In vitro and in vivo characterization of tumor-educated human monocytic myeloid-derived suppressor cells.

Tumor-driven immune suppression can arise from the recruitment and activation of immunosuppressive cell populations, including myeloid-derived suppressor cells (MDSCs). MDSCs are immature myeloid cells that accumulate in tumor microenvironment and dampen the antitumor immune response. However, the mechanisms of MDSC induction remain poorly understood. A greater understanding of their induction and characterization will enable the development of treatments with the potential to combat tumor-induced immune suppression. We characterized the phenotype and function of human tumor cell line (SKMEL5) educated monocytic MDSCs (M-MDSC) in vitro and in vivo. M-MDSCs were generated via coculture of human PBMCs with SKMEL5 tumor cells. The induced suppressive cells were assessed with respect to their cell surface phenotype, cytokine secretion profile, and T-cell suppression (measured by 3H-Thymidine proliferation assay and IFNy secretion). We further identified molecular markers that distinguish M-MDSCs from naive CD33 myeloid cells. In vivo, humanized mice bearing SKMEL5 tumors were characterized for infiltration of immune cells. Results from in vitro model indicated that tumor cell line educated M-MDSCs expressed low levels of HLA-DR and CD86, high levels of CD33, CD11b and suppressed both CD8+T-cell proliferation and IFNy secretion. Comparing M-MDSC and naive CD33 myeloid gene expression revealed upregulation of IL-10, PDL1/2, and VEGFR1 in M-MDSCs. Furthermore, M-MDSC expressed high levels of two new drug targets, immunoglobulin-like transcript 3 (ILT3, also known as LILRB4) and immunoglobulin-like transcript 4 (ILT4 also known as LILRB2). We investigated how ILT-targeting could modulate M-MDSC cell function. In addition, In vivo results from humanized mice bearing SKMEL5 tumors demonstrated expansion of multiple subtypes of tumor-associated myeloid cells with increased ILT3 and ILT4 expression. These human MDSC models provide a platform for building biologic understanding of emerging myeloid targets expressed in pembrolizumab nonresponders and tumors rich in myeloid-suppressive cells. Citation Format: Latika Singh, Peter Stivers, Anthony Palmieri, Mark Zhang, Barbara Joyce-Shaikh, Jie Zhang-Hoover, Yujie Qu, Alan Byford, Michael Meehl, Philip Brandish. In vitro and in vivo characterization of tumor-educated human monocytic myeloid-derived suppressor cells [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B84.