TNF-α induces the late-phase airway hyperresponsiveness and airway inflammation through cytosolic phospholipase A₂ activation

Background: Late-phase airway hyperresponsiveness (AHR) in asthma is considered the event leading to persistent inflammation in the lungs, but the molecular mechanisms involved in this process are poorly understood. Objective: To examine the role of TNF-α in the development of a late AHR and airway inflammation in asthma. Methods: We established a murine model of asthma with not only biphasic AHR to methacholine but also airway eosinophilia. The effect of TNF-α blockade was determined by using anti-TNF-α antibody and TNF-α knockout mice. Cytosolic phospholipase A₂ (cPLA ₂) mRNA expression and activity were assessed by using RT-PCR and 1-stearoyl-2-[1-¹⁴C] arachidonyl-sn-glycero-3-phosphocholine as the substrate, respectively. Results: TNF-α blockade resulted in significant inhibition of the late AHR without affecting the early AHR, and reduction in airway eosinophilia and inflammation. cPLA₂ activity was increased in asthmatic lungs in a TNF-α-dependent way, and cPLA₂ inhibitor blocked late AHR and airway eosinophilia. TNF-α also stimulated the synthesis of cPLA₂ metabolites such as leukotriene B₄ and platelet-activating factor in the airway. Specific inhibitors of cPLA ₂ metabolites inhibited the late AHR and airway eosinophilia. Conclusions: TNF-α is the proximal key cytokine capable of developing late-phase AHR and subsequent airway inflammation through expression/activation of cPLA₂.