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

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-1 beta (IL-1 beta), tumor necrosis factor alpha, keratinocyte-derived cytokine/growth-related oncogene alpha, and transforming growth factor beta 1 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-1 beta 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.