IN-VIVO EFFECTS OF ENDOTOXIN ON DNA-SYNTHESIS IN RAT NASAL EPITHELIUM

Airway inflammation in bacterial infections is characterized by the pr esence of neutrophils and often epithelial injury and repair. Release of endotoxin from bacteria may contribute to these processes. The purp ose of this study was to determine the in vivo effects of repeated end otoxin exposure on DNA synthesis in rat nasal epithelium in the presen ce and absence of neutrophilic influx. Rats were intranasally instille d, once a day for 3 days, with endotoxin or saline (controls). Before the first and third instillations, half of the saline and endotoxin-in stilled animals were depleted of circulating blood neutrophils by admi nistering a rabbit anti-rat neutrophil antiserum. Rats were sacrificed 6 or 24 h after the last instillation. Two hours prior to sacrifice, rats were intraperitoneally injected with bromodeoxyuridine (BrdU), an analog of thymidine that is incorporated in the nucleus of cells in t he S-phase of the cell cycle. Nasal tissues were processed for light m icroscopy and immunohistochemical detection of BrdU in nasal epithelia l cells. The numbers of nasal epithelial cells, BrdU-labeled epithelia l nuclei, and neutrophils per millimeter of basal lamina in the epithe lium lining the nasal turbinates in the proximal nasal passages were d etermined by morphometric analysis. We did not observe a neutrophilic influx in the nasal tissues of neutrophil-depleted rats at 6 or 24 h a fter the last endotoxin instillation; however, the numbers of nasal ep ithelial cells and the BrdU-labeling index were significantly increase d compared to saline-instilled controls. In contrast, non-neutrophil-d epleted rats instilled with endotoxin had a marked neutrophilic influx , but no significant differences in the number of nasal epithelial cel ls at 6 or 24 h, compared to controls. In addition, the BrdU-labeling index in neutrophil-sufficient rats was increased only 6 h after the l ast instillation, compared to controls. These results indicate that 1) endotoxin induces an increase in epithelial DNA synthesis that is neu trophil-independent, and 2) without a neutrophilic influx, there is su stained epithelial cell proliferation, resulting in a hyperplastic epi thelium after repeated endotoxin exposure. (C) 1993 Wiley-Liss, Inc.