HGF and pulmonary emphysema: To further determine the mechanisms underlying the increased susceptibility of PPE-instilled IL-33−/− mice to PPE-induced emphysema, we measured the levels of neutrophil- and macrophage-related chemokines and proteinases related to the destruction of alveoli [13, 20, 23], as well as HGF and VEGF, which protect against the development of emphysema [5, 6], in BAL fluid at different time points.