Endotoxin enhancement of ozone-induced mucous cell metaplasia is neutrophil-dependent in rat nasal epithelium

Ozone, the primary oxidant gas in photochemical smog, causes neutrophilic i nflammation and mucous cell metaplasia (MCM) in the nasal transitional epit helium (NTE) of rats and monkeys. Bacterial endotoxin is another common air borne agent that induces acute neutrophilic inflammation, but not MCM, in N TE. It does, however, enhance ozone-induced MCM in rat nasal airways (Fanuc chi et al., 1998, Toxicol. Appl. Pharmacol. 152, 1-9). In the present study , F344 rats exposed to filtered air or 0.5 ppm ozone (8 h/day for 3 days) w ere intranasally instilled with sterile saline or 100 mug endotoxin 24 h an d 48 h after the third ozone exposure. To determine the role of neutrophili c inflammation in endotoxin-induced potentiation of the MCM caused by ozone , half of the rats were depleted of circulating neutrophils prior to saline or endotoxin instillations. Rats were killed 6 h or 3 days after the last intranasal instillation, and nasal tissues were processed for (1) light mic roscopy and morphometric analysis to determine the number of infiltrating n eutrophils and the volume amount (density) of stored mucosubstances in the NTE, and (2) quantitative RT-PCR analysis of steady-state mucin gene (rMuc- 5AC) mRNA levels in the NTE. Endotoxin induced a transient influx of neutro phils in both air- and ozone-exposed rats that was completely blocked by ne utrophil depletion. Endotoxin increased rMuc-5AC mRNA levels in the NTE of ozone-exposed rats. Neutrophil depletion, however, had no effect on endotox in-induced upregulation of mucin gene mRNA levels. Endotoxin enhanced the o zone-induced increase in stored mucosubstances (4-fold increase), but only in neutrophil-sufficient rats. These data indicate that endotoxin enhanceme nt of ozone-induced upregulation of rMuc-5AC mRNA levels is neutrophil-inde pendent, while its effects on intraepithelial production and storage of muc us glycoproteins is dependent on the presence of neutrophils.