Immunization with a modified vaccinia virus expressing simian immunodeficiency virus (SIV) Gag-Pol primes for an anamnestic Gag-specific cytotoxic T-lymphocyte response and is associated with reduction of viremia after SIV challenge.

Evidence for a critical role of cytotoxic T lymphocytes (CTLs) in the containment of human immunodeficiency virus (HIV) infection (16, 23, 36, 39) has led to a consensus among those attempting to develop an AIDS vaccine that such a vaccine should generate CTL in addition to broadly neutralizing antibodies (26). An additional hurdle for an AIDS vaccine is the long-term maintenance of levels of CTL and antibody that will be necessary for protection (26). Both effector CTL and neutralizing antibodies induced by vaccination tend to be transient. Therefore, it may not be feasible to maintain immune responses essential for preventing infection. The importance of the magnitude of the vaccine-elicited memory and postinfection anamnestic immune responses thus become a critical issue in developing an AIDS vaccine. At present, viable vaccine strategies that might effectively stimulate CTL include viral vectors, peptides, and DNA immunization (26). Viral vectors under investigation include adenovirus (10, 48), alphaviruses such as Semliki Forest virus (8, 35) and Venezuelan equine encephalitis virus (11), poliovirus replicons (23), and various poxviruses (12, 21, 29, 30, 34, 43). Among these approaches, use of the poxviruses is a particularly promising vaccine strategy to express viral proteins. Studies with conventional New York Board of Health vaccinia virus demonstrated that priming with a vaccinia virus recombinant expressing simian immunodeficiency virus (SIV) envelope and/or core proteins, followed by boosting the antibody response with recombinant envelope protein, provided protection against a homologous SIV challenge with a biologically cloned strain of limited pathogenicity (SIVmne/E11S) (20). However this approach provided only partial protection against a more pathogenic and heterogeneous SIV challenge (SIVmne) (44) and blunting of viremia in macaques challenged with the highly pathogenic SIVmac251 (2). Since there are side effects associated with using conventional vaccinia viruses that become potentially life threatening when used in immunocompromised individuals (34), the use of attenuated poxviruses is an attractive alternative (20, 34, 43). A number of attenuated poxvirus strains have been developed as vaccine vectors: the avipoxviruses (43), canarypox virus, fowlpox virus, and the attenuated vaccinia virus derivatives, NYVAC (43), and modified vaccinia virus Ankara (MVA) (13, 32, 34, 43). The attenuated poxviruses appear to be safe in immunosuppressed animals (30), although their bases for attenuation differ. Thus, the avipoxviruses are genetically quite distinct from vaccinia viruses and do not complete an entire replication cycle in mammalian cells. NYVAC is a genetically engineered derivative of the Copenhagen strain with deletion of host range genes (43). MVA is a spontaneously derived attenuated variant of the Ankara strain that has multiple deletions in host range genes and genes involved in suppressing vaccinia virus-elicited immune responses (29, 30, 34). Perhaps due to these latter deletions, MVA appears to be as immunogenic as wild-type vaccinia virus strains, despite limited replication in mammalian cells (5, 9, 13, 32). In addition, MVA has an excellent safety record in humans, having been used without incident as a smallpox vaccine in approximately 100,000 individuals (30). Each of these attenuated poxvirus vectors has been evaluated in primate models and has shown some degree of efficacy (1, 4, 7, 14, 19, 22, 24, 37, 49, 50, 54). In previous studies, we explored the use of MVA as a viral vector to express SIV antigens and have evaluated its efficacy in the SIVsm-macaque model (19). The SIV-macaque model is a highly relevant system in which to evaluate the efficacy of partially protective vaccines since it provides valid disease endpoints. In addition, the level at which plasma viremia stabilizes after primary infection (or viral set point) is a highly significant prognostic surrogate for the rate of disease progression in both HIV infection (31) and SIV infection (19, 52, 55). While the SIV-HIV (SHIV) chimeras provide a system to evaluate the role of envelope-specific immune response in vaccine protection, there is no particular advantage to the use of SHIV in evaluating Gag-Pol-specific immunity. Indeed, the pathogenesis of SIV infection of macaques more closely models the pathogenesis of human AIDS (18) than the rapid CD4+ T-cell depletion observed with the pathogenic SHIVs (21, 28, 51). Prior immunization with MVA expressing the SIVsmH4 Gag-Pol and Env followed by boosting with whole inactivated SIV particles, administered without adjuvant, resulted in significant modulation of viremia and disease progression in macaques subsequently challenged with pathogenic SIVsmE660 (19). Two of these MVA-SIV vaccinees have maintained low viremia and normal CD4+ T-lymphocyte numbers throughout the 4 years since challenge, analogous to HIV type 1-infected clinical long-term nonprogressor humans (12, 42). The recent development of technology for measuring effector CTL by flow cytometric analyses of CD8+ T lymphocytes that bind specific peptide epitope-major histocompatibility complex (MHC) class I tetrameric complexes (3) has revolutionized the ability to quantitate CTL responses in SIV-infected macaques (24, 25). This technology is particularly useful for evaluation of CTL responses in recombinant vaccinia virus-immunized macaques since the background cytolysis that is seen in functional CTL assays with the use of vaccinia virus expression of viral proteins in target cells can be avoided. In a previous study we used tetramer technology to evaluate the ability of an MVA recombinant expressing SIV Gag-Pol to elicit CTL (50). In the present study, we evaluated immunogenicity of this recombinant through two subsequent boosts and evaluated the efficacy following intravenous challenge with pathogenic, uncloned, SIVsmE660.