NGF does not affect HIV-1 adsorption and penetration. Macrophages were exposed to an R5-tropic HIV-1 isolate (Ba-L) and treated with NGF (10 ng/mL) or sCD4 (1 μg/mL) for 3 hours at 4°C. The inoculum was then removed, cells were washed and lysed, and the adsorbed HIV-1 p24 Ag was measured with an HIV-1 p24 ELISA (adsorption). Alternatively, after the incubation at 4°C, the temperature was raised to 37°C for an additional 2 hours and cells were then washed and lysed to measure internalized HIV-1 p24 Ag as above (penetration). Data represent mean ± SEM of 3 independent experiments.