Lymphatic proliferation ameliorates pulmonary fibrosis after lung injury.

Although there have been many reports on pulmonary blood vessels in lung fibrosis, the contribution of lung lymphatics is unknown. We examined the mechanism and consequences of lymphatic remodeling in mice with lung fibrosis after bleomycin injury or telomere dysfunction. Prox1-EGFP mice and immunohistochemistry revealed widespread lymphangiogenesis after bleomycin and in fibrotic lungs of transgenic mice with telomere dysfunction due to lung epithelial cell-specific deletion of telomere shelterin protein complex (Trf1). In loss-of-function studies, blocking antibodies revealed that lymphangiogenesis 14 days after bleomycin was dependent on vascular endothelial growth factor (Vegf) receptor 3 signaling, but not on Vegf receptor 2. Vegfc gene and protein expression increased, but not Vegfa or Vegfd. Extensive extravasated plasma, platelets, and macrophages at sites of lymphatic growth were potential sources of Vegfc. Lymphangiogenesis peaked at 14 to 28 days after bleomycin, was accompanied by doubling of chemokine (C-C motif) ligand 21 in lung lymphatics and tertiary lymphoid organ formation, and then decreased as lung injury resolved by 56 days. In gain-of-function studies, expansion of the lung lymphatic network by transgenic overexpression of Vegfc in CCSP/VEGF-C mice reduced macrophage accumulation and fibrosis and accelerated recovery after bleomycin. These findings provide evidence that lymphatics have an overall protective effect in lung injury and fibrosis and fit with a mechanism whereby lung lymphatic network expansion reduces lymph stasis and increases clearance of fluid and cells, including profibrotic macrophages.