Distinct patterns of pulmonary injury and fibrosis induced by intratracheal and subcutaneous bleomycin in the mouse: Relevance for distinct forms of human lung fibrosis

Background: Murine models can be useful for understanding complex diseases such as pulmonary fibrosis. Bleomycin, administered via a wide variety of routes, is commonly used to induce lung fibrosis in rodents. We compared fibrotic responses in the lung elicited by bleomycin given by subcutaneous injection and intratracheal administration. Mice were treated with one dose of intratracheal bleomycin or with subcutaneous bleomycin daily for 14 days. Lungs and skin were analyzed for fibrosis by histopathology and immunostaining for alpha-smooth muscle actin, and transforming growth factor-beta protein level and apoptosis were evaluated. Results: While intratracheal and subcutaneous approaches yielded comparable severity of pulmonary fibrosis, distinct spatial and temporal evolution of pulmonary changes was noted. Intratracheal bleomycin induced early onset and rapid resolution of inflammation, while subcutaneous bleomycin caused more sustained lung inflammation with slow resolution of injury. Moreover, subcutaneous bleomcyin caused a predominantly peri-vascular pattern of lung fibrosis, whereas intratracheal instillation of bleomycin additionally caused injury around the airways. Early apoptotic responses were primarily centered around the airways in mice given intratracheal bleomycin but were seen around blood vessels with subcutaneous bleomycin. Conclusions: Subcutaneous administration of bleomycin causes a pattern of lung injury and fibrosis that is histologically distinct from that induced by the intratracheal route. These two distinct modes of bleomycin delivery might have utility for modeling interstitial lung disease associated with airway injury, such as idiopathic pulmonary fibrosis, versus systemic autoimmunity, such as systemic sclerosis.