Monoclonal antibody against Toll-like receptor 4 attenuates ventilator-induced lung injury in rats by inhibiting MyD88-and NF-κB-dependent signaling

The mechanisms through which mechanical ventilation causes non-infectious inflammatory diseases and lung injury are poorly understood. Animals models of this type of injury suggest that it involves signaling mediated by Toll-like receptor (TLR)4 and 9. In this study, in order to gain further insight into the involvement of TLR4 in this type of injury, we performed in vivo and in vitro experiments to determine the mechanisms through which TLR4 triggers inflammation. We also examined whether the use of TLR4 monoclonal antibody (mAb) can alleviate this type of injury. For this purpose, rats were tracheotomized and administered intratracheal injections of anti TLR4 mAb or saline, and then ventilated for 4 h at a high tidal volume (HTV) of 40 ml/kg or allowed to breathe spontaneously for the same period of time (controls). Alveolar macrophages (AMs) were isolated from the bronchoalveolar lavage fluid (BALF) of the rats and stimulated for 16 h with tumor necrosis factor (TNF)-alpha in the presence or absence of anti-TLR4 mAb. Lung injury was assessed by examining lung histopathology, lung wet/dry weight ratio, BALF total protein and cytokine levels in BALF and plasma. The mRNA and protein expression levels of TLR4, TLR9, myeloid differentiation factor 88 (Myd88) and nuclear factor (NF)-kappa B were measured in cultured macrophages. Compared to the controls (spontaneous breathing), the ventilated rats exhibited greater pulmonary permeability, more severe inflammatory cell infiltration/lung edema, and higher levels of interleukin (IL)-1 beta, IL-6 and TNF-alpha in BALF and plasma. The AMs from the ventilated rats expressed higher mRNA and protein levels of TLR4, TLR9, Myd88 and NF-kappa B compared with the macrophages from the spontaneously breathing rats. The ventilated rats pre-treated with anti TLR4 mAb exhibited markedly attenuated signs of ventilation-induced injury, such as less lung inflammation and pulmonary edema, fewer cells in BALF, and lower levels of ILs and TNF-alpha in BALF and plasma. Similarly, the TNF-alpha-dependent increases in the mRNA and protein expression of TLR4, Myd88 and NF-kappa B in AMs were attenuated when TNF-alpha was co-administered with anti TLR4 mAb than when TNF-alpha was administered alone. Co-administering anti-TLR4 mAb also reduced the TNF-alpha-dependent secretion of ILs. On the whole, our data demonstrate that TLR4 contributes significantly to ventilation-induced lung injury by activating the Myd88/NF-kappa B pathway, and pre-treating rats with anti-TLR4 mAb partially protects them against this type of injury by inhibiting Myd88/NF-kappa B signaling.