Rapid complementation assays measuring replicative potential of human immunodeficiency virus type 1 envelope glycoprotein mutants

Rapid assays which measure the ability of mutant human immunodeficiency virus type 1 envelope glycoproteins to mediate cell-free and/or cell-to-cell transmission of virus are described. By using these assays, envelope glycoprotein mutants with varying degrees of syncytium-forming ability were tested for ability to complement viral replication in trans. As expected, mutants that dramatically affect association of the gp120-gp41 envelope subunits, CD4 binding, or membrane fusion were unable to form syncytia or to support cell-free or cell-to-cell transmission. Surprisingly, some membrane fusion-defective mutants significantly attenuated in syncytium-forming ability were able to complement viral replication. Conversely, mutations in the carboxyl terminus of gp41 transmembrane glycoprotein, although not affecting syncytium-forming ability, significantly attenuated both forms of virus transmission. These results indicate that syncytium formation is not sufficient for cell-to-cell transmission of human immunodeficiency virus type 1. Furthermore, virus transmission appears to be less sensitive to inhibition of membrane fusion than is syncytium formation.