Interleukin-33 from Monocytes Recruited to the Lung Contributes to House Dust Mite-Induced Airway Inflammation in a Mouse Model

Background
Interleukin-33 (IL-33) activates group 2 innate lymphoid cells (ILC2), resulting in T-helper-2 inflammation in bronchial asthma. Airway epithelial cells were reported as sources of IL-33 during apoptosis and necrosis. However, IL-33 is known to be from sources other than airway epithelial cells such as leukocytes, and the mechanisms of IL-33 production and release are not fully understood. The aim of this study was to clarify the role of IL-33 production by monocytes in airway inflammation.
Methods
BALB/c mice were sensitized and challenged with a house dust mite (HDM) preparation. Airway inflammation was assessed by quantifying inflammatory cells in bronchoalveolar lavage (BAL) fluid, and IL-25, IL-33, and thymic stromal lymphopoietin (TSLP) levels in lung. Immunohistochemistry for IL-33 in lung sections was also performed. Ly6c, CD11b, and CD11c expression was examined by flow cytometry. Clodronate liposomes were used in the HDM-airway inflammation model to deplete circulating monocytes.
Results
The IL-33, but not IL-25 or TSLP, level in lung homogenates was markedly increased in HDM mice compared to control mice. IL-33-positive cells in the lungs were identified using immunohistochemistry and were increased in areas surrounding bronchi and vasculature. Furthermore, IL-33 levels were increased in mononuclear cells derived from lungs of HDM mice compared to controls. The expression of Ly6c in mononuclear cells was significantly higher in HDM mice than in controls. Treatment with clodronate liposomes led to inhibition of not only inflammatory cells in BAL fluid, airway hyper reactivity and Th2 cytokines in lung, but also IL-33 in lung.
Conclusion
IL-33 from monocytes recruited to the lung may contribute to the pathogenesis of HDM-induced airway inflammation.

背景
白细胞介素-33（Interleukin-33, IL-33）可激活2型先天淋巴细胞（group 2 innate lymphoid cells, ILC2），进而引发支气管哮喘中的Th2型炎症。既往研究表明，气道上皮细胞在凋亡及坏死过程中可作为IL-33的来源。然而，IL-33还可来源于气道上皮细胞以外的其他细胞类型（如白细胞），目前其产生与释放的具体机制尚未完全阐明。本研究旨在明确单核细胞产生的IL-33在气道炎症中的作用。

方法
本研究采用BALB/c小鼠，以屋尘螨（house dust mite, HDM）提取物进行致敏与攻击造模。通过计数支气管肺泡灌洗液（bronchoalveolar lavage, BAL）中的炎症细胞、检测肺组织中IL-25、IL-33及胸腺基质淋巴细胞生成素（thymic stromal lymphopoietin, TSLP）的水平，评估气道炎症程度；同时对肺组织切片进行IL-33免疫组化染色。采用流式细胞术检测单核细胞表面Ly6c、CD11b及CD11c的表达水平。在HDM诱导的气道炎症模型中，使用氯膦酸盐脂质体清除循环单核细胞。

结果
与对照组小鼠相比，HDM造模小鼠的肺组织匀浆中IL-33水平显著升高，而IL-25与TSLP水平无明显变化。免疫组化结果显示，肺组织中IL-33阳性细胞数量增加，且主要分布于支气管与血管周围区域。进一步检测发现，HDM造模小鼠肺部来源的单核细胞中IL-33水平较对照组显著升高，且其表面Ly6c的表达水平同样显著高于对照组。经氯膦酸盐脂质体处理后，不仅支气管肺泡灌洗液中的炎症细胞数量、气道高反应性及肺组织中的Th2型细胞因子水平受到抑制，肺组织中的IL-33水平也显著降低。

结论
募集至肺部的单核细胞所产生的IL-33，可能参与了HDM诱导的气道炎症的发病过程。