MMP9 and pulmonary fibrosis: In this study, we found that the expression of COL-1 and MMP-9/TIMP-1 were significantly decreased in lung tissue at 14-day injection of MSCs, EPO, especially, MSCs+EPO in BPD model, these results indicated that MSCs, EPO and MSCs in combination with EPO could reduce the lung fibrosis, interestingly, MSCs+EPO co-treatment more effective in repairing hyperoxia-induced alveoli dysplasia injury than MSCs or EPO treatment alone.