Type I interferon reaction to viral infection in interferon-competent, immortalized cell lines from the African fruit bat Eidolon helvum

IRF7 from the black flying fox induces a STAT1-independent ISG response in unstimulated cell lines that protects against diverse RNA viruses

Bats are considered unique in their ability to harbor large numbers of viruses and serve as reservoirs for zoonotic viruses that have the potential to spill over into humans. However, these animals appear relatively resistant to the pathogenic effects of many viruses. Mounting evidence suggests that bats may tolerate viral infections due to unique immune features. These include evolutionary innovations in inflammatory pathways and in the molecules involved in viral sensing, interferon induction, and downstream interferon-induced antiviral effectors. We sought to determine whether interferon-stimulated genes (ISGs) from the black flying fox ( Pteropus alecto ) encoded proteins with unique antiviral activity relative to their human orthologs. Accordingly, we compared the antiviral activity of over 50 ISG human-bat ortholog pairs to identify differences in individual effector functions. We identified IRF7 from Pteropus alecto (Pa.IRF7) as a potent and broad-acting antiviral molecule that provides robust antiviral protection without prior activation. We show that Pa.IRF7 uniquely induces a subset of protective ISGs independent of canonical IFN signaling, which leads to protection from alphaviruses, a flavivirus, a rhabdovirus, and a paramyxovirus. In uninfected cells, Pa.IRF7 partially localizes to the nucleus and can directly bind interferon-sensitive regulatory elements (ISREs). Compared to human IRF7, Pa.IRF7 also has additional serines in its C terminal domain that contribute to antiviral activity and may serve as unique phosphorylation hubs for activation. These properties constitute major differences between bat and human IRF7 that offer additional insight into the potential uniqueness of the black flying fox immune system.

In vitro and in vivo effects of Pelargonium sidoides DC. root extract EPs® 7630 and selected constituents against SARS-CoV-2 B.1, Delta AY.4/AY.117 and Omicron BA.2

The occurrence of immune-evasive SARS-CoV-2 strains emphasizes the importance to search for broad-acting antiviral compounds. Our previous in vitro study showed that Pelargonium sidoides DC. root extract EPs® 7630 has combined antiviral and immunomodulatory properties in SARS-CoV-2-infected human lung cells. Here we assessed in vivo effects of EPs® 7630 in SARS-CoV-2-infected hamsters, and investigated properties of EPs® 7630 and its functionally relevant constituents in context of phenotypically distinct SARS-CoV-2 variants. We show that EPs® 7630 reduced viral load early in the course of infection and displayed significant immunomodulatory properties positively modulating disease progression in hamsters. In addition, we find that EPs® 7630 differentially inhibits SARS-CoV-2 variants in nasal and bronchial human airway epithelial cells. Antiviral effects were more pronounced against Omicron BA.2 compared to B.1 and Delta, the latter two preferring TMPRSS2-mediated fusion with the plasma membrane for cell entry instead of receptor-mediated low pH-dependent endocytosis. By using SARS-CoV-2 Spike VSV-based pseudo particles (VSVpp), we confirm higher EPs® 7630 activity against Omicron Spike-VSVpp, which seems independent of the serine protease TMPRSS2, suggesting that EPs® 7630 targets endosomal entry. We identify at least two molecular constituents of EPs® 7630, i.e., (-)-epigallocatechin and taxifolin with antiviral effects on SARS-CoV-2 replication and cell entry. In summary, our study shows that EPs® 7630 ameliorates disease outcome in SARS-CoV-2-infected hamsters and has enhanced activity against Omicron, apparently by limiting late endosomal SARS-CoV-2 entry.

Interaction between Old World fruit bats and humans: From large scale ecosystem services to zoonotic diseases

The Old World tropical and subtropical frugivorous bat genus Rousettus (Pteropodidae) contains species with broad distributions, as well as those occurring in restricted geographical areas, particularly islands. Herein we review the role of Rousettus as a keystone species from a global "One Health" approach and related to ecosystem functioning, zoonotic disease and public health. Rousettus are efficient at dispersing seeds and pollinating flowers; their role in forest regeneration is related to their ability to fly considerable distances during nightly foraging bouts and their relatively small body size, which allows them to access fruits in forested areas with closed vegetation. Rousettus are also reservoirs for various groups of pathogens (viruses, bacteria, fungi, protozoa), which, by definition, are infectious agents causing disease. The study of day roosts of different species of Rousettus and the successful establishment of captive breeding colonies have provided important details related to the infection dynamics of their associated pathogens. Large-scale conversion of forested areas into agricultural landscapes has increased contact between humans and Rousettus, therefore augmenting the chances of infectious agent spillover. Many crucial scientific details are still lacking related to members of this genus, which have direct bearing on the prevention of emerging disease outbreaks, as well as the conservation of these bats. The public should be better informed on the capacity of fruit bats as keystone species for large scale forest regeneration and in spreading pathogens. Precise details on the transmission of zoonotic diseases of public health importance associated with Rousettus should be given high priority.

Reduced IFN-ß inhibitory activity of Lagos bat virus phosphoproteins in human compared to Eidolon helvum bat cells

Eidolon helvum bats are reservoir hosts for highly pathogenic lyssaviruses often showing limited disease upon natural infection. An enhanced antiviral interferon (IFN) response combined with reduced inflammation might be linked to the apparent virus tolerance in bats. Lyssavirus phosphoproteins inhibit the IFN response with virus strain-specific efficiency. To date, little is known regarding the lyssavirus P-dependent anti-IFN countermeasures in bats, mainly due to a lack of in vitro tools. By using E. helvum bat cell cultures in a newly established bat-specific IFN-promoter activation assay, we analyzed the IFN-ß inhibitory activity of multiple lyssavirus P in E. helvum compared to human cells. Initial virus infection studies with a recently isolated E. helvum-borne Lagos bat virus street strain from Ghana showed enhanced LBV propagation in an E. helvum lung cell line compared to human A549 lung cells at later time points suggesting effective viral countermeasures against cellular defense mechanisms. A direct comparison of the IFN-ß inhibitory activity of the LBV-GH P protein with other lyssavirus P proteins showed that LBV-GH P and RVP both strongly inhibited the bat IFN-β promotor activation (range 75–90%) in EidLu/20.2 and an E. helvum kidney cell line. Conversely, LBV-GH P blocked the activation of the human IFN-β promoter less efficiently compared to a prototypic Rabies virus P protein (range LBV P 52–68% vs RVP 71–95%) in two different human cell lines (HEK-293T, A549). The same pattern was seen for two prototypic LBV P variants suggesting an overall reduced LBV P IFN-ß inhibitory activity in human cells as compared to E. helvum bat cells. Increased IFN-ß inhibition by lyssavirus P in reservoir host cells might be a result of host-specific adaptation processes towards an enhanced IFN response in bat cells.

Evidence that two instead of one defective interfering RNA in influenza A virus-derived defective interfering particles (DIPs) does not enhance antiviral activity

Influenza A virus (IAV) infection constitutes a significant health threat. Defective interfering particles (DIPs) can arise during IAV infection and inhibit spread of wild type (WT) IAV. DIPs harbor defective RNA segments, termed DI RNAs, that usually contain internal deletions and interfere with replication of WT viral RNA segments. Here, we asked whether DIPs harboring two instead of one DI RNA exert increased antiviral activity. For this, we focused on DI RNAs derived from segments 1 and 3, which encode the polymerase subunits PB2 and PA, respectively. We demonstrate the successful production of DIPs harboring deletions in segments 1 and/or 3, using cell lines that co-express PB2 and PA. Further, we demonstrate that DIPs harboring two instead of one DI RNA do not exhibit increased ability to inhibit replication of a WT RNA segment. Similarly, the presence of two DI RNAs did not augment the induction of the interferon-stimulated gene MxA and the inhibition of IAV infection. Collectively, our findings suggest that the presence of multiple DI RNAs derived from genomic segments encoding polymerase subunits might not result in increased antiviral activity.

Glycoproteins of Predicted Amphibian and Reptile Lyssaviruses Can Mediate Infection of Mammalian and Reptile Cells

Lyssaviruses are neurotropic rhabdoviruses thought to be restricted to mammalian hosts, and to originate from bats. The identification of lyssavirus sequences from amphibians and reptiles by metatranscriptomics thus comes as a surprise and challenges the mammalian origin of lyssaviruses. The novel sequences of the proposed American tree frog lyssavirus (ATFLV) and anole lizard lyssavirus (ALLV) reveal substantial phylogenetic distances from each other and from bat lyssaviruses, with ATFLV being the most distant. As virus isolation has not been successful yet, we have here studied the functionality of the authentic ATFLV- and ALLV-encoded glycoproteins in the context of rabies virus pseudotype particles. Cryogenic electron microscopy uncovered the incorporation of the plasmid-encoded G proteins in viral envelopes. Infection experiments revealed the infectivity of ATFLV and ALLV G-coated RABV pp for a broad spectrum of cell lines from humans, bats, and reptiles, demonstrating membrane fusion activities. As presumed, ATFLV and ALLV G RABV pp escaped neutralization by human rabies immune sera. The present findings support the existence of contagious lyssaviruses in poikilothermic animals, and reveal a broad cell tropism in vitro, similar to that of the rabies virus.

Development and Characterization of a cDNA-Launch Recombinant Simian Hemorrhagic Fever Virus Expressing Enhanced Green Fluorescent Protein: ORF 2b' Is Not Required for In Vitro Virus Replication

Simian hemorrhagic fever virus (SHFV) causes acute, lethal disease in macaques. We developed a single-plasmid cDNA-launch infectious clone of SHFV (rSHFV) and modified the clone to rescue an enhanced green fluorescent protein-expressing rSHFV-eGFP that can be used for rapid and quantitative detection of infection. SHFV has a narrow cell tropism in vitro, with only the grivet MA-104 cell line and a few other grivet cell lines being susceptible to virion entry and permissive to infection. Using rSHFV-eGFP, we demonstrate that one cricetid rodent cell line and three ape cell lines also fully support SHFV replication, whereas 55 human cell lines, 11 bat cell lines, and three rodent cells do not. Interestingly, some human and other mammalian cell lines apparently resistant to SHFV infection are permissive after transfection with the rSHFV-eGFP cDNA-launch plasmid. To further demonstrate the investigative potential of the infectious clone system, we introduced stop codons into eight viral open reading frames (ORFs). This approach suggested that at least one ORF, ORF 2b’, is dispensable for SHFV in vitro replication. Our proof-of-principle experiments indicated that rSHFV-eGFP is a useful tool for illuminating the understudied molecular biology of SHFV.

[Biological characteristics and permissiveness to viruses of diploid kidney cells strain from the bat Nathusius' pipistrelle (Pipistrellus nathusii Keyserling & Blasius, 1839; Chiroptera: Microchiroptera: Vespertilionidae)]

INTRODUCTION

Bats are an epidemiologically important natural reservoir of viruses of various taxonomic groups, including causative agents of especially dangerous infections of humans and animals. Considering the relevance of arbovirus infections, it seems advisable to study the spectrum of the sensitivity of cells derived from bats inhabiting and migrating on the territory of the Russian Federation to causative agents of vector-borne diseases of animals.The study aimed to obtain a diploid strain of cells from renal tissue of bats Pipistrellus nathusii and to investigate its biological characteristics, as well as to assess its permissiveness for bluetongue (BTV); Rift Valley fever (RVFV); lumpy skin disease (LSDV); rabbit myxoma (Myxomatosis cuniculi); rabbit, or Shope fibroma (RFV); African horse sickness (AHSV) and African swine fever (ASFV) viruses.

MATERIAL AND METHODS

There were 2 clinically healthy male individuals of P. nathusii who taken as donors of organs. To obtain diploid kidney cell culture strain and to study its properties, the level of the 6th passage was investigated by conventional cytological, virological, and molecular methods. The permissiveness of the obtained cell culture for BTV, RVFV, LSDV, Myxomatosis cuniculi, RFV, AHSV and ASFV was determined.

RESULTS

The formation of a confluent monolayer was observed after 72 hours, while the proliferation index was 2.7-3.3. The cell monolayer had been maintained without changing the medium for 45 days (observation period). The stability of the karyotype had been demonstrated in continuous subculturing at the 36th passage. The cell culture named «Diploid cell line Pipistrellus nathusii kidney», and its permissiveness to BTV, RVFV, LSDV and Myxomatosis cuniculi had been demonstrated.

DISCUSSION

The sensitivity of the strain to BTV and RVFV is consistent with the data on the identification of reovirus and RVFV in Egyptian fruit bats (Rousettus aegyptiacus), and its permissiveness for LSDV and rabbits myxoma virus is consistent with the results of detection of poxviruses in big brown bat (Eptesicus fuscus).

CONCLUSION

A diploid kidney cell strain derived from P. nathusii was obtained and certified. Its permissiveness to BTV, RVFV, LSDV and rabbits myxoma viruses makes it possible to use this strain for isolation and studies of these viruses. Reproduction of the viruses in diploid kidney cells strain derived from P. nathusii living and migrating in the European part of the Russian Federation indicates their potential role in the epidemiology of significant infections, especially transmissible ones.

Functional Studies with Primary Cells Provide a System for Genome-to-Phenome Investigations in Marine Mammals

Marine mammals exhibit some of the most dramatic physiological adaptations in their clade and offer unparalleled insights into the mechanisms driving convergent evolution on relatively short time scales. Some of these adaptations, such as extreme tolerance to hypoxia and prolonged food deprivation, are uncommon among most terrestrial mammals and challenge established metabolic principles of supply and demand balance. Non-targeted omics studies are starting to uncover the genetic foundations of such adaptations, but tools for testing functional significance in these animals are currently lacking. Cellular modeling with primary cells represents a powerful approach for elucidating the molecular etiology of physiological adaptation, a critical step in accelerating genome-to-phenome studies in organisms in which transgenesis is impossible (e.g., large-bodied, long-lived, fully aquatic, federally protected species). Gene perturbation studies in primary cells can directly evaluate whether specific mutations, gene loss, or duplication confer functional advantages such as hypoxia or stress tolerance in marine mammals. Here, we summarize how genetic and pharmacological manipulation approaches in primary cells have advanced mechanistic investigations in other non-traditional mammalian species, and highlight the need for such investigations in marine mammals. We also provide key considerations for isolating, culturing, and conducting experiments with marine mammal cells under conditions that mimic in vivo states. We propose that primary cell culture is a critical tool for conducting functional mechanistic studies (e.g., gene knockdown, over-expression, or editing) that can provide the missing link between genome- and organismal-level understanding of physiological adaptations in marine mammals.

Egyptian Rousette IFN-ω Subtypes Elicit Distinct Antiviral Effects and Transcriptional Responses in Conspecific Cells

Bats host a number of viruses that cause severe disease in humans without experiencing overt symptoms of disease themselves. While the mechanisms underlying this ability to avoid sickness are not known, deep sequencing studies of bat genomes have uncovered genetic adaptations that may have functional importance in the antiviral response of these animals. Egyptian rousette bats (Rousettus aegyptiacus) are the natural reservoir hosts of Marburg virus (MARV). In contrast to humans, these bats do not become sick when infected with MARV. A striking difference to the human genome is that Egyptian rousettes have an expanded repertoire of IFNW genes. To probe the biological implications of this expansion, we synthesized IFN-ω4 and IFN-ω9 proteins and tested their antiviral activity in Egyptian rousette cells. Both IFN-ω4 and IFN-ω9 showed antiviral activity against RNA viruses, including MARV, with IFN-ω9 being more efficient than IFN-ω4. Using RNA-Seq, we examined the transcriptional response induced by each protein. Although the sets of genes induced by the two IFNs were largely overlapping, IFN-ω9 induced a more rapid and intense response than did IFN-ω4. About 13% of genes induced by IFN-ω treatment are not found in the Interferome or other ISG databases, indicating that they may be uniquely IFN-responsive in this bat.

Accelerated viral dynamics in bat cell lines, with implications for zoonotic emergence

Bats host virulent zoonotic viruses without experiencing disease. A mechanistic understanding of the impact of bats’ virus hosting capacities, including uniquely constitutive immune pathways, on cellular-scale viral dynamics is needed to elucidate zoonotic emergence. We carried out virus infectivity assays on bat cell lines expressing induced and constitutive immune phenotypes, then developed a theoretical model of our in vitro system, which we fit to empirical data. Best fit models recapitulated expected immune phenotypes for representative cell lines, supporting robust antiviral defenses in bat cells that correlated with higher estimates for within-host viral propagation rates. In general, heightened immune responses limit pathogen-induced cellular morbidity, which can facilitate the establishment of rapidly-propagating persistent infections within-host. Rapidly-transmitting viruses that have evolved with bat immune systems will likely cause enhanced virulence following emergence into secondary hosts with immune systems that diverge from those unique to bats.

Bats can carry viruses that are deadly to other mammals without themselves showing serious symptoms. In fact, bats are natural reservoirs for viruses that have some of the highest fatality rates of any viruses that people acquire from wild animals – including rabies, Ebola and the SARS coronavirus.

Bats have a suite of antiviral defenses that keep the amount of virus in check. For example, some bats have an antiviral immune response called the interferon pathway perpetually switched on. In most other mammals, having such a hyper-vigilant immune response would cause harmful inflammation. Bats, however, have adapted anti-inflammatory traits that protect them from such harm, include the loss of certain genes that normally promote inflammation. However, no one has previously explored how these unique antiviral defenses of bats impact the viruses themselves.

Now, Brook et al. have studied this exact question using bat cells grown in the laboratory. The experiments made use of cells from one bat species – the black flying fox – in which the interferon pathway is always on, and another – the Egyptian fruit bat – in which this pathway is only activated during an infection. The bat cells were infected with three different viruses, and then Brook et al. observed how the interferon pathway helped keep the infections in check, before creating a computer model of this response.

The experiments and model helped reveal that the bats’ defenses may have a potential downside for other animals, including humans. In both bat species, the strongest antiviral responses were countered by the virus spreading more quickly from cell to cell. This suggests that bat immune defenses may drive the evolution of faster transmitting viruses, and while bats are well protected from the harmful effects of their own prolific viruses, other creatures like humans are not.

The findings may help to explain why bats are often the source for viruses that are deadly in humans. Learning more about bats' antiviral defenses and how they drive virus evolution may help scientists develop better ways to predict, prevent or limit the spread of viruses from bats to humans. More studies are needed in bats to help these efforts. In the meantime, the experiments highlight the importance of warning people to avoid direct contact with wild bats.

Entry, Replication, Immune Evasion, and Neurotoxicity of Synthetically Engineered Bat-Borne Mumps Virus

Bats harbor a plethora of viruses with an unknown zoonotic potential. In-depth functional characterization of such viruses is often hampered by a lack of virus isolates. The genome of a virus closely related to human mumps viruses (hMuV) was detected in African fruit bats, batMuV. Efforts to characterize batMuV were based on directed expression of the batMuV glycoproteins or use of recombinant chimeric hMuVs harboring batMuV glycoprotein. Although these studies provided initial insights into the functionality of batMuV glycoproteins, the host range, replication competence, immunomodulatory functions, virulence, and zoonotic potential of batMuV remained elusive. Here, we report the successful rescue of recombinant batMuV. BatMuV infects human cells, is largely resistant to the host interferon response, blocks interferon induction and TNF-α activation, and is neurotoxic in rats. Anti-hMuV antibodies efficiently neutralize batMuV. The striking similarities between hMuV and batMuV point at the putative zoonotic potential of batMuV.

Activation of RNase L in Egyptian Rousette Bat-Derived RoNi/7 Cells Is Dependent Primarily on OAS3 and Independent of MAVS Signaling

Many RNA viruses that are highly pathogenic in humans are relatively apathogenic in their bat reservoirs, making it important to compare innate immune responses in bats to those well characterized in humans. One such antiviral response is the OAS-RNase L pathway. OASs, upon sensing dsRNA, produce 2-5A, leading to activation of RNase L which degrades viral and host RNA, limiting viral replication. Analysis of Egyptian Rousette bat sequences revealed three OAS genes expressing OAS1, OAS2, and OAS3 proteins. Interferon treatment or viral infection induces all three bat OAS mRNAs. In these bat cells as in human cells, RNase L activation and its antiviral activity are dependent primarily on OAS3 while MAVS signaling is not required. Importantly, our findings indicate the OAS-RNase L system is a primary response to virus rather than a secondary effect of interferon signaling and therefore can be activated early in infection or while interferon signaling is antagonized.

Bats are reservoirs for many RNA viruses that are highly pathogenic in humans yet relatively apathogenic in the natural host. It has been suggested that differences in innate immunity are responsible. The antiviral OAS-RNase L pathway is well characterized in humans, but there is little known about its activation and antiviral activity in bats. During infection, OASs, upon sensing double-stranded RNA (dsRNA), produce 2′-5′ oligoadenylates (2-5A), leading to activation of RNase L which degrades viral and host RNA, limiting viral replication. Humans encode three active OASs (OAS1 to -3). Analysis of the Egyptian Rousette bat genome combined with mRNA sequencing from bat RoNi/7 cells revealed three homologous OAS proteins. Interferon alpha treatment or viral infection induced all three OAS mRNAs, but RNase L mRNA is constitutively expressed. Sindbis virus (SINV) or vaccinia virus (VACVΔE3L) infection of wild-type (WT) or OAS1-KO (knockout), OAS2-KO, or MAVS-KO RoNi/7 cells, but not RNase L-KO or OAS3-KO cells, induces robust RNase L activation. SINV replication is 100- to 200-fold higher in the absence of RNase L or OAS3 than in WT cells. However, MAVS-KO had no detectable effect on RNA degradation or replication. Thus, in RoNi/7 bat cells, as in human cells, activation of RNase L during infection and its antiviral activity are dependent primarily on OAS3 while MAVS signaling is not required for the activation of RNase L and restriction of infection. Our findings indicate that OAS proteins serve as pattern recognition receptors (PRRs) to recognize viral dsRNA and that this pathway is a primary response to virus rather than a secondary effect of interferon signaling.

Fusogenicity of the Ghana Virus (Henipavirus: Ghanaian bat henipavirus) Fusion Protein is Controlled by the Cytoplasmic Domain of the Attachment Glycoprotein

The Ghana virus (GhV) is phylogenetically related to the zoonotic henipaviruses Nipah (NiV) and Hendra virus. Although GhV uses the highly conserved receptor ephrin-B2, the fusogenicity is restricted to cell lines of bat origin. Furthermore, the surface expression of the GhV attachment glycoprotein (G) is reduced compared to NiV and most of this protein is retained in the endoplasmic reticulum (ER). Here, we generated truncated as well as chimeric GhV G proteins and investigated the influence of the structural domains (cytoplasmic tail, transmembrane domain, ectodomain) of this protein on the intracellular transport and the fusogenicity following coexpression with the GhV fusion protein (F). We demonstrate that neither the cytoplasmic tail nor the transmembrane domain is responsible for the intracellular retention of GhV G. Furthermore, the cytoplasmic tail of GhV G modulates the fusogenicity of GhV F and therefore controls the species-restricted fusogenicity of the GhV surface glycoproteins.

Nipah virus induces two inclusion body populations: Identification of novel inclusions at the plasma membrane

Formation of cytoplasmic inclusion bodies (IBs) is a hallmark of infections with non-segmented negative-strand RNA viruses (order Mononegavirales). We show here that Nipah virus (NiV), a bat-derived highly pathogenic member of the Paramyxoviridae family, differs from mononegaviruses of the Rhabdo-, Filo- and Pneumoviridae families by forming two types of IBs with distinct localizations, formation kinetics, and protein compositions. IBs in the perinuclear region form rapidly upon expression of the nucleocapsid proteins. These IB_peri are highly mobile and associate with the aggresome marker y-tubulin. IB_peri can recruit unrelated overexpressed cytosolic proteins but do not contain the viral matrix (M) protein. Additionally, NiV forms an as yet undescribed IB population at the plasma membrane (IB^PM) that is y-tubulin-negative but contains the M protein. Infection studies with recombinant NiV revealed that IB^PM require the M protein for their formation, and most likely represent sites of NiV assembly and budding. The identification of this novel type of plasma membrane-associated IBs not only provides new insights into NiV biology and may open new avenues to develop novel antiviral approaches to treat these highly pathogenic viruses, it also provides a basis for a more detailed characterization of IBs and their role in virus assembly and replication in infections with other Mononegavirales.

Inclusion bodies (IBs) induced by non-segmented negative-strand RNA viruses (Mononegavirales) are described as mobile cytosolic compartments that concentrate viral proteins and represent the main viral replication sites in infected cells. This general concept is mainly based on studies with mononegaviruses from the Rhabdo-, Filo- and Pneumoviridae families. IBs induced by members of the Paramyxoviridae family are much less well characterized, and this study provides evidence that paramyxoviral IBs may have different compositions and functions. The main finding of this study is that Nipah virus (NiV), a highly pathogenic member of the genus Henipavirus in the family Paramyxoviridae, forms a novel type of IB whose formation at plasma membrane assembly sites depends on the viral matrix protein, and suggests a role for IBs not yet described for other Mononegavirales. This discovery clearly extents the current concept of IB functions and illustrates the need to further investigate IBs formed by other paramyxoviruses.

Human, Nonhuman Primate, and Bat Cells Are Broadly Susceptible to Tibrovirus Particle Cell Entry

In 2012, the genome of a novel rhabdovirus, Bas-Congo virus (BASV), was discovered in the acute-phase serum of a Congolese patient with presumed viral hemorrhagic fever. In the absence of a replicating virus isolate, fulfilling Koch's postulates to determine whether BASV is indeed a human virus and/or pathogen has been impossible. However, experiments with vesiculoviral particles pseudotyped with Bas-Congo glycoprotein suggested that BASV particles can enter cells from multiple animals, including humans. In 2015, genomes of two related viruses, Ekpoma virus 1 (EKV-1) and Ekpoma virus 2 (EKV-2), were detected in human sera in Nigeria. Isolates could not be obtained. Phylogenetic analyses led to the classification of BASV, EKV-1, and EKV-2 in the same genus, Tibrovirus, together with five biting midge-borne rhabdoviruses [i.e., Beatrice Hill virus (BHV), Bivens Arm virus (BAV), Coastal Plains virus (CPV), Sweetwater Branch virus (SWBV), and Tibrogargan virus (TIBV)] not known to infect humans. Using individual recombinant vesiculoviruses expressing the glycoproteins of all eight known tibroviruses and more than 75 cell lines representing different animal species, we demonstrate that the glycoproteins of all tibroviruses can mediate vesiculovirus particle entry into human, bat, nonhuman primate, cotton rat, boa constrictor, and Asian tiger mosquito cells. Using four of five isolated authentic tibroviruses (i.e., BAV, CPV, SWBV, and TIBV), our experiments indicate that many cell types may be partially resistant to tibrovirus replication after virion cell entry. Consequently, experimental data solely obtained from experiments using tibrovirus surrogate systems (e.g., vesiculoviral pseudotypes, recombinant vesiculoviruses) cannot be used to predict whether BASV, or any other tibrovirus, infects humans.

Interferon Regulatory Factor 3-Mediated Signaling Limits Middle-East Respiratory Syndrome (MERS) Coronavirus Propagation in Cells from an Insectivorous Bat

Insectivorous bats are speculated to be ancestral hosts of Middle-East respiratory syndrome (MERS) coronavirus (CoV). MERS-CoV causes disease in humans with thirty-five percent fatality, and has evolved proteins that counteract human antiviral responses. Since bats experimentally infected with MERS-CoV do not develop signs of disease, we tested the hypothesis that MERS-CoV would replicate less efficiently in bat cells than in human cells because of its inability to subvert antiviral responses in bat cells. We infected human and bat (Eptesicus fuscus) cells with MERS-CoV and observed that the virus grew to higher titers in human cells. MERS-CoV also effectively suppressed the antiviral interferon beta (IFNβ) response in human cells, unlike in bat cells. To determine if IRF3, a critical mediator of the interferon response, also regulated the response in bats, we examined the response of IRF3 to poly(I:C), a synthetic analogue of viral double-stranded RNA. We observed that bat IRF3 responded to poly(I:C) by nuclear translocation and post-translational modifications, hallmarks of IRF3 activation. Suppression of IRF3 by small-interfering RNA (siRNA) demonstrated that IRF3 was critical for poly(I:C) and MERS-CoV induced induction of IFNβ in bat cells. Our study demonstrates that innate antiviral signaling in E. fuscus bat cells is resistant to MERS-CoV-mediated subversion.

Bats and Coronaviruses

Bats are speculated to be reservoirs of several emerging viruses including coronaviruses (CoVs) that cause serious disease in humans and agricultural animals. These include CoVs that cause severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), porcine epidemic diarrhea (PED) and severe acute diarrhea syndrome (SADS). Bats that are naturally infected or experimentally infected do not demonstrate clinical signs of disease. These observations have allowed researchers to speculate that bats are the likely reservoirs or ancestral hosts for several CoVs. In this review, we follow the CoV outbreaks that are speculated to have originated in bats. We review studies that have allowed researchers to identify unique adaptation in bats that may allow them to harbor CoVs without severe disease. We speculate about future studies that are critical to identify how bats can harbor multiple strains of CoVs and factors that enable these viruses to “jump” from bats to other mammals. We hope that this review will enable readers to identify gaps in knowledge that currently exist and initiate a dialogue amongst bat researchers to share resources to overcome present limitations.

Attenuation of replication by a 29 nucleotide deletion in SARS-coronavirus acquired during the early stages of human-to-human transmission

A 29 nucleotide deletion in open reading frame 8 (ORF8) is the most obvious genetic change in severe acute respiratory syndrome coronavirus (SARS-CoV) during its emergence in humans. In spite of intense study, it remains unclear whether the deletion actually reflects adaptation to humans. Here we engineered full, partially deleted (-29 nt), and fully deleted ORF8 into a SARS-CoV infectious cDNA clone, strain Frankfurt-1. Replication of the resulting viruses was compared in primate cell cultures as well as Rhinolophus bat cells made permissive for SARS-CoV replication by lentiviral transduction of the human angiotensin-converting enzyme 2 receptor. Cells from cotton rat, goat, and sheep provided control scenarios that represent host systems in which SARS-CoV is neither endemic nor epidemic. Independent of the cell system, the truncation of ORF8 (29 nt deletion) decreased replication up to 23-fold. The effect was independent of the type I interferon response. The 29 nt deletion in SARS-CoV is a deleterious mutation acquired along the initial human-to-human transmission chain. The resulting loss of fitness may be due to a founder effect, which has rarely been documented in processes of viral emergence. These results have important implications for the retrospective assessment of the threat posed by SARS.

The papain-like protease determines a virulence trait that varies among members of the SARS-coronavirus species

SARS-coronavirus (CoV) is a zoonotic agent derived from rhinolophid bats, in which a plethora of SARS-related, conspecific viral lineages exist. Whereas the variability of virulence among reservoir-borne viruses is unknown, it is generally assumed that the emergence of epidemic viruses from animal reservoirs requires human adaptation. To understand the influence of a viral factor in relation to interspecies spillover, we studied the papain-like protease (PLP) of SARS-CoV. This key enzyme drives the early stages of infection as it cleaves the viral polyprotein, deubiquitinates viral and cellular proteins, and antagonizes the interferon (IFN) response. We identified a bat SARS-CoV PLP, which shared 86% amino acid identity with SARS-CoV PLP, and used reverse genetics to insert it into the SARS-CoV genome. The resulting virus replicated like SARS-CoV in Vero cells but was suppressed in IFN competent MA-104 (3.7-fold), Calu-3 (2.6-fold) and human airway epithelial cells (10.3-fold). Using ectopically-expressed PLP variants as well as full SARS-CoV infectious clones chimerized for PLP, we found that a protease-independent, anti-IFN function exists in SARS-CoV, but not in a SARS-related, bat-borne virus. This PLP-mediated anti-IFN difference was seen in primate, human as well as bat cells, thus independent of the host context. The results of this study revealed that coronavirus PLP confers a variable virulence trait among members of the species SARS-CoV, and that a SARS-CoV lineage with virulent PLPs may have pre-existed in the reservoir before onset of the epidemic.

The Egyptian Rousette Genome Reveals Unexpected Features of Bat Antiviral Immunity

Bats harbor many viruses asymptomatically, including several notorious for causing extreme virulence in humans. To identify differences between antiviral mechanisms in humans and bats, we sequenced, assembled, and analyzed the genome of Rousettus aegyptiacus, a natural reservoir of Marburg virus and the only known reservoir for any filovirus. We found an expanded and diversified KLRC/KLRD family of natural killer cell receptors, MHC class I genes, and type I interferons, which dramatically differ from their functional counterparts in other mammals. Such concerted evolution of key components of bat immunity is strongly suggestive of novel modes of antiviral defense. An evaluation of the theoretical function of these genes suggests that an inhibitory immune state may exist in bats. Based on our findings, we hypothesize that tolerance of viral infection, rather than enhanced potency of antiviral defenses, may be a key mechanism by which bats asymptomatically host viruses that are pathogenic in humans.

Development of molecular and cellular tools to decipher the type I IFN pathway of the common vampire bat

Though the common vampire bat, Desmodus rotundus, is known as the main rabies virus reservoir in Latin America, no tools are available to investigate its antiviral innate immune system. To characterize the IFN-I pathway, we established an immortalized cell line from a D. rotundus fetal lung named FLuDero. Then we molecularly characterized some of the Toll-like receptors (TLR3, 7, 8 and 9), the three RIG-I-like receptor members, as well as IFNα1 and IFNβ. Challenging the FLuDero cell line with poly (I:C) resulted in an up-regulation of both IFNα1 and IFNβ and the induction of expression of the different pattern recognition receptors characterized. These findings provide evidence of the intact dsRNA recognition machinery and the IFN-I signaling pathway in our cellular model. Herein, we generated a sum of insightful specific molecular and cellular tools that will serve as a useful model to study virus-host interactions of the common vampire bat.

Tools to study pathogen-host interactions in bats

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Bats are important reservoir hosts for emerging zoonotic viruses.

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Viruses detected in bats are difficult to isolate using traditional cell lines.

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Bat cell lines provide critical tools to dissect host pathogen interactions.

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Little is known about immune cell populations and their responses in bats.

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Sharing reagents and cell lines will accelerate research and virus discovery.

Bats are natural reservoirs for a variety of emerging viruses that cause significant disease in humans and domestic animals yet rarely cause clinical disease in bats. The co-evolutionary history of bats with viruses has been hypothesized to have shaped the bat-virus relationship, allowing both to exist in equilibrium. Progress in understanding bat-virus interactions and the isolation of bat-borne viruses has been accelerated in recent years by the development of susceptible bat cell lines. Viral sequences similar to severe acute respiratory syndrome corona virus (SARS-CoV) have been detected in bats, and filoviruses such as Marburg virus have been isolated from bats, providing definitive evidence for the role of bats as the natural host reservoir. Although viruses can be readily detected in bats using molecular approaches, virus isolation is far more challenging. One of the limitations in using traditional culture systems from non-reservoir species is that cell types and culture conditions may not be compatible for isolation of bat-borne viruses. There is, therefore, a need to develop additional bat cell lines that correspond to different cell types, including less represented cell types such as immune cells, and culture them under more physiologically relevant conditions to study virus host interactions and for virus isolation. In this review, we highlight the current progress in understanding bat-virus interactions in bat cell line systems and some of the challenges and limitations associated with cell lines. Future directions to address some of these challenges to better understand host-pathogen interactions in these intriguing mammals are also discussed, not only in relation to viruses but also other pathogens carried by bats including bacteria and fungi.

Unexpected Functional Divergence of Bat Influenza Virus NS1 Proteins

Recently, two influenza A virus (FLUAV) genomes were identified in Central and South American bats. These sequences exhibit notable divergence from classical FLUAV counterparts, and functionally, bat FLUAV glycoproteins lack canonical receptor binding and destroying activity. Nevertheless, other features that distinguish these viruses from classical FLUAVs have yet to be explored. Here, we studied the viral nonstructural protein NS1, a virulence factor that modulates host signaling to promote efficient propagation. Like all FLUAV NS1 proteins, bat FLUAV NS1s bind double-stranded RNA and act as interferon antagonists. Unexpectedly, we found that bat FLUAV NS1s are unique in being unable to bind host p85β, a regulatory subunit of the cellular metabolism-regulating enzyme, phosphoinositide 3-kinase (PI3K). Furthermore, neither bat FLUAV NS1 alone nor infection with a chimeric bat FLUAV efficiently activates Akt, a PI3K effector. Structure-guided mutagenesis revealed that the bat FLUAV NS1-p85β interaction can be reengineered (in a strain-specific manner) by changing two to four NS1 residues (96L, 99M, 100I, and 145T), thereby creating a hydrophobic patch. Notably, ameliorated p85β-binding is insufficient for bat FLUAV NS1 to activate PI3K, and a chimeric bat FLUAV expressing NS1 with engineered hydrophobic patch mutations exhibits cell-type-dependent, but species-independent, propagation phenotypes. We hypothesize that bat FLUAV hijacking of PI3K in the natural bat host has been selected against, perhaps because genes in this metabolic pathway were differentially shaped by evolution to suit the unique energy use strategies of this flying mammal. These data expand our understanding of the enigmatic functional divergence between bat FLUAVs and classical mammalian and avian FLUAVs.

IMPORTANCE The potential for novel influenza A viruses to establish infections in humans from animals is a source of continuous concern due to possible severe outbreaks or pandemics. The recent discovery of influenza A-like viruses in bats has raised questions over whether these entities could be a threat to humans. Understanding unique properties of the newly described bat influenza A-like viruses, such as their mechanisms to infect cells or how they manipulate host functions, is critical to assess their likelihood of causing disease. Here, we characterized the bat influenza A-like virus NS1 protein, a key virulence factor, and found unexpected functional divergence of this protein from counterparts in other influenza A viruses. Our study dissects the molecular changes required by bat influenza A-like virus NS1 to adopt classical influenza A virus properties and suggests consequences of bat influenza A-like virus infection, potential future evolutionary trajectories, and intriguing virus-host biology in bat species.

Evolution and Antiviral Specificities of Interferon-Induced Mx Proteins of Bats against Ebola, Influenza, and Other RNA Viruses

Bats serve as a reservoir for various, often zoonotic viruses, including significant human pathogens such as Ebola and influenza viruses. However, for unknown reasons, viral infections rarely cause clinical symptoms in bats. A tight control of viral replication by the host innate immune defense might contribute to this phenomenon. Transcriptomic studies revealed the presence of the interferon-induced antiviral myxovirus resistance (Mx) proteins in bats, but detailed functional aspects have not been assessed. To provide evidence that bat Mx proteins might act as key factors to control viral replication we cloned Mx1 cDNAs from three bat families, Pteropodidae, Phyllostomidae, and Vespertilionidae. Phylogenetically these bat Mx1 genes cluster closely with their human ortholog MxA. Using transfected cell cultures, minireplicon systems, virus-like particles, and virus infections, we determined the antiviral potential of the bat Mx1 proteins. Bat Mx1 significantly reduced the polymerase activity of viruses circulating in bats, including Ebola and influenza A-like viruses. The related Thogoto virus, however, which is not known to infect bats, was not inhibited by bat Mx1. Further, we provide evidence for positive selection in bat Mx1 genes that might explain species-specific antiviral activities of these proteins. Together, our data suggest a role for Mx1 in controlling these viruses in their bat hosts.IMPORTANCE Bats are a natural reservoir for various viruses that rarely cause clinical symptoms in bats but are dangerous zoonotic pathogens, like Ebola or rabies virus. It has been hypothesized that the interferon system might play a key role in controlling viral replication in bats. We speculate that the interferon-induced Mx proteins might be key antiviral factors of bats and have coevolved with bat-borne viruses. This study evaluated for the first time a large set of bat Mx1 proteins spanning three major bat families for their antiviral potential, including activity against Ebola virus and bat influenza A-like virus, and we describe here their phylogenetic relationship, revealing patterns of positive selection that suggest a coevolution with viral pathogens. By understanding the molecular mechanisms of the innate resistance of bats against viral diseases, we might gain important insights into how to prevent and fight human zoonotic infections caused by bat-borne viruses.

A Polymorphism within the Internal Fusion Loop of the Ebola Virus Glycoprotein Modulates Host Cell Entry

The large scale of the Ebola virus disease (EVD) outbreak in West Africa in 2013-2016 raised the question whether the host cell interactions of the responsible Ebola virus (EBOV) strain differed from those of other ebolaviruses. We previously reported that the glycoprotein (GP) of the virus circulating in West Africa in 2014 (EBOV2014) exhibited reduced ability to mediate entry into two nonhuman primate (NHP)-derived cell lines relative to the GP of EBOV1976. Here, we investigated the molecular determinants underlying the differential entry efficiency. We found that EBOV2014-GP-driven entry into diverse NHP-derived cell lines, as well as human monocyte-derived macrophages and dendritic cells, was reduced compared to EBOV1976-GP, although entry into most human- and all bat-derived cell lines tested was comparable. Moreover, EBOV2014 replication in NHP but not human cells was diminished relative to EBOV1976, suggesting that reduced cell entry translated into reduced viral spread. Mutagenic analysis of EBOV2014-GP and EBOV1976-GP revealed that an amino acid polymorphism in the receptor-binding domain, A82V, modulated entry efficiency in a cell line-independent manner and did not account for the reduced EBOV2014-GP-driven entry into NHP cells. In contrast, polymorphism T544I, located in the internal fusion loop in the GP2 subunit, was found to be responsible for the entry phenotype. These results suggest that position 544 is an important determinant of EBOV infectivity for both NHP and certain human target cells.IMPORTANCE The Ebola virus disease outbreak in West Africa in 2013 entailed more than 10,000 deaths. The scale of the outbreak and its dramatic impact on human health raised the question whether the responsible virus was particularly adept at infecting human cells. Our study shows that an amino acid exchange, A82V, that was acquired during the epidemic and that was not observed in previously circulating viruses, increases viral entry into diverse target cells. In contrast, the epidemic virus showed a reduced ability to enter cells of nonhuman primates compared to the virus circulating in 1976, and a single amino acid exchange in the internal fusion loop of the viral glycoprotein was found to account for this phenotype.

Understanding the interaction between henipaviruses and their natural host, fruit bats: Paving the way toward control of highly lethal infection in humans

Hendra virus and Nipah virus (NiV) are highly pathogenic zoonotic paramyxoviruses, from henipavirus genus, that have emerged in late 1990s in Australia and South-East Asia, respectively. Since their initial identification, numerous outbreaks have been reported, affecting both domestic animals and humans, and multiple rounds of person-to-person NiV transmission were observed. Widely distributed fruit bats from Pteropodidae family were found to be henipavirus natural reservoir. Numerous studies have reported henipavirus seropositivity in pteropid bats, including bats in Africa, thus expanding notably the geographic distribution of these viruses. Interestingly, henipavirus infection in bats seems to be asymptomatic, in contrast to severe disease induced in numerous other mammals. Unique among the mammals by their ability to fly, these intriguing animals are natural reservoir for many other emerging and remerging viruses highly pathogenic for humans. This feature, combined with absence of clinical symptoms, has attracted the interest of scientific community to virus-bat interactions. Therefore, several bat genomes were sequenced and particularities of the bat immune system have been intensively analyzed during the last decade to understand their coexistence with viruses in the absence of disease. The peculiarities in inflammasome activation, a constitutive expression of interferon alpha, and some differences in adaptive immunity have been recently reported in fruit bats. Studies on virus-bat interactions have thus emerged as an exciting novel area of research that should shed new light on the mechanisms that regulate viral infection and may allow development of novel therapeutic approaches to control this highly lethal emerging infectious disease in humans.

Unlocking bat immunology: establishment of Pteropus alecto bone marrow-derived dendritic cells and macrophages

Bats carry and shed many emerging infectious disease agents including Ebola virus and SARS-like Coronaviruses, yet they rarely display clinical symptoms of infection. Bat epithelial or fibroblast cell lines were previously established to study the bat immune response against viral infection. However, the lack of professional immune cells such as dendritic cells (DC) and macrophages has greatly limited the significance of current investigations. Using Pteropus alecto (P. alecto) GM-CSF plus IL4, FLT3L and CSF-1, we successfully generated bat bone marrow-derived DC and macrophages. Cells with the phenotype, morphology and functional features of monocyte-derived DC, bona fide DC or macrophages were obtained in GM-CSF/IL4, FLT3L or CSF-1 cultures, respectively. The successful generation of the first bat bone marrow-derived immune cells paves the way to unlocking the immune mechanisms that confer host resilience to pathogens in bats.

Generation and Characterization of Eptesicus fuscus (Big brown bat) kidney cell lines immortalized using the Myotis polyomavirus large T-antigen

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Eptesicus fuscus kidney cells immortalized using Myotis polyomavirus T-antigen.

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E. fuscus interferon competent kidney cell line supports the growth of vesicular stomatitis virus, porcine epidemic diarrhea virus, herpes simplex virus and Middle-East respiratory syndrome coronavirus.

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All cell lines exhibit a marker for fibroblasts (vimentin), some also exhibit an epithelial marker (cytokeratin).

It is speculated that bats are important reservoir hosts for numerous viruses, with 27 viral families reportedly detected in bats. Majority of these viruses have not been isolated and there is little information regarding their biology in bats. Establishing a well-characterized bat cell line supporting the replication of bat-borne viruses would facilitate the analysis of virus-host interactions in an in vitro model. Currently, few bat cell lines have been developed and only Tb1-Lu, derived from Tadarida brasiliensis is commercially available. Here we describe a method to establish and immortalize big brown bat (Eptesicus fuscus) kidney (Efk3) cells using the Myotis polyomavirus T-antigen. Subclones of this cell line expressed both epithelial and fibroblast markers to varying extents. Cell clones expressed interferon beta in response to poly(I:C) stimulation and supported the replication of four different viruses, namely, vesicular stomatitis virus (VSV), porcine epidemic diarrhea coronavirus (PED-CoV), Middle-East respiratory syndrome coronavirus (MERS-CoV) and herpes simplex virus (HSV). To our knowledge, this is the first bat cell line from a northern latitude insectivorous bat developed using a novel technology. The cell line has the potential to be used for isolation of bat viruses and for studying virus-bat interactions in culture.

Epithelial cell lines of the cotton rat (Sigmodon hispidus) are highly susceptible in vitro models to zoonotic Bunya-, Rhabdo-, and Flaviviruses

Background

Small mammals such as bats and rodents have been increasingly recognized as reservoirs of novel potentially zoonotic pathogens. However, few in vitro model systems to date allow assessment of zoonotic viruses in a relevant host context. The cotton rat (Sigmodon hispidus) is a New World rodent species that has a long-standing history as an experimental animal model due to its unique susceptibility to human viruses. Furthermore, wild cotton rats are associated with a large variety of known or potentially zoonotic pathogens.

Methods

A method for the isolation and culture of airway epithelial cell lines recently developed for bats was applied for the generation of rodent airway and renal epithelial cell lines from the cotton rat. Continuous cell lines were characterized for their epithelial properties as well as for their interferon competence. Susceptibility to members of zoonotic Bunya-, Rhabdo-, and Flaviviridae, in particular Rift Valley fever virus (RVFV), vesicular stomatitis virus (VSV), West Nile virus (WNV), and tick-borne encephalitis virus (TBEV) was tested. Furthermore, novel arthropod-derived viruses belonging to the families Bunya-, Rhabdo-, and Mesoniviridae were tested.

Results

We successfully established airway and kidney epithelial cell lines from the cotton rat, and characterized their epithelial properties. Cells were shown to be interferon-competent. Viral infection assays showed high-titre viral replication of RVFV, VSV, WNV, and TBEV, as well as production of infectious virus particles. No viral replication was observed for novel arthropod-derived members of the Bunya-, Rhabdo-, and Mesoniviridae families in these cell lines.

Conclusion

In the current study, we showed that newly established cell lines from the cotton rat can serve as host-specific in vitro models for viral infection experiments. These cell lines may also serve as novel tools for virus isolation, as well as for the investigation of virus-host interactions in a relevant host species.

The Hemagglutinin of Bat-Associated Influenza Viruses Is Activated by TMPRSS2 for pH-Dependent Entry into Bat but Not Human Cells

New World bats have recently been discovered to harbor influenza A virus (FLUAV)-related viruses, termed bat-associated influenza A-like viruses (batFLUAV). The internal proteins of batFLUAV are functional in mammalian cells. In contrast, no biological functionality could be demonstrated for the surface proteins, hemagglutinin (HA)-like (HAL) and neuraminidase (NA)-like (NAL), and these proteins need to be replaced by their human counterparts to allow spread of batFLUAV in human cells. Here, we employed rhabdoviral vectors to study the role of HAL and NAL in viral entry. Vectors pseudotyped with batFLUAV-HAL and -NAL were able to enter bat cells but not cells from other mammalian species. Host cell entry was mediated by HAL and was dependent on prior proteolytic activation of HAL and endosomal low pH. In contrast, sialic acids were dispensable for HAL-driven entry. Finally, the type II transmembrane serine protease TMPRSS2 was able to activate HAL for cell entry indicating that batFLUAV can utilize human proteases for HAL activation. Collectively, these results identify viral and cellular factors governing host cell entry driven by batFLUAV surface proteins. They suggest that the absence of a functional receptor precludes entry of batFLUAV into human cells while other prerequisites for entry, HAL activation and protonation, are met in target cells of human origin.

The Glycoproteins of All Filovirus Species Use the Same Host Factors for Entry into Bat and Human Cells but Entry Efficiency Is Species Dependent

Ebola and marburgviruses, members of the family Filoviridae, can cause severe hemorrhagic fever in humans. The ongoing Ebola virus (EBOV) disease epidemic in Western Africa claimed more than 11,300 lives and was associated with secondary cases outside Africa, demonstrating that filoviruses pose a global health threat. Bats constitute an important natural reservoir of filoviruses, including viruses of the recently identified Cuevavirus genus within the Filoviridae family. However, the interactions of filoviruses with bat cells are incompletely understood. Here, we investigated whether filoviruses employ different strategies to enter human and bat cells. For this, we examined host cell entry driven by glycoproteins (GP) from all filovirus species into cell lines of human and fruit bat origin. We show that all GPs were able to mediate entry into human and most fruit bat cell lines with roughly comparable efficiency. In contrast, the efficiency of entry into the cell line EidNi/41 derived from a straw-colored fruit bat varied markedly between the GPs of different filovirus species. Furthermore, inhibition studies demonstrated that filoviruses employ the same host cell factors for entry into human, non-human primate and fruit bat cell lines, including cysteine proteases, two pore channels and NPC1 (Niemann-Pick C1 molecule). Finally, processing of GP by furin and the presence of the mucin-like domain in GP were dispensable for entry into both human and bat cell lines. Collectively, these results show that filoviruses rely on the same host cell factors for entry into human and fruit bat cells, although the efficiency of the usage of these factors might differ between filovirus species.

The Role of Bats as Reservoir Hosts of Emerging Neuroviruses

Recent studies have clearly shown that bats are the reservoir hosts of a wide diversity of novel viruses with representatives from most of the known animal virus families. In many respects bats make ideal reservoir hosts for viruses: they are the only mammals that fly, thus assisting in virus dispersal; they roost in large numbers, thus aiding transmission cycles; some bats hibernate over winter, thus providing a mechanism for viruses to persist between seasons; and genetic factors may play a role in the ability of bats to host viruses without resulting in clinical disease. Within the broad diversity of viruses found in bats are some important neurological pathogens, including rabies and other lyssaviruses, and Hendra and Nipah viruses, two recently described viruses that have been placed in a new genus, Henipaviruses in the family Paramyxoviridae. In addition, bats can also act as alternative hosts for the flaviviruses Japanese encephalitis and St Louis encephalitis viruses, two important mosquito-borne encephalitogenic viruses, and bats can assist in the dispersal and over-wintering of these viruses. Bats are also the reservoir hosts of progenitors of SARS and MERS coronaviruses, although other animals act as spillover hosts. This chapter presents the physiological and ecological factors affecting the ability of bats to act as reservoirs of neurotropic viruses, and describes the major transmission cycles leading to human infection.

Filovirus receptor NPC1 contributes to species-specific patterns of ebolavirus susceptibility in bats

Biological factors that influence the host range and spillover of Ebola virus (EBOV) and other filoviruses remain enigmatic. While filoviruses infect diverse mammalian cell lines, we report that cells from African straw-colored fruit bats (Eidolon helvum) are refractory to EBOV infection. This could be explained by a single amino acid change in the filovirus receptor, NPC1, which greatly reduces the affinity of EBOV-NPC1 interaction. We found signatures of positive selection in bat NPC1 concentrated at the virus-receptor interface, with the strongest signal at the same residue that controls EBOV infection in Eidolon helvum cells. Our work identifies NPC1 as a genetic determinant of filovirus susceptibility in bats, and suggests that some NPC1 variations reflect host adaptations to reduce filovirus replication and virulence. A single viral mutation afforded escape from receptor control, revealing a pathway for compensatory viral evolution and a potential avenue for expansion of filovirus host range in nature.

DOI:http://dx.doi.org/10.7554/eLife.11785.001

Ebola virus and other filoviruses can cause devastating diseases in humans and other apes. Numerous small outbreaks of Ebola virus disease have occurred in Africa over the past 40 years. However, in 2013–2015, the largest outbreak on record took place in three Western African nations with no previous history of the disease.

Human outbreaks of Ebola virus disease likely begin when a person encounters an infected wild animal. Though it remains unclear precisely which animals harbor Ebola virus between outbreaks, and how they transmit the virus to humans or other primates, recent work showed that some filoviruses do infect specific types of bats in nature.

Ng, Ndungo, Kaczmarek et al. sought to identify the genes that influence whether or not a type of bat is susceptible to infection by Ebola virus and other filoviruses. Several filoviruses, including Ebola virus, were tested to see if they could infect cells that had been collected from four types of African fruit bats. These bats are all found in areas where outbreaks have occurred in the past.

The tests revealed that a small change in the sequence of the NPC1 gene in some bat cells greatly reduced their susceptibility to Ebola virus. NPC1 encodes a protein that mammals need in order to move cholesterol within their cells. In humans, the loss of the protein encoded by NPC1 causes a rare but very severe disease called Niemann-Pick type C disease. This protein also turns out to be a receptor that the filoviruses must bind to before they can infect the cells. Further analysis then revealed that NPC1 has evolved rapidly in bats, with changes concentrated in the parts of the receptor that interact with Ebola virus.

Ng, Ndungo, Kaczmarek et al. went on to discover some changes in the genome sequence of Ebola virus that could compensate for the changes in the bat’s NPC1 gene. These findings hint at one way that a filovirus could evolve to better infect a host with receptors that were less than optimal.

Following on from this work, the next challenges will be to expand the investigation to include additional types of bats, other types of mammals, and other host genes that could influence filovirus infection and disease. Further studies could also examine the other side of the arms race – that is, the evolution of viral genes in bats. However, such studies would be complicated by the lack of viral sequences that have been collected from bats, because to date most have been isolated from humans and other primates instead.

DOI:http://dx.doi.org/10.7554/eLife.11785.002

Cloning, expression, and antiviral activity of interferon β from the Chinese microbat, Myotis davidii

Bats are natural reservoir hosts for many viruses that produce no clinical symptoms in bats. Therefore, bats may have evolved effective mechanisms to control viral replication. However, little information is available on bat immune responses to viral infection. Type I interferon (IFN) plays a key role in controlling viral infections. In this study, we report the cloning, expression, and biological activity of interferon β (IFNβ) from the Chinese microbat species, Myotis davidii. We demonstrated the upregulation of IFNB and IFN-stimulated genes in a kidney cell line derived from M. davidii after treatment with polyI:C or infection with Sendai virus. Furthermore, the recombinant IFNβ inhibited vesicular stomatitis virus and bat adenovirus replication in cell lines from two bat species, M. davidii and Rhinolophus sinicus. We provide the first in vitro evidence of IFNβ antiviral activity in microbats, which has important implications for virus interactions with these hosts.

Immunology of bats and their viruses: challenges and opportunities

Bats are reservoir hosts of several high-impact viruses that cause significant human diseases, including Nipah virus, Marburg virus and rabies virus. They also harbor many other viruses that are thought to have caused disease in humans after spillover into intermediate hosts, including SARS and MERS coronaviruses. As is usual with reservoir hosts, these viruses apparently cause little or no pathology in bats. Despite the importance of bats as reservoir hosts of zoonotic and potentially zoonotic agents, virtually nothing is known about the host/virus relationships; principally because few colonies of bats are available for experimental infections, a lack of reagents, methods and expertise for studying bat antiviral responses and immunology, and the difficulty of conducting meaningful field work. These challenges can be addressed, in part, with new technologies that are species-independent that can provide insight into the interactions of bats and viruses, which should clarify how the viruses persist in nature, and what risk factors might facilitate transmission to humans and livestock.

Transcriptome Profiling of the Virus-Induced Innate Immune Response in Pteropus vampyrus and Its Attenuation by Nipah Virus Interferon Antagonist Functions

Bats are important reservoirs for several viruses, many of which cause lethal infections in humans but have reduced pathogenicity in bats. As the innate immune response is critical for controlling viruses, the nature of this response in bats and how it may differ from that in other mammals are of great interest. Using next-generation transcriptome sequencing (mRNA-seq), we profiled the transcriptional response of Pteropus vampyrus bat kidney (PVK) cells to Newcastle disease virus (NDV), an avian paramyxovirus known to elicit a strong innate immune response in mammalian cells. The Pteropus genus is a known reservoir of Nipah virus (NiV) and Hendra virus (HeV). Analysis of the 200 to 300 regulated genes showed that genes for interferon (IFN) and antiviral pathways are highly upregulated in NDV-infected PVK cells, including genes for beta IFN, RIG-I, MDA5, ISG15, and IRF1. NDV-infected cells also upregulated several genes not previously characterized to be antiviral, such as RND1, SERTAD1, CHAC1, and MORC3. In fact, we show that MORC3 is induced by both IFN and NDV infection in PVK cells but is not induced by either stimulus in human A549 cells. In contrast to NDV infection, HeV and NiV infection of PVK cells failed to induce these innate immune response genes. Likewise, an attenuated response was observed in PVK cells infected with recombinant NDVs expressing the NiV IFN antagonist proteins V and W. This study provides the first global profile of a robust virus-induced innate immune response in bats and indicates that henipavirus IFN antagonist mechanisms are likely active in bat cells.

IMPORTANCE

Bats are the reservoir host for many highly pathogenic human viruses, including henipaviruses, lyssaviruses, severe acute respiratory syndrome coronavirus, and filoviruses, and many other viruses have also been isolated from bats. Viral infections are reportedly asymptomatic or heavily attenuated in bat populations. Despite their ecological importance to viral maintenance, research into their immune system and mechanisms for viral control has only recently begun. Nipah virus and Hendra virus are two paramyxoviruses associated with high mortality rates in humans and whose reservoir is the Pteropus genus of bats. Greater knowledge of the innate immune response of P. vampyrus bats to viral infection may elucidate how bats serve as a reservoir for so many viruses.

Functional properties and genetic relatedness of the fusion and hemagglutinin-neuraminidase proteins of a mumps virus-like bat virus

A bat virus with high phylogenetic relatedness to human mumps virus (MuV) was identified recently at the nucleic acid level. We analyzed the functional activities of the hemagglutinin-neuraminidase (HN) and the fusion (F) proteins of the bat virus (batMuV) and compared them to the respective proteins of a human isolate. Transfected cells expressing the F and HN proteins of batMuV were recognized by antibodies directed against these proteins of human MuV, indicating that both viruses are serologically related. Fusion, hemadsorption, and neuraminidase activities were demonstrated for batMuV, and either bat-derived protein could substitute for its human MuV counterpart in inducing syncytium formation when coexpressed in different mammalian cell lines, including chiropteran cells. Cells expressing batMuV glycoproteins were shown to have lower neuraminidase activity. The syncytia were smaller, and they were present in lower numbers than those observed after coexpression of the corresponding glycoproteins of a clinical isolate of MuV (hMuV). The phenotypic differences in the neuraminidase and fusion activity between the glycoproteins of batMuV and hMuV are explained by differences in the expression level of the HN and F proteins of the two viruses. In the case of the F protein, analysis of chimeric proteins revealed that the signal peptide of the bat MuV fusion protein is responsible for the lower surface expression. These results indicate that the surface glycoproteins of batMuV are serologically and functionally related to those of hMuV, raising the possibility of bats as a reservoir for interspecies transmission.

IMPORTANCE

The recently described MuV-like bat virus is unique among other recently identified human-like bat-associated viruses because of its high sequence homology (approximately 90% in most genes) to its human counterpart. Although it is not known if humans can be infected by batMuV, the antigenic relatedness between the bat and human forms of the virus suggests that humans carrying neutralizing antibodies against MuV are protected from infection by batMuV. The close functional relationship between MuV and batMuV is demonstrated by cooperation of the respective HN and F proteins to induce syncytium formation in heterologous expression studies. An interesting feature of the glycoproteins of batMuV is the downregulation of the fusion activity by the signal peptide of F, which has not been reported for other paramyxoviruses. These results are important contributions for risk assessment and for a better understanding of the replication strategy of batMuV.

A novel rhabdovirus isolated from the straw-colored fruit bat Eidolon helvum, with signs of antibodies in swine and humans

Bats have been implicated as reservoirs of emerging viruses. Bat species forming large social groups and roosting in proximity to human communities are of particular interest. In this study, we sampled a colony of ca. 350,000 individuals of the straw-colored fruit bat Eidolon helvum in Kumasi, the second largest city of Ghana. A novel rhabdovirus (Kumasi rhabdovirus [KRV]) was isolated in E. helvum cell cultures and passaged to Vero cells as well as interferon-competent human and primate cells (A549 and MA104). Genome composition was typical for a rhabdovirus. KRV was detected in 5.1% of 487 animals, showing association with the spleen but not the brain. Antibody prevalence was 11.5% by immunofluorescence and 6.4% by plaque reduction virus neutralization test (PRNT). Detection throughout 3 sampling years was pronounced in both annual wet seasons, of which only one overlaps the postparturition season. Juvenile bats showed increased viral prevalence. No evidence of infection was obtained in 1,240 female mosquitos (6 different genera) trapped in proximity to the colony to investigate potential vector association. Antibodies were found in 28.9% (5.4% by PRNT) of 107 swine sera but not in similarly large collections of sheep, goat, or cattle sera. The antibody detection rate in human subjects with occupational exposure to the bat colony was 11% (5/45 persons), which was significantly higher than in unexposed adults (0.8% [1/118]; chi square, P < 0.001). KRV is a novel bat-associated rhabdovirus potentially transmitted to humans and swine. Disease associations should be investigated.

IMPORTANCE

Bats are thought to carry a huge number of as-yet-undiscovered viruses that may pose epidemic threats to humans and livestock. Here we describe a novel dimarhabdovirus which we isolated from a large colony of the straw-colored fruit bat Eidolon helvum in Ghana. As these animals are exposed to humans and several livestock species, we looked for antibodies indicating infection in humans, cattle, swine, sheep, and goats. Signs of infection were found in swine and humans, with increased antibody findings in humans who are occupationally exposed to the bat colony. Our data suggest that it is worthwhile to look for diseases caused by the novel virus in humans and livestock.

Molecular characterization of RIG-I, STAT-1 and IFN-beta in the horseshoe bat

Wild Chinese horseshoe bats have been proven to be natural reservoirs of SARS-like coronaviruses. However, the molecular characterization of key proteins in bats still needs to be explored further. In this study, we used cloning and bioinformatics to analyze the sequence of RIG-I, STAT-1 and IFN-β in the immortalized cell lines from Rhinolophus affinis and Rhinolophus sinicus. Then, we treated different bat cells, mouse embryonic fibroblasts (MEF) and splenocytes with polyinosinic–polycytidylic acid (polyI:C) and vesicular stomatitis virus (VSV) to assess and compare antiviral immune responses between bats and mice. Our results demonstrated that bat RIG-I, STAT-1 and IFN-β showed close homology with human, mouse, pig and rhesus monkey. RIG-I and STAT-1 were both highly expressed in bat spleen. Furthermore, IFN-β was induced by polyI:C and VSV in both bat and mouse cells. These findings have provided new insight into the potential characteristics of the bat innate immune system against viral infection.

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The genes of RIG-I, STAT-1 and IFN-β in Chinese horseshoe bats were sequenced and analyzed.

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The expression patterns of RIG-I, STAT-1 and IFN-β were appraised in bat tissues.

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The virus induced IFN-β expression is higher in bat cells than in mouse cells.

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The replication of invaded VSV is more in mouse cells than in bat cells.

Bat and pig IFN-induced transmembrane protein 3 restrict cell entry by influenza virus and lyssaviruses

IFN-induced transmembrane protein 3 (IFITM3) is a restriction factor that blocks cytosolic entry of numerous viruses that utilize acidic endosomal entry pathways. In humans and mice, IFITM3 limits influenza-induced morbidity and mortality. Although many IFITM3-sensitive viruses are zoonotic, whether IFITMs function as antiviral restriction factors in mammalian species other than humans and mice is unknown. Here, IFITM3 orthologues in the microbat (Myotis myotis) and pig (Sus scrofa domesticus) were identified using rapid amplification of cDNA ends. Amino acid residues known to be important for IFITM3 function were conserved in the pig and microbat orthologues. Ectopically expressed pig and microbat IFITM3 co-localized with transferrin (early endosomes) and CD63 (late endosomes/multivesicular bodies). Pig and microbat IFITM3 restricted cell entry mediated by multiple influenza haemagglutinin subtypes and lyssavirus glycoproteins. Expression of pig or microbat IFITM3 in A549 cells reduced influenza virus yields and nucleoprotein expression. Conversely, small interfering RNA knockdown of IFITM3 in pig NPTr cells and primary microbat cells enhanced virus replication, demonstrating that these genes are functional in their species of origin at endogenous levels. In summary, we showed that IFITMs function as potent broad-spectrum antiviral effectors in two mammals - pigs and bats - identified as major reservoirs for emerging viruses.

CD26/DPP4 cell-surface expression in bat cells correlates with bat cell susceptibility to Middle East respiratory syndrome coronavirus (MERS-CoV) infection and evolution of persistent infection

Middle East respiratory syndrome coronavirus (MERS-CoV) is a recently isolated betacoronavirus identified as the etiologic agent of a frequently fatal disease in Western Asia, Middle East respiratory syndrome. Attempts to identify the natural reservoirs of MERS-CoV have focused in part on dromedaries. Bats are also suspected to be reservoirs based on frequent detection of other betacoronaviruses in these mammals. For this study, ten distinct cell lines derived from bats of divergent species were exposed to MERS-CoV. Plaque assays, immunofluorescence assays, and transmission electron microscopy confirmed that six bat cell lines can be productively infected. We found that the susceptibility or resistance of these bat cell lines directly correlates with the presence or absence of cell surface-expressed CD26/DPP4, the functional human receptor for MERS-CoV. Human anti-CD26/DPP4 antibodies inhibited infection of susceptible bat cells in a dose-dependent manner. Overexpression of human CD26/DPP4 receptor conferred MERS-CoV susceptibility to resistant bat cell lines. Finally, sequential passage of MERS-CoV in permissive bat cells established persistent infection with concomitant downregulation of CD26/DPP4 surface expression. Together, these results imply that bats indeed could be among the MERS-CoV host spectrum, and that cellular restriction of MERS-CoV is determined by CD26/DPP4 expression rather than by downstream restriction factors.

Influenza A virus polymerase is a site for adaptive changes during experimental evolution in bat cells

The recent identification of highly divergent influenza A viruses in bats revealed a new, geographically dispersed viral reservoir. To investigate the molecular mechanisms of host-restricted viral tropism and the potential for transmission of viruses between humans and bats, we exposed a panel of cell lines from bats of diverse species to a prototypical human-origin influenza A virus. All of the tested bat cell lines were susceptible to influenza A virus infection. Experimental evolution of human and avian-like viruses in bat cells resulted in efficient replication and created highly cytopathic variants. Deep sequencing of adapted human influenza A virus revealed a mutation in the PA polymerase subunit not previously described, M285K. Recombinant virus with the PA M285K mutation completely phenocopied the adapted virus. Adaptation of an avian virus-like virus resulted in the canonical PB2 E627K mutation that is required for efficient replication in other mammals. None of the adaptive mutations occurred in the gene for viral hemagglutinin, a gene that frequently acquires changes to recognize host-specific variations in sialic acid receptors. We showed that human influenza A virus uses canonical sialic acid receptors to infect bat cells, even though bat influenza A viruses do not appear to use these receptors for virus entry. Our results demonstrate that bats are unique hosts that select for both a novel mutation and a well-known adaptive mutation in the viral polymerase to support replication.

IMPORTANCE

Bats constitute well-known reservoirs for viruses that may be transferred into human populations, sometimes with fatal consequences. Influenza A viruses have recently been identified in bats, dramatically expanding the known host range of this virus. Here we investigated the replication of human influenza A virus in bat cell lines and the barriers that the virus faces in this new host. Human influenza A and B viruses infected cells from geographically and evolutionarily diverse New and Old World bats. Viruses mutated during infections in bat cells, resulting in increased replication and cytopathic effects. These mutations were mapped to the viral polymerase and shown to be solely responsible for adaptation to bat cells. Our data suggest that replication of human influenza A viruses in a nonnative host drives the evolution of new variants and may be an important source of genetic diversity.

Establishment of Myotis myotis cell lines--model for investigation of host-pathogen interaction in a natural host for emerging viruses

Bats are found to be the natural reservoirs for many emerging viruses. In most cases, severe clinical signs caused by such virus infections are normally not seen in bats. This indicates differences in the virus-host interactions and underlines the necessity to develop natural host related models to study these phenomena. Due to the strict protection of European bat species, immortalized cell lines are the only alternative to investigate the innate anti-virus immune mechanisms. Here, we report about the establishment and functional characterization of Myotis myotis derived cell lines from different tissues: brain (MmBr), tonsil (MmTo), peritoneal cavity (MmPca), nasal epithelium (MmNep) and nervus olfactorius (MmNol) after immortalization by SV 40 large T antigen. The usefulness of these cell lines to study antiviral responses has been confirmed by analysis of their susceptibility to lyssavirus infection and the mRNA patterns of immune-relevant genes after poly I:C stimulation. Performed experiments indicated varying susceptibility to lyssavirus infection with MmBr being considerably less susceptible than the other cell lines. Further investigation demonstrated a strong activation of interferon mediated antiviral response in MmBr contributing to its resistance. The pattern recognition receptors: RIG-I and MDA5 were highly up-regulated during rabies virus infection in MmBr, suggesting their involvement in promotion of antiviral responses. The presence of CD14 and CD68 in MmBr suggested MmBr cells are microglia-like cells which play a key role in host defense against infections in the central nervous system (CNS). Thus the expression pattern of MmBr combined with the observed limitation of lyssavirus replication underpin a protective mechanism of the CNS controlling the lyssavirus infection. Overall, the established cell lines are important tools to analyze antiviral innate immunity in M. myotis against neurotropic virus infections and present a valuable tool for a broad spectrum of future investigations in cellular biology of M. myotis.

Poxviruses in bats … so what?

Poxviruses are important pathogens of man and numerous domestic and wild animal species. Cross species (including zoonotic) poxvirus infections can have drastic consequences for the recipient host. Bats are a diverse order of mammals known to carry lethal viral zoonoses such as Rabies, Hendra, Nipah, and SARS. Consequent targeted research is revealing bats to be infected with a rich diversity of novel viruses. Poxviruses were recently identified in bats and the settings in which they were found were dramatically different. Here, we review the natural history of poxviruses in bats and highlight the relationship of the viruses to each other and their context in the Poxviridae family. In addition to considering the zoonotic potential of these viruses, we reflect on the broader implications of these findings. Specifically, the potential to explore and exploit this newfound relationship to study coevolution and cross species transmission together with fundamental aspects of poxvirus host tropism as well as bat virology and immunology.

Anti-lyssaviral activity of interferons κ and ω from the serotine bat, Eptesicus serotinus

Interferons (IFNs) are cytokines produced by host cells in response to the infection with pathogens. By binding to the corresponding receptors, IFNs trigger different pathways to block intracellular replication and growth of pathogens and to impede the infection of surrounding cells. Due to their key role in host defense against viral infections, as well as for clinical therapies, the IFN responses and regulation mechanisms are well studied. However, studies of type I IFNs have mainly focused on alpha interferon (IFN-α) and IFN-β subtypes. Knowledge of IFN-κ and IFN-ω is limited. Moreover, most studies are performed in humans or mouse models but not in the original host of zoonotic pathogens. Bats are important reservoirs and transmitters of zoonotic viruses such as lyssaviruses. A few studies have shown an antiviral activity of IFNs in fruit bats. However, the function of type I IFNs against lyssaviruses in bats has not been studied yet. Here, IFN-κ and IFN-ω genes from the European serotine bat, Eptesicus serotinus, were cloned and functionally characterized. E. serotinus IFN-κ and IFN-ω genes are intronless and well conserved between microchiropteran species. The promoter regions of both genes contain essential regulatory elements for transcription factors. In vitro studies indicated a strong activation of IFN signaling by recombinant IFN-ω, whereas IFN-κ displayed weaker activation. Noticeably, both IFNs inhibit to different extents the replication of different lyssaviruses in susceptible bat cell lines. The present study provides functional data on the innate host defense against lyssaviruses in endangered European bats.

IMPORTANCE

We describe here for the first time the molecular and functional characterization of two type I interferons (IFN-κ and -ω) from European serotine bat (Eptesicus serotinus). The importance of this study is mainly based on the fact that very limited information about the early innate immune response against bat lyssaviruses in their natural host serotine bats is yet available. Generally, whereas the antiviral activity of other type I interferons is well studied, the functional involvement of IFN-κ and -ω has not yet been investigated.

More novel hantaviruses and diversifying reservoir hosts--time for development of reservoir-derived cell culture models?

Due to novel, improved and high-throughput detection methods, there is a plethora of newly identified viruses within the genus Hantavirus. Furthermore, reservoir host species are increasingly recognized besides representatives of the order Rodentia, now including members of the mammalian orders Soricomorpha/Eulipotyphla and Chiroptera. Despite the great interest created by emerging zoonotic viruses, there is still a gross lack of in vitro models, which reflect the exclusive host adaptation of most zoonotic viruses. The usually narrow host range and genetic diversity of hantaviruses make them an exciting candidate for studying virus-host interactions on a cellular level. To do so, well-characterized reservoir cell lines covering a wide range of bat, insectivore and rodent species are essential. Most currently available cell culture models display a heterologous virus-host relationship and are therefore only of limited value. Here, we review the recently established approaches to generate reservoir-derived cell culture models for the in vitro study of virus-host interactions. These successfully used model systems almost exclusively originate from bats and bat-borne viruses other than hantaviruses. Therefore we propose a parallel approach for research on rodent- and insectivore-borne hantaviruses, taking the generation of novel rodent and insectivore cell lines from wildlife species into account. These cell lines would be also valuable for studies on further rodent-borne viruses, such as orthopox- and arenaviruses.

Bat airway epithelial cells: a novel tool for the study of zoonotic viruses

Bats have been increasingly recognized as reservoir of important zoonotic viruses. However, until now many attempts to isolate bat-borne viruses in cell culture have been unsuccessful. Further, experimental studies on reservoir host species have been limited by the difficulty of rearing these species. The epithelium of the respiratory tract plays a central role during airborne transmission, as it is the first tissue encountered by viral particles. Although several cell lines from bats were established recently, no well-characterized, selectively cultured airway epithelial cells were available so far. Here, primary cells and immortalized cell lines from bats of the two important suborders Yangochiroptera and Yinpterochiroptera, Carollia perspicillata (Seba's short-tailed bat) and Eidolon helvum (Straw-colored fruit bat), were successfully cultured under standardized conditions from both fresh and frozen organ specimens by cell outgrowth of organ explants and by the use of serum-free primary cell culture medium. Cells were immortalized to generate permanent cell lines. Cells were characterized for their epithelial properties such as expression of cytokeratin and tight junctions proteins and permissiveness for viral infection with Rift-Valley fever virus and vesicular stomatitis virus Indiana. These cells can serve as suitable models for the study of bat-borne viruses and complement cell culture models for virus infection in human airway epithelial cells.

Characterization of African bat henipavirus GH-M74a glycoproteins

In recent years, novel henipavirus-related sequences have been identified in bats in Africa. To evaluate the potential of African bat henipaviruses to spread in non-bat mammalian cells, we compared the biological functions of the surface glycoproteins G and F of the prototype African henipavirus GH-M74a with those of the glycoproteins of Nipah virus (NiV), a well-characterized pathogenic member of the henipavirus genus. Glycoproteins are central determinants for virus tropism, as efficient binding of henipavirus G proteins to cellular ephrin receptors and functional expression of fusion-competent F proteins are indispensable prerequisites for virus entry and cell-to-cell spread. In this study, we analysed the ability of the GH-M74a G and F proteins to cause cell-to-cell fusion in mammalian cell types readily permissive to NiV or Hendra virus infections. Except for limited syncytium formation in a bat cell line derived from Hypsignathus monstrosus, HypNi/1.1 cells, we did not observe any fusion. The highly restricted fusion activity was predominantly due to the F protein. Whilst GH-M74a G protein was found to interact with the main henipavirus receptor ephrin-B2 and induced syncytia upon co-expression with heterotypic NiV F protein, GH-M74a F protein did not cause evident fusion in the presence of heterotypic NiV G protein. Pulse-chase and surface biotinylation analyses revealed delayed F cleavage kinetics with a reduced expression of cleaved and fusion-active GH-M74a F protein on the cell surface. Thus, the F protein of GH-M74a showed a functional defect that is most likely caused by impaired trafficking leading to less efficient proteolytic activation and surface expression.

Middle East respiratory syndrome coronavirus accessory protein 4a is a type I interferon antagonist

Middle East respiratory syndrome coronavirus (MERS-CoV) causes severe acute respiratory infection with as yet unclear epidemiology. We previously showed that MERS-CoV counteracts parts of the innate immune response in human bronchiolar cells. Here we analyzed accessory proteins 3, 4a, 4b, and 5 for their abilities to inhibit the type I interferon response. Accessory protein 4a was found to block interferon induction at the level of melanoma differentiation-associated protein 5 (MDA5) activation presumably by direct interaction with double-stranded RNA.