50-59%) of amino acid sequence homology with a computer-aligned consensus sequence of the influenza nucleoprotein gene family. These and other structural features of the RNA 5 protein suggest that RNA 5 of Dhori viruses codes for the nucleoprotein. The data also suggest that Dhori viruses are orthomyxoviruses, but that they are more distantly related to the influenza viruses than type A, B, and C viruses are to each other.

A novel mode of arbovirus transmission involving a nonviremic host

In nature, infected and uninfected arthropod vectors often feed together on an animal. In mimicking this scenario in the laboratory, uninfected vectors were found to acquire virus while cofeeding on the same host as infected vectors. However, the vertebrate host on which they fed did not develop detectable levels of virus in its blood. These observations were made with Thogoto virus, an influenza-like virus of medical and veterinary significance. Rhipicephalus appendiculatus ticks were used as the vector and guinea pigs as the vertebrate host. The results demonstrate that a vertebrate that is apparently refractory to infection by an arthropod-borne virus can still play an important role in the epidemiology of the virus, and they suggest a novel mode of arthropod-borne virus transmission.

3'-Terminal sequences of influenza C virion RNA. Brief report

The 3'-terminal nucleotides of the genome segments of influenza C/Taylor/47. C/Bavaria/79 and C/Johannesburg/1/66 were identified by two RNA sequencing techniques. These comprised 11 nucleotides (3' UCGUCUUCGUC) which were found to be conserved among the genome segments of each virus.