Differentiation of Mesenchymal Stem/ Stromal Cells into CD45+ Macrophage-like Cells: Expanding Insights into MSC Plasticity

Mesenchymal stem/stromal cells (MSCs) are notable for their differentiation potential and immunomodulatory properties. They share several features with macrophages that are also similar to those of fibrocytes. This study aimed to investigate the relationship between MSCs, fibrocytes, and macrophages, focusing on the hypothesis of a possible direct lineage. This study focuses on a freshly isolated murine MSC population known as PαSmMSCs (CD45 ⁻ TER119 ⁻ Sca-1 ⁺ PDGFRα ⁺ ). Transplanting PαSmMSCs from transgenic GFP mice into irradiated recipients produced GFP ⁺ cells expressing the macrophage markers CD45, CD68, and CD115, indicating a macrophage-like phenotype. Further analysis showed that PαSmMSCs cultured in macrophage differentiation media acquired characteristics of M2 macrophages (CD45, CD68, and CD206). These results were confirmed in two different mouse strains as well as in human MSCs (hMSCs). Single-cell RNA sequencing revealed macrophage-like and fibrocyte-like populations among the PαSmMSCs. These findings demonstrate the phenotypic plasticity of freshly isolated MSCs to transition into macrophage-like and/or fibrocyte-like states, which has not been reported, particularly in long-term cultured MSCs. This study provides new insights into MSC plasticity and its potential roles in immune regulation and tissue repair.