Host range of human T-cell leukemia virus type I analyzed by a cell fusion-dependent reporter gene activation assay

We constructed a sensitive and quantitative assay system to examine human T-cell leukemia virus type I (HTLV-I) envelope (env) glycoprotein-mediated cell fusion in which T7 RNA polymerase in donor cells coexpressing env glycoproteins activates a reporter gene in recipient cells upon cell fusion. An efficient expression of HTLV-I env glycoproteins (gp46 and gp21) was observed in 293T cells transfected with an expression plasmid by both immunoblot and immunofluorescence analyses. The cells expressing env glycoproteins also exhibited self-fusion. By cocultivating the donor cells with recipient cells transfected with a reporter plasmid possessing the luciferase gene under the T7 promoter, the expression of luciferase was observed upon cell fusion. The activation of the luciferase gene was inhibited by either anti-env neutralizing antibody or synthetic peptide corresponding to env gp21, thus indicating the cell fusion to be specifically mediated by the HTLV-I env glycoproteins expressed in the donor cells. A broad range of cell lines exhibited susceptibility to HTLV-I env-mediated cell fusion by this assay. This newly established assay system may thus provide an efficient way both to study the fusion mechanisms mediated by HTLV-I env glycoproteins and to identify the HTLV-I receptor(s).