S100A8 and rheumatoid arthritis: Despite its lower efficacy, S100A8/A9 stimulation is believed to be effective in joints of patients with RA, where the protein is present at 54.8 ± 28.6 μg/ml, because in vitro-differentiated macrophages, as well as freshly isolated monocytes (data not shown), when stimulated with 2 μM (50 μg/ml) S100A8/A9, could produce significant levels of TNF-α.