A potential role for macrophages in maintaining lipopolysaccharide-induced subacute airway inflammation in rats

Bacterial infection is a key factor in airway inflammation. The present study describes the time-dependent changes in the leukocyte counts and cytokine levels of the bronchoalveolar lavage fluid (BALF) following subacute airway inflammation induced by lipopolysaccharide (LPS), a major component of the outer membranes of Gram-negative bacteria. LPS (200 μg/rat) or saline was intratracheally administered to rats which were sacrificed 2, 4 or 7 days after LPS treatment. Airway inflammation was evaluated using hematoxylin and eosin staining, cell counts and proinflammatory cytokine levels in the BALF. Rat airways obtained from the LPS group exhibited marked airway wall thickening and infiltration of inflammatory cells compared with the control group, as well as elevated cell counts (neutrophils, macrophages, lymphocytes) and proinflammatory cytokine levels [(tumor necrosis factor (TNF)-α, interleukin (IL)-1β, cytokine-induced neutrophil chemoattractant (CINC)-1)] in the BALF, which peaked on day 2 and subsequently decreased until the experimental endpoint. Notably, IL-1β levels induced by LPS changed in a similar manner to macrophage cell counts, but not neutrophil and lymphocyte counts. Moreover, TNF-α and CINC-1 levels did not decrease as rapidly as neutrophil counts after peaking. These findings suggest that macrophages may play a significant role in maintaining subacute inflammatory responses induced by LPS in rat airways.