pDCs are the cellular source of IL-4–enhanced IFN-λ1. (A) Purified pDCs were cocultured with autologous monocytes at a ratio of 9:1 (pDC/monocyte). SNs were harvested from these cultures after overnight incubation with HSV in the presence or absence of IL-4; IFN-λ1 levels were determined by ELISA. (B) Monocytes were stimulated with IL-4 (50 ng/mL) for 1 hour then washed. Cells were then cocultured with autologous pDCs and stimulated overnight with HSV in the presence or absence of IL-4 as shown. One representative experiment of 3 is shown; mean of triplicate wells ± SD.