Fusion from without directed by human immunodeficiency virus particles.

Fusion from without is the process through which particles of some enveloped viruses can direct fusion of target cells in the absence of viral replication. We demonstrate here that human immunodeficiency virus (HIV) particles can efficiently promote fusion from without. Using HeLa-CD4 cells carrying a Tat-inducible lacZ gene, we observed syncytia as early as 6 h after exposure to HIV particles, before HIV gene expression could be detected. Efficient syncytium formation could be obtained when cells were treated with zidovudine, which prevented HIV replication and expression but not cell-cell fusion. Fusion was also observed when cells were exposed to particles of a replication-defective HIV integrase mutant. Fusion from without by HIV particles could be blocked by a monoclonal antibody specific for the V3 loop of the HIV-1 envelope glycoprotein and by soluble CD4. This mechanism of cytopathicity, which can involve cells that do not actively replicate HIV and can be directed by replication-defective particles, could participate in the pathogenicity of the CD4 cell depletion that characterizes HIV infection.