CXCR4 and fibrosis: In accordance with previous studies (21, 22, 24, 25), we observed increased CXCR4 expression, decreased Ly6G expression, and reduced cell size and granularity on aged neutrophils from both fibrotic and nonfibrotic mice (Fig. 2C), validating our method of distinguishing neutrophil age in the bleomycin model of fibrosis.