NONINFECTIOUS VIRUS-LIKE PARTICLES PRODUCED BY MOLONEY MURINE LEUKEMIA VIRUS-BASED RETROVIRUS PACKAGING CELLS DEFICIENT IN VIRAL ENVELOPE BECOME INFECTIOUS IN THE PRESENCE OF LIPOFECTION REAGENTS

Retrovirus packaging cell lines expressing the Moloney murine leukemia virus gag and pol genes but lacking virus envelope genes produce viru s-like particles constitutively, whether or not they express a transcr ipt from an integrated retroviral provirus, In the absence of a provir al transcript, the assembled particles contain processed gag and rever se transcriptase, and particles made by cells expressing an integrated lacZ provirus also contain viral RNA. The virus-like particles from b oth cell types are enveloped and are secreted/budded into the extracel lular space but are noninfectious. Their physicochemical properties ar e similar to those of mature retroviral particles, The noninfectious g ag pol RNA particles can readily be made infectious by the addition of lipofection reagents to produce preparations with titers of up to 10( 5) colony-forming units per ml.