Fibroblast-like synoviocytes (FLS) stimulated by monosodium urate (MSU) crystals in the presence of high-density lipoproteins (HDL) retain intracellular CCL2. FLS were stimulated or not with MSU crystals (50 μg/ml) in the presence or absence of the indicated concentration of HDL. After 24 hours, cells were fixed and subjected to immunostaining with anti-CCL2 antibodies. (a) Resting FLS; (b, c) FLS stimulated with MSU crystals in the absence (b) or presence of HDL (c). Original magnification × 600. (d) FLS were treated (white columns) or not (grey columns) with 10 μg/ml cycloheximide (CHX) for 30 minutes and then activated for the indicated time with 50 μg/ml MSU crystals. Results represent mean ± SD of three independent experiments.