Modifications of HIV-1 retrovirus-like particles to enhance safety and immunogenicity

HIV-1 retrovirus-like particles can be produced in VERO cells that have been transfected with an expression construct encoding HIV-1 structural proteins. The particles are entirely non-infectious although structurally they resemble infectious virus particles. This makes them a promising candidate for use as an HIV-1 vaccine. In order to ensure their safety and enhance their immunogenicity, the retrovirus-like particles were modified in a number of ways. A large deletion in the HIV-1 pol gene has eliminated reverse transcriptase and integrase activities. Deletion of RNA packaging signals in the RNA untranslated leader sequence and in Gag reduced packaged RNA to 5% of that in HIV-1 virus. Replacement of the existing HIV-1LAI envelope protein with that of HIV-1MN has ensured that immune responses to the particles are relevant to those against the majority of HIV-1 clade B isolates. In addition to these changes in particle composition, yields of the modified particles were increased using a superior method of inducing the expression construct promoter, and an effective scheme for particle purification was developed. Immunization of non-human primates demonstrated that the particles were capable of generating anti-HIV-1 neutralizing antibodies. The technological refinements reported here will permit retrovirus-like particles to be tested safely in humans, and the change in envelope proteins should allow a more realistic evaluation of the immunogenicity of these particles. Experience gained in engineering these refinements will greatly facilitate other modifications that may be required to achieve maximum efficacy as a vaccine against HIV-1.