Potential for broad-spectrum protection against feline calicivirus using an attenuated myxoma virus expressing a chimeric FCV capsid protein

Oclacitinib and Myxoma Virus Therapy in Dogs with High-Grade Soft Tissue Sarcoma

Human rhabdomyosarcomas are rarely cured by surgical resection alone. This is also true for high-grade soft tissue sarcomas in dogs. Dogs with spontaneous sarcoma are good models for clinical responses to new cancer therapies. Strategic combinations of immunotherapy and oncolytic virotherapy (OV) could improve treatment responses in canine and human cancer patients. To develop an appropriate combination of immunotherapy and OV for dogs with soft tissue sarcoma (STS), canine cancer cells were inoculated with myxoma viruses (MYXVs) and gene transcripts were quantified. Next, the cytokine concentrations in the canine cancer cells were altered to evaluate their effect on MYXV replication. These studies indicated that, as in murine and human cells, type I interferons (IFN) play an important role in limiting MYXV replication in canine cancer cells. To reduce type I IFN production during OV, oclacitinib (a JAK1 inhibitor) was administered twice daily to dogs for 14 days starting ~7 days prior to surgery. STS tumors were excised, and MYXV deleted for serp2 (MYXV∆SERP2) was administered at the surgical site at two time points post-operatively to treat any remaining microscopic tumor cells. Tumor regrowth in dogs treated with OV was decreased relative to historical controls. However, regrowth was not further inhibited in patients given combination therapy.

Viral Vector Vaccines against Bluetongue Virus

Bluetongue virus (BTV), the prototype member of the genus Orbivirus (family Reoviridae), is the causative agent of an important livestock disease, bluetongue (BT), which is transmitted via biting midges of the genus Culicoides. To date, up to 29 serotypes of BTV have been described, which are classified as classical (BTV 1–24) or atypical (serotypes 25–27), and its distribution has been expanding since 1998, with important outbreaks in the Mediterranean Basin and devastating incursions in Northern and Western Europe. Classical vaccine approaches, such as live-attenuated and inactivated vaccines, have been used as prophylactic measures to control BT through the years. However, these vaccine approaches fail to address important matters like vaccine safety profile, effectiveness, induction of a cross-protective immune response among serotypes, and implementation of a DIVA (differentiation of infected from vaccinated animals) strategy. In this context, a wide range of recombinant vaccine prototypes against BTV, ranging from subunit vaccines to recombinant viral vector vaccines, have been investigated. This article offers a comprehensive outline of the live viral vectors used against BTV.

Treatment of an Alveolar Rhabdomyosarcoma Allograft with Recombinant Myxoma Virus and Oclacitinib

Purpose

Rhabdomyosarcomas (RMS) are difficult tumors to treat with conventional therapies. Publications indicate that oncolytic virotherapy (OV) could benefit cancer patients with tumors that are refractory to conventional treatments. It is believed that the efficacy of OV can be enhanced when used in combination with other treatments. This study evaluated the response of mice with aggressive alveolar RMS (ARMS) allografts to treatment with an OV [recombinant myxoma virus (MYXVΔserp2)] in combination with a Janus kinase (JAK) inhibitor (oclacitinib). Oclacitinib is known to inhibit JAK1 and JAK2 cell signaling pathways, which should limit the antiviral Type I interferon response. However, oclacitinib does not inhibit immune pathways that promote antigen presentation, which help stimulate an anti-cancer immune response.

Materials and Methods

To determine if MYXVΔserp2 and oclacitinib could improve outcomes in animals with ARMS, nude mice were inoculated subcutaneously with murine ARMS cells to establish tumors. Immune responses, tumor growth, and clinical signs in mice treated with combination therapy were compared to mice given placebo therapy and mice treated with OV alone.

Results

Combination therapy was safe; no viral DNA was detected in off-target organs, only within tumors. As predicted, viral DNA was detected in tumors of mice given oclacitinib and MYXVΔserp2 for a longer time period than mice treated with OV alone. Although tumor growth rates and median survival times were not significantly different between groups, clinical signs were less severe in mice treated with OV.

Conclusion

Our data indicate that MYXVΔserp2 treatment benefits mice with ARMS by reducing clinical signs of disease and improving quality of life.

Myxoma Virus-Encoded Host Range Protein M029: A Multifunctional Antagonist Targeting Multiple Host Antiviral and Innate Immune Pathways

Myxoma virus (MYXV) is the prototypic member of the Leporipoxvirus genus of the Poxviridae family of viruses. In nature, MYXV is highly restricted to leporids and causes a lethal disease called myxomatosis only in European rabbits (Oryctologous cuniculus). However, MYXV has been shown to also productively infect various types of nonrabbit transformed and cancer cells in vitro and in vivo, whereas their normal somatic cell counterparts undergo abortive infections. This selective tropism of MYXV for cancer cells outside the rabbit host has facilitated its development as an oncolytic virus for the treatment of different types of cancers. Like other poxviruses, MYXV possesses a large dsDNA genome which encodes an array of dozens of immunomodulatory proteins that are important for host and cellular tropism and modulation of host antiviral innate immune responses, some of which are rabbit-specific and others can function in nonrabbit cells as well. This review summarizes the functions of one such MYXV host range protein, M029, an ortholog of the larger superfamily of poxvirus encoded E3-like dsRNA binding proteins. M029 has been identified as a multifunctional protein involved in MYXV cellular and host tropism, antiviral responses, and pathogenicity in rabbits.

Recombinant Myxoma Virus Expressing Walleye Dermal Sarcoma Virus orfC Is Attenuated in Rabbits

The poxvirus, myxoma virus (MYXV) has shown efficacy as an oncolytic virus (OV) in some cancer models. However, MYXV replication within murine cancer models and spontaneous canine sarcomas is short-lived. In mice, successful treatment of tumors requires frequent injections with MYXV. We hypothesize that treatment of cancer with a recombinant MYXV that promotes apoptosis could improve the efficacy of MYXV. The orfC gene of walleye dermal sarcoma virus (WDSV), which induces apoptosis, was recombined into the MYXV genome (MYXVorfC). A marked increase in apoptosis was observed in cells infected with MYXVorfC. To ensure that expression of WDSV orfC by MYXV does not potentiate the pathogenesis of MYXV, we evaluated the effects of MYXVorfC inoculation in the only known host of MYXV, New Zealand white rabbits. Virus dissemination in rabbit tissues was similar for MYXVorfC and MYXV. Virus titers recovered from tissues were lower in MYXVorfC-infected rabbits as compared to MYXV-infected rabbits. Importantly, rabbits infected with MYXVorfC had a delayed onset of clinical signs and a longer median survival time than rabbits infected with MYXV. This study indicates that MYXVorfC is attenuated and suggests that MYXVorfC will be safe to use as an OV therapy in future studies.

The potential of the combined use of targeted type I interferon pathway inhibitors and oncolytic viruses to treat sarcomas

Replicating oncolytic viruses (OVs) are appealing, new, FDA-approved, therapeutic options for humans with head and neck cancers and melanomas. These treatments are not yet available for veterinary patients, but recent clinical trials have shown several OVs to be safe in dogs and cats. Specific viruses being used to treat sarcomas in dogs include modified canine adenovirus 2, myxoma virus, vesicular stomatitis virus and reovirus. In cats with vaccine-associated sarcomas, poxviruses have been injected postoperatively and a reduced rate of tumour recurrence was documented. To date, the response rates of canine and feline patients to OV therapy have been variable (as they are in people). Optimal methods of OV administration and dosing schedules continue to be evaluated. One way to improve outcomes of OV therapy in veterinary patients may be to use OVs in combination with other immunomodulatory therapies. This review discusses the potential utility of concurrent therapy with an OV and an inhibitor of the type I interferon pathway.

Safety of an Oncolytic Myxoma Virus in Dogs with Soft Tissue Sarcoma

Many oncolytic viruses that are efficacious in murine cancer models are ineffective in humans. The outcomes of oncolytic virus treatment in dogs with spontaneous tumors may better predict human cancer response and improve treatment options for dogs with cancer. The objectives of this study were to evaluate the safety of treatment with myxoma virus lacking the serp2 gene (MYXVΔserp2) and determine its immunogenicity in dogs. To achieve these objectives, dogs with spontaneous soft tissue sarcomas were treated with MYXVΔserp2 intratumorally (n = 5) or post-operatively (n = 5). In dogs treated intratumorally, clinical scores were recorded and tumor biopsies and swabs (from the mouth and virus injection site) were analyzed for viral DNA at multiple time-points. In all dogs, blood, urine, and feces were frequently collected to evaluate organ function, virus distribution, and immune response. No detrimental effects of MYXVΔserp2 treatment were observed in any canine cancer patients. No clinically significant changes in complete blood profiles, serum chemistry analyses, or urinalyses were measured. Viral DNA was isolated from one tumor swab, but viral dissemination was not observed. Anti-MYXV antibodies were occasionally detected. These findings provide needed safety information to advance clinical trials using MYXVΔserp2 to treat patients with cancer.

Myxoma virus therapy for human embryonal rhabdomyosarcoma in a nude mouse model

Rhabdomyosarcoma (RMS) is a devastating tumor of young people that is difficult to cure. To determine if oncolytic virus therapy can improve outcomes in individuals with RMS, myxoma virus expressing a red fluorescent protein (MYXV-red) was evaluated for antitumoral effects using a murine model of RMS. Fluorescent protein was expressed in four RMS cell lines inoculated with MYXV-red, indicating that these cells were semipermissive to MYXV infection. MYXV-red replication and cytopathic effects were further evaluated using human embryonal RMS (CCL-136) cells. Logarithmic growth of MYXV-red and significant cell death were observed 72 hours after inoculation with MYXV. The oncolytic effects of MYXV-red were then studied in nude mice that were injected subcutaneously with CCL-136 cells to establish RMS xenografts. Once tumors measured 5 mm in diameter, mice were treated with multiple intratumoral injections of MXYV-red or saline. The average final tumor volume and rate of tumor growth were significantly decreased, and median survival time was significantly increased in MYXV-red-treated mice (P-values =0.0416, 0.0037, and 0.0004, respectively). Histologic sections of MYXV-red-treated tumors showed increased inflammation compared to saline-treated tumors (P-value =0.0002). In conclusion, MXYV-red treatment of RMS tumors was successful in individual mice as it resulted in decreased tumor burden in eight of eleven mice with nearly complete tumor remission in five of eleven mice. These data hold promise that MYXV-red treatment may be beneficial for people suffering from RMS. To our knowledge, this is the first report of successful treatment of RMS tumors using an oncolytic poxvirus.

On the potential of oncolytic virotherapy for the treatment of canine cancers

Over 6 million dogs are diagnosed with cancer in the USA each year. Treatment options for many of these patients are limited. It is important that the veterinary and scientific communities begin to explore novel treatment protocols for dogs with cancer. Oncolytic viral therapy is a promising treatment option that may prove to be relatively inexpensive and effective against several types of cancer. The efficacy of oncolytic virus therapies has been clearly demonstrated in murine cancer models, but the positive outcomes observed in mice are not always seen in human cancer patients. These therapies should be thoroughly evaluated in dogs with spontaneously arising cancers to provide needed information about the potential effectiveness of virus treatment for human cancers and to promote the health of our companion animals. This article provides a review of the results of oncolytic virus treatment of canine cancers.

The evolution of poxvirus vaccines

After Edward Jenner established human vaccination over 200 years ago, attenuated poxviruses became key players to contain the deadliest virus of its own family: Variola virus (VARV), the causative agent of smallpox. Cowpox virus (CPXV) and horsepox virus (HSPV) were extensively used to this end, passaged in cattle and humans until the appearance of vaccinia virus (VACV), which was used in the final campaigns aimed to eradicate the disease, an endeavor that was accomplished by the World Health Organization (WHO) in 1980. Ever since, naturally evolved strains used for vaccination were introduced into research laboratories where VACV and other poxviruses with improved safety profiles were generated. Recombinant DNA technology along with the DNA genome features of this virus family allowed the generation of vaccines against heterologous diseases, and the specific insertion and deletion of poxvirus genes generated an even broader spectrum of modified viruses with new properties that increase their immunogenicity and safety profile as vaccine vectors. In this review, we highlight the evolution of poxvirus vaccines, from first generation to the current status, pointing out how different vaccines have emerged and approaches that are being followed up in the development of more rational vaccines against a wide range of diseases.

Oncolysis of canine tumor cells by myxoma virus lacking the serp2 gene

OBJECTIVE

To determine the oncolytic efficacy of an attenuated form of myxoma virus lacking the serp2 gene in canine tumor cells.

SAMPLE

Primary cells were isolated from tumors that were surgically removed from dogs and from connective tissue obtained from the cadaver of a dog. Cells of various established cell lines from tumors and nontumorous tissues were obtained.

PROCEDURES

Experiments were performed with cells in monolayer culture. Cell cultures were inoculated with wild-type myxoma viruses or myxoma viruses lacking the serp2 gene, and measures of cytopathic effects, viral growth kinetics, and cell death and apoptosis were determined.

RESULTS

Myxoma viruses replicated in cells of many of the primary and established canine tumor cell lines. Canine tumor cells in which expression of activated protein kinase B was upregulated were more permissive to myxoma virus infection than were cells in which expression of activated protein kinase B was not upregulated. Myxoma viruses lacking the serp2 gene caused more cytopathic effects in canine tumor cells because of apoptosis than did wild-type myxoma viruses.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of the present study indicated myxoma viruses lacking the serp2 gene may be useful for treatment of cancer in dogs. Impact for Human Medicine-Results of the present study may be useful for development of novel oncolytic treatments for tumors in humans.

Myxoma virus induces apoptosis in cultured feline carcinoma cells

There is growing interest in utilizing replicating oncolytic viruses as cancer therapeutics agents. The effectiveness of myxoma virus-induced oncolysis was evaluated in two feline cancer cell cultures. Although myxoma virus is a rabbit-specific pathogen, protein expression driven by myxoma virus and production of infectious viral particles were detected. Cell death occurred in primary feline cancer cells within 48 h of inoculation with myxoma virus. Future studies to determine if other feline neoplasms are susceptible to myxoma virus infection are warranted.

Infection of nonhost species dendritic cells in vitro with an attenuated myxoma virus induces gene expression that predicts its efficacy as a vaccine vector

Recombinant myxoma virus (MYXV) can be produced without a loss of infectivity, and its highly specific host range makes it an ideal vaccine vector candidate, although careful examination of its interaction with the immune system is necessary. Similar to rabbit bone marrow-derived dendritic cells (BM-DCs), ovine dendritic cells can be infected by SG33, a MYXV vaccine strain, and support recombinant antigen expression. The frequency of infected cells in the nonhost was lower and the virus cycle was abortive in these cell types. Among BM-DC subpopulations, Langerhans cell-like DCs were preferentially infected at low multiplicities of infection. Interestingly, ovine BM-DCs remained susceptible to MYXV after maturation, although apoptosis occurred shortly after infection as a function of the virus titer. When gene expression was assessed in infected BM-DC cultures, type I interferon (IFN)-related and inflammatory genes were strongly upregulated. DC gene expression profiles were compared with the profiles produced by other poxviruses in interaction with DCs, but very few commonalities were found, although genes that were previously shown to predict vaccine efficacy were present. Collectively, these data support the idea that MYXV permits efficient priming of adaptive immune responses and should be considered a promising vaccine vector along with other poxviruses.

Genome comparison of a nonpathogenic myxoma virus field strain with its ancestor, the virulent Lausanne strain

One of the best-studied examples of host-virus coevolution is the release of myxoma virus (MV) for biological control of European rabbits in Australia and Europe. To investigate the genetic basis of MV adaptation to its new host, we sequenced the genome of 6918, an attenuated Spanish field strain, and compared it with that of Lausanne, the strain originally released in Europe in 1952. Although isolated 43 years apart, the genomes were highly conserved (99.95% identical). Only 32 of the 159 MV predicted proteins revealed amino acid changes. Four genes (M009L, M036L, M135R, and M148R) in 6918 were disrupted by frameshift mutations.

Safety and immunogenicity of myxoma virus as a new viral vector for small ruminants

Myxoma virus (MYXV), a leporide-specific poxvirus, represents an attractive candidate for the generation of safe and non-replicative vaccine vectors for other species. With the aim of developing new recombinant vaccines for ruminants, we evaluated the safety and the immunogenicity of recombinant MYXV in sheep. In vitro studies indicated that ovine primary fibroblasts were not permissive for MYXV and that infection of ovine peripheral blood mononuclear cells occurred at a low rate. Although non-specific activation significantly improved the susceptibility of lymphocytes, MYXV infection remained abortive. Histological and immunohistochemical examination at the inoculation sites revealed the development of an inflammatory process and allowed the detection of sparse infected cells in the dermis. In addition, inoculated sheep developed an antibody response directed against MYXV and the product of the transgene. Overall, these results provide the first line of evidence on the potential of MYXV as a viral vector for ruminants.

In vitro permissivity of bovine cells for wild-type and vaccinal myxoma virus strains

Myxoma virus (MYXV), a leporide-specific poxvirus, represents an attractive candidate for the generation of safe, non-replicative vaccine vector for non-host species. However, there is very little information concerning infection of non-laboratory animals species cells with MYXV. In this study, we investigated interactions between bovine cells and respectively a wild type strain (T1) and a vaccinal strain (SG33) of MYXV. We showed that bovine KOP-R, BT and MDBK cell lines do not support MYXV production. Electron microscopy observations of BT-infected cells revealed the low efficiency of viral entry and the production of defective virions. In addition, infection of bovine peripheral blood mononuclear cells (PBMC) occurred at a very low level, even following non-specific activation, and was always abortive. We did not observe significant differences between the wild type strain and the vaccinal strain of MYXV, indicating that SG33 could be used for new bovine vaccination strategies.

Feline calicivirus

Feline calicivirus (FCV) is an important and highly prevalent pathogen of cats. It belongs to the family Caliciviridae which includes other significant pathogens of man and animals. As an RNA virus, high polymerase error rates convey upon FCV a high genome plasticity, and allow the virus to respond rapidly to environmental selection pressures. This makes the virus very adaptable and has important implications for clinical disease and its control. Being genetically diverse, FCV is associated with a range of clinical syndromes from inapparent infections to relatively mild oral and upper respiratory tract disease with or without acute lameness. More recently, highly virulent forms of the virus have emerged associated with a systemic infection that is frequently fatal. A proportion of FCV infected cats that recover from acute disease, remain persistently infected. In such cats, virus evolution is believed to help the virus to evade the host immune response. Such long-term carriers may only represent a minority of the feline population but are likely to be crucial to the epidemiology of the virus. Vaccination against FCV has been available for many years and has effectively reduced the incidence of clinical disease. However, the vaccines do not prevent infection and vaccinated cats can still become persistently infected. In addition, FCV strain variability means that not all strains are protected against equally. Much progress has been made in understanding the biology and pathogenesis of this important feline virus. Challenges for the future will necessarily focus on how to control the variability of this virus particularly in relation to emerging virulent strains and vaccination.