Preparation of MSU crystals

MSU crystals were prepared according to a previous report [²⁶]. Uric acid (1 g; Sigma Aldrich USA) was dissolved in 180 ml of 0.01 M NaOH and heated to 70°C. The pH of the solution was maintained at 7.2, and the solution was cooled by continuous stirring at room temperature for 24 h. The MSU crystals were collected in a sterilized container, washed with ethanol, dried, and re-suspended in PBS by sonication. The MSU crystals prepared this way contained < 0.005 EU/mL endotoxin, as detected by a Pierce Chromogenic Endotoxin Quant Kit (Thermo Fisher Scientific, USA).