Properties of virus-like particles produced by SIV-chronically infected human cell clones.

SIVsm chronically infected cultures were obtained after infection of CEMX174 cells with either SIVsmH3 or SIVsmE660. These phenotypically CD4 cells, formed syncytia but only when cocultivated with CD4+ cells. Single cell clones were derived from these cultures and examined for the production of virus-specific proteins. The majority of the clones expressed SIV p27 antigen and low levels of virus reverse transcriptase activity. Western blot analysis, performed with either monoclonal or polyclonal sera, showed that a chronically infected clone (B7) produced particles which contained envelope (gp135 and gp43), gag precursors and gag proteins (p27, p16 and p8). However, these particles (SIVsmB7) lacked detectable levels of vpx and of integrase, and contained several fusion proteins which expressed viral protease antigens. This defective virus failed to infect established CD4+ cell lines, as well as primary cultures of macrophages and of peripheral blood lymphocytes, obtained both from humans and from rhesus macaques. Lack of infection correlated with lack of viral DNA detection by PCR amplification of genomic DNA extracted from these cell cultures. In addition, SIVsmB7 virus lacked infectivity in vivo. Rhesus macaques inoculated with high concentrations of SIVsmB7 showed no viremia and their PBMC were PCR negative. Thus, B7 cells produced stable, non-infectious virus mutants, which contained env and gag proteins, but lacked detectable amounts of vpx and of enzymes required for virus replication. Due to the high constitutive expression of this virus-like particle, we are now testing this preparation as a vaccine.