CD4 and infection: At 48 h post-transfection, cells were lysed, and the levels of cell-to-cell infection were quantified by measuring luciferase activity, while the supernatants were harvested, filtered through 0.45-μm pore size filters, and used to infect freshly cultured 293T/CD4/X4 target cells for another 48 h (cell-free infection) (Figure 2A).