Abstract B26: Sarcoma microenvironment blocks DC but promotes macrophage differentiation from tumor-infiltrating monocytes
Authors: Devalaraja Samir , Folkert Ian , Li Minghong , Tada Yuma , Jain Kanika , Haldar Malay 
Published: August 2018

Abstract
Sarcomas are rare and heterogeneous cancer of mesenchymal origin. Treatment options have largely remained unchanged over decades and metastatic sarcoma is essentially incurable. There are more than 60 distinct diagnostic subtypes of sarcoma, many of which do not harbor consistent mutations. These factors make the development of targeted therapy particularly challenging in sarcomas. Therefore, a new therapeutic approach broadly targeting sarcomas of various subtypes is highly desirable. Immunotherapy offers a viable approach in cancer treatment, which has not been adequately explored in sarcomas. The immune system can distinguish malignant from normal cells and mount an antitumor T-cell response. However, tumors create a highly immunosuppressive microenvironment, which counteracts both endogenous as well as therapy-induced antitumor immune responses. Therefore, understanding the cellular and molecular basis of immunosuppression in sarcoma microenvironment is important for the development and success of any immunotherapeutic approach.
Macrophages (MACs) and dendritic cells (DC) are antigen-presenting cells (APC) of the innate immune system that play key role in shaping immune responses. Studies in carcinomas have shown that immunosuppressive MACs greatly outnumber immunostimulatory DCs in the tumor microenvironment. Currently, it is unclear whether sarcomas also harbor such polarized distribution of APC in their microenvironment. Tumor-associated MACs can develop from circulating monocytes or from preexisting tissue macrophages. Likewise, DCs can develop from circulating monocytes or bone marrow-derived precursors. However, the origin of sarcoma-infiltrating MACs and DCs is unclear. Insights into the distribution and origin of tumor-infiltrating APCs is critical for devising therapeutic approaches targeting these cells to counteract immunosuppression in tumor microenvironment. We have addressed this issue by using several distinct murine models of sarcomas.
Based on genetic background sarcomas can be divided into (1) translocation-associated sarcomas, (2) sarcomas harboring mutations in oncogenes and tumor suppressors, and (3) sarcomas with complex karyotypes displaying significant genomic instability. We selected one murine model to represent each of the three subtypes. An SYT-SSX fusion oncogene-driven mouse model of synovial sarcoma represented group 1, loss of P53 coupled with activation of oncogenic KRAS-driven mouse model of undifferentiated pleomorphic sarcoma represented group 2, and syngeneic transplant of sarcoma cell lines derived from methylcholanthrene-induced fibrosarcomas represented group 3. In all three murine models, APCs were found to comprise the majority of leukocyte infiltrates in tumors. Using multiparametric flow cytometry, gene-expression profiling, and lineage labeling we found that the vast majority of APCs in sarcoma were MACs with DCs comprising a very small subset. Next, using a combination of in vivo lineage tracing and lineage ablation we uncovered that the majority of MACs and DCs were derived from circulating monocytes. These results suggest that monocytes preferentially differentiate into macrophages but not DCs in sarcoma microenvironment.
These findings lay the groundwork for future efforts in our laboratory aimed at understanding the molecular basis for such polarized macrophage differentiation in the tumor microenvironment. The overarching goal is to translate our findings into therapeutic approaches aimed at increasing the frequency of immunostimulatory DCs while reducing immunosuppressive MACs in sarcoma microenvironment. The clinical benefits of such an approach will be two-fold: (1) reduced immunosuppression in the tumor microenvironment increasing the efficacy of therapeutically generated T cells, and (2) increased probability of generating endogenous antitumor immune responses via the activity of DCs in the tumor microenvironment.
Citation Format: Samir Devalaraja, Ian Folkert, Minghong Li, Yuma Tada, Kanika Jain, Malay Haldar. Sarcoma microenvironment blocks DC but promotes macrophage differentiation from tumor-infiltrating monocytes [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr B26.