Arabinofuranosyl Thymine Derivatives-Potential Candidates against Cowpox Virus: A Computational Screening Study

Cowpox is caused by a DNA virus known as the cowpox virus (CPXV) belonging to the Orthopoxvirus genus in the family Poxviridae. Cowpox is a zoonotic disease with the broadest host range among the known poxviruses. The natural reservoir hosts of CPXV are wild rodents. Recently, the cases of orthopoxviral infections have been increasing worldwide, and cowpox is considered the most common orthopoxviral infection in Europe. Cowpox is often a self-limiting disease, although cidofovir or anti-vaccinia gammaglobulin can be used in severe and disseminated cases of human cowpox. In this computational study, a molecular docking analysis of thymine- and arabinofuranosyl-thymine-related structures (1-21) on two cowpox-encoded proteins was performed with respect to the cidofovir standard and a 3D ligand-based pharmacophore model was generated. Three chemical structures (PubChem IDs: 123370001, 154137224, and 90413364) were identified as potential candidates for anti-cowpox agents. Further studies combining in vitro and in silico molecular dynamics simulations to test the stability of these promising compounds could effectively improve the future design of cowpox virus inhibitors, as molecular docking studies are not sufficient to consider a ligand a potential drug.

[Modeling spatial structures of variola and cowpox virus TNF-binding CrmB proteins bound to murine or human TNF]

Orthopoxviruses bear in their genomes several genes coding for homologous secreted proteins able to bind tumor necrosis factor. Different species of the genus possess different sets of these tumor necrosis factor-binding proteins. Viriola virus encodes the only one of them named CrmB. Despite sharing high sequence identity, CrmB proteins belonging to distinct orthopoxviral species were shown to significantly differ by their physico-chemical and biological properties. We modeled spatial structures of tumor necrosis factor receptor domains of variola and cowpox virus CrmB proteins bound to either murine, or human or mutated human tumor necrosis factor. In the sequence of last the arginine residue at position 31 is substituted with glutamine that is characteristic for murine tumor necrosis factor. Theoretical analysis of modeled ligand-receptor complexes revealed that the least stable should be the complex of cowpox virus CrmB with human tumor necrosis factor, and that arginine to glutamine substitution at position 31 should significantly stabilize binding of corresponding human tumor necrosis factor mutant to cowpox virus CrmB. Experimental evaluation of recombinant variola and cowpox virus CrmB efficiencies in inhibiting cytotoxic effect of all these tumor necrosis factors have approved our predictions.

Virus and Infections 2010 - BIT's first world congress

The World Congress of Virus and Infections, held in Busan, South Korea, included topics reviewing the field of zoonoses. This conference report highlights selected presentations on surveillance, epidemiology and measures for the control and prevention of zoonotic diseases. Topics discussed include human factors influencing zoonoses, the molecular epidemiology of Crimean-Congo hemorrhagic fever, the emerging Nipah virus, and the re-emergence of cowpox virus.

Cowpox virus outbreak in banded mongooses (Mungos mungo) and jaguarundis (Herpailurus yagouaroundi) with a time-delayed infection to humans

Background

Often described as an extremely rare zoonosis, cowpox virus (CPXV) infections are on the increase in Germany. CPXV is rodent-borne with a broad host range and contains the largest and most complete genome of all poxviruses, including parts with high homology to variola virus (smallpox). So far, most CPXV cases have occurred individually in unvaccinated animals and humans and were caused by genetically distinguishable virus strains.

Methodology/Principal Findings

Generalized CPXV infections in banded mongooses (Mungos mungo) and jaguarundis (Herpailurus yagouaroundi) at a Zoological Garden were observed with a prevalence of the affected animal group of 100% and a mortality of 30%. A subsequent serological investigation of other exotic animal species provided evidence of subclinical cases before the onset of the outbreak. Moreover, a time-delayed human cowpox virus infection caused by the identical virus strain occurred in a different geographical area indicating that handling/feeding food rats might be the common source of infection.

Conclusions/Significance

Reports on the increased zoonotic transmission of orthopoxviruses have renewed interest in understanding interactions between these viruses and their hosts. The list of animals known to be susceptible to CPXV is still growing. Thus, the likely existence of unknown CPXV hosts and their distribution may present a risk for other exotic animals but also for the general public, as was shown in this outbreak. Animal breeders and suppliers of food rats represent potential multipliers and distributors of CPXV, in the context of increasingly pan-European trading. Taking the cessation of vaccination against smallpox into account, this situation contributes to the increased incidence of CPXV infections in man, particularly in younger age groups, with more complicated courses of clinical infections.

Specific inhibition of orthopoxvirus replication by a small interfering RNA targeting the D5R gene

BACKGROUND

Concerns about the potential use of smallpox in bioterrorism have stimulated interest in the development of novel antiviral treatments. Currently, there are no effective therapies against smallpox and new treatment strategies are greatly needed.

METHODS

In this study, specifically designed small interfering RNAs (siRNAs), targeting five proteins essential for orthopoxvirus replication, were investigated for their ability to inhibit vaccinia virus strain Western Reserve (VACVWR) replication.

RESULTS

Among these siRNAs, 100 nM siD5R-2, an siRNA targeting the D5 protein, decreased VACVWR replication up to 90% when used either prophylactically or therapeutically in human lung carcinoma A549 cells. This siRNA induced a striking concentration-dependent inhibition of VACVWR replication and a prolonged prophylactic antiviral effect that lasted for 72 h, at a concentration of 100 nM. Confocal microscopy of Alexa-siD5R-2-treated VACVWR-infected cells confirmed a decrease in viral replication. Furthermore, siD5R-2 was shown to specifically reduce the D5R mRNA and protein expression using real-time reverse transcriptase-PCR and western blotting analysis, without inducing interferon-13 in A549 cells. We also demonstrated the antiviral potency of siD5R-2 against different pathogenic orthopoxviruses, such as cowpox and monkeypox viruses, which were inhibited up to 70% at the lowest concentration (1 nM) tested. Finally, siD5R-2 showed antiviral effects in VACVWR-infected human keratinocyte and fibroblast cell cultures.

CONCLUSIONS

These results suggest that siD5R-2 could be a potential candidate to treat poxvirus infections.

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.

Properties of the recombinant TNF-binding proteins from variola, monkeypox, and cowpox viruses are different

Tumor necrosis factor (TNF), a potent proinflammatory and antiviral cytokine, is a critical extracellular immune regulator targeted by poxviruses through the activity of virus-encoded family of TNF-binding proteins (CrmB, CrmC, CrmD, and CrmE). The only TNF-binding protein from variola virus (VARV), the causative agent of smallpox, infecting exclusively humans, is CrmB. Here we have aligned the amino acid sequences of CrmB proteins from 10 VARV, 14 cowpox virus (CPXV), and 22 monkeypox virus (MPXV) strains. Sequence analyses demonstrated a high homology of these proteins. The regions homologous to cd00185 domain of the TNF receptor family, determining the specificity of ligand–receptor binding, were found in the sequences of CrmB proteins. In addition, a comparative analysis of the C-terminal SECRET domain sequences of CrmB proteins was performed. The differences in the amino acid sequences of these domains characteristic of each particular orthopoxvirus species were detected. It was assumed that the species-specific distinctions between the CrmB proteins might underlie the differences in these physicochemical and biological properties. The individual recombinant proteins VARV-CrmB, MPXV-CrmB, and CPXV-CrmB were synthesized in a baculovirus expression system in insect cells and isolated. Purified VARV-CrmB was detectable as a dimer with a molecular weight of 90 kDa, while MPXV- and CPXV-CrmBs, as monomers when fractioned by non-reducing SDS-PAGE. The CrmB proteins of VARV, MPXV, and CPXV differed in the efficiencies of inhibition of the cytotoxic effects of human, mouse, or rabbit TNFs in L929 mouse fibroblast cell line. Testing of CrmBs in the experimental model of LPS-induced shock using SPF BALB/c mice detected a pronounced protective effect of VARV-CrmB. Thus, our data demonstrated the difference in anti-TNF activities of VARV-, MPXV-, and CPXV-CrmBs and efficiency of VARV-CrmB rather than CPXV- or MPXV-CrmBs against LPS-induced mortality in mice.

The cowpox virus host range gene, CP77, affects phosphorylation of eIF2 alpha and vaccinia viral translation in apoptotic HeLa cells

Host restriction of vaccinia virus has been previously described in CHO and RK13 cells in which a cowpox virus CP77 gene rescues vaccinia virus growth at the viral protein translation level. Here we investigate the restrictive stage of vaccinia virus in HeLa cells using a vaccinia mutant virus (VV-hr) that contains a deletion of 18-kb genome sequences resulting in no growth in HeLa cells. Insertion of CP77 gene into VV-hr generated a recombinant virus (VV-36hr) that multiplied well in HeLa cells. Both viruses could enter cells, initiate viral DNA replication and intermediate gene transcription. However, translation of viral intermediate gene was only detected in cells infected with VV-36hr, indicating that CP77 relieves host restriction at the intermediate gene translation stage in HeLa cells. Caspase-2 and -3 activation was observed in HeLa cells infected with VV-hr coupled with dramatic morphological alterations and cleavage of the translation initiation factor eIF4G. Caspase activation was reduced in HeLa cells infected with VV-36hr, indicating that CP77 acts upstream of caspase activation. Enhanced phosphorylation of PKR and eIF2alpha was also observed in cells infected with VV-hr and was suppressed by CP77. Suppression of eIF4G cleavage with the caspase inhibitor ZVAD did not rescue virus translation, whereas expression of a mutant eIF2alpha protein with an alanine substitution of serine at amino acid position 51 (eIF2alphaS51A) partially restored viral translation and moderately increased virus growth in HeLa cells.

Identification of the orthopoxvirus p4c gene, which encodes a structural protein that directs intracellular mature virus particles into A-type inclusions

The orthopoxvirus gene p4c has been identified in the genome of the vaccinia virus strain Western Reserve. This gene encodes the 58-kDa structural protein P4c present on the surfaces of the intracellular mature virus (IMV) particles. The gene is disrupted in the genome of cowpox virus Brighton Red (BR), demonstrating that although the P4c protein may be advantageous for virus replication in vivo, it is not essential for virus replication in vitro. Complementation and recombination analyses with the p4c gene have shown that the P4c protein is required to direct the IMV into the A-type inclusions (ATIs) produced by cowpox virus BR. The p4c gene is highly conserved among most members of the orthopoxvirus genus, including viruses that produce ATIs, such as cowpox, ectromelia, and raccoonpox viruses, as well as those such as variola, monkeypox, vaccinia, and camelpox viruses, which do not. The conservation of the p4c gene among the orthopoxviruses, irrespective of their capacities to produce ATIs, suggests that the P4c protein provides functions in addition to that of directing IMV into ATIs. These findings, and the presence of the P4c protein in IMV but not extracellular enveloped virus (D. Ulaeto, D. Grosenbach, and D. E. Hruby, J. Virol. 70:3372-3377, 1996), suggest a model in which the P4c protein may play a role in the retrograde movement of IMV particles, thereby contributing to the retention of IMV particles within the cytoplasm and within ATIs when they are present. In this way, the P4c protein may affect both viral morphogenesis and processes of virus dissemination.

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.

Treatment of lethal cowpox virus respiratory infections in mice with 2-amino-7-[(1,3-dihydroxy-2-propoxy)methyl]purine and its orally active diacetate ester prodrug

The acyclic purine nucleoside analog, 2-amino-7-[(1,3-dihydroxy-2-propoxy)methyl]purine (S2242) and its orally active diacetate ester prodrug (HOE961) were reported to be potent inhibitors of vaccinia virus replication in cell culture and in infected mice. These compounds were evaluated further, using infections with the related cowpox virus. Against a wild-type (WT) cowpox virus strain in mouse C127I cell culture, 50% effective concentrations (EC(50), determined by plaque reduction assays) of S2242 and cidofovir (a positive control) were 3.5 and 1.0 microM, respectively. EC(50) values obtained against a cidofovir-resistant strain of the virus were 33 and 230 microM, respectively. Compounds were at least ten-fold less potent against WT virus in Vero cells than C127I cells. S2242 and cidofovir were 50% inhibitory to the proliferation of uninfected C127I cells at 340 and 180 microM, respectively, but neither compound inhibited Vero cell growth at 1000 microM. Mice were lethally infected with cowpox virus by intranasal inoculation, followed 24 h later by antiviral treatment for 5 consecutive days. Once or twice daily intraperitoneal (i.p.) treatments with either S2242 or HOE961 at 100 mg/kg per day resulted in > or = 70 survival compared with no survivors in the placebo group. Lower doses of these compounds (10 and 30 mg/kg per day) were not protective, however. Cidofovir was 100% protective at 30 mg/kg per day. A 10-day course of treatment gave comparable survival results and demonstrated the oral efficacy of HOE961. Treatments with S2242 (100 mg/kg per day) and cidofovir (30 mg/kg per day) each reduced lung and nasal virus titers by approximately ten-fold, whereas, HOE961 (100 mg/kg per day) was less active. Overall, S2242 and HOE961 were found to be effective against cowpox virus infections in mice but were less potent than cidofovir. Since, HOE961 was orally active, it may have advantages over the other parenterally administered compounds for treating orthopoxvirus infections.

Orthopoxvirus IL-18 binding proteins: affinities and antagonist activities

The affinities of purified recombinant human IL-18 binding protein (BP) and ectromelia and cowpox virus homologs for human and murine IL-18 were compared by plasmon resonance. The dissociation constants of human IL-18BP were similar for murine and human IL-18. By contrast, the dissociation constants of the viral proteins for murine IL-18 were 12- to 50-fold lower than that for human IL-18. The ectromelia and cowpox virus proteins were biologically active, as judged by their ability to inhibit induction of interferon-gamma by murine and human IL-18. The relative affinities of the orthopoxvirus IL-18BPs are consistent with the rodent host range of the viruses.

Ectromelia, vaccinia and cowpox viruses encode secreted interleukin-18-binding proteins

Interleukin-18 (IL-18) is a proinflammatory cytokine that plays a key role in the activation of natural killer and T helper 1 cell responses principally by inducing interferon-gamma (IFN-gamma). Human and mouse secreted IL-18-binding proteins (IL-18BPs) have recently been described which block IL-18 activity but have no sequence similarity to membrane IL-18 receptors. Several poxvirus genes encode proteins with sequence similarity to IL-18BPs. Here we show that vaccinia, ectromelia and cowpox viruses secrete from infected cells a soluble IL-18BP (vIL-18BP) that may modulate the host antiviral response. The ectromelia virus protein was found to block NF-kappaB activation and induction of IFN-gamma in response to IL-18. The highly attenuated vaccinia virus modified virus Ankara encodes IL-18-binding activity, and thus deletion of the vIL-18BP may improve further the safety and immunogenicity of this promising human vaccine candidate. We confirm that molluscum contagiosum virus, a molluscipoxvirus that produces small skin tumours in immunocompetent individuals and opportunistic infections in immunodeficient AIDS patients, also encodes a related, larger vIL-18BP (gene MC54L). This protein may contribute to the lack of inflammatory response characteristic of molluscum contagiosum virus lesions. The expression of vIL-18BPs by distinct poxvirus genera that cause local or general viral dissemination, or persistent or acute infections in the host, emphasizes the importance of IL-18 in response to viral infections.

Novel acylation of poxvirus A-type inclusion proteins

Myristylation is one of several post-translational modifications that occur on vaccinia virus (VV) proteins. Previously, time course labeling of VV-infected cells with myristic acid had indicated that five late proteins (17, 25, 36, 38 and 92 kDa) are myristylated. Four of these proteins were mapped to the E7R, L1R, AI6L and G9R open-reading frames, respectively, because of the predicted presence of the N-myristyltransferase recognition sequence (M-G-X-X-X-S/T/A) at their amino termini. In contrast, computer analyses of large (80-100 kDa) VV open reading frames did not reveal any predicted species with this N-terminal motif. By immunoprecipitation with monospecific sera and transient expression of cloned gene products, the myristylated 92-kDa protein has been demonstrated to be the A-type inclusion protein encoded by the Western Reserve (WR) strain of VV. Labeling of cowpox virus (CPV) infected cells with myristic acid indicated that the 160-kDa A-type inclusion protein appears to be myristylated as well. Both the VV 92-kDa and the CPV 160-kDa A-type inclusion proteins labeled with myristic acid were stable to hydroxylamine treatment, suggesting an amide linkage between the fatty acid and the acceptor protein. HPLC analysis confirmed that the 92-kDa protein was in fact myristylated. This data suggests that poxvirus ATI proteins may be subject to a novel type of internal myristylation modification, and the roles such modifications may play in the replication cycles of these viruses is discussed.

The Amsacta moorei entomopoxvirus spheroidin gene is improperly transcribed in vertebrate poxviruses

The Amsacta moorei entomopoxvirus (AmEPv) spheroidin is the most highly expressed late viral gene product in infected insect cells. However, when a cassette containing the spheroidin gene and putative promoter was inserted into cowpox (CPV) or vaccinia viruses, only very low levels of spheroldin gene expression were observed. Primer extension analysis suggests much lower spheroidin gene transcript levels than seen either for the highly expressed CPV A-type inclusion gene or for the spheroidin gene within infected insect cells, indicating that in vertebrate cells, the spheroidin promoter functions poorly if at all. Examination of the spheroidin mRNA synthesized in recombinant CPV shows that the 5' start site of the spheroidin transcript was also unexpectedly imprecise and upstream (approximately 31 bp) of the well-defined start site normally observed in AmEPV-infected insect cells. Sequencing of the 5' terminus of the CPV recombinant spheroidin mRNA suggested that 5' poly(A), a characteristic feature of late poxvirus mRNAs and spheroidin mRNA derived from insect cells, was absent, despite the presence of the typical vertebrate poxvirus late promoter consensus sequence, TAAATG. Our results indicate that insect and vertebrate poxvirus promoters may not be universally interchangeable and imply that there are regulatory features of gene expression unique to the infected insect cell environment.

Complementation of a vaccinia virus host-range K1L gene deletion by the nonhomologous CP77 gene

We investigated the host-range restriction of a vaccinia virus (VV) K1L deletion mutant in rabbit kidney RK13 cells and the ability of the nonhomologous cowpox virus CP77 gene to overcome this block. Viral early mRNAs were made by K1L- VV but early protein synthesis was arrested consistent with a translational block. Replication of viral DNA did not occur and neither intermediate nor late mRNAs or proteins were detected. These results indicated that host-range restriction occurs earlier in RK13 cells than in Chinese hamster ovary cells (CHO) cells infected with CP77- VV, where the block occurs at translation of intermediate stage mRNA. We confirmed a report (Perkus et al., Virology 179, 276-286, 1990) that the CP77 gene, which allows VV replication in CHO cells, could replace the K1L gene for plaque formation in RK13 cells. However, the size of the plaques formed by K1L-CP77+ VV was smaller than those formed by K1L+CP77- VV. Single-step growth curves also showed that the CP77 gene could functionally replace the K1L gene, although formation of infectious virus was delayed and did not reach the same level as that of K1L+ VV. Most surprisingly, the dramatic shutoff of viral and host gene expression was similar in RK13 cells infected with K1L-CP77- VV and K1L-CP77+ VV and little difference was noted for the first 6 hr. Subsequently, in cells infected with the K1L-CP77+ VV, viral early protein synthesis was spontaneously resurrected and the replication cycle proceeded. Despite the absence of homology, K1L and CP77 gene products appear to be acting in a common virus/cell interaction pathway.

Vaccinia and cowpox viruses encode a novel secreted interleukin-1-binding protein

Supernatants from vaccinia virus (VV)-infected CV-1 cells were examined and found to contain a 33 kd protein capable of binding murine interleukin-1 beta (mIL-1 beta). A VV open reading frame (ORF) that exhibits 30% amino acid identity to the type II IL-1 receptor was expressed in CV-1-EBNA cells and shown specifically to bind mIL-1 beta. A similar ORF from cowpox virus was expressed and also specifically bound mIL-1 beta. A recombinant VV was constructed in which this ORF was disrupted (vB15RKO). Supernatants from vB15RKO-infected cells did not contain an IL-1-binding protein. Supernatants from VV-infected CV-1 cells were capable of inhibiting IL-1-induced murine lymphocyte proliferation in vitro while supernatants from vB15RKO infected cells did not. Intracranial inoculation of mice with vB15RKO suggests that this ORF is involved in VV virulence. The possible role of a virus-encoded IL-1-binding protein in the pathology of a poxvirus infection and its relationship to other poxvirus-encoded immune modulators is discussed.