Recombinant modified vaccinia virus Ankara generating excess early double-stranded RNA transiently activates protein kinase R and triggers enhanced innate immune responses.

UNLABELLED: Double-stranded RNA (dsRNA) is an important molecular pattern associated with viral infection and is detected by various extra- and intracellular recognition molecules. Poxviruses have evolved to avoid producing dsRNA early in infection but generate significant amounts of dsRNA late in infection due to convergent transcription of late genes. Protein kinase R (PKR) is activated by dsRNA and triggers major cellular defenses against viral infection, including protein synthesis shutdown, apoptosis, and type I interferon (IFN-I) production. The poxviral E3 protein binds and sequesters viral dsRNA and is a major antagonist of the PKR pathway. We found that the highly replication-restricted modified vaccinia virus Ankara (MVA) engineered to produce excess amounts of dsRNA early in infection showed enhanced induction of IFN-ß in murine and human cells in the presence of an intact E3L gene. IFN-ß induction required a minimum overlap length of 300 bp between early complementary transcripts and was strongly PKR dependent. Excess early dsRNA produced by MVA activated PKR early but transiently in murine cells and induced enhanced systemic levels of IFN-α, IFN-γ, and other cytokines and chemokines in mice in a largely PKR-dependent manner. Replication-competent chorioallantois vaccinia virus Ankara (CVA) generating excess early dsRNA also enhanced IFN-I production and was apathogenic in mice even at very high doses but showed no in vitro host range defect. Thus, genetically adjuvanting MVA and CVA to generate excess early dsRNA is an effective method to enhance innate immune stimulation by orthopoxvirus vectors and to attenuate replicating vaccinia virus in vivo. IMPORTANCE: Efficient cellular sensing of pathogen-specific components, including double-stranded RNA (dsRNA), is an important prerequisite of an effective antiviral immune response. The prototype poxvirus vaccinia virus (VACV) and its derivative modified vaccinia virus Ankara (MVA) produce dsRNA as a by-product of viral transcription. We found that inhibition of cellular dsRNA recognition established by the virus-encoded proteins E3 and K3 can be overcome by directing viral overexpression of dsRNA early in infection without compromising replication of MVA in permissive cells. Early dsRNA induced transient activation of the cellular dsRNA sensor protein kinase R (PKR), resulting in enhanced production of interferons and cytokines in cells and mice. Enhancing the capacity of MVA to activate the innate immune system is an important approach to further improve the immunogenicity of this promising vaccine vector.

The vaccinia virus O1 protein is required for sustained activation of extracellular signal-regulated kinase 1/2 and promotes viral virulence.

Sustained activation of the Raf/MEK/extracellular signal-regulated kinase (ERK) pathway in infected cells has been shown to be crucial for full replication efficiency of orthopoxviruses in cell culture. In infected cells, this pathway is mainly activated by the vaccinia virus growth factor (VGF), an epidermal growth factor (EGF)-like protein. We show here that chorioallantois vaccinia virus Ankara (CVA), but not modified vaccinia virus Ankara (MVA), induced sustained activation of extracellular signal-regulated kinase 1/2 (ERK1/2) in infected human 293 cells, although both viruses direct secretion of functional VGF. A CVA mutant lacking the O1L gene (CVA-ΔO1L) demonstrated that the O1 protein was required for sustained upregulation of the ERK1/2 pathway in 293 cells as well as in other mammalian cell lines. The highly conserved orthopoxvirus O1L gene encodes a predicted 78-kDa protein with a hitherto-unknown function. CVA-ΔO1L showed reduced plaque size and an attenuated cytopathic effect (CPE) in infected cell cultures and reduced virulence and spread from lungs to ovaries in intranasally infected BALB/c mice. Reinsertion of an intact O1L gene into MVA, which in its original form harbors a fragmented O1L open reading frame (ORF), restored ERK1/2 activation in 293 cells but did not increase replication and spread of MVA in human or other mammalian cell lines. Thus, the O1 protein was crucial for sustained ERK1/2 activation in CVA- and MVA-infected human cells, complementing the autocrine function of VGF, and enhanced virulence in vivo.

Introduction of the six major genomic deletions of modified vaccinia virus Ankara (MVA) into the parental vaccinia virus is not sufficient to reproduce an MVA-like phenotype in cell culture and in mice.

Modified vaccinia virus Ankara (MVA) has a highly restricted host range in cell culture and is apathogenic in vivo. MVA was derived from the parental chorioallantois vaccinia virus Ankara (CVA) by more than 570 passages in chicken embryo fibroblast (CEF) cells. During CEF cell passaging, six major deletions comprising 24,668 nucleotides occurred in the CVA genome. We have cloned both the MVA and the parental CVA genome as bacterial artificial chromosomes (BACs) and have sequentially introduced the six major MVA deletions into the cloned CVA genome. Reconstituted mutant CVA viruses containing up to six major MVA deletions showed no detectable replication restriction in 12 of 14 mammalian cell lines tested; the exceptions were rabbit cell lines RK13 and SIRC. In mice, CVA mutants with up to three deletions showed slightly enhanced virulence, suggesting that gene deletion in replicating vaccinia virus (VACV) can result in gain of fitness in vivo. CVA mutants containing five or all six deletions were still pathogenic, with a moderate degree of attenuation. Deletion V was mainly responsible for the attenuated phenotype of these mutants. In conclusion, loss or truncation of all 31 open reading frames in the six major deletions is not sufficient to reproduce the specific MVA phenotype of strong attenuation and highly restricted host range. Mutations in viral genes outside or in association with the six major deletions appear to contribute significantly to this phenotype. Host range restriction and avirulence of MVA are most likely a cooperative effect of gene deletions and mutations involving the major deletions.