Vaccinia and other poxvirus expression vectors

Six host-range restricted poxviruses from three genera induce distinct gene expression profiles in an in vivo mouse model

BACKGROUND

Host-range restricted poxviruses make promising vaccine vectors due to their safety profile and immunogenicity. An understanding of the host innate immune responses produced by different poxvirus vectors would aid in the assessment, selection and rational design of improved vaccines for human and veterinary applications. Novel avipoxviruses are being assessed to determine if they are different from other poxvirus vectors. Analysis of the transcriptome induced in a mouse model would aid in determining if there were significant differences between different poxvirus vectors which may reflect different adjuvant potential as well as establish if they should be further evaluated as vaccine vectors.

RESULTS

We compared host transcript abundance in the spleens of BALB/c mice twenty four hours after intravenous infection (10(5) pfu/mouse) with six host-restricted poxvirus species from three genera, namely Lumpy Skin Disease virus (LSDV), Canarypox virus (CNPV), Fowlpox virus (FWPV), modified vaccinia Ankara (MVA) and two novel South African avipoxviruses, Feral Pigeonpox virus (FeP2) and Penguinpox virus (PEPV). These six viruses produced qualitatively and quantitatively distinct host responses with LSDV, followed by MVA, inducing the greatest interferon (IFN) response. FeP2 and PEPV caused very little change to host transcript abundance compared to the other 4 viruses tested. CNPV and FWPV induced the up regulation of two immunoglobulin genes (Ighg and Ighg3 (IgG3)) with CNPV inducing a third, Ighm (IgM). HIV-1-specific IgG3 antibodies have been correlated with decreased risk of HIV-1 infection in the RV144 trial, which included a CNPV-based vector (Yates et al. (Sci Transl Med, 6(228) p228, 2014). Up regulation of IgG3 by CNPV and FWPV but not the other poxviruses tested in vivo, implies that these two avipoxvirus-vector backbones may be involved in stimulation of the clinically important IgG3 antibody subclass. Differential transcript abundance associated with the different poxviruses is further discussed with particular emphasis on responses related to immune responses.

CONCLUSION

Six, genetically diverse host-restricted poxviruses produce different responses in a mouse model early after infection. These differences may affect the immune response induced to vaccine antigen in vectors based on these viruses. The two novel avipoxviruses were clearly distinguishable from the other viruses.

Avipoxviruses: infection biology and their use as vaccine vectors

Avipoxviruses (APVs) belong to the Chordopoxvirinae subfamily of the Poxviridae family. APVs are distributed worldwide and cause disease in domestic, pet and wild birds of many species. APVs are transmitted by aerosols and biting insects, particularly mosquitoes and arthropods and are usually named after the bird species from which they were originally isolated. The virus species Fowlpox virus (FWPV) causes disease in poultry and associated mortality is usually low, but in flocks under stress (other diseases, high production) mortality can reach up to 50%. APVs are also major players in viral vaccine vector development for diseases in human and veterinary medicine. Abortive infection in mammalian cells (no production of progeny viruses) and their ability to accommodate multiple gene inserts are some of the characteristics that make APVs promising vaccine vectors. Although abortive infection in mammalian cells conceivably represents a major vaccine bio-safety advantage, molecular mechanisms restricting APVs to certain hosts are not yet fully understood. This review summarizes the current knowledge relating to APVs, including classification, morphogenesis, host-virus interactions, diagnostics and disease, and also highlights the use of APVs as recombinant vaccine vectors.

Role of cell signaling in poxvirus-mediated foreign gene expression in mammalian cells

Poxviruses have been extensively used as a promising vehicle to efficiently deliver a variety of antigens in mammalian hosts to induce immune responses against infectious diseases and cancer. Using recombinant vaccinia virus (VV) and canarypox virus (ALVAC) expressing enhanced green fluorescent protein (EGFP) or multiple HIV-1 gene products, we studied the role of four cellular signaling pathways, the phosphoinositide-3-OH kinase (PI3K), extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (p38 MAPK), and c-Jun N-terminal kinase (JNK), in poxvirus-mediated foreign gene expression in mammalian cells. In nonpermissive infection (human monocytes), activation of PI3K, ERK, p38 MAPK, and JNK was observed in both VV and ALVAC and blocking PI3K, p38 MAKP, and JNK pathways with their specific inhibitors significantly reduced viral and vaccine antigen gene expression. Whereas, blocking the ERK pathway had no significant effect. Among these cellular signaling pathways studied, PI3K was the most critical pathway involved in gene expression by VV- or ALVAC-infected monocytes. The important role of PI3K in poxvirus-mediated gene expression was further confirmed in mouse epidermal cells stably transfected with dominant-negative PI3K mutant, as poxvirus-mediated targeted gene expression was significantly decreased in these cells when compared with their parental cells. Signaling pathway activation influenced gene expression at the mRNA level rather than virus binding. In permissive mammalian cells, however, VV DNA copies were also significantly decreased in the absence of normal function of the PI3K pathway. Poxvirus-triggered activation of PI3K pathway could be completely abolished by atazanavir, a new generation of antiretroviral protease inhibitors (PIs). As a consequence, ALVAC-mediated EGFP or HIV-1 gag gene expression in infected primary human monocytes was significantly reduced in the presence of atazanavir. These findings implicate that antiretroviral therapy (ART), also known as highly active antiretroviral therapy (HAART), may negatively impact the efficacy of live poxvirus vector-based vaccines and should be carefully considered when administering such live vaccines to individuals on ART.

Poxvirus tropism for primary human leukocytes and hematopoietic cells

Poxviruses including canarypox (ALVAC) and vaccinia viruses have, in recent years, received considerable attention as live vectors for the development of vaccines against infectious diseases such as AIDS, malaria, and tuberculosis. However, the cellular targets for viral infection within the human immune system and the consequences of infection for cells involved in the generation of immune responses have not been clearly delineated. Using recombinant enhanced green fluorescence protein (EGFP)-expressing ALVAC and vaccinia viruses, we have focused here on a side-by-side comparison of ALVAC and vaccinia virus tropism for cells from human peripheral blood and bone marrow. Both ALVAC and vaccinia viruses showed a strong bias toward monocyte infection. ALVAC minimally infected CD19(+) B cells and was unable to infect ex vivo NK cells and T lymphocytes, whereas vaccinia virus could infect B lymphocytes and NK cell populations. Vaccinia virus was also able to infect T lymphocytes at low but detectable levels which could be enhanced upon their activation. Both ALVAC and vaccinia viruses could infect immature monocyte-derived dendritic cells (MDDCs), but only ALVAC infection induced their subsequent maturation. Infection in human bone marrow cells showed that ALVAC infection was restricted to a myelomonocytoid cell-specific CD33(+) cell population, while vaccinia virus showed a strong, but not exclusive, preference for these cells.

Comparative analysis of tropism between canarypox (ALVAC) and vaccinia viruses reveals a more restricted and preferential tropism of ALVAC for human cells of the monocytic lineage

The poxviruses including canarypox (ALVAC) and vaccinia viruses are promising vaccine vectors in humans, but little is known about their biology in human cells. Using recombinant enhanced green fluorescence protein (EGFP)-expressing ALVAC and vaccinia viruses, we have focused here on a side-by-side comparison of ALVAC and vaccinia virus tropism for cells from human peripheral blood and bone marrow. Both ALVAC and vaccinia viruses showed a strong bias towards monocyte infection. ALVAC minimally infected CD19+ B cells and was unable to infect ex vivo NK cells and T lymphocytes, whereas vaccinia virus could infect B lymphocytes and NK cell populations. Vaccinia virus was also able to infect T lymphocytes at low, but detectable levels that could be enhanced upon their activation. The observed preferential infection of ALVAC or vaccinia virus to monocytes was the result of preferential binding to this population, rather than lineage-specific differences in the expression of viral genes. Moreover, the level of CD14 expression on monocytes correlated with their preference to be infected with ALVAC or vaccinia virus. Both ALVAC and vaccinia viruses could infect immature monocyte derived dendritic cells (MDDCs), but only ALVAC infection induced their subsequent maturation. Vaccinia virus, however, showed greater tropism for mature MDDCs compared to ALVAC. Infection in human bone marrow cultures showed that ALVAC infection was restricted to a myelomonocytoid cell-specific CD33(+) cell population, while vaccinia virus showed a strong, but not exclusive, preference for these cells. These findings have implications in terms of choosing optimal pox virus derived vectors as vaccines in terms of reducing clinical reactogenicity and inducing dendritic cell (DC) maturation.

Modulation of the antibody response to the HIV envelope subunit by co-administration of infectious or heat-inactivated canarypoxvirus (ALVAC) preparations

Poxviruses are large DNA viruses capable of infecting a broad range of animal species. Infection is generally accompanied by an inflammatory response in the host, the extent of which varies considerably with the specific poxvirus and host species. Regarding ALVAC, a poxvirus derived from the canarypox vaccine strain, Kanapox, and which represents a promising immunization vehicle in humans, nothing is known about its inflammatory capacity. The present study was aimed at documenting this issue in rodents, including mice and guinea pigs. It was then attempted to evaluate how such properties could influence the immunogenicity of an antigen concomitantly administered with ALVAC preparations using the HIV envelope subunit, rgp160, as the model immunogen. The results revealed that ALVAC, either infectious or heat-inactivated, induced in both animal species an early inflammatory response, as evidenced by a rapid migration of neutrophils to the site of inoculation. In parallel, the canarypoxvirus was shown to strongly adjuvant the co-administered immunogen, resulting in a marked increase in Env-specific IgG, IgG1 and particularly IgG2(a) serum titers. Of further interest, the heat-inactivated preparation of ALVAC retained this immunostimulatory activity. Whether or not a link between the inflammatory and immunomodulatory properties of ALVAC exists remains to be established, but such features are clearly interesting with respect to the potential use of ALVAC as an immunization vehicle.

Overview of vector design for mammalian gene expression

The expression of cloned genes in mammalian cells is a basic tool for understanding gene expression, protein structure, and function, and biological regulatory mechanisms. The level of protein expression from heterologous genes introduced into mammalian cells depends upon multiple factors including DNA copy number, efficiency of transportation, mRNA processing, mRNA transport, mRNA stability, and translational efficiency, and protein processing, transport, and stability. Different genes exhibit different rate limiting steps for efficient expression. Multiple strategies are available to obtain high level expression in mammalian cells. This article reviews vector design for expression of foreign genes in mammalian cells.

Low-level transforming activity of an activated Ras gene under the control of a vaccinia virus p40 promoter is abrogated by truncation of the Ras cDNA

Many human cancers have been shown to contain activated forms of the Ras proto-oncogene. Mutations comprising amino acid changes at codons 12, 13 and 61 therefore represent unique targets for cancer immunotherapy. Recombinant Vaccinia viruses encoding point mutated Ras oncogenes have raised issues concerning the safety and transforming ability of these recombinant vaccines. Vaccinia virus, a representative of the orthopox virus genus, is a large DNA virus that is cytopathogenic and that replicates in the cytoplasm of the infected cell. However, it remains unclear whether orthopox viruses are capable of genetic interactions with infected cells. Our studies show that DNA isolated from cells infected with a recombinant Vaccinia virus expressing mutated Ras constituted a poor reagent for transfection into NIH3T3 cells for transformation analysis. Stable integration of a recombinant Vaccinia virus expressing mutant Ras DNA was not detected in recipient cells. This study also demonstrates that the crossover plasmids used to generate the recombinant virus where the activated Ras gene is under the control of a Vaccinia virus early promoter had low but detectable transforming efficiency in the NIH3T3 transformation assay. Analysis of the transfected cells indicated that Ras transcription was initiated upstream of the Vaccinia virus promoter. The introduction of wobble mutations as well as the truncation of the Ras protein removed the transforming capabilities of the crossover vector. This study demonstrates the potential problems and solutions in the use of point mutated oncogenes in live vectors for cancer vaccine development.

Highly attenuated poxviruses induce functional priming of neutrophils in vitro

Highly attenuated poxvirus samples were used to examine the influence of potential poxvirus expression vector systems on neutrophil function. Exposure to the viruses alters the number of membrane bound complement-and Fc gamma-receptors and led to functional priming of neutrophils to subsequent stimuli.