A mouse model of aerosol-transmitted orthopoxviral disease: morphology of experimental aerosol-transmitted orthopoxviral disease in a cowpox virus-BALB/c mouse system

Olfactory immunology: the missing piece in airway and CNS defence

The olfactory mucosa is a component of the nasal airway that mediates the sense of smell. Recent studies point to an important role for the olfactory mucosa as a barrier to both respiratory pathogens and to neuroinvasive pathogens that hijack the olfactory nerve and invade the CNS. In particular, the COVID-19 pandemic has demonstrated that the olfactory mucosa is an integral part of a heterogeneous nasal mucosal barrier critical to upper airway immunity. However, our insufficient knowledge of olfactory mucosal immunity hinders attempts to protect this tissue from infection and other diseases. This Review summarizes the state of olfactory immunology by highlighting the unique immunologically relevant anatomy of the olfactory mucosa, describing what is known of olfactory immune cells, and considering the impact of common infectious diseases and inflammatory disorders at this site. We will offer our perspective on the future of the field and the many unresolved questions pertaining to olfactory immunity.

Orally available nucleoside analog UMM-766 provides protection in a murine model of orthopox disease

Although smallpox has been eradicated, other orthopoxviruses continue to be a public health concern as exemplified by the ongoing Mpox (formerly monkeypox) global outbreak. While medical countermeasures (MCMs) previously approved by the Food and Drug Administration for the treatment of smallpox have been adopted for Mpox, previously described vulnerabilities coupled with the questionable benefit of at least one of the therapeutics during the 2022 Mpox outbreak reinforce the need for identifying and developing other MCMs against orthopoxviruses. Here, we screened a panel of Merck proprietary small molecules and identified a novel nucleoside inhibitor with potent broad-spectrum antiviral activity against multiple orthopoxviruses. Efficacy testing of a 7-day dosing regimen of the orally administered nucleoside in a murine model of severe orthopoxvirus infection yielded a dose-dependent increase in survival. Treated animals had greatly reduced lesions in the lung and nasal cavity, particularly in the 10 µg/mL dosing group. Viral levels were also markedly lower in the UMM-766-treated animals. This work demonstrates that this nucleoside analog has anti-orthopoxvirus efficacy and can protect against severe disease in a murine orthopox model.IMPORTANCEThe recent monkeypox virus pandemic demonstrates that members of the orthopoxvirus, which also includes variola virus, which causes smallpox, remain a public health issue. While currently FDA-approved treatment options exist, risks that resistant strains of orthopoxviruses may arise are a great concern. Thus, continued exploration of anti-poxvirus treatments is warranted. Here, we developed a template for a high-throughput screening assay to identify anti-poxvirus small-molecule drugs. By screening available drug libraries, we identified a compound that inhibited orthopoxvirus replication in cell culture. We then showed that this drug can protect animals against severe disease. Our findings here support the use of existing drug libraries to identify orthopoxvirus-targeting drugs that may serve as human-safe products to thwart future outbreaks.

What a Difference a Gene Makes: Identification of Virulence Factors of Cowpox Virus

Cowpox virus (CPXV) is a zoonotic orthopoxvirus (OPV) that causes spillover infections from its animal hosts to humans. In 2009, several human CPXV cases occurred through transmission from pet rats. An isolate from a diseased rat, RatPox09, exhibited significantly increased virulence in Wistar rats and caused high mortality compared to that caused by the mildly virulent laboratory strain Brighton Red (BR). The RatPox09 genome encodes four genes which are absent in the BR genome. We hypothesized that their gene products could be major factors influencing the high virulence of RatPox09. To address this hypothesis, we employed several BR-RatPox09 chimeric viruses. Using Red-mediated mutagenesis, we generated BR-based knock-in mutants with single or multiple insertions of the respective RatPox09 genes. High-throughput sequencing was used to verify the genomic integrity of all recombinant viruses, and transcriptomic analyses confirmed that the expression profiles of the genes that were adjacent to the modified ones were unaltered. While the in vitro growth kinetics were comparable to those of BR and RatPox09, we discovered that a knock-in BR mutant containing the four RatPox09-specific genes was as virulent as the RatPox09 isolate, causing death in over 75% of infected Wistar rats. Unexpectedly, the insertion of gCPXV0030 (g7tGP) alone into the BR genome resulted in significantly higher clinical scores and lower survival rates matching the rate for rats infected with RatPox09. The insertion of gCPXV0284, encoding the BTB (broad-complex, tramtrack, and bric-à-brac) domain protein D7L, also increased the virulence of BR, while the other two open reading frames failed to rescue virulence independently. In summary, our results confirmed our hypothesis that a relatively small set of four genes can contribute significantly to CPXV virulence in the natural rat animal model.IMPORTANCE With the cessation of vaccination against smallpox and its assumed cross-protectivity against other OPV infections, waning immunity could open up new niches for related poxviruses. Therefore, the identification of virulence mechanisms in CPXV is of general interest. Here, we aimed to identify virulence markers in an experimental rodent CPXV infection model using bacterial artificial chromosome (BAC)-based virus recombineering. We focused our work on the recent zoonotic CPXV isolate RatPox09, which is highly pathogenic in Wistar rats, unlike the avirulent BR reference strain. In several animal studies, we were able to identify a novel set of CPXV virulence genes. Two of the identified virulence genes, encoding a putative BTB/POZ protein (CPXVD7L) and a B22R-family protein (CPXV7tGP), respectively, have not yet been described to be involved in CPXV virulence. Our results also show that single genes can significantly affect virulence, thus facilitating adaptation to other hosts.

Dynamics of Pathological and Virological Findings During Experimental Calpox Virus Infection of Common Marmosets (Callithrix jacchus)

Experimental intranasal infection of marmosets (Callithrix jacchus) with calpox virus results in fatal disease. Route and dose used for viral inoculation of the test animals mimics the natural transmission of smallpox, thus representing a suitable model to study pathogenesis and to evaluate new vaccines against orthopoxvirus infection. However, the pathogenic mechanisms leading to death are still unclear. Therefore, our study aimed at investigating the kinetics of pathological alterations to clarify the pathogenesis in calpox virus infection. Following intranasal inoculation with two different viral doses, common marmosets were sacrificed on days 3, 5, 7, 10 and 12 post inoculation. Collected tissue was screened using histopathology, immunohistochemistry, transmission electron microscopy, and virological assays. Our data suggest that primary replication took place in nasal and bronchial epithelia followed by secondary replication in submandibular lymph nodes and spleen. Parallel to viremia at day 7, virus was detectable in many organs, mainly located in epithelial cells and macrophages, as well as in endothelial cells. Based on the onset of clinical signs, the histological and ultrastructural lesions and the immunohistochemical distribution pattern of the virus, the incubation period was defined to last 11 days, which resembles human smallpox. In conclusion, the data indicate that the calpox model is highly suitable for studying orthopoxvirus-induced disease.

Cowpox Virus: A New and Armed Oncolytic Poxvirus

Oncolytic virus therapy has recently been recognized as a promising new therapeutic approach for cancer treatment. In this study, we are proposing for the first time to evaluate the in vitro and in vivo oncolytic capacities of the Cowpox virus (CPXV). To improve the tumor selectivity and oncolytic activity, we developed a thymidine kinase (TK)-deleted CPXV expressing the suicide gene FCU1, which converts the non-toxic prodrug 5-fluorocytosine (5-FC) into cytotoxic 5-fluorouracil (5-FU) and 5-fluorouridine-5′-monophosphate (5-FUMP). This TK-deleted virus replicated efficiently in human tumor cell lines; however, it was notably attenuated in normal primary cells, thus displaying a good therapeutic index. Furthermore, this new recombinant poxvirus rendered cells sensitive to 5-FC. In vivo, after systemic injection in mice, the TK-deleted variant caused significantly less mortality than the wild-type strain. A biodistribution study demonstrated high tumor selectivity and low accumulation in normal tissues. In human xenograft models of solid tumors, the recombinant CPXV also displayed high replication, inducing relevant tumor growth inhibition. This anti-tumor effect was improved by 5-FC co-administration. These results demonstrated that CPXV is a promising oncolytic vector capable of expressing functional therapeutic transgenes.

Evaluation of Taterapox Virus in Small Animals

Taterapox virus (TATV), which was isolated from an African gerbil (Tatera kempi) in 1975, is the most closely related virus to variola; however, only the original report has examined its virology. We have evaluated the tropism of TATV in vivo in small animals. We found that TATV does not infect Graphiurus kelleni, a species of African dormouse, but does induce seroconversion in the Mongolian gerbil (Meriones unguiculatus) and in mice; however, in wild-type mice and gerbils, the virus produces an unapparent infection. Following intranasal and footpad inoculations with 1 × 10⁶ plaque forming units (PFU) of TATV, immunocompromised stat1^−/− mice showed signs of disease but did not die; however, SCID mice were susceptible to intranasal and footpad infections with 100% mortality observed by Day 35 and Day 54, respectively. We show that death is unlikely to be a result of the virus mutating to have increased virulence and that SCID mice are capable of transmitting TATV to C57BL/6 and C57BL/6 stat1^−/− animals; however, transmission did not occur from TATV inoculated wild-type or stat1^−/− mice. Comparisons with ectromelia (the etiological agent of mousepox) suggest that TATV behaves differently both at the site of inoculation and in the immune response that it triggers.

Histopathological and Immunohistochemical Studies of Cowpox Virus Replication in a Three-Dimensional Skin Model

Human cowpox virus (CPXV) infections are rare, but can result in severe and sometimes fatal outcomes. The majority of recent cases were traced back to contacts with infected domestic cats or pet rats. The aim of the present study was to evaluate a three-dimensional (3D) skin model as a possible replacement for animal experiments. We monitored CPXV lesion formation, viral gene expression and cell cycle patterns after infection of 3D skin cultures with two CPXV strains of different pathogenic potential: a recent pet rat isolate (RatPox09) and the reference Brighton red strain. Infected 3D skin cultures exhibited histological alterations that were similar to those of mammal skin infections, but there were no differences in gene expression patterns and tissue damage between the two CPXV strains in the model system. In conclusion, 3D skin cultures reflect the development of pox lesions in the skin very well, but seem not to allow differentiation between more or less virulent virus strains, a distinction that is made possible by experimental infection in suitable animal models.

Highly immunogenic variant of attenuated vaccinia virus

The LIVPΔ6 strain of vaccinia virus (VACV) was created by genetic engineering on the basis of previously obtained attenuated 1421ABJCN strain by target deletion of the A35R gene encoding an inhibitor of antigen presentation by the major histocompatibility complex class II. 1421ABJCN is the LIVP strain of VACV with five inactivated virulence genes encoding hemagglutinin (A56R), γ-interferon-binding protein (B8R), thymidine kinase (J2R), complement-binding protein (C3L), and Bcl2-like inhibitor of apoptosis (N1L). The highly immunogenic LIVPΔ6 strain could be an efficient fourth-generation attenuated vaccine against smallpox and other orthopoxvirus infections.

Biology and pathogenesis of cytomegalovirus in periodontal disease

Human periodontitis is associated with a wide range of bacteria and viruses and with complex innate and adaptive immune responses. Porphyromonas gingivalis, Tannerella forsythia, Aggregatibacter actinomycetemcomitans, Treponema denticola, cytomegalovirus and other herpesviruses are major suspected pathogens of periodontitis, and a combined herpesvirus–bacterial periodontal infection can potentially explain major clinical features of the disease. Cytomegalovirus infects periodontal macrophages and T‐cells and elicits a release of interleukin‐1β and tumor necrosis factor‐α. These proinflammatory cytokines play an important role in the host defense against the virus, but they also have the potential to induce alveolar bone resorption and loss of periodontal ligament. Gingival fibroblasts infected with cytomegalovirus also exhibit diminished collagen production and release of an increased level of matrix metalloproteinases. This article reviews innate and adaptive immunity to cytomegalovirus and suggests that immune responses towards cytomegalovirus can play roles in controlling, as well as in exacerbating, destructive periodontal disease.

Humoral immunity to smallpox vaccines and monkeypox virus challenge: proteomic assessment and clinical correlations

Despite the eradication of smallpox, orthopoxviruses (OPV) remain public health concerns. Efforts to develop new therapeutics and vaccines for smallpox continue through their evaluation in animal models despite limited understanding of the specific correlates of protective immunity. Recent monkeypox virus challenge studies have established the black-tailed prairie dog (Cynomys ludovicianus) as a model of human systemic OPV infections. In this study, we assess the induction of humoral immunity in humans and prairie dogs receiving Dryvax, Acam2000, or Imvamune vaccine and characterize the proteomic profile of immune recognition using enzyme-linked immunosorbent assays (ELISA), neutralization assays, and protein microarrays. We confirm anticipated similarities of antigenic protein targets of smallpox vaccine-induced responses in humans and prairie dogs and identify several differences. Subsequent monkeypox virus intranasal infection of vaccinated prairie dogs resulted in a significant boost in humoral immunity characterized by a shift in reactivity of increased intensity to a broader range of OPV proteins. This work provides evidence of similarities between the vaccine responses in prairie dogs and humans that enhance the value of the prairie dog model system as an OPV vaccination model and offers novel findings that form a framework for examining the humoral immune response induced by systemic orthopoxvirus infection.

Clinical course and pathology in rats (Rattus norvegicus) after experimental cowpox virus infection by percutaneous and intranasal application

Recently, several cases of human cowpox virus (CPXV) infections were reported in France and Germany, which had been acquired through close contact with infected pet rats. The animals exhibited respiratory signs or skin lesions and died shortly after purchase. After natural infection of white rats with CPXV in the USSR in 1978, a peracute pulmonary form, a milder dermal form, and a mixed form exhibiting features of both have been described. To the best of the authors' knowledge, 3 experimental cowpox virus infection studies using rats have been performed to date; however, neither results of histomorphological examinations nor immunohistochemical analyses have yet been reported in rats after experimental infections. To investigate the impact of the infection route on the clinical course, the development of lesions, and tropism, rats were infected intradermally, intranasally, or by a combination of both routes. The authors found a correlation between clinical manifestation, pathology, and infection routes. Intradermal and contact exposure yielded a mild dermal form, characterized by the development of vesiculopustular dermatitis. In contrast, intranasally infected animals died peracutely, showing severe dyspnea. Occasionally, a combination of the dermal and the respiratory form occurred after intranasal infection. Immunohistochemically, CPXV antigen was detected in the epithelial and mesenchymal cells of the upper respiratory tract and affected skin lesions and rarely in mesenchymal cells of lymph nodes. This is the first histomorphological and immunohistochemical analysis of CPXV in rats after experimental infection.

Intrabronchial inoculation of cynomolgus macaques with cowpox virus

The public health threat of orthopoxviruses from bioterrorist attacks has prompted researchers to develop suitable animal models for increasing our understanding of viral pathogenesis and evaluation of medical countermeasures (MCMs) in compliance with the FDA Animal Efficacy Rule. We present an accessible intrabronchial cowpox virus (CPXV) model that can be evaluated under biosafety level-2 laboratory conditions. In this dose-ranging study, utilizing cynomolgus macaques, signs of typical orthopoxvirus disease were observed with the lymphoid organs, liver, skin (generally mild) and respiratory tract as target tissues. Clinical and histopathological evaluation suggests that intrabronchial CPXV recapitulated many of the features of monkeypox and variola virus, the causative agent of smallpox, infections in cynomolgus macaque models. These similarities suggest that CPXV infection in non-human primates should be pursued further as an alternative model of smallpox. Further development of the CPXV primate model, unimpeded by select agent and biocontainment restrictions, should facilitate the development of MCMs for smallpox.

Deletion of major nonessential genomic regions in the vaccinia virus Lister strain enhances attenuation without altering vaccine efficacy in mice

The vaccinia virus (VACV) Lister strain was one of the vaccine strains that enabled smallpox eradication. Although the strain is most often harmless, there have been numerous incidents of mild to life-threatening accidents with this strain and others. In an attempt to further attenuate the Lister strain, we investigated the role of 5 genomic regions known to be deleted in the modified VACV Ankara (MVA) genome in virulence in immunodeficient mice, immunogenicity in immunocompetent mice, and vaccine efficacy in a cowpox virus challenge model. Lister mutants were constructed so as to delete each of the 5 regions or various combinations of these regions. All of the mutants replicated efficiently in tissue culture except region I mutants, which multiplied more poorly in human cells than the parental strain. Mutants with single deletions were not attenuated or only moderately so in athymic nude mice. Mutants with multiple deletions were more highly attenuated than those with single deletions. Deleting regions II, III, and V together resulted in total attenuation for nude mice and partial attenuation for SCID mice. In immunocompetent mice, the Lister deletion mutants induced VACV specific humoral responses equivalent to those of the parental strain but in some cases lower cell-mediated immune responses. All of the highly attenuated mutants protected mice from a severe cowpox virus challenge at low vaccine doses. The data suggest that several of the Lister mutants combining multiple deletions could be used in smallpox vaccination or as live virus vectors at doses equivalent to those used for the traditional vaccine while displaying increased safety.

N1L is an ectromelia virus virulence factor and essential for in vivo spread upon respiratory infection

The emergence of zoonotic orthopoxvirus infections and the threat of possible intentional release of pathogenic orthopoxviruses have stimulated renewed interest in understanding orthopoxvirus infections and the resulting diseases. Ectromelia virus (ECTV), the causative agent of mousepox, offers an excellent model system to study an orthopoxvirus infection in its natural host. Here, we investigated the role of the vaccinia virus ortholog N1L in ECTV infection. Respiratory infection of mice with an N1L deletion mutant virus (ECTVΔN1L) demonstrated profound attenuation of the mutant virus, confirming N1 as an orthopoxvirus virulence factor. Upon analysis of virus dissemination in vivo, we observed a striking deficiency of ECTVΔN1L spreading from the lungs to the livers or spleens of infected mice. Investigating the immunological mechanism controlling ECTVΔN1L infection, we found the attenuated phenotype to be unaltered in mice deficient in Toll-like receptor (TLR) or RIG-I-like RNA helicase (RLH) signaling as well as in those missing the type I interferon receptor or lacking B cells. However, in RAG-1(-/-) mice lacking mature B and T cells, ECTVΔN1L regained virulence, as shown by increasing morbidity and virus spread to the liver and spleen. Moreover, T cell depletion experiments revealed that ECTVΔN1L attenuation was reversed only by removing both CD4(+) and CD8(+) T cells, so the presence of either cell subset was still sufficient to control the infection. Thus, the orthopoxvirus virulence factor N1 may allow efficient ECTV infection in mice by interfering with host T cell function.

Treatment of Vaccinia and Cowpox Virus Infections in Mice with CMX001 and ST-246

Although a large number of compounds have been identified with antiviral activity against orthopoxviruses in tissue culture systems, it is highly preferred that these compounds have activity in vivo before they can be seriously considered for further development. One of the most commonly used animal models for the confirmation of this activity has been the use of mice infected with either vaccinia or cowpox viruses. These model systems have the advantage that they are relatively inexpensive, readily available and do not require any special containment facilities; therefore, relatively large numbers of compounds can be evaluated in vivo for their activity. The two antiviral agents that have progressed from preclinical studies to human safety trials for the treatment of orthopoxvirus infections are the cidofovir analog, CMX001, and an inhibitor of extracellular virus formation, ST-246. These compounds are the ones most likely to be used in the event of a bioterror attack. The purpose of this communication is to review the advantages and disadvantages of using mice infected with vaccinia and cowpox virus as surrogate models for human orthopoxvirus infections and to summarize the activity of CMX001 and ST-246 in these model infections.

Development of ST-246® for Treatment of Poxvirus Infections

ST-246 (Tecovirimat) is a small synthetic antiviral compound being developed to treat pathogenic orthopoxvirus infections of humans. The compound was discovered as part of a high throughput screen designed to identify inhibitors of vaccinia virus-induced cytopathic effects. The antiviral activity is specific for orthopoxviruses and the compound does not inhibit the replication of other RNA- and DNA-containing viruses or inhibit cell proliferation at concentrations of compound that are antiviral. ST-246 targets vaccinia virus p37, a viral protein required for envelopment and secretion of extracellular forms of virus. The compound is orally bioavailable and protects multiple animal species from lethal orthopoxvirus challenge. Preclinical safety pharmacology studies in mice and non-human primates indicate that ST-246 is readily absorbed by the oral route and well tolerated with the no observable adverse effect level (NOAEL) in mice measured at 2000 mg/kg and the no observable effect level (NOEL) in non-human primates measured at 300 mg/kg. Drug substance and drug product processes have been developed and commercial scale batches have been produced using Good Manufacturing Processes (GMP). Human phase I clinical trials have shown that ST-246 is safe and well tolerated in healthy human volunteers. Based on the results of the clinical evaluation, once a day dosing should provide plasma drug exposure in the range predicted to be antiviral based on data from efficacy studies in animal models of orthopoxvirus disease. These data support the use of ST-246 as a therapeutic to treat pathogenic orthopoxvirus infections of humans.

Animal models of orthopoxvirus infection

Smallpox was one of the most devastating diseases known to humanity. Although smallpox was eradicated through a historically successful vaccination campaign, there is concern in the global community that either Variola virus (VARV), the causative agent of smallpox, or another species of Orthopoxvirus could be used as agents of bioterrorism. Therefore, development of countermeasures to Orthopoxvirus infection is a crucial focus in biodefense research, and these efforts rely on the use of various animal models. Smallpox typically presented as a generalized pustular rash with 30 to 40% mortality, and although smallpox-like syndromes can be induced in cynomolgus macaques with VARV, research with this virus is highly restricted; therefore, animal models with other orthopoxviruses have been investigated. Monkeypox virus causes a generalized vesiculopustular rash in rhesus and cynomolgus macaques and induces fatal systemic disease in several rodent species. Ectromelia virus has been extensively studied in mice as a model of orthopoxviral infection in its natural host. Intranasal inoculation of mice with some strains of vaccinia virus produces fatal bronchopneumonia, as does aerosol or intranasal inoculation of mice with cowpox virus. Rabbitpox virus causes pneumonia and fatal systemic infections in rabbits and can be naturally transmitted between rabbits by an aerosol route similar to that of VARV in humans. No single animal model recapitulates all known aspects of human Orthopoxvirus infections, and each model has its advantages and disadvantages. This article provides a brief review of the Orthopoxvirus diseases of humans and the key pathologic features of animal models of Orthopoxvirus infections.

Phenotypic and genetic diversity of the traditional Lister smallpox vaccine

As an initial step in the development of a second-generation smallpox vaccine derived from the Lister strain, to be prepared for a variola virus threat, diversity of the traditional vaccine was examined by characterizing a series of ten viral clones. In vitro and in vivo phenotypic studies showed that the biological behavior of the clones diverged from each other and in most cases diverged from the vaccinia virus (VACV) Lister parental population. Taken together, these results demonstrate the heterogeneity of the viral population within the smallpox vaccine and highlight the difficulty in choosing one clone which would meet the current requirements for a safe and effective vaccine candidate.

The role of the cowpox virus crmA gene during intratracheal and intradermal infection of C57BL/6 mice

Intratracheal (i.t.) infection of mice with cowpox virus (CPXV), is lethal at a lower dose than intranasal (i.n.) inoculation. CPXV deleted for cytokine response modifier A (CPXVDeltacrmA) was attenuated compared to CPXV after i.t. inoculation. This attenuation could not be attributed to differences in virus replication, immunomodulators, or cells infiltrating the lungs. Deletion of crmA also caused attenuation during intradermal (i.d.) infection. In contrast to i.t.-inoculated virus, deletion of crmA reduced virus replication at the site of infection. This difference correlated to increased numbers of CD3(+) cells in CPXVDeltacrmA-associated dermal lesions. Thus, crmA is a virulence factor in mice during either pulmonary or dermal cowpox infection; however the influence of crmA is more evident during i.d. inoculation. This suggests that the host immune response differs in the two routes of infection and emphasizes the need to consider the effect of route of infection when examining functions of virulence factors in vivo.

Progress in the Discovery of Compounds Inhibiting Orthopoxviruses in Animal Models

Surrogate animal models must be used for testing antiviral agents against variola (smallpox) virus infections. Once developed, these compounds can be stockpiled for use in the event of a bioterrorist incident involving either variola or monkeypox virus, or used to treat an occasional serious orthopoxvirus infection, such as disseminated vaccinia complication following expo-sure to the live virus vaccine. Recently, considerable progress has been made in the discovery of novel anti-viral agents found active against orthopoxviruses in vivo. This includes the development of new animal models or refinement of existing ones for compound efficacy testing. Current mouse models employ ectromelia, cowpox and vaccinia (WR and IHD strains) viruses with respiratory (lung) or tail lesion infections commonly studied. Rabbitpox and vaccinia (WR strain) viruses are available for rabbit infections. Monkeypox and variola viruses are used for infecting monkeys. This review describes these and other animal models, and covers compounds found active in vivo from 2003 to date. Cidofovir, known to be active against orthopox virus infections prior to 2003, has been studied extensively over recent years. New compounds showing promise are orally active inhibitors of orthopoxvirus infections that include ether lipid prodrugs of cidofovir and ( S)-HPMPA, ST-246, N-meth-anocarbathymidine ( N-MCT) and SRI 21950 (a 4'-thio derivative of iododeoxyuridine). Another compound with high activity but requiring parenteral administration is HPMPO-DAPy. Further development of these compounds is warranted.

Differential pathogenesis of cowpox virus intranasal infections in mice induced by low and high inoculum volumes and effects of cidofovir treatment

The causes of death from intranasal cowpox virus infections in mice remain unclear. Hypotheses include severe pneumonitis, hepatitis and/or hyperproduction of cytokines and chemokines. This work explores these hypotheses by studying the influence of low- and high-volume virus inocula on viral pathogenesis. BALB/c mice were infected intranasally with a syncytium-forming variant of cowpox virus in 5 microL or 50 microL volumes containing the same infectious virus challenge dose. The 50 microL infection produced a more rapidly lethal disease associated with severe pneumonitis, high lung and nasal virus titres and increased cytokine and chemokine levels in the lungs and nasal tissue, whilst liver infection was minimal. The 5 microL inoculum infection was also lethal, but the infection was primarily confined to the upper respiratory tract and included elevated nasal cytokine and chemokine levels. Levels of the pro-inflammatory cytokine interleukin-6 were particularly high in both infections. Treatment of the infections with cidofovir (100mg/kg/day for 2 days starting 24h after virus exposure) led to survival and suppression of tissue virus titres. Treatment reduced pneumonitis in the 50 microL infection and lessened cytokine hyperproduction in both infections. We conclude that a 5 microL volume inoculum of cowpox virus causes a lethal upper respiratory tract infection, whilst the 50 microL inoculum targets both upper and lower respiratory tracts, with excessive release of systemic pro-inflammatory factors. Cidofovir effectively treated both infections and slowed viral replication sufficiently to subdue the exaggerated release of pro-inflammatory mediators.

Cowpox virus exploits the endoplasmic reticulum retention pathway to inhibit MHC class I transport to the cell surface

Major histocompatibility complex (MHC) class I molecules assemble with peptides in the ER lumen and are transported via Golgi to the plasma membrane for recognition by T cells. Inhibiting MHC assembly, transport, and surface expression are common viral strategies of evading immune recognition. Cowpox virus, a clinically relevant orthopoxvirus, downregulates MHC class I expression on infected cells. However, the viral protein(s) and mechanisms responsible are unknown. We identify CPXV203 as a cowpox virus protein that associates with fully assembled MHC class I molecules and blocks their transport through the Golgi. A C-terminal KTEL motif in CPXV203 closely resembles the canonical ER retention motif KDEL and is required for CPXV203 function, indicating that a physiologic pathway is exploited to retain MHC class I in the ER. This viral mechanism for MHC class I downregulation may explain virulence differences between clinical isolates of orthopoxviruses.

Intranasal cowpox virus infection of the mouse as a model for preclinical evaluation of smallpox vaccines

The intranasal infection of mice with cowpox virus (CPXV) has been evaluated as a model for smallpox infection in man. Administration of a lethal dose of CPXV allowed time for development of T-cell responses but antibodies could not be detected before death occurred. In contrast, infection with a sublethal dose was associated with an early T-cell response followed by neutralising antibodies which correlated with virus clearance. Comparison of two first generation smallpox vaccines revealed no significant differences in terms of immunogenicity, protection and post-challenge virus clearance. These studies show that the CPXV/mouse model is valuable for the initial assessment of smallpox vaccines.

Comparative study of pathological changes in the mouse viscera in infection caused by two orthopoxviruses

The mechanisms of infection development in intraperitoneal inoculation of mice by ectromelia virus strain K-1 and cowpox strain EP-2 were studied. Ultrastructural parameters of virus assembly and maturation are described. Differences in the types of cells replicating the viruses and in the type of visceral injuries were detected. The studies showed a local type of strain EP-2 cowpox infection and dissemination of ectromelia strain K-1.

Smallpox antiviral drug development: satisfying the animal efficacy rule

Concerns over the potential use of variola virus as a biological weapon have prompted new interest in the development of small molecule therapeutics to prevent and treat smallpox infection. Since smallpox is no longer endemic, human clinical trials designed to link antiviral efficacy to clinical outcome have been supplanted by antiviral efficacy evaluations in animal models of orthopoxvirus disease. This poses a unique challenge for drug development; how can animal efficacy data with a surrogate virus be used to establish clinical correlates predictive of human disease outcome? This review will examine the properties of selected animal models that are being used to evaluate poxvirus antiviral drug candidates, and discuss how data from these models can be used to link drug efficacy to clinical correlates of human disease.

Enhanced immunogenicity and protective effect conferred by vaccination with combinations of modified vaccinia virus Ankara and licensed smallpox vaccine Dryvax in a mouse model

Significant adverse events are associated with vaccination with the currently licensed smallpox vaccine. Candidate new-generation smallpox vaccines such as the replication-defective modified vaccinia virus Ankara (MVA) produce very few adverse events in experimental animals and in limited human clinical trials conducted near the end of the smallpox eradication campaign. Efficacy evaluation of such new-generation vaccines will be extraordinarily complex, however, since the eradication of smallpox precludes a clinical efficacy trial and the correlates of protection against smallpox are unknown. A combination of relevant animal efficacy studies along with thorough comparative immunogenicity studies between traditional and new-generation smallpox vaccines will be necessary for vaccine licensure. In the present study, a variety of immune responses elicited by MVA and the licensed smallpox vaccine Dryvax in a murine model were compared, with a focus on mimicking conditions and strategies likely to be employed in human vaccine trials. Immunization of mice with MVA, using several relevant vaccination routes including needle-free delivery, elicited humoral and cellular immune responses qualitatively similar to those elicited by vaccination with Dryvax. Similar levels of vaccinia-specific IgG and neutralizing antibody were elicited by Dryvax and MVA when higher doses (approximately 1 log) of MVA were used for immunization. Antibody levels peaked at about 6 weeks post-immunization and remained stable for at least 15 weeks. A booster immunization of either MVA or Dryvax following an initial priming immunization with MVA resulted in an enhanced IgG titer and neutralizing antibody response. In addition, both Dryvax and various MVA vaccination protocols elicited antibody responses to the extracellular enveloped form of the virus and afforded protection against a lethal intranasal challenge with vaccinia virus WR.

Bioluminescence imaging of vaccinia virus: effects of interferon on viral replication and spread

Whole animal imaging allows viral replication and localization to be monitored in intact animals, which provides significant advantages for determining viral and host factors that determine pathogenesis. To investigate effects of interferons on spatial and temporal progression of vaccinia infection, we generated recombinant viruses that express firefly luciferase or a monomeric orange fluorescent protein. These viruses allow vaccinia infection to be monitored with bioluminescence or fluorescence imaging, respectively. The recombinant viruses were not attenuated in vitro or in vivo relative to a control WR virus. In cell culture, reporters could be detected readily by 4 h post-infection, showing that these viruses can be used as early markers of infection. The magnitude of firefly luciferase activity measured with bioluminescence imaging in vitro and in vivo correlated directly with increasing titers of vaccinia virus, validating imaging data as a marker of viral infection. Replication of vaccinia was significantly greater in mice lacking receptors for type I interferons (IFN I R-/-) compared with wild-type mice, although both genotypes of mice developed focal infections in lungs and brain after intranasal inoculation. IFN I R-/- mice had greater dissemination of virus to liver and spleen than wild-type animals even when mortality occurred at the same time point after infection. Protective effects of type I interferons were mediated primarily through parenchymal cells rather than hematopoietic cells as analyzed by bone marrow transplant experiments. Collectively, our data define a new function for type I interferon signaling in systemic dissemination of vaccinia and validate these reporter viruses for studies of pathogenesis.

Effects of deletions of kelch-like genes on cowpox virus biological properties

Cowpox virus (CPXV) strain GRI-90 contains six genes encoding kelch-like proteins. All six proteins contain both, the N-terminal BTB domain and the C-terminal kelch domain. We constructed mutant variants of a CPXV strain with targeted deletions of one to four genes of the kelch family, namely D11L, C18L, G3L, and A57R. As kelch genes are located in terminal variable regions of the CPXV genome, we studied the relationship of these genes with integral biological characteristics such as virulence, host range, reproduction in vitro and in ovo (in chicken embryos). It was demonstrated that the following effects occurred in a gene dose dependent manner with an increase of the number of genes deleted: (1) range of sensitive cells altered--deletion mutants lacking three genes displayed a considerably decreased ability to reproduce in MDCK cells; mutants lacking four genes lost this ability completely; (2) analysis of pocks formed by mutants with deletion of three and four kelch-like genes on chorioallantoic membranes of chicken embryos demonstrated that pock size and virus yield were significantly decreased; (3) light microscopic analysis of the pocks revealed impaired proliferation and reduced vascularisation in the pock region. More alterations were detected by electron microscopic analysis: the reproduction of mutants results in a reduction of the number of mature virions formed, and in many cells this process was arrested at the stage of assembly of immature virions; and (4) the evaluation of LD(50) and body weight loss in BALB/c mice infected intranasally with CPXVs revealed a reduction of the virulence of the deletion mutants, which became statistically significant when four kelch-like genes were excised.

Effects of four antiviral substances on lethal vaccinia virus (IHD strain) respiratory infections in mice

Intranasal infection of BALB/c mice with the IHD strain of vaccinia virus was found to cause pneumonia, profound weight loss and death. Cidofovir, hexadecyloxypropyl-cidofovir (HDP-CDV), the diacetate ester prodrug of 2-amino-7-[(1,3-dihydroxy-2-propoxy)methyl]purine (HOE961), and ribavirin were used to treat the infections starting 24h after virus exposure. Single intraperitoneal (i.p.) cidofovir treatments of 100 and 30 mg/kg led to 90-100% survival compared with no survivors in the placebo group, whereas a 10 mg/kg dose was ineffective. The 100 mg/kg treatment reduced lung and snout virus titres on day 3 of the infection by 20- and 8-fold, respectively. Mean arterial oxygen saturation levels in these two cidofovir treatment groups were significantly higher than placebo on days 4 through 6 of the infection, indicating an improvement in lung function. Effects of cidofovir on viral pathogenesis were studied on days 1, 3 and 5 of the infection, and demonstrated statistically significant reductions in lung consolidation scores, lung weights, lung virus titre and snout virus titres on days 3 and 5. Cidofovir treatment also reduced virus titres in other tissues and body fluid, including blood, brain, heart, liver, salivary gland and spleen. HDP-CDV was given by oral gavage at 100, 50 and 25mg/kg doses one time only, resulting in 80-100% survival. Lower daily oral doses of 10 and 5mg/kg per day given for 5 days protected only 30% of animals from death. Oral doses (100, 50 and 25 mg/kg per day) of HOE961 for 5 days protected all animals, whereas equivalent oral doses of ribavirin were completely ineffective. The rapidity of recovery from weight loss during the infection was a function of dose of compound administered. These data indicate the utility of parenteral cidofovir, oral HDP-CDV and oral HOE961 in treating severe respiratory infections caused by this virus.

Exploring the potential of variola virus infection of cynomolgus macaques as a model for human smallpox

Smallpox virus (variola) poses a significant threat as an agent of bioterrorism. To mitigate this risk, antiviral drugs and an improved vaccine are urgently needed. Satisfactory demonstration of protective efficacy against authentic variola will require development of an animal model in which variola produces a disease course with features consistent with human smallpox. Toward this end, cynomolgus macaques were exposed to several variola strains through aerosol and/or i.v. routes. Two strains, Harper and India 7124, produced uniform acute lethality when inoculated i.v. in high doses (10(9) plaque-forming units). Lower doses resulted in less fulminant, systemic disease and lower mortality. Animals that died had profound leukocytosis, thrombocytopenia, and elevated serum creatinine levels. After inoculation, variola was disseminated by means of a monocytic cell-associated viremia. Distribution of viral antigens by immunohistochemistry correlated with the presence of replicating viral particles demonstrated by electron microscopy and pathology in the lymphoid tissues, skin, oral mucosa, gastrointestinal tract, reproductive system, and liver. These particles resembled those seen in human smallpox. High viral burdens in target tissues were associated with organ dysfunction and multisystem failure. Evidence of coagulation cascade activation (D dimers) corroborated histologic evidence of hemorrhagic diathesis. Depletion of T cell-dependent areas of lymphoid tissues occurred, probably as a consequence of bystander apoptotic mechanisms initiated by infected macrophages. Elaboration of cytokines, including IL-6 and IFN-gamma, contribute to a cytokine storm formerly known as "toxemia." A more precise understanding of disease pathogenesis should provide targets for therapeutic intervention, to be used alone or in combination with inhibitors of variola virus replication.

Efficacy of multiple- or single-dose cidofovir against vaccinia and cowpox virus infections in mice

Orthopoxviruses, including variola and monkeypox, pose risks to human health through natural transmission or potential bioterrorist activities. Since vaccination has not recently been utilized for control of these infections, there is renewed effort in the development of antiviral agents not only for postexposure smallpox therapy but also for treatment of adverse reactions following vaccination. The objectives of this study were to expand on the results of others that cidofovir (CDV) is effective in mice inoculated with cowpox virus (CV) or vaccinia virus (VV) and to document the efficacy of single and interval dosing beginning prior to or after infection, particularly including evaluations using suboptimal doses of CDV. We utilized BALB/c or SCID mice inoculated with CV or VV as models for systemic poxvirus infections. BALB/c mice were inoculated intranasally with CV or VV and treated with CDV prior to or after virus inoculation. CDV, at concentrations as low as 0.7 to 6.7 mg/kg of body weight/day for 5 days, conferred significant protection when treatment was initiated as late as 72 to 96 h postinfection. A single-dose pretreatment or posttreatment with CDV at 3 to 100 mg/kg was effective when given as early as 5 days prior to infection or as late as 3 days after infection with either VV or CV. Interval treatments given every third day beginning 72 h postinfection using 6.7 or 2 mg of CDV/kg also proved effective against CV infections. When SCID mice were inoculated intraperitoneally with CV or VV and treated for 7 to 30 days with CDV, all the mice eventually died during or after cessation of treatment; however, significant delays in time to death and reduction of virus replication in organs occurred in most treated groups, and no resistance to CDV was detected.

Clonal vaccinia virus grown in cell culture as a new smallpox vaccine

Although the smallpox virus was eradicated over 20 years ago, its potential release through bioterrorism has generated renewed interest in vaccination. To develop a modern smallpox vaccine, we have adapted vaccinia virus that was derived from the existing Dryvax vaccine for growth in a human diploid cell line. We characterized six cloned and one uncloned vaccine candidates. One clone, designated ACAM1000, was chosen for development based on its comparability to Dryvax when tested in mice, rabbits and monkeys for virulence and immunogenicity. By most measures, ACAM1000 was less virulent than Dryvax. We compared ACAM1000 and Dryvax in a randomized, double-blind human clinical study. The vaccines were equivalent in their ability to produce major cutaneous reactions ('takes') and to induce neutralizing antibody and cell-mediated immunity against vaccinia virus.

Aerosolized cidofovir is retained in the respiratory tract and protects mice against intranasal cowpox virus challenge

We employed a murine model to test the concept of using an aerosolized, long-acting antiviral drug to protect humans against smallpox. We previously showed that a low dose of aerosolized cidofovir (HPMPC [Vistide]) was highly protective against subsequent aerosolized cowpox virus challenge and was more effective than a much larger dose of drug given by injection, suggesting that aerosolized cidofovir is retained in the lung. Because the nephrotoxicity of cidofovir is a major concern in therapy, delivering the drug directly to the respiratory tract might be an effective prophylactic strategy that maximizes the tissue concentration at the site of initial viral replication, while minimizing its accumulation in the kidneys. In the present study, we found that treating mice with aerosolized (14)C-labeled cidofovir ((14)C-cidofovir) resulted in the prolonged retention of radiolabeled drug in the lungs at levels greatly exceeding those in the kidneys. In contrast, subcutaneous injection produced much higher concentrations of (14)C-cidofovir in the kidneys than in the lungs over the 96-h time course of the study. As further evidence of the protective efficacy of aerosolized cidofovir, we found that aerosol treatment before or after infection was highly protective in mice challenged intranasally with cowpox virus. All or nearly all mice that were treated once by aerosol, from 2 days before to 2 days after challenge, survived intranasal infection, whereas all placebo-treated animals died.

Esterification of cidofovir with alkoxyalkanols increases oral bioavailability and diminishes drug accumulation in kidney

Smallpox was eradicated by vaccination in the 1970s. However, concerns have arisen about the potential use of variola virus as a biological weapon. Most of the world's population has little residual immunity because systematic vaccination against smallpox ceased in the early 1970s. Vaccination of key elements of the population against smallpox is again being considered. However, there are now large numbers of persons who cannot be safely vaccinated with the current vaccine because of AIDS, immunosuppressive drugs, and certain common skin disorders. It would be useful to have a potent orally active drug as an alternative for these persons in case of an outbreak of smallpox. Alkoxyalkyl esters of cidofovir (CDV) have been shown to be highly active and selective against poxviruses in vitro with activities several logs greater than the activity of unmodified CDV. This is due in large part to increased cellular penetration and conversion to CDV-diphosphate, the active antiviral. In this paper, the oral pharmacokinetics of 14C-labeled hexadecyloxypropyl-cidofir (HDP-CDV), octadecyloxyethyl-cidofir (ODP-CDV), and oleyloxypropyl-cidofir (OLP-CDV) are examined and oral bioavailability and tissue distribution assessed and compared with parenteral CDV. The alkoxyalkyl CDVs are highly orally bioavailable and do not concentrate in kidney, the site of the dose-limiting toxicity of CDV. Plasma and tissue drug levels are many times greater than the in vitro EC(50s) for variola, cowpox, and vaccinia viruses. Thus, the compounds are good candidates for further development for prevention and treatment of smallpox infection and the complications of vaccination.

A review of compounds exhibiting anti-orthopoxvirus activity in animal models

Several animal models using mice (most frequently), rabbits, or monkeys have been used to identify compounds active against orthopoxvirus infections. The treatment of vaccinia virus infections has been well studied in models involving infection of scarified skin or eyes, or resulting from intravenous, intraperitoneal, intracerebral, or intranasal virus inoculation. Cowpox virus has been used in intranasal or aerosol infection studies to evaluate the treatment of lethal respiratory infections. Rabbitpox, monkeypox, and variola viruses have been employed to a lesser extent than the other viruses in chemotherapy experiments. A review of the literature over the past 50 years has identified a number of compounds effective in treating one or more of these infections, which include thiosemicarbazones, nucleoside and nucleotide analogs, interferon, interferon inducers, and other unrelated compounds. Substances that appear to have the greatest potential as anti-orthopoxvirus agents are the acyclic nucleotides, (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (cidofovir, HPMPC) and 1-[((S)-2-hydroxy-2-oxo-1,4,2-dioxaphosphorinan-5-yl)methyl]cytosine (cyclic HPMPC), and the acyclic nucleoside analog, 2-amino-7-[(1,3-dihydroxy-2-propoxy)methyl]purine (S2242). Other classes of compounds that have not been sufficiently studied in lethal infection models and deserve further consideration are thiosemicarbazones related to methisazone, and analogs of adenosine-N(1)-oxide and 1-(benzyloxy)adenosine.

Poxvirus immune modulators: functional insights from animal models

Poxviruses express several different classes of immune modulators that suppress the host response to infection, including soluble cytokine binding proteins, serpins, chemokine binding proteins, a complement control protein, and members of the semaphorin and Toll/IL-1 receptor families. Biochemical activity of these proteins has been demonstrated by many in vitro studies. Conservation in evolution of poxvirus immune modulators implies that these genes are functional in vivo, but the results of infecting animals with knockout viruses have not always been clear cut. Studies involving different animal models are reviewed, and the criteria for suitable models are discussed. Challenges include finding an appropriate animal host, and using an inoculation route that resembles the process of natural infection. The fact that multiple immune modulators can target the same pathway at different steps may explain why single knockout mutants are not always attenuated in animals.

Antivaccinia activities of acyclic nucleoside phosphonate derivatives in epithelial cells and organotypic cultures

Organotypic "raft" cultures of epithelial cells allow the reconstitution of a skin equivalent that is easily infectible with different viruses with cutaneous tropism. Among these, poxvirus and particularly vaccinia virus (VV) are good candidates for use in antiviral tests, giving histological pictures comparable to those observed in humans infected with smallpox. Therefore, we decided to evaluate a series of phosphonate derivatives for their ability to inhibit VV growth in epithelial cell monolayers, and the most powerful derivatives were tested in the organotypic cultures. The most active compound was 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]adenine [(S)-HPMPA], followed by 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]-2,6-diaminopurine, cyclic (S)-HPMPA, 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine [(S)-HPMPC; cidofovir, Vistide], and cyclic (S)-HPMPC. Cidofovir, which is on the market for the treatment of human cytomegalovirus retinitis in immunocompromised patients, is potentially a good candidate for the treatment of a poxvirus outbreak, in the absence of any vaccination.

Effect of 5-iodo-2'-deoxyuridine on vaccinia virus (orthopoxvirus) infections in mice

There is a concern that there may be unregistered stocks of smallpox that can be used for bioterrorism or biological warfare. According to the WHO Advisory Committee on Variola Research, there is a need to develop strategies to treat smallpox infections should they reappear. It would also be important to have an effective drug at hand for the treatment of monkeypox disease in humans. We show here that 5-iodo-2'-deoxyuridine (IDU) is a potent inhibitor of vaccinia virus (VV) replication and that IDU inhibits VV DNA synthesis in a dose-dependent way. The in vivo protective effect of IDU was assessed in the VV tail lesion model in immunocompetent mice and in a lethal model for VV infection in SCID (severe combined immune deficiency) mice that had been infected either intranasally, intraperitoneally, or intravenously. Subcutaneous treatment with IDU at 150 and 100 mg/kg of body weight markedly reduced the number of tail lesions in immunocompetent NMRI mice. Untreated intranasally VV-infected SCID mice died at 20.8 +/- 3.1 days after infection (mean +/- standard deviation). Treatment with IDU (subcutaneously, 150 mg/kg/day [from day 0 to 4] and 75 mg/kg/day [from day 6 to 11]) delayed-virus induced mortality by 15 days (mean day of death +/- standard deviation, 35.8 +/- 6.7; P < 0.0001). This protective effect was associated with (i) an improvement of lung histology and (ii) a marked reduction in lung viral titers. IDU also delayed VV-induced mortality when mice had either been infected intraperitoneally or intravenously. Even when the start of treatment with IDU (in intraperitoneally VV-infected mice) was postponed until 2 or 4 days after infection, an important delay in virus-induced mortality was noted.

Treatment of aerosolized cowpox virus infection in mice with aerosolized cidofovir

The Brighton strain of cowpox virus causes lethal bronchopneumonia when delivered as a small-particle (1 microm) aerosol to weanling BALB/c mice. We showed previously that this disease can be prevented or cured with one subcutaneous injection of cidofovir (HPMPC, Vistide). To determine whether even better results could be obtained by delivering the drug directly to the respiratory tract, we administered cidofovir by small-particle aqueous aerosol before or after aerosolized cowpox infection. In a series of five experiments, aerosol doses of 0.5-5 mg/kg were always more effective than 25 mg/kg and sometimes more effective than 100 mg/kg injected subcutaneously, as measured by changes in body and lung weight, lung viral titers, pulmonary pathology and survival. A cyclic analog ((1-[(S)-2-hydroxy-2-oxo-1,4,2-dioxaphosphorinan-5-yl)methyl] cytosine) (cHPMPC) was less protective. The results suggest that aerosolized cidofovir would be effective for prophylaxis or early post-exposure therapy of human smallpox or monkeypox virus infection.

The pathology of experimental aerosolized monkeypox virus infection in cynomolgus monkeys (Macaca fascicularis)

Cynomolgus monkeys (Macaca fascicularis) were exposed by fine-particle aerosol to lethal doses of monkeypox virus, Zaire strain. Death, attributable to fibrinonecrotic bronchopneumonia, occurred 9 to 17 days postexposure. Lower airway epithelium served as the principal target for primary infection. The relative degree of involvement among lymphoid tissues suggested that tonsil, mediastinal, and mandibular lymph nodes were also infected early in the course of the disease, and may have served as additional, although subordinate, sites of primary replication. The distribution of lesions was consistent with lymphatogenous spread to the mediastinal lymph nodes and systemic dissemination of the virus through a monocytic cell-associated viremia. This resulted in lesions affecting other lymph nodes, the thymus, spleen, skin, oral mucosa, gastrointestinal tract, and reproductive system. The mononuclear phagocyte/dendritic cell system was the principal target within lymphoid tissues and may also have provided the means of entry into other systemic sites. Hepatic involvement was uncommon. Lesions at all affected sites were characterized morphologically as necrotizing. Terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate nick-end labeling (TUNEL) staining of select lesions suggested that cell death within lymphoid and epithelial tissues was due in large part to apoptosis. Skin and mucosal surfaces of the respiratory and gastrointestinal tracts also exhibited variable proliferation of epithelial cells and subjacent fibroblasts. Epithelial intracytoplasmic inclusion bodies, consistent with Guarnieri bodies, were usually inconspicuous by light microscopy, but when present, were most readily apparent in the stratified squamous epithelium of the oral mucosa and epidermis. Multinucleated syncytial cells were also occasionally observed in the stratified squamous epithelium of the tongue, tonsil, and skin, and in the intestinal mucosa. Monkeypox virus antigen was readily demonstrated by immunohistochemistry using anti-vaccinia mouse polyclonal antibodies as well as anti-monkeypox rabbit polyclonal antibodies. Detectable poxviral antigen was limited to sites exhibiting obvious morphologic involvement and was most prominent within epithelial cells, macrophages, dendritic cells, and fibroblasts of affected tissues. The presence of poxviral antigen, as determined by immunohistochemistry, correlated with ultrastructural identification of replicating virus. Concurrent bacterial septicemia, present in one monkey, was associated with increased dissemination of the virus to the liver, spleen, and bone marrow and resulted in a more rapidly fatal clinical course.

Effects of cidofovir on the pathogenesis of a lethal vaccinia virus respiratory infection in mice

Intranasal infection of BALB/c mice with the WR strain of vaccinia virus leads to pneumonia, profound weight loss, and death. Although the major sites of virus replication are in the lungs and nasal tissue, dissemination of the virus to other visceral organs and brain occurs via the blood. In this report the effects of cidofovir on the pathogenesis of the infection was studied. Mice were infected intranasally with virus followed 1 day later by a single intraperitoneal treatment with cidofovir (100 mg/kg) or placebo. Placebo-treated mice were dead by day 8, whereas all cidofovir-treated animals survived through 21 days. Cidofovir treatment did not prevent profound weight loss from occurring during the acute phase of the infection, but the mice gained weight quickly after the 8th day. Significantly higher arterial oxygen saturation levels, as determined by pulse oximetry, were seen in cidofovir-treated animals compared to placebos on days 4-7. Cidofovir treatment markedly improved lung consolidation scores and prevented lung weights from increasing during the infection. Virus titers in lungs and nasal tissue were high starting from the first day of the infection, whereas the titers in liver, spleen, brain, and blood was low for 3 days then markedly rose between days 4 and 6. Lung and nasal virus titers were reduced 10-30-fold by cidofovir treatment on days 2, 4 and 6. Virus titers in the other tissues and blood at their peak (day 6) were 30- to >1000-fold less than in tissues of placebos. These results illustrate the ability of a single cidofovir treatment to control the pathogenesis of an acute lethal infection in various tissues during the vaccinia virus infection in mice.

Treatment of lethal vaccinia virus respiratory infections in mice with cidofovir

Intranasal infection of BALB/c mice with the WR strain of vaccinia virus leads to pneumonia, profound weight loss and death. Five days after intranasal inoculation, virus from untreated mice was recovered from 11 organs, tissues and whole blood. The highest titres [>10(8) plaque forming units (pfu)/g] were in lungs and nose/sinus tissue, with about 10(7) pfu/g in spleen and blood. Seven other organs contained 30- to > or = 50-fold lower amounts of virus. Mice infected with the related cowpox virus (for comparative purposes) had the majority of virus located in the respiratory tract. The vaccinia mouse model was used to study the efficacy of cidofovir treatments on the infection. Subcutaneous injections of 30 or 100 mg/kg/day, given on days 1 and 4 after virus challenge, reduced mortality by 60-100%. However, lung virus titres on days 2-5 were reduced no more than 10-fold by these treatments. A moderate improvement in drug efficacy occurred with daily treatments for 5 days. The efficacy of cidofovir also increased as the virus challenge dose decreased, where subcutaneous or intraperitoneal treatment routes showed similar degrees of protection. Although it has been known for many years that the WR strain of vaccinia virus can cause lethal infections by intranasal route, its application to antiviral therapy represents a new model for studying anti-orthopoxvirus agents.

Treatment of cowpox virus respiratory infections in mice with ribavirin as a single agent or followed sequentially by cidofovir

To better understand the potential of ribavirin in the treatment of orthopoxvirus infections (such as those acquired through bioterrorist activities), the efficacy of the drug was studied in a cowpox respiratory infection model in mice under varying disease severity. Mice did not survive a high intranasal cowpox virus challenge [3 x 10(6) plaque forming units (pfu)/animal] treated with subcutaneous ribavirin (100 mg/kg/day for 5 days), but lived 3.9 days longer than placebos. In contrast, 100% of animals receiving the same dose of drug survived a 3 x 10(5) pfu challenge compared with 0% survival of those that received placebo. Survival rates of 50 and 30% occurred with ribavirin doses of 50 and 25 mg/kg/day, respectively. At the 100 mg/kg/day dose, ribavirin reduced lung virus titres 40-fold on day 6 of the infection relative to titres in the placebo group. Weight loss resulting from illness and mean lung weights of mice treated with ribavirin were also significantly reduced. Mice were infected intranasally with the high 3 x 10(6) pfu virus challenge dose and treated with 100 mg/kg/day ribavirin for 5 days, followed by single injections of 75 mg/kg cidofovir on day 6, 7, 8 or 9. Cidofovir alone (without ribavirin) administered on day 6 had no beneficial effect on disease outcome. Ribavirin alone increased the mean time to death by 3.7 days. Ribavirin treatment for 5 days followed by cidofovir treatment on days 6 and 7 significantly increased the mean time to death beyond that achieved with ribavirin alone by 8.2 and 4.4 days, respectively, with 30 and 40% of mice surviving the infection. These results suggest that many individuals infected with an orthopoxvirus by aerosol route would benefit by a course of ribavirin therapy. Later, the fewer number of very sick individuals could be treated with intravenous cidofovir.