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 (proIL-1), 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, 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 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, tumor necrosis factor ␣, IL-6, CCL2 (monocyte chemotactic protein 1), and CXCL1 (cytokine-induced neutrophil chemoattractant) following ex vivo MSU crystal restimulation, and leading to IL-1 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.
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.Acute gout is an intensely painful form of inflammatory arthritis caused by the formation of monosodium urate monohydrate (MSU) crystals in the joint and connective tissues. An acute attack of gout is characterized by intense pain and swelling and reddening of the skin around the affected area. Without treatment this inflammatory response will resolve naturally over 7-10 days.Interleukin-1 (IL-1) has been identified as a pivotal cytokine in gout and MSU crystal-induced inflammation (1-3). More recently, studies have identified IL-1 production following activation of the NLRP3 inflammasome in resident macrophages as a key event in the initiation of an attack of acute gout (4-7). However, there is a growing body of evidence indicating that cells of the monocyte/macrophage lineage play a pivotal role not only in the initiation but also in the progression and resolution of acute gouty inflammation.It has been proposed that the self-limiting nature of an acute gout attack is linked to the differentiation state of the MSU crystal-recruited monocyte population. In...