To further determine the importance of endotoxin in grain dust-induced inflammation of the lower respiratory tract, we evaluated the efficacy of pentaacylated diphosphoryl lipid A derived from the lipopolysaccharide of Rhodobacter sphaeroides (RsDPLA) as a partial agonist of grain dust-induced airway inflammation. RsDPLA is a relatively inactive compound compared with lipid A derived from Escherichia coli (LPS) and has been demonstrated to act as a partial agonist of LPS-induced inflammation. To assess the potential stimulatory effect of RsDPLA in relation to LPS, we incubated THP-1 cells with RsDPLA (0.001-100 micrograms/ml), LPS (0.02 microgram endotoxin activity/ml), or corn dust extract (CDE; 0.02 microgram endotoxin activity/ml). Incubation with RsDPLA revealed a tumor necrosis factor (TNF)-alpha stimulatory effect at 100 micrograms/ml. In contrast, incubation with LPS or CDE resulted in TNF-alpha release at 0.02 microgram/ml. Pretreatment of THP-1 cells with varying concentrations of RsDPLA before incubation with LPS or CDE (0.02 microgram endotoxin activity/ml) resulted in a dose-dependent reduction in the LPS- or CDE-induced release of TNF-alpha with concentrations of RsDPLA of up to 10 micrograms/ml but not at 100 micrograms/ml. To further understand the role of endotoxin in grain dust-induced airway inflammation, we utilized the unique LPS inhibitory property of RsDPLA to determine the inflammatory response to inhaled CDE in mice in the presence of RsDPLA. Ten micrograms of RsDPLA intratracheally did not cause a significant inflammatory response compared with intratracheal saline. However, pretreatment of mice with 10 micrograms of RsDPLA intratracheally before exposure to CDE (5.4 and 0.2 micrograms/m3) or LPS (7.2 and 0.28 micrograms/m3) resulted in significant reductions in the lung lavage concentrations of total cells, neutrophils, and specific proinflammatory cytokines compared with mice pretreated with sterile saline. These results confirm the LPS-inhibitory effect of RsDPLA and support the role of endotoxin as the principal agent in grain dust causing airway inflammation.