Viruses and renal carcinoma of Rana pipiens. IX. The influence of temperature and host cell on replication of frog polyhedral cytoplasmic deoxyribovirus (PCDV)

Temperature affects the host hematological and cytokine response following experimental ranavirus infection in red-eared sliders (Trachemys scripta elegans)

Pathogen-host interactions are important components of epidemiological research, but are scarcely investigated in chelonians. Red-eared sliders (Trachemys scripta elegans), are recognized as a model for frog virus-3 infection (FV3), a ranavirus in the family Iridoviridae that infects multiple classes of ectothermic vertebrates. Previous challenge studies observed differences in disease outcome based on environmental temperature in this species, but the host response was minimally evaluated. We challenged red-eared sliders with an FV3-like ranavirus at both 28°C and 22°C. We monitored several host response variables for 30 days, including: survival (binary outcome and duration), clinical signs, total and differential leukocytes, and select cytokine transcription in the buffy coat (IL-1β, TNFα, IFYg, IL-10). After 30 days, 17% of challenged turtles survived at 28°C (Median survival time [MST]: 15 days, range: 10–30 days) and 50% survived (MST: 28.5 days, range: 23–30 days) at 22°C (range 23–30 days). The most common clinical signs were injection site swelling, palpebral swelling, and lethargy. The heterophil/lymphocyte ratio at 22°C and interleukin-1 beta (IL1β) transcription at both 22°C and 28°C were significantly greater on days 9, 16, and 23 in FV3 challenged groups. Tumor necrosis factor alpha and interleukin-10 were transcribed at detectable levels, but did not display significant differences in mean relative transcription quantity over time. Overall, evidence indicates an over-robust immune response leading to death in the challenged turtles. FV3 remains a risk for captive and free-ranging chelonian populations, and insight to host/pathogen interaction through this model helps to elucidate the timing and intensity of the host response that contribute to mortality.

Class A Scavenger Receptors Are Used by Frog Virus 3 During Its Cellular Entry

Frog virus 3 (FV3) is the type species of the genus Ranavirus (family Iridoviridae). FV3 and FV3-like viruses are globally distributed infectious agents with the capacity to replicate in three vertebrate classes (teleosts, amphibians, and reptiles). At the cellular level, FV3 and FV3-like viruses can infect cells from virtually all vertebrate classes. To date, the cellular receptors that are involved in the FV3 entry process are unknown. Class A scavenger receptors (SR-As) are a family of evolutionarily conserved cell-surface receptors that bind a wide range of chemically distinct polyanionic ligands and can function as cellular receptors for other DNA viruses, including vaccinia virus and herpes simplex virus. The present study aimed to determine whether SR-As are involved in FV3 cellular entry. By using well-defined SR-A competitive and non-competitive ligand-blocking assays and absolute qPCR, we demonstrated that the SR-A competitive ligands drastically reduced the quantities of cell-associated viral loads in frog cells. Moreover, inducing the expression of a human SR-AI in an SR-A null cell line significantly increased FV3⁻cell association. Together, our results indicate that SR-As are utilized by FV3 during the cellular entry process.

Water Temperature Affects Susceptibility to Ranavirus

The occurrence of emerging infectious diseases in wildlife populations is increasing, and changes in environmental conditions have been hypothesized as a potential driver. For example, warmer ambient temperatures might favor pathogens by providing more ideal conditions for propagation or by stressing hosts. Our objective was to determine if water temperature played a role in the pathogenicity of an emerging pathogen (ranavirus) that infects ectothermic vertebrate species. We exposed larvae of four amphibian species to a Frog Virus 3 (FV3)-like ranavirus at two temperatures (10 and 25°C). We found that FV3 copies in tissues and mortality due to ranaviral disease were greater at 25°C than at 10°C for all species. In a second experiment with wood frogs (Lithobates sylvaticus), we found that a 2°C change (10 vs. 12°C) affected ranaviral disease outcomes, with greater infection and mortality at 12°C. There was evidence that 10°C stressed Cope's gray tree frog (Hyla chrysoscelis) larvae, which is a species that breeds during summer-all individuals died at this temperature, but only 10% tested positive for FV3 infection. The greater pathogenicity of FV3 at 25°C might be related to faster viral replication, which in vitro studies have reported previously. Colder temperatures also may decrease systemic infection by reducing blood circulation and the proportion of phagocytes, which are known to disseminate FV3 through the body. Collectively, our results indicate that water temperature during larval development may play a role in the emergence of ranaviruses.

Use of cell lines and primary cultures to explore the capacity of rainbow trout to be a host for frog virus 3 (FV3)

The capacity of rainbow trout, Oncorhynchus mykiss, to be a host for frog virus 3 (FV3) was evaluated at the cellular level. Cell cultures from this species were tested for their ability to express FV3 major capsid protein (MCP) gene, to develop cytopathic effect (CPE), and to produce FV3. After FV3 addition, MCP transcripts were detected in six of six cell lines and in primary macrophage cultures. CPE developed in all cell culture systems, except primary lymphocytes. For the macrophage cell line, RTS11, and primary macrophages, cell death was by apoptosis because DNA laddering and Annexin staining were detected. By contrast, markers of apoptosis did not accompany CPE in three epithelial cell lines from the gill (RTgill-W1), intestine (RTgut-GC), and liver (RTL-W1) and in two fibroblast cell lines from gonads (RTG-2) and skin (RTHDF). Therefore, FV3 was able to enter and begin replicating in several cell types. Yet, FV3 was produced in only two cell lines, RTG-2 and RTL-W1, and only modestly. Overall, these results suggest that if tissue accessibility were possible, FV3 would have the capacity to induce injury, but the ability to replicate would be limited, likely making rainbow trout a poor host for FV3.

Intraerythrocytic iridovirus in central bearded dragons (Pogona vitticeps)

Three adult central bearded dragons ( Pogona vitticeps) originating from a commercial breeding facility presented with clinical signs, including anorexia, dehydration, white multifocal lesions on the dorsal aspect of the tongue, blepharospasm, and weight loss. In 1 of 3 lizards, a marked regenerative anemia was noted, and all 3 bearded dragons had erythrocytic intracytoplasmic inclusion bodies. Nine bearded dragons housed in contact also had identical, but fewer intraerythrocytic inclusions. Inclusion bodies examined by electron microscopy had particles consistent with iridoviruses. Attempts to culture the virus were unsuccessful; however, amplification and sequencing of regions of the viral DNA polymerase by polymerase chain reaction confirmed the presence of an iridovirus. One of the bearded dragons died, while the 2 others showing clinical signs were euthanized. The remaining 9 infected bearded dragons of the teaching colony were also euthanized. Postmortem examination revealed a moderate, multifocal, lymphoplasmacytic or mononuclear adenitis of the tongue in the 3 bearded dragons, and a lymphohistiocytic hepatitis with bacterial granulomas in 2 lizards.

Expression of frog virus 3 genes is impaired in mammalian cell lines

Frog virus 3 (FV3) is a large DNA virus that is the prototypic member of the family Iridoviridae. To examine levels of FV3 gene expression we generated a polyclonal antibody against the FV3 protein 75L. Following a FV3 infection in fathead minnow (FHM) cells 75L was found in vesicles throughout the cytoplasm as early as 3 hours post-infection. While 75L expressed strongly in FHM cells, our findings revealed no 75L expression in mammalian cells lines despite evidence of a FV3 infection. One explanation for the lack of gene expression in mammalian cell lines may be inefficient codon usage. As a result, 75L was codon optimized and transfection of the codon optimized construct resulted in detectable expression in mammalian cells. Therefore, although FV3 can infect and replicate in mammalian cell lines, the virus may not express its full complement of genes due to inefficient codon usage in mammalian species.

ISOLATION OF A RANAVIRUS FROM A GECKO (UROPLATUS FIMBRIATUS)

A virus was isolated from the liver and stomach of a leaf-tailed gecko (Uroplatus fimbriatus) with granulomatous lesions on the tongue and hepatitis. The virus was identified as an iridovirus on the basis of morphology by electron microscopy, restriction endonuclease assay, and sequencing of a large portion of the major capsid protein gene. Comparative analysis revealed that this isolate is related to frog virus 3, the type species of the genus Ranavirus.

Comparative genomic analyses of frog virus 3, type species of the genus Ranavirus (family Iridoviridae)

Frog virus 3 (FV3) is the type species member of the genus Ranavirus (family Iridoviridae). To better understand the molecular mechanisms involved in the replication of FV3, including transcription of its highly methylated DNA genome, we have determined the complete nucleotide sequence of the FV3 genome. The FV3 genome is 105903 bp long excluding the terminal redundancy. The G + C content of FV3 genome is 55% and it encodes 98 nonoverlapping potential open reading frames (ORFs) containing 50-1293 amino acids. Eighty-four ORFs have significant homology to known proteins of other iridoviruses, whereas twelve of these unique FV3 proteins do not share homology to any known protein. A microsatellite containing a stretch of 34 tandemly repeated CA dinucleotide in a noncoding region was detected. To date, no such sequence has been reported in any animal virus.

Restriction of frog virus 3 polypeptide synthesis to immediate early and delayed early species by supraoptimal temperatures

Multiplication of frog virus 3 (FV 3) occurs in mammalian cells provided they are incubated at temperatures lower than 33 degrees. The expression of the viral genome at supraoptimal temperatures was followed by analyzing the polypeptides produced in CHO-infected cells and comparing with those obtained under restrictive conditions provoked by amino acid analogs or metabolic inhibitors. Late polypeptides were not detected at 33 degrees and the number of delayed early species decreased gradually with increasing temperatures consequently the synthesis of all delayed early proteins was not turned on in response to a unique event. At 37 degrees the synthesis was limited to the immediate early species, i.e., the proteins synthesized after cycloheximide reversal. Temperature shift experiments suggested that delayed early genes remained untranscribed at 37 degrees. Thus, incubation of FV 3-infected mammalian cells at 37 degrees provides a unique way of limiting viral synthesis to immediate early proteins without the side-effect provoked by inhibitors.

trans activation of an immediate-early frog virus 3 promoter by a virion protein

We investigated the protein and DNA sequence requirements for the expression of an immediate-early frog virus 3 (FV3) gene, infected-cell RNA (ICR) 169. We used a plasmid containing the 78 nucleotides 5' to the transcription start site of ICR-169 placed upstream from the coding sequence for the bacterial enzyme chloramphenicol acetyltransferase (CAT). This construction, when introduced by CaPO4-mediated transfection into various eucaryotic cell lines, promoted CAT synthesis only if the transfected cells were subsequently infected with FV3. Dot-blot hybridization of RNA extracted from transfected, FV3-infected cells with a radioactive CAT probe showed that the induction of CAT synthesis by FV3 was at the level of transcription. When transfected cells were infected with FV3 in the presence of cycloheximide, induction of CAT-specific RNA still occurred, demonstrating that a virion protein was responsible for the trans activation. FV3-induced CAT synthesis was inhibited by alpha-amanitin in wild-type Chinese hamster ovary (CHO) cells but not in CHO cells with an alpha-amanitin-resistant RNA polymerase II. The results suggest that a virion protein alters either the DNA template or the host polymerase to allow transcription from immediate-early FV3 promoters.