Although the concept of M2 polarization is an oversimplification of the complex gene expression changes that occurs in vivo during lung fibrosis (5, 50), we show increased expression of Arg1, Retnla, and Msr1 (CD204) in lungs from WT mice given bleomycin, suggesting a contribution of an M2 phenotype in the development of pulmonary fibrosis. Here, ARG1 is linked to pulmonary fibrosis.