Agglomerates of Ultrafine Particles of Elemental Carbon and TiO2Induce Generation of Lipid Mediators in Alveolar Macrophages

Agglomerates of ultrafine particles (AUFPs) may cause adverse health effects because of their large surface area. To evaluate physiologic responses of immune cells, we studied whether agglomerates of 77-nm elemental carbon [(EC); specific surface area 750 m2/ g] and 21 nm titanium dioxide ( TiO2) particles (specific surface area 50 m2/ g) affect the release of lipid mediators by alveolar macrophages (AMs). After 60-min incubation with 1 μg/mL AUFP-EC (corresponding to 7.5 cm2particle surface area), canine AMs (1× 106cells/mL) released arachidonic acid (AA) and the cyclooxygenase (COX) products prostaglandin E2( PGE2), thromboxane B2, and 12-hydroxyheptadecatrienoic acid but not 5-lipoxygenase (5-LO) products. AUFP- TiO2with a 10-fold higher mass (10 μg/mL) than AUFP-EC, but a similar particle surface area (5 cm2) also induced AMs to release AA and COX products. Agglomerates of 250 nm TiO2particles (specific surface area 6.5 m2/ g) at 100 μg/mL mass concentration (particle surface area 6.5 cm2) showed the same response. Interestingly, 75 cm2/ mL surface area of AUFP-EC and 16 cm2/ mL surface area of AUFP- TiO2additionally induced the release of the 5-LO products leukotriene B4and 5-hydroxyeicosatetraenoic acid. Respiratory burst activity of stimulated canine neutrophils was partially suppressed by supernatants of AMs treated with various mass concentrations of the three types of particles. Inhibition of neutrophil activity was abolished by supernatants of AMs treated with COX inhibitors prior to AUFP-incubation. This indicates that anti-inflammatory properties of PGE2dominate the overall response of lipid mediators released by AUFP-affected AMs. In conclusion, our data indicate that surface area rather than mass concentration determines the effect of AUFPs, and that activation of phospholipase A2and COX pathway occurs at a lower particle surface area than that of 5-LO-pathway. We hypothesize a protective role of PGE2in downregulating potential inflammatory reactions induced by ultrafine particles.