Mesenchymal stem cell-mediated immunomodulation of recruited mononuclear phagocytes during acute lung injury: a high-dimensional analysis study

Rationale: Acute lung injury (ALI)-recruited mononuclear phagocytes play a pivotal role in lung injury and repair. This study investigated the types of recruited mononuclear phagocytes and the immunotherapeutic effects of allograft mesenchymal stem cells (MSCs) in a mouse model of lipopolysaccharide (LPS)-induced ALI. Methods: C57BL/6 mice were orotracheally instilled with LPS (20 mg/kg). Compact bone-derived MSCs were administered orotracheally 4 h after LPS inhalation. Mononuclear phagocytes recruited in the lung tissues were characterized at different timepoints by high-dimensional analysis including flow cytometry, mass cytometry, and single-cell RNA sequencing. Results: Eight mononuclear phagocyte subsets recruited to LPS-challenged lungs were precisely identified. On day 3 after LPS administration, both Ly6C(hi)CD38(+) and Ly6C(low)CD38(+) monocytes were recruited into acutely injured lungs, which was associated with increased secretion of neutrophil chemokines. Ly6C(hi)CD38(+) monocytes differentiated into M1 macrophages on day 3, and subsequently differentiated into CD38(+) monocyte-derived dendritic cells (mo-DCs) on day 7, while Ly6C(low)CD38(+) monocytes differentiated into CD11b(+)CD38(+) DCs on day 7. When ALI mice were treated with MSCs, the mortality significantly reduced. Notably, MSCs reduced the amount of M1 macrophages and reduced the secretion of neutrophil chemokines on day 3. Furthermore, MSCs reduced the number of CD38(+) mo-DCs and CD11b(+)CD38(+) DCs on day 7, suppressing the antigen presentation process. Recruited mononuclear phagocyte subsets with a high level of CD38 exhibited an activated phenotype and could secrete higher levels of cytokines and chemokines. Conclusions: This study characterized the dynamic functions and phenotypes of recruited mononuclear phagocytes in ALI mice and MSC-treated ALI mice.