Basic Calcium Phosphate Crystals Induce Monocyte/Macrophage IL-1β Secretion through the NLRP3 Inflammasome In Vitro

Abstract

Basic calcium phosphate (BCP) crystals are associated with severe osteoarthritis and acute periarticular inflammation. Three main forms of BCP crystals have been identified from pathological tissues: octacalcium phosphate, carbonate-substituted apatite, and hydroxyapatite. We investigated the proinflammatory effects of these BCP crystals in vitro with special regard to the involvement of the NLRP3–inflammasome in THP-1 cells, primary human monocytes and macrophages, and mouse bone marrow-derived macrophages (BMDM). THP-1 cells stimulated with BCP crystals produced IL-1β in a dose-dependent manner. Similarly, primary human cells and BMDM from wild-type mice also produced high concentrations of IL-1β after crystal stimulation. THP-1 cells transfected with short hairpin RNA against the components of the NLRP3 inflammasome and mouse BMDM from mice deficient for NLRP3, apoptosis-associated speck-like protein, or caspase-1 did not produce IL-1β after BCP crystal stimulation. BCP crystals induced macrophage apoptosis/necrosis as demonstrated by MTT and flow cytometric analysis. Collectively, these results demonstrate that BCP crystals induce IL-1β secretion through activating the NLRP3 inflammasome. Furthermore, we speculate that IL-1 blockade could be a novel strategy to inhibit BCP-induced inflammation in human disease.

Inflammasome Priming in Sterile Inflammatory Disease

“…Cholesterol crystals (CC) [52] , MSU, and calcium phosphate crystals have all been found to trigger NLRP3 activation through lysosomal damage and ROS production [53] . The complement system has also been shown to recognize crystalline material, resulting in the production of C5a [54] .…”

Calcifying nanoparticles promote mineralization in vascular smooth muscle cells: implications for atherosclerosis

“…Notably, the initiating factor for this activation appears to be uptake of calcium phosphate nanocrystals that precipitate into the medium independently of cell activitynot soluble phosphate. 14,42 Within the context of the present study, BMP-2 is particularly significant since it initiates a program that enhances the expression of multiple osteogenic transcription factors including Msx-2, Osterix, and runtrelated transcription factor 2. 43 In healthy smooth muscle cells, the osteoinductive properties of BMP-2 are neutralized by its formation of a complex with γ-carboxylated MGP, a potent mineralization inhibitor produced by smooth muscle cells, which itself is downregulated in vascular calcification.…”

Inflammasome Activation by Cystine Crystals

“…6 NLRP3 is activated by endogenous crystals or particle formats, including monosodium urate, calcium phosphate, hydroxyapatite, silica, asbestos, and cholesterol crystals. [7][8][9][10][11][12] Although the exact mechanism by which NLRP3 is activated is still unknown, NLRP3 activation by crystalline structures has been demonstrated to require phagolysosomal destabilization, generation of reactive oxygen species (ROS), and potassium efflux. 13 To understand whether the inflammasome is activated by cystine crystals, we investigated whether cystine crystals induce IL-1b secretion from cultured PBMCs.…”