Brief Report: Granulocyte–Macrophage Colony‐Stimulating Factor Drives Monosodium Urate Monohydrate Crystal–Induced Inflammatory Macrophage Differentiation and NLRP3 Inflammasome Up‐Regulation in an In Vivo Mouse Model

Objective To determine the role of granulocyte–macrophage colony-stimulating factor (GM-CSF) in the differentiation of inflammatory macrophages in an in vivo model of monosodium urate monohydrate (MSU) crystal–induced inflammation. Methods C57BL/6J mice were treated with either clodronate liposomes to deplete peritoneal macrophages or GM-CSF antibody and were then challenged by intraperitoneal injection of MSU crystals. Peritoneal lavage fluid was collected, and cellular infiltration was determined by flow cytometry. Purified resident and MSU crystal–recruited monocyte/macrophages were stimulated ex vivo with MSU crystals. The interleukin-1β (IL-1β) levels in lavage fluids and ex vivo assay supernatants were measured. GM-CSF–derived and macrophage colony-stimulating factor (M-CSF)–derived macrophages were generated in vitro from bone marrow cells. Protein expression of IL-1β, caspase 1, NLRP3, and ASC by in vitro– and in vivo–generated monocyte/macrophages was analyzed by Western blotting. Results Depletion of resident macrophages lowered MSU crystal–induced IL-1β and GM-CSF levels in vivo as well as IL-1β production by MSU crystal–recruited monocytes stimulated ex vivo. GM-CSF neutralization in vivo decreased MSU crystal–induced IL-1β levels and neutrophil infiltration. MSU crystal–recruited monocyte/macrophages from GM-CSF–neutralized mice expressed lower levels of the macrophage marker CD115 and produced less IL-1β following ex vivo stimulation. These monocytes exhibited decreased expression of NLRP3, pro/active IL-1β, and pro/active caspase 1. In vitro–derived GM-CSF–differentiated macrophages expressed higher levels of NLRP3, pro/active IL-1β, and pro/active caspase 1 compared to M-CSF–differentiated macrophages. Conclusion GM-CSF plays a key role in the differentiation of MSU crystal–recruited monocytes into proinflammatory macrophages. GM-CSF production may therefore contribute to the exacerbation of inflammation in gout.