Abstract A02: Altered myeloid cells in the tumor microenvironment promote growth of T cell acute lymphoblastic leukemia.

Though many cell-intrinsic genetic and genomic alterations contributing to tumorigenesis of T cell acute lymphoblastic leukemia/lymphoma (T-ALL) have been uncovered, the contributions of non-tumor cells and their associated signals in the tumor microenvironment have remained obscure. To address whether the tumor microenvironment contributes to T-ALL tumor growth, we first analyzed the thymic microenvironment during T-ALL progression in murine models of T-ALL. The cellularity and architecture of the thymic stroma becomes increasingly aberrant during initiation and progression of disease, such that tumor cells accumulate and proliferate in epithelial-free regions at the periphery of the thymus. In addition to proliferating tumor cells, these thymic epithelial free regions contain vasculature, fibroblast networks, and an accumulation of dendritic cells. Interestingly, a subset of conventional thymic dendritic cells is greatly expanded in tumor-bearing thymi. To determine if cellular subsets in the tumor microenvironment contribute to T-ALL growth, we established a co-culture system containing primary T-ALL cells and purified stromal subsets taken directly from the tumor microenvironment. We found that ex vivo lymphoma cells require the presence of tumor-associated stroma for survival and proliferation. Strikingly, tumor-associated myeloid cells are necessary and sufficient to support ex vivo lymphoma survival, with tumor-associated dendritic cells most potently promoting T-ALL growth. Tumor-associated dendritic cells, but not wild-type thymic dendritic cells, are uniquely capable of supporting T-ALL survival. Global gene expression profiling reveals that tumor-associated dendritic cells exhibit limited altered gene-expression profiles, such that they upregulate transcripts associated with tumor-associated macrophages. Importantly, we find that T-ALL cells from metastatic tumor sites retain their dependence on stromal cells from the tumor microenvironment, and dendritic cells from metastatic tumors are sufficient to promote T-ALL survival. Thus, dendritic cells both at the primary tumor site and at sites of disseminated disease are sufficient to support growth of T-ALL cells. To determine if T-ALL cells preferentially contact dendritic cells in an intact thymic microenvironment, we used 2-photon microscopy to visualize interactions of thymic dendritic cells with T-ALL cells versus wild-type thymocytes. T-ALL cells interact with thymic dendritic cells with increased frequency and duration relative to wild-type thymocytes. Taken together, these studies strongly implicate endogenous myeloid cells, and particularly dendritic cells in the tumor microenvironment as potent stimulators of T-ALL growth, and suggest tumor-associated dendritic cells or their associated mitogenic signals could serve as novel therapeutic targets. Citation Format: Todd A. Triplett, Kim T. Cardenas, Jessica N. Jones, Hilary J. Selden, Guadalupe J. Jasso, Sadhana Balasubramanyam, Zicheng Hu, Li LiQi, Paul E. Love, Lauren IR Ehrlich. Altered myeloid cells in the tumor microenvironment promote growth of T cell acute lymphoblastic leukemia. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr A02.