The antiviral piRNA response in mosquitoes?

There are several RNA interference (RNAi) pathways in insects. The small interfering RNA pathway is considered to be the main antiviral mechanism of the innate immune system; however, virus-specific P-element-induced Wimpy testis gene (PIWI)-interacting RNAs (vpiRNAs) have also been described, especially in mosquitoes. Understanding the antiviral potential of the RNAi pathways is important, given that many human and animal pathogens are transmitted by mosquitoes, such as Zika virus, dengue virus and chikungunya virus. In recent years, significant progress has been made to characterize the piRNA pathway in mosquitoes (including the possible antiviral activity) and to determine the differences between mosquitoes and the model organism Drosophila melanogaster. The new findings, especially regarding vpiRNA in mosquitoes, as well as important questions that need to be tackled in the future, are discussed in this review.

Detection, infection dynamics and small RNA response against Culex Y virus in mosquito-derived cells.

Many insect cell lines are persistently infected with insect-specific viruses (ISV) often unrecognized by the scientific community. Considering recent findings showing the possibility of interference between arbovirus and ISV infections, it is important to pay attention to ISV-infected cell lines. One example is the Entomobirnavirus, Culex Y virus (CYV). Here we describe the detection of CYV using a combination of small RNA sequencing, electron microscopy and PCR in mosquito cell lines Aag2, U4.4 and C7-10. We found CYV-specific small RNAs in all three cell lines. Interestingly, the magnitude of the detected viral RNA genome is variable among cell passages and leads to irregular detection via electron microscopy. Gaining insights into the presence of persistent ISV infection in commonly used mosquito cells and their interactions with the host immune system is beneficial for evaluating the outcome of co-infections with arboviruses of public health concern.

Experimental transmission of Zika virus by mosquitoes from central Europe.

Mosquitoes collected in Germany in 2016, including Culex pipiens pipiens biotype pipiens, Culex torrentium and Aedes albopictus, as well as Culex pipiens pipiens biotype molestus (in colony since 2011) were experimentally infected with Zika virus (ZIKV) at 18 °C or 27 °C. None of the Culex taxa showed vector competence for ZIKV. In contrast, Aedes albopictus were susceptible for ZIKV but only at 27 °C, with transmission rates similar to an Aedes aegypti laboratory colony tested in parallel.

Physico-Chemical Characteristics of Culex pipiens sensu lato and Culex torrentium (Diptera: Culicidae) Breeding Sites in Germany.

Culex pipiens sensu lato (L.) and Culex torrentium Martini, 1925 are two widely distributed mosquito species in Europe. Due to difficulties in morphological discrimination, the current knowledge on differences in the breeding ecology is fragmentary. Therefore, this study evaluated the relation between the presence-absence of both species at various types of breeding habitats in response to physico-chemical parameters, using a recently developed molecular assay that allows reliable species-specific typing of larvae and pupae. The results revealed that the two species often occur in sympatry at the studied breeding sites, and there were no substantial differences concerning presence-absence of the two species with regard to the various environmental parameters investigated.

Aedes japonicus japonicus (Diptera: Culicidae) from Germany have vector competence for Japan encephalitis virus but are refractory to infection with West Nile virus.

The interplay between arthropod-borne (arbo) viruses and their vectors is usually complex and often exert unique relationships. Aedes japonicus japonicus (Hulecoeteomyia japonica or Ochlerotatus japonicus japonicus), an invasive mosquito species with laboratory proven vector competence for a number of emerging viruses has been newly introduced to Germany and is currently expanding its range throughout the country. On the other hand, West Nile virus (WNV), an emerging arbovirus originating from Africa, is already circulating in several European countries and might soon be introduced to Germany. Because newly introduced and rapidly expanding vector species pose a potential risk for public health in Germany, we assessed the vectorial capacity of German Ae. j. japonicus populations for WNV and Japanese encephalitis virus (JEV). The results indicate that German Ae. j. japonicus are susceptible for JEV but are refractory to infection with WNV. Of 67 Ae. j. japonicus females challenged by feeding of WNV-containing blood, none had measurable amounts of WNV-RNA (0% infection rate) on day 14 post-infection. In contrast, all females challenged with JEV were positive for JEV-RNA (100% infection rate) on day 14 post-infection. The reason for WNV resistance remains to be determined but is independent from co-infection with other flaviviruses or the presence of endosymbiotic Wolbachia, since we found no evidence for other flavivirus infections within 1,033 tested A. j. japonicus females from the sampling region, nor detectable Wolbachia infection within 30 randomly selected individuals.

Antiviral RNAi Response in Culex quinquefasciatus-Derived HSU Cells.

Culex spp. mosquitoes are important vectors of viruses, such as West Nile virus, Eastern equine encephalitis virus and Rift valley fever virus. However, their interactions with innate antiviral immunity, especially RNA interference (RNAi), are not well known. Most research on RNAi pathways in mosquitoes is focused on the tropical vector mosquito Aedes aegypti. Here, we investigated the production of arbovirus-specific small RNAs in Cx. quinquefasciatus-derived HSU cells. Furthermore, by silencing RNAi-related proteins, we investigated the antiviral role of these proteins for two different arboviruses: Semliki Forest virus (SFV) and Bunyamwera orthobunyavirus (BUNV). Our results showed an expansion of Ago2 and Piwi6 in Cx. quinquefasciatus compared to Ae. aegypti. While silencing Ago2a and Ago2b increased BUNV replication, only Ago2b showed antiviral activity against SFV. Our results suggest differences in the function of Cx. quinquefasciatus and Ae. aegypti RNAi proteins and highlight the virus-specific function of these proteins in Cx. quinquefasciatus.

Agua Salud Alphavirus Infection, Dissemination and Transmission in Aedes aegypti Mosquitoes.

Mosquitoes are competent vectors for many important arthropod-borne viruses (arboviruses). In addition to arboviruses, insect-specific viruses (ISV) have also been discovered in mosquitoes. ISVs are viruses that replicate in insect hosts but are unable to infect and replicate in vertebrates. They have been shown to interfere with arbovirus replication in some cases. Despite the increase in studies on ISV-arbovirus interactions, ISV interactions with their hosts and how they are maintained in nature are still not well understood. In the present study, we investigated the infection and dissemination of the Agua Salud alphavirus (ASALV) in the important mosquito vector Aedes aegypti through different infection routes (per oral infection, intrathoracic injection) and its transmission. We show here that ASALV infects the female Ae. aegypti and replicates when mosquitoes are infected intrathoracically or orally. ASALV disseminated to different tissues, including the midgut, salivary glands and ovaries. However, we observed a higher virus load in the brain than in the salivary glands and carcasses, suggesting a tropism towards brain tissues. Our results show that ASALV is transmitted horizontally during adult and larval stages, although we did not observe vertical transmission. Understanding ISV infection and dissemination dynamics in Ae. aegypti and their transmission routes could help the use of ISVs as an arbovirus control strategy in the future.

The Aedes aegypti RNA interference response against Zika virus in the context of co-infection with dengue and chikungunya viruses.

Since its detection in 2015 in Brazil, Zika virus (ZIKV) has remained in the spotlight of international public health and research as an emerging arboviral pathogen. In addition to single infection, ZIKV may occur in co-infection with dengue (DENV) and chikungunya (CHIKV) viruses, with whom ZIKV shares geographic distribution and the mosquito Aedes aegypti as a vector. The main mosquito immune response against arboviruses is RNA interference (RNAi). It is unknown whether or not the dynamics of the RNAi response differ between single arboviral infections and co-infections. In this study, we investigated the interaction of ZIKV and DENV, as well as ZIKV and CHIKV co-infections with the RNAi response in Ae. aegypti. Using small RNA sequencing, we found that the efficiency of small RNA production against ZIKV -a hallmark of antiviral RNAi-was mostly similar when comparing single and co-infections with either DENV or CHIKV. Silencing of key antiviral RNAi proteins, showed no change in effect on ZIKV replication when the cell is co-infected with ZIKV and DENV or CHIKV. Interestingly, we observed a negative effect on ZIKV replication during CHIKV co-infection in the context of Ago2-knockout cells, though his effect was absent during DENV co-infection. Overall, this study provides evidence that ZIKV single or co-infections with CHIKV or DENV are equally controlled by RNAi responses. Thus, Ae. aegypti mosquitoes and derived cells support co-infections of ZIKV with either CHIKV or DENV to a similar level than single infections, as long as the RNAi response is functional.

Interaction of Mesonivirus and Negevirus with arboviruses and the RNAi response in Culex tarsalis-derived cells.

BACKGROUND: Mosquito-specific viruses (MSVs) comprise a variety of different virus families, some of which are known to interfere with infections of medically important arboviruses. Viruses belonging to the family Mesoniviridae or taxon Negevirus harbor several insect-specific viruses, including MSVs, which are known for their wide geographical distribution and extensive host ranges. Although these viruses are regularly identified in mosquitoes all over the world, their presence in mosquitoes in Germany had not yet been reported. METHODS: A mix of three MSVs (Yichang virus [Mesoniviridae] and two negeviruses [Daeseongdong virus and Dezidougou virus]) in a sample that contained a pool of Coquillettidia richiardii mosquitoes collected in Germany was used to investigate the interaction of these viruses with different arboviruses in Culex-derived cells. In addition, small RNA sequencing and analysis of different mosquito-derived cells infected with this MSV mix were performed. RESULTS: A strain of Yichang virus (Mesoniviridae) and two negeviruses (Daeseongdong virus and Dezidougou virus) were identified in the Cq. richiardii mosquitoes sampled in Germany, expanding current knowledge of their circulation in central Europe. Infection of mosquito-derived cells with these three viruses revealed that they are targeted by the small interfering RNA (siRNA) pathway. In Culex-derived cells, co-infection by these three viruses had varying effects on the representative arboviruses from different virus families (Togaviridae: Semliki forest virus [SFV]; Bunyavirales: Bunyamwera orthobunyavirus [BUNV]; or Flaviviridae: Usutu virus [USUV]). Specifically, persistent MSV co-infection inhibited BUNV infection, as well as USUV infection (but the latter only at specific time points). However, the impact on SFV infection was only noticeable at low multiplicity of infection (MOI 0.1) and at specific time points in combination with the infection status. CONCLUSIONS: Taken together, these results are important findings that will lead to a better understanding of the complex interactions of MSVs, mosquitoes and arboviruses.

Antiviral RNAi Response against the Insect-Specific Agua Salud Alphavirus.

Arboviruses transmitted by mosquitoes are responsible for the death of millions of people each year. In addition to arboviruses, many insect-specific viruses (ISVs) have been discovered in mosquitoes in the last decade. ISVs, in contrast to arboviruses transmitted by mosquitoes to vertebrates, cannot replicate in vertebrate cells even when they are evolutionarily closely related to arboviruses. The alphavirus genus includes many arboviruses, although only a few ISVs have been discovered from this genus so far. Here, we investigate the interactions of a recently isolated insect-specific alphavirus, Agua Salud alphavirus (ASALV), with its mosquito host. RNA interference (RNAi) is one of the essential antiviral responses against arboviruses, although there is little knowledge on the interactions of RNAi with ISVs. Through the knockdown of transcripts of the different key RNAi pathway (small interfering RNA [siRNA], microRNA [miRNA], and P-element-induced wimpy testis [PIWI]-interacting RNA [piRNA]) proteins, we show the antiviral role of Ago2 (siRNA), Ago1 (miRNA), and Piwi4 proteins against ASALV in Aedes aegypti-derived cells. ASALV replication was increased in Dicer2 and Ago2 knockout cells, confirming the antiviral role of the siRNA pathway. In infected cells, mainly ASALV-specific siRNAs are produced, while piRNA-like small RNAs, with the characteristic nucleotide bias resulting from ping-pong amplification, are produced only in Dicer2 knockout cells. Taken together, ASALV interactions with the mosquito RNAi response differ from those of arthropod-borne alphaviruses in some aspects, although they also share some commonalities. Further research is needed to understand whether the identified differences can be generalized to other insect-specific alphaviruses. IMPORTANCE Mosquitoes are efficient vectors for many arboviruses that cause emergent infectious diseases in humans. Many insect-specific viruses (ISVs) that can infect mosquitoes but cannot infect vertebrates have been discovered in the last decade. ISVs have attracted great attention due to their potential use in mosquito or arbovirus control, by either decreasing mosquito fitness or restricting arbovirus replication and transmission to humans. However, ISV-mosquito interactions are not well understood. RNA interference (RNAi) is the most important innate immune response against many arboviruses, while it is unknown if it is antiviral against ISVs. Here, we investigate in detail the antiviral effect of the RNAi response in mosquitoes against an ISV for the first time. Using a recently isolated insect-specific alphavirus, we show that the regulation of virus replication was different from that for arthropod-borne alphaviruses despite some similarities. The differences in mosquito-virus interactions could drive the different transmission modes, which could eventually drive the evolution of arboviruses. Hence, an understanding of mosquito-ISV interactions can shed light on the ecology and evolution of both ISVs and the medically important arboviruses.

Mosquito-Specific Viruses-Transmission and Interaction.

Mosquito-specific viruses (MSVs) are a subset of insect-specific viruses that are found to infect mosquitoes or mosquito derived cells. There has been an increase in discoveries of novel MSVs in recent years. This has expanded our understanding of viral diversity and evolution but has also sparked questions concerning the transmission of these viruses and interactions with their hosts and its microbiome. In fact, there is already evidence that MSVs interact with the immune system of their host. This is especially interesting, since mosquitoes can be infected with both MSVs and arthropod-borne (arbo) viruses of public health concern. In this review, we give an update on the different MSVs discovered so far and describe current data on their transmission and interaction with the mosquito immune system as well as the effect MSVs could have on an arboviruses-co-infection. Lastly, we discuss potential uses of these viruses, including vector and transmission control.

Culex torrentium: A Potent Vector for the Transmission of West Nile Virus in Central Europe.

The continuous circulation of West Nile virus (WNV) in Central, South and East Europe and its recent detection in several dead birds and two horses in Germany highlights the need for information on WNV vector competence of mosquitoes from Central Europe. Therefore, three common Culex species (Culex pipiens biotype pipiens, Culex pipiens biotype molestus and Culex torrentium) from Germany were orally infected with WNV and kept at 18 °C, 21 °C, 24 °C or 27 °C for 14 or 21 days post infection (dpi). Thereafter viable WNV was present in the saliva in all tested taxa, but only at incubation temperatures of 24 °C or 27 °C and predominantly at the extended incubation period of 21 dpi. Highest transmission efficiency rates of 17 % (24 °C) and 24% (27 °C) were found for Cx. torrentium. Culex p. pipiens and Cx. p. molestus showed low transmission efficiencies with a maximum of only 3%. Consequently, temperatures above 21 °C support transmission of WNV, which matches the predominant distribution of human WNV cases around the Mediterranean Sea and in South-East Europe. Culex torrentium has been identified as a potent vector for WNV in Central and Northern Europe, which highlights the need for surveillance of mosquito-borne viruses north of the Alps.

RNAi-mediated antiviral immunity in insects and their possible application.

Antiviral RNA interference (RNAi) in insects is known for some time. Previously, data from the model organism Drosophila was lightly transferred to other insects. However, since more research is performed on other insects, both similarities and important differences among the RNAi pathways of insects are revealed. The piRNA pathway, for example, is suggested to have antiviral functions in mosquitoes, which is unknown for other insects. Moreover, production of viral cDNA from RNA viruses during infection and their possible incorporation into the genome hint towards potential inheritable immunity in mosquitoes. Increased knowledge of antiviral RNAi pathways has initiated efforts to engineer insects resistant to viral infections. New advances in genome editing will further stimulate this process.

Forced Salivation As a Method to Analyze Vector Competence of Mosquitoes.

Vector competence is defined as the potential of a mosquito species to transmit a mosquito-borne virus (mobovirus) to a vertebrate host. Viable virus particles are transmitted during a blood meal via the saliva of an infected mosquito. Forced salivation assays allow determining the vector potential on the basis of single mosquitoes, avoiding the use of animal experiments. The method is suitable to analyze a large number of mosquitoes in one experiment within a short period of time. Forced salivation assays were used to analyze 856 individual mosquitoes trapped in Germany, including two different Culex pipiens pipiens biotypes, Culex torrentium as well as Aedes albopictus, which were experimentally infected with Zika virus (ZIKV) and incubated at 18 °C or 27 °C for two and three weeks. The results indicated the lack of vector competence of the different Culex taxa for ZIKV. In contrast, Aedes albopictus was susceptible to ZIKV, but only at 27 °C, with transmission rates similar to an Aedes aegypti laboratory colony tested in parallel.

Culex pipiens and Culex torrentium populations from Central Europe are susceptible to West Nile virus infection.

West Nile virus (WNV), a Flavivirus with an avian primary host, is already widespread in Europe and might also pose an infection risk to Germany, should competent mosquito vectors be present. Therefore, we analysed the ability of WNV to infect German Culex mosquitoes with special emphasis on field collected specimens of Culex torrentium and Culex pipiens biotype pipiens. We collected egg rafts of Culex mosquitoes over two subsequent seasons at two geographically distinct sampling areas in Germany and differentiated the samples by molecular methods. Adult females, reared from the various egg rafts, were challenged with WNV by feeding of artificial blood meals. WNV infection was confirmed by real-time RT-PCR and virus titration. The results showed that field collected C. pipiens biotype pipiens and C. torrentium mosquitoes native to Germany are susceptible to WNV infection at 25 °C as well as 18 °C incubation temperature. C. torrentium mosquitoes, which have not been established as WNV vector so far, were the most permissive species tested with maximum infection rates of 96% at 25 °C. Furthermore, a disseminating infection was found in up to 94% of tested C. pipiens biotype pipiens and 100% of C. torrentium. Considering geographical variation of susceptibility, C. pipiens biotype pipiens mosquitoes from Southern Germany were more susceptible to WNV infection than corresponding populations from Northern Germany. All in all, we observed high infection and dissemination rates even at a low average ambient temperature of 18 °C. The high susceptibility of German Culex populations for WNV indicates that an enzootic transmission cycle in Germany could be possible.

Natural NS3 resistance polymorphisms occur frequently prior to treatment in HIV-positive patients with acute hepatitis C.

NS3 protease inhibitors are set to improve sustained virological response rates in HIV-positive patients with hepatitis C. We measured the prevalence of natural resistance polymorphisms in 38 acutely infected treatment-naive patients using direct and deep sequencing. Twenty six percent of patients (10/38) had a majority variant resistance mutation (in order of frequency; Q80K - 16%, V36M - 5%, T54S - 3%, V55A - 3%, and D168A - 3%). Low-frequency mutations were detected in all samples. Further studies are required to determine threshold levels associated with treatment failure.