Monosodium Urate Monohydrate Crystal-Recruited Noninflammatory Monocytes Differentiate Into M1-like Proinflammatory Macrophages in a Peritoneal Murine Model of Gout

Objective. To profile monosodium urate monohydrate (MSU) crystal-recruited monocyte inflammatory function during the course of in vivo differentiation, in a murine model of peritoneal MSU crystal-induced inflammation. Methods. C57BL/6J mice were injected intraperitoneally with MSU crystals, and the peritoneal cells were harvested at different time points. The MSU crystal-recruited monocyte/macrophage population was analyzed for the expression of differentiation and activation markers, cytokine production following MSU crystal restimulation ex vivo and in vivo, expression of NLRP3-associated proteins (ASC, caspase 1) and prointerleukin-1 beta (proIL-1 beta), and phagocytic capacity. Results. Monocytes recruited 8 hours after MSU crystal stimulation (F4/80(low)Gr-1(int)7/4+) exhibited poor phagocytic capacity, expressed low levels of proIL-1 beta, and failed to produce proinflammatory cytokines in response to MSU crystal restimulation. In the absence of MSU crystal restimulation, differentiating monocytes produced low levels of transforming growth factor beta 1 ex vivo, and this was abrogated following MSU crystal restimulation. Over time these cells developed a proinflammatory phenotype in vivo, characterized by the production of IL-1 beta, tumor necrosis factor alpha, IL-6, CCL2 (monocyte chemotactic protein 1), and CXCL1 (cytokine-induced neutrophil chemoattractant) following ex vivo MSU crystal restimulation, and leading to IL-1 beta production and cell infiltration following MSU crystal rechallenge in vivo. Proinflammatory function was associated with differentiation toward a macrophage phenotype (F4/80(high)Gr-1-7/4-), an increase in phagocytic capacity, and an increase in the expression of proIL-1 beta. Conclusion. MSU crystal-recruited monocytes differentiate into proinflammatory M1-like macrophages in vivo. This proinflammatory macrophage phenotype is likely to play a key role in perpetuating inflammation in gouty arthritis in the presence of ongoing deposition of fresh MSU crystals.