Vaccinia Virus Mutant Research Articles

Vaccinia Virus Nucleoside Triphosphate Phosphohydrolase I Is an Essential Viral Early Gene Transcription Termination Factor

Deng and Shuman (J. Biol Chem.271, 29386 (1996)) reported that an ATPase different from the known viral termination factor, VTF, is required for vaccinia virus early gene transcription termination. Properties of this ATPase were similar to those of a known vaccinia virus enzyme, nucleoside triphosphate phosphohydrolase I (NPH I) the product of gene D11L. Transcription-competent cell-free extracts were prepared from A549 cells infected with wild-type or mutant vaccinia virus harboring ts mutations in gene D11L. These extracts were employed to investigate the role of NPH I in early gene transcription termination. Extracts prepared under nonpermissive conditions from both wild-type virus and ts mutant virus-infected cells exhibited high levels of early and intermediate gene transcription activity but were incapable of supporting late gene transcription. ts mutant extracts lacked signal-dependent early gene transcription termination activity, which was restored by the addition of either free NPH I or a GST-NPH I fusion protein. A comparison of the NPH I amino acid sequence to the protein databases revealed the presence of a set of sequences characteristic of nucleic acid helicase superfamily II members. A series of site-specific mutations in the helicase motifs and N-terminal and C-terminal deletion mutations were expressed as GST fusion proteins and their activities assessed. Of the mutations in helicase motifs I to VI, alteration of all but motif III reduced the ATPase activity. Removal of as few as 24 amino acids from the N-terminal end eliminated ATPase activity, while deletion of 68 C-terminal amino acids exhibited only a modest decrease in ATP hydrolysis. Larger C-terminal deletions eliminated ATPase activity. Each deletion mutation, and site-specific mutations other than the motif III mutation, failed to support transcription terminationin vitro.Mutations in motifs I, II, V, and VI inhibit wild-type NPH I transcription termination activity. However, deletion of up to 68 amino acids from the C-terminal end eliminates this inhibitory property. This observation is particularly interesting since these C-terminal deletions retain both ATPase activity and single-stranded DNA binding activity. Their failure to inhibit transcription termination suggests that these C-terminal deletion mutations eliminate a site required for a function other than from DNA binding or ATP hydrolysis.

Roles of vaccinia virus EEV-specific proteins in intracellular actin tail formation and low pH-induced cell-cell fusion.

During vaccinia virus (VV) morphogenesis intracellular mature virus (IMV) is wrapped by two additional membranes to form intracellular enveloped virus (IEV). IEV particles can nucleate the formation of actin tails which aid movement of IEVs to the cell surface where the outer IEV membrane fuses with the plasma membrane forming cell-associated enveloped virus (CEV) which remains attached to the cell, or extracellular enveloped virus (EEV) which is shed from the cell. In this report, we have used a collection of VV mutants lacking individual EEV-specific proteins to compare the roles of these proteins in the formation of IEV and IEV-associated actin tails and fusion of infected cells after a low pH shock. Data presented here show that p45-50 (A36R) is not required for IEV formation or for acid-induced cell-cell fusion, but is required for formation of IEV-associated actin tails. In contrast, gp86 (A56R), the virus haemagglutinin, is not required for formation of either IEV or IEV-associated actin tails. Data presented also confirm that p37 (gene F13L), gp42 (B5R) and gp22-24 (A34R) are needed for formation of IEV-associated actin tails and for cell-cell fusion after low pH shock. The phenotypes of these mutants were not affected by the host cell type as similar results were obtained in a range of different cells. Lastly, comparisons of the phenotypes of VV strains Western Reserve, deltaA34R and deltaA36R demonstrate that actin tails are not required for low pH-induced cell-cell fusion.

Role for the Vaccinia Virus A36R Outer Envelope Protein in the Formation of Virus-Tipped Actin-Containing Microvilli and Cell-to-Cell Virus Spread

A small-plaque-forming vaccinia virus mutant with a deletion in the A36R gene encoding an outer envelope protein (Parkinson and Smith,Virology,204, 376–390, 1994) was shown to assemble wrapped forms of intra- and extracellular virus particles and to mediate acid-induced polykaryon formation. The intracellular virions, however, did not acquire actin tails and those on the cell surface were not associated with specialized microvilli. This phenotype is similar to that of the A34R (E. J. Wolffe, E. Katz, A. Weisberg, and B. Moss,J. Virol.71, 3904–3915, 1997) and A33R (R. Roper, E. J. Wolffe, A. Weisberg, and B. Moss,J. Virol.,in press) deletion mutants. Taken together, these data support a model in which the envelope proteins encoded by the A33R, A34R, and A36R genes are all required for nucleation of actin tails, which facilitate dissemination rather than egress of virus particles.

Vaccinia Virus E3L Protein Is an Inhibitor of the Interferon (IFN)-Induced 2-5A Synthetase Enzyme

Induction of apoptosis in mammalian cells by double-stranded (ds) RNA-dependent enzymes, protein kinase (PKR), and 2-5A-synthetase/RNase L (referred to as the 2-5A system) might be a mechanism mediating anticellular and antiviral actions of interferon (IFN). To counteract the effect of IFN, animal viruses have acquired genes that block specific IFN pathways. Among poxviruses, vaccinia virus (VV) encodes E3L, a dsRNA-binding protein, which inhibits activation of IFN-induced PKR. It has been proposed that E3L might also block activation of the 2-5A system, but direct proof is lacking. To establish if E3L inhibits the 2-5A system, we have developed a method to assay apoptosis induced by increased production of enzymes in the 2-5A pathway, as well as of their putative modulators. This assay is based on the use of cells derived from homozygous PKR knockout mice (Pkr−/−) infected with a VV mutant lacking E3L (ΔE3L) and transiently transfected with a luciferase reporter gene together with plasmid vectors expressing 2-5A-synthetase, RNase L, or E3L, all controlled by the same inducible promoter. We found that expression of 2-5A-synthetase inhibited luciferase activity in a dose–response manner, reaching inhibition values of 80% relative to transfections with control plasmids. Similar results were obtained by transfection with an RNase L vector, although in this case the extent of inhibition was further enhanced upon coexpression of 2-5A-synthetase and RNase L. Inhibition of protein synthesis mediated by the 2-5A system correlated well with induction of apoptosis. Transfection of cells with a plasmid vector expressing E3L together with 2-5A-synthetase completely prevented apoptosis induced by this enzyme. We conclude that VV E3L acts as an inhibitor of the IFN-induced 2-5A-synthetase enzyme.

Activation of antiviral protein kinase leads to immunoglobulin E class switching in human B cells.

An epidemiologic association between viral infections and the onset of asthma and allergy has been documented. Also, evidence from animal and human studies has suggested an increase in antigen-specific immunoglobulin E (IgE) production during viral infections, and elevated levels of IgE are characteristic of human asthma and allergy. Here, we provide molecular evidence for the roles of viral infection and of activation of the antiviral protein kinase (PKR) (double-stranded-RNA [dsRNA]-activated protein kinase) in the induction of IgE class switching. The presence of dsRNA, a known component of viral infection and an activator of PKR, induced IgE class switching as detected by the expression of germ line epsilon in the human Ramos B-cell line. Furthermore, dsRNA treatment of Ramos cells resulted in the activation of PKR and in vivo activation of the NF-kappaB complex. Interestingly, infection of Ramos cells with rhinovirus (common cold virus) serotypes 14 and 16 resulted in the induction of germ line epsilon expression. To further evaluate the role of PKR in the viral induction of IgE class switching, we infected Ramos cells with two different vaccinia virus (cowpox virus) strains. Infection with wild-type vaccinia virus failed to induce germ line epsilon expression; however, a deletion mutant of vaccinia virus (VP1080) lacking the PKR-inhibitory polypeptide E3L induced the expression of germ line epsilon. Collectively, the results of our study define a common molecular mechanism underlying the role of viral infections in IgE class switching and subsequent induction of IgE-mediated disorders such as allergy and asthma.

Analysis of a Temperature-Sensitive Vaccinia Virus Mutant in the Viral mRNA Capping Enzyme Isolated by Clustered Charge-to-Alanine Mutagenesis and Transient Dominant Selection

We have previously reported the successful development of a targeted genetic method for the creation of temperature-sensitive vaccinia virus mutants [D. E. Hassett and R. C. Condit (1994)Proc. Natl. Acad. Sci. USA91, 4554–4558]. This method has now been applied to the large subunit of the multifunctional vaccinia virus capping enzyme, encoded by gene D1R. Ten clustered charge-to-alanine mutations were created in a cloned copy of D1R. Four of these mutations were successfully transferred into the viral genome using transient dominant selection, and each of these four mutations yielded viruses with plaque phenotypes different from that of wild-type virus. Two of the mutant viruses, 516 and 793, were temperature sensitive in a plaque assay. Mutant 793 was also temperature sensitive in a one-step growth experiment. Phenotypic characterization of the 793 virus under both permissive and nonpermissive conditions revealed nearly normal patterns of viral protein and mRNA synthesis. Under nonpermissive conditions the 793 virus was defective in telomere resolution and blocked at an intermediate stage of viral morphogenesis.In vitroassays of various capping enzyme activities revealed that in permeabilized virions, enzyme guanylylate intermediate formation was reduced and methyltransferase activity was thermolabile, while in solubilized virion extracts enzyme guanylylate activity was reduced and both guanylyltransferase and methyltransferase activities were absent. Thus, the 793 mutation affects at least two separate enzymatic activities of the capping enzyme, guanylyltransferase and methyltransferase, and when incorporated into the virus genome, the mutation yields a virus that is temperature sensitive for growth, telomere resolution, and virion morphogenesis.

Poxviruses as expression vectors

Poxviruses are widely used for the cytoplasmic expression of recombinant genes in mammalian cells. Recent improvements allow high expression and simplify the integration of multiple foreign genes. Vaccinia virus mutants and other poxviruses that undergo abortive infection in mammalian cells are receiving special attention because of their diminished cytopathic effects and increased safety. New replicating and ‘non-replicating’ vectors, encoding the bacteriophage T7 RNA polymerase for transcription of recombinant genes, have been engineered.

Construction of a vaccinia virus deficient in the essential DNA repair enzyme uracil DNA glycosylase by a complementing cell line.

The vaccinia virus D4R open reading frame, encoding the essential DNA repair enzyme uracil DNA glycosylase, was expressed in two permanent cell lines, the rabbit kidney cell line RK13 and the human fibroblast cell line 293. The temperature-sensitive vaccinia virus mutant ts4149, which maps within D4R, was able to grow under restrictive conditions in both of these transformed cell lines. Cell clones complemented D4R function to various degrees, demonstrating complementation of an essential vaccinia virus gene by a cell line constitutively expressing the essential function. Thus, the complementing host cells allowed the rescue of a virus defective in the D4R gene, demonstrating that this system may be used for the propagation of defective cytoplasmic DNA viruses. The defective virus grew to high yields only in the engineered cell lines. The data support the hypothesis that early gene products, such as uracil DNA glycosylase, supplied in trans can fully complement essential viral functions.

Complementation of Deletion of the Vaccinia Virus E3L Gene by theEscherichia coliRNase III Gene

This work investigated whether theEscherichia coliRNase III gene,rnc+, could complement vp1080, a mutant vaccinia virus that is deleted of its E3L gene. Like E3L,rnc+ codes for a dsRNA binding protein that contains an additional nucleolytic activity.Rncgenes were cloned into the eukaryotic expression vector pMTVa−, expressed in COS-1 cells, and shown to be functional. Transient rescue experiments in HeLa cells demonstrated that the cleavage function of thernc+ gene was necessary for full rescue of vp1080. Thernc70 gene, which encodes a product deficient in catalytic activity but still capable of binding to dsRNA, rescued vp1080 weakly. Thernc105 gene, which encodes a product that cannot bind or cleave dsRNA, was unable to rescue vp1080. Therncgenes were also inserted into the E3L locus of vp1080. While recombinants containing thernc+ gene or thernc70 gene regained the IFN resistance phenotype in RK13cells, full host range of vaccinia virus was only restored in the recombinant containing thernc+ gene. Thus, the ability of RNase III to process dsRNA appears to be necessary to restore the host range phenotype. The vp-rnc 105 recombinant behaved similarly to vp1080.

Phenotypic characterization of mutants in vaccinia virus gene G2R, a putative transcription elongation factor.

The phenotypic defects of two mutants of vaccinia virus, the lesions of which map to gene G2R, were characterized in vivo, and the results suggest a role for the G2R protein in viral transcription elongation. Both a temperature-sensitive mutant, Cts56, and an isatin-beta-thiosemicarbazone-dependent deletion mutant, G2A, in gene G2R have a characteristic and unique defect in late viral gene expression. The G2R mutants synthesize early viral RNA, early viral proteins, and viral DNA normally under nonpermissive conditions. In G2R mutants, late viral protein synthesis begins at the normal time, low-molecular-weight viral proteins are synthesized in normal quantities, but synthesis of high-molecular-weight viral proteins is reduced in amount. Intermediate and late promoter utilization is normal in G2R mutants, but intermediate and late RNAs are reduced in size. The reduction in length of the intermediate and late mRNAs represents a truncation of mRNA 3' ends. Thus, intermediate and late RNAs are too short to encode large proteins but long enough to encode small proteins, therefore accounting for the protein synthesis phenotype. These results suggest that the G2R protein acts to regulate the elongation potential of the viral RNA polymerase late during a vaccinia virus infection.

Characterization of ts 16, a temperature-sensitive mutant of vaccinia virus.

We have characterized a temperature-sensitive mutant of vaccinia virus, ts16, originally isolated by Condit et al. (Virology 128:429-443, 1983), at the permissive and nonpermissive temperatures. In a previous study by Kane and Shuman (J. Virol 67:2689-2698, 1993), the mutation of ts16 was mapped to the I7 gene, encoding a 47-kDa protein that shows partial homology to the type II topoisomerase of Saccharomyces cerevisiae. The present study extends previous electron microscopy analysis, showing that in BSC40 cells infected with ts16 at the restrictive temperature (40 degrees C), the assembly was arrested at a stage between the spherical immature virus and the intracellular mature virus (IMV). In thawed cryosections, a number of the major proteins normally found in the IMV were subsequently localized to these mutant particles. By using sucrose density gradients, the ts16 particles were purified from cells infected at the permissive and nonpermissive temperatures. These were analyzed by immunogold labelling and negative-staining electron microscopy, and their protein composition was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. While the ts16 virus particles made at the permissive temperature appeared to have a protein pattern identical to that of wild-type IMV, in the mutant particles the three core proteins, p4a, p4b, and 28K, were not proteolytically processed. Consistent with previous data the sucrose-purified particles could be labelled with [3H]thymidine. In addition, anti-DNA labelling on thawed cryosections suggested that most of the mutant particles had taken up DNA. On thawed cryosections of cells infected at the permissive temperature, antibodies to I7 labelled the virus factories, the immature viruses, and the IMVs, while under restrictive conditions these structures were labelled much less, if at all. Surprisingly, however, by Western blotting (immunoblotting) the I7 protein was present in similar amounts in the defective particles and in the IMVs isolated at the permissive temperature. Finally, our data suggest that at the nonpermissive temperature the assembly of ts16 is irreversibly arrested in a stage at which the DNA is in the process of entering but before the particle has completely sealed, as monitored by protease experiments.

Identification and characterization of the thymidine kinase gene of Yaba virus.

DNA of Yaba virus, a member of the poxviruses, was mapped by cross-hybridization between fragments of various restriction enzymes. The genome was approximately 135 kb in length and possessed two characteristic features of poxviruses: cross-links and inverted terminal repeats at both termini. Hybridization of fragments of Yaba virus DNA to known vaccinia virus DNA fragments indicated that the thymidine kinase (TK) gene mapped within the 0.9 kb XhoI-HincII fragment between 52.5 and 53.5 kb from the left end of the genome. The fragment could rescue the TK+ phenotype in TK- cells preinfected with a TK vaccinia virus mutant. Nucleotide sequencing of the fragment revealed an ORF capable of encoding 181 amino acids. The sequence TAAAAATGAAAAATTA upstream of the ORF was considered to be the promoter and the downstream sequence TTTTTAT to be the early transcription termination signal. These sequences are in good accord with the consensus regulatory sequences for the expression of early genes of other known poxviruses. The amino acid sequence similarity among the poxvirus TK genes suggests that Yaba virus is most closely related to swinepox virus and less similar to fowlpox virus.

Stable expression of the vaccinia virus K1L gene in rabbit cells complements the host range defect of a vaccinia virus mutant.

Modified vaccinia virus Ankara (MVA), having acquired genomic deletions during passage in chicken embryo fibroblasts, is highly attenuated and unable to productively infect most mammalian cell lines. Multiplication in rabbit kidney-derived RK13 cells, but not other nonpermissive cells, can be restored by insertion of the vaccinia virus K1L gene into the MVA genome. During nonproductive infection of RK13 cells by MVA, transcription of representative viral early genes was revealed by Northern (RNA) blotting, whereas synthesis of an intermediate mRNA and replication of viral DNA could not be detected. Despite the persistence of viral early mRNA for at least several hours, synthesis of virus-induced polypeptides occurred only during the first hour and was followed by abrupt inhibition of all protein synthesis. Transfection of RK13 cells with a eukaryotic expression plasmid that contained the K1L gene allowed MVA infection to proceed to late stages of viral protein synthesis. Moreover, RK13 cell lines that stably expressed the K1L gene were permissive for MVA as well as a K1E deletion mutant of the WR strain of vaccinia virus. This is the first description of the complementation of a poxvirus mutant by cells that stably express a viral gene.

Products of the porcine group C rotavirus NSP3 gene bind specifically to double-stranded RNA and inhibit activation of the interferon-induced protein kinase PKR.

The porcine group C rotavirus (Cowden strain) NSP3 protein (the group C equivalent of the group A gene 7 product, formerly called NS34) shares homology with known double-stranded RNA-binding proteins, such as the interferon-induced, double-stranded RNA-dependent protein kinase PKR. A clone of NSP3, expressed both in vitro and in COS-1 cells, led to the synthesis of minor amounts of a product with an M(r) of 45,000 (the expected full-length M(r) of NSP3) and major amounts of products with M(r)s of 38,000 and 8,000. Restriction enzyme digestion analysis prior to expression in vitro and amino-terminal sequence analysis suggest that the products with M(r)s of 38,000 and 8,000 are cleavage products of the protein with an M(r) of 45,000. The full-length protein and the product with an M(r) of 8,000, both of which contain the motif present in double-stranded RNA-binding proteins, bound specifically to double-stranded RNA. The products with M(r)s of 45,000 and 8,000 were also detected in Cowden strain-infected MA104 cells. NSP3 products expressed in COS-1 cells were capable of inhibiting activation of the double-stranded RNA-dependent protein kinase similar to other double-stranded RNA-binding proteins, and NSP3 products expressed in HeLa cells were capable of rescuing the replication of an interferon-sensitive deletion mutant of vaccinia virus.

TAR RNA-binding protein is an inhibitor of the interferon-induced protein kinase PKR.

A cDNA encoding a double-stranded-RNA (dsRNA)-binding protein was isolated by screening a HeLa cell cDNA expression library for proteins that bind the HIV-1 Rev-responsive-element RNA. The cDNA encoded a protein that was identical to TRBP, the previously reported cellular protein that binds the transactivation response element (TAR) RNA of human immunodeficiency virus type 1. TRBP inhibited phosphorylation of the interferon-induced ribosome-associated protein kinase PKR and of the eukaryotic translation initiation factor eIF-2 alpha in a transient-expression system in which the translation of a reporter gene was inhibited by the localized activation of PKR. TRBP expression in HeLa cells complemented the growth and protein-synthesis defect of a vaccinia virus mutant lacking the expression of the dsRNA-binding protein E3L. These results implicate TRBP as a cellular regulatory protein that binds RNAs containing specific secondary structure(s) to mediate the inhibition of PKR activation and stimulate translation in a localized manner.

The Interferon-induced Double-Stranded RNA-Activated Protein Kinase Induces Apoptosis

Interferons (IFNs) exert antitumor activities, but the molecular mechanism underlying these effects is poorly understood. IFN-induced, double-stranded (ds) RNA-activated protein kinase (p68 kinase) has long been implicated in mediating the antiproliferative effects of IFN. In addition, recent studies suggest that p68 kinase may function as a tumor suppressor gene. In this investigation we showed that expression of p68 kinase in HeLa cells resulted in a rapid cell death characteristic of apoptosis. Rapid cell death was not observed in cells which expressed a mutant form of p68 kinase (lys 296 → arg) indicating that cell death observed is the result of p68 kinase expression and activation. Moreover, infection of HeLa cells with the mutant vaccinia virus lacking E3L gene, which encodes a dsRNA binding protein that acts as an inhibitor of p68 kinase, also resulted in apoptosis. Thus, we propose that human p68 kinase functions as a tumor suppressor gene by actively participating in apoptosis.

Assembly of vaccinia virus: effects of rifampin on the intracellular distribution of viral protein p65.

The cytoplasmic assembly of vaccinia virus is reversibly blocked by the antibiotic rifampin, leading to the accumulation of partially membrane-delineated rifampin bodies in infected cells. Rifampin-resistant vaccinia virus mutants have point mutations in the D13L gene, which is controlled by a late promoter and expresses a 65-kDa protein, designated p65. To further characterize the mechanism of rifampin inhibition and the function of p65 in virus assembly, we raised antibodies to this protein. Immunoreactive p65 was expressed at late times of infection, and neither its expression nor its turnover was affected by rifampin. Virus-associated p65 could be extracted only with denaturing detergents from purified virions, suggesting that it is an integral viral component. Immunofluorescence studies showed that p65 is localized to the sites of virus assembly. Also, immunoelectron microscopy showed p65 to be associated with viral crescents as well as spherical, immature virions, in both cases predominantly on the inner or concave surface. In the presence of rifampin, p65 was found in large, cytoplasmic inclusion bodies that were distinct from rifampin bodies. The rifampin bodies themselves were labeled with p65 antibodies only after reversal of the rifampin block, predominantly on the viral crescents which rapidly formed following removal of the drug. We propose that p65 functions as an internal scaffold in the formation of viral crescents and immature virions, analogously to the matrix proteins of other viruses.

Identification of Viral Membrane Proteins Required for Cell Fusion and Viral Dissemination That Are Modified during Vaccinia Virus Persistence

Wild-type vaccinia virus WR strain forms non-fusogenic (F-) large plaques and is hemagglutinin positive (HA+) under normal conditions of virus infection. We have analyzed a collection of spontaneous, highly attenuated mutants of vaccinia virus isolated from persistently infected Friend erythroleukemia cells (E. Paez, S. Dallo, and M. Esteban, J. Virol. 61, 2642-2647, 1987) for the ability to express HA during virus infection. After 14 cell passages, all the mutants isolated were hemadsorption negative (HAD-) and did not synthesize a HA that could be recognized by anti-HA monoclonal antibodies. All these HA- mutants induced extensive cell-cell fusion (F+), with the exception of two mutants (65-16 and 101-14) isolated from late cell passages. Nucleotide sequence analysis of the HA gene in these two mutants confirmed the HA- phenotype. A frameshift mutation very close to the initiation codon resulted in premature translational termination. The truncated gene now only encodes the first 25 amino acids. Analysis of progeny from "wild-type," like early serial passage virus (5-3) X mutant back crosses, shows that for one late passage non-fusogenic small-plaque mutant (101-14) among large plaque progeny there is good correspondence between the ability to fuse and the absence of a viral HA and that each large plaque mutant contains a normal 14 kDa membrane protein. However, with a second serial passage mutant 65-16, which, like 101-14, is a nonfusogenic small-plaque variant, there is again an excellent correlation between the inability to synthesize HA and the ability to fuse, but there is no correlation of plaque size with a normal 14 kDa viral membrane protein, as most large plaque mutants encode a larger, i.e., 17 kDa protein. Rescue experiments of 65-16 with bona fide cloned 14 kDa protein gene confirm that the ability to regulate plaque size and cell fusion in this mutant is due to a protein other than the 14 kDa protein. Marker rescue experiments indicated that the map position of the additional mutation coincided with a small deletion occurring in the HindIII F fragment. This deletion affected the 5′-end and promoter sequences of the 37 kDa envelope protein gene and produced a lack of expression of this protein, shown by others to be involved in the formation of extracellular enveloped virus in infected cells. These results shown that viruses with mutations of viral membrane proteins required for cell fusion and viral dissemination are selected during vaccinia virus persistence in cell culture.

In Vitro and in Vivo Study of the Ectromelia Virus Homolog of the Vaccinia Virus K1L Host Range Gene

Ectromelia virus encodes a protein which is homologous to the product of the vaccinia virus host range gene, K1L, except for eight conservative and two non-conservative substitutions and an additional threonine residue at the carboxyl terminus. Unlike the vaccinia virus gene, the ectromelia virus homolog failed to support optimal virus replication in RK-13 cells and appeared to be expressed 20-fold loss efficiently. This lower level of expression was not due to the genetic background of the virus, K1L RNA transcription, sequence of the K1L RNA leader, or stability of K1L RNA or protein. Infections of RK-13 cells with ectromelia or vaccinia virus mutants lacking an intact K1L gene resulted in transient expression of early genes followed by a rapid and irreversible cessation of both virus and host protein synthesis. Infections of the disease-susceptible ANCR or -resistant C57BL/6 mice with the K1L-lacking ectromelia virus yielded a pathogenesis pattern indistinguishable from wild-type, suggesting that the ectromelia virus homolog of vaccinia virus K1L is not important for ectromelia virus in vivo replication and spread.

Recombinant vaccinia viruses expressing GP46/M-2 protect against Leishmania infection

Leishmania is a genus of parasitic protozoa capable of causing a spectrum of human diseases. The GP46/M-2 membrane glycoprotein has been demonstrated in a murine model system to elicit a protective immune response against infection with Leishmania amazonensis; in highly susceptible BALB/c mice, immunization leads to significant protection against infection. In the present study, for induction of long-term immunological effects, two recombinant vaccinia viruses, derived from the wild type and attenuated variant 48-7 and expressing the GP46/M-2 protein, were constructed; to ensure safety, we used the attenuated vaccinia virus mutant (48-7) as a live vector. Susceptible BALB/c mice immunized with either GP46/M-2-recombinant vaccinia virus were significantly protected against infection with L. amazonensis; 45 to 76% of the animals were completely protected (sterile) against a challenge inoculum of 10(3) infective organisms. The protectively immunized animals demonstrated T- and B-cell-dependent immunological responses; both lymphokine responses as well as antibody responses and long-term memory are indicative of T-cell activation. This first report of the use of a recombinant vaccinia virus to induce protection against a Leishmania infection indicates that recombinant vaccinia viruses should be of value in the design of a safe and effective vaccine against this parasitic disease.