Discovery of calcite as a new pro-inflammatory calcium-containing crystal in human osteoarthritic synovial fluid

Abstract

ObjectiveWe aimed to characterize calcium-containing crystals present in synovial fluid from patients with knee osteoarthritis (OA) using Raman spectroscopy, and specifically investigate the biological effects of calcite crystals. DesignThirty-two synovial fluid samples were collected pre-operatively from knee OA patients undergoing total joint arthroplasty. An integrated Raman polarized light microscope was used for identification of crystals in synovial fluid. Human peripheral-blood mononuclear cells (PBMC’s), human OA articular chondrocytes (HACs) and fibroblast-like synoviocytes (FLSs) were exposed to calcite crystals. Expression of relevant cytokines and inflammatory genes were measured using ELISA and real-time PCR. ResultsVarious calcium-containing crystals were identified, including calcium pyrophosphate (37.5%) and basic calcium phosphate (21.8%), but they were never found simultaneously in the same OA synovial fluid sample. For the first time, we discovered the presence of calcite crystals in 93.8% of the samples, while dolomite was detected in 25% of the cases. Characterization of the cellular response to calcite crystal exposure revealed increased production of innate immune-derived cytokines by peripheral blood mononuclear cells (PBMC’s), when co-stimulated with lipopolysaccharide (LPS). Additionally, calcite crystal stimulation of HACS and FLSs resulted in enhanced secretion of pro-inflammatory molecules and alterations in the expression of extracellular matrix remodeling enzymes. ConclusionsThis study highlights the unique role of Raman spectroscopy in OA crystal research and identified calcite as a novel pro-inflammatory crystal type in OA synovial fluid. Understanding the role of specific crystal species in the OA joint may open new avenues for pharmacological interventions and personalized approaches to treating OA.