Alveolar macrophage cytokine and growth factor production in a rat model of crocidolite‐induced pulmonary inflammation and fibrosis

Abstract

The present study was undertaken to further define the role of alveolar macrophages (AM) in the pulmonary response to crocidolite fibers. Briefly, groups of 4 male F344 rats were intratracheally instilled with saline or saline suspensions of crocidolite at 2 or 20 mg/kg body weight. Animals were sacrificed 3, 7, 14, and 28 d after exposure and the lung response was characterized by analysis of bronchoalveolar lavage fluid (BALF) for markers of lung injury and inflammation. AM obtained in BALF were cultured and their production of the pro‐inflammatory cytokines, tumor necrosis factor α (TNFα), and interleukin‐1 (IL‐1) were characterized along with fibronectin, a protein known to stimulate fibroblast migration and proliferation. Lung hydroxyproline content was determined 28 d after exposure and lung histopathology was characterized on d 28 and 90 after exposure. Crocidolite instillation resulted in transient dose‐related pulmonary inflammation as evidenced by increased numbers of BALF neutrophils at the low dose and neutrophils, macrophages, and lymphocytes at the high dose. Cytotoxicity and increased permeability were demonstrated by increased levels of BALF lactate dehydrogenase (LDH) and total protein, respectively. AM TNFα and IL‐1 production were increased only at the high crocidolite dose. This cytokine response was greatest at d 3 and decreased thereafter. AM TNFα and IL‐1 release were positively correlated with the increased BALF neutrophils. In contrast to TNFα and IL‐1, AM fibronectin release was increased at both the low and high doses, with the magnitude of response increasing over time. Consistent with previous acute asbestos inhalation studies, histopathology revealed inflammation localized at the level of the terminal bronchioles and alveolar ducts. Fibrosis was demonstrated at both doses by increased trichrome staining of lung tissue sections. Only the high dose resulted in a detectable increase in lung hydroxyproline. Given the bioactivities of TNFα, IL‐1, and fibronectin, their increased production after crocidolite exposure indicates they contribute to the pulmonary inflammation and fibrosis occurring with this mineral fiber. In addition, the correlation of increased AM TNFα and IL‐1 production with increased BALF neutrophils supports a role for these cytokines in crocidolite‐induced inflammatory cell recruitment. Lastly, association of a persistent increase in AM fibronectin production with an eventual increase in lung collagen deposition extends the growing database indicating this response is a predictive marker of pulmonary fibrosis.