In contrast a single instillation of BLM results
In contrast, a single instillation of BLM results in a progression of inflammatory events that culminates in marked remodeling of the lung interstitium. This process takes 2–3 weeks, with the fibrotic lesions becoming most pronounced within the first month . Consequently, the BLM model offers the opportunity to investigate the effects of ERAs in the broader context of lung injury and repair. The ability of a single pretreatment with HJP272 to preferentially mitigate the fibrotic response would suggest that the natural progression of the disease process is determined at a very early stage. This might explain why ERAs have not been shown to be an effective treatment for human pulmonary fibrosis , , , .
Discussion The reduction in neutrophil infiltrates seen with HJP272 suggests that its anti-inflammatory activity may be largely dependent on blocking the effects of ET-1 on the vascular compartment. However, other factors may also be responsible for this finding, including decreased expression of macrophage TNFR1. The loss of this receptor may limit the activity of TNF-alpha, a cytokine that can activate a number of macrophage proinflammatory genes, including those responsible for the synthesis of metalloproteinases that play a role in the migration of inflammatory 132 6 to the lung , , , . Another mechanism that may be responsible for the effect of ET-1 on neutrophils involves increased synthesis of both p-selectin and intercellular adhesion molecule 1, which facilitates attachment of these cells to vascular endothelium , . ET-1 may also increase neutrophil expression of CXCR2, a cell-surface receptor that binds interleukin-8, which is a potent activator of these cells . Either of these processes could be enhanced by the fact that ET-1 can change the F-actin content of neutrophils, thereby promoting their sequestration in pulmonary microvessels . Our laboratory has also investigated the anti-inflammatory effects of phosphoramidon, an endothelin-converting enzyme inhibitor, on LPS-induced lung injury . Using either an IP or IT route of administration, it was shown that phosphoramidon can significantly reduce the influx of neutrophils into the lung. However, delaying treatment with this agent until 1 h after instillation of LPS abrogated this effect . A similar phenomenon was observed in the current investigation. The disparate findings with regard to pretreatment and post-treatment with HJP272 suggest that very early inflammatory events are responsible for programming the lung for subsequent fibrosis. A single treatment with this ERA leads to less inflammation, and decreased fibrosis several weeks later, despite the generally rapid clearance of these agents from the circulation . The relationship between the inflammatory and fibrotic reactions involves a number of different cytokines. Early recruitment of inflammatory cells results in elevated levels of PDGF, TGF-β, and CXCL12, which promote subsequent fibrosis , , , , , , , , . Migration of lung fibroblasts to the site of injury is facilitated by expression of PDGF and TGF-β, and is followed by differentiation of these cells into myofibroblasts , , , . Furthermore, PDGF promotes mesenchymal cell synthesis of glycosaminoglycans, while TGF-β is responsible for epithelial-mesenchymal transitions that require dedifferentiation of pulmonary epithelial cells into mesenchymal stem cells . CXCL12, in contrast, attracts extrapulmonary fibrocytes to the lung, which then undergo differentiation into myofibroblasts , , . This cytokine is a member of the CXC family of mediators that are involved in multiple injury and repair processes, including recruitment of leukocytes, vascular remodeling, and mobilization of mesenchymal progenitor cells such as fibrocytes . The observed reductions in PDGF, TGF-β, and CXCL12 following pretreatment with HJP272 may therefore contribute to the decrease in parenchymal fibrosis and lung collagen deposition.