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A4493 - Clusterin Modulates the Recruitment and Function of Lung Macrophages
Author Block: K. Bernard, Y. Wang, N. J. Logsdon, D. Kurundkar, J. S. Deshane, V. J. Thannickal; Pulmonary, Allergy and Critical Care Medicine, UAB, Birmingham, AL, United States.
Rationale: Idiopathic pulmonary fibrosis (IPF) is a fatal lung disease that results from the loss of lung function due to excessive accumulation of extracellular matrix (ECM). ECM turnover is a complex process involving proteolytic enzymes, extracellular chaperone-mediated phagocytosis and lysosomal degradation by macrophages. Clusterin is an extracellular chaperone that promotes the endocytosis and lysosomal degradation of unfolded extracellular proteins and functions as a chemoattractant for macrophages. In this study, we sought to establish a link between clusterin deficiency and the pathogenesis of non-resolving fibrosis. Methods: We analyzed macrophage subsets in the BAL and lung tissue of wild-type (WT) and clusterin-deficient mice using flow-cytometry. We assessed clusterin levels in the BAL and lung tissue by ELISA and Western-blotting. We utilized the bleomycin-injury model of lung fibrosis in mice. The accumulation of lung collagen in response to bleomycin injury was assessed using a colorimetric assay that quantifies lung hydroxyproline. The phagocytic capacity of macrophages towards collagen or particulates of E.Coli was determined using flow cytometry and a fluorescence-based assay. Results: The accumulation of lung collagen measured at peak fibrosis in response to bleomycin-injury was similar between WT and clusterin-deficient mice. However, while WT mice resolved bleomycin-induced collagen deposition, clusterin-deficient mice continued accumulating lung collagen up to 6 weeks post-injury. Clusterin-deficient mice demonstrated significantly reduced numbers of total macrophages in the BAL and lung tissue, compared to WT. In response to bleomycin-injury, we observed a decrease in the number of resident alveolar macrophages and an increase in the number of monocyte-derived macrophages in the BAL and lung tissue of WT mice at peak fibrosis. These changes in macrophage numbers were associated with an increase in clusterin levels in the BAL and lung tissue. During the resolution phase of fibrosis, there were no significant differences in the number of resident alveolar macrophages and monocyte-derived macrophages between controls and bleomycin-injured mice. Clusterin levels were also downregulated during this phase compared to peak fibrosis. Additionally, we observed impaired in vivo phagocytosis of collagen by clusterin-deficient macrophages. Conclusions: Our study suggests that clusterin regulates macrophage recruitment and function in the BAL and lung tissue in homeostasis and in the context of pulmonary fibrosis. Clusterin deficiency may impair the clearance of ECM at sites of matrix remodeling to limit fibrosis resolution. This study provides evidence for a protective role of clusterin in fibrosis progression by promoting fibrosis-resolving activities of macrophages. Funding: NIH grants, P01 HL114470, R01 AG046210