Diesel exhaust particles suppress macrophage function and slow the pulmonary clearance of Listeria monocytogenes in rats

In this study, we tested the hypothesis that exposure to diesel exhaust particles (DEP) may increase susceptibility of the host to pulmonary infection. Male Sprague-Dawley rats received a single dose of DEP (5 mg/kg), carbon black (CB, 5 mg/kg), or saline intratracheally. Three days later, the rats were inoculated intratracheally with similar to5,000 Listeria monocytogenes and sacrificed at 3, 5, and 7 days postinfection, and we determined the number of viable Listeria in the left lobe of lungs. The remaining lungs underwent bronchoalveolar lavage (BAL) and the retrieved BAL cells were identified and counted. Luminol-dependent chemiluminescence, a measure of reactive oxygen species (ROS) formation, generated by BAL cells was monitored and the levels of nitric oxide and tumor necrosis factor (TNF)-alpha produced by macrophages in culture were determined. At 7 days postinfection, we excised the lung-draining lymph nodes and phenotyped the lymphocyte subpopulations. Exposure of rats to DEP, but not to CB, decreased the clearance of Listeria from the lungs. Listeria-induced generation of luminol-dependent chemiluminescence by pulmonary phagocytes decreased by exposure to DEP but not CB. Similarly, Listeria-induced production of NO by alveolar macrophages was negated at 3, 5, and 7 days after inoculation in DEP-exposed rats. In contrast, CB exposure had no effect on Listeria-induced NO production at 3 days after infection and had a substantially smaller effect than DEP at later days. Exposure to DEP or CB resulted in enlarged lung-draining lymph nodes and increased the number and percentage of CD4(+) and CD8(+) T cells. These results showed that exposure to DEP decreased the ability of macrophages to produce antimicrobial oxidants in response to Listeria, which may play a role in the increased susceptibility of rats to pulmonary infection. This DEP-induced suppression is caused partially by chemicals adsorbed onto the carbon core of DEP, because impaired macrophage function and decreased Listeria clearance were not observed following exposure to CB.