Expression of vaccinia E3L and K3L genes by a novel recombinant canarypox HIV vaccine vector enhances HIV-1 pseudovirion production and inhibits apoptosis in human cells

Novel Modified Vaccinia Virus Ankara Vector Expressing Anti-apoptotic Gene B13R Delays Apoptosis and Enhances Humoral Responses

Modified vaccinia virus Ankara (MVA), an attenuated poxvirus, has been developed as a potential vaccine vector for use against cancer and multiple infectious diseases, including human immunodeficiency virus (HIV). MVA is highly immunogenic and elicits strong cellular and humoral responses in preclinical models and humans. However, there is potential to further enhance the immunogenicity of MVA, as MVA-infected cells undergo rapid apoptosis, leading to faster clearance of recombinant antigens and potentially blunting a greater response. Here, we generated MVA-B13R by replacing the fragmented 181R/182R genes of MVA with a functional anti-apoptotic gene, B13R, and confirmed its anti-apoptotic function against chemically induced apoptosis in vitro In addition, MVA-B13R showed a significant delay in induction of apoptosis in muscle cells derived from mice and humans, as well as in plasmacytoid dendritic cells (pDCs) and CD141⁺ DCs from rhesus macaques, compared to the induction of apoptosis in MVA-infected cells. MVA-B13R expressing simian immunodeficiency virus (SIV) Gag and Pol and HIV envelope (SHIV) (MVA-B13R/SHIV) produced higher levels of envelope in the supernatants than MVA/SHIV-infected DF-1 cells in vitro Immunization of BALB/c mice showed induction of higher levels of envelope-specific antibody-secreting cells and memory B cells, higher IgG antibody titers, and better persistence of antibody titers with MVA-B13R/SHIV than with MVA/SHIV. Gene set enrichment analysis of draining lymph node cells from day 1 after immunization showed negative enrichment for interferon responses in MVA-B13R/SHIV-immunized mice compared to the responses in MVA/SHIV-immunized mice. Taken together, these results demonstrate that restoring B13R functionality in MVA significantly delays MVA-induced apoptosis in muscle and antigen-presenting cells in vitro and augments vaccine-induced humoral immunity in mice.IMPORTANCE MVA is an attractive viral vector for vaccine development due to its safety and immunogenicity in multiple species and humans even under conditions of immunodeficiency. Here, to further improve the immunogenicity of MVA, we developed a novel vector, MVA-B13R, by replacing the fragmented anti-apoptotic genes 181R/182R with a functional version derived from vaccinia virus, B13R Our results show that MVA-B13R significantly delays apoptosis in antigen-presenting cells and muscle cells in vitro and augments vaccine-induced humoral immunity in mice, leading to the development of a novel vector for vaccine development against infectious diseases and cancer.

Fowlpox virus as a recombinant vaccine vector for use in mammals and poultry

Live vaccines against fowlpox virus, which causes moderate pathology in poultry and is the type species of the Avipoxvirus genus, were developed in the 1920s. Development of recombinant fowlpox virus vector vaccines began in the 1980s, for use not only in poultry, but also in mammals including humans. In common with other avipoxviruses, such as canarypox virus, fowlpox virus enters mammalian cells and expresses proteins, but replicates abortively. The use of fowlpox virus as a safe vehicle for expression of foreign antigens and host immunomodulators, is being evaluated in numerous clinical trials of vaccines against cancer, malaria, tuberculosis and AIDS, notably in heterologous prime-boost regimens. In this article, technical approaches to, and issues surrounding, the use of fowlpox virus as a recombinant vaccine vector in poultry and mammals are reviewed.

Poxviruses and the evolution of host range and virulence

Poxviruses as a group can infect a large number of animals. However, at the level of individual viruses, even closely related poxviruses display highly diverse host ranges and virulence. For example, variola virus, the causative agent of smallpox, is human-specific and highly virulent only to humans, whereas related cowpox viruses naturally infect a broad spectrum of animals and only cause relatively mild disease in humans. The successful replication of poxviruses depends on their effective manipulation of the host antiviral responses, at the cellular-, tissue- and species-specific levels, which constitutes a molecular basis for differences in poxvirus host range and virulence. A number of poxvirus genes have been identified that possess host range function in experimental settings, and many of these host range genes target specific antiviral host pathways. Herein, we review the biology of poxviruses with a focus on host range, zoonotic infections, virulence, genomics and host range genes as well as the current knowledge about the function of poxvirus host range factors and how their interaction with the host innate immune system contributes to poxvirus host range and virulence. We further discuss the evolution of host range and virulence in poxviruses as well as host switches and potential poxvirus threats for human and animal health.

A survey of host range genes in poxvirus genomes

Poxviruses are widespread pathogens, which display extremely different host ranges. Whereas some poxviruses, including variola virus, display narrow host ranges, others such as cowpox viruses naturally infect a wide range of mammals. The molecular basis for differences in host range are poorly understood but apparently depend on the successful manipulation of the host antiviral response. Some poxvirus genes have been shown to confer host tropism in experimental settings and are thus called host range factors. Identified host range genes include vaccinia virus K1L, K3L, E3L, B5R, C7L and SPI-1, cowpox virus CP77/CHOhr, ectromelia virus p28 and 022, and myxoma virus T2, T4, T5, 11L, 13L, 062R and 063R. These genes encode for ankyrin repeat-containing proteins, tumor necrosis factor receptor II homologs, apoptosis inhibitor T4-related proteins, Bcl-2-related proteins, pyrin domain-containing proteins, cellular serine protease inhibitors (serpins), short complement-like repeats containing proteins, KilA-N/RING domain-containing proteins, as well as inhibitors of the double-stranded RNA-activated protein kinase PKR. We conducted a systematic survey for the presence of known host range genes and closely related family members in poxvirus genomes, classified them into subgroups based on their phylogenetic relationship and correlated their presence with the poxvirus phylogeny. Common themes in the evolution of poxvirus host range genes are lineage-specific duplications and multiple independent inactivation events. Our analyses yield new insights into the evolution of poxvirus host range genes. Implications of our findings for poxvirus host range and virulence are discussed.

ALVAC-HIV as a prophylactic and therapeutic vaccine: highlights from over a decade of clinical trials

ALVAC-HIV vaccines (vCP125, vCP205, vCP300, vCP1433, vCP1452 and vCP1521) are preparations of a modified recombinant canarypox virus designed to induce or augment CD8+ immune responses. As the focus of several Phase I–III trials, they have been the most extensively studied live vector-based HIV vaccines. Overall, ALVAC-HIV induces modest CD8+ T-cell responses in approximately 20–50% of recipients. The addition of IL-2, recombinant glycoprotein 120 or 160, Remune or LIPO-6T to ALVAC-HIV does not appear to enhance overall CD8+ T-cell immune responses. The ability of ALVAC-HIV to induce interclade immunity and immunogenicity in newborns with perinatal exposure to HIV has important implications for the control of HIV worldwide. Experience from clinical trials in over 10,000 HIV-infected and noninfected individuals has shown that ALVAC constructs are safe, with reactogenicity profiles similar to those reported for currently licensed vaccines. Despite seemingly modest immunogenicity at the present time, studies to date have set the stage for further exploration of the potential of ALVAC-HIV vaccines. This report highlights findings from clinical trials using ALVAC-HIV, alone and in combination with other agents, as both a prophylactic and a therapeutic vaccine.

Co-expression of miRNA targeting the expression of PERK, but not PKR, enhances cellular immunity from an HIV-1 Env DNA vaccine

Small non-coding micro-RNAs (miRNA) are important post-transcriptional regulators of mammalian gene expression that can be used to direct the knockdown of expression from targeted genes. We examined whether DNA vaccine vectors co-expressing miRNA with HIV-1 envelope (Env) antigens could influence the magnitude or quality of the immune responses to Env in mice. Human miR-155 and flanking regions from the non-protein encoding gene mirhg155 were introduced into an artificial intron within an expression vector for HIV-1 Env gp140. Using the miR-155-expressing intron as a scaffold, we developed novel vectors for miRNA-mediated targeting of the cellular antiviral proteins PKR and PERK, which significantly down-modulated target gene expression and led to increased Env expression in vitro. Finally, vaccinating BALB/c mice with a DNA vaccine vector delivering miRNA targeting PERK, but not PKR, was able to augment the generation of Env-specific T-cell immunity. This study provides proof-of-concept evidence that miRNA effectors incorporated into vaccine constructs can positively influence vaccine immunogenicity. Further testing of vaccine-encoded miRNA will determine if such strategies can enhance protective efficacy from vaccines against HIV-1 for eventual human use.

The adjuvancy of OX40 ligand (CD252) on an HIV-1 canarypox vaccine

The immunogenicity of current human immunodeficiency virus-1 (HIV-1) canarypox vaccines is weak and needs to be improved. Ligation of OX40 (CD134), a member of tumor necrosis factor receptor superfamily (TNFRSF), by its ligand OX40L (CD252), a tumor necrosis factor superfamily (TNFSF) molecule, has been demonstrated to provide a pivotal costimulatory signal to enhance CD4(+) T cell help of humoral and cytotoxic T cell immune responses. The present study examined whether an OX40L-expressing vector could boost the immunogenicity of the HIV-1 canarypox vaccine, vCP1452, in mice. Co-immunization of mice with OX40L-expressing canarypox and vCP1452 augmented HIV-1 specific CD8(+) T cell responses in terms of frequency and cytokine expression. OX40L-expressing canarypox enhanced the frequency of antigen specific CD8(+) T cells with an effector (CD127(-)CD62L(-)) phenotype, which was associated with an ex vivo expansion of HIV-1 specific CD4(+) T cells. This was in contrast to our previous work in which a CD40L-expressing construct preferentially enhanced antigen specific memory responses [Liu J, Yu Q, Stone GW, Yue FY, Ngai N, Jones RB, et al. CD40L expressed from the canarypox vector, ALVAC, can boost immunogenicity of HIV-1 canarypox vaccine in mice and enhance the in vitro expansion of viral specific CD8+ T cell memory responses from HIV-1-infected and HIV-1-uninfected individuals. Vaccine 2008;26(32):4062-72]. Surprisingly, OX40L did not enhance antibody responses elicited by the HIV-1 canarypox vaccine. We saw no added benefit by combining OX40L and CD40L vectors as an adjuvant strategy for vCP1452. Our results indicate that, similar to CD40L, canarypox vectors expressing OX40L can enhance the cellular but not humoral immunogenicity of HIV-1 canarypox vaccines. In summary, our findings show that OX40L can be used as a molecular adjuvant to enhance T cell immune responses.

Lessons From a Multisite International Trial in the Caribbean and South America of an HIV-1 Canarypox Vaccine (ALVAC-HIV vCP1452) With or Without Boosting With MN rgp120

BACKGROUND

The first multicenter, international National Institutes of Allergy and Infectious Diseases (NIAID)-sponsored HIV vaccine trial took place in Brazil, Haiti, Peru and Trinidad. This randomized, double-blind, placebo-controlled, phase 2 trial evaluated the safety and immunogenicity of a clade B-derived, live canarypox HIV vaccine, vCP1452. vCP1452 was administered alone or with a heterologous boost of MN rgp120 glycoprotein. The trial was pivotal in deciding whether these vaccines advanced to phase 3 efficacy trials.

METHODS

Forty seronegative volunteers per site were randomized to ALVAC alone, ALVAC plus MN rgp120, or placebo in a 0, 1, 3, and 6 month schedule. Immunogenicity was assayed by chromium-release cytotoxic T lymphocyte (CTL) responses; interferon-gamma (IFN-gamma) enzyme-linked immunosorbent spot assays (ELISpot); lymphocyte proliferation assays (LPA); neutralization; and enzyme-linked immunosorbent assays (ELISA).

RESULTS

Enrollment and follow-up were excellent. Both vaccines were well tolerated. Neutralizing antibody to the laboratory-adapted MN strain was detected. Cellular immune responses, as measured by CTL, ELISpot, and LPA, did not differ between vaccines and placebos.

CONCLUSIONS

The observation of disappointing immunogenicity in this and a parallel domestic study has informed future vaccine development. Equally important, challenges to doing an integrated trial across countries, cultures, languages, and differing at-risk populations were overcome. The identification of specific safety, ethical, logistic, and immunological issues in this trial established the foundation for current larger international studies.

Direct comparison of antigen production and induction of apoptosis by canarypox virus- and modified vaccinia virus ankara-human immunodeficiency virus vaccine vectors

Recombinant poxvirus vectors are undergoing intensive evaluation as vaccine candidates for a variety of infectious pathogens. Avipoxviruses, such as canarypox virus, are replication deficient in mammalian cells by virtue of a poorly understood species-specific restriction. Highly attenuated vaccinia virus strains such as modified vaccinia virus Ankara (MVA) are similarly unable to complete replication in most mammalian cells but have an abortive-late phenotype, in that the block to replication occurs post-virus-specific DNA replication. In this study, an identical expression cassette for human immunodeficiency virus gag, pro, and env coding sequences was placed in canarypox virus and MVA vector backbones in order to directly compare vector-borne expression and to analyze differences in vector-host cell interactions. Antigen production by recombinant MVA was shown to be greater than that from recombinant canarypox virus in the mammalian cell lines and in the primary human cells tested. This observation was primarily due to a longer duration of antigen production in recombinant MVA-infected cells. Apoptosis induction was found to be more profound with the empty canarypox virus vector than with MVA. Remarkably, however, the inclusion of a gag/pro/env expression cassette altered the kinetics of apoptosis induction in recombinant MVA-infected cells to levels equal to those found in canarypox virus-infected cells. Antigen production by MVA was noted to be greater in human dendritic cells and resulted in enhanced T-cell stimulation in an in vitro antigen presentation assay. These results reveal differences in poxvirus vector-host cell interactions that should be relevant to their use as immunization vehicles.

Phase 2 study of an HIV-1 canarypox vaccine (vCP1452) alone and in combination with rgp120: negative results fail to trigger a phase 3 correlates trial

BACKGROUND

A goal of T-cell HIV vaccines is to define the correlation between a vaccine-induced immune response and protection from HIV infection. We conducted a phase 2 trial to determine if a canarypox vaccine candidate (vCP1452) administered with rgp120 subunit protein would "qualify" for a trial to define a correlate of efficacy.

METHODS

A total of 330 healthy volunteers were enrolled into 4 groups: 120 received vCP1452 alone (0, 1, 3, and 6 months), 120 received vCP1452 with 2 different regimens of rgp120 coadministration, and 90 received placebo. HIV-specific antibody responses were measured by enzyme-linked immunoassay (ELISA) and neutralizing activity. T-cell responses were measured by chromium release and interferon-gamma (IFNgamma) enzyme-linked immunospot (ELISpot) assay.

RESULTS

Significant neutralizing antibody responses to the HIV MN strain were detected in all vaccine groups, with net responses ranging from 57% (95% confidence interval [CI]: 40% to 71%) to 94% (95% CI: 85% to 99%). Net cumulative HIV-specific CD8 IFNgamma ELISpot assay responses were 13% (95% CI: -1% to 26%) for recipients of vCP1452 alone and 16% (95% CI: 2% to 29%) for recipients of vCP1452 plus rgp120.

CONCLUSIONS

Overall, the HIV-specific CD8 cytotoxic T lymphocyte (CTL) response was not sufficient to qualify the regimen for a subsequent trial designed to detect an immune correlate of protection requiring a minimum CD8 CTL frequency of 30%.

The prospective preventative HIV vaccine based on modified poliovirus

In order to control HIV pandemic, many vaccines are invented. Although none first verified its efficacy in clinic, we hypothesize that HIV vaccine based on poliovirus is potential to develop the promising one, because it can elicit the broad immune response including the main mucosal, humoral and cellular reaction. However, the viral neural virulence is one major concern. The attenuated Sabin strain is a better candidate. While partial poliovirus genes are replaced by HIV antigen genes, the defective interfering particle will fail to produce progeny virions, which may further ensure its security. Although the vaccinal immune efficacy was verified in some similar animal experiments based on poliovirus to express the exogenous genes, more animal and clinical immune trials about HIV-poliovirus chimeric minireplicons are to be carried out and the hypotheses are to be validated.

HIV-1 vaccine induced immune responses in newborns of HIV-1 infected mothers

OBJECTIVE

Breast milk transmission continues to account for a large proportion of cases of mother-to-child transmission of HIV-1 worldwide. An effective HIV-1 vaccine coupled with either passive immunization or short-term antiretroviral prophylaxis represents a potential strategy to prevent breast milk transmission. This study evaluated the safety and immunogenicity of ALVAC HIV-1 vaccine with and without a subunit envelope boost in infants born to HIV-1-infected women.

DESIGN

: Placebo-controlled, double-blinded study.

METHODS

Infants born to HIV-1-infected mothers in the US were immunized with a prime-boost regimen using a canarypox virus HIV-1 vaccine (vCP1452) and a recombinant glycoprotein subunit vaccine (rgp120). Infants (n = 30) were randomized to receive: vCP1452 alone, vCP1452 + rgp120, or corresponding placebos.

RESULTS

Local reactions were mild or moderate and no significant systemic toxicities occurred. Subjects receiving both vaccines had gp120-specific binding serum antibodies that were distinguishable from maternal antibody. Repeated gp160-specific lymphoproliferative responses were observed in 75%. Neutralizing activity to HIV-1 homologous to the vaccine strain was observed in 50% of the vCP1452 + rgp120 subjects who had lost maternal antibody by week 24. In some infants HIV-1-specific proliferative and antibody responses persisted until week 104. HIV-1-specific cytotoxic T lymphocyte responses were detected in two subjects in each treatment group; the frequency of HIV-1 specific cytotoxic T lymphocyte responses did not differ between vaccine and placebo recipients.

CONCLUSION

The demonstration of vaccine-induced immune responses in early infancy supports further study of HIV-1 vaccination as a strategy to reduce breast milk transmission.

Pseudovirion particle production by live poxvirus human immunodeficiency virus vaccine vector enhances humoral and cellular immune responses

Live-vector-based human immunodeficiency virus (HIV) vaccines are an integral part of a number of HIV vaccine regimens currently under evaluation. Live vectors that carry an intact gag gene are capable of eliciting HIV pseudovirion particle formation from infected host cells. The impact of pseudovirion particle formation on the immune response generated by live HIV vaccine vectors has not been established. In this study, a canarypox HIV vaccine candidate vector expressing HIV gag and env genes, vCP205, was modified by the introduction of a glycine-to-alanine coding change in the N-terminal myristylation site of gag to create Myr- vCP205. This substitution effectively eliminated particle formation without altering the level of protein production. vCP205 and Myr- vCP205 were then directly compared for the ability to induce HIV-specific immune responses in mice. The particle-competent vector vCP205 elicited higher levels of CD8+ T-cell responses, as indicated by gamma interferon enzyme-linked immunospot (ELISPOT) assay and intracellular cytokine staining. Humoral responses to Gag and Env were also markedly higher from animals immunized with the particle-competent vector. Furthermore, HIV-specific CD4+ T-cell responses were greater among animals immunized with the particle-competent vector. Using a human dendritic cell model of antigen presentation in vitro, vCP205 generated greater ELISPOT responses than Myr- vCP205. These results demonstrate that pseudovirion particle production by live-vector HIV vaccines enhances HIV-specific cellular and humoral immune responses.

Poxvirus tropism

Despite the success of the WHO-led smallpox eradication programme a quarter of a century ago, there remains considerable fear that variola virus, or other related pathogenic poxviruses such as monkeypox, could re-emerge and spread disease in the human population. Even today, we are still mostly ignorant about why most poxvirus infections of vertebrate hosts show strict species specificity, or how zoonotic poxvirus infections occur when poxviruses occasionally leap into novel host species. Poxvirus tropism at the cellular level seems to be regulated by intracellular events downstream of virus binding and entry, rather than at the level of specific host receptors as is the case for many other viruses. This review summarizes our current understanding of poxvirus tropism and host range, and discusses the prospects of exploiting host-restricted poxvirus vectors for vaccines, gene therapy or tissue-targeted oncolytic viral therapies for the treatment of human cancers.

Poxvirus host range varies markedly ? some viruses, such as variola and molluscum contagiosum virus (both of which are human-specific), exhibit strict species tropism, whereas others such as cowpox virus are able to infect multiple host species.

Members of four of the eight genera of chordopoxviruses can zoonotically infect man. For example, monkeypox virus can cause severe smallpox-like disease in humans that clinically resembles variola virus.

The species tropism that is exhibited by many poxviruses in terms of causing disease is frequently quite different from the range of cultured cells that can be infected by these viruses.

Specific host-cell receptors do not mediate the distinction between cells that are permissive as opposed to non-permissive for poxvirus infection. Rather, restrictive host cells fail to support the full replication cycle of the infecting poxvirus at a point downstream of binding and entry.

A variety of poxviral host-range genes have been identified that contribute to the control of permissive versus non-permissive infection of cultured mammalian cells. The gene products of these host-range genes regulate the ability of the virus to complete its cytoplasmic replication cycle.

The development of host-restricted vaccines, like modified vaccinia Ankara (MVA), that do not replicate in humans but that retain potent immunogenicity, will provide safer platforms for recombinant vaccines.

Another advance has been the development of poxvirus-based oncolytic vectors that replicate preferentially in human tumour cells.

The vaccinia virus B1R kinase induces p53 downregulation by an Mdm2-dependent mechanism

Poxvirus infection has a strong effect on cellular functions. To understand viral pathogenesis, it is necessary to know how viral proteins interact with host proteins. The B1R kinase is an early viral gene required for vaccinia virus DNA synthesis and replication, but no cellular substrate is known for this viral kinase. B1R is able to hyperphosphorylate p53 in several residues in the N-terminal transactivation domain, including Ser15 and Thr18. B1R does not phosphorylate Mdm2. B1R promotes an increase in p53 ubiquitination and a reduction of p53 acetylation by p300. The over-expressed B1R protein induces the degradation of p53 in a concentration-dependent manner and is lost when Ser15 and Th18 are changed to alanine or when the B1R kinase is inactivated by introducing the K149Q substitution. The B1R-induced downregulation of p53 requires Mdm2. The hyperphosphorylated p53 is transcriptionally active, and this activity also falls as B1R increases. The BAX gene promoter is more sensitive to this reduction of transcription than p21 or 14-3-3 gene promoters. This effect of B1R on p53 can be one of the mechanisms by which vaccinia virus exerts its role in infected cells.

The genome of canarypox virus

Here we present the genomic sequence, with analysis, of a canarypox virus (CNPV). The 365-kbp CNPV genome contains 328 potential genes in a central region and in 6.5-kbp inverted terminal repeats. Comparison with the previously characterized fowlpox virus (FWPV) genome revealed avipoxvirus-specific genomic features, including large genomic rearrangements relative to other chordopoxviruses and novel cellular homologues and gene families. CNPV also contains many genomic differences with FWPV, including over 75 kbp of additional sequence, 39 genes lacking FWPV homologues, and an average of 47% amino acid divergence between homologues. Differences occur primarily in terminal and, notably, localized internal genomic regions and suggest significant genomic diversity among avipoxviruses. Divergent regions contain gene families, which overall comprise over 49% of the CNPV genome and include genes encoding 51 proteins containing ankyrin repeats, 26 N1R/p28-like proteins, and potential immunomodulatory proteins, including those similar to transforming growth factor beta and beta-nerve growth factor. CNPV genes lacking homologues in FWPV encode proteins similar to ubiquitin, interleukin-10-like proteins, tumor necrosis factor receptor, PIR1 RNA phosphatase, thioredoxin binding protein, MyD116 domain proteins, circovirus Rep proteins, and the nucleotide metabolism proteins thymidylate kinase and ribonucleotide reductase small subunit. These data reveal genomic differences likely affecting differences in avipoxvirus virulence and host range, and they will likely be useful for the design of improved vaccine vectors.

Poxviruses and immune evasion

Large DNA viruses defend against hostile assault executed by the host immune system by producing an array of gene products that systematically sabotage key components of the inflammatory response. Poxviruses target many of the primary mediators of innate immunity including interferons, tumor necrosis factors, interleukins, complement, and chemokines. Poxviruses also manipulate a variety of intracellular signal transduction pathways such as the apoptotic response. Many of the poxvirus genes that disrupt these pathways have been hijacked directly from the host immune system, while others have demonstrated no clear resemblance to any known host genes. Nonetheless, the immunological targets and the diversity of strategies used by poxviruses to disrupt these host pathways have provided important insights into diverse aspects of immunology, virology, and inflammation. Furthermore, because of their anti-inflammatory nature, many of these poxvirus proteins hold promise as potential therapeutic agents for acute or chronic inflammatory conditions.

Replication of modified vaccinia virus Ankara in primary chicken embryo fibroblasts requires expression of the interferon resistance gene E3L

Highly attenuated modified vaccinia virus Ankara (MVA) serves as a candidate vaccine to immunize against infectious diseases and cancer. MVA was randomly obtained by serial growth in cultures of chicken embryo fibroblasts (CEF), resulting in the loss of substantial genomic information including many genes regulating virus-host interactions. The vaccinia virus interferon (IFN) resistance gene E3L is among the few conserved open reading frames encoding viral immune defense proteins. To investigate the relevance of E3L in the MVA life cycle, we generated the deletion mutant MVA-DeltaE3L. Surprisingly, we found that MVA-DeltaE3L had lost the ability to grow in CEF, which is the first finding of a vaccinia virus host range phenotype in this otherwise highly permissive cell culture. Reinsertion of E3L led to the generation of revertant virus MVA-E3rev and rescued productive replication in CEF. Nonproductive infection of CEF with MVA-DeltaE3L allowed viral DNA replication to occur but resulted in an abrupt inhibition of viral protein synthesis at late times. Under these nonpermissive conditions, CEF underwent apoptosis starting as early as 6 h after infection, as shown by DNA fragmentation, Hoechst staining, and caspase activation. Moreover, we detected high levels of active chicken alpha/beta IFN (IFN-alpha/beta) in supernatants of MVA-DeltaE3L-infected CEF, while moderate IFN quantities were found after MVA or MVA-E3rev infection and no IFN activity was present upon infection with wild-type vaccinia viruses. Interestingly, pretreatment of CEF with similar amounts of recombinant chicken IFN-alpha inhibited growth of vaccinia viruses, including MVA. We conclude that efficient propagation of MVA in CEF, the tissue culture system used for production of MVA-based vaccines, essentially requires conserved E3L gene function as an inhibitor of apoptosis and/or IFN induction.

Poxviruses and apoptosis: a time to die

All known apoptosis modulators in poxviruses have been shown to function as inhibitors. The mechanistic classes of these poxvirus-encoded inhibitors are quite diverse, and indicate that a wide variety of distinct host proteins in cellular apoptotic pathways have been targeted for inhibition by individual poxviruses.