Activation of Porcine Alveolar Macrophages by Actinobacillus pleuropneumoniae Lipopolysaccharide via the Toll-Like Receptor 4/NF-κB-Mediated Pathway

Actinobacillus pleuropneumoniae is the causative agent of porcine contagious pleuropneumonia. Overproduction of proinflammatory cytokines, like interleukin-1 beta (IL-1 beta), IL-6, tumor necrosis factor alpha, and resistin, in the lung is an important feature of A. pleuropneumoniae infection. These proinflammatory cytokines enhance inflammatory and immunological responses. However, the mechanism that leads to cytokine production remains unclear. As a major virulence factor of A. pleuropneumoniae, lipopolysaccharide (LPS) may act as a potent stimulator of Toll-like receptor 4 (TLR4), triggering a number of intracellular signaling pathways that lead to the synthesis of proinflammatory cytokines. Porcine alveolar macrophages (PAMs) are the first line of defense against pathogenic microbes during pathogen invasion. The results of the present study demonstrate that A. pleuropneumoniae LPS induces PAMs to produce inflammatory cytokines in time-and dose-dependent manners. Moreover, PAMs were activated by A. pleuropneumoniae LPS, resulting in upregulation of signaling molecules, including TLR4, MyD88, TRIF-related adaptor molecule, and NF-kappa B. In contrast, the activation effects of A. pleuropneumoniae LPS on PAMs could be suppressed by specific inhibitors, like small interfering RNA and Bay11-7082. Taken together, our data indicate that A. pleuropneumoniae LPS can induce PAMs to produce proinflammatory cytokines via the TLR4/NF-kappa B-mediated pathway. These findings partially reveal the mechanism of the overproduction of proinflammatory cytokines in the lungs of swine with A. pleuropneumoniae infection and may provide targets for the prevention of A. pleuropneumoniae-induced pneumonia. All the data could be used as a reference for the pathogenesis of respiratory infection.