Modified Vaccinia Virus Ankara Immunization Protects against Lethal Challenge with Recombinant Vaccinia Virus Expressing Murine Interleukin-4

The continued threat of bioterrorism has led to concern over the reemergence of smallpox. Global immunity against poxviruses has declined over the last 20 years, as routine immunization against smallpox ceased in the early 1980s following the declaration of smallpox eradication by the World Health Organization. The consequences of a deliberate smallpox release would be great, as mortality for nonimmune persons has been reported to be near 30% (10). While historical experience supports the efficacy of replication-competent vaccinia virus immunization against natural smallpox, the current Food and Drug Administration (FDA)-licensed vaccine Dryvax presents some safety concerns. Previous studies have described a significant rate of serious complications following Dryvax administration, including death, in 1 per 1 million vaccinees (26, 27). This rate could be even higher if mass vaccination were instituted today because of the large and growing number of persons for whom Dryvax is contraindicated. This has led to hesitation within the medical community for the use of widespread vaccination (9, 33). As a result, there have been renewed investigations into the development of a safer second-generation smallpox vaccine. Previous strategies for the development of safe smallpox vaccines have focused on less virulent vaccinia virus strains, including recombinant vaccinia viruses with selected deletions of virulence genes or insertions of proinflammatory cytokines (13, 14, 34, 40) and empirically attenuated live vaccinia virus strains (21, 25, 29, 30, 37). The modified vaccinia virus Ankara (MVA) was attenuated by >570 passages in chicken embryo fibroblasts, resulting in the loss of approximately 15% of its parent genome, including several host range genes (2, 29, 31). MVA is consequently unable to replicate effectively in mammalian cells, which reduces the risk of dissemination and transmission (8, 35). In addition, MVA no longer encodes many of the soluble inhibitors of cytokine and chemokine function as well as other factors that play a role in immune evasion (1, 6, 39). However, epitopes that are known to elicit neutralizing antibodies are conserved (16, 32, 44), and recently three human CD8+ cytotoxic T lymphocyte (CTL) epitopes restricted to HLA-A*0201 have been identified that are present in MVA, the Copenhagen strain of vaccinia virus, and variola virus (11, 41). Thus, MVA can induce significant vaccinia virus-specific immune responses that are unmodified by normal vaccinia virus immune evasion mechanisms. Earlier work with MVA demonstrated its safety and its ability to protect against the development of poxvirus infections in several animal models (22, 30, 43). Recently, MVA immunization has been shown to provide protection against a pulmonary vaccinia virus challenge (4, 11). With the threat of bioterrorism and the potential for exposure to genetically manipulated weaponized smallpox, the ability of a new vaccine to protect against pathogens with enhanced virulence may be necessary. Type 2 cytokines have been shown to diminish CTL activity in vivo and to inhibit viral clearance (2, 12, 24, 42). The coexpression of interleukin-4 (IL-4) in the presence of vaccinia virus infection results in the downregulation of type 1 cytokines, reduces cytolytic activity, and delays viral clearance (2, 3, 24, 36). The demonstration of potent immunity and in vivo protection by novel second-generation vaccines against vaccinia virus strains with enhanced virulence would lend further support to the development of a new approach to smallpox immunization. We sought to evaluate the comparative efficacies of MVA and a replication-competent vaccinia virus strain, vSC8, against both intradermal and pulmonary challenges of vaccinia virus in a mouse model. MVA immunization elicited both humoral and cellular immunity equivalent to that elicited by replication-competent vaccinia virus and protected mice from the illness associated with poxvirus challenge, including a lethal intranasal challenge with a recombinant vaccinia virus, vSC8-mIL4. MVA immunization reduced viral replication in the lung tissue and reduced the pathology associated with vaccinia virus pulmonary infections. After the selective depletion of B- and T-cell subsets, mice immunized with MVA retained sufficient immunity for protection, suggesting that the immunologic correlate of protection from vaccinia virus is complex and redundant.