Abstract 452: Monocytic Gr-1+/CD11b+ myeloid cells are necessary for natural killer cells to eradicate glioma and are inhibited by tumor-derived galectin-1
Authors: Baker Gregory J. , Chockley Peter , Zamler Daniel , Yadav Viveka Nand , Castro Maria G. , Lowenstein Pedro R. 
Published: August 2015

Abstract
We have recently demonstrated that natural killer (NK) cells eradicate galectin-1 (gal-1)-deficient glioma (Baker et al, Cancer Research. 2014. Sep 15;74(18):5079-90). Here we present our progress towards understanding the cellular mechanism(s) by which NK cells eradicate such tumors. We demonstrate that monocytic Gr-1+/CD11b+ myeloid cells are required to stimulate NK-mediated lysis of gal-1-deficient glioma. Immunodepletion of Gr-1+ cells in RAG1-/- mice permits lethal gal-1-deficient glioma formation despite the presence of NK cells, demonstrating that Gr-1+ cells (i.e. myeloid cells) are necessary for NK-mediated tumor rejection in vivo. We also demonstrate that the effect of glioma-derived gal-1 is to conceal glioma cells from recognition by NK cells by inhibiting myeloid-NK cell crosstalk. In vitro experiments reveal that gal-1-deficient glioma cells stimulate cross-activation between NK cells and monocytic Gr-1+/CD11b+ myeloid cells, causing the myeloid cells to morph into a phenotypically distinct cell population reminiscent of macrophages or dendritic cells. Cell-killing assays show that monocytic Gr-1+/CD11b+ myeloid cells significantly enhance NK-mediated glioma cell lysis in vitro. Recombinant mouse gal-1 protein inhibits the myeloid cell enhancement of NK-mediated tumor lysis, but fails to suppress intrinsic NK-mediated tumor lysis. This result strongly suggests that the role of glioma-derived gal-1 is to suppress the ability of monocytic myeloid cells to stimulate cytotoxic potential in NK cells. Further in vivo experiments also reveal that RAG1-/- mice bearing gal-1-deficient glioma have 7-fold more Gr-1+/CD11b+ myeloid cells present within the tumor microenvironment 48hrs after tumor implantation compared to gal-1-expressing tumors. Together our data show that glioma-derived gal-1 acts to potently suppress anti-glioma NK immune surveillance through a tripartite mechanism involving: (1) the inhibition of myeloid cell recruitment into the brain tumor microenvironment, (2) the suppression of enhanced NK-mediated glioma lysis stimulated by monocytic myeloid cells, and (3) an increased resistance to NK-mediated glioma cell lysis. We now aim to identify the molecular factors used by gal-1-deficient glioma cells to stimulate monocytic myeloid cell activation and the factors that myeloid cells use to in turn amplify NK cell cytotoxicity.
Citation Format: Gregory J. Baker, Peter Chockley, Daniel Zamler, Viveka Nand Yadav, Maria G. Castro, Pedro R. Lowenstein. Monocytic Gr-1+/CD11b+ myeloid cells are necessary for natural killer cells to eradicate glioma and are inhibited by tumor-derived galectin-1. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 452. doi:10.1158/1538-7445.AM2015-452