Mouse lung inflammation after instillation of particulate matter collected from a working dairy barn

Coarse and fine particulate matter (PM_2.5-10 and PM_2.5, respectively) are regulated ambient air pollutants thought to have major adverse health effects in exposed humans. The role of endotoxin and other bioaerosol components in the toxicity of PM from ambient air is controversial. This study evaluated the inflammatory lung response in mice instilled intratracheally with PM_2.5-10 and PM_2.5 emitted from a working dairy barn, a source presumed to have elevated concentrations of endotoxin. PM_2.5-10 was more pro-inflammatory on an equal weight basis than was PM_2.5; both fractions elicited a predominantly neutrophilic response. The inflammatory response was reversible, with a peak response to PM_2.5-10 observed at 24 h after instillation, and a return to control values by 72 h after instillation. The major active pro-inflammatory component in whole PM_2.5-10, but not in whole PM_2.5, is heat-labile, consistent with it being endotoxin. A heat treatment protocol for the gradual inactivation of biological materials in the PM fractions over a measurable time course was developed and optimized in this study using pure lipopolysaccharide (LPS) as a model system. The time course of heat inactivation of pure LPS and of endotoxin activity in PM_2.5-10 as measured by Limulus bioassay is identical. The active material in both PM_2.5-10 and PM_2.5 remained in the insoluble fraction when the whole PM samples were extracted with physiological saline solution. Histological analysis of lung sections from mice instilled with PM_2.5-10 or PM_2.5 showed evidence of inflammation consistent with the cellular responses observed in lung lavage fluid. The major pro-inflammatory components present in endotoxin-rich PM were found in the insoluble fraction of PM_2.5-10; however, in contrast with PM_2.5-10 isolated from ambient air in the Central Valley of California, the active components in the insoluble fraction were heat-labile.