NGF, proNGF, and BDNF enhance HIV-1 replication. (A) Macrophages were infected by an R5-tropic HIV-1 isolate (Ba-L), treated only once with different concentrations of neurotrophins as indicated, and virus replication was measured in the culture supernatants by an HIV-1 p24 ELISA 14 days after infection. (B) HIV-1 production was also measured at different time points after infection and addition of NGF (10 ng/mL). Data represent means ± SEM of 6 independent experiments. Virus production in the positive control (HIV-1–infected cells cultured only with medium): 105 ± 27 ng/mL of p24 Ag. *P < .05; **P < .01.