Abstract 5403: Snail+ bone-metastatic cancer cells impair CTL induction via induction of immunoregulatory MSCs
Authors: Kudo-Saito Chie , Kawakami Yutaka 
Published: August 2012

Abstract
Purpose: Immunosuppressive mechanism in cancer patients is still unintelligible due to its complexity including various immunoregulatory members such as MDSCs, Treg cells and mesenchymal stem cells (MSCs). Particularly, the phenotype markers and the functional role of MSCs are not fully defined yet in tumor immunology, although demonstrated in regenerative medicine fields. We recently found that Snail+ tumor cells undergoing EMT preferentially metastasized into bone marrow (BM) than other organs such as spleen and lymph node, accompanying increase of MSC-like cells. In this study, we elucidated the molecular mechanism and the MSC features for the further understanding of immunosuppressive mechanism in host with cancers. Results: When BM cells of C57BL/6 mice were cultured with supernatant of murine snail-transduced melanoma B16-F10 cells for 7-14 days, CD45-Vimentin+ mesenchymal cells were significantly increased as compared with the control with mock transfectant. These cells showed the ability to differentiate into adipocytes, chondrocytes and osteocytes, indicating MSC generation by Snail+ tumor cells. CD45- MSCs were also increased in BM of the mice 2-3 weeks after Snail+ tumor implantation. These Snail-induced MSCs (sMSCs) showed much higher suppressive activity on T-cell proliferation followed by Treg increase than the MSCs stimulated with mock-transfected B16-F10 supernatant. When B16-F10 cells were subcutaneously co-injected with the sMSCs in mice, tumor growth and metastasis was significantly promoted in C57BL/6 mice, but not in immunodeficient mice, implying requirement of other immune cells for the sMSC efficacy. Utilizing EGFP+ mice or GFP+ B16-F10 tumor cells, we found that the co-injected sMSCs mainly migrated and accumulated in lymphatic organs such as spleen. In the spleen, CD11b+Gr1+ MDSCs and Foxp3+ Treg cells were increased, and CD8+ T cells were disabled for IFNγ induction and tumor-killing, indicating CTL impairment following accumulation of various immunoregulatory cells. The sMSCs also significantly suppressed T-cell proliferation in vitro. On the other hand, the co-injected tumor cells rapidly metastasized into BM of the mice, suggesting EMT induction by the sMSCs. We identified the sMSC markers and the sMSC inducers that produced from Snail+ tumor cells. Blocking of the sMSC inducer with the specific siRNA resulted in decrease of the sMSCs and tumor cells in BM, accompanying improvement of anti-tumor immune responses in the mice. Conclusions: These results suggest that Snail+ bone-metastatic tumor cells increase sMSCs that induce MDSCs and Treg cells leading to systemic spread of immunosuppression in host. The sMSCs also stimulate noninvasive tumor cells to migrate into BM, resulting in further amplification of the immunosuppressive cascade. Elimination of the sMSCs would be an useful strategy for treatment of cancers.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5403. doi:1538-7445.AM2012-5403