Cytoarchitecture of ex vivo midgut cultures of unfed Ixodes scapularis infected with a tick-borne flavivirus

A bite from an infected tick is the primary means of transmission for tick-borne flaviviruses (TBFV). Ticks ingest the virus while feeding on infected blood. The traditional view is that the virus first replicates in and transits the tick midgut prior to dissemination to other organs, including salivary glands. Thus, understanding TBFV infection in the tick midgut is a key first step in identifying potential countermeasures against infection. Ex vivo midgut cultures prepared from unfed adult female Ixodes scapularis ticks were viable and remained morphologically intact for more than 8 days. The midgut consisted of two clearly defined cell layers separated by a basement membrane: an exterior network of smooth muscle cells and an internal epithelium composed of digestive generative cells. The smooth muscle cells were arranged in a stellate circumferential pattern spaced at regular intervals along the long axis of midgut diverticula. When the cultures were infected with the TBFV Langat virus (LGTV), virus production increased by two logs with a peak at 96 hours post-infection. Infected cells were readily identified by immunofluorescence staining for the viral envelope protein, nonstructural protein 3 (NS3) and dsRNA. Microscopy of the stained cultures suggested that generative cells were the primary target for virus infection in the midgut. Infected cells exhibited an expansion of membranes derived from the endoplasmic reticulum; a finding consistent with TBFV infected cell cultures. Electron microscopy of infected cultures revealed virus particles in the basolateral region between epithelial cells. These results demonstrated LGTV replication in midgut generative cells of artificially infected, ex vivo cultures of unfed adult female I. scapularis ticks.

Of Murines and Humans: Modeling Persistent Powassan Disease in C57BL/6 Mice

Powassan infection is caused by two closely related, tick-transmitted viruses of the genus Flavivirus (family Flaviviridae): Powassan virus lineage I (POWV) and lineage II (known as deer tick virus [DTV]). Infection is typically asymptomatic or mild but can progress to neuroinvasive disease. Approximately 10% of neuroinvasive cases are fatal, and half of the survivors experience long-term neurological sequelae. Understanding how these viruses cause long-term symptoms as well as the possible role of viral persistence is important for developing therapies. We intraperitoneally inoculated 6-week-old C57BL/6 mice (50% female) with 103 focus-forming units (FFU) DTV and assayed for infectious virus, viral RNA, and inflammation during acute infection and 21, 56, and 84 days postinfection (dpi). Although most mice (86%) were viremic 3 dpi, only 21% of the mice were symptomatic and 83% recovered. Infectious virus was detected only in the brains of mice sampled during the acute infection. Viral RNA was detected in the brain until 84 dpi, but the magnitude decreased over time. Meningitis and encephalitis were visible in acute mice and from mice sampled at 21 dpi. Inflammation was observed until 56 dpi in the brain and 84 dpi in the spinal cord, albeit at low levels. These results suggest that the long-term neurological symptoms associated with Powassan disease are likely caused by lingering viral RNA and chronic inflammation in the central nervous system rather than by a persistent, active viral infection. The C57BL/6 model of persistent Powassan mimics illness in humans and can be used to study the mechanisms of chronic disease. IMPORTANCE Half of Powassan infection survivors experience long-term, mild to severe neurological symptoms. The progression from acute to chronic Powassan disease is not well understood, severely limiting treatment and prevention options. Infection of C57BL/6 mice with DTV mimics clinical disease in humans, and the mice exhibit CNS inflammation and viral RNA persistence until at least 86 dpi, while infectious virus is undetectable after 12 dpi. These findings suggest that the long-term neurological symptoms of chronic Powassan disease are in part due the persistence of viral RNA and the corresponding long-term inflammation of the brain and spinal cord. Our work demonstrates that C57BL/6 mice can be used to study the pathogenesis of chronic Powassan disease.

Genetic sequencing of a 1944 Rocky Mountain spotted fever vaccine

Rocky Mountain spotted fever (RMSF) is a rapidly progressive and often fatal tick-borne disease caused by Rickettsia rickettsii. Its discovery and characterization by Howard Ricketts has been hailed as a remarkable historical example of detection and control of an emerging infectious disease, and subsequently led to the establishment of the Rocky Mountain Laboratories (RML). Here, we examined an unopened bottle of a vaccine, labeled as containing RMSF inactivated by phenol-formalin of infected ticks, developed prior to 1944 at RML by DNA analysis using Illumina high throughput sequencing technology. We found that it contains DNA from the Rocky Mountain wood tick (Dermacentor andersoni), the vector of RMSF, the complete genome of Rickettsia rickettsii, the pathogen of RMSF, as well as the complete genome of Coxiella burnetii, the pathogen of Q-fever. In addition to genomic reads of Rickettsia rickettsii and Coxiella burnetii, smaller percentages of the reads are from Rickettsia rhipicephali and Arsenophonus nasoniae, suggesting that the infected ticks used to prepare the vaccine carried more than one pathogen. Together, these findings suggest that this early vaccine was likely a bivalent vaccine for RMSF and Q-fever. This study is the among the first molecular level examinations of an historically important vaccine.

Tick-borne encephalitis virus capsid protein induces translational shut-off as revealed by its structural-biological analysis

Tick-borne encephalitis virus (TBEV) is the most medically relevant tick-transmitted Flavivirus in Eurasia, targeting the host central nervous system and frequently causing severe encephalitis. The primary function of its capsid protein (TBEVC) is to recruit the viral RNA and form a nucleocapsid. Additional functionality of Flavivirus capsid proteins has been documented, but further investigation is needed for TBEVC. Here, we show the first capsid protein 3D structure of a member of the tick-borne flaviviruses group. The structure of monomeric Δ16-TBEVC was determined using high-resolution multidimensional NMR spectroscopy. Based on natural in vitro TBEVC homodimerization, the dimeric interfaces were identified by hydrogen deuterium exchange mass spectrometry (MS). Although the assembly of flaviviruses occurs in endoplasmic reticulum-derived vesicles, we observed that TBEVC protein also accumulated in the nuclei and nucleoli of infected cells. In addition, the predicted bipartite nuclear localization sequence in the TBEVC C-terminal part was confirmed experimentally, and we described the interface between TBEVC bipartite nuclear localization sequence and import adapter protein importin-alpha using X-ray crystallography. Furthermore, our coimmunoprecipitation coupled with MS identification revealed 214 interaction partners of TBEVC, including viral envelope and nonstructural NS5 proteins and a wide variety of host proteins involved mainly in rRNA processing and translation initiation. Metabolic labeling experiments further confirmed that TBEVC and other flaviviral capsid proteins are able to induce translational shutoff and decrease of 18S rRNA. These findings may substantially help to design a targeted therapy against TBEV.

Kinetics of tick infection by the relapsing fever spirochete Borrelia hermsii acquired through artificial membrane feeding chambers

The relapsing fever agent Borrelia hermsii is transmitted by the tick Ornithodoros hermsi. To study the B. hermsii-tick interactions required for pathogen acquisition and transmission we developed an artificial membrane feeding system for O. hermsi nymphs and adults that results in a high percentage of engorgement. This system provides the nutritional requirements necessary for the tick to develop, mate, and produce viable eggs. By inoculating the blood with B. hermsii, we were able to obtain infected ticks for quantitative studies on pathogen acquisition and persistence. These ticks subsequently transmitted the spirochetes to mice, validating this system for both acquisition and transmission studies. Using this feeding method, a mutant of the antigenic variation locus of B. hermsii (Vmp-) that is incapable of persisting in mice was acquired by ticks at equivalent densities as the wild-type. Furthermore, Vmp is not required for persistence in the tick, as the mutant and wild-type strains are maintained at similar numbers after ecdysis and subsequent feeding. These results support the theory that Vmp is an adaptation for mammalian infection but unnecessary for survival within the tick. Interestingly, B. hermsii numbers severely declined after acquisition, though these ticks still transmitted the infection to mice. This procedure reduces animal use and provides a safe, highly controlled and well-contained alternative method for feeding and maintaining O. hermsi colonies. Importantly, this system permits quantitative studies with B. hermsii strains through ingestion during the blood meal, and thus more closely recapitulates pathogen acquisition in nature than other artificial systems.

The liver X receptor agonist LXR 623 restricts flavivirus replication

The vector-borne flaviviruses (VBFVs) are well known for causing great misery and death in humans worldwide. The VBFVs include those transmitted by mosquitos, such as Zika virus (ZIKV), dengue virus; and those transmitted by ticks including the tick-borne flavivirus serocomplex and Powassan virus (POWV). Two of our recent reports showed that intracranial POWV infection in the reservoir host, Peromyscus leucopus, was restricted and caused no overt clinical disease. Several modes of analyses suggested activation of the LXR pathway. Activation of the LXR pathway leads to increased efflux of cholesterol from cells and consequent disturbances in membrane biogenesis. Because VBFV replication is dependent on membrane biogenesis, we evaluated the effect of an LXR agonist (LXR623) on POWV and ZIKV infection and observed that the compound impaired permissive replication of both viruses in a human neuroblastoma SK-N-SH cell line. The LXR agonist resulted in failure of the viruses to induce ER expansion and elaborate vesicle formation, suggesting that the efflux of cholesterol was part of the antiviral mechanism. We also observed that the LXR agonist contributed to the mechanism of virus suppression by increased expression of mRNAs encoding for the antiviral cytokines CXCL10, RANTES and IFN1β. In sharp contrast, a LXR antagonist (GSK2033) had no significant effect on VBFV replication. We conclude that LXR623 impairs flavivirus replication by stimulating cellular antiviral factors.

Characterization of flavivirus infection in salivary gland cultures from male Ixodes scapularis ticks

Infected Ixodes scapularis (black-legged tick) transmit a host of serious pathogens via their bites, including Borrelia burgdorferi, Babesia microti, and tick-borne flaviviruses (TBFVs), such as Powassan virus (POWV). Although the role of female I. scapularis ticks in disease transmission is well characterized, the role of male ticks is poorly understood. Because the pathogens are delivered in tick saliva, we studied the capacity of male salivary glands (SGs) to support virus replication. Ex vivo cultures of SGs from unfed male I. scapularis were viable for more than a week and maintained the characteristic tissue architecture of lobular ducts and acini. When SG cultures were infected with the TBFVs Langat virus (LGTV) or POWV lineage II (deer tick virus), the production of infectious virus was demonstrated. Using a green fluorescent protein-tagged LGTV and confocal microscopy, we demonstrated LGTV infection within SG acinus types II and III. The presence of LGTV in the acini and lobular ducts of the cultures was also shown via immunohistochemistry. Furthermore, the identification by in situ hybridization of both positive and negative strand LGTV RNA confirmed that the virus was indeed replicating. Finally, transmission electron microscopy of infected SGs revealed virus particles packaged in vesicles or vacuoles adjacent to acinar lumina. These studies support the concept that SGs of male I. scapularis ticks support replication of TBFVs and may play a role in virus transmission, and further refine a useful model system for developing countermeasures against this important group of pathogens.

Powassan disease has greatly increased in frequency since its discovery in Powassan, Ontario in 1958. Powassan virus (lineage I; POWV) and Powassan virus lineage II (deer tick virus; DTV) are endemic to North America and there were 133 reported cases between 2009 and 2018, the majority since 2016. Nymphal and adult Ixodes scapularis ticks are thought to be the primary vectors of POWV/DTV to humans. However, little is known regarding DTV infection of male Ixodes ticks or their potential as vectors. In this study we characterized LGTV, a model tick-borne flavivirus, and DTV infection and propagation in male I. scapularis salivary gland cultures using an ex vivo organ culture system. This work provides insight into potential flavivirus transmission by the male I. scapularis tick.

Tick-Borne Flaviviruses Depress AKT Activity during Acute Infection by Modulating AKT1/2

Tick-borne flaviviruses (TBFVs) are reemerging public health threats. To develop therapeutics against these pathogens, increased understanding of their interactions with the mammalian host is required. The PI3K-AKT pathway has been implicated in TBFV persistence, but its role during acute virus infection remains poorly understood. Previously, we showed that Langat virus (LGTV)-infected HEK 293T cells undergo a lytic crisis with a few surviving cells that become persistently infected. We also observed that AKT2 mRNA is upregulated in cells persistently infected with TBFV. Here, we investigated the virus-induced effects on AKT expression over the course of acute LGTV infection and found that total phosphorylated AKT (pAKT), AKT1, and AKT2 decrease over time, but AKT3 increases dramatically. Furthermore, cells lacking AKT1 or AKT2 were more resistant to LGTV-induced cell death than wild-type cells because they expressed higher levels of pAKT and antiapoptotic proteins, such as XIAP and survivin. The differential modulation of AKT by LGTV may be a mechanism by which viral persistence is initiated, and our results demonstrate a complicated manipulation of host pathways by TBFVs.

Sharing the Ride: Ixodes scapularis Symbionts and Their Interactions

The deer tick Ixodes scapularis transmits a variety of disease agents in the United States, spreading the bacteria that causes Lyme borreliosis, the protozoan agent of babesiosis, and viruses such as Powassan. However, a variety of other organisms have also evolved symbiotic relationships with this tick species, and it seems likely that some of these microbes have simultaneously coevolved mechanisms to impact each other and their tick host. The number of organisms identified as I. scapularis symbionts has increased seemingly exponentially with the advent of PCR and next generation sequencing technologies, but convincing arguments have proposed that some of these are of environmental origin, unadapted to surviving the physiological conditions of the tick or that they are artifacts of ultrasensitive detection methods. In this review, we examine the diversity of the known microbes occurring within the I. scapularis microbiome, the evidence for interactions between microbes, and discuss whether some organisms reported to be symbionts of I. scapularis are experimental artifacts.

PERK-Mediated Unfolded Protein Response Signaling Restricts Replication of the Tick-Borne Flavivirus Langat Virus

The unfolded protein response (UPR) maintains protein-folding homeostasis in the endoplasmic reticulum (ER) and has been implicated as both beneficial and detrimental to flavivirus infection. Protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK), a sensor of the UPR, is commonly associated with antiviral effects during mosquito-borne flavivirus (MBFV) infection, but its relation to tick-borne flavivirus (TBFV) infection remains largely unexplored. In this study, we identified changes in UPR and autophagic activity during Langat virus (LGTV) infection. LGTV robustly activated UPR and altered autophagic flux. Knockdown of endogenous PERK in human cells resulted in increased LGTV replication, but not that of closely related Powassan virus (POWV). Finally, on examining changes in protein levels of components associated with UPR and autophagy in the absence of PERK, we could show that LGTV-infected cells induced UPR but did not lead to expression of C/EBP homologous protein (CHOP), an important downstream transcription factor of multiple stress pathways. From these data, we hypothesize that LGTV can antagonize other kinases that target eukaryotic initiation factor 2α (eIF2α), but not PERK, implicating PERK as a potential mediator of intrinsic immunity. This effect was not apparent for POWV, a more pathogenic TBFV, suggesting it may be better equipped to mitigate the antiviral effects of PERK.

Routes of Zika virus dissemination in the testis and epididymis of immunodeficient mice

Sexual transmission and persistence of Zika virus (ZIKV) in the male reproductive tract (MRT) poses new challenges for controlling virus outbreaks and developing live-attenuated vaccines. To elucidate routes of ZIKV dissemination in the MRT, we here generate microRNA-targeted ZIKV clones that lose the infectivity for (1) the cells inside seminiferous tubules of the testis, or (2) epithelial cells of the epididymis. We trace ZIKV dissemination in the MRT using an established mouse model of ZIKV pathogenesis. Our results support a model in which ZIKV infects the testis via a hematogenous route, while infection of the epididymis can occur via two routes: (1) hematogenous/lymphogenous and (2) excurrent testicular. Co-targeting of the ZIKV genome with brain-, testis-, and epididymis-specific microRNAs restricts virus infection of these organs, but does not affect virus-induced protective immunity in mice and monkeys. These defined alterations of ZIKV tropism represent a rational design of a safe live-attenuated ZIKV vaccine.

The Role of Mammalian Reservoir Hosts in Tick-Borne Flavivirus Biology

Small-to-medium sized mammals and large animals are lucrative sources of blood meals for ixodid ticks that transmit life-threatening tick-borne flaviviruses (TBFVs). TBFVs have been isolated from various organs obtained from wild-caught Myodes and Apodemus species in Europe and Asia. Thus, these rodents are well-established reservoirs of TBFVs. Wild-caught Peromyscus species have demonstrated seropositivity against Powassan virus, the only TBFV known to circulate in North America, suggesting that they may play an important role in the biology of the virus in this geographic region. However, virus isolation from Peromyscus species is yet to be demonstrated. Wild-caught medium-sized mammals, such as woodchucks (Marmota monax) and skunks (Mephitis mephitis) have also demonstrated seropositivity against POWV, and virus was isolated from apparently healthy animals. Despite the well-established knowledge that small-to-medium sized animals are TBFV reservoirs, specific molecular biology addressing host-pathogen interactions remains poorly understood. Elucidating these interactions will be critical for gaining insight into the mechanism(s) of viral pathogenesis and/or resistance.

Viral Determinants of Virulence in Tick-Borne Flaviviruses

Tick-borne flaviviruses have a global distribution and cause significant human disease, including encephalitis and hemorrhagic fever, and often result in neurologic sequelae. There are two distinct properties that determine the neuropathogenesis of a virus. The ability to invade the central nervous system (CNS) is referred to as the neuroinvasiveness of the agent, while the ability to infect and damage cells within the CNS is referred to as its neurovirulence. Examination of laboratory variants, cDNA clones, natural isolates with varying pathogenicity, and virally encoded immune evasion strategies have contributed extensively to our understanding of these properties. Here we will review the major viral determinants of virulence that contribute to pathogenesis and influence both neuroinvasiveness and neurovirulence properties of tick-borne flaviviruses, focusing particularly on the envelope protein (E), nonstructural protein 5 (NS5), and the 3′ untranslated region (UTR).

The Use of Ex Vivo Organ Cultures in Tick-Borne Virus Research

Each year there are more than 15 000 cases of human disease caused by infections with tick-borne viruses (TBVs). These illnesses occur worldwide and can range from very mild illness to severe encephalitis and hemorrhagic fever. Although TBVs are currently identified as neglected vector-borne pathogens and receive less attention than mosquito-borne viruses, TBVs are expanding into new regions, and infection rates are increasing. Furthermore, effective vaccines, diagnostic tools, and other countermeasures are limited. The application of contemporary technologies to TBV infections presents an excellent opportunity to develop improved, effective countermeasures. Experimental tick and mammal models of infection can be used to characterize determinants of infection, transmission, and virulence and to test candidate countermeasures. The use of ex vivo tick cultures in TBV research provides a unique way to look at infection in specific tick organs. Mammal ex vivo organ slice and, more recently, organoid cultures are additional models that can be used to elucidate direct tissue-specific responses to infection. These ex vivo model systems are convenient for testing methods involving transcript knockdown and small molecules under tightly controlled conditions. They can also be combined with in vitro and in vivo studies to tease out possible host factors and potential vaccine or therapeutic candidates. In this brief perspective, we describe how ex vivo cultures can be combined with modern technologies to advance research on TBV infections.

Flavivirus Infection of Ixodes scapularis (Black-Legged Tick) Ex Vivo Organotypic Cultures and Applications for Disease Control

Ixodes scapularis ticks transmit many infectious agents that cause disease, including tick-borne flaviviruses (TBFVs). TBFV infections cause thousands of human encephalitis cases worldwide annually. In the United States, human TBFV infections with Powassan virus (POWV) are increasing and have a fatality rate of 10 to 30%. Additionally, Langat virus (LGTV) is a TBFV of low neurovirulence and is used as a model TBFV. TBFV replication and dissemination within I. scapularis organs are poorly characterized, and a deeper understanding of virus biology in this vector may inform effective countermeasures to reduce TBFV transmission. Here, we describe short-term, I. scapularis organ culture models of TBFV infection. Ex vivo organs were metabolically active for 9 to 10 days and were permissive to LGTV and POWV replication. Imaging and videography demonstrated replication and spread of green fluorescent protein-expressing LGTV in the organs. Immunohistochemical staining confirmed LGTV envelope and POWV protein synthesis within the infected organs. LGTV- and POWV-infected organs produced infectious LGTV and POWV; thus, the ex vivo cultures were suitable for study of virus replication in individual organs. LGTV- and POWV-infected midgut and salivary glands were subjected to double-stranded RNA (dsRNA) transfection with dsRNA to the LGTV 3' untranslated region (UTR), which reduced infectious LGTV and POWV replication, providing a proof-of-concept use of RNA interference in I. scapularis organ cultures to study the effects on TBFV replication. The results contribute important information on TBFV localization within ex vivo I. scapularis organs and provide a significant translational tool for evaluating recombinant, live vaccine candidates and potential tick transcripts and proteins for possible therapeutic use and vaccine development to reduce TBFV transmission.IMPORTANCE Tick-borne flavivirus (TBFV) infections cause neurological and/or hemorrhagic disease in humans worldwide. There are currently no licensed therapeutics or vaccines against Powassan virus (POWV), the only TBFV known to circulate in North America. Evaluating tick vector targets for antitick vaccines directed at reducing TBFV infection within the arthropod vector is a critical step in identifying efficient approaches to controlling TBFV transmission. This study characterized infection of female Ixodes scapularis tick organ cultures of midgut, salivary glands, and synganglion with the low-neurovirulence Langat virus (LGTV) and the more pathogenic POWV. Cell types of specific organs were susceptible to TBFV infection, and a difference in LGTV and POWV replication was noted in TBFV-infected organs. This tick organ culture model of TBFV infection will be useful for various applications, such as screening of tick endogenous dsRNA corresponding to potential control targets within midgut and salivary glands to confirm restriction of TBFV infection.

BiP: Master Regulator of the Unfolded Protein Response and Crucial Factor in Flavivirus Biology

Flaviviruses have an intimate relationship with their host cells, utilizing host proteins during replication. Much of viral genome replication and virion assembly occurs on and within the endoplasmic reticulum (ER). As a cellular protein folding hub, the ER provides an ideal environment for flaviviruses to replicate. Flaviviruses can interact with several ER processes, including the unfolded protein response (UPR), a cellular stress mechanism responsible for managing unfolded protein accumulation and ER stress. The UPR can alter the ER environment in several ways, including increasing ER volume and quantity of available chaperones, both of which can favor viral replication. BiP, a chaperone and master regulator of the UPR, has been demonstrated to play a key role in several flavivirus infections. Here we describe what is known in regard to BiP, its implicated role with flavivirus infection, and what remains to be discovered.

Cytoarchitecture of Zika virus infection in human neuroblastoma and Aedes albopictus cell lines

The Zika virus (ZIKV) pandemic is a global concern due to its role in the development of congenital anomalies of the central nervous system. This mosquito-borne flavivirus alternates between mammalian and mosquito hosts, but information about the biogenesis of ZIKV is limited. Using a human neuroblastoma cell line (SK-N-SH) and an Aedes albopictus mosquito cell line (C6/36), we characterized ZIKV infection by immunofluorescence, transmission electron microscopy (TEM), and electron tomography (ET) to better understand infection in these disparate host cells. ZIKV replicated well in both cell lines, but infected SK-N-SH cells suffered a lytic crisis. Flaviviruses scavenge host cell membranes to serve as replication platforms and ZIKV showed the hallmarks of this process. Via TEM, we identified virus particles and 60-100nm spherular vesicles. ET revealed these vesicular replication compartments contain smaller 20-30nm spherular structures. Our studies indicate that SK-N-SH and C6/36 cells are relevant models for viral cytoarchitecture study.

Modeling Powassan virus infection in Peromyscus leucopus, a natural host

The tick-borne flavivirus, Powassan virus (POWV) causes life-threatening encephalitis in humans in North America and Europe. POWV is transmitted by ixodid tick vectors that feed on small to medium-sized mammals, such as Peromyscus leucopus mice, which may serve as either reservoir, bridge or amplification hosts. Intraperitoneal and intracranial inoculation of 4-week old Peromyscus leucopus mice with 103 PFU of POWV did not result in overt clinical signs of disease. However, following intracranial inoculation, infected mice seroconverted to POWV and histopathological examinations revealed that the mice uniformly developed mild lymphocytic perivascular cuffing and microgliosis in the brain and spinal cord from 5 to 15 days post infection (dpi), suggesting an early inflammatory response. In contrast, intracranial inoculation of 4-week old C57BL/6 and BALB/c mice was lethal by 5 dpi. Intraperitoneal inoculation was lethal in BALB/c mice, but 40% (2/5) of C57BL/6 mice survived. We concluded that Peromyscus leucopus mice infected i.c. with a lethal dose of POWV support a limited infection, restricted to the central nervous system and mount an antibody response to the virus. However, they fail to develop clinical signs of disease and are able to control the infection. These results suggest the involvement of restriction factors, and the mechanism by which Peromyscus leucopus mice restrict POWV infection remains under study.

Analysis of the Langat Virus Genome in Persistent Infection of an Ixodes scapularis Cell Line

Tick-borne flaviviruses (TBFVs) cause a broad spectrum of disease manifestations ranging from asymptomatic to mild febrile illness and life threatening encephalitis. These single-stranded positive-sense (ss(+)) RNA viruses are naturally maintained in a persistent infection of ixodid ticks and small-medium sized mammals. The development of cell lines from the ixodid ticks has provided a valuable surrogate system for studying the biology of TBFVs in vitro. When we infected ISE6 cells, an Ixodes scapularis embryonic cell line, with Langat virus (LGTV) we observed that the infection proceeded directly into persistence without any cytopathic effect. Analysis of the viral genome at selected time points showed that no defective genomes were generated during LGTV persistence by 10 weeks of cell passage. This was in contrast to LGTV persistence in 293T cells in which defective viral genomes are detectable by five weeks of serial cell passage. We identified two synonymous nucleotide changes i.e., 1893A→C (29% of 5978 reads at 12 h post infection (hpi)) and 2284T→A (34% of 4191 reads at 12 hpi) in the region encoding for the viral protein E. These results suggested that the mechanisms supporting LGTV persistence are different between tick and mammalian cells.

Stability of a Tick-Borne Flavivirus in Milk

The tick-borne flaviviruses (TBFV) occur worldwide and the tick-borne encephalitis virus (TBEV) members of the group often cause severe, debilitating neurological disease in humans. Although the primary route of infection is through the bite of an infected tick, alimentary infection through the consumption of TBEV-contaminated dairy products is also well-documented and is responsible for some disease in endemic areas. Experimental infection of goats, cattle, and sheep with TBEV shows that the virus can be excreted in the milk of infected animals. Additionally, the virus remains infectious after exposure to low pH levels, similar to those found in the stomach. To evaluate the survival of virus in milk, we studied the stability of the BSL-2 TBFV, Langat virus, in unpasteurized goat milk over time and after different thermal treatments. Virus was stable in milk maintained under refrigeration conditions; however, there was a marked reduction in virus titer after incubation at room temperature. High temperature, short time pasteurization protocols completely inactivated the virus. Interestingly, simulation of a typical thermal regime utilized for cheese did not completely inactivate the virus in milk. These findings stress the importance of proper milk handling and pasteurization processes in areas endemic for TBEV.

The role of viral persistence in flavivirus biology

In nature, vector borne flaviviruses are persistently cycled between either the tick or mosquito vector and small mammals such as rodents, skunks, and swine. These viruses account for considerable human morbidity and mortality worldwide. Increasing and substantial evidence of viral persistence in humans, which includes the isolation of RNA by RT PCR and infectious virus by culture, continues to be reported. Viral persistence can also be established in vitro in various human, animal, arachnid, and insect cell lines in culture. Although some research has focused on the potential roles of defective virus particles, evasion of the immune response through the manipulation of autophagy and/or apoptosis, the precise mechanism of flavivirus persistence is still not well understood. We propose additional research for further understanding of how viral persistence is established in different systems. Avenues for additional studies include determining whether the multifunctional flavivirus protein NS5 has a role in viral persistence, the development of relevant animal models of viral persistence, and investigating the host responses that allow vector borne flavivirus replication without detrimental effects on infected cells. Such studies might shed more light on the viral–host relationships and could be used to unravel the mechanisms for establishment of persistence.

Persistent infections by vector borne flaviviruses are an important, but inadequately studied topic.

A three-dimensional comparison of tick-borne flavivirus infection in mammalian and tick cell lines

Tick-borne flaviviruses (TBFV) are sustained in nature through cycling between mammalian and tick hosts. In this study, we used African green monkey kidney cells (Vero) and Ixodes scapularis tick cells (ISE6) to compare virus-induced changes in mammalian and arthropod cells. Using confocal microscopy, transmission electron microscopy (TEM), and electron tomography (ET), we examined viral protein distribution and the ultrastructural changes that occur during TBFV infection. Within host cells, flaviviruses cause complex rearrangement of cellular membranes for the purpose of virus replication. Virus infection was accompanied by a marked expansion in endoplasmic reticulum (ER) staining and markers for TBFV replication were localized mainly to the ER in both cell lines. TEM of Vero cells showed membrane-bound vesicles enclosed in a network of dilated, anastomosing ER cisternae. Virions were seen within the ER and were sometimes in paracrystalline arrays. Tubular structures or elongated vesicles were occasionally noted. In acutely and persistently infected ISE6 cells, membrane proliferation and vesicles were also noted; however, the extent of membrane expansion and the abundance of vesicles were lower and no viral particles were observed. Tubular profiles were far more prevalent in persistently infected ISE6 cells than in acutely infected cells. By ET, tubular profiles, in persistently infected tick cells, had a cross-sectional diameter of 60–100 nm, reached up to 800 nm in length, were closed at the ends, and were often arranged in fascicle-like bundles, shrouded with ER membrane. Our experiments provide analysis of viral protein localization within the context of both mammalian and arthropod cell lines as well as both acute and persistent arthropod cell infection. Additionally, we show for the first time 3D flavivirus infection in a vector cell line and the first ET of persistent flavivirus infection.

Internal polyadenylation of parvoviral precursor mRNA limits progeny virus production

Aleutian Mink Disease Virus (AMDV) is the only virus in the genus Amdovirus of family Parvoviridae. In adult mink, AMDV causes a persistent infection associated with severe dysfunction of the immune system. Cleavage of AMDV capsid proteins has been previously shown to play a role in regulating progeny virus production (Fang Cheng et al., J. Virol. 84:2687-2696, 2010). The present study shows that AMDV has evolved a second strategy to limit expression of capsid proteins by preventing processing of the full-length capsid protein-encoding mRNA transcripts. Characterization of the cis-elements of the proximal polyadenylation site [(pA)p] in the infectious clone of AMDV revealed that polyadenylation at the (pA)p site is controlled by an upstream element (USE) of 200 nts in length, the AAUAAA signal, and a downstream element (DSE) of 40 nts. A decrease in polyadenylation at the (pA)p site, either by mutating the AAUAAA signal or the DSE, which does not affect the encoding of amino acids in the infectious clone, increased the expression of capsid protein VP1/VP2 and thereby increased progeny virus production approximately 2-3-fold. This increase was accompanied by enhanced replication of the AMDV genome. Thus, this study reveals correlations among internal polyadenylation, capsid production, viral DNA replication and progeny virus production of AMDV, indicating that internal polyadenylation is a limiting step for parvovirus replication and progeny virus production.

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.

Modern dosimetric tools for (60)Co irradiation at high containment laboratories

PURPOSE

To evaluate an innovative photo-fluorescent film as a routine dosimetric tool during (60)Co irradiations at a high containment biological research laboratory, and to investigate whether manufacturer-provided chamber exposure rates can be used to accurately administer a prescribed dose to biological specimens.

MATERIALS AND METHODS

Photo-fluorescent, lithium fluoride film dosimeters and National Institutes of Standards and Technology (NIST) transfer dosimeters were co-located in a self-shielded (60)Co irradiator and exposed to γ-radiation with doses ranging from 5-85 kGy. Film dose-response relationships were developed for varying temperatures simulating conditions present when irradiating infectious biological specimens. Dose measurement results from NIST transfer dosimeters were compared to doses predicted using manufacturer-provided irradiator chamber exposure rates.

RESULTS

The film dosimeter exhibited a photo-fluorescent response signal that was consistent and nearly linear in relationship to γ-radiation exposure over a wide dose range. The dosimeter response also showed negligible effects from dose fractionization and humidity. Significant disparities existed between manufacturer-provided chamber exposure rates and actual doses administered.

CONCLUSION

This study demonstrates the merit of utilizing dosimetric tools to validate the process of exposing dangerous and exotic biological agents to γ-radiation at high containment laboratories. The film dosimeter used in this study can be utilized to eliminate potential for improperly administering γ-radiation doses.

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.

Preparing a community hospital to manage work-related exposures to infectious agents in BioSafety level 3 and 4 laboratories

Training increased willingness of healthcare workers to care for patients with all types of communicable diseases.

Construction of new BioSafety Level (BSL) 3 and 4 laboratories has raised concerns regarding provision of care to exposed workers because of healthcare worker (HCW) unfamiliarity with precautions required. When the National Institutes of Health began construction of a new BSL-4 laboratory in Hamilton, Montana, USA, in 2005, they contracted with St. Patrick Hospital in Missoula, Montana, for care of those exposed. A care and isolation unit is described. We developed a training program for HCWs that emphasized the optimal use of barrier precautions and used pathogen-specific modules and simulations with mannequins and fluorescent liquids that represented infectious body fluids. The facility and training led to increased willingness among HCWs to care for patients with all types of communicable diseases. This model may be useful for other hospitals, whether they support a BSL-4 facility, are in the proximity of a BSL-3 facility, or are interested in upgrading their facilities to prepare for exotic and novel infectious diseases.

Potential impact of a 2-person security rule on BioSafety Level 4 laboratory workers

DOI: 10.3201/eid1507.081523

Directors of all major BioSafety Level 4 (BSL-4) laboratories in the United States met in 2008 to review the current status of biocontainment laboratory operations and to discuss the potential impact of a proposed 2-person security rule on maximum-containment laboratory operations. Special attention was paid to the value and risks that would result from a requirement that 2 persons be physically present in the laboratory at all times. A consensus emerged indicating that a video monitoring system represents a more efficient, economical standard; provides greater assurance that pathogens are properly manipulated; and offers an increased margin of employee safety and institutional security. The 2-person security rule (1 to work and 1 to observe) may decrease compliance with dual responsibilities of safety and security by placing undue pressure on the person being observed to quickly finish the work, and by placing the observer in the containment environment unnecessarily.

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.

Framework for leadership and training of Biosafety Level 4 laboratory workers

One-sentence summary for table of contents: Training should include theoretical consideration of biocontainment principles, practical hands-on training, and mentored on-the-job experience.

Construction of several new Biosafety Level 4 (BSL-4) laboratories and expansion of existing operations have created an increased international demand for well-trained staff and facility leaders. Directors of most North American BSL-4 laboratories met and agreed upon a framework for leadership and training of biocontainment research and operations staff. They agreed on essential preparation and training that includes theoretical consideration of biocontainment principles, practical hands-on training, and mentored on-the-job experiences relevant to positional responsibilities as essential preparation before a person’s independent access to a BSL-4 facility. They also agreed that the BSL-4 laboratory director is the key person most responsible for ensuring that staff members are appropriately prepared for BSL-4 operations. Although standardized certification of training does not formally exist, the directors agreed that facility-specific, time-limited documentation to recognize specific skills and experiences of trained persons is needed.

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.

Identification of residues critical for the interferon antagonist function of Langat virus NS5 reveals a role for the RNA-dependent RNA polymerase domain

All pathogenic flaviviruses examined thus far inhibit host interferon (IFN) responses by suppressing the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway. Both Langat virus (LGTV; a member of the tick-borne encephalitis virus serogroup) and Japanese encephalitis virus use the nonstructural protein NS5 to suppress JAK-STAT signaling. However, NS5 is also critical to virus replication, contributing methyltransferase and RNA-dependent RNA polymerase (RdRP) activities. The specific amino acid residues of NS5 involved in IFN antagonism are not known. Here, we demonstrate that the LGTV NS5 JAK-STAT inhibitory domain is contained between amino acids 355 and 735 (of 903), a range which lies within the RdRP domain. Furthermore, we identified two noncontiguous stretches of specific amino acids within the RdRP, 374 to 380 and 624 to 647, as critical for inhibition of JAK-STAT signaling. Despite considerable separation on the linear NS5 sequence, these residues localized adjacent to each other when modeled on the West Nile virus RdRP crystal structure. Due to the general conservation of RdRP structures, these results suggest that the specific residues identified act cooperatively to form a unique functional site on the RdRP responsible for JAK-STAT inhibition. This insight into the mechanism underlying flavivirus IFN evasion strategies will facilitate the design of antiviral therapeutics that potentiate the action of IFN during infection.

Action and reaction: the arthropod-borne flaviviruses and host interferon responses

The arthropod-borne flaviviruses include tick- and mosquito-borne viruses that are causes of globally significant emerging diseases. These single-stranded RNA viruses are exquisitely sensitive to the antiviral effects of host interferons. However, both the tick- and mosquito-borne flaviviruses are capable of modulating the interferon response. Despite the high degree of similarity among members of the flavivirus genus, the mechanisms employed by individual viruses to modulate interferon responses differ. This review considers the arthropod-borne flaviviruses and the host interferon response as a pair of forces, the action and the reaction. The interaction of these two forces has led to a complex relationship between virus and host. An increased understanding of these interactions will likely facilitate the rational design of novel vaccines and therapeutics.

Pathogenesis of aleutian mink disease parvovirus and similarities to b19 infection

Aleutian mink disease parvovirus (ADV) is an unusual member of the autonomous parvoviruses in both its replication and pathogenesis. Infection of newborn mink kits results in an acute disease typified by virus replication in type II pneumocytes in the lung. This replication is permissive and cytopathic, characterized by the production of high levels of viral replicative intermediates and infectious progeny. However, infection of adult Aleutian mink leads to a chronic form of the disease termed Aleutian disease (AD). In this case, virus replication occurs predominantly in lymph node macrophages and is restricted, with viral DNA replication, RNA transcription, protein expression and production of infectious progeny occurring at low levels. B19 is the only autonomous parvovirus known to infect humans. The primary site of virus replication in both children and adults is in erythrocyte precursors in the blood and bone marrow, although viral genomes have been detected in various other tissues. B19 infection often causes a self-limiting disease although persistent infection of B19 can occur in both immuno-compromised and -competent people. Perhaps the most striking similarity between infection with ADV or with B19 is the important role the humoral immune response to infection has in pathogenesis. It can be both protective and pathogenic. Due to of the central role of antibody in the disease caused by either virus, understanding the specific roles of antibody production in protection, antibody-mediated enhancement of infection, the establishment of persistent infection and immune-mediated pathology will provide insight into the pathogenesis of these infections. A second similarity between the two viruses is the ability to establish persistent infection. Persistence of ADV is associated with restricted replication. Although many cellular factors may contribute to restricted virus replication, the interactions between the major non-structural protein, NS1, and the cells are likely to be critical. Parallels exist between the expression and post-translational modification of ADV and B19 NS1 proteins that may contribute to restriction of virus replication. Thus, a study of the regulation of NS1 expression and its interactions with cell signalling pathways may lead to increased understanding of the restricted replication of these two viruses, and perhaps of persistent infection.

Two mink parvoviruses use different cellular receptors for entry into CRFK cells

Mink enteritis virus (MEV) and Aleutian mink disease parvovirus (ADV) are two mink parvoviruses that replicate permissively in Crandell feline kidney (CRFK) cells. We have used this cell model to examine if these two mink parvoviruses use the same cellular receptor. Whereas the cellular receptor for MEV is expected to be the transferrin receptor (TfR), the cellular receptor for ADV has not been clearly identified. We used short hairpin RNAs (shRNAs) produced from plasmids to trigger RNA interference (RNAi), specifically and effectively reducing TfR expression in CRFK cells. TfR expression was reduced to levels undetectable by immunofluorescence in the majority of cells. In viral infection assays, we show that TfR expression was necessary for MEV infection but was not required for ADV infection. Thus, our results demonstrate that TfR is the cellular receptor for MEV, but not the cellular receptor for ADV. The use of two different receptors by MEV and ADV to infect the same cell line is yet another difference between these two parvoviruses that may contribute to their unique pathogenesis in mink.

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.

Caspase cleavage of the nonstructural protein NS1 mediates replication of Aleutian mink disease parvovirus

Virus-induced apoptosis of infected cells can limit both the time and the cellular machinery available for virus replication. Hence, many viruses have evolved strategies to specifically inhibit apoptosis. However, Aleutian mink disease parvovirus (ADV) is the first example of a DNA virus that not only induces apoptosis but also utilizes caspase activity to facilitate virus replication. To determine the function of caspase activity during ADV replication, virus-infected cell lysates or purified ADV proteins were incubated with various purified caspases. Caspases cleaved the major nonstructural protein of ADV (NS1) at two caspase recognition sequences, whereas ADV structural proteins could not be cleaved. Importantly, the NS1 products could be identified in ADV-infected cells but were not present in infected cells pretreated with caspase inhibitors. By mutating putative caspase cleavage sites (D to E), we mapped the two cleavage sites to amino acid residues NS1:227 (INTD downward arrow S) and NS1:285 (DQTD downward arrow S). Replication of ADV containing either of these mutations was reduced 10(3)- to 10(4)-fold compared to that of wild-type virus, and a construct containing both mutations was replication defective. Immunofluorescent studies revealed that cleavage was required for nuclear localization of NS1. The requirement for caspase activity during permissive replication suggests that limitation of caspase activation and apoptosis in vivo may be a novel approach to restricting virus replication.

The role of helix 1 aspartates and salt bridges in the stability and conversion of prion protein

A key event in the pathogenesis of transmissible spongiform encephalopathies is the conversion of PrP-sen to PrP-res. Morrissey and Shakhnovich (Morrissey, M. P., and Shakhnovich, E. I. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 11293-11298) proposed that the conversion mechanism involves critical interactions at helix 1 (residues 144-153) and that the helix is stabilized on PrP-sen by intra-helix salt bridges between two aspartic acid-arginine ion pairs at positions 144 and 148 and at 147 and 151, respectively. Mutants of the hamster prion protein were constructed by replacing the aspartic acids with either asparagines or alanines to destabilize the proposed helix 1 salt bridges. Thermal and chemical denaturation experiments using circular dichroism spectroscopy indicated the overall structures of the mutants are not substantially destabilized but appear to unfold differently. Cell-free conversion reactions performed using ionic denaturants, detergents, and salts (conditions unfavorable to salt bridge formation) showed no significant differences between conversion efficiencies of mutant and wild type proteins. Using conditions more favorable to salt bridge formation, the mutant proteins converted with up to 4-fold higher efficiency than the wild type protein. Thus, although spectroscopic data indicate the salt bridges do not substantially stabilize PrP-sen, the cell-free conversion data suggest that Asp-144 and Asp-147 and their respective salt bridges stabilize PrP-sen from converting to PrP-res.

Caspase activation is required for permissive replication of Aleutian mink disease parvovirus in vitro

Aleutian mink disease parvovirus (ADV) is distinct among the parvoviruses as infection in vivo is persistent, restricted, and noncytopathic. In contrast, infections with other more prototypic parvoviruses, like mink enteritis virus (MEV), are acute, cytopathic, and characterized by permissive replication in vivo. Although apoptosis results in the death of cells acutely infected by parvoviruses, the role of apoptosis in ADV infections is unknown. Permissive infection of ADV resulted in apoptosis of Crandell feline kidney (CrFK) cells as indicated by TUNEL staining, Annexin-V staining, and characteristic changes in cell morphology. Pretreatment of infected cells with caspase 3 or broad-spectrum caspase inhibitors prevented apoptosis. In addition, treatment of infected cells with these inhibitors caused a 2 log(10) reduction in the yield of infectious virus compared to untreated cultures. This block in replication preceded substantial viral DNA amplification and gene expression. However, inhibitors of caspases 1, 6, and 8 did not have this effect. MEV also induced caspase-dependent apoptosis following infection of CrFK cells, although production of infectious progeny was not affected by inhibition of apoptosis. Thus, permissive replication of ADV in vitro depended upon activation of specific caspases. If ADV infection of cells in vivo fails to initiate caspase activation, the requirement of caspase activity for replication may not be met, thus providing a possible mechanism for persistent, restricted infection.

Identification of aleutian mink disease parvovirus capsid sequences mediating antibody-dependent enhancement of infection, virus neutralization, and immune complex formation

Aleutian mink disease parvovirus (ADV) causes a persistent infection associated with circulating immune complexes, immune complex disease, hypergammaglobulinemia, and high levels of antiviral antibody. Although antibody can neutralize ADV infectivity in Crandell feline kidney cells in vitro, virus is not cleared in vivo, and capsid-based vaccines have proven uniformly ineffective. Antiviral antibody also enables ADV to infect macrophages, the target cells for persistent infection, by Fc-receptor-mediated antibody-dependent enhancement (ADE). The antibodies involved in these unique aspects of ADV pathogenesis may have specific targets on the ADV capsid. Prominent differences exist between the structure of ADV and other, more-typical parvoviruses, which can be accounted for by short peptide sequences in the flexible loop regions of the capsid proteins. In order to determine whether these short sequences are targets for antibodies involved in ADV pathogenesis, we studied heterologous antibodies against several peptides present in the major capsid protein, VP2. Of these antibodies, a polyclonal rabbit antibody to peptide VP2:428-446 was the most interesting. The anti-VP2:428-446 antibody aggregated virus particles into immune complexes, mediated ADE, and neutralized virus infectivity in vitro. Thus, antibody against this short peptide can be implicated in key facets of ADV pathogenesis. Structural modeling suggested that surface-exposed residues of VP2:428-446 are readily accessible for antibody binding. The observation that antibodies against a single target peptide in the ADV capsid can mediate both neutralization and ADE may explain the failure of capsid-based vaccines.

Effect of a valine residue at codon 352 of the VP2 capsid protein on in vivo replication and pathogenesis of Aleutian disease parvovirus in mink

OBJECTIVE

To determine whether a group of 3 genetic differences in the nonstructural protein (NS1) or 1 genetic difference in the structural protein (VP2) of Aleutian disease parvovirus (ADV) is responsible for an increase in the in vivo replication and pathogenicity of G/U-8, a chimera of ADV-G (nonpathogenic) and ADV-Utah (pathogenic), compared with G/U-10.

ANIMALS

32 eight-month-old female sapphire mink (Mustela vison).

PROCEDURE

Chimeric viruses were constructed, propagated in vitro, and used to inoculate mink. Antiviral antibody responses, presence of serum viral nucleic acid, and serum gamma globulin concentrations were monitored for 120 days following inoculation. Histologic examination of the liver, kidneys, spleen, and mesenteric lymph nodes was performed after necropsy.

RESULTS

A chimera containing only the 3 amino acid substitutions in NS1 did not elicit measurable responses indicative of replication or pathogenicity in inoculated mink. Serum antiviral antibody responses, frequency of detection of viral nucleic acid in serum, gamma globulin response, and histologic changes in mink inoculated with chimeras containing a valine residue at codon 352 (352V) of VP2 capsid were increased, compared with values from mink inoculated with chimeric viruses that did not contain 352V.

CONCLUSIONS AND CLINICAL RELEVANCE

A valine residue at codon 352 in the VP2 capsid protein of ADV affects in vivo viral replication and pathogenicity. This amino acid may be part of an incompletely defined pathogenic determinant of ADV. Further characterization of the pathogenic determinant may allow future development of focused preventive and therapeutic interventions for Aleutian disease of mink.

Parvovirus infections in wild carnivores

Various parvoviruses infect carnivores and can cause disease. In this review article the knowledge about infections of free-ranging or captive carnivores with the feline parvoviruses, feline panleukopenia virus, and canine parvovirus, including the antigenic types CPV-2a and -2b, as well as Aleutian disease of mink virus and minute virus of canines are summarized. Particular emphasis is placed on description of the evolution of canine parvovirus which apparently involved wild carnivore hosts.

Aleutian mink disease parvovirus in wild riparian carnivores in Spain

Serious declines in populations of native European mink (Mustela lutreola) have occurred in Europe. One responsible factor may be infectious diseases introduced by exotic American mink (Mustela vison). In order to investigate a possible role for Aleutian mink disease parvovirus (ADV), we surveyed native riparian carnivores and feral American mink. When serum samples from 12 free-ranging European and 16 feral American mink were tested, antibodies to ADV were detected from three of nine European mink. ADV DNA was detected by polymerase chain reaction in whole cell DNA from four of seven carcasses; two American mink, one European mink and a Eurasian otter (Lutra lutra). Lesions typical of Aleutian disease were present in one of the American mink. A portion of the ADV VP2 capsid gene was sequenced and the results suggested that two sequence types of ADV were circulating in Spain, and that the Spanish ADVs differed from other described isolates from North America and Europe. Future conservation and restoration efforts should include measures to avoid introduction or spread of ADV infection to native animals.

Nonsuppurative meningoencephalitis associated with Aleutian mink disease parvovirus infection in ranch mink

Severe nonsuppurative meningoencephalitis associated with Aleutian mink disease parvovirus (ADV) infection was observed in adult ranch mink. Brain lesions included severe, locally extensive to coalescing lymphoplasmacytic meningoencephalitis with accompanying gliosis, satellitosis, and mild extension of inflammation into the leptomeninges. ADV was identified in mesenteric lymph node, spleen, brain, and liver of affected mink by polymerase chain reaction techniques. Sequences of the ADV isolate (TH5) revealed 2 unique residues in the region of the viral genome that determines pathogenicity. These findings suggest that certain strains of ADV may preferentially cause disease in the nervous system. ADV infection should be considered in the differential diagnosis of neurologic disorders in mink.

Replication of Aleutian mink disease parvovirus in mink lymph node histocultures

Aleutian mink disease parvovirus (ADV), causes an immune disorder with a persistent infection of lymphoid organs in adult mink. We studied replication of ADV in gel-supported histocultures prepared from adult mink mesenteric lymph node (MLN). Evidence of virus replication in the histocultures was first observed by indirect immunofluorescence 72 h after incubation with virus. Cells resembling lymphocytes and macrophages contained both ADV capsid (VP2) and nonstructural (NS1 and NS2) proteins, and were present in a distribution suggestive of infected cells within germinal centres. ADV replicative form and encapsidated virion DNA were also detected in infected histocultures at time-points after 72 h. In addition, we were able to passage ADV-Utah to a new round of histocultures. These results suggested that the infected cells were actual target cells for ADV replication and that productive ADV-Utah replication, complete with the generation of virus, was occurring in the histocultures. The mink MLN histocultures provide a system to study the replication and molecular pathogenesis of ADV in target tissues.