Semen inhibits trans-infection of T cells by DC-SIGN. B-THP-1-DC-SIGN cells were treated with the indicated concentration of SE, SE-F or SE-P for 30 min, subsequently exposed to R5 HIV-1 for 30 min, washed and cocultivated with CEM-M7 cells. The levels of infection mediated by B-THP-1 cells, which do not express DC-SIGN, and the absence of cells (medium) are also shown as controls. Shown are average values ± SD derived from triplicate infections. Stars indicate cytotoxicity, infection rates obtained after infection with 2% and 10% SE, SE-F or SE-P treated virus were close to background luciferase activities of uninfected cells.