S100A8 and rheumatoid arthritis: Next, in vitro-differentiated macrophages, prepared by incubating monocytes for 7 days with or without GM-CSF or IFN-γ, were stimulated for 24 hours by 2 μM S100A8/A9 (50 μg/ml; equivalent to approximately the mean concentration in RA SF), and TNF-α production was determined.