Punta Toro virus infection of C57BL/6J mice: a model for phlebovirus-induced disease

Heartland Virus Epidemiology, Vector Association, and Disease Potential

First identified in two Missouri farmers exhibiting low white-blood-cell and platelet counts in 2009, Heartland virus (HRTV) is genetically closely related to severe fever with thrombocytopenia syndrome virus (SFTSV), a tick-borne phlebovirus producing similar symptoms in China, Korea, and Japan. Field isolations of HRTV from several life stages of unfed, host-seeking Amblyomma americanum, the lone star tick, implicated it as a putative vector capable of transstadial transmission. Laboratory vector competence assessments confirmed transstadial transmission of HRTV, demonstrated vertical infection, and showed co-feeding infection between A. americanum. A vertical infection rate of 33% from adult females to larvae in the laboratory was observed, while only one of 386 pools of molted nymphs (1930) reared from co-feeding larvae was positive for HRTV (maximum-likelihood estimate of infection rate = 0.52/1000). Over 35 human HRTV cases, all within the distribution range of A. americanum, have been documented. Serological testing of wildlife in areas near the index human cases, as well as in widely separated regions of the eastern United States where A. americanum occur, indicated many potential hosts such as raccoons and white-tailed deer. Attempts, however, to experimentally infect mice, rabbits, hamsters, chickens, raccoons, goats, and deer failed to produce detectable viremia. Immune-compromised mice and hamsters are the only susceptible models. Vertical infection augmented by co-feeding transmission could play a role in maintaining the virus in nature. A more complete assessment of the natural transmission cycle of HRTV coupled with serosurveys and enhanced HRTV disease surveillance are needed to better understand transmission dynamics and human health risks.

Vertebrate Host Susceptibility to Heartland Virus

Virus-infected Ag129 mice could be a useful model for identifying tick infection or virus transmission.

Heartland virus (HRTV) is a recently described phlebovirus initially isolated in 2009 from 2 humans who had leukopenia and thrombocytopenia. Serologic assessment of domestic and wild animal populations near the residence of 1 of these persons showed high exposure rates to raccoons, white-tailed deer, and horses. To our knowledge, no laboratory-based assessments of viremic potential of animals infected with HRTV have been performed. We experimentally inoculated several vertebrates (raccoons, goats, chickens, rabbits, hamsters, C57BL/6 mice, and interferon-α/β/γ receptor–deficient [Ag129]) mice with this virus. All animals showed immune responses against HRTV after primary or secondary exposure. However, neutralizing antibody responses were limited. Only Ag129 mice showed detectable viremia and associated illness and death, which were dose dependent. Ag129 mice also showed development of mean peak viral antibody titers >8 log₁₀ PFU/mL, hemorrhagic hepatic lesions, splenomegaly, and large amounts of HRTV antigen in mononuclear cells and hematopoietic cells in the spleen.

Rift Valley fever virus NSs protein functions and the similarity to other bunyavirus NSs proteins

Rift Valley fever is a mosquito-borne zoonotic disease that affects both ruminants and humans. The nonstructural (NS) protein, which is a major virulence factor for Rift Valley fever virus (RVFV), is encoded on the S-segment. Through the cullin 1-Skp1-Fbox E3 ligase complex, the NSs protein promotes the degradation of at least two host proteins, the TFIIH p62 and the PKR proteins. NSs protein bridges the Fbox protein with subsequent substrates, and facilitates the transfer of ubiquitin. The SAP30-YY1 complex also bridges the NSs protein with chromatin DNA, affecting cohesion and segregation of chromatin DNA as well as the activation of interferon-β promoter. The presence of NSs filaments in the nucleus induces DNA damage responses and causes cell-cycle arrest, p53 activation, and apoptosis. Despite the fact that NSs proteins have poor amino acid similarity among bunyaviruses, the strategy utilized to hijack host cells are similar. This review will provide and summarize an update of recent findings pertaining to the biological functions of the NSs protein of RVFV as well as the differences from those of other bunyaviruses.

A Comparison of Pyrimidinone Analogue Immunomodulators for Treatment of Phlebovirus Infections in Mice

The substituted pyrimidinone immunomodulators 5-bromo-2,3-dihydro-2-imino-6-phenyl-4(1H)-pyrimidinone (ABPP), 5-iodo-2,3-dihydro-2-imino-6-phenyl-4(1H)-pyrimidinone (AIPP), 5-bromo-2,3-dihydro-2-imino-6-methyl-4(1H)-pyrimidinone (ABMP), 5-chloro-2,3-dihydro-2-imino-6-phenyl-4(1H)-pyrimidinone (ACPP), 5-bromo-2,3-dihydro-2-iminp-6-(3-fluorophenyl)-4(1H)-pyrimidinone (ABmFPP) and 5-chloro-2,3-dihydro-2-imino-6-(2,3-difluorophenyl)-4(1H)-pyrimidinone (AComF2PP) were evaluated against the hepatotropic and immunosuppressive infection of mice induced by Punta Toro virus, a Phlebovirus related to Rift Valley fever virus. Using intraperitoneal (i.p.) treatments once daily for 3 days beginning 1 day prior to virus exposure, the order of efficacy was ABPP > ABmFPP > AComF2PP > AIPP > ABMP > ACPP, with prevention of death as end-point. Using single i.p. treatment 1 day prior to virus exposure, antiviral activity was seen in the order ABPP > ABmFPP > ABMP > AIPP > ACPP > AComF2PP. Three active pyrimidonones, ABPP, ABMP and AIPP, were given by gavage (p.o.) using the above 3-day treatment schedule. In this experiment, expanded parameters were used, with prevention of death seen as well as decreased hepatic icterus, as evidenced by lowered liver scores and serum glutamic oxalacetic and pyruvic transaminases. In addition, recoverable virus titres were reduced in the livers and sera of the mice. Greatest activity was seen using ABPP, followed by ABMP and then by AIPP. ABPP in doses of 100–800 mg kg−1 was markedly effective when administered p.o. in single treatments given as late as 24 hr after virus inoculation; a single dose, 400 mg kg−1, prevented death when given as late as 48 hr after virus exposure. In comparison of three once-daily, every-other-day, or every 2-day p.o. treatments, ABPP was most effective using the every-other-day regimen.

Inhibition of Phlebovirus Infections in vivo by Tiazofurin and Selenazofurin

The c-nucleosides tiazofurin and selenazofurin were evaluated in mice against the Adames strain of Punta Toro virus, a Phlebovirus related to Rift Valley fever and sandfly fever viruses. When administered subcutaneously (s.c.) twice a day for 5 days starting 4h before virus inoculation, tiazofurin reduced mortality, hepatic icterus scores, serum aspartate aminotransferase (AST) levels, and serum virus titres at 250–500 mg kg−1 day−1. Liver virus titres were not reduced in these treated animals, however. Oral tiazofurin treatment was effective by the same criteria at 375 and 750 mg kg−1, except that liver virus titres were reduced at 750 mg kg−1. Similar effects were observed with selenazofurin given s.c. (80–160 mg kg−1) or orally (80–320 mg kg−1) for 5 days. Selenazofurin suppressed liver virus titres by oral but not s.c. treatment. In a time-course study tiazofurin was effective s.c. at 250–1000 mg kg−1 when administered once only at 48 h after virus inoculation, whereas treatments at 4, 24, 72 and 96 h were less or not effective. By comparison with published results, tiazofurin and selenazofurin do not appear to be as active or selective as ribavirin and ribamidine against Punta Toro virus infections in mice.

Combination Chemotherapy of Punta Toro Virus Infections in Mice Using Ribavirin and 7-thia-8-oxoguanosine

Ribavirin was combined with the immunoenhancing agent 7-thia-8-oxoguanosine (TOGuo) to treat lethal experimental Punta Toro virus infections in mice. Lethal hepatic disease caused by the Adames strain of the virus was treated orally with ribavirin and intraperitoneally with TOGuo, with both treatments starting 24 h after virus inoculation. By themselves, ribavirin (25 mg/kg) and TOGuo (12.5 and 25 mg/kg) protected 80–100% of mice from mortality, but lower doses of each were ineffective. Combinations of the two agents (ribavirin, 25 mg/kg plus TOGuo at 6.25 and 12.5 mg/kg) produced synergistic decreases in hepatic icterus scores, liver and spleen virus titres, and serum alanine aminotransferase and aspartate aminotransferase activities. Lower dosage combinations were not synergistic. Of particular interest was the observation that TOGuo (25 mg/kg) could reverse the lethal toxicity of a high dose of ribavirin (1250 mg/kg) both in Punta Toro virus-infected and uninfected mice. These data suggest that TOGuo increased the therapeutic index of ribavirin by decreasing toxicity and increasing antiviral activity in this model.

Phleboviruses and the Type I Interferon Response

The genus Phlebovirus of the family Bunyaviridae contains a number of emerging virus species which pose a threat to both human and animal health. Most prominent members include Rift Valley fever virus (RVFV), sandfly fever Naples virus (SFNV), sandfly fever Sicilian virus (SFSV), Toscana virus (TOSV), Punta Toro virus (PTV), and the two new members severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland virus (HRTV). The nonstructural protein NSs is well established as the main phleboviral virulence factor in the mammalian host. NSs acts as antagonist of the antiviral type I interferon (IFN) system. Recent progress in the elucidation of the molecular functions of a growing list of NSs proteins highlights the astonishing variety of strategies employed by phleboviruses to evade the IFN system.

Attenuation of pathogenic Rift Valley fever virus strain through the chimeric S-segment encoding sandfly fever phlebovirus NSs or a dominant-negative PKR

Rift Valley fever is a mosquito-borne zoonotic disease affecting ruminants and humans. Rift Valley fever virus (RVFV: family Bunyaviridae, genus Phlebovirus) causes abortions and fetal malformations in ruminants, and hemorrhagic fever, encephalitis, or retinitis in humans. The live-attenuated MP-12 vaccine is conditionally licensed for veterinary use in the US. However, this vaccine lacks a marker for the differentiation of vaccinated from infected animals (DIVA). NSs gene is dispensable for RVFV replication, and thus, rMP-12 strains lacking NSs gene is applicable to monitor vaccinated animals. However, the immunogenicity of MP-12 lacking NSs was not as high as parental MP-12. Thus, chimeric MP-12 strains encoding NSs from either Toscana virus (TOSV), sandfly fever Sicilian virus (SFSV) or Punta Toro virus Adames strain (PTA) were characterized previously. Although chimeric MP-12 strains are highly immunogenic, the attenuation through the S-segment remains unknown. Using pathogenic ZH501 strain, we aimed to demonstrate the attenuation of ZH501 strain through chimeric S-segment encoding either the NSs of TOSV, SFSV, PTA, or Punta Toro virus Balliet strain (PTB). In addition, we characterized rZH501 encoding a human dominant-negative PKR (PKRΔE7), which also enhances the immunogenicity of MP-12. Study done on mice revealed that attenuation of rZH501 occurred through the S-segment encoding either PKRΔE7 or SFSV NSs. However, rZH501 encoding either TOSV, PTA, or PTB NSs in the S-segment uniformly caused lethal encephalitis. Our results indicated that the S-segments encoding PKRΔE7 or SFSV NSs are attenuated and thus applicable toward next generation MP-12 vaccine candidates that encode a DIVA marker.

Animal models for viral haemorrhagic fever

Viral haemorrhagic fever can be caused by one of a diverse group of viruses that come from four different families of RNA viruses. Disease severity can vary from mild self-limiting febrile illness to severe disease characterized by high fever, high-level viraemia, increased vascular permeability that can progress to shock, multi-organ failure and death. Despite the urgent need, effective treatments and preventative vaccines are currently lacking for the majority of these viruses. A number of factors preclude the effective study of these diseases in humans including the high virulence of the agents involved, the sporadic nature of outbreaks of these viruses, which are typically in geographically isolated areas with underserviced diagnostic capabilities, and the requirements for high level bio-containment. As a result, animal models that accurately mimic human disease are essential for advancing our understanding of the pathogenesis of viral haemorrhagic fevers. Moreover, animal models for viral haemorrhagic fevers are necessary to test vaccines and therapeutic intervention strategies. Here, we present an overview of the animal models that have been established for each of the haemorrhagic fever viruses and identify which aspects of human disease are modelled. Furthermore, we discuss how experimental design considerations, such as choice of species and virus strain as well as route and dose of inoculation, have an influence on animal model development. We also bring attention to some of the pitfalls that need to be avoided when extrapolating results from animal models.

Rift Valley fever virus infection in golden Syrian hamsters

Rift Valley fever virus (RVFV) is a formidable pathogen that causes severe disease and abortion in a variety of livestock species and a range of disease in humans that includes hemorrhagic fever, fulminant hepatitis, encephalitis and blindness. The natural transmission cycle involves mosquito vectors, but exposure can also occur through contact with infected fluids and tissues. The lack of approved antiviral therapies and vaccines for human use underlies the importance of small animal models for proof-of-concept efficacy studies. Several mouse and rat models of RVFV infection have been well characterized and provide useful systems for the study of certain aspects of pathogenesis, as well as antiviral drug and vaccine development. However, certain host-directed therapeutics may not act on mouse or rat pathways. Here, we describe the natural history of disease in golden Syrian hamsters challenged subcutaneously with the pathogenic ZH501 strain of RVFV. Peracute disease resulted in rapid lethality within 2 to 3 days of RVFV challenge. High titer viremia and substantial viral loads were observed in most tissues examined; however, histopathology and immunostaining for RVFV antigen were largely restricted to the liver. Acute hepatocellular necrosis associated with a strong presence of viral antigen in the hepatocytes indicates that fulminant hepatitis is the likely cause of mortality. Further studies to assess the susceptibility and disease progression following respiratory route exposure are warranted. The use of the hamsters to model RVFV infection is suitable for early stage antiviral drug and vaccine development studies.

Favipiravir (T-705) protects against peracute Rift Valley fever virus infection and reduces delayed-onset neurologic disease observed with ribavirin treatment

Rift Valley fever is a zoonotic, arthropod-borne disease that affects livestock and humans. The etiologic agent, Rift Valley fever virus (RVFV; Bunyaviridae, Phlebovirus) is primarily transmitted through mosquito bites, but can also be transmitted by exposure to infectious aerosols. There are presently no licensed vaccines or therapeutics to prevent or treat severe RVFV infection in humans. We have previously reported on the activity of favipiravir (T-705) against the MP-12 vaccine strain of RVFV and other bunyaviruses in cell culture. In addition, efficacy has also been documented in mouse and hamster models of infection with the related Punta Toro virus. Here, hamsters challenged with the highly pathogenic ZH501 strain of RVFV were used to evaluate the activity of favipiravir against lethal infection. Subcutaneous RVFV challenge resulted in substantial serum and tissue viral loads and caused severe disease and mortality within 2-3 days of infection. Oral favipiravir (200 mg/kg/day) prevented mortality in 60% or greater of hamsters challenged with RVFV when administered within 1 or 6h post-exposure and reduced RVFV titers in serum and tissues relative to the time of treatment initiation. In contrast, although ribavirin (75 mg/kg/day) was effective at protecting animals from the peracute RVFV disease, most ultimately succumbed from a delayed-onset neurologic disease associated with high RVFV burden observed in the brain in moribund animals. When combined, T-705 and ribavirin treatment started 24 h post-infection significantly improved survival outcome and reduced serum and tissue virus titers compared to monotherapy. Our findings demonstrate significant post-RVFV exposure efficacy with favipiravir against both peracute disease and delayed-onset neuroinvasion, and suggest added benefit when combined with ribavirin.

Pathogenesis of emerging severe fever with thrombocytopenia syndrome virus in C57/BL6 mouse model

The discovery of an emerging viral disease, severe fever with thrombocytopenia syndrome (SFTS), caused by SFTS virus (SFTSV), has prompted the need to understand pathogenesis of SFTSV. We are unique in establishing an infectious model of SFTS in C57/BL6 mice, resulting in hallmark symptoms of thrombocytopenia and leukocytopenia. Viral RNA and histopathological changes were identified in the spleen, liver, and kidney. However, viral replication was only found in the spleen, which suggested the spleen to be the principle target organ of SFTSV. Moreover, the number of macrophages and platelets were largely increased in the spleen, and SFTSV colocalized with platelets in cytoplasm of macrophages in the red pulp of the spleen. In vitro cellular assays further revealed that SFTSV adhered to mouse platelets and facilitated the phagocytosis of platelets by mouse primary macrophages, which in combination with in vivo findings, suggests that SFTSV-induced thrombocytopenia is caused by clearance of circulating virus-bound platelets by splenic macrophages. Thus, this study has elucidated the pathogenic mechanisms of thrombocytopenia in a mouse model resembling human SFTS disease.

The pathogenesis of Rift Valley fever

Rift Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the Rift Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis.

Host genetic variation in susceptibility to Punta Toro virus

Infection of small laboratory animals by Punta Toro virus (PTV), family Bunyaviridae, genus Phlebovirus, is a model for the study of the human pathogen Rift Valley fever virus (RVFV). We have identified inbred mouse strains with significant differences in host response to the Adames strain of PTV. Nine inbred strains of mice representing major branches in the Mus musculus phylogeny were inoculated subcutaneously with a high dose of PTV in survival experiments. Two inbred strains of mice, NZW/LacJ and 129S1/SvImJ, died ~4 days after PTV infection, whereas 7 other strains survived the challenge and showed no clinical signs of disease. Histologically, 129S1/SvImJ mice showed massive hepatocellular necrosis and had additional lesions in lung, brain, and spleen, whereas NZW/LacJ mice had mild piecemeal hepatocellular necrosis. PTV viral loads in the livers of infected mice were determined by reverse transcriptase quantitative PCR. Inbred mice from strains that showed clinical signs and succumbed to PTV infection had higher liver viral loads than did mice of resistant strains. Hybrid F₁ mice were generated by crossing susceptible 129S1 and resistant FVB/N mice and tested for susceptibility. The hybrid F₁ mice showed significantly higher viral loads in the liver than the resistant parental FVB/N mice, suggesting that susceptibility is dominant. These findings will enable an unbiased genetic approach to identify host genes mediating susceptibility to PTV.

Efficacy of favipiravir (T-705) and T-1106 pyrazine derivatives in phlebovirus disease models

Several studies have reported favipiravir (T-705) to be effective in treating a number of viral diseases modeled in rodent systems. Notably, the related pyrazine derivative, T-1106, was found to be more effective than T-705 in treating yellow fever virus infection in hamsters. Based on these findings, we hypothesized that T-1106 may be more effective in treating hepatotropic Punta Toro virus (PTV, Phlebovirus) infection in rodents. In cell culture, the inhibitory concentrations of the compounds against various phleboviruses ranged from 3 to 55microM for T-705 and from 76 to 743microM for T-1106. In PTV-challenged hamsters, a model that generally presents with high liver viral loads, T-1106 was more effective at reducing mortality. However, in mice infected with PTV, a model wherein systemic infection is more prominent, the greater efficacy exhibited by T-1106 in the hamster system was not apparent. In contrast, T-705 was superior in preventing mortality in hamsters challenged with Pichinde virus (PICV, Arenavirus), an infection characterized as diffuse and pantropic. Remarkably, T-1106 has proven more active in vivo than would have been expected from our cell culture results, and our in vivo findings suggest that it is more effective in infections characterized predominantly by high levels of hepatic viral burden.

Punta Toro virus (Bunyaviridae, Phlebovirus) infection in mice: strain differences in pathogenesis and host interferon response

The Adames strain of Punta Toro virus (PTV-A, Bunyaviridae, Phlebovirus) causes an acute lethal disease in hamsters and mice. The Balliet strain of the virus (PTV-B) is generally considered to be avirulent. The difference in hamster susceptibility is likely due to the ability of PTV-A to suppress interferon (IFN)-beta similarly to that described for Rift Valley fever virus. Here we investigated strain differences in PTV pathogenesis and the IFN response in mice. Although PTV-B infection in mice did not induce systemic IFN-beta release, primary macrophages produced dramatically higher levels when exposed to the virus in culture. The importance of IFN in resistance to PTV infection was borne out in studies employing STAT-1 knock-out mice. Also, a number of genes specific to IFN response pathways were upregulated in PTV-B-infected macrophages. Our findings provide new insights into the type I IFN response during PTV infection in the mouse model of phleboviral disease.

A biotechnological product and its potential as a new immunomodulator for treatment of animal phlebovirus infection: Punta Toro virus

Intracellular pathogens with widespread drug-resistance contribute substantially to the increasing rates in morbidity and mortality due to emerging and reemerging diseases. Thus, the development of new drugs, including those that can enhance the immune response, is urgently needed. The immunomodulator, P-MAPA, a proteinaceous aggregate of ammonium and magnesium phospholinoleate-palmitoleate anhydride derived from Aspergillus oryzae, have been shown to induce antitumor activities. The ability of this compound to elicit protective immunity against viral infections has not been fully explored. Here, we report findings on the use of P-MAPA as an antiviral agent in a mouse model of acute phleboviral (Punta Toro virus) disease. A dose administered i.p. 24h post-infectious challenge (100mg/kg dose of P-MAPA) was remarkably effective at preventing death due to Punta Toro virus infection. This dose also reduced systemic viral burden and liver discoloration assayed on day 3 of infection. Taken together, our findings indicate that non-specific immunotherapy with P-MAPA appears to be an effective treatment for blocking Punta Toro virus-induced disease and suggest that further exploration with other viral disease models is warranted.

Prophylaxis with cationic liposome-DNA complexes protects hamsters from phleboviral disease: importance of liposomal delivery and CpG motifs

Cationic liposome-DNA complexes (CLDC) are cationic/neutral lipid carriers complexed with plasmid DNA that when administered systemically results in a robust T(H)1 cytokine response. CLDC have been shown to be effective in prophylaxis and therapeutic treatment of animal models of viral disease. To determine the contribution of liposomal delivery and CpG content of the plasmid DNA to the efficacy of CLDC; plasmid, CpG-free plasmid DNA, or CpG-containing oligodeoxynucleotides (ODN) with and without liposomes, as well as poly(I:C(12)U), were evaluated for their ability to elicit protection against lethal Punta Toro virus (PTV, Bunyaviridae, phlebovirus) challenge in hamsters. CLDC-containing plasmid significantly improved survival, decreased systemic and liver viral loads, and reduced liver damage due to progression of viral infection. Mouse-reactive ODNs complexed with liposomes failed to protect hamsters, whereas ODNs known to cross-react with human and mouse (CpG 2006) or non-liposomal poly(I:C(12)U) showed survival benefit but did not limit liver injury. Liposomes complexed with a non-CpG motif-containing plasmid reduced liver viral load and tissue damage, but did not protect hamsters from death. To evaluate the mechanisms of the enhanced activity of CLDC, microarray experiments examined differences in the gene expression profile. The results suggest a broad T(H)1 response elicited by liposomal delivery of a diverse sequence containing CpG and non-CpG elements may be a more effective antiviral treatment than other nucleic acid based immunotherapeutics.

Immunoprophylaxis of Punta Toro virus (Phlebovirus, Bunyaviridae) infection in hamsters with recombinant Eimeria profilin-like antigen

Recombinant Eimeria antigen (rEA) has been shown to have potent anticancer and antiviral activity in respective mouse disease models, presumably through robust immune stimulation that occurs via TLR11, a pattern recognition receptor that recognizes profilin-like proteins expressed on apicomplexan protozoans. Comparable immunostimulatory activity in other species has yet to be demonstrated. Since rEA is known to be highly effective in treating Punta Toro virus (PTV) infection in mice, its ability to elicit protective immunity in the hamster PTV infection model was investigated. rEA was given alone, or in combination with IL-18 or IL-2, and virally challenged hamsters were observed for mortality. Cytokine transcript profiles for IL-12p40, IL-21, IFN-gamma and TNF-alpha were assessed to evaluate the induction of these inflammatory mediators known to be induced in mice following exposure to rEA. A dose of 100 microg of rEA, given once 4 h prior to viral challenge, and a second time on day 3 of the infection, was found to be the most effective prophylactic therapy protecting 60% of treated hamsters from mortality, compared to only 5-10% observed in animals receiving placebo. Increased expression of IFN-gamma and IL-12p40 was evident following treatment with rEA. The data suggest that rEA does induce host antiviral responses in hamsters that result in significant protection from death, although determining the most appropriate dose for intervention in other species, including humans, will likely be challenging.

Animal models of highly pathogenic RNA viral infections: hemorrhagic fever viruses

A diverse group of highly pathogenic RNA viruses cause a severe multisystemic illness in humans commonly referred to as viral hemorrhagic fever (VHF). Although they can vary widely in clinical presentation, all VHFs share certain features that include intense fever, malaise, bleeding and shock. Effective antiviral therapies for most of the VHFs are lacking. Complicating development of intervention strategies is the relative infrequency and unpredictability of VHF outbreaks making human clinical trials extremely challenging or unfeasible. Therefore, animal models that can recapitulate human disease are essential to the development of effective antivirals and vaccines. In general, a good animal model of VHF will demonstrate systemic dispersion of the virus through infection of mononuclear phagocytes and dendritic cells, which induces the release of inflammatory mediators that increase vascular permeability and facilitate coagulation. The culmination of this process leads to significant loss of plasma volume and terminal hypovolemic shock. Although it is clear that nonhuman primate models are the most faithful to human disease, the more accessible and less costly rodent models, including those based on infection with related surrogate viruses, can reproduce certain components of VHF and can serve as suitable preclinical models for initial development of effective countermeasures. Such models are sufficient for testing of drugs that directly block viral replication, but may be inadequate for evaluating therapies that depend for their success on the activation or inhibition of host responses.

TLR3 is essential for the induction of protective immunity against Punta Toro Virus infection by the double-stranded RNA (dsRNA), poly(I:C12U), but not Poly(I:C): differential recognition of synthetic dsRNA molecules

In the wake of RNA virus infections, dsRNA intermediates are often generated. These viral pathogen-associated molecular patterns can be sensed by a growing number of host cell cytosolic proteins and TLR3, which contribute to the induction of antiviral defenses. Recent evidence indicates that melanoma differentiation-associated gene-5 is the prominent host component mediating IFN production after exposure to the dsRNA analog, poly(I:C). We have previously reported that Punta Toro virus (PTV) infection in mice is exquisitely sensitive to treatment with poly(I:C(12)U), a dsRNA analog that has a superior safety profile while maintaining the beneficial activity of the parental poly(I:C) in the induction of innate immune responses. The precise host factor(s) mediating protective immunity following its administration remain to be elucidated. To assess the role of TLR3 in this process, mice lacking the receptor were used to investigate the induction of protective immunity, type I IFNs, and IL-6 following treatment. Unlike wild-type mice, those lacking TLR3 were not protected against PTV infection following poly(I:C(12)U) therapy and failed to produce IFN-alpha, IFN-beta, and IL-6. In contrast, poly(I:C) treatment significantly protected TLR3(-/-) mice from lethal challenge despite some deficiencies in cytokine induction. There was no indication that the lack of protection was due to the fact that TLR3-deficient mice had a reduced capacity to fight infection because they were not found to be more susceptible to PTV. We conclude that TLR3 is essential to the induction of antiviral activity elicited by poly(I:C(12)U), which does not appear to be recognized by the cytosolic sensor of poly(I:C), melanoma differentiation-associated gene-5.

The S segment of Punta Toro virus (Bunyaviridae, Phlebovirus) is a major determinant of lethality in the Syrian hamster and codes for a type I interferon antagonist

Two strains of Punta Toro virus (PTV), isolated from febrile humans in Panama, cause a differential pathogenesis in Syrian hamsters, which could be a useful model for understanding the virulence characteristics and differential outcomes in other phleboviral infections such as Rift Valley fever virus. Genetic reassortants produced between the lethal Adames (A/A/A) and nonlethal Balliet (B/B/B) strains were used in this study to investigate viral genetic determinants for pathogenesis and lethality in the hamster model. The S segment was revealed to be a critical genome segment, determining lethality with log(10) 50% lethal doses for each PTV genotype as follows (L/M/S convention): A/A/A, <0.7; B/A/A, <0.7; A/B/A, 1.5; B/B/A, 2.2; B/A/B, 4.7; A/B/B, >4.7; A/A/B, >4.7; B/B/B, >4.7. In addition, the Adames strain inhibits the induction of alpha/beta interferon (IFN-alpha/beta) in vivo and in vitro and inhibits the activation of the IFN-beta promoter. Expression of the PTV Adames NSs protein, encoded by the S RNA segment, inhibited the virus-mediated induction of an IFN-beta promoter-driven reporter gene, suggesting that PTV NSs functions as a type I IFN antagonist. Taken together, these data indicate a mechanism of pathogenesis in which the suppression of the type I IFN response early during PTV infection leads to early and uncontrolled viral replication and, ultimately, hamster death. This study contributes to our understanding of Phlebovirus pathogenesis and identifies potential targets for immune modulation to increase host survival.

TLR3 Deletion Limits Mortality and Disease Severity due to Phlebovirus Infection

TLR3 was the first member of the TLR family of pattern recognition receptors found to detect a conserved viral molecular pattern, dsRNA, yet supporting evidence for a major role in host defense against viral pathogens is limited. Punta Toro virus (PTV) has been shown to produce severe infection in mice, modeling disease caused by the related highly pathogenic Rift Valley fever phlebovirus in humans and domesticated ungulates. Using TLR3-deficient mice, we investigated the involvement of TLR3 in host defense against PTV infection. Compared with wild-type, TLR3(-/-) mice demonstrate increased resistance to lethal infection and have reduced liver disease associated with hepatotropic PTV infection. Infectious challenge produced comparable peak liver and serum viral loads; however, TLR3(-/-) mice were able to clear systemic virus at a slightly faster rate. Cytokine profiling suggests that TLR3 plays an important role in PTV pathogenesis through the overproduction of inflammatory mediators, which may be central to the observed differences in survival and disease severity. Compared with TLR3-deficient mice, IL-6, MCP-1, IFN-gamma, and RANTES were all present at higher levels in wild-type animals. Most dramatic was the exaggerated levels of IL-6 found systemically and in liver tissue of infected wild-type mice; however, IL-6-deficient animals were found to be more susceptible to lethal PTV infection. Taken together, we conclude that the TLR3-mediated response to PTV infection is detrimental to disease outcome and propose that IL-6, although critical to establishing antiviral defense, contributes to pathogenesis when released in excess, necessitating its controlled production as is seen with TLR3(-/-) mice.

Recombinant Eimeria protozoan protein elicits resistance to acute phlebovirus infection in mice but not hamsters

A protein antigen from an Eimeria protozoan has recently been reported to induce antitumor activity in mice. This activity most likely results from the strong induction of interkeukin-12 (IL-12) and gamma interferon (IFN-gamma), which are also essential factors in the establishment of protective immunity against viral infection. We evaluated recombinant Eimeria antigen (rEA) as a potential immunotherapeutic agent in mouse and hamster models of acute phleboviral disease. Punta Toro virus (PTV) was highly sensitive to a single dose of nanogram quantities of rEA in the mouse infection model. Intraperitoneal treatment with rEA also reduced virus load and liver damage associated with PTV infection. IL-12 was elicited following exposure of uninfected mice to quantities of rEA of 10 ng or greater, and the levels peaked at between 3 and 8 h postexposure. IFN-gamma release was induced more slowly and required less rEA (1 ng) to produce a significant rise in systemic levels. The induction of IL-12 and IFN-gamma involved in the coordination of innate and adaptive immune responses to microbial pathogens required myeloid differentiation factor 88, a signaling adaptor shared by most members of the Toll-like receptor (TLR) family. Despite encouraging results in the murine system, rEA failed to protect hamsters challenged with PTV. Our findings suggest that hamsters may lack functional TLR11, which has recently been shown to recognize a profilin-like protein homologous to rEA from the protozoan Toxoplasma gondii. Further investigation into the immunostimulatory capacity of rEA in other mammalian systems is necessary.

Viruses of the Bunya- and Togaviridae families: potential as bioterrorism agents and means of control

When considering viruses of potential importance as tools for bioterrorism, several viruses in the Bunya- and Togaviridae families have been cited. Among those in the Bunyaviridae family are Rift Valley fever, Crimean-Congo hemorrhagic fever, hanta, and sandfly fever viruses, listed in order of priority. Those particularly considered in the Togaviridae family are Venezuelan, eastern and western equine encephalitis viruses. Factors affecting the selection of these viruses are the ability for them to induce a fatal or seriously incapacitating illness, their ease of cultivation in order to prepare large volumes, their relative infectivity in human patients, their ability to be transmitted by aerosol, and the lack of measures available for their control. Each factor is fully considered in this review. Vaccines for the control of infections induced by these viruses are in varying stages of development, with none universally accepted to date. Viruses in the Bunyaviridae family are generally sensitive to ribavirin, which has been recommended as an emergency therapy for infections by viruses in this family although has not yet been FDA-approved. Interferon and interferon inducers also significantly inhibit these virus infections in animal models. Against infections induced by viruses in the Togaviridae family, interferon-alpha would appear to currently be the most useful for therapy.

Expression, processing, and immunogenicity of the structural proteins of Venezuelan equine encephalitis virus from recombinant baculovirus vectors

Recombinant baculoviruses expressing the structural proteins of Venezuelan equine encephalitis virus (VEE) have been constructed and the intracellular processing, antigenicity, and immunogenicity of the expression products have been assessed. Baculoviruses expressing the entire structural protein region (C-E3-E2-6K-E1), or the complete glycoprotein region (E3-E2-6K-E1), generated products in Sf9 cells that were accurately and completely processed, and resulted in mature proteins that were antigenically and electrophoretically indistinguishable from authentic viral proteins. These products were highly immunogenic in BALB/c mice, induced efficient VEE neutralizing responses, and protected these animals against challenge with virulent VEE. Expression of individual glycoprotein regions (E3-E2 and 6K-E1) generated products that were accurately but incompletely processed, and induced non-neutralizing antibodies that reacted more efficiently with denatured than native VEE proteins. Nonetheless, immunization with the 6K-E1 expression product provided complete protection against VEE challenge.

Antiviral and immunomodulating inhibitors of experimentally-induced Punta Toro virus infections

A major component of a US Army Medical Research and Development Command-supported program to discover and develop new drugs for the treatment of Rift Valley fever, sandfly fever, and Crimean-Congo hemorrhagic fever has been to study candidate test materials against hepatotropic infections of C57BL/6 mice induced by the related but less biohazardous Punta Toro virus (PTV). The effects of 75 compounds, some of which were considered immunomodulators in their primary mechanism of activity, were studied in the PTV infection model. Of these, ribavirin, ribamidine, ribavirin 2',3',5'-triacetate, tiazofurin, tiazofurin-5'-monophosphate, tiazofurin-2',3',5'-triacetate, selenazofurin, pyrazofurin, 3-deazaguanine, and 3-deazaguanosine were considered significantly inhibitory, acting against the infection by a direct antiviral (non-immunomodulatory) fashion. These compounds had therapeutic indices (TI) ranging from > or = 5 to 65, using increased survivors as the evaluation parameter. Immunomodulators considered significantly inhibitory to this infection were poly (ICLC), ampligen, human recombinant interferon-alpha-A/D, MVE-1, MVE-2, AM-3, AM-5, mannozym, bropirimine, CL246,738, phenyleneamine, and 7-thia-8-oxoguanosine. Utilizing increased survivor numbers as measure of activity, these inhibitors had TI ranging from > or = 16 to 1000. Other antiviral effects exerted by the active compounds included reduction of hepatic icterus, lowered serum glutamic oxaloacetic and pyruvic acid transaminases, and inhibition of recoverable serum and liver virus titers. The active immunomodulators were significantly effective when therapy was initiated as late as 48 h after virus inoculation, at a time when clinical signs of the PTV disease were being manifested in the animal.

Molecular approaches for the treatment of hemorrhagic fever virus infections

Viruses causing hemorrhagic fevers in man belong to the following virus groups: togavirus (Chikungunya), flavivirus (dengue, yellow fever, Kyasanur Forest disease, Omsk hemorrhagic fever), arenavirus (Argentinian hemorrhagic fever, Bolivian hemorrhagic fever, Lassa fever), filovirus (Ebola, Marburg), phlebovirus (Rift Valley fever), nairovirus (Crimian-Congo hemorrhagic fever) and hantavirus (hemorrhagic fever with renal syndrome, nephropathic epidemia). Hemorrhagic fever virus infections can be approached by different therapeutic strategies: (i) vaccination; (ii) administration of high-titered antibodies; and (iii) treatment with antiviral drugs. Depending on the molecular target of their interaction, antiviral agents could be classified as follows: IMP dehydrogenase inhibitors (i.e., ribavirin and its derivatives); OMP decarboxylase inhibitors (i.e., pyrazofurin); CTP synthetase inhibitors (i.e., cyclopentylcytosine and cyclopentenylcytosine); SAH hydrolase inhibitors (i.e., neplanocin A); polyanionic substances (i.e., sulfated polymers); interferon and immunomodulators.

Persistent infection of rabbits with bovine immunodeficiency-like virus

Chronic infection of rabbits was induced by a single intraperitoneal injection of bovine immunodeficiency-like virus (BIV)-infected cells. Ten BIV-infected animals were monitored serologically for up to 2 years. Results of serologic and virus rescue assays indicated that all animals became infected and demonstrated a rapid and sustained BIV-specific humoral response. BIV was rescued by cocultivation from spleen, lymph nodes, and peripheral blood leukocytes of infected animals. Viral DNA in immune tissues was confirmed by polymerase chain reaction amplification of BIV sequences. These data and specific immunohistochemical staining of mononuclear cells of the spleen for BIV antigen suggest that the infection is targeted to immune system cells.

Potential role of immunomodulators for treatment of phlebovirus infections of animals

Rift Valley fever (RVFV) is a major phlebovirus-induced epizootic disease of domestic animals (primarily cattle and sheep) in Africa. No therapies for the disease are known. A related phlebovirus, Punta Toro virus (PTV), has been adapted to induce an RVFV-like disease in C57BL/6 mice. This PTV infection has been used as a model for RVFV because it is reasonably safe and does not require high-level biologic containment. The infection model has been used to study the potential role of immunomodulating substances as therapies. A spectrum of immunomodulators has been studied; those immunomodulators most capable of preventing death and other disease manifestations are ampligen, bropirimine, poly (ICLC), AM-3, P-136, and 7-thia-8-oxoguanosine. An immunologic parameter common to all these substances has been their ability to induce interferon. Timing studies have indicated that these active substances may be administered therapeutically as well as prophylactically to inhibit markedly the progress of the disease. Further work is needed in the development of these materials for use in treating viral infections in domestic animals. As a next step, studies need to be run to compare the immunologic profiles induced by each substance in domestic animals and in mice.

Effect of macrophage source and activation on susceptibility in an age-dependent model of murine hepatitis caused by a phlebovirus, Punta Toro

The Adames strain of a bunyavirus, Punta Toro virus (PTV), is an hepatotrophic virus that has been described to produce an age-dependent lethal hepatic necrosis in 3-4 week old C57BL/6 mice, but 8 week old mice survive with minimal necrosis. The course of PTV infection in vitro in macrophages derived from these mice served as a model to study the pathogenesis of phlebovirus infection. Peripheral blood monocytes, resident or elicited peritoneal macrophages, and Kupffer cell liver macrophages, as well as hepatocytes, were able to support replication of PTV in vitro to a variable extent. Kupffer cells were the only population of macrophages, however, that expressed an age-related ability to affect viral infection and replication in vitro, suggesting that liver macrophages may have a unique modulatory effect on the occurrence and severity of PTV-induced hepatitis in mice. Whereas PTV showed minimal replication in resident peritoneal macrophages, the virus could replicate effectively in peritoneal macrophages elicited by thioglycolate. Activation of peritoneal macrophages with endotoxin resulted in a significant inhibition of intrinsic PTV replication (p less than 0.001), and a modest extrinsic inhibitory effect on PTV replication in cocultured hepatocytes. Both effects persisted in the presence of anti-interferon. These results indicate that the source and state of activation of macrophage/monocyte populations can influence the course of infection in vitro by the phlebovirus, Punta Toro, and can modulate infection in cocultured target cells.

Effect of human, recombinant interleukin 2 on Punta Toro virus infections in C57BL/6 mice

The effect of human recombinant interleukin-2 (rIL-2) on Punta Toro virus (PTV) infection was investigated in C57BL/6 mice. Immunologic and viral parameters were assessed after mice were treated i.p. with rIL-2 for 5 days. Treatment of mice with 25000 and 12500 units/mouse of rIL-2 resulted in significant inhibition of the disease as indicated by increases in survival of mice as well as decreases in liver and serum virus titers. Serum glutamic oxalic acid and pyruvic acid transaminase levels were also lowered indicating reduced liver damage. Murine IL-2 production returned to normal or above-normal levels in rIL-2 treated mice. Natural killer cell activity was also moderately stimulated by rIL-2 treatment. Significant amounts of interferon were not detected in the sera of treated mice. Weight gain and survival rates were similar for both toxicity and normal controls indicating that rIL-2 treatments had no toxic effect.

Prophylactic and therapeutic activities of 7-thia-8-oxoguanosine against Punta Toro virus infections in mice

The biological response modifier 7-thia-8-oxoguanosine was evaluated in mice against the hepatotropic Adames strain of Punta Toro virus. When administered intraperitoneally in divided doses, significant protection from death was conferred at doses of 50 and 100 mg/kg/day given 24 and 17 h pre-virus inoculation, 25-100 mg/kg/day administered 4 h pre- and 3 h post-virus challenge, and 12.5 to 100 mg/kg/day administered 24 and 31 h after virus inoculation. These doses preventing death reduced liver icterus scores, serum alanine aminotransferase and aspartate aminotransferase levels, and liver and serum virus titers relative to placebo controls. Full daily doses administered at 24 h were somewhat less protective to mice than divided daily doses starting at the same time. The initiation of treatment could be delayed as late as 36 h after virus inoculation, resulting in complete protection from mortality at 100 mg/kg/day. This prevention of death occurred despite the acute nature of the infection which resulted in deaths by 96 h in the placebo-treated controls. These results show that 7-thia-8-oxoguanosine has both prophylactic and therapeutic potential as an anti-Phlebovirus agent. Interferon induction appears to be the reason for antiviral activity in this model, since up to 10,000 units of interferon/ml were induced in mice 1 h after treatment with 100 mg 7-thia-8-oxoguanosine per kg, and antibody to interferon alpha/beta administered shortly after treatment with the nucleoside negated the antiviral effect.

Role of hepatocytes and Kupffer cells in age-dependent murine hepatitis caused by a phlebovirus, Punta Toro

Punta Toro virus (PTV) infection of C57BL/6 mice results in fulminant hepatic necrosis and death in 3-week-old susceptible mice, but survival with minimal hepatocellular necrosis in 8-week-old resistant mice. Susceptibility in 3-week-old mice is associated with an earlier rise of viral titers in liver and serum than that occurring in 8-week-old resistant mice. There is also an earlier and more rapid accumulation of infectious progeny in serum vs. liver after PTV infection in both age groups, suggesting that the virus may replicate in extrahepatic sites as well as the liver. PTV infection of isolated hepatocytes and Kupffer cells from 3- and 8-week-old mice demonstrates a significant age-related difference in the ability of these cells to support replication of PTV in vitro (P less than 0.05). The age-related difference in liver cell-PTV interaction appears to be an inherent difference in the liver cells themselves, since there are no age-related differences in viral adsorption, morphogenesis, cytopathic effect, or interferon action within these cells. Thus, age-related differences in PTV replication or dissemination at extrahepatic sites, and the ability of the virus to replicate in intrahepatic sites, may be additive factors in the expression of age-related susceptibility to PTV in C57BL/6 mice.

Pathogenesis of a phleboviral infection (Punta Toro virus) in golden Syrian hamsters

The hamster, Mesocricetus auratus, was examined as a possible model for investigating the poorly defined pathogenesis of the family Bunyaviridae, genus Phlebovirus. Punta Toro virus (PTV) isolates from Eastern Panama were highly virulent for two outbred and five inbred hamster strains, while isolates from western Panama were of low virulence. The Adames strain (eastern Panama) of PTV (LD50 approximately 1 PFU, sc) caused an acute fatal disease (average survival time, 3.8 days) in 10-week-old Lak: LVG (SYR) hamsters. Severe necrosis of the liver, spleen, and small intestine was associated with extensive expression of viral antigen in these organs. The Balliet strain (western Panama) of PTV (LD50 greater than 6 log10 PFU, subcutaneously) caused a mild hepatocellular infection with peak viral liver titers of 3-4 log10 PFU/g compared to 8-9 log10 PFU/g for the Adames strain. We observed histological lesions in the red pulp of the spleen or the lamina propria of the small intestine with the Adames strain. Lesions in the hamsters had characteristics of disseminated intravascular coagulation (DIC). The PTV-hamster model shares similarities to Rift Valley fever (phleboviral disease), which causes fatal disease in man and domesticated ruminants.

Effects of ribamidine, a 3-carboxamidine derivative of ribavirin, on experimentally induced Phlebovirus infections

Ribamidine (1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamidine) was inhibitory in rhesus monkey kidney (LLC-MK2 derivative) cells to Adames and Balliet strains of Punta Toro virus (PTV), a Phlebovirus related to Rift Valley fever and sandfly fever viruses. The 50% effective dose was 8 and 12 micrograms/ml against each respective virus strain; the 50% cytotoxic dose was 320 micrograms/ml, giving selectivity indices of 40 and 27 against each virus strain. The virus ratings were 1.2 and 1.0, respectively. In radiolabel uptake studies, ribamidine had a moderate effect on [3H]leucine uptake at dosages down to 1 microgram/ml, but [3H]thymidine, [32P], and [3H]uridine were inhibited at high (100-1000 micrograms/ml) doses only. Subcutaneous (s.c.) and oral treatments of Adames PTV-infected mice were equally highly effective, as evidenced particularly by 100% survivors. Reduced hepatic icterus, serum oxalic acid transaminase, serum glutamic pyruvic acid transaminase, and recoverable virus titers from livers and sera of infected mice were also seen as a result of ribamidine treatment. Twice daily treatment for 5 days could be started as late as 72 h post-virus inoculation (p.v.i.) with significant inhibition of PTV infection seen. Single s.c. treatments administered as late as 48 h p.v.i. were similarly effective. Using the chronic therapy schedule, the maximum tolerated dose was 1000 mg/kg/day and the minimum effective dose was 31.3 to 62.5 mg/kg/day. Using single treatment, a maximum tolerated dose was greater than 1000 mg/kg, and the minimum effective dose was 125 mg/kg. Ribamidine s.c. treatment of mice infected intracerebrally with the Balliet strain of PTV resulted in a moderate infection-inhibitory effect, seen especially by reduced virus titers in the brains of the infected, treated mice.