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Induction of AIDS Virus-Specific CTL Activity in Fresh, Unstimulated Peripheral Blood Lymphocytes from Rhesus Macaques Vaccinated with a DNA Prime/Modified Vaccinia Virus Ankara Boost Regimen¹

Todd M. Allen^2,*, Thorsten U. Vogel^*, Deborah H. Fuller, Bianca R. Mothé^*, Susan Steffen^*, Jon E. Boyson^3,*, Tim Shipley, Jim Fuller, Tomas Hanke^§, Alessandro Sette^¶, John D. Altman^||, Bernard Moss^#, Andrew J. McMichael^§ and David I. Watkins^*,

^* Wisconsin Regional Primate Research Center and Department of Pathology and Laboratory Medicine, University of Wisconsin, Madison, WI 53715; PowderJect, Inc., Madison, WI 53711; ^§ Institute of Molecular Medicine, University of Oxford, Oxford, United Kingdom; ^¶ Epimmune, Inc., San Diego, CA 92121; ^|| Emory Vaccine Center, Emory University School of Medicine, Atlanta, GA 30322; and ^# Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892

The observed role of CTL in the containment of AIDS virus replication suggests that an effective HIV vaccine will be required to generate strong CTL responses. Because epitope-based vaccines offer several potential advantages for inducing strong, multispecific CTL responses, we tested the ability of an epitope-based DNA prime/modified vaccinia virus Ankara (MVA) boost vaccine to induce CTL responses against a single SIVgag CTL epitope. As assessed using both ⁵¹Cr release assays and tetramer staining of in vitro stimulated PBMC, DNA vaccinations administered to the skin with the gene gun induced and progressively increased p11C, CM (CTPYDINQM)-specific CD8⁺ T lymphocyte responses in six of six Mamu-A*01⁺ rhesus macaques. Tetramer staining of fresh, unstimulated PBMC from two of the DNA-vaccinated animals indicated that as much as 0.4% of all CD3⁺/CD8⁺ T lymphocytes were specific for the SIVgag CTL epitope. Administration of MVA expressing the SIVgag CTL epitope further boosted these responses, such that 0.8–20.0% of CD3⁺/CD8⁺ T lymphocytes in fresh, unstimulated PBMC were now Ag specific. Enzyme-linked immunospot assays confirmed this high frequency of Ag-specific cells, and intracellular IFN- staining demonstrated that the majority of these cells produced IFN- after peptide stimulation. Moreover, direct ex vivo SIV-specific cytotoxic activity could be detected in PBMC from five of the six DNA/MVA-vaccinated animals, indicating that this epitope-based DNA prime/MVA boost regimen represents a potent method for inducing high levels of functionally active, Ag-specific CD8⁺ T lymphocytes in non-human primates.