Galectin-9 regulates dendritic cell polarity and uropod contraction by modulating RhoA activity.

Adaptive immunity relies on dendritic cell (DC) migration to transport antigens from tissues to lymph nodes. Galectins, a family of β-galactoside-binding proteins, control cell membrane organization, exerting crucial roles in multiple physiological processes. Here, we report a novel mechanism underlying cell polarity and uropod retraction by demonstrating that galectin-9 regulates basal and chemokine-driven DC migration in humans and mice. Galectin-9 depletion caused a defect in RhoA signaling that resulted in impaired cell rear contractility. Mechanistically, galectin-9 interacts with and organizes CD44 at the cell surface, in turn modulating RhoA binding to GEF-H1 and the initiation of downstream signaling. Analysis of DC motility in the 3D tumor microenvironment revealed galectin-9 is also required for DC recruitment and infiltration. Exogenous galectin-9 rescued the motility of tumor-immunocompromised human blood DCs, validating the physiological relevance of galectin-9 in DC migration. Our results identify galectin-9 as a necessary mechanistic component for DC motility by regulating cell polarity and contractility, and underscore its implications for DC-based immunotherapies.