Pathogenic role of basic calcium phosphate crystals in destructive arthropathies

Area of Science:

• Biochemistry
• Immunology
• Rheumatology

Background:

• Basic calcium phosphate (BCP) crystals are prevalent in osteoarthritis (OA) joints and linked to cartilage damage.
• BCP crystals stimulate in vitro catabolic responses, including metalloprotease and interleukin-1 (IL-1) production.
• In vivo, BCP crystal-induced IL-1 production involves both NLRP3 inflammasome-dependent and -independent mechanisms.

Purpose of the Study:

• To investigate the role of BCP crystals in cartilage destruction.
• To elucidate the involvement of IL-1 and the NLRP3 inflammasome in BCP crystal-related cartilage degradation.

Main Methods:

• Analysis of synovial membranes from OA knees using alizarin Red and FTIR.
• Intra-articular injection of BCP crystals into wild-type (WT), NLRP3-/-, ASC-/-, IL-1α-/-, and IL-1β-/- mice.
• Assessment of articular destruction, synovial inflammation, proteoglycan loss, and chondrocyte apoptosis at various time points.
• Treatment of WT mice with anakinra (IL-1 receptor antagonist) to evaluate IL-1's role.

Main Results:

• BCP crystals were frequently detected in OA synovial membranes, even in low-grade cases.
• BCP crystal injection induced synovial inflammation, cartilage degradation (proteoglycan loss, chondrocyte apoptosis), and persistent macrophage infiltration.
• BCP crystal-induced synovitis and cartilage degradation were independent of IL-1α, IL-1β, and NLRP3 inflammasome components.
• Anakinra treatment did not prevent the effects of BCP crystals.

Conclusions:

• Intra-articular BCP crystals directly contribute to synovial inflammation and cartilage degradation in OA.
• The pathogenic effects of BCP crystals in OA are not mediated by IL-1 or the NLRP3 inflammasome.