Objectives: To assess whether cytokine response to monosodium urate (MSU) crystals is conditioned by the UA level in an in vitro model, and to replicate this phenomenon in gouty patients.
Methods: A) In vitro: Primary human neutrophils were cultured with increasing concentrations of UA (1, 2, 4, 6, 7, and 8mg/dL). Cells were stimulated with MSU crystals at a constant dose (0.4ug/mL). The production of IL-2, IL-4, IL-6, IL-10, tumour necrosis factor-alpha (TNFa), interferon-gamma (IFNg), and IL-17 was measured by duplicate. B) For the in vivo study, consecutive patients with gout seen in our clinics and under urate-lowering therapy (ULT) were screened. Those whose ULT dose adjustment was considered by the clinician after long-term treatment were selected. Blood tests were taken at two different time-points: time 1, during the standard ULT; and time 2, after ULT dose adjustment (discontinuation or dose reduction). Serum levels of UA, IL-1b, TNFa, and IL-6 were measured at both time-points. Wilcoxon signed-rank test was used to compare matched samples, and Spearman’s rho to assess correlation between serum UA and cytokines levels.
Results: A) In vitro: At increasing UA concentrations, TNFa, IL-6, IFNg, and IL-2 levels showed a progressively rising after MSU crystal stimulation, while IL-4, IL-10, and IL-17 levels did not modify [Figure, left]. Interestingly, highest cytokine levels were achieved with UA at 6mg/dL, and further increases in UA were not followed by further parallel cytokine levels increase. For the B) in vivo study, 14 gouty patients were enrolled, median (p25-75) aged 64.5 (60.0, 70.3) years, 92.9% males. Median disease duration was 10.0 (5.8, 17.8) years, being 21.4% tophaceous. Blood tests showed a significant increase in serum UA after ULT dose modification/discontinuation (p=0.001), TNFa (p=0.004), and IL-6 (p=0.03), and a non-significant trend for IL-1b (p=0.055) [Figure, right]. TNFa levels strongly correlated with SUA levels (r=0.77, p=0.001), while the correlation of the other cytokines with SUA was found to be moderate (r=0.45, p=0.13 for IL-6; r=0.31, p=0.30 for IL-1b).
Conclusions: Both in vitro and in vivo studies confirm that UA is able to enhance MSU-induced cytokine inflammatory response in a concentration-dependent manner.