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Obesity-associated adipose tissue inflammation contributes to the development of type 2 diabetes. Chronic activation of IL-1β system in adipose tissue on metabolic disorders is well demonstrated. However, a mechanism for its expression and activation in the tissue has remained unexplored. Here we demonstrate that IL-1β transcript was enriched in neutrophils of white adipose tissue (WAT), and the expression of IL-1β was up-regulated by a high-fat diet in the WAT but not in other tissues, including bone marrow, peripheral blood, and spleen. The co-culture of neutrophils with 3T3-L1 adipocytes indicate that contact interaction with adipocytes is critical for the expression of IL-1β via NF-κB activation in adipose tissue neutrophils. Lipolysis of adipocytes accumulated neutrophils prior to macrophages in the WAT, and produced high levels of IL-1β via an ASC-caspase-1-mediated inflammasome pathway. Leukotriene B4 production in the WAT also contributed to neutrophil accumulation. Furthermore, neutrophils and IL-1β contributed to the expression of chemotactic molecules involved in high-fat diet-induced macrophage infiltration into the WAT. We have identified previously unappreciated roles for neutrophils in the development of adipose tissue inflammation: robust IL-1β production and infiltration of macrophages to initiate chronic inflammation.

MD-1 is a secreted protein that forms a complex with radioprotective 105 (RP105) on B cells, macrophages and dendritic cells. This complex plays a crucial role in lipopolysaccharide (LPS) recognition by B cells. Disease progression is known to improve in RP105-deficient lupus-prone MRLlpr/lpr mice. Furthermore, a soluble form of the homologous MD-2 protein is present in the plasma of septic patients and can opsonize gram-negative bacteria in cooperation with Toll-like receptor 4. We have now established a flow cytometry-based assay to detect endogenous soluble form of murine MD-1 (sMD-1) and explored potential roles in autoimmunity. The assay was quantitative and validated with sera from MD-1-deficient mice. Interestingly, heat-inactivated murine serum diminished the ability of sMD-1 to bind RP105. The sMD-1 was secreted by bone marrow-derived macrophages from C57BL/6 mice. Autoimmune prone MRLlpr/lpr mice had higher levels of serum sMD-1 than control MRL+/+ mice, and levels markedly increased with disease progression. Expression of MD-1 but not MD-2 mRNA increased with age in the liver and kidney of MRLlpr/lpr mice. Finally, immunohistochemical analyses revealed that MD-1 was present in infiltrated macrophages within perivascular lesions of the MRLlpr/lpr kidney. This correlation suggests that sMD-1 may contribute to pathogenesis in this autoimmune disease model mouse. Also, sMD-1 may be feasible to monitor autoimmune disease severity.

Accumulating evidence suggest that innate immunity is associated with obesity-induced chronic inflammation and metabolic disorders. For example, the TLR4/MD-2 complex recognizes free fatty acids (FAAs) derived from adipose tissue and promotes adipose tissue inflammation and insulin resistance. The RP105/MD-1 complex is a homolog of the TLR4/MD-2 complex. However, the roles of RP105/MD-1 complex in adipose tissue inflammation have remained elusive. We now report that the RP105/MD-1 complex contributes to high-fat diet (HFD)-induced obesity, adipose tissue inflammation, and insulin resistance independent of TLR4 signaling. RP105 mRNA expression is dramatically increased by HFD in stromal vascular fraction of epididymal white adipose tissue (eWAT) in wild-type (WT) mice. In human, RP105 mRNA expression is also increased in the visceral adipose tissue of obese subjects. The RP105/MD-1 complex is expressed by most adipose tissue macrophages (ATMs). HFD increases RP105/MD-1 expression on the M1 ATMs in eWAT. Increased RP105 and MD-1 mRNA expression in macrophages is induced by co-culture with 3T3L-1 adipocytes. RP105 KO and MD-1 KO mice have less HFD-induced adipose tissue inflammation, hepatic steatosis and insulin resistance compared to these in WT and TLR4 KO mice. Finally, the saturated FAAs palmitic and stearic acids activate TLR4/MD-2 but not RP105/MD-1. Thus, the RP105/MD-1 complex is a major mediator of adipose tissue inflammation independent of TLR4 signaling.