The following is a summary of “Discovery of calcite as a new pro-inflammatory calcium-containing crystal in human osteoarthritic synovial fluid,” published in the May 2024 issue of Rheumatology by Niessink et al.

The researchers conducted a study to comprehensively analyze calcium-containing crystals present in synovial fluid from patients with knee osteoarthritis (OA) using Raman spectroscopy, particularly investigating calcite crystals’ biological implications.

Synovial fluid samples were collected from 32 knee patients with OA before total joint arthroplasty. Crystals in synovial fluid were identified using an integrated Raman polarized light microscope. Subsequently, human peripheral blood mononuclear cells (PBMCs), human OA articular chondrocytes (HACs), and fibroblast-like synoviocytes (FLSs) were exposed to calcite crystals. The expression of relevant cytokines and inflammatory genes was assessed using enzyme-linked immunosorbent assay (ELISA) and real-time polymerase chain reaction (PCR).

The study group identified calcium-containing crystals, including calcium pyrophosphate (37.5%) and basic calcium phosphate (21.8%), although they were not co-present in the same OA synovial fluid sample. Notably, calcite crystals were detected in 93.8% of the samples, with dolomite present in 25% of cases. Examination of the cellular responses to calcite crystal exposure revealed heightened production of innate immune-derived cytokines by PBMCs when co-stimulated with lipopolysaccharide (LPS). Furthermore, stimulation of HACs and FLSs with calcite crystals led to increased secretion of pro-inflammatory molecules and altered expression of enzymes involved in extracellular matrix remodeling.

This research underscores the pivotal role of Raman spectroscopy in elucidating OA crystal compositions and, importantly, identifies calcite as a previously unrecognized pro-inflammatory crystal type in OA synovial fluid. Understanding the specific roles of different crystal species within the OA joint may pave the way for novel pharmacological strategies and personalized treatment approaches for patients with OA.

Source: sciencedirect.com/science/article/pii/S1063458424012032