Background/Purpose:

Rheumatoid arthritis (RA) is an autoimmune disease characterized by synovitis and periarticular bone erosion. Bony erosions and cell necrosis might lead to a local increase in the concentration of extracellular calcium. We have shown in previous studies that increased extracellular calcium activates the Nlrp3 inflammasome of monocytes. Aim of the study was to evaluate the contribution of extracellular calcium to the pathogenesis of rheumatoid arthritis.

Methods:

Monocytes from 42 RA patients fulfilling the EULAR/ACR 2010 diagnostic criteria and from 24 age-matched controls were isolated from the peripheral blood using magnetic separation (Miltenyi). Monocytes were differentiated into macrophages for 7 days using human serum. Synovial fluid mononuclear cells (SFMC) and peripheral blood mononuclear cells (PBMC) were isolated using density gradient centrifugation. Cells were stimulated with lipopolysaccharide (LPS) and increasing calcium concentrations (0.9-1.7 mM), and cytokine concentrations were determined in the supernatant by ELISA. Th17 cells were expanded in vitro using a co-culture of CD4+ T cells and LPS-activated monocytes. Arthritis was induced in 18 DBA/1 mice using collagen (CIA), 5 DBA/1 mice were used as controls.

Results:

The ionized calcium concentration is increased in the synovial fluid of RA patients (n=22) compared to control patients with psoriatic arthritis/ankylosing spondylitis (n=16) (1.1 mM vs. 0.95 mM, p=0.0004) while the ionized calcium concentration in the serum is not different. Blood monocytes from RA patients responded with higher IL-1b, IL-1a, IL-18 and TNF production to an increased calcium concentration (1.7 mM) compared to healthy controls. Importantly, RA monocytes already responded to a calcium concentration of 1.1 mM with a high production of IL-1b, IL-1a and IL-18 whereas control monocytes did not produce cytokines in response to this calcium concentration. SFMCs from RA patients produced even more IL-1b in response to calcium compared to the corresponding blood PBMCs. Monocyte-derived macrophages from RA patients also secreted more IL-1b in response to an increased calcium concentration as well as mouse monocytes/macrophages in the collagen-induced arthritis mouse model. Calcium-induced monocytic IL-1b production correlated with the CIA disease score (r=0.704, p=0.001). The in vitro expansion of Th17 cells, which is in part dependent on monocytic IL-1b production, is also increased when a high calcium concentration is present.

Conclusion:

The ionized calcium concentration is increased in the synovial fluid of RA patients, and monocytes/macrophages from RA patients and CIA mice respond with a higher cytokine production to ionized calcium which in turn leads to an increased Th17 expansion. This might point to a contribution of ionized calcium in the pathogenesis of RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/monocytes-and-macrophages-of-patients-with-rheumatoid-arthritis-respond-to-pathologically-increased-ionized-calcium-with-proinflammatory-cytokine-production/