The anti‐inflammatory effects of mesenchymal stem cells attenuate diffuse pulmonary hemorrhage

There are few effective treatment options for diffuse pulmonary hemorrhage (DPH). We aimed to elucidate the therapeutic role and underlying mechanisms of mesenchymal stem cells (MSCs) and MSC‐derived extracellular vesicles (MSC‐EVs) in DPH. Therapeutic effects of MSCs/MSC‐EVs in pristane‐induced DPH mice were evaluated via pulmonary function testing and histopathology. Transcriptome sequencing analyzed differentially expressed genes in control, DPH, and MSC groups. The proportion of macrophage polarization was evaluated in vivo and in vitro via fluorescence‐activated cell sorting in control, DPH, MSC, MSC‐EV inhalation, and MSC‐EV intravenous groups. Intraperitoneal injection of pristane induced diffuse alveolar hemorrhage, early fibrosis, and inflammation in C57BL/6 mice. Monocytes were depleted in the peripheral blood in DPH mice and MSCs were recruited to the lungs, resulting in significantly attenuated diffuse alveolar hemorrhage and suppressed immunological response. This was more effective in the hyperacute hemorrhage phase than the early inflammatory phase. An MSC treatment‐mediated anti‐inflammatory effect was observed in DPH mice. Furthermore, MSC‐EVs inhalation or tail‐vein injection could effectively reduce DPH injury. MSCs could suppress macrophage M1 polarization in DPH in vivo and in vitro. MSCs displayed significant therapeutic effects in pristane‐induced DPH, which may be a promising cell‐free therapeutic approach. Graphical and Lay Summary Intraperitoneal injection of pristane induced diffuse alveolar hemorrhage, early fibrosis, and inflammation in C57BL/6 mice. Either MSCs, MSC‐EVs inhalation or tail‐vein injection displayed therapeutic effects in pristane‐induced DPH. Moreover, MSCs could suppress macrophage M1 polarization in DPH in vivo and in vitro, which may be a promising cell‐free therapeutic approach.