MMP9 and pulmonary fibrosis: Our previous studies demonstrated that intratracheal transplantation of human umbilical cord-derived mesenchymal stromal cells (HUMSCs) could attenuate pulmonary fibrosis through three mechanisms: suppressing inflammation and myofibroblast activation, promoting macrophage-derived MMP-9 to degrade collagen, and upregulating TLR-4 to stimulate epithelial regeneration [11,12,13].