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Abstract

Objective

To determine the mechanisms involved in inflammatory responses to octacalcium phosphate (OCP) crystals in vivo.

Methods

OCP crystal–induced inflammation was monitored using a peritoneal model of inflammation in mice with different deficiencies affecting interleukin-1 (IL-1) secretion (IL-1α–/–, IL-1β–/–, ASC–/–, and NLRP3–/– mice) or in mice pretreated with IL-1 inhibitors (anakinra [recombinant IL-1 receptor antagonist] and anti–IL-1β). The production of IL-1α, IL-1β, and myeloid-related protein 8 (MRP-8)–MRP-14 complex was determined by enzyme-linked immunosorbent assay. Peritoneal neutrophil recruitment and cell viability were determined by flow cytometry. Depletion of mast cells or resident macrophages was performed by pretreatment with compound 48/80 or clodronate liposomes, respectively.

Results

OCP crystals induced peritoneal inflammation, as demonstrated by neutrophil recruitment and up-modulation of IL-1α, IL-1β, and MRP-8–MRP-14 complex, to levels comparable with those induced by monosodium urate monohydrate crystals. This OCP crystal–induced inflammation was both IL-1α– and IL-1β–dependent, as shown by the inhibitory effects of anakinra and anti–IL-1β antibody treatment. Accordingly, OCP crystal stimulation resulted in milder inflammation in IL-1α–/– and IL-1β–/– mice. Interestingly, ASC–/– and NLRP3–/– mice did not show any alteration in their inflammation status in response to OCP crystals. Depletion of the resident macrophage population resulted in a significant decrease in crystal-induced neutrophil infiltration and proinflammatory cytokine production in vivo, whereas mast cell depletion had no effect. Finally, OCP crystals induced apoptosis/necrosis of peritoneal cells in vivo.

Conclusion

These data indicate that macrophages, rather than mast cells, are important for initiating and driving OCP crystal–induced inflammation. Additionally, OCP crystals induce IL-1–dependent peritoneal inflammation without requiring the NLRP3 inflammasome.