Mesenchymal Stem Cells Promote Tumor Progression By Inducing The Suppressive Function Of Bone Marrow-derived Myeloid Cells

Background:Mesenchymal stem cells, isolated from many different tissues, are multipotential nonhematopoietic stem cells with the capacity for self-renewal and mutilineage differentiation. They can be cultured in vitro and expanded in large quantities, and can be differentiated to produce multiple cell types such as nerve cells, osteoblasts, chondrocytes, myocyte and adipocyte under certain conditions. These capacities present them as a promising stem cell candidate for cell replacement therapy and tissue engineering, having broad prospects for clinical application. In addition MSCs are a class of immune-deficient cell have low immunogenic and immunoregulatory properties. They play immunosuppressive function primarily through cell-contact and soluble factors, and research hotspots of the field of transplantation and treatment of autoimmune disease. Proinflammatory cytokines are required to induce immunosuppression by MSCs through the concerted action of chemokines and NO and PGE2induced by TNF-α. Otherwise, MSCs can recruit monocytes, macrophages and neutrophils through secreting large amounts of cykines induced by TNF-a and influence the growth of tumor. However, the action of MSCs on the CD11b+Grl+cells from Bone Marrow and the influence on tumor progression need to be further investigated.Objective:To investigate the role of mesenchymal stem cell (MSCs) in tumor progression by inducing suppressive function of bone marrow-derived CD11b+Gr1+cells (BM-CD11b+Gr1+cells).Methods:The immunophenotypes of CD11b+Grl+cells were tested after being cocultured with MSCs. The influence of MSCs on CD11b+Grl+cells was evaluated by T cell proliferation assay after cultured with or without MSCs. The4T1breast tumor model was established to explore the effect on tumor growth.Results:MSCs can promote the survival of CD11b+Gr1+cells and induce the generation of CD11b+Gr1+cells from CD11b-Gr1-cells sorted from bone marrow. MSCs can induce the ability of BM-CD11b+Gr1+cells to suppress T cells proliferation and activation. CD11b+Gr1+cells cocultured with MSCs can promote4T1tumor growth and accelerate death of tumor-bearing mice.Conclusion:MSCs promote tumor progression by inducing suppressive function of CDl1b+Grl+cells from bone marrow. Background and aim:Mesenchymal stromal cells (MSCs) have potent regenerative capacity and serve as a major niche component for hematopoietic stem/progenitor cells in the bone marrow. MSCs also possess potent immunomodulatory properties and they have been shown to interact with a variety of immune cells including T cells, B cells, dendritic cells, and monocytes/macrophages. MSCs are one of the key elements of tumor microenvironment and are also believed to be the precursors of tumor-associated fibroblasts. Recent studies demonstrated that MSCs constitutively secreted a large number of cytokines, and contributed to greater recruitment of CD11b+Ly6C+Ly6G-monocytes, F4/80+macrophages, and CD11b+Ly6CdimLy6G+neutrophils to the tumor to contribute to tumor growth. In addition, it is also demonstrated that MSCs are capable of skewing monocytes/macrophages into an immunosuppressive phenotype, which might promote tumor progression.Neutrophils as important effector cells in the innate immune response against invading micro-organisms are always abundant in tumor microenvironment, yet their impact on tumor development still remains elusive. It has been recently recognized that neutrophils could display multiple function phenotype. In fact, only after priming (typically by a cytokine, chemokine or bioactive lipid) can neutrophils optimally exert pro-inflammatory functions such as the generation of a respiratory burst induced by fMLF or chemotaxis. In addition, neutrophils can be switched from non-suppressive to immunosuppressive phenotypes. Phenotype switching of neutrophils has recently been reported by the addition of granulocyte macrophage colony-stimulating factor (GM-CSF) to mature or immature murine bone marrow-derived neutrophils. Recent studies also showed that tumor-associated neutrophils are driven by TGF-β to switch to the tumor-promoting phenotype. Neverthelss, it is still poorly understood how neutrophils switch into the suppressive phenotypes, especially during cancer development.MSCs have been reported to promote the survival of neutrophils, yet the direct effect of MSCs on the function of neutrophils has not been addressed. Due to the key role of MSCs in promoting tumor progression in many conditions, we hypothesize that neutrophils are a major type of cells through which MSCs might exert their tumor-promoting effect. In this study, we investigated the effect of MSCs on the immunophenotype and functional characteristics of CD11b+Ly6G+neutrophils, regarding its impact on tumor progression.Methods:Immunosuppressive function of neutrophils was evaluated by T cell proliferation assay and4T1breast tumor model; molecular mechanisms were explored by transcriptional profiling, Real-time RT-PCR, arginase activity assay, and iNOS inhibition experiments.Results:After being cocultured with MSCs primed by TNF-a (TNF-MSCs), CD11b+Ly6G+neutrophils isolated from bone marrow of normal mice or spleen of tumor-bearing mice obtained immunosuppressive function to inhibit T cell proliferation in vitro, and to enhance4T1tumor progression in vivo. Moreover, arginase activity and expression of iNOS, saa3, some cytokines and chemokines and their receptors, were upregulated in neutrophils after co-culture with TNF-MSCs. Inhibition of iNOS activity attenuated the suppressive effect of TNF-MSC pre-cocultured neutrophils on T cell proliferation.Conclusion:MSCs program neutrophils into an immunosuppressive and tumor-promoting phenotype.