Opposite effects of coinjection and distant injection of mesenchymal stem cells on breast tumor cell growth

Mesenchymal stem cells (MSCs) usually promote tumor growth and metastasis. By using a breast tumor 4T1 cell-based animal model, this study determined that coinjection and distant injection of allogeneic bone marrow-derived MSCs with tumor cells could exert different effects on tumor growth. Whereas the coinjection of MSCs with 4T1 cells promoted tumor growth, surprisingly, the injection of MSCs at a site distant from the 4T1 cell inoculation site suppressed tumor growth. We further observed that, in the distant injection model, MSCs decreased the accumulation of myeloid-derived suppressor cells and regulatory T cells in tumor tissues by enhancing proinflammatory factors such as interferon-ɤ, tumor necrosis factor-α, Toll-like receptor (TLR)-3, and TLR-4, promoting host antitumor immunity and inhibiting tumor growth. Unlike previous reports, this is the first study reporting that MSCs may exert opposite roles on tumor growth in the same animal model by modulating the host immune system, which may shed light on the potential application of MSCs as vehicles for tumor therapy and other clinical applications.