Tumor-derived DLK1 restrains tumor progression through remodeling of the tumor immune microenvironment.

The tumor immune microenvironment (TIME) plays a pivotal role in tumor progression and therapeutic responsiveness, yet the tumor-derived factors governing immune remodeling remain incompletely understood. Here, we identified delta-like homolog 1 (DLK1), a noncanonical Notch ligand, as a candidate regulator of the TIME through comparative transcriptomic analyses of murine tumors. DLK1 expression progressively declined during tumor progression and was inversely associated with tumor hypoxia. The hypoxia-responsive microRNA miR-338-5p contributed, at least in part, to the downregulation of DLK1 expression. Ectopic expression of DLK1 significantly suppressed the growth of B16 melanoma and LLC1 lung carcinoma tumors in syngeneic mice without substantially affecting tumor cell proliferation, apoptosis, or stem-like properties in vitro, consistent with a predominantly tumor cell-extrinsic mechanism. DLK1 expression was associated with immunostimulatory remodeling of the TIME, characterized by increased T cell infiltration and activation, reduced accumulation of myeloid-derived suppressor cells, and activation of tumor-associated macrophages toward an immunostimulatory phenotype. Consistently, recombinant DLK1-Fc directly enhanced macrophage activation and pro-inflammatory cytokine production in a contact-dependent manner. Collectively, our findings identify DLK1 as a previously unrecognized tumor-derived regulator of the TIME and suggest that hypoxia-driven loss of DLK1 may contribute to the establishment of an immunosuppressive microenvironment during tumor progression.