The immunogenicity and protective efficacy of a modified vaccinia virus Ankara (MVA) recombinant expressing the simian immunodeficiency virus (SIV) Gag-Pol proteins (MVA-gag-pol) was explored in rhesus monkeys expressing the major histocompatibility complex (MHC) class I allele, MamuA*01. Macaques received four sequential intramuscular immunizations with the MVA-gag-pol recombinant virus or nonrecombinant MVA as a control. Gag-specific cytotoxic T-lymphocyte (CTL) responses were detected in all MVA-gagpol-immunized macaques by both functional assays and flow cytometric analyses of CD8 ؉ T cells that bound a specific MHC complex class I-peptide tetramer, with levels peaking after the second immunization. Following challenge with uncloned SIVsmE660, all macaques became infected; however, viral load set points were lower in MVA-gag-pol-immunized macaques than in the MVA-immunized control macaques. MVA-gag-pol-immunized macaques exhibited a rapid and substantial anamnestic CTL response specific for the p11C, C-M Gag epitope. The level at which CTL stabilized after resolution of primary viremia correlated inversely with plasma viral load set point (P ‫؍‬ 0.03). Most importantly, the magnitude of reduction in viremia in the vaccinees was predicted by the magnitude of the vaccine-elicited CTL response prior to 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 app...

The utility of modified vaccinia virus Ankara (MVA) as a vector for eliciting AIDS virus-specific cytotoxic T lymphocytes (CTL) was explored in the simian immunodeficiency virus (SIV)͞rhesus monkey model. After two intramuscular immunizations with recombinant MVA-SIV SM gag pol, the monkeys developed a Gag epitope-specific CTL response readily detected in peripheral blood lymphocytes by using a functional killing assay. Moreover, those immunizations also elicited a population of CD8؉ T lymphocytes in the peripheral blood that bound a specific major histocompatibility complex class I͞peptide tetramer. These Gag epitopespecific CD8؉ T lymphocytes also were demonstrated by using both functional and tetramer-binding assays in lymph nodes of the immunized monkeys. These observations suggest that MVA may prove a useful vector for an HIV-1 vaccine. They also suggest that tetramer staining may be a useful technology for monitoring CTL generation in vaccine trials in nonhuman primates and in humans.