Engagement of CD14 mediates the inflammatory potential of monosodium urate crystals

Phagocyte ingestion of monosodium urate (NISU) crystals can induce proinflammatory responses and trigger acute gouty inflammation. Alternatively, the uptake of MSU crystals by mature macrophages can be noninflammatory and promote resolution of gouty inflammation. Macrophage activation by extracellular MSU crystals involves apparent recognition and ingestion mediated by TLR2 and TLR4, with subsequent intracellular recognition linked to caspase-1 activation and IL-1 beta processing driven by the NACHT-LRR-PYD-containing protein-3 inflammasome. In this study, we examined the potential role in gouty inflammation of CD14, a phagocyte-expressed pattern recognition receptor that functionally interacts with both TLR2 and TLR4. NISU crystals, but not latex beads, directly bound recombinant soluble (s) CD14 in vitro. CD14(-/-) bone marrow-derived macrophages (BMDMs) demonstrated unimpaired phagocytosis of MSU crystals but reduced p38 phosphorylation and similar to 90% less IL-1 beta and CXCL1 release. Attenuated NISU crystal-induced IL-1 beta release in CD14(-/-) BMDMs was mediated by decreased pro-IL-1 beta protein expression and additionally by decreased caspase-1 activation and IL-1 beta processing consistent with diminished NACHT-LRR-PYD-containing protein-3 inflammasome activation. Coating of MSU crystals with sCD14, but not sTLR2 or sTLR4, restored IL-1 beta and CXCL1 production in CD14(-/-) BMDMs in vitro. Gain of function of CD14 directly enhanced TLR4-mediated signaling in response to MSU crystals in transfected Chinese hamster ovary cells in vitro. Last, MSU crystal-induced leukocyte influx at 6 h was reduced by similar to 75%, and local induction of IL-1 beta decreased by > 80% in CD14(-/-) mouse s.c. air pouches in vivo. We conclude that engagement of CD14 is a central determinant of the inflammatory potential of MSU crystals.