Octacalcium phosphate crystals induce inflammation in vivo through interleukin‐1 but independent of the NLRP3 inflammasome in mice

Abstract

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