FNG inhibits HIV expression in PMA-stimulated U1 cells and in U937 cells acutely infected with an X4 HIV-1. (A) U1 cells were incubated with the indicated concentrations of FNG and then stimulated with PMA. (B) Cell disruption rescues the RT activity levels of U1 cells stimulated with PMA in the presence of FNG, as detected after 4 days of culture (arrowhead in panel A). (C) U937 cells were incubated with FNG (100 μg/mL) and then infected with the X4 strain HIV-1_LAI/IIIB; FNG was added to the cell cultures every 72 hours, when 50% of the culture supernatant was replaced with culture medium. (D) Cell disruption rescued the RT activity levels of acutely infected U937 cells in the presence of FNG at the indicated days after infection (arrowheads in panel C; *P < .001). The results shown were derived from 1 experiment, representative of 3 (A,B) and 2 (C,D) independently performed experiments. cpm indicates counts per minute.