Galectin-9 Mediates the Functions of Microglia in the Hypoxic Brain Tumor Microenvironment

Galectin-9 is a multifaceted regulator of various pathophysiological processes that exerts positive or negative effects in a context-dependent manner. Here, we elucidated the distinctive functional properties of galectin-9 on myeloid cells within the brain tumor microenvironment. Galectin-9-expressing cells were abundant at the hypoxic tumor edge in the tumor-bearing ipsilateral hemisphere compared to the contralateral hemisphere in an intracranial mouse brain tumor model. Galectin-9 was highly expressed in microglia and macrophages in tumor-infiltrating cells. In primary glia, both the expression and secretion of galectin-9 were influenced by tumors. Analysis of a human glioblastoma bulk RNA-sequencing dataset and a single-cell RNA-sequencing dataset from a murine glioma model revealed a correlation between galectin-9 expression and glial cell activation. Notably, the galectin-9high microglial subset was functionally distinct from the galectin-9neg/low subset in the brain tumor microenvironment. Galectin-9high microglia exhibited properties of inflammatory activation and higher rates of cell death, whereas galectin-9neg/low microglia displayed a superior phagocytic ability against brain tumor cells. Blockade of galectin-9 suppressed tumor growth and altered the activity of glial and T cells in a mouse glioma model. Additionally, glial galectin-9 expression was regulated by Hif-2α in the hypoxic brain tumor microenvironment. Myeloid-specific Hif-2α deficiency led to attenuated tumor progression. Together, these findings reveal that galectin-9 on myeloid cells is an immunoregulator and putative therapeutic target in brain tumors.