内毒素降解酶AOAH对过敏性哮喘的保护作用及免疫学机制研究

Allergic asthma, the most common form of asthma, is a major public health problem. The adaptive immune responses to allergens (hypersensitivity), which leads to airway inflammation and airway hyper-reactivity, are believed to be the core of asthma pathogenesis. Gram-negative bacterial endotoxin (LPS) is ubiquitous in our living environment and many epidemiology studies have shown that elevated endotoxin exposure is a major risk factor of allergic asthma. Acyloxyacyl hydrolase (AOAH) is a lipase, produced by host myeloid phagocytes, that degrades and inactivates LPS. AOAH is the only known endogenous enzyme that detoxifies LPS in tissues. Our published studies demonstrate that AOAH limits LPS-induced innate inflammation. However, it is not clear whether AOAH regulates adaptive immunity, such as hypersensitivity in the lung or whether AOAH affects asthma. Our preliminary data suggest that AOAH plays a protective role in allergic asthma; AOAH diminishes dendritic cell (DC) number and T cell activation in the lung. We hypothesize that AOAH degrades inhaled LPS, regulates DC functions, and limits hypersensitivity to ameliorate asthma. We will use two classical allergic asthma mouse models and AOAH-null mice to study AOAH-mediated LPS degradation in the lung; characterize the protective role that AOAH plays in asthma and elucidate the mechanisms; test the therapeutic potential of AOAH and an LPS antagonist in allergic asthma. This study will shed light on AOAH functions in the lung; elucidate a gene-environment interaction that regulates asthma pathogenesis and may open new avenue toward prevention and treatment of allergic asthma.

过敏性哮喘为困扰人类的难治慢性病，其核心为过敏原诱导的适应性免疫反应即超敏反应。流行病学研究表明环境中革兰氏阴性菌内毒素（LPS）暴露的增加是哮喘的重要危险因素，酰基羧酸水解酶(AOAH)为目前发现的唯一能在组织中降解LPS的内源性酶，我们已发表的研究表明AOAH限制LPS诱导的固有免疫反应，但其对肺超敏反应的调节及与哮喘的关系有待阐明。前期数据提示AOAH对过敏性哮喘起保护作用，AOAH减少肺树突状细胞数量及T细胞活化，由此，我们提出假说，AOAH通过降解吸入的LPS，调节树突状细胞功能，限制超敏反应而缓解哮喘。本课题拟利用两种经典的小鼠哮喘模型和AOAH基因缺失小鼠，系统研究肺部LPS降解、AOAH对过敏性哮喘的保护作用、阐明AOAH的调节机制和潜在的防治作用。本研究将解析AOAH在肺部的功能，并从AOAH-LPS这个新视角探索哮喘发病的调节机制，为临床防治开拓新途径。

过敏性哮喘是由Th2型细胞介导的慢性气道炎症性疾病。多项研究表明幼儿时期暴露于生活环境中革兰氏阴性菌脂多糖与过敏性哮喘的发生呈负相关，其机制可能是由于吸入脂多糖诱导了肺上皮细胞的耐受。宿主具有的一种高度保守且独特的酯酶，叫酰基羧酸水解酶（AOAH），AOAH通过去除脂多糖核心部分脂质A的两条脂肪酸链而使LPS失去生物活性。在此项研究中，我们发现AOAH缺失（AOAH KO）小鼠对屋尘螨提取物（HDM extract)诱导的过敏性哮喘有抵抗作用。我们进一步研究了其免疫学机制，发现AOAH KO小鼠肺上皮细胞对HDM刺激的应答减弱，从而造成下游树突状细胞的活化以及Th2细胞的激活受限。喂饮含抗生素的水减少肠道革兰氏阴性菌后能够恢复AOAH KO小鼠对HDM的应答，而经直肠给LPS又使得肺上皮细胞产生耐受，提示肠道菌来源的LPS诱导肺上皮细胞耐受。此外，我们发现AOAH KO小鼠粪便、血浆和肺的LPS活性均高于野生型小鼠。因此，通过降解肠道菌来源的LPS，AOAH防止了过量活性LPS进入血液循环和肺，免除肺上皮细胞耐受，使其能够对HDM产生应答。有趣的是，我们另有研究表明AOAH对肺的免疫防御功能以及LPS诱导的急性肺损伤的恢复起重要作用，因而这些研究体现了感染和过敏的对抗平衡关系。