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3 Protective Immunity Induced by Whole Recombinant Yeast-Based HIV Vaccine
R. C. Duke1,2, A. S. Stubbs2, J. Sun2, D. Chan2, T. Whisenand1, K. Hance1, D. Bellgrau1,2, C. C. Wilson2, and A. Franzusoff*1
1GlobeImmune, Inc., Denver, CO, USA and 2Univ. of Colorado Hlth. Sci. Ctr., Denver, USA
Background: Therapeutic vaccines that stimulate cytotoxic T lymphocytes (CTLs) are being sought that can trigger and maintain cell-mediated immunity in HIV-infected individuals. Optimal stimulation of CTLs requires presentation of antigens by dendritic cells (DCs). DCs are unique in their ability to process particulate antigens into the MHC class I pathway for presentation to CTLs. Vaccine strategies that target or activate DCs are the subject of intense research. We have developed a novel vaccine formulation based on using whole, recombinant, nonpathogenic Saccharomyces cerevisiae yeast as a vector. Initial studies showed that yeast were rapidly internalized by DCs, leading to DC maturation, up-regulation of costimulatory molecules, and secretion of IL-12. DCs that had internalized yeast efficiently presented yeast-associated antigens to both MHC class I- and class II-restricted T cells. In addition, yeast provided a potent adjuvant effect, allowing DCs to efficiently present non-yeast-associated, exogenous antigens to MHC class I-restricted T cells. Here we show how the yeast-based formulation has been developed for use in clinical trials as a therapeutic HIV vaccine.
Methods: Recombinant yeast were engineered to express HIV-1HXB2 p55 gag protein: inducibly in a plasmid construct (HIVAX-2.2); constitutively expressed in a plasmid construct (HIVAX-2.3); and as constitutively expressed constructs integrated as multiple copies in the yeast genome (HIVAX-2.5).
Results: Gag protein expression in the HIVAX-2 yeast was confirmed by immunoblots. C57Bl/6 mice were immunized subcutaneously with 20 million intact live or heat-killed yeast. No adverse effects were observed following vaccination of immunocompetent or -deficient (scid) mice. Vaccine-induced immune responses were demonstrated by intracellular IFN-gamma staining, by CTL-mediated killing assays, and by T-cell proliferation assays. To demonstrate that these immune responses conferred protective immunity, an HIV-gag dependent tumor model was utilized. Mice vaccinated with the various HIVAX-2 preparations were protected against subsequent challenge with syngeneic, B16 melanoma cells stably expressing HIV-1HXB2 p55 gag. Protection was not observed in mice vaccinated with YVEC or in mice challenged with nontransfected B16 melanoma cells.
Conclusions: Our results show that vaccination of animals with whole recombinant yeast triggers protective, antigen-specific immunity against tumor and viral antigens. The studies described here and in a recent Nature Medicine article demonstrate that the recombinant yeast formulation represents a platform vaccine strategy for the potent induction of broad-based cellular immune responses. The HIVAX-2 yeast-based formulation is currently being prepared by GlobeImmune, Inc. for use as a therapeutic HIV vaccine in a phase I clinical trial.
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