Mesenchymal Stromal Cells Regulate M1/M2 Macrophage Polarization in Mice with Immune Thrombocytopenia

Mesenchymal stromal cells have shown promising effects in the treatment of immune thrombocytopenia. However, the underlying mechanisms are not fully understood. In this study, we investigated the therapeutic effects of human bone marrow mesenchymal stromal cells (hBMSCs) and analyzed their unique role in regulating the M1/M2 macrophage ratio. We established a passive immune thrombocytopenia (ITP) mouse model and showed that there was a significant M1/M2 imbalance in ITP model mice by assessing the M1/M2 ratios in the liver, spleen, and bone marrow; we observed excessive activation of M1 cells and decreased M2 cell numbers in vivo. We have shown that systemic infusion of hBMSCs effectively elevated platelet levels after disease onset. Further analysis revealed that hBMSCs treatment significantly suppressed the number of proinflammatory M1 macrophages and enhanced the number of anti-inflammatory M2 macrophages; in addition, the levels of proinflammatory factors, such as interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), were significantly decreased in vivo, while the levels of the anti-inflammatory factor interleukin-10 (IL-10) were increased. In conclusion, our data suggest that hBMSCs treatment can effectively increase platelet counts, and the mechanism is related to the induction of macrophage polarization toward the anti-inflammatory M2 phenotype and the decrease in proinflammatory cytokine production, which together ameliorate innate immune disorders.