Abstract: The invention relates to retrovirus-like particles having an altered nucleocapsid domain that inhibits packaging of genomic RNA into the retrovirus-like particle, and a protease having reduced enzymatic activity. The invention also provides nucleic acid constructs that encode the retrovirus-like particles. Also provided are immunological compositions, vaccines, and DNA vaccines containing the retrovirus-like particles and nucleic acid constructs that can be used to immunize a mammal against infection by a retrovirus. Methods to produce retrovirus-like particles are also provided.