Abstract 183: Immune dysfunction cascades caused by ALCAM+ mesenchymal stem cells toward tumor progression

Abstract

Abstract Purposes: We have been investigating the interplay between cancer cells and host immunity, and previously demonstrated that Snail expression in tumor cells accelerates metastasis not only by enhancing tumor motility but also by inducing immune dysfunction through increase of a variety of immunoregulatory cells using murine tumor models implanted with snail-transduced B16-F10 melanoma (F10-snail+). We additionally found that Snail+ tumor cells frequently metastasize into bone marrow (BM) of the mice, and pluripotent CD45-ALCAM+ mesenchymal stem cells (MSCs) consequently increased in the mice. In this study, we attempted to characterize the Snail+ tumor-induced CD45-ALCAM+ MSCs (sMSCs). Results: The sMSCs were prepared by stimulating BMCs with F10-snail+ tumor supernatant for 5-7 days, or by sorting CD45-ALCAM+ cells from BM or spleen of the F10-snail+ tumor cells 2-3 weeks after implantation. When the mixture of the sMSCs and F10-mock tumor cells was subcutaneously injected in C57BL/6 mice, tumor growth was aggressively promoted, and spontaneous tumor dissemination in BM indicating de novo bone metastasis was observed, accompanied by increase of sMSCs in BM. In the tumor milieu and spleen of these mice, functionally impaired CD8(low) T cells and immunosuppressive Treg-inducible CD11b+Gr1+ MDSCs predominantly increased. Indeed, CD8(low) T cells were generated in the culture with splenic T cells and sMSCs, and MDSCs were expanded in the culture with BM cells and sMSCs. We identified key molecules responsible for the sMSC-induced events: ALCAM, ANGPT2 and FSTL1, which significantly increased only in the sMSC but not F10-mock-derived MSCs and normal MSCs. FSTL1 inhibited bone metastasis and consequent increase of sMSCs, CD8(low) T cells and MDSCs. Blocking ALCAM with mAb inhibited both sMSC expansion and CD8(low) T-cell induction. Blocking ANGPT2 inhibited expansion of MDSCs and Tregs rather than angiogenesis. Blocking FSTL1, which is a key trigger to initiate these cascades, in combination with elimination of other sMSC-associated factors significantly induced anti-tumor immune responses, resulting in complete inhibition of tumor growth and metastasis. Conclusions: These results suggest that sMSCs play a central role in the cancer EMT-induced immune dysfunction leading to tumor progression. Snail is reported to regulate acquisition of stem-like properties in tumor cells. However, how cancer stem cells would modulate the BM microenvironment and alter anti-tumor immune responses also remains unclear. Targeting sMSCs and its associated molecules may be a promising strategy for abrogating even cancer stem cells through reprogramming and activating anti-tumor immune responses appropriately. Citation Format: Chie Kudo-Saito, Takafumi Fuwa, Kouichi Murakami. Immune dysfunction cascades caused by ALCAM+ mesenchymal stem cells toward tumor progression. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 183. doi:10.1158/1538-7445.AM2014-183