Innate immunity conferred by toll‐like receptors 2 and 4 and myeloid differentiation factor 88 expression is pivotal to monosodium urate monohydrate crystal–induced inflammation

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Abstract
Objective: In gout, incompletely defined molecular factors alter recognition of dormant articular and bursal monosodium urate monohydrate (MSU) crystal deposits, thereby inducing self‐limiting bouts of characteristically severe neutrophilic inflammation. To define primary determinants of cellular recognition, uptake, and inflammatory responses to MSU crystals, we conducted a study to test the role of Toll‐like receptor 2 (TLR‐2), TLR‐4, and the cytosolic TLR adapter protein myeloid differentiation factor 88 (MyD88), which are centrally involved in innate immune recognition of microbial pathogens.Methods: We isolated bone marrow–derived macrophages (BMDMs) in TLR‐2−/−, TLR‐4−/−, MyD88−/−, and congenic wild‐type mice, and assessed phagocytosis and cytokine expression in response to endotoxin‐free MSU crystals under serum‐free conditions. MSU crystals also were injected into mouse synovium‐like subcutaneous air pouches.Results: TLR‐2−/−, TLR‐4−/−, and MyD88−/− BMDMs demonstrated impaired uptake of MSU crystals in vitro. MSU crystal–induced production of interleukin‐1β (IL‐1β), tumor necrosis factor α, keratinocyte‐derived cytokine/growth‐related oncogene α, and transforming growth factor β1 also were significantly suppressed in TLR‐2−/− and TLR‐4−/− BMDMs and were blunted in MyD88−/− BMDMs in vitro. Neutrophil influx and local induction of IL‐1β in subcutaneous air pouches were suppressed 6 hours after injection of MSU crystals in TLR‐2−/− and TLR‐4−/− mice and were attenuated in MyD88−/− mice.Conclusion: The murine host requires TLR‐2, TLR‐4, and MyD88 for macrophage activation and development of full‐blown neutrophilic, air pouch inflammation in response to MSU crystals. Our findings implicate innate immune cellular recognition of naked MSU crystals by specific TLRs as a major factor in determining the inflammatory potential of MSU crystal deposits and the course of gouty arthritis.