Monosodium urate crystals (MSU), the etiological agent of gout, are one of the most potent proinflammatory stimuli for neutrophils. The modulation of MSU-induced neutrophil activation by inhibitory receptors remains poorly characterized. The expression of the myeloid inhibitory C-type lectin-like receptor (MICL) in neutrophils is downregulated by several proinflammatory stimuli, suggestive of a role for this receptor in neutrophil function. We thus investigated the potential role of MICL in MSU-induced neutrophil activation.
The expression of MICL was monitored in human neutrophils by flow cytometry and Western blot analysis after stimulation with MSU. Protein tyrosine phosphorylation was also assessed by Western blot analysis and the production of IL-1 and IL-8 by enzyme-linked immunosorbent assay. Changes in the concentration of cytoplasmic free calcium were monitored with the Fura-2-acetoxymethyl ester calcium indicator. MICL expression was modulated with an anti-MICL antibody in neutrophils and siRNA in the PLB-985 neutrophil-like cell line.
MSU induced the downregulation of MICL expression in neutrophils. A diminution in the expression of MICL induced by antibody cross-linking or siRNA enhanced the MSU-dependent increase in cytoplasmic calcium levels, protein tyrosine phosphorylation and IL-8 but not IL-1 production. Pretreatment of neutrophils with colchicine inhibited the MSU-induced downregulation of MICL expression.
Our findings strongly suggest that MICL acts as an inhibitory receptor in human neutrophils since the downregulation of MICL expression enhances MSU-induced neutrophil activation. Since MSU downregulates the expression of MICL, MICL may play a pathogenic role in gout by enhancing neutrophil effector functions. In support of this notion, colchicine counteracts the MSU-induced loss of MICL expression. Our findings thus also provide further insight into the potential molecular mechanisms behind the anti-inflammatory properties of this drug.