Efficient depletion of alveolar macrophages using intratracheally inhaled aerosols of liposome-encapsulated clodronate

Rat alveolar macrophages (AMs) were depleted via intratracheal inhalation (ITIH) of clodronate-containing liposomes. AM depletion following ITIH delivery of clodronate liposomes was 33.2 +/- 14.2 on day 1, 88.1 +/- 6.2 on day 3, and 91.4 +/- 1.8 on day 4 relative to control rats given saline-containing liposomes. Almost all (approximately 99%) of the AMs remaining at the 3-day time point were peroxidase negative, suggesting that immature macrophages were not recruited from the circulation to replace those undergoing cell death on that day. Only 0.5% +/- 0.5% of bronchoalveolar lavage (BAL) cells were neutrophils at this time (normalized to controls). Whole-body inhalation did not induce as much AM depletion at 3 days (37.6% +/- 10.1%) and required larger amounts of liposome-encapsulated clodronate compared to ITIH. Intratracheal instillation (as opposed to inhalation) of clodronate liposomes produced a significant inflammatory response characterized by the influx of both polymorphonuclear neutrophils (PMNs) and macrophages. In subsequent pilot studies, the response to intratracheally instilled crystalline silica (75 microg) was found to be markedly reduced in rats depleted of AMs by the ITIH method. We conclude that ITIH of clodronate liposomes in rats is both efficient and useful for examining the role of AMs in pulmonary toxicology.