Synovial fluid derived from patients with RA and PsA, but not OA, induces monocyte differentiation into DCs in the presence of NK cells. (A) CD14⁺ PB monocytes derived from a healthy donor were cultured alone (top row) or together with freshly isolated autologous PB NK cells (bottom row) in medium containing 10% cell-free SF derived from patients with RA (n = 3), PsA (n = 1) or OA (n = 4) as indicated on the top. Cells were stained on day 4 for DC-SIGN and CD40. A combined gate was set on monocytes/DCs based on their CD56⁻ and FSC:SSC profiles, and quadrants are set according to the isotype control staining. One of 2 experiments performed with similar results is shown. (B) The light gray histograms depict the CD14 levels on monocytes derived from healthy donors when cultured alone in the presence of medium supplemented with 10% cell-free SF from the patients in panel A, as indicated. The bold lines indicate the CD14 levels on cells cocultured with autologous NK cells in the presence of 10% SF as indicated. The black histogram indicates the isotype control staining of monocytes cultured in medium alone. (C) The IL-15 levels as measured by ELISA in cell-free SF derived from the patients in panel A as indicated.