Macrophages participate in lymphangiogenesis in idiopathic diffuse alveolar damage through CCL19-CCR7 signal

An abnormal healing process is believed to be involved in lung tissue damage in pulmonary fibrosis. Lymphangiogenesis has been determined to play a role in structural remodeling in diffuse alveolar damage. However, the mechanism of lymphangiogenesis remains unclear. The aim of this study is to investigate the cellular mechanisms of lymphangiogenesis in diffuse alveolar damage, focusing on the roles of macrophages. Formalin-fixed and paraffin-embedded lung tissues from 13 autopsy cases with idiopathic diffuse alveolar damage were used. Antibodies specific for D2-40 and CD68 were mainly used as the markers for lymphatics and macrophages, respectively, and immunohistochemical examinations and morphometric analyses were performed. Immunohistochemistry showed the aggregation of numerous CD68+ macrophages around newly formed lymphatics in the intraalveolar fibrotic lesions in the proliferative stage, and some of the CD68+ macrophages were colocalized with the lymphatic endothelium. These macrophages were characterized by the expression of vascular endothelial growth factor-C. Among the 3 stages of diffuse alveolar damage, the number of Ki-67+ cells on the lymphatic endothelium tended to increase in newly formed lymphatics of the proliferative stage. In addition, the number of CD68+ macrophages on the lymphatic endothelium was significantly increased in the newly formed lymphatics of the proliferative stage more than those of the fibrotic stage. The CD68+ macrophages around the newly formed lymphatics coexpressed CCR7. Dual immunostaining showed the coexpression of CCL19-a ligand for CCR7-on the lymphatic endothelium. Thus, macrophages may participate in lymphangiogenesis in diffuse alveolar damage, which is facilitated by CCL19 and CCR7.