The animals were euthanized 7 or 21 days after bleomycin challenge. Treated humanized mice with human MSCs showed an attenuation of pulmonary fibrosis development. MSCs are suggested to suppress T-cell overactivation via PD-1 and PD-L1 interaction. Human MSCs have a therapeutic effect only in the early phase of pulmonary fibrosis. This evidence concerns the gene CD274 and pulmonary fibrosis.