Table 1 Inclusion and exclusion criteria for participants with severe bronchopulmonary dysplasia (BPD)

Inclusion criteria

1. Participants who are male or female and whose age is 0–1 years old

2. Participants who are diagnosed as severe BPD according to diagnostic criteria of BPD made by the National Institute of Child Health and Human Development (NICHD )[5].

3. Participants who have abnormal respiratory manifestations and the Silverman-Anderson score [32] is more than 3 points

4. Written informed consent signed by a legal representative or a parent

Exclusion criteria

1. Participants whose age is more than 1 year old

2. Participants who have no signs of dyspnea or BPD-related changes in pulmonary imaging, such as central apnea or diaphragmatic paralysis although mechanical ventilation or oxygen are required

3. Participants who have concurrent cyanotic or acyanotic congenital heart diseases, except for patent ductus arteriosus, and atrial septal defect and ventricular septal defect with defect < 5 mm

4. Participants whose important laboratory test (liver and kidney functions tests, cardiac markers, hematology and immunity tests, urinalysis, etc.) abnormalities are more than three times compared with the normal value

5. Participants who have severe pulmonary hypertension confirmed by cardiac ultrasound at the time of assessment

6. Participants who have severe respiratory tract malformation, such as Pierre-Robin syndrome, tracheobronchomalacia, vascular ring syndrome, congenital tracheal stenosis, tracheo-esophageal fistula, pulmonary emphysema, pulmonary sequestration, congenital pulmonary dysplasia, congenital pulmonary cyst, congenital spasm, etc.

7. Participants who have severe chromosome anomalies (such as Edward syndrome, Patau syndrome, Down syndrome) or severe congenital malformation (such as hydrocephalus, encephalocele) or hereditary diseases

8. Participants who have severe congenital infection such as Herpes simplex, toxoplasmosis, rubella, syphilis, AIDS, etc.

9. Participants who have severe active infection when C-reactive protein (CRP) > 30 mg/dL, or suffer sepsis or septic shock

10. Participants who are going to have surgery within 72 h before/after this study hUC-MSCs administration

11. Participants who have surfactant administration within 24 h before this hUC-MSCs administration.

12. Participants who have severe intracranial hemorrhage ≥ grade 3 or active pneumorrhagia or active air-leak syndrome

13. Participants who are using hormones or needing hormones within and after 7 days of hUC-MSCs administration

14. Participants who are participating in other interventional clinical trials

15. Participants who are considered inappropriate by the investigators or whose parents cannot provide informed consent