Structure and immunogenicity of alternative forms of the simian immunodeficiency virus gag protein expressed using Venezuelan equine encephalitis virus replicon particles

Abstract

Venezuelan equine encephalitis virus replicon particles (VRP) were engineered to express different forms of SIV Gag to compare expression in vitro, formation of intra- and extracellular structures and induction of humoral and cellular immunity in mice. The three forms examined were full-length myristylated SIV Gag (Gag myr+), full-length Gag lacking the myristylation signal (Gag myr−) or a truncated form of Gag myr− comprising only the matrix and capsid domains (MA/CA). Comparison of VRP-infected primary mouse embryo fibroblasts, mouse L929 cells and primate Vero cells showed comparable expression levels for each protein, as well as extracellular virus-like particles (VRP-Gag myr+) and distinctive cytoplasmic aggregates (VRP-Gag myr−) with each cell type. VRP were used to immunize BALB/c mice, and immune responses were compared using an interferon (IFN)-γ ELISPOT assay and a serum antibody ELISA. Although all three VRP generated similar levels of IFN-γ-producing cells at 1 week post-boost, at 10 weeks post-boost the MA/CA-VRP-induced response was maintained at a significantly higher level relative to that induced by Gag myr+-VRP. Antibody responses to MA/CA-VRP and Gag myr+-VRP were not significantly different.