Toll-Like Receptor Ligands LPS and Poly (I:C) Exacerbate Airway Hyperresponsiveness in a Model of Airway Allergy in Mice, Independently of Inflammation

It is well-established that bacterial and viral infections have an exacerbating effect on allergic asthma, particularly aggravating respiratory symptoms, such as airway hyperresponsiveness (AHR). The mechanism by which these infections alter AHR is unclear, but some studies suggest that Toll-like receptors (TLRs) play a role. In this study, we investigated the impact of TLR3 and TLR4 ligands on AHR and airway inflammation in a model of pre-established allergic inflammation. Female BALB/c mice were sensitised and challenged intranasally (i.n.) with either PBS or ovalbumin (OVA) and subsequently i.n. challenged with poly (I:C) (TLR3) or LPS (TLR4) for four consecutive days. The response to methacholine was measured in vivo; cellular and inflammatory mediators were measured in blood, lung tissue and broncheoalveolar lavage fluid (BALF). OVA challenge resulted in an increase in AHR to methacholine, as well as increased airway eosinophilia and TH2 cytokine production. Subsequent challenge with TLR agonists resulted in a significant increase in AHR, but decreased TLR-specific cellular inflammation and production of immune mediators. Particularly evident was a decline in LPS-induced neutrophilia and neutrophil-associated cytokines following LPS and poly (I:C) treatment. The present data indicates that TLRs may play a pivotal role in AHR in response to microbial infection in allergic lung inflammation. These data also demonstrate that aggravated AHR occurs in the absence of an exacerbation in airway inflammation and that allergic inflammation impedes a subsequent inflammatory response to TLRs. These results may parallel clinical signs of microbial asthma exacerbation, including an extended duration of illness and increased respiratory symptoms.已有研究证实，细菌与病毒感染可加重过敏性哮喘，尤其会加剧呼吸道症状，例如气道高反应性（airway hyperresponsiveness, AHR）。目前此类感染改变AHR的具体机制尚不明确，但部分研究提示Toll样受体（Toll-like receptors, TLRs）参与其中。本研究基于预先构建的过敏性炎症模型，探究了TLR3与TLR4配体对AHR及气道炎症的影响。实验选用雌性BALB/c小鼠，经鼻（i.n.）分别以磷酸盐缓冲液（phosphate-buffered saline, PBS）或卵清蛋白（ovalbumin, OVA）进行致敏与激发，随后连续4天经鼻给予聚肌胞苷酸（poly(I:C)，TLR3配体）或脂多糖（lipopolysaccharide, LPS，TLR4配体）激发。采用体内法检测小鼠对乙酰甲胆碱的反应；同时采集血液、肺组织及支气管肺泡灌洗液（bronchoalveolar lavage fluid, BALF），检测其中的细胞组分与炎症介质水平。OVA激发可使小鼠对乙酰甲胆碱的AHR升高，同时伴随气道嗜酸性粒细胞增多及Th2型细胞因子生成增加。后续给予TLR激动剂激发后，AHR显著升高，但TLR特异性细胞炎症及免疫介质的生成却出现下调。尤为显著的是，经LPS与poly(I:C)处理后，LPS诱导的中性粒细胞增多及中性粒细胞相关细胞因子水平均显著降低。本研究数据表明，TLRs在过敏性肺部炎症中针对微生物感染的AHR应答过程中可能发挥关键作用。此外，本研究数据还显示，尽管气道炎症未进一步加重，但AHR却出现了加剧；同时过敏性炎症会抑制机体对TLRs的后续炎症应答。上述研究结果与微生物诱导的哮喘加重临床特征相符，包括病程延长及呼吸道症状加重。