Fluorescently labeled R837 activates human pDCs. Human pDCs, purified from fresh buffy coats and precultured with IL-3 for 24 hours, were stimulated with different concentrations of R837, labeled or not with TAMRA or AF488. After overnight incubation, cytokine concentrations in the culture supernatant fluids were assayed, and cells were analyzed by flow cytometry for CD80 and CD86 expression and for compound uptake (TAMRA or AF488 fluorescence). Data shown are from one donor and are representative of ≥ 3 independent donors.