Background Macrophages initiate the inflammatory response to monosodium urate (MSU) crystals and produce inflammatory cytokines and chemokines that induce migration of blood monocytes to further amplify the inflammatory response. One of the questions that remain unanswered is why some patients with hyperuricemia, and even with MSU deposits, remain clinically asymptomatic. Different subpopulations of blood monocytes have been recognised: classical (CD14++CD16-), intermediate (CD14++CD16+) and non-classical (CD14+CD16++); characteristically intermediate monocytes are expanded in infectious and inflammatory diseases and are associated with atherosclerosis.^[1][2]

Objectives To investigate the possibility that inflammasome reactivity to MSU crystals could influence the clinical expression of gout and to analyse the distribution of monocyte subpopulations in patients with gout.

Methods 17 patients with gout (13 asymptomatic and 4 during an acute flare) and 19 healthy controls were selected. Inflammasome activation after stimulation with MSU (200 μg/ml), ATP (5mM) and nigericin (20 μg/mL) was assessed in peripheral blood mononuclear cells (PBMCs) by flow cytometry using Caspase-1 FLICA™ Detection Kit (Immunochemistry Technologies). Phagocytosis of MSU crystals was quantified by flow cytometry. PBMCs were cultured for 24 hours in the presence of MSU (200 μg/ml), LPS (100 ng/ml) or both and IL-1β was quantified by ELISA. Samples of peripheral blood were stained with CD45, HLA-DR, CD16, and CD14 antibodies. Uric acid, creatinine and C-reactive protein (CRP) were quantified in sera. Flow cytometry and statistics analysis were performed with the FACSDiva and GraphPad Prism 5.

Results No differences were observed in active caspase-1 at baseline. However, when stimulated with MSU, monocytes from gout patients exhibited a higher increase of active caspase-1 (mean ± SEM, gout 2.20±0.15, controls 1.66±0.18, p=0.0419). Differences in phagocytosis of MSU crystals were excluded, as in both groups monocytes exhibited equivalent phagocytic capability. Unexpectedly, PBMCs of patients with gout exhibited diminished IL-1β production when challenged with LPS and MSU (p=0.0473). Regarding monocyte subpopulations, the intermediate phenotype was expanded in patients with gout during an acute flare and a weak correlation between intermediate monocytes and CRP was observed.

Conclusions Monocytes of patients with gout exhibit an enhanced inflammasome activation with MSU crystals, suggesting that a reduced threshold in the inflammatory response could be involved in the development of clinical gout. However, for unknown reasons they produced less IL-1β upon stimulation. The expansion of intermediate monocytes was observed during gout flares and, although it cannot be excluded that intermediate monocytes could be “innocent bystanders” in the context of inflammation, it could suggest that these monocytes participate in the inflammatory response to MSU crystals.

References

1. Heine G. Nat Rev Nephrol 2012;8(6):362-9.

2. Rossol M. Arthritis Rheum 2012;64(3):671-7.

Disclosure of Interest None declared