Assembly of virus particles during mixed infection with wild-type vaccinia and a rifampicin-resistant mutant

Poxvirus under the eyes of electron microscope

Zoonotic poxvirus infections pose significant threat to human health as we have witnessed recent spread of monkeypox. Therefore, insights into molecular mechanism behind poxvirus replication cycle are needed for the development of efficient antiviral strategies. Virion assembly is one of the key steps that determine the fate of replicating poxviruses. However, in-depth understanding of poxvirus assembly is challenging due to the complex nature of multi-step morphogenesis and heterogeneous virion structures. Despite these challenges, decades of research have revealed virion morphologies at various maturation stages, critical protein components and interactions with host cell compartments. Transmission electron microscopy has been employed as an indispensable tool for the examination of virion morphology, and more recently for the structure determination of protein complexes. In this review, we describe some of the major findings in poxvirus morphogenesis and the contributions of continuously advancing electron microscopy techniques.

Virological and Immunological Outcomes of Coinfections

Coinfections involving viruses are being recognized to influence the disease pattern that occurs relative to that with single infection. Classically, we usually think of a clinical syndrome as the consequence of infection by a single virus that is isolated from clinical specimens. However, this biased laboratory approach omits detection of additional agents that could be contributing to the clinical outcome, including novel agents not usually considered pathogens. The presence of an additional agent may also interfere with the targeted isolation of a known virus. Viral interference, a phenomenon where one virus competitively suppresses replication of other coinfecting viruses, is the most common outcome of viral coinfections. In addition, coinfections can modulate virus virulence and cell death, thereby altering disease severity and epidemiology. Immunity to primary virus infection can also modulate immune responses to subsequent secondary infections. In this review, various virological mechanisms that determine viral persistence/exclusion during coinfections are discussed, and insights into the isolation/detection of multiple viruses are provided. We also discuss features of heterologous infections that impact the pattern of immune responsiveness that develops.

A novel mode of poxvirus superinfection exclusion that prevents fusion of the lipid bilayers of viral and cellular membranes

Superinfection exclusion is a widespread phenomenon that prevents secondary infections by closely related viruses. The vaccinia virus A56 and K2 proteins in the cell membrane can prevent superinfection by interacting with the entry-fusion complex of subsequent viruses. Here, we described another form of exclusion that is established earlier in infection and does not require the A56 or K2 protein. Cells infected with one or more infectious virions excluded hundreds of superinfecting vaccinia virus particles. A related orthopoxvirus, but neither a flavivirus nor a rhabdovirus, was also excluded, indicating selectivity. Although superinfecting vaccinia virus bound to cells, infection was inhibited at the membrane fusion step, thereby preventing core entry into the cytoplasm and early gene expression. In contrast, A56/K2 protein-mediated exclusion occurred subsequent to membrane fusion. Induction of resistance to superinfection depended on viral RNA and protein synthesis by the primary virus but did not require DNA replication. Although superinfection resistance correlated with virus-induced changes in the cytoskeleton, studies with mutant vaccinia viruses indicated that the cytoskeletal changes were not necessary for resistance to superinfection. Interferon-inducible transmembrane proteins, which can inhibit membrane fusion in other viral systems, did not prevent vaccinia virus membrane fusion, suggesting that these interferon-inducible proteins are not involved in superinfection exclusion. While the mechanism remains to be determined, the early establishment of superinfection exclusion may provide a "winner-take-all" reward to the first poxvirus particles that successfully initiate infection and prevent the entry and genome reproduction of defective or less fit particles.

IMPORTANCE

The replication of a virus usually follows a defined sequence of events: attachment, entry into the cytoplasm or nucleus, gene expression, genome replication, assembly of infectious particles, and spread to other cells. Although multiple virus particles may enter a cell at the same time, mechanisms exist to prevent infection by subsequent viruses. The latter phenomenon, known as superinfection exclusion, can occur by a variety of mechanisms that are not well understood. We showed that superinfection by vaccinia virus was prevented at the membrane fusion step, which closely followed virion attachment. Thus, neither gene expression nor genome replication of the superinfecting virus occurred. Expression of early proteins by the primary virus was necessary and sufficient to induce the superinfection-resistant state. Superinfection exclusion may be beneficial to vaccinia virus by selecting particles that can infect cells rapidly, excluding defective particles and synchronizing the replication cycle.

Poxvirus cell entry: how many proteins does it take?

For many viruses, one or two proteins enable cell binding, membrane fusion and entry. The large number of proteins employed by poxviruses is unprecedented and may be related to their ability to infect a wide range of cells. There are two main infectious forms of vaccinia virus, the prototype poxvirus: the mature virion (MV), which has a single membrane, and the extracellular enveloped virion (EV), which has an additional outer membrane that is disrupted prior to fusion. Four viral proteins associated with the MV membrane facilitate attachment by binding to glycosaminoglycans or laminin on the cell surface, whereas EV attachment proteins have not yet been identified. Entry can occur at the plasma membrane or in acidified endosomes following macropinocytosis and involves actin dynamics and cell signaling. Regardless of the pathway or whether the MV or EV mediates infection, fusion is dependent on 11 to 12 non-glycosylated, transmembrane proteins ranging in size from 4- to 43-kDa that are associated in a complex. These proteins are conserved in poxviruses making it likely that a common entry mechanism exists. Biochemical studies support a two-step process in which lipid mixing of viral and cellular membranes is followed by pore expansion and core penetration.

Effect of virulence on immunogenicity of single and double vaccinia virus recombinants expressing differently immunogenic antigens: antibody-response inhibition induced by immunization with a mixture of recombinants differing in virulence

It has been shown recently that the residual virulence of vaccinia virus (VV) is an important factor that influences the outcome of immunization with VV recombinants. This study focused on the correlation of the residual virulence of several VV recombinants with antibody responses against the strongly immunogenic extrinsic glycoprotein E of varicella-zoster virus and the weakly immunogenic extrinsic protein preS2-S of hepatitis B virus and against VV proteins, with mice used as a model organism. Furthermore, the effects of mixing different recombinants on the antibody response were studied. The results obtained indicated that: (i) the antibody response depended on the residual virulence of the recombinants, more so in the case of the weakly immunogenic protein; (ii) the residual virulence, the growth rate of the VV recombinants in extraneural tissues and the immunogenicity were associated features; (iii) immunization with mixtures of two differently virulent recombinants or with unequal amounts of two similarly virulent recombinants sometimes led to the suppression of antibody response. The appearance of this suppression was dependent on three factors: the residual virulence of the recombinants, the immunogenicity of the extrinsic proteins and the ratio of the recombinants in the mixtures. Thus, the data obtained demonstrate that there are various limitations to the use of replicating VV recombinants for immunization purposes.

Immature viral envelope formation is interrupted at the same stage by lac operator-mediated repression of the vaccinia virus D13L gene and by the drug rifampicin

Specific missense mutations of the vaccinia virus D13L gene confer resistance to the effects of rifampicin on virion morphogenesis. We constructed a recombinant vaccinia virus in which elements of the Escherichia coli lac operator system were used to regulate the D13L gene. Replication of the recombinant vaccinia virus was dependent on addition of the inducer isopropyl beta-D-thiogalactoside (IPTG) and the virus yield was decreased by more than 99% when IPTG was omitted. Under the nonpermissive condition, transcription of the D13L gene was reduced and synthesis of the 65,000-Da protein product was inhibited by more than 95%. Consequently, virion morphogenesis was blocked at an early stage and uncoated membrane precursors of the immature viral envelope and uncleaved precursors of the major core proteins accumulated. The phenotype of the conditional lethal mutant virus, in the absence of IPTG, closely resembled that of wild-type virus in cells treated with rifampicin.

Superinfection exclusion of vaccinia virus in virus-infected cell cultures

Vaccinia virus-infected BSC 40 cells do not permit the replication of superinfecting vaccinia virus. The extent of superinfecting virus propagation depends on the time of superinfection; there is 90% exclusion by 4 hr after the initial infection, and more than 99% by 6 hr. When superinfection is attempted at 6 hr after infection, the superinfecting virus is incapable of carrying out DNA replication or early gene transcription, demonstrating that an early event in the virus life cycle is inhibited. The rate of adsorption of the superinfecting virus is unaltered which shows that exclusion is affected at a point between adsorption and early gene transcription. In order to exclude superinfection, the primary infecting virus does not require replication of its DNA or expression of its late genes but it must express one or more early genes.

Rifampicin prevents virosome localization of L65, an essential vaccinia virus polypeptide

In contrast to its irreversible effect on the Escherichia coliRNA polymerase beta-subunit, the antibiotic rifampicin reversibly inhibits vaccinia virus morphogenesis at a step during the formation of immature viral particles. The protein affected by the presence of rifampicin is L65, a major late vaccinia polypeptide to which mutations that confer rifampicin resistance have been mapped. We now provide evidence using a monospecific anti-L65 serum in concert with immunofluorescence and sucrose gradient analysis that the mechanism of action of rifampicin on vaccinia virus replication involves the inhibition of localization of L65 to the viral factories (virosomes) thereby blocking further development. Studies on the expression and distribution of L65 during the infection cycle reveal that L65 is a stable, nonglycosylated late protein associated with virions. These results are discussed in relationship to the possible in vivo functions of the L65 protein.

Genetic map of the vaccinia virus HindIII D Fragment

Seventeen ts mutants of vaccinia virus known to map to the viral HindIII D fragment (R. C. Condit and A. Motyczka, 1981, Virology 113, 224-241; R. C. Condit, A. Motyczka, and G. Spizz, 1983, Virology 128, 429-443; M. J. Ensinger and M. Rovinsky, 1983, J. Virol. 48, 419-428) have been sorted into seven complementation groups. The precise location of each mutant on the HindIII D DNA fragment has been identified by either one-step or two-step marker rescue. By a comparison of this genetic map and the known sequence of this DNA fragment (E. G. Niles et al., 1986, Virology 153, 96-112; S. L. Weinrich and D. E. Hruby, 1986, Nucleic Acids Res. 14, 3003-3016), each mutant has been assigned to a single gene in the HindIII D fragment. In several cases, the map position of a mutant has been localized to a region of fewer than 300 bp in length. The complementation groups are evenly distributed along the DNA. However, within a single gene, the mutants are often clustered.

Resistance of vaccinia virus to rifampicin conferred by a single nucleotide substitution near the predicted NH2 terminus of a gene encoding an Mr 62,000 polypeptide

Marker transfer procedures were used to locate the site of mutation in the genome of a previously characterized (B. Moss, E. N. Rosenblum, and P. Grimley, 1971), Virology 45, 135-148) rifampicin-resistant (RifR) vaccinia virus isolate. Starting with a cosmid library prepared from the mutant genome, recombination with successively smaller DNA fragments was shown to transfer drug resistance to wild-type vaccinia virus. In this manner, the mutation was mapped within a 485-bp DNA segment in the central region of the genome at the extreme right end of the HindIII D fragment. Nucleotide sequencing indicated that this DNA segment differed from the homologous region of wild-type DNA by a single C/G----A/T substitution. Sequencing of the flanking 2195 bp revealed two tandem nonoverlapping open reading frames (ORFs) encoding putative polypeptides of Mr 16,908 and 61,840. The RifR mutation resulted in a predicted glutamine----lysine change only 27 amino acids from the NH2 terminus of the longer ORF. A predicted asparagine to aspartic acid substitution, found in another RifR vaccinia virus mutant by J. Tartaglia and E. Paoletti (Virology 147, 394-404, 1985), mapped near the carboxyl terminus of the same ORF. These data suggest a model in which head-to-tail interaction between Mr 61,840 polypeptides occurs and in which rifampicin blocks virus assembly by preventing this association.

Vaccinia virus rifampicin-resistance locus specifies a late 63,000 Da gene product

The genetic locus specifying rifampicin-resistance (RifR) in a vaccinia virus mutant has been localized by marker rescue analysis (J. Tartaglia and E. Paoletti (1985) Virology 147, 394-404). The mutation was defined by DNA sequence analysis as an AT to GC transition occurring 56 bp to the left of the unique XhoI site within HindIII D. The point mutation resulted in an asparagine to aspartic acid substitution 60 amino acids from the predicted C-terminus. Specific DNA probes were used to characterize the RifR designated gene at the transcriptional and translational levels. This region is transcriptionally active only after vaccinia virus DNA synthesis, but not in the presence of cytosine arabinoside suggesting that the RifR function is a late gene product. Translation of hybrid selected RNA to DNA surrounding the mutant marker directed the synthesis of a polypeptide with an apparent mol wt of 63 kDa. Transcriptional and translational mapping studies showed that the mRNA encoding this 63-kDa polypeptide was initiated approximately 460 bp to the right of the HindIII D-A junction and was transcribed in a leftward direction into the HindIII D region.

Physical mapping and DNA sequence analysis of the rifampicin resistance locus in vaccinia virus

Rifampicin has been shown to inhibit the maturation of poxviruses at a discrete step in envelope formation (Moss et al., 1969; Pennington et al., 1970; Nagayama et al., 1970; Grimley et al., 1970). A rifampicin-resistant vaccinia virus mutant (RifR) was selected for its ability to grow in the presence of 100 micrograms/ml of rifampicin. Utilizing intact DNA or endonuclease restricted cloned DNA subfragments derived from the RifR mutant virus, the locus specifying rifampicin resistance was physically mapped by marker rescue analysis leftward of the unique XhoI site within the HindIII D fragment. DNA sequencing of a 445 bp fragment encompassing this region revealed an AT to GC transition when compared with the equivalent wild-type DNA fragment. Analysis of the six potential open reading frames within the 445-bp fragment indicated only one available open reading frame. On this basis, the rifampicin-resistant vaccinia virus mutant was shown to have a codon transition from asparagine to aspartic acid.

Rifampin and vaccinia DNA

The effect of rifampin on the replication of vaccinia DNA was studied in mouse L cells by a cytochemical techinque and by alkaline sucrose sedimentation analysis of newly synthesized viral DNA molecules. By the use of a fluorescent DNA-binding compound (Hoechst 33258), the sequential appearance, size, and location of the viral "factories" in rifampin-treated, virus-infected cells were found to be indistinguishable from those observed in untreated, infected cells. Sedimentation analysis in alkaline scurose gradients of the viral DNA molecules labeled in pulse-chase experiments showed that formation of small fragments, elongation into "intermediate"-sized molecules, and maturation into full-length viral DNA and, finally, into cross-linked viral DNA molecules occurred in the absence or presence of rifampin. The results support the view that the primary effect of the drug is related to assembly or morphogenesis.

Rifamycins: modulation of specific anti-poxviral activity by small substitutions on the piperazinyliminomethyl side chain

Rifamycin derivatives differing in the substitutent at the 4 position of the piperazinyliminomethyl side chain were tested for anti-poxviral activity. The effects of each derivative on wild-type vaccinia virus and on a mutant selected for resistance to rifampin were determined. Antiviral activity was measured in tissue culture by plaque inhibition, reduction in virus yield, and specific interruption of virus morphogenesis. Rifamycin derivatives containing H, ethyl, or propyl groups at the 4 position of the piperazinyliminomethyl side chain were much less active than rifampin, which has a methyl group at this position. Thus, minimal shortening or lengthening of the methyl piperazinyliminomethyl side chain of rifampin led to loss of specific antiviral activity. In contrast, the derivative containing an amino group at the 4 position of the piperazinyliminomethyl side chain had enhanced anti-poxviral activity.