Attenuation of B5R mutants of rabbitpox virus in vivo is related to impaired growth and not an enhanced host inflammatory response

Cross-species transmission and host range genes in poxviruses

The persistent epidemic of human mpox, caused by mpox virus (MPXV), raises concerns about the future spread of MPXV and other poxviruses. MPXV is a typical zoonotic virus which can infect human and cause smallpox-like symptoms. MPXV belongs to the Poxviridae family, which has a relatively broad host range from arthropods to vertebrates. Cross-species transmission of poxviruses among different hosts has been frequently reported and resulted in numerous epidemics. Poxviruses have a complex linear double-strand DNA genome that encodes hundreds of proteins. Genes related to the host range of poxvirus are called host range genes (HRGs). This review briefly introduces the taxonomy, phylogeny and hosts of poxviruses, and then comprehensively summarizes the current knowledge about the cross-species transmission of poxviruses. In particular, the HRGs of poxvirus are described and their impacts on viral host range are discussed in depth. We hope that this review will provide a comprehensive perspective about the current progress of researches on cross-species transmission and HRG variation of poxviruses, serving as a valuable reference for academic studies and disease control in the future.

Poxvirus Host Range Genes and Virus-Host Spectrum: A Critical Review

The Poxviridae family is comprised of double-stranded DNA viruses belonging to nucleocytoplasmic large DNA viruses (NCLDV). Among the NCLDV, poxviruses exhibit the widest known host range, which is likely observed because this viral family has been more heavily investigated. However, relative to each member of the Poxviridae family, the spectrum of the host is variable, where certain viruses can infect a large range of hosts, while others are restricted to only one host species. It has been suggested that the variability in host spectrum among poxviruses is linked with the presence or absence of some host range genes. Would it be possible to extrapolate the restriction of viral replication in a specific cell lineage to an animal, a far more complex organism? In this study, we compare and discuss the relationship between the host range of poxvirus species and the abundance/diversity of host range genes. We analyzed the sequences of 38 previously identified and putative homologs of poxvirus host range genes, and updated these data with deposited sequences of new poxvirus genomes. Overall, the term host range genes might not be the most appropriate for these genes, since no correlation between them and the viruses' host spectrum was observed, and a change in nomenclature should be considered. Finally, we analyzed the evolutionary history of these genes, and reaffirmed the occurrence of horizontal gene transfer (HGT) for certain elements, as previously suggested. Considering the data presented in this study, it is not possible to associate the diversity of host range factors with the amount of hosts of known poxviruses, and this traditional nomenclature creates misunderstandings.

New effective chemically synthesized anti-smallpox compound NIOCH-14

Antiviral activity of the new chemically synthesized compound NIOCH-14 (a derivative of tricyclodicarboxylic acid) in comparison with ST-246 (the condensed derivative of pyrroledione) was observed in experiments in vitro and in vivo using orthopoxviruses including highly pathogenic ones. After oral administration of NIOCH-14 to outbred ICR mice infected intranasally with 100 % lethal dose of ectromelia virus, it was shown that 50 % effective doses of NIOCH-14 and ST-246 did not significantly differ. The 'therapeutic window' varied from 1 day before infection to 6 days post-infection (p.i.) to achieve 100-60 % survival rate. The administration of NIOCH-14 and ST-246 to mice resulted in a significant reduction of ectromelia virus titres in organs examined as compared with the control and also reduced pathological changes in the lungs 6 days p.i. Oral administration of NIOCH-14 and ST-246 to ICR mice and marmots challenged with monkeypox virus as compared with the control resulted in a significant reduction of virus production in the lungs and the proportion of infected mice 7 days p.i. as well as the absence of disease in marmots. Significantly lower proportions of infected mice and virus production levels in the lungs as compared with the control were demonstrated in experiments after oral administration of NIOCH-14 and ST-246 to ICR mice and immunodeficient SCID mice challenged with variola virus 3 and 4 days p.i., respectively. The results obtained suggest good prospects for further study of the chemical compound NIOCH-14 to create a new smallpox drug on its basis.

Genetic characterization and phylogenetic analysis of host-range genes of Camelpox virus isolates from India

Camelpox virus (CMLV), a close variant of variola virus (VARV) infects camels worldwide. The zoonotic infections reported from India signify the need to study the host-range genes-responsible for host tropism. We report sequence and phylogenetic analysis of five host-range genes: cytokine response modifier B (crmB), chemokine binding protein (ckbp), viral schlafen-like (v-slfn), myxomavirus T4-like (M-T4-like) and b5r of CMLVs isolated from outbreaks in India. Comparative analysis revealed that these genes are conserved among CMLVs and shared 94.5-100 % identity at both nucleotide (nt) and amino acid (aa) levels. All genes showed identity (59.3-98.4 %) with cowpox virus (CPXV) while three genes-crmB, ckbp and b5r showed similarity (92-96.5 %) with VARVs at both nt and aa levels. Interestingly, three consecutive serine residue insertions were observed in CKBP protein of CMLV-Delhi09 isolate which was similar to CPXV-BR and VACVs, besides five point mutations (K53Q, N67I, F84S, A127T and E182G) were also similar to zoonotic OPXVs. Further, few inconsistent point mutation(s) were also observed in other gene(s) among Indian CMLVs. These indicate that different strains of CMLVs are circulating in India and these mutations could play an important role in adaptation of CMLVs in humans. The phylogeny revealed clustering of all CMLVs together except CMLV-Delhi09 which grouped separately due to the presence of specific point mutations. However, the topology of the concatenated phylogeny showed close evolutionary relationship of CMLV with VARV and TATV followed by CPXV-RatGer09/1 from Germany. The availability of this genetic information will be useful in unveiling new strategies to control emerging zoonotic poxvirus infections.

Mutational analysis of vaccinia virus E3 protein: the biological functions do not correlate with its biochemical capacity to bind double-stranded RNA

Vaccinia E3 protein has the biochemical capacity of binding to double-stranded RNA (dsRNA). The best characterized biological functions of the E3 protein include its host range function, suppression of cytokine expression, and inhibition of interferon (IFN)-induced antiviral activity. Currently, the role of the dsRNA binding capacity in the biological functions of the E3 protein is not clear. To further understand the mechanism of the E3 protein biological functions, we performed alanine scanning of the entire dsRNA binding domain of the E3 protein to examine the link between its biochemical capacity of dsRNA binding and biological functions. Of the 115 mutants examined, 20 were defective in dsRNA binding. Although the majority of the mutants defective in dsRNA binding also showed defective replication in HeLa cells, nine mutants (I105A, Y125A, E138A, F148A, F159A, K171A, L182A, L183A, and I187/188A) retained the host range function to various degrees. Further examination of a set of representative E3L mutants showed that residues essential for dsRNA binding are not essential for the biological functions of E3 protein, such as inhibition of protein kinase R (PKR) activation, suppression of cytokine expression, and apoptosis. Thus, data described in this communication strongly indicate the E3 protein performs its biological functions via a novel mechanism which does not correlate with its dsRNA binding activity.

IMPORTANCE

dsRNAs produced during virus replication are important pathogen-associated molecular patterns (PAMPs) for inducing antiviral immune responses. One of the strategies used by many viruses to counteract such antiviral immune responses is achieved by producing dsRNA binding proteins, such as poxvirus E3 family proteins, influenza virus NS1, and Ebola virus V35 proteins. The most widely accepted model for the biological functions of this class of viral dsRNA binding proteins is that they bind to and sequester viral dsRNA PAMPs; thus, they suppress the related antiviral immune responses. However, no direct experimental data confirm such a model. In this study of vaccinia E3 protein, we found that the biological functions of the E3 protein are not necessarily linked to its biochemical capacity of dsRNA binding. Thus, our data strongly point to a new concept of virus modulation of cellular antiviral responses triggered by dsRNA PAMPs.

Protective properties of vaccinia virus-based vaccines: skin scarification promotes a nonspecific immune response that protects against orthopoxvirus disease

The process of vaccination introduced by Jenner generated immunity against smallpox and ultimately led to the eradication of the disease. Procedurally, in modern times, the virus is introduced into patients via a process called scarification, performed with a bifurcated needle containing a small amount of virus. What was unappreciated was the role that scarification itself plays in generating protective immunity. In rabbits, protection from lethal disease is induced by intradermal injection of vaccinia virus, whereas a protective response occurs within the first 2 min after scarification with or without virus, suggesting that the scarification process itself is a major contributor to immunoprotection. importance: These results show the importance of local nonspecific immunity in controlling poxvirus infections and indicate that the process of scarification should be critically considered during the development of vaccination protocols for other infectious agents.

Poxviruses and the evolution of host range and virulence

Poxviruses as a group can infect a large number of animals. However, at the level of individual viruses, even closely related poxviruses display highly diverse host ranges and virulence. For example, variola virus, the causative agent of smallpox, is human-specific and highly virulent only to humans, whereas related cowpox viruses naturally infect a broad spectrum of animals and only cause relatively mild disease in humans. The successful replication of poxviruses depends on their effective manipulation of the host antiviral responses, at the cellular-, tissue- and species-specific levels, which constitutes a molecular basis for differences in poxvirus host range and virulence. A number of poxvirus genes have been identified that possess host range function in experimental settings, and many of these host range genes target specific antiviral host pathways. Herein, we review the biology of poxviruses with a focus on host range, zoonotic infections, virulence, genomics and host range genes as well as the current knowledge about the function of poxvirus host range factors and how their interaction with the host innate immune system contributes to poxvirus host range and virulence. We further discuss the evolution of host range and virulence in poxviruses as well as host switches and potential poxvirus threats for human and animal health.

A survey of host range genes in poxvirus genomes

Poxviruses are widespread pathogens, which display extremely different host ranges. Whereas some poxviruses, including variola virus, display narrow host ranges, others such as cowpox viruses naturally infect a wide range of mammals. The molecular basis for differences in host range are poorly understood but apparently depend on the successful manipulation of the host antiviral response. Some poxvirus genes have been shown to confer host tropism in experimental settings and are thus called host range factors. Identified host range genes include vaccinia virus K1L, K3L, E3L, B5R, C7L and SPI-1, cowpox virus CP77/CHOhr, ectromelia virus p28 and 022, and myxoma virus T2, T4, T5, 11L, 13L, 062R and 063R. These genes encode for ankyrin repeat-containing proteins, tumor necrosis factor receptor II homologs, apoptosis inhibitor T4-related proteins, Bcl-2-related proteins, pyrin domain-containing proteins, cellular serine protease inhibitors (serpins), short complement-like repeats containing proteins, KilA-N/RING domain-containing proteins, as well as inhibitors of the double-stranded RNA-activated protein kinase PKR. We conducted a systematic survey for the presence of known host range genes and closely related family members in poxvirus genomes, classified them into subgroups based on their phylogenetic relationship and correlated their presence with the poxvirus phylogeny. Common themes in the evolution of poxvirus host range genes are lineage-specific duplications and multiple independent inactivation events. Our analyses yield new insights into the evolution of poxvirus host range genes. Implications of our findings for poxvirus host range and virulence are discussed.

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.

Roles of vaccinia virus genes E3L and K3L and host genes PKR and RNase L during intratracheal infection of C57BL/6 mice

The importance of the 2'-5' oligoadenylate synthetase (OAS)/RNase L and double-stranded RNA (dsRNA)-dependent protein kinase (PKR) pathways in host interferon induction resulting from virus infection in response to dsRNA has been well documented. In poxvirus infections, the interactions between the vaccinia virus (VV) genes E3L and K3L, which target RNase L and PKR, respectively, serve to prevent the induction of the dsRNA-dependent induced interferon response in cell culture. To determine the importance of these host genes in controlling VV infections, mouse single-gene knockouts of RNase L and PKR and double-knockout mice were studied following intratracheal infection with VV, VVΔK3L, or VVΔE3L. VV caused lethal disease in all mouse strains. The single-knockout animals were more susceptible than wild-type animals, while the RNase L(-/-) PKR(-/-) mice were the most susceptible. VVΔE3L infections of wild-type mice were asymptomatic, demonstrating that E3L plays a critical role in controlling the host immune response. RNase L(-/-) mice showed no disease, whereas 20% of the PKR(-/-) mice succumbed at a dose of 10(8) PFU. Lethal disease was routinely observed in RNase L(-/-) PKR(-/-) mice inoculated with 10(8) PFU of VVΔE3L, with a distinct pathology. VVΔK3L infections exhibited no differences in virulence among any of the mouse constructs, suggesting that PKR is not the exclusive target of K3L. Surprisingly, VVΔK3L did not disseminate to other tissues from the lung. Hence, the cause of death in this model is respiratory disease. These results also suggest that an unanticipated role of the K3L gene is to facilitate virus dissemination.

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.

Poxvirus host range genes

As a family of viruses, poxviruses collectively exhibit a broad host range and most of the individual members are capable of replicating in a wide array of cell types from various host species, at least in vitro. At the cellular level, poxvirus tropism is dependent not upon specific cell surface receptors, but rather upon: (1) the ability of the cell to provide intracellular complementing factors needed for productive virus replication, and (2) the ability of the specific virus to successfully manipulate intracellular signaling networks that regulate cellular antiviral processes downstream of virus entry. The large genomic coding capacity of poxviruses enables the virus to express a unique collection of viral proteins that function as host range factors, which specifically target and manipulate host signaling pathways to establish optimal cellular conditions for viral replication. Functionally, the known host range factors from poxviruses have been associated with manipulation of a diverse array of cellular targets, which includes cellular kinases and phosphatases, apoptosis, and various antiviral pathways. To date, only a small number of poxvirus host range genes have been identified and studied, and only a handful of these have been functionally characterized. For this reason, poxvirus host range factors represent a potential gold mine for the discovery of novel pathogen-host protein interactions. This review summarizes our current understanding of the mechanisms by which the known poxvirus host range genes, and their encoded factors, expand tropism through the manipulation of host cell intracellular signaling pathways.

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.

Variola virus immune evasion proteins

Variola virus, the causative agent of smallpox, encodes approximately 200 proteins. Over 80 of these proteins are located in the terminal regions of the genome, where proteins associated with host immune evasion are encoded. To date, only two variola proteins have been characterized. Both are located in the terminal regions and demonstrate immunoregulatory functions. One protein, the smallpox inhibitor of complement enzymes (SPICE), is homologous to a vaccinia virus virulence factor, the vaccinia virus complement-control protein (VCP), which has been found experimentally to be expressed early in the course of vaccinia infection. Both SPICE and VCP are similar in structure and function to the family of mammalian complement regulatory proteins, which function to prevent inadvertent injury to adjacent cells and tissues during complement activation. The second variola protein is the variola virus high-affinity secreted chemokine-binding protein type II (CKBP-II, CBP-II, vCCI), which binds CC-chemokine receptors. The vaccinia homologue of CKBP-II is secreted both early and late in infection. CKBP-II proteins are highly conserved among orthopoxviruses, sharing approximately 85% homology, but are absent in eukaryotes. This characteristic sets it apart from other known virulence factors in orthopoxviruses, which share sequence homology with known mammalian immune regulatory gene products. Future studies of additional variola proteins may help illuminate factors associated with its virulence, pathogenesis and strict human tropism. In addition, these studies may also assist in the development of targeted therapies for the treatment of both smallpox and human immune-related diseases.

The effects of targeting the vaccinia virus B5R protein to the endoplasmic reticulum on virus morphogenesis and dissemination

The consequence of redirecting the vaccinia virus (VV) B5R protein to the endoplasmic reticulum (ER) has been investigated by the addition of an ER retrieval signal KKSL (K(2)X(2)) to the B5R C-terminus. This mutant B5R gene and a version of the gene with the inactive ER retrieval sequence KKSLAL (K(2)X(4)) were inserted into the thymidine kinase locus of a VV mutant lacking the B5R gene, vDeltaB5R. Similar levels of B5R protein were made by each virus, but the B5R-K(2)X(2) protein remained sensitive to endoglycosidase H and colocalised with protein disulphide isomerase in the ER. In contrast, the B5R-K(2)X(4) protein colocalised with 1, 4-galactosyltransferase in the trans-Golgi network. Electron microscopy revealed that even when the B5R protein was redirected to the ER, intracellular mature virus particles were wrapped by cellular membranes to form intracellular enveloped virus particles, although more incompletely wrapped particles were evident compared with wild type. These intracellular enveloped virus particles were, however, unable to efficiently induce the polymerisation of actin and the plaque size formed by vB5R-K(2)X(2) was small. Nevertheless, the amount and specific infectivity of EEV produced by vB5R-K(2)X(2) were similar to those of wild type, despite the dramatic reduction in the amount of B5R protein present in vB5R-K(2)X(2) EEV.

Functional analysis of vaccinia virus B5R protein: essential role in virus envelopment is independent of a large portion of the extracellular domain

Vaccinia virus has two forms of infectious virions: the intracellular mature virus and the extracellular enveloped virus (EEV). EEV is critical for cell-to-cell and long-range spread of the virus. The B5R open reading frame (ORF) encodes a membrane protein that is essential for EEV formation. Deletion of the B5R ORF results in a dramatic reduction of EEV, and as a consequence, the virus produces small plaques in vitro and is highly attenuated in vivo. The extracellular portion of B5R is composed mainly of four domains that are similar to the short consensus repeats (SCRs) present in complement regulatory proteins. To determine the contribution of these putative SCR domains to EEV formation, we constructed recombinant vaccinia viruses that replaced the wild-type B5R gene with a mutated gene encoding a B5R protein lacking the SCRs. The resulting recombinant viruses produced large plaques, indicating efficient cell-to-cell spread in vitro, and gradient centrifugation of supernatants from infected cells confirmed that EEV was formed. In contrast, phalloidin staining of infected cells showed that the virus lacking the SCR domains was deficient in the induction of thick actin bundles. Thus, the highly conserved SCR domains present in the extracellular portion of the B5R protein are dispensable for EEV formation. This indicates that the B5R protein is a key viral protein with multiple functions in the process of virus envelopment and release. In addition, given the similarity of the extracellular domain to complement control proteins, the B5R protein may be involved in viral evasion from host immune responses.