Erratum: AC-DC Electropenetrography as a Tool to Quantify Probing and Ingestion Behaviors of the Yellow Fever Mosquito (Aedes aegypti) on Mice in Biocontainment.

This corrects the article DOI: 10.30802/AALAS-CM-23-000037
When the above article was first published in the Vol 3 No 6 (December 2023) issue of Comparative Medicine, figure images were incorrectly associated with the figure legends. The correct version of this article has been reprinted in full in volume 74, issue 1 of the February issue of Comparative Medicine.
The publisher apologizes for this error and any inconvenience caused.

An electropenetrography waveform library for the probing and ingestion behaviors of Culex tarsalis on human hands.

Culex tarsalis Coquillett (Diptera: Culicidae) mosquitoes are capable of vectoring numerous pathogens affecting public and animal health. Unfortunately, the probing behaviors of mosquitoes are poorly understood because they occur in opaque tissues. Electropenetrography (EPG) has the potential to elucidate these behaviors by recording the electrical signals generated during probing. We used an AC-DC EPG with variable input resistors (Ri levels) to construct a waveform library for Cx. tarsalis feeding on human hands. Biological events associated with mosquito probing were used to characterize waveforms at four Ri levels and with two electrical current types. The optimal settings for EPG recordings of Cx. tarsalis probing on human hands was an Ri level of 107 Ohms using an applied signal of 150 millivolts alternating current. Waveforms for Cx. tarsalis included those previously observed and associated with probing behaviors in Aedes aegypti L. (Diptera: Culicidae): waveform families J (surface salivation), K (stylet penetration through the skin), L (types 1 and 2, search for a blood vessel/ingestion site), M (types 1 and 2, ingestion), N (type 1, an unknown behavior which may be a resting and digestion phase), and W (withdrawal). However, we also observed variations in the waveforms not described in Ae. aegypti, which we named types L3, M3, M4, and N2. This investigation enhances our understanding of mosquito probing behaviors. It also provides a new tool for the automated calculation of peak frequency. This work will facilitate future pathogen acquisition and transmission studies and help identify new pest and disease management targets.

In Vitro Infection Dynamics of Japanese Encephalitis Virus in Established Porcine Cell Lines.

Japanese encephalitis virus (JEV) is a zoonotic mosquito-borne pathogen that regularly causes severe neurological disease in humans in Southeast Asia and the Western Pacific region. Pigs are one of the main amplifying hosts of JEV and play a central role in the virus transmission cycle. The objective of this study was to identify in vitro cell systems to investigate early effects of JEV infection including viral replication and host cell death. Here, we demonstrate the susceptibility of several porcine cell lines to the attenuated genotype III JEV strain SA14-14-2. Monolayers of porcine nasal turbinate (PT-K75), kidney (SK-RST), testis (ST), and monocyte-derived macrophage (CΔ2+) cells were infected with SA14-14-2 for up to five days at a multiplicity of infection (MOI) of 0.1. The hamster kidney cell line BHK-21, previously shown to be susceptible to SA14-14-2, was used as a positive control. Culture supernatants and cells were collected between 0 and 120 h post infection (hpi), and monolayers were observed for cytopathic effect (CPE) using brightfield microscopy. The number of infectious virus particles was quantified by plaque assay and cell viability was determined using trypan blue staining. An indirect immunofluorescence assay was used to detect the presence of JEV NS1 antigens in cells infected at 1 MOI. All four porcine cell lines demonstrated susceptibility to SA14-14-2 and produced infectious virus by 12 hpi. Virus titers peaked at 48 hpi in CΔ2+, BHK-21, and SK-RST cells, at 72 hpi in PT-K75, and at 120 hpi in ST cells. CPE was visible in infected CΔ2+ and BHK-21 cells, but not the other three cell lines. The proportion of viable cells, as measured by trypan blue exclusion, declined after 24 hpi in BHK-21 and 48 hpi in CΔ2+ cells, but did not substantially decline in SK-RST, PT-K75 or ST cells. At 48 hpi, JEV NS1 was detected in all infected cell lines by fluorescence microscopy. These findings demonstrate several porcine cell lines which have the potential to serve as useful research tools for investigating JEV infection dynamics and host cell mechanisms in a natural amplifying host species, such as pigs, in vitro.

Mechanical transmission of SARS-CoV-2 by house flies.

BACKGROUND: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a recently emerged coronavirus that is the causative agent of the coronavirus disease 2019 (COVID-19) pandemic. COVID-19 in humans is characterized by a wide range of symptoms that range from asymptomatic to mild or severe illness including death. SARS-CoV-2 is highly contagious and is transmitted via the oral-nasal route through droplets and aerosols, or through contact with contaminated fomites. House flies are known to transmit bacterial, parasitic and viral diseases to humans and animals as mechanical vectors. Previous studies have shown that house flies can mechanically transmit coronaviruses, such as turkey coronavirus; however, the house fly's role in SARS-CoV-2 transmission has not yet been explored. The goal of this work was to investigate the potential of house flies to mechanically transmit SARS-CoV-2. For this purpose, it was determined whether house flies can acquire SARS-CoV-2, harbor live virus and mechanically transmit the virus to naive substrates and surfaces. METHODS: Two independent studies were performed to address the study objectives. In the first study, house flies were tested for infectivity after exposure to SARS-CoV-2-spiked medium or milk. In the second study, environmental samples were tested for infectivity after contact with SARS-CoV-2-exposed flies. During both studies, samples were collected at various time points post-exposure and evaluated by SARS-CoV-2-specific RT-qPCR and virus isolation. RESULTS: All flies exposed to SARS-CoV-2-spiked media or milk substrates were positive for viral RNA at 4 h and 24 h post-exposure. Infectious virus was isolated only from the flies exposed to virus-spiked milk but not from those exposed to virus-spiked medium. Moreover, viral RNA was detected in environmental samples after contact with SARS-CoV-2 exposed flies, although no infectious virus was recovered from these samples. CONCLUSIONS: Under laboratory conditions, house flies acquired and harbored infectious SARS-CoV-2 for up to 24 h post-exposure. In addition, house flies were able to mechanically transmit SARS-CoV-2 genomic RNA to the surrounding environment up to 24 h post-exposure. Further studies are warranted to determine if house fly transmission occurs naturally and the potential public health implications of such events.

Susceptibility of Midge and Mosquito Vectors to SARS-CoV-2.

SARS-CoV-2 is a recently emerged, highly contagious virus and the cause of the current COVID-19 pandemic. It is a zoonotic virus, although its animal origin is not clear yet. Person-to-person transmission occurs by inhalation of infected droplets and aerosols, or by direct contact with contaminated fomites. Arthropods transmit numerous viral, parasitic, and bacterial diseases; however, the potential role of arthropods in SARS-CoV-2 transmission is not fully understood. Thus far, a few studies have demonstrated that SARS-CoV-2 replication is not supported in cells from certain insect species nor in certain species of mosquitoes after intrathoracic inoculation. In this study, we expanded the work of SARS-CoV-2 susceptibility to biting insects after ingesting a SARS-CoV-2-infected bloodmeal. Species tested included Culicoides sonorensis (Wirth & Jones) (Diptera: Ceratopogonidae) biting midges, as well as Culex tarsalis (Coquillett) and Culex quinquefasciatus (Say) mosquitoes (Diptera: Culicidae), all known biological vectors for numerous RNA viruses. Arthropods were allowed to feed on SARS-CoV-2-spiked blood and at a time point postinfection analyzed for the presence of viral RNA and infectious virus. Additionally, cell lines derived from C. sonorensis (W8a), Aedes aegypti (Linnaeus) (Diptera: Culicidae) (C6/36), Cx. quinquefasciatus (HSU), and Cx. tarsalis (CxTrR2) were tested for SARS-CoV-2 susceptibility. Our results indicate that none of the biting insects, nor the insect cell lines evaluated support SARS-CoV-2 replication, suggesting that these species are unable to be biological vectors of SARS-CoV-2.

Age-Dependent Susceptibility to Pulmonary Fibrosis Is Associated with NLRP3 Inflammasome Activation.

Aging has been implicated in the development of pulmonary fibrosis, which has seen a sharp increase in incidence in those older than 50 years. Recent studies demonstrate a role for the nucleotide-binding domain and leucine rich repeat containing family, pyrin domain containing 3 (NLRP3) inflammasome and its regulated cytokines in experimental lung fibrosis. In this study, we tested the hypothesis that age-related NLRP3 inflammasome activation is an important predisposing factor in the development of pulmonary fibrosis. Briefly, young and aged wild-type and NLRP3(-/-) mice were subjected to bleomycin-induced lung injury. Pulmonary fibrosis was determined by histology and hydroxyproline accumulation. Bone marrow and alveolar macrophages were isolated from these mice. NLRP3 inflammasome activation was assessed by co-immunoprecipitation experiments. IL-1ß and IL-18 production was measured by ELISA. The current study demonstrated that aged wild-type mice developed more lung fibrosis and exhibited increased morbidity and mortality after bleomycin-induced lung injury, when compared with young mice. Bleomycin-exposed aged NLRP3(-/-) mice had reduced fibrosis compared with their wild-type age-matched counterparts. Bone marrow-derived and alveolar macrophages from aged mice displayed higher levels of NLRP3 inflammasome activation and caspase-1-dependent IL-1ß and IL-18 production, which was associated with altered mitochondrial function and increased production of reactive oxygen species. Our study demonstrated that age-dependent increases in alveolar macrophage mitochondrial reactive oxygen species production and NLRP3 inflammasome activation contribute to the development of experimental fibrosis.

Age-enhanced endoplasmic reticulum stress contributes to increased Atg9A inhibition of STING-mediated IFN-ß production during Streptococcus pneumoniae infection.

Pneumococcal infections remain a leading cause of death in persons ≥ 65 y of age. Recent reports have illustrated detrimental changes in the endoplasmic reticulum stress response or unfolded protein response in aging and age-related diseases; however, the relationship between aging, the unfolded protein response, and innate immune responses to Streptococcus pneumoniae has not been fully elucidated. Our results illustrate that stimulator of IFN genes-mediated production of IFN-ß during S. pneumoniae infection is decreased in aged hosts. Enhanced endoplasmic reticulum stress in response to S. pneumoniae augmented inositol-requiring protein 1/X-box binding protein 1-mediated production of autophagy-related gene 9 (Atg9a). Knockdown of Atg9a or treatment with gemcitabine HCl resulted in enhanced stimulator of IFN genes-mediated production of IFN-ß by aged macrophages. Consecutive treatments with gemcitabine during in vivo S. pneumoniae infection decreased morbidity and mortality in aged hosts, which was associated with decreased Atg9a expression, increased IFN-ß production, and improved bacterial clearance from lung tissue. Taken together, data presented in this study provide new evidence as to why older persons are more susceptible to S. pneumoniae, and provide a possible mechanism to enhance these responses, thereby decreasing morbidity and mortality in this population.

Human metapneumovirus inhibits the IL-6-induced JAK/STAT3 signalling cascade in airway epithelium.

The host cytokine IL-6 plays an important role in host defence and prevention of lung injury from various pathogens, making IL-6 an important mediator in the host's susceptibility to respiratory infections. The cellular response to IL-6 is mediated through a Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) signal transduction pathway. Human metapneumovirus (hMPV) is an important causative agent of viral respiratory infections known to inhibit the IFN-mediated activation of STAT1. However, little is known about the interactions between this virus and other STAT signalling cascades. Herein, we showed that hMPV can attenuate the IL-6-mediated JAK/STAT3 signalling cascade in lung epithelial cells. HMPV inhibited a key event in this pathway by impeding the phosphorylation and nuclear translocation of STAT3 in A549 cells and in primary normal human bronchial epithelial cells. Further studies established that hMPV interrupted the IL-6-induced JAK/STAT pathway early in the signal transduction pathway by blocking the phosphorylation of JAK2. By antagonizing the IL-6-mediated JAK/STAT3 pathway, hMPV perturbed the expression of IL-6-inducible genes important for apoptosis, cell differentiation and growth. Infection with hMPV also differentially regulated the effects of IL-6 on apoptosis. Thus, hMPV regulation of these genes could usurp the protective roles of IL-6, and these data provide insight into an important element of viral pathogenesis.

Differential salivary gland transcript expression profile in Ixodes scapularis nymphs upon feeding or flavivirus infection.

Ixodid ticks are vectors of human diseases such as Lyme disease, babesiosis, anaplasmosis, and tick-borne encephalitis. These diseases cause significant morbidity and mortality worldwide and are transmitted to humans during tick feeding. The tick-host-pathogen interface is a complex environment where host responses are modulated by the molecules in tick saliva to enable the acquisition of a blood meal. Disruption of host responses at the site of the tick bite may also provide an advantage for pathogens to survive and replicate. Thus, the molecules in tick saliva not only aid the tick in securing a nutrient-rich blood meal, but can also enhance the transmission and acquisition of pathogens. To investigate the effect of feeding and flavivirus infection on the salivary gland transcript expression profile in ticks, a first-generation microarray was developed using ESTs from a cDNA library derived from Ixodes scapularis salivary glands. When the salivary gland transcript profile in ticks feeding over the course of 3 days was compared to that in unfed ticks, a dramatic increase in transcripts related to metabolism was observed. Specifically, 578 transcripts were up-regulated compared to 151 down-regulated transcripts in response to feeding. When specific time points post attachment were analyzed, a temporal pattern of gene expression was observed. When Langat virus-infected ticks were compared to mock-infected ticks, transcript expression changes were observed at all 3 days of feeding. Differentially regulated transcripts include putative secreted proteins, lipocalins, Kunitz domain-containing proteins, anti-microbial peptides, and transcripts of unknown function. These studies identify salivary gland transcripts that are differentially regulated during feeding or in the context of flavivirus infection in Ixodes scapularis nymphs, a medically important disease vector. Further analysis of these transcripts may identify salivary factors that affect the transmission or replication of tick-borne flaviviruses.

Replacement of the 3' untranslated variable region of mosquito-borne dengue virus with that of tick-borne Langat virus does not alter vector specificity.

The four major flavivirus clades are transmitted by mosquitoes, ticks, directly between vertebrates or directly between arthropods, respectively, but the molecular determinants of mode of transmission in flaviviruses are unknown. To assess the role of the UTRs in transmission, we generated chimeric genomes in which the 5' UTR, capsid and/or 3' UTR of mosquito-borne dengue virus serotype 4 (rDENV-4) were replaced, separately or in combination, with those of tick-borne Langat virus (rLGTV). None of the chimeric genomes yielded detectable virus following transfection. Replacement of the variable region (VR) in the rDENV-4 3' UTR with that of rLGTV generated virus rDENV-4-rLGTswapVR, which showed lower replication than its wild-type parents in mammalian but not mosquito cells in culture and was able to infect mosquitoes in vivo. Neither rDENV-4 nor rDENV-4-rLGTswapVR could infect larval Ixodes scapularis ticks immersed in virus, while rLGTV was highly infectious via this route.

Chimeric tick-borne encephalitis/dengue virus is attenuated in Ixodes scapularis ticks and Aedes aegypti mosquitoes.

In an effort to derive an efficacious live attenuated vaccine against tick-borne encephalitis, we generated a chimeric virus bearing the structural protein genes of a Far Eastern subtype of tick-borne encephalitis virus (TBEV) on the genetic background of recombinant dengue 4 (DEN4) virus. Introduction of attenuating mutations into the TBEV envelope protein gene, as well as the DEN4 NS5 protein gene and 3' noncoding region in the chimeric genome, results in decreased neurovirulence and neuroinvasiveness in mice, and restricted replication in mouse brain. Since TBEV and DEN4 viruses are transmitted in nature by ticks and mosquitoes, respectively, it was of interest to investigate the infectivity of the chimeric virus for both arthropod vectors. Therefore, parental and chimeric viruses were tested for growth in mosquito and tick cells and for oral infection in vivo. Although all chimeric viruses demonstrated moderate levels of replication in C6/36 mosquito cells, they were unable to replicate in ISE6 tick cells. Further, the chimeric viruses were unable to infect or replicate in Aedes aegypti mosquitoes and Ixodes scapularis tick larvae. The poor infectivity for both potential vectors reinforces the safety of chimeric virus-based vaccine candidates for the environment and for use in humans.

Tick-borne flaviviruses: dissecting host immune responses and virus countermeasures.

The tick-borne encephalitis (TBE) serocomplex of viruses, genus Flavivirus, includes a number of important human pathogens that cause serious neurological illnesses and hemorrhagic fevers. These viruses pose a significant public health problem due to high rates of morbidity and mortality, their emergence to new geographic areas, and the recent rise in the incidence of human infections. The most notable member of the TBE serocomplex is tick-borne encephalitis virus (TBEV), a neurotropic flavivirus that causes debilitating and sometimes fatal encephalitis. Although effective prophylactic anti-TBEV vaccines have been developed, there is currently no specific treatment for infection. To identify new targets for therapeutical intervention, it is imperative to understand interactions between TBEV and the host immune response to infection. Interferon (IFN) has a critical role in controlling flavivirus replication. Dendritic cells (DCs) represent an early target of TBEV infection and are major producers of IFN. Thus, interactions between DCs, IFN responses, and the virus are likely to substantially influence the outcome of infection. Early IFN and DC responses are modulated not only by the virus, but also by the tick vector and immunomodulatory compounds of tick saliva inoculated with virus into the skin. Our laboratory is examining interactions between the triad of virus, tick vector, and mammalian host that contribute to the pathogenesis of tick-borne flaviviruses. This work will provide a more detailed understanding of early events in virus infection and their impact on flavivirus pathogenesis.

Identification of genetic determinants of a tick-borne flavivirus associated with host-specific adaptation and pathogenicity.

Tick-borne flaviviruses are maintained in nature in an enzootic cycle involving a tick vector and a vertebrate host. Thus, the virus replicates in two disparate hosts, each providing selective pressures that can influence virus replication and pathogenicity. To identify viral determinants associated with replication in the individual hosts, plaque purified Langat virus (TP21pp) was adapted to growth in mouse or tick cell lines to generate two virus variants, MNBp20 and ISEp20, respectively. Virus adaptation to mouse cells resulted in four amino acid changes in MNBp20 relative to TP21pp, occurring in E, NS4A and NS4B. A comparison between TP21pp and ISEp20 revealed three amino acid modifications in M, NS3 and NS4A of ISEp20. ISEp20, but not MNBp20, was attenuated following intraperitoneal inoculation of mice. Following isolation from mice brains, additional mutations reproducibly emerged in E and NS3 of ISEp20 that were possibly compensatory for the initial adaptation to tick cells. Thus, our data implicate a role for E, M, NS3, NS4A and NS4B in host adaptation and pathogenicity of tick-borne flaviviruses.

Tick-borne flavivirus infection in Ixodes scapularis larvae: development of a novel method for synchronous viral infection of ticks.

Following a bite from an infected tick, tick-borne flaviviruses cause encephalitis, meningitis and hemorrhagic fever in humans. Although these viruses spend most of their time in the tick, little is known regarding the virus-vector interactions. We developed a simple method for synchronously infecting Ixodes scapularis larvae with Langat virus (LGTV) by immersion in media containing the virus. This technique resulted in approximately 96% of ticks becoming infected. LGTV infection and replication were demonstrated by both viral antigen expression and the accumulation of viral RNA. Furthermore, ticks transmitted LGTV to 100% of the mice and maintained the virus through molting into the next life stage. This technique circumvents limitations present in the current methods by mimicking the natural route of infection and by using attenuated virus strains to infect ticks, thereby making this technique a powerful tool to study both virus and tick determinants of replication, pathogenesis and transmission.

Inhibition of interferon-stimulated JAK-STAT signaling by a tick-borne flavivirus and identification of NS5 as an interferon antagonist.

The tick-borne encephalitis (TBE) complex of viruses, genus Flavivirus, can cause severe encephalitis, meningitis, and/or hemorrhagic fevers. Effective interferon (IFN) responses are critical to recovery from infection with flaviviruses, and the mosquito-borne flaviviruses can inhibit this response. However, little is known about interactions between IFN signaling and TBE viruses. Langat virus (LGTV), a member of the TBE complex of viruses, was found to be highly sensitive to the antiviral effects of IFN. However, LGTV infection inhibited IFN-induced expression of a reporter gene driven by either IFN-alpha/beta- or IFN-gamma-responsive promoters. This indicated that LGTV can inhibit the IFN-mediated JAK-STAT (Janus kinase-signal transducer and activator of transcription) pathway of signal transduction. The mechanism of inhibition was due to blocks in the phosphorylation of both Janus kinases, Jak1 and Tyk2, during IFN-alpha signaling and at least a failure of Jak1 phosphorylation following IFN-gamma stimulation. To determine the viral protein(s) responsible, we individually expressed all nonstructural (NS) proteins and examined their ability to inhibit signal transduction. Expression of NS5 alone inhibited STAT1 phosphorylation in response to IFN, thus identifying NS5 as a potential IFN antagonist. Examination of interactions between NS5 and cellular proteins revealed that NS5 associated with IFN-alpha/beta and -gamma receptor complexes. Importantly, inhibition of JAK-STAT signaling and NS5-IFN receptor interactions were demonstrated in LGTV-infected human monocyte-derived dendritic cells, important target cells for early virus replication. Because NS5 may interfere with both innate and acquired immune responses to virus infection, this protein may have a significant role in viral pathogenesis.

Translational regulation of rotavirus gene expression.

Rotavirus mRNAs are transcribed from 11 genomic dsRNA segments within a subviral particle. The mRNAs are extruded into the cytoplasm where they serve as mRNA for protein synthesis and as templates for packaging and replication into dsRNA. The molecular steps in the replication pathway that regulate the levels of viral gene expression are not well defined. We have investigated potential mechanisms of regulation of rotavirus gene expression by functional evaluation of two differentially expressed viral mRNAs. NSP1 (gene 5) and VP6 (gene 6) are expressed early in infection, and VP6 is expressed in excess over NSP1. We formulated the hypothesis that the amounts of NSP1 and VP6 were regulated by the translational efficiencies of the respective mRNAs. We measured the levels of gene 5 and gene 6 mRNA and showed that they were not significantly different, and protein analysis indicated no difference in stability of NSP1 compared with VP6. Polyribosome analysis showed that the majority of gene 6 mRNA was present on large polysomes. In contrast, sedimentation of more than half of the gene 5 mRNA was subpolysomal. The change in distribution of gene 5 mRNA in polyribosome gradients in response to treatment with low concentrations of cycloheximide suggested that gene 5 is a poor translation initiation template compared with gene 6 mRNA. These data define a regulatory mechanism for the difference in amounts of VP6 and NSP1 and provide evidence for post-transcriptional control of rotavirus gene expression mediated by the translational efficiency of individual viral mRNAs.

Interferon regulatory factor 3 is a cellular partner of rotavirus NSP1.

The rotavirus nonstructural protein NSP1 is the least conserved protein in the rotavirus genome, and its function in the replication cycle is not known. We employed NSP1 as bait in the yeast two-hybrid interaction trap to identify candidate cellular partners of NSP1 that may provide clues to its function. Interferon regulatory factor 3 (IRF-3) was identified as an NSP1 interactor. NSP1 synthesized in rotavirus-infected cells bound IRF-3 in a glutathione S-transferase pull-down assay, indicating that the interaction was not unique to the two-hybrid system. NSP1 of murine rotavirus strain EW also interacted with IRF-3. NSP1 deletion and point mutants were constructed to map domains important in the interaction between NSP1 and IRF-3. The data suggest that a binding domain resides in the C terminus of NSP1 and that the N-terminal conserved zinc finger is important but not sufficient to mediate binding to IRF-3. We predict that a role for NSP1 in rotavirus-infected cells is to inhibit activation of IRF-3 and diminish the cellular interferon response.