Double-stranded RNA is a potent inducer of interferon, a modulator of the expression of a number of other genes involved in the response of cells to virus infection, an activator of the interferon-induced antiviral state, and may be involved in differentiation, induction of apoptosis, and control of oncogenic transformation. This review will attempt to summarize what is known about the cellular proteins that act to mediate the response of cells to double-stranded RNA and the viral and cellular macromolecules that may be able to modulate these responses.

A vaccinia virus-encoded double-stranded RNA-binding protein, p25, has been previously implicated in inhibition of the interferon-induced, double-stranded RNAactivated protein kinase. In this study, we have identified the vaccinia viral gene (WR strain) that encodes p25. Amino acid sequence analysis of a chymotryptic fragment of p25 revealed a close match to the vaccinia virus (Copenhagen strain) E3L gene. The WR strain E3L gene was cloned and expressed either in COS-1 cells or in rabbit reticulocyte lysates in vitro. A M, 25,000 polypeptide that could bind to poly(rI) poly(rC)-agarose and that reacted with p25-specific antiserum was produced in each case. In addition, COS cells expressing E3L gene products inhibited activation of the double-stranded RNA-activated protein kinase in extracts from interferon-treated cells. Removal of E3L-encoded products by adsorption with anti-p25 antiserum resulted in loss of kinase inhibitory activity. These results demonstrate that the vaccinia virus E3L gene encodes p25 and that the products of the E3L gene have kinase inhibitory activity. Comparison of the deduced amino acid sequence of the E3L gene products with the protein sequence data base revealed a region closely related to the human interferon-induced, double-stranded RNA-activated protein kinase.

The N-terminal domain of the E3L protein of vaccinia virus has sequence similarity to a family of Z-DNA binding proteins of defined three-dimensional structure and it is necessary for pathogenicity in mice. When other Z-DNA-binding domains are substituted for the similar E3L domain, the virus retains its lethality after intracranial inoculation. Mutations decreasing Z-DNA binding in the chimera correlate with decreases in viral pathogenicity, as do analogous mutations in wild-type E3L. A chimeric virus incorporating a related protein that does not bind Z-DNA is not pathogenic, but a mutation that creates Z-DNA binding makes a lethal virus. The ability to bind the Z conformation is thus essential to E3L activity. This finding may allow the design of a class of antiviral agents, including agents against variola (smallpox), which has an almost identical E3L.

Vaccinia virus (VACV) has been used more extensively for human immunization than any other vaccine. For almost two centuries, VACV was employed to provide cross-protection against variola virus, the causative agent of smallpox, until the disease was eradicated in the late 1970s. Since that time, continued research on VACV has produced a number of modified vaccines with improved safety profiles. Attenuation has been achieved through several strategies, including sequential passage in an alternative host, deletion of specific genes or genetic engineering of viral genes encoding immunomodulatory proteins. Some highly attenuated third-and fourth-generation VACV vaccines are now being considered for stockpiling against a possible re-introduction of smallpox through bioterrorism. Researchers have also taken advantage of the ability of the VACV genome to accommodate additional genetic material to produce novel vaccines against a wide variety of infectious agents, including a recombinant VACV encoding the rabies virus glycoprotein that is administered orally to wild animals. This review provides an in-depth examination of these successive generations of VACV vaccines, focusing on how the understanding of poxviral replication and viral gene function permits the deliberate modification of VACV immunogenicity and virulence.

The vaccinia virus (VV) E3L gene is responsible for providing interferon (IFN) resistance and a broad host range to VV in cell culture. The E3L gene product contains two distinct domains. A conserved carboxy-terminal domain, which is required for the IFN resistance and broad host range of the virus, has been shown to bind double-stranded RNA (dsRNA) and inhibit the antiviral dsRNA-dependent protein kinase, PKR. The aminoterminal domain, while conserved among orthopoxviruses, is dispensable in cell culture. To study the role of E3L in whole-animal infections, WR strain VV recombinants either lacking E3L (VV⌬E3L) or expressing an amino-terminal (VVE3L⌬83N) or carboxy-terminal (VVE3L⌬26C) truncation of E3L were constructed. Whereas wild-type VV had a 50% lethal dose of approximately 10 4 PFU after intranasal infection, and elicited severe weight loss and morbidity, VV⌬E3L was apathogenic, leading to no death, weight loss, or morbidity. VV⌬E3L was also apathogenic after intracranial injection. Although the amino-terminal domain of E3L is dispensable for infection of cells in culture, both the amino-and carboxy-terminal domains of E3L were required for full pathogenesis in intranasal infections. These results demonstrate that the entire E3L gene is required for pathogenesis in the mouse model.