Role of TLR2, TLR4, and MyD88 in murine ozone-induced airway hyperresponsiveness and neutrophilia

Exposure to air pollutants such as ozone (O3) induces airway hyperresponsiveness (AHR) and airway inflammation. Toll-like receptors (TLR) are first-line effector molecules in innate immunity to infections and signal via adapter proteins, including myeloid differentiation factor-88 (MyD88). We investigated the sensing of ozone by TLR2, TLR4, and MyD88. Ozone induced AHR in wild-type (WT) C57BL/6 mice, but AHR was absent in TLR2−/−, TLR4−/−, and MyD88−/− mice. Bronchoalveolar lavage neutrophilia induced by ozone was inhibited at 3 h but not at 24 h in TLR2−/− and TLR4−/− mice, while in MyD88−/− mice, this was inhibited at 24 h. We investigated the expression of inflammatory cytokines and TLR2, TLR4, and MyD88 in these mice. Ozone induced time-dependent increases in inflammatory gene expression of keratinocyte chemoattractant (KC) and IL-6 and of TLR2, TLR4, and MyD88 in WT mice. IL-6 and KC expression induced by ozone was inhibited in TLR2−/−, TLR4−/−, and MyD88−/− mice. Expression of MyD88 was increased in TLR2−/− and TLR4−/− mice, while induction of TLR2 or TLR4 was reduced in TLR2−/− and TLR4−/− mice, respectively. TLR2 and TLR4 mediate AHR induced by oxidative stress such as ozone, while the adapter protein MyD88, but not TLR2 or TLR4, is important in mediating ozone-induced neutrophilia. TLR2 and TLR4 may also be important in regulating the speed of neutrophilic response. Therefore, ozone may induce murine AHR and neutrophilic inflammation through the activation of the Toll-like receptor pathway that may sense noninfectious stimuli such as oxidative stress.