The initial stimulus for inflammatory cell recruitment and the mechanisms responsible for the perpetuation and evolution of chronic inflammation, granulation tissue formation, and fibrosis have not been fully elucidated. Although interleukin (IL)-13, a Th2 cytokine, has been shown to have direct effects on fibroblasts that support fibroproliferation, it is also a potent inducer of a novel CC chemokine, C10, which is chemotactic for mononuclear phagocytes. The macrophage/mononuclear phagocyte has been shown to have a role in the pathogenesis of pulmonary fibrosis, serving as an important source of growth factors that regulate extracellular matrix synthesis. In this study we demonstrate that IL-13 and C10 are elevated in the pathogenesis of bleomycin-induced pulmonary fibrosis. Neutralization of IL-13, but not IL-4, attenuated bleomycin-induced pulmonary fibrosis and levels of C10, suggesting that IL-13 has an important role in the development of pulmonary fibrosis. IL-13 is a potent inducer of C10 in vivo, and neutralization of C10 attenuated bleomycin-induced pulmonary fibrosis and intrapulmonary macrophage numbers. This suggests that IL-13 has a role in the development of pulmonary fibrosis that is independent of its direct effect on fibroblasts and is evidence for an interaction between Th2 cytokines and specific CC chemokines.

Increasing evidence suggests that the development of pulmonary fibrosis is a Th2-mediated process. We hypothesized that the CC chemokines that are associated with a Th2 profile (CCL17 and CCL22) have an important role in the development of pulmonary fibrosis. We measured CCL17 and CCL22 during the pathogenesis of bleomycin-induced pulmonary fibrosis. We found that both CCL17 and CCL22 were significantly elevated through day 20 as compared with control mice. Peak expression of CCL22 preceded the peak levels of CCL17, as measured by real-time quantitative PCR. CCR4 is the receptor for CCL17 and CCL22 therefore, to further characterize the role of CCL17 and CCL22, we measured CCR4 mRNA in lung tissue of bleomycin-treated mice by real-time quantitative PCR. CCR4 was significantly elevated in bleomycin-treated mice as compared with control mice. Immunolocalization demonstrated that CCR4 was expressed predominantly on macrophages. Neutralization of CCL17, but not CCL22, led to a reduction in pulmonary fibrosis. Immunolocalization of bleomycin-treated lung tissue and human idiopathic pulmonary fibrosis tissue specimens showed that epithelial cells expressed CCL17. These findings demonstrate a central role for Th2 chemokines and the macrophage in the pathogenesis of pulmonary fibrosis and are further support for the role of a Th2 phenotype in the pathogenesis of pulmonary fibrosis.

Hyperoxia-induced lung injury is characterized by infiltration of activated neutrophils in conjunction with endothelial and epithelial cell injury, followed by fibrogenesis. Specific mechanisms recruiting neutrophils to the lung during hyperoxia-induced lung injury have not been fully elucidated. Because CXCL1 and CXCL2/3, acting through CXCR2, are potent neutrophil chemoattractants, we investigated their role in mediating hyperoxia-induced lung injury. Under variable concentrations of oxygen, murine survival during hyperoxia-induced lung injury was dose dependent. Eighty percent oxygen was associated with 50% mortality at 6 days, while greater oxygen concentrations were more lethal. Using 80% oxygen, we found that lungs harvested at day 6 demonstrated markedly increased neutrophil sequestration and lung injury. Expression of CXCR2 ligands paralleled neutrophil recruitment to the lung and CXCR2 mRNA expression. Inhibition of CXC chemokine ligands/CXCR2 interaction using CXCR2−/− mice exposed to hyperoxia significantly reduced neutrophil sequestration and lung injury, and led to a significant survival advantage as compared with CXCR2+/+ mice. These findings demonstrate that CXC chemokine ligand/CXCR2 biological axis is critical during the pathogenesis of hyperoxia-induced lung injury.

Diffuse alveolar damage is the histopathological hallmark of acute respiratory distress syndrome (ARDS) and is a stereotypic response to a variety of etiologies. Moreover, a significant proportion of ARDS survivors have residual pulmonary fibrosis and compromised pulmonary function. This suggests that the pathogenesis of diffuse alveolar damage that ultimately leads to the chronic fibrosis of ARDS has features of dysregulated repair exemplified by exaggerated intra-alveolar angiogenesis and fibrogenesis (i.e., fibroproliferation and deposition of extracellular matrix), leading to progressive alveolar fibrosis and impaired lung function. We obtained bronchoalveolar lavage fluid (BALF) from patients with ARDS or ventilated control patients and assessed CXC chemokine levels by ELISA. We found an imbalance in the expression of ELR+ as compared with ELR− CXC chemokines from BALF of patients with ARDS as compared with controls. This imbalance correlated with angiogenic activity as assessed by the corneal micropocket assay. Furthermore, these levels correlated with both procollagen I and procollagen III levels in BALF. In contrast, while BALF levels of vascular endothelial growth factor were elevated, vascular endothelial growth factor did not appear to be significantly contributing to the angiogenic activity. These findings suggest that CXC chemokines have an important role in the fibroproliferative phase of ARDS via the regulation of angiogenesis.

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