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Instillation of crystalline silica into the lungs of mice is a common experimental model of pulmonary fibrosis. Typically, a suspension of silica in saline is injected into the trachea via intubation or surgical tracheostomy. These techniques require a high degree of technical skill, have a lengthy training period, and can suffer from a high failure rate. In oropharyngeal aspiration, a droplet of liquid is placed in the animal's mouth while simultaneously holding its tongue (to block the swallow reflex) and pinching its nose shut, forcing it to breathe through its mouth, aspirating the liquid. To determine whether oropharyngeal aspiration (OA) could replace intratracheal instillation (IT) in a model of silica-induced fibrosis, a comparison was performed. Crystalline silica was introduced into the lungs of male C57BL/6 mice by the IT or OA procedure, and the resulting inflammation and fibrosis was assessed after 3 weeks. IT and OA instillation of silica both resulted in neutrophilic inflammation and fibrotic changes, including interstitial fibrosis and dense fibrotic foci. Mice treated via IT demonstrated a few large lesions proximal to conducting airways with little involvement of the distal parenchyma and large interanimal variability. In contrast, OA resulted in a diffuse pathology with numerous fibrotic foci distributed throughout the lung parenchyma, which is more representative of human fibrotic lung disease. OA- but not IT-treated mice exhibited significantly increased lung collagen content. Furthermore, the interanimal variability within the OA group was significantly less than in the IT group. Oropharyngeal aspiration should be considered as an alternative to intratracheal instillation of silica and other particulates in studies of respiratory toxicity and lung disease.

Ionizing radiation leads to a progressive injury in which a monocyte/macrophage-rich pneumonitis is followed by a chronic progressive fibrosis. In the present study, the role of macrophage/monocyte recruitment in the genesis of radiation-induced pulmonary fibrosis was examined. The objectives were threefold: (i) characterize the inflammatory cells recruited into the lung during the development of radiation-induced fibrosis; (ii) investigate changes in lung response following depletion of resident alveolar macrophages in vivo prior to radiation treatment; (iii) assess if inhalation of low levels of endotoxin would potentiate the radiation-initiated injury. One group of fibrosis-sensitive C57BL/6 mice was irradiated with a single dose of 15 Gy to the thorax. In a second group, resident inflammatory cells were depleted using clodronate, encapsulated into liposomes, 48 hours prior to irradiation with a single dose of 15 Gy to the thorax. Control animals were sham irradiated. All groups of animals then were examined 8, 16, or 24 weeks post irradiation. No difference in total cell numbers or cell differentials was observed between irradiated mice or those that were both liposome treated and irradiated at any time point. At 16 weeks, mice that received radiation showed a 5- to 6-fold increase in lymphocytes regardless of treatment as compared to control animals. At 24 weeks post irradiation, select groups were exposed to lipopolysaccharide (LPS) and examined 24 hours post inhalation. Lavageable protein was increased several fold in mice that received both radiation and LPS exposure as compared to 15 Gy or LPS exposure alone. These results demonstrate: (i) macrophages and lymphocytes are the predominately recruited cell types through 24 weeks post irradiation; (ii) recovery of inflammatory cells, regardless of prior macrophage depletion, were similar, suggesting that early responses are primarily driven by parenchymal cell injury; (iii) thoracic irradiation-induced injury can cause sensitization to a secondary stimulus that may result in injuries/responses not predicted by evaluating exposures individually.