Kinetics of chemokine secretion by U937 cells. U937 cells were cultivated with (○) medium alone, (•) IL-6 (0.24 nmol/L), (▪) IFN-γ (1.5 nmol/L), or (▴) TNF-α (0.02 nmol/L) for 72 hours. Supernatants were harvested daily, aliquoted, and frozen at −80°C until tested for the presence of MCP-1.