Allergy Asthma Immunol Res.  2020 Jul;12(4):608-625. 10.4168/aair.2020.12.4.608.

Toll-like Receptor 4 Deficiency Aggravates Airway Hyperresponsiveness and Inflammation by Impairing Neutrophil Apoptosis in a Toluene Diisocyanate-Induced Murine Asthma Model

Affiliations
  • 1The Second Affiliated Hospital, Guangdong Provincial Key Laboratory of Allergy & Clinical Immunology, State Key Laboratory of Respiratory Disease, Guangzhou Medical University, Guangzhou, China
  • 2Center for Immunology, Inflammation & Immune-Mediated Disease, Guangzhou Medical University, Guangzhou, China
  • 3Guangzhou First People's Hospital, School of Medicine, South China University of Technology, Guangzhou, China
  • 4State Key Laboratory of Respiratory Diseases, Guangzhou Institute of Respiratory Health, The First Affiliated Hospital, Guangzhou Medical University, Guangzhou, China

Abstract

Purpose
Accumulating evidence has suggested that toll-like receptor 4 (TLR4) is critically involved in the pathogenesis of asthma. The aim of this study was to investigate the role of TLR4 in toluene diisocyanate (TDI)-induced allergic airway inflammation.
Methods
TLR4−/− and wild-type (WT) C57BL/10J mice were sensitized and challenged with TDI to generate a TDI-induced asthma model. B-cell lymphoma 2 (Bcl-2) inhibitors, ABT-199 (4 mg/kg) and ABT-737 (4 mg/kg), were intranasally given to TDI-exposed TLR4−/− mice after each challenge.
Results
TDI exposure led to increased airway hyperresponsiveness (AHR), granulocyte flux, bronchial epithelial shedding and extensive submucosal collagen deposition, which were unexpectedly aggravated by TLR4 deficiency. Following TDI challenge, TLR4−/− mice exhibited down-regulated interleukin-17A and increased colony-stimulating factor 3 in bronchoalveolar lavage fluid (BALF), while WT mice did not. In addition, TLR4 deficiency robustly suppressed the expression of NOD-like receptor family pyrin domain containing 3 and NLR family CARD domain containing 4, decreased caspase-1 activity in TDI-exposed mice, but had no effect on the level of high mobility group box 1 in BALF. Flow cytometry revealed that TDI hampered both neutrophil and eosinophil apoptosis, of which neutrophil apoptosis was further inhibited in TDI-exposed TLR4−/− mice, with marked up-regulation of Bcl-2. Moreover, inhibition of Bcl-2 with either ABT-199 or ABT-737 significantly alleviated neutrophil recruitment by promoting apoptosis.
Conclusions
These data indicated that TLR4 deficiency promoted neutrophil infiltration by impairing its apoptosis via up-regulation of Bcl-2, thereby resulting in deteriorated AHR and airway inflammation, which suggests that TLR4 could be a negative regulator of TDI-induced neutrophilic inflammation.

Keyword

Toll-like receptor 4; asthma; toluene diisocyanate; neutrophlic inflammation; apoptosis
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