MMP9 and breast cancer: Alveolar epithelial type II cells took up the breast-cancer-derived exosomal miR-200b-3p, and induced high expression of chemokine ligand 2 (CCL2), S100A8/9, matrix metalloproteinase 9 (MMP9), and colony-stimulating factor 1 (CSF-1) to recruit myeloid-derived suppressor cells (MDSCs) and promote inflammatory PMN formation [43].