Our design of the cell-cell fusion assay is described below. HEK293T cells, as
effector cells, were co-transfected with plasmids encoding for HIV-1 Env and HIV-1 Tat
proteins. HeLa T4+ cells, as target cells, were transfected with a plasmid harboring the HIV-1
LTR-driven copGFP reporter gene. Co-cultivation of the effector cells and target cells
enabled HIV-1 gp120 on the effector cell surface to bind to CD4 and CXCR4 co-receptor on
the target cell. The bindings triggered conformational changes in HIV-1 gp41 that finally
became fusion competent, mediating cell-cell membrane fusion processes. This allowed the
contents of the two cells to mix each other; HIV-1 Tat protein from the effector cells could
bind to the LTR-promoter in the target cells, resulting in increased transcription of copGFP
mRNA. The number of fusion events therefore is directly proportional to the expression of
copGFP protein. A schematic representation of the HIV-1 Env-mediated cell-cell fusion
assay in this study is shown in Figure 1.