FIG. 4.

HIV-1 induced fusion from without of Sup T1 cells. (A) Sup T1 cells, highly susceptible to CD4-dependent, HIV-1 envelope glycoprotein-mediated cell fusion. (B) Following a 3-h incubation with concentrated native HIV-1, characteristic syncytia are seen, reflecting virion-mediated fusion from without. (C) Fusion mediated by native virions is inhibited by prior incubation of cells with anti-Leu 3a. (D) AT-2-inactivated virions mediate fusion from without comparable to native virus. (E) Fusion mediated by AT-2-inactivated virions is inhibited by anti-Leu 3a. Magnification, ×200.