Age- and Sex-Associated Pathogenesis of Cell Culture-Passaged Kemerovo Virus in IFNAR(-/-) Mice.

Kemerovo virus (KEMV) is a tick-borne orbivirus transmitted by ticks of the genus Ixodes. Previous animal experimentation studies with orbiviruses, in particular the interferon receptor double knock-out (IFNAR(-/-)) mouse model, did not indicate bias that is related to age or sex. We endeavoured to assess the effect of serial and alternated passages of KEMV in mammalian or Ixodes cells on virus replication and potential virulence in male or female IFNAR(-/-) mice, with important age differences: younger males (4-5 months old), older males (14-15 months old), and old females (14-15 months old). After 30 serial passages in mammalian or tick cells, or alternated passages in the two cell types, older female mice which were inoculated with the resulting virus strains were the first to show clinical signs and die. Younger males behaved differently from older males whether they were inoculated with the parental strain of KEMV or with any of the cell culture-passaged strains. The groups of male and female mice inoculated with the mammalian cell culture-adapted KEMV showed the lowest viraemia. While older female and younger male mice died by day 6 post-inoculation, surprisingly, the older males survived until the end of the experiment, which lasted 10 days. RNA extracted from blood and organs of the various mice was tested by probe-based KEMV real-time RT-PCR. Ct values of the RNA extracts were comparable between older females and younger males, while the values for older males were >5 Ct units higher for the various organs, indicating lower levels of replication. It is noteworthy that the hearts of the old males were the only organs that were negative for KEMV RNA. These results suggest, for the first time, an intriguing age- and sex-related bias for an orbivirus in this animal model. Changes in the amino acid sequence of the RNA-dependent RNA polymerase of Kemerovo virus, derived from the first serial passage in Ixodes cells (KEMV Ps.IRE1), were identified in the vicinity of the active polymerase site. This finding suggests that selection of a subpopulation of KEMV with better replication fitness in tick cells occurred.

Orbivirus NS4 Proteins Play Multiple Roles to Dampen Cellular Responses.

Non-structural protein 4 (NS4) of insect-borne and tick-borne orbiviruses is encoded by genome segment 9, from a secondary open reading frame. Though a protein dispensable for bluetongue virus (BTV) replication, it has been shown to counter the interferon response in cells infected with BTV or African horse sickness virus. We further explored the functional role(s) of NS4 proteins of BTV and the tick-borne Great Island virus (GIV). We show that NS4 of BTV or GIV helps an E3L deletion mutant of vaccinia virus to replicate efficiently in interferon-treated cells, further confirming the role of NS4 as an interferon antagonist. Our results indicate that ectopically expressed NS4 of BTV localised with caspase 3 within the nucleus and was found in a protein complex with active caspase 3 in a pull-down assay. Previous studies have shown that pro-apoptotic caspases (including caspase 3) suppress type I interferon response by cleaving mediators involved in interferon signalling. Our data suggest that orbivirus NS4 plays a role in modulating the apoptotic process and/or regulating the interferon response in mammalian cells, thus acting as a virulence factor in pathogenesis.

The phenuivirus Toscana virus makes an atypical use of vacuolar acidity to enter host cells.

Toscana virus is a major cause of arboviral disease in humans in the Mediterranean basin during summer. However, early virus-host cell interactions and entry mechanisms remain poorly characterized. Investigating iPSC-derived human neurons and cell lines, we found that virus binding to the cell surface was specific, and 50% of bound virions were endocytosed within 10 min. Virions entered Rab5a+ early endosomes and, subsequently, Rab7a+ and LAMP-1+ late endosomal compartments. Penetration required intact late endosomes and occurred within 30 min following internalization. Virus entry relied on vacuolar acidification, with an optimal pH for viral membrane fusion at pH 5.5. The pH threshold increased to 5.8 with longer pre-exposure of virions to the slightly acidic pH in early endosomes. Strikingly, the particles remained infectious after entering late endosomes with a pH below the fusion threshold. Overall, our study establishes Toscana virus as a late-penetrating virus and reveals an atypical use of vacuolar acidity by this virus to enter host cells.

New Cell Lines Derived from Laboratory Colony Triatoma infestans and Rhodnius prolixus, Vectors of Trypanosoma cruzi, Do Not Harbour Triatoma Virus.

Triatomine bugs of the genera Triatoma and Rhodnius are vectors of Chagas disease, a neglected tropical disease of humans in South America caused by Trypanosoma cruzi. Triatoma virus (TrV), a natural pathogen of Triatoma infestans, has been proposed as a possible tool for the bio-control of triatomine bugs, but research into this virus has been hampered by a lack of suitable host cells for in vitro propagation. Here we report establishment and partial characterisation of continuous cell lines from embryos of T. infestans (TIE/LULS54) and Rhodnius prolixus (RPE/LULS53 and RPE/LULS57). RNAseq screening by a sequence-independent, single primer amplification approach confirmed the absence of TrV and other RNA viruses known to infect R. prolixus, indicating that these new cell lines could be used for propagation of TrV.

Coinfection of Two Rickettsia Species in a Single Tick Species Provides New Insight into Rickettsia-Rickettsia and Rickettsia-Vector Interactions.

Rickettsiae are obligate intracellular bacteria that can cause life-threatening illnesses. There is an ongoing debate as to whether established infections by one Rickettsia species preclude the maintenance of the second species in ticks. Here, we identified two Rickettsia species in inoculum from Haemaphysalis montgomeryi ticks and subsequently obtained pure isolates of each species by plaque selection. The two isolates were classified as a transitional group and spotted fever group rickettsiae and named Rickettsia hoogstraalii str CS and Rickettsia rhipicephalii str EH, respectively. The coinfection of these two Rickettsia species was detected in 25.6% of individual field-collected H. montgomeryi. In cell culture infection models, R. hoogstraalii str CS overwhelmed R. rhipicephalii str EH with more obvious cytopathic effects, faster plaque formation, and increased cellular growth when cocultured, and R. hoogstraalii str CS seemed to polymerize actin tails differently from R. rhipicephalii str EH in vitro. This work provides a model to investigate the mechanisms of both Rickettsia-Rickettsia and Rickettsia-vector interactions. IMPORTANCE The rickettsiae are a group of obligate intracellular Gram-negative bacteria that include human pathogens causing an array of clinical symptoms and even death. There is an important question in the field, that is whether one infection can block the superinfection of other rickettsiae. This work demonstrated the coinfection of two Rickettsia species in individual ticks and further highlighted that testing the rickettsial competitive exclusion hypothesis will undoubtedly be a promising area as methods for bioengineering and pathogen biocontrol become amenable for rickettsiae.

Discovery of Tick-Borne Karshi Virus Implies Misinterpretation of the Tick-Borne Encephalitis Virus Seroprevalence in Northwest China.

Despite few human cases of tick-borne encephalitis virus (TBEV), high rates of TBEV seroprevalence were reported among humans and animals in Xinjiang Uygur Autonomous Region in Northwestern China. In this study, the Karshi virus (KSIV) was identified and isolated from Hyalomma asiaticum ticks in Xinjiang. It belongs to the genus Flavivirus of the family Flaviviridae and is closely related to TBEV. KSIV infects cell lines from humans, other mammals and ticks, and causes encephalitis in suckling mice. High minimum infection rates (4.96%) with KSIV were detected among tick groups. KSIV infections have occurred in sheep and marmots, resulting in antibody-positive rates of 2.43 and 2.56%, respectively. We further found that, of the KSIV antibody-positive serum samples from animals, 13.9% had TBEV exposure showing cross-reaction to KSIV, and 11.1% had KSIV infection resulting in cross-reaction to TBEV; 8.3% were likely to have co-exposure to both viruses (or may be infected with one of them and present cross-reactivity with the other). The results revealed a substantial KSIV prevalence among ticks in Xinjiang, indicating exposure of animals to KSIV and TBEV. The findings implied misinterpretation of the high rates of TBEV seroprevalence among humans and animals in previous studies. There is a need to develop detection methods to distinguish KSIV from TBEV and to perform an in-depth investigation of KSIV and TBEV prevalence and incidence in Northwestern China, which would enhance our preparation to provide medical treatment of emerging diseases caused by tick-borne viral pathogens such as KSIV.

New Cell Lines Derived from European Tick Species.

Tick cell lines are important tools for research on ticks and the pathogens they transmit. Here, we report the establishment of ten new cell lines from European ticks of the genera Argas, Dermacentor, Hyalomma, Ixodes and Rhipicephalus originating from Germany and Spain. For each cell line, the method used to generate the primary culture, a morphological description of the cells and species confirmation by sequencing of the partial 16S rRNA gene are presented. Further molecular analysis of the two new Ixodes ricinus cell lines and three existing cell lines of the same species revealed genetic variation between cell lines derived from ticks collected in the same or nearby locations. Collectively, these new cell lines will support research into a wide range of viral, bacterial and protozoal tick-borne diseases prevalent in Europe.

Detection of Antibodies to Ehrlichia spp. in Dromedary Camels and Co-Grazing Sheep in Northern Kenya Using an Ehrlichia ruminantium Polyclonal Competitive ELISA.

A disease with clinical and post-mortem presentation similar to those seen in heartwater, a tick-borne disease of domestic and wild ruminants caused by the intracellular bacterium Ehrlichia ruminantium, was first reported in dromedary camels in Kenya in 2016; investigations carried out at the time to determine the cause were inconclusive. In the present study, we screened sera from Kenyan camels collected before (2015) and after (2020) the 2016 disease outbreak for antibodies to Ehrlichia spp. using an E. ruminantium polyclonal competitive ELISA (PC-ELISA). Median antibody levels were significantly higher (p < 0.0001) amongst camels originating from areas where the heartwater-like disease was reported than from disease-free areas, for animals sampled in both 2015 and 2020. Overall median seropositivity was higher in camels sampled in 2015 than in 2020, which could have been due to higher mean age in the former group. Camels that were PCR-positive for Candidatus Ehrlichia regneryi had significantly lower (p = 0.03) median antibody levels than PCR-negative camels. Our results indicate that Kenyan camels are frequently exposed to E. ruminantium from an early age, E. ruminantium was unlikely to have been the sole cause of the outbreak of heartwater-like disease; and Ca. E. regneryi does not appreciably cross-react with E. ruminantium in the PC-ELISA.

Differential characteristics of mammalian and tick-derived promoters to trigger protein expression in transfected tick cell lines.

The transfection of plasmids into cell lines for the transient expression of exogenous proteins is a fundamental method for characterizing their functions, cellular localization and interactions. Currently, only a few reports on tick transfection systems and expression plasmids specifically constructed for tick cell lines have been published. In this study, the transcriptome of the tick cell line IDE8 was analyzed to screen for highly-expressed genes. The upstream sequences of these genes were selected as possible tick-derived promoters, and their promoter activity was evaluated using a luciferase assay. Four IDE8-derived sequences with promoter activity were identified, and the promoter activities of three common mammalian promoters, CMV, PGK and CAG, were studied and compared in the IDE8 and IRE/CTVM19 tick cell lines. In the two tick cell lines, the efficiency of the CAG promoter was considerably higher than that of CMV, PGK and the four newly-identified tick promoters. Additionally, time course experiments revealed that the protein expression driven by mammalian promoters reached peak levels on day 3, while the protein expression driven by our constructed tick-derived promoters reached peak levels on day 2 in tick cells. By comparing the transfection efficiency of three transfection reagents with different mechanisms in tick cell lines, we identified Effectene (with Enhancer, Qiagen) as the most effective reagent for tick cells. The findings of this study suggested that there are differences between tick and mammalian cell lines in their response to the transfection system. These findings will contribute to future studies on topics including tick protein function, tick genetic modification and tick-host-pathogen interactions.

Virus-Derived DNA Forms Mediate the Persistent Infection of Tick Cells by Hazara Virus and Crimean-Congo Hemorrhagic Fever Virus.

Crimean-Congo hemorrhagic fever (CCHF) is a severe disease of humans caused by CCHF virus (CCHFV), a biosafety level (BSL)-4 pathogen. Ticks of the genus Hyalomma are the viral reservoir, and they represent the main vector transmitting the virus to its hosts during blood feeding. We have previously shown that CCHFV can persistently infect Hyalomma-derived tick cell lines. However, the mechanism allowing the establishment of persistent viral infections in ticks is still unknown. Hazara virus (HAZV) can be used as a BSL-2 model virus instead of CCHFV to study virus/vector interactions. To investigate the mechanism behind the establishment of a persistent infection, we developed an in vitro model with Hyalomma-derived tick cell lines and HAZV. As expected, HAZV, like CCHFV, persistently infects tick cells without any sign of cytopathic effect, and the infected cells can be cultured for more than 3 years. Most interestingly, we demonstrated the presence of short viral-derived DNA forms (vDNAs) after HAZV infection. Furthermore, we demonstrated that the antiretroviral drug azidothymine triphosphate could inhibit the production of vDNAs, suggesting that vDNAs are produced by an endogenous retrotranscriptase activity in tick cells. Moreover, we collected evidence that vDNAs are continuously synthesized, thereby downregulating viral replication to promote cell survival. Finally, vDNAs were also detected in CCHFV-infected tick cells. In conclusion, vDNA synthesis might represent a strategy to control the replication of RNA viruses in ticks allowing their persistent infection. IMPORTANCE Crimean-Congo hemorrhagic fever (CCHF) is an emerging tick-borne viral disease caused by CCHF virus (CCHFV). Ticks of the genus Hyalomma can be persistently infected with CCHFV representing the viral reservoir, and the main vector for viral transmission. Here we showed that tick cells infected with Hazara virus, a nonpathogenic model virus closely related to CCHFV, contained short viral-derived DNA forms (vDNAs) produced by endogenous retrotranscriptase activity. vDNAs are transitory molecules requiring viral RNA replication for their continuous synthesis. Interestingly, vDNA synthesis seemed to be correlated with downregulation of viral replication and promotion of tick cell viability. We also detected vDNAs in CCHFV-infected tick cells suggesting that they could represent a key element in the cell response to nairovirus infection and might represent a more general mechanism of innate immunity against RNA viral infection.

Isolation in Natural Host Cell Lines of Wolbachia Strains wPip from the Mosquito Culex pipiens and wPap from the Sand Fly Phlebotomus papatasi.

Endosymbiotic intracellular bacteria of the genus Wolbachia are harboured by many species of invertebrates. They display a wide range of developmental, metabolic and nutritional interactions with their hosts and may impact the transmission of arboviruses and protozoan parasites. Wolbachia have occasionally been isolated during insect cell line generation. Here, we report the isolation of two strains of Wolbachia, wPip and wPap, during cell line generation from their respective hosts, the mosquito Culex pipiens and the sand fly Phlebotomus papatasi. wPip was pathogenic for both new C. pipiens cell lines, CPE/LULS50 and CLP/LULS56, requiring tetracycline treatment to rescue the lines. In contrast, wPap was tolerated by the P. papatasi cell line PPL/LULS49, although tetracycline treatment was applied to generate a Wolbachia-free subline. Both Wolbachia strains were infective for a panel of heterologous insect and tick cell lines, including two novel lines generated from the sand fly Lutzomyia longipalpis, LLE/LULS45 and LLL/LULS52. In all cases, wPip was more pathogenic for the host cells than wPap. These newly isolated Wolbachia strains, and the novel mosquito and sand fly cell lines reported here, will add to the resources available for research on host-endosymbiont relationships, as well as on C. pipiens, P. papatasi, L. longipalpis and the pathogens that they transmit.

A novel nairovirus associated with acute febrile illness in Hokkaido, Japan.

The increasing burden of tick-borne orthonairovirus infections, such as Crimean-Congo hemorrhagic fever, is becoming a global concern for public health. In the present study, we identify a novel orthonairovirus, designated Yezo virus (YEZV), from two patients showing acute febrile illness with thrombocytopenia and leukopenia after tick bite in Hokkaido, Japan, in 2019 and 2020, respectively. YEZV is phylogenetically grouped with Sulina virus detected in Ixodes ricinus ticks in Romania. YEZV infection has been confirmed in seven patients from 2014-2020, four of whom were co-infected with Borrelia spp. Antibodies to YEZV are found in wild deer and raccoons, and YEZV RNAs have been detected in ticks from Hokkaido. In this work, we demonstrate that YEZV is highly likely to be the causative pathogen of febrile illness, representing the first report of an endemic infection associated with an orthonairovirus potentially transmitted by ticks in Japan.

Replication Kinetics of Rickettsia raoultii in Tick Cell Lines.

Rickettsia raoultii is one of the causative agents of tick-borne lymphadenopathy in humans. This bacterium was previously isolated and propagated in tick cell lines; however, the growth characteristics have not been investigated. Here, we present the replication kinetics of R. raoultii in cell lines derived from different tick genera (BME/CTVM23, RSE/PILS35, and IDE8). Tick cell cultures were infected in duplicate with cryopreserved R. raoultii prepared from homologous cell lines. By 12-14 days post infection, 100% of the cells were infected, as visualized in Giemsa-stained cytocentrifuge smears. R. raoultii growth curves, determined by rickettsiae-specific gltA qPCR, exhibited lag, exponential, stationary and death phases. Exponential phases of 4-12 days and generation times of 0.9-2.6 days were observed. R. raoultii in BME/CTVM23 and RSE/PILS35 cultures showed, respectively, 39.5- and 37.1-fold increases compared to the inoculum. In contrast, multiplication of R. raoultii in the IDE8 cultures was 110.1-fold greater than the inoculum with a 7-day stationary phase. These findings suggest variation in the growth kinetics of R. raoultii in the different tick cell lines tested, amongst which IDE8 cells could tolerate the highest levels of R. raoultii replication. Further studies of R. raoultii are needed for a better understanding of its persistence within tick populations.

Ticks and Tick-Borne Pathogens Associated with Dromedary Camels (Camelus dromedarius) in Northern Kenya.

Ticks and tick-borne pathogens (TBPs) are major constraints to camel health and production, yet epidemiological data on their diversity and impact on dromedary camels remain limited. We surveyed the diversity of ticks and TBPs associated with camels and co-grazing sheep at 12 sites in Marsabit County, northern Kenya. We screened blood and ticks (858 pools) from 296 camels and 77 sheep for bacterial and protozoan TBPs by high-resolution melting analysis and sequencing of PCR products. Hyalomma (75.7%), Amblyomma (17.6%) and Rhipicephalus (6.7%) spp. ticks were morphologically identified and confirmed by molecular analyses. We detected TBP DNA in 80.1% of blood samples from 296 healthy camels. "Candidatus Anaplasma camelii", "Candidatus Ehrlichia regneryi" and Coxiella burnetii were detected in both camels and associated ticks, and Ehrlichia chaffeensis, Rickettsia africae, Rickettsia aeschlimannii and Coxiella endosymbionts were detected in camel ticks. We also detected Ehrlichia ruminantium, which is responsible for heartwater disease in ruminants, in Amblyomma ticks infesting camels and sheep and in sheep blood, indicating its endemicity in Marsabit. Our findings also suggest that camels and/or the ticks infesting them are disease reservoirs of zoonotic Q fever (C. burnetii), ehrlichiosis (E. chaffeensis) and rickettsiosis (R. africae), which pose public health threats to pastoralist communities.

How relevant are in vitro culture models for study of tick-pathogen interactions?

Although tick-borne infectious diseases threaten human and animal health worldwide, with constantly increasing incidence, little knowledge is available regarding vector-pathogen interactions and pathogen transmission. In vivo laboratory study of these subjects using live, intact ticks is expensive, labor-intensive, and challenging from the points of view of biosafety and ethics. Several in vitro models have been developed, including over 70 continuous cell lines derived from multiple tick species and a variety of tick organ culture systems, facilitating many research activities. However, some limitations have to be considered in the translation of the results from the in vitro environment to the in vivo situation of live, intact ticks, and vertebrate hosts. In this review, we describe the available in vitro models and selected results from their application to the study of tick-borne viruses, bacteria, and protozoa, where possible comparing these results to studies in live, intact ticks. Finally, we highlight the strengths and weaknesses of in vitro tick culture models and their essential role in tick-borne pathogen research.

Geographical and Tick-Dependent Distribution of Flavi-Like Alongshan and Yanggou Tick Viruses in Russia.

The genus Flavivirus includes related, unclassified segmented flavi-like viruses, two segments of which have homology with flavivirus RNA-dependent RNA polymerase NS5 and RNA helicase-protease NS3. This group includes such viruses as Jingmen tick virus, Alongshan virus, Yanggou tick virus and others. We detected the Yanggou tick virus in Dermacentor nuttalli and Dermacentor marginatus ticks in two neighbouring regions of Russia. The virus prevalence ranged from 0.5% to 8.0%. We detected RNA of the Alongshan virus in 44 individuals or pools of various tick species in eight regions of Russia. The virus prevalence ranged from 0.6% to 7.8%. We demonstrated the successful replication of the Yanggou tick virus and Alongshan virus in IRE/CTVM19 and HAE/CTVM8 tick cell lines without a cytopathic effect. According to the phylogenetic analysis, we divided the Alongshan virus into two groups: an Ixodes persulcatus group and an Ixodes ricinus group. In addition, the I. persulcatus group can be divided into European and Asian subgroups. We found amino acid signatures specific to the I. ricinus and I. persulcatus groups and also distinguished between the European and Asian subgroups of the I. persulcatus group.

Spiroplasma Isolated From Third-Generation Laboratory Colony Ixodes persulcatus Ticks.

Spiroplasma are vertically-transmitted endosymbionts of ticks and other arthropods. Field-collected Ixodes persulcatus have been reported to harbour Spiroplasma, but nothing is known about their persistence during laboratory colonisation of this tick species. We successfully isolated Spiroplasma from internal organs of 6/10 unfed adult ticks, belonging to the third generation of an I. persulcatus laboratory colony, into tick cell culture. We screened a further 51 adult male and female ticks from the same colony for presence of Spiroplasma by genus-specific PCR amplification of fragments of the 16S rRNA and rpoB genes; 100% of these ticks were infected and the 16S rRNA sequence showed 99.8% similarity to that of a previously-published Spiroplasma isolated from field-collected I. persulcatus. Our study shows that Spiroplasma endosymbionts persist at high prevalence in colonised I. persulcatus through at least three generations, and confirms the usefulness of tick cell lines for isolation and cultivation of this bacterium.

Tick Importin-α Is Implicated in the Interactome and Regulome of the Cofactor Subolesin.

Ticks and tick-borne diseases (TBDs) represent a burden for human and animal health worldwide. Currently, vaccines constitute the safest and most effective approach to control ticks and TBDs. Subolesin (SUB) has been identified as a vaccine antigen for the control of tick infestations and pathogen infection and transmission. The characterization of the molecular function of SUB and the identification of tick proteins interacting with SUB may provide the basis for the discovery of novel antigens and for the rational design of novel anti-tick vaccines. In the present study, we used the yeast two-hybrid system (Y2H) as an unbiased approach to identify tick SUB-interacting proteins in an Ixodes ricinus cDNA library, and studied the possible role of SUB as a chromatin remodeler through direct interaction with histones. The Y2H screening identified Importin-α as a potential SUB-interacting protein, which was confirmed in vitro in a protein pull-down assay. The sub gene expression levels in tick midgut and fat body were significantly higher in unfed than fed female ticks, however, the importin-α expression levels did not vary between unfed and fed ticks but tended to be higher in the ovary when compared to those in other organs. The effect of importin-α RNAi was characterized in I. ricinus under artificial feeding conditions. Both sub and importin-α gene knockdown was observed in all tick tissues and, while tick weight was significantly lower in sub RNAi-treated ticks than in controls, importin-α RNAi did not affect tick feeding or oviposition, suggesting that SUB is able to exert its function in the absence of Importin-α. Furthermore, SUB was shown to physically interact with histone 4, which was corroborated by protein pull-down and western blot analysis. These results confirm that by interacting with numerous tick proteins, SUB is a key cofactor of the tick interactome and regulome. Further studies are needed to elucidate the nature of the SUB-Importin-α interaction and the biological processes and functional implications that this interaction may have.