Poxviruses Utilize Multiple Strategies to Inhibit Apoptosis.

Cells have multiple means to induce apoptosis in response to viral infection. Poxviruses must prevent activation of cellular apoptosis to ensure successful replication. These viruses devote a substantial portion of their genome to immune evasion. Many of these immune evasion products expressed during infection antagonize cellular apoptotic pathways. Poxvirus products target multiple points in both the extrinsic and intrinsic apoptotic pathways, thereby mitigating apoptosis during infection. Interestingly, recent evidence indicates that poxviruses also hijack cellular means of eliminating apoptotic bodies as a means to spread cell to cell through a process called apoptotic mimicry. Poxviruses are the causative agent of many human and veterinary diseases. Further, there is substantial interest in developing these viruses as vectors for a variety of uses including vaccine delivery and as oncolytic viruses to treat certain human cancers. Therefore, an understanding of the molecular mechanisms through which poxviruses regulate the cellular apoptotic pathways remains a top research priority. In this review, we consider anti-apoptotic strategies of poxviruses focusing on three relevant poxvirus genera: Orthopoxvirus, Molluscipoxvirus, and Leporipoxvirus. All three genera express multiple products to inhibit both extrinsic and intrinsic apoptotic pathways with many of these products required for virulence.

Tumorigenic poxviruses up-regulate intracellular superoxide to inhibit apoptosis and promote cell proliferation.

Tumorigenic leporipoxviruses encode catalytically inactive homologs of cellular Cu-Zn superoxide dismutase (SOD1). The function of the orthologous myxoma virus M131R and Shope fibroma virus S131R gene products is uncertain, but they inhibit SOD1 activity by a process linked to binding its copper chaperone. Using a superoxide-sensitive dye (hydroethidine), we observed that virus infection increased intracellular superoxide levels in an M/S131R-dependent manner. To see whether this effect promotes infection, we deleted the Shope fibroma virus S131R gene and compared the clinical manifestations of wild-type and mutant virus infections in rabbits. S131RDelta virus produced significantly smaller fibroxanthosarcoma-like growths in vivo and, at a point where these growths were already receding, wild-type infections still showed extensive leukocyte infiltration, necrosis, and fibromatous cell proliferation. Coincidentally, whereas Jurkat cells are protected from mitochondria- and Fas-mediated apoptosis by wild-type myxoma virus in vitro, M131RDelta virus could not block Fas-initiated apoptosis as judged by DNA laddering, terminal deoxynucleotidyltransferase-mediated dUTP-fluorescein nick end labeling, and caspase 3 cleavage assays. These data suggest that tumorigenic poxviruses can modulate intracellular redox status to their advantage to stimulate infected cell growth and inhibit programmed cell death.

Construction of recombinant vaccinia viruses using leporipoxvirus-catalyzed recombination and reactivation of orthopoxvirus DNA.

Poxvirus DNA is not infectious because the initiation of the infective process requires proteins encapsidated along with the virus genome. However, infectious virus can be produced if purified poxvirus DNA is transfected into cells previously infected with another poxvirus. This process is termed heterologous reactivation if the infecting virus is different from the transfected virus. We describe a method in which the high-frequency recombination and replication reactions catalyzed by the Leporipoxvirus, Shope fibroma virus (SFV), can be coupled with SFV-promoted reactivation reactions to rapidly construct recombinant vaccinia viruses in high yields (25-100% recombinant progeny). The reactivated vaccinia viruses are easily purified free of the SFV helper virus by plating mixed populations of virus on cells that support only the growth of vaccinia virus. These heterologous reactivation reactions can be used to manipulate the structure of virus genomes and produce viruses that express recombinant proteins at high levels. We illustrate the method by polymerase chain reaction (PCR) cloning the gene encoding green fluorescent protein (GFP), then using double-strand break repair reactions to produce a recombinant virus that expresses high levels of GFP.

High-frequency genetic recombination and reactivation of orthopoxviruses from DNA fragments transfected into leporipoxvirus-infected cells.

Poxvirus DNA is not infectious because establishing an infection requires the activities of enzymes packaged in the virion. This barrier can be overcome by transfecting virus DNA into cells previously infected with another poxvirus, since the resident virus can provide the trans-acting systems needed to reactivate transfected DNA. In this study we show that cells infected with a leporipoxvirus, Shope fibroma virus (SFV), can reactivate vaccinia virus DNA. Similar heterologous packaging systems which used fowlpox-infected cells to reactivate vaccinia virus have been described, but SFV-infected cells promoted a far more efficient reaction that can produce virus titers exceeding 10(6) PFU/ micro g of transfected DNA. SFV-promoted reactions also exploit the hyperrecombinogenic systems previously characterized in SFV-infected cells, and these coupled recombination and reactivation reactions could be used to delete nonessential regions of the vaccinia virus genome and to reconstruct vaccinia virus from overlapping DNA fragments. SFV-catalyzed recombination reactions need only two 18- to 20-bp homologies to target PCR amplicons to restriction enzyme-cut vaccinia virus vectors, and this reaction feature was used to rapidly clone and express a gene encoding fluorescent green protein without the need for plaque purification or selectable markers. The ability of SFV-infected cells to reactivate fragments of vaccinia virus was ultimately limited by the number of recombinational exchanges required and one cannot reconstruct vaccinia virus from multiple PCR fragments spanning essential portions of the genome. These observations suggest that recombination is an integral part of poxvirus reactivation reactions and provide a useful new technique for altering the structure of poxvirus genomes.

Comparative effects of virulent and avirulent poxviruses on cell cycle progression.

We studied the impact of tumorigenic poxviral infection on key regulators of cell cycle progression. Malignant fibroma virus (MV) is a virulent poxvirus that causes severe immunological impairment in vivo and in vitro. It also directs expression of important cellular regulatory proteins, such as p53. Its avirulent relative, Shope fibroma virus (SFV), has little effect on the immune system or p53. Accordingly we examined the effects of MV and SFV on the cell cycle in RK-13 rabbit kidney fibroblasts. MV caused an accumulation of cells in G2/M phase and decreased the percentage of cells in G0/G1. Prolongation of G2/M phase was associated with increased levels of cyclin B protein, decreases in cyclin A and cdc2 proteins, and diminished cdc2 activity. In contrast SFV did not affect cellular cycling detectably. SFV infection was accompanied by large increases in cyclin A and cdc2 proteins and increased cdc2 activity. Thus alterations in cell cycle transit during virus infection may reflect active direction in which virus induces changes in cell cycle regulators. Such changes may be important in the differences in virulence between MV and SFV.