Galectin-8 is a major ligand of LILRB4 prompting MDSC functions in the tumor microenvironment

The LILRB4 myeloid receptor has been implicated in an immunosuppressive microenvironment, with specific antibodies under preclinical or clinical development for tumor immunotherapy. However, it remains largely unknown which natural ligand may trigger LILRB4 to expand myeloid derived suppressive cells (MDSC), and the relevant downstream signaling pathways are also under debate. Here we show that Galectin-8 is a high-affinity functional ligand of LILRB4, and its ligation induces MDSC by activating STAT3 as well as inhibiting NF-{kappa}B. Importantly, Galectin-8 but not APOE could induce MDSC, and both ligands bind LILRB4 in a non-competitive manner. Antibodies recognizing a defined epitope on LILRB4 could efficiently block Galectin-8 binding and neutralize its effects on MDSC induction and relevant signaling pathways. Galectin-8 expression promoted B16 tumor growth in mice, and knockout of LILRB4 attenuated tumor growth in this context. The LILRB4-specific Galectin-8 blocking antibody efficiently suppressed MDSC expansion and tumor growth in vivo. These results identify Galectin-8 as a functionally important ligand of LILRB4, highlighting the blockade of LILRB4-Galectin-8 interaction as a promising strategy for cancer immunotherapy.