BLyS promotes monocyte activation and differentiation. Cells were stimulated with the indicated stimuli for 24 hours before isolation of cell-free supernatants and assessment of secreted (A) IL-6, (B) TNF-α, and (C) IL-1β levels by ELISA. ^*Values that were below the limits of detection. (A-C) Data shown represent mean ± SD of 3 different experiments. (D) Isolated monocytes were cultured ± 200 ng/mL _LBLyS for 2 days before assessment of expression of the indicated surface costimulatory molecules on viable cells by flow cytometry. Each symbol represents the ΔMFI, which is a reflection of fold induction of each molecule relative to the isotype control. (E) Monocyte differentiation was determined by measuring surface expression of CD14 and CD1a at day 6. Cell morphology was analyzed by flow cytometry and by light microscopy using an Olympus AX70 microscope (Olympus America, Center Valley, PA) equipped with a UPlan FL objective at a magnification of 60×(1.25 numeric aperture) under oil. Images were collected using a Diagnostic Instruments camera (model 1.4.0; Sterling Heights, MI) and SPOT image acquisition software version 3.0.4 (Diagnostic Instruments).