Kinetics of HIV-1 fusion in engineered and natural target cells. HXB2 pseudoviruses were spinoculated at 4°C onto TZM-bl (A), U87.CD4.CCR5 (B), CEMss (C), PM1 (D), Jurkat (E), and primary CD4+ T cells (F). Particles pseudotyped with JRFL Env were also used in panels A and B, and BaL pseudotypes were employed in panel A. Fusion was allowed to proceed for 90 min at 37°C in DMEM/10% FBS and measured by the BlaM assay. Virus fusion was stopped either by adding C52L (1 μM) or by placing cells on ice (TB) after varied times of incubation. At the end of the incubation period, cells were placed on ice, loaded with the BlaM substrate, and the extent of fusion was measured after an overnight incubation at 12°C. Unless indicated otherwise, data points are means ± standard errors of the mean (SEM) from triplicate measurements.