Type I interferons are partially responsible for the inhibition of HIV-1 replication by PDC supernatant. PM1 T cells and PDC supernatant were preincubated with type I IFN blocking antibodies (or a control antibody), followed by HIV-1 infection and subsequent replication of the virus. A control experiment was performed with mock supernatant (DC culturing medium). Viral spread was followed by CA-p24 ELISA; the results of day 5 are shown. Antibodies were used in a recommended 1:100 dilution, or at twice that concentration (1:50). Error bars represent standard deviations. *P < .001 compared with respective controls with mock supernatant. #P < .001.