Innate Immune Modulation via TLR3 Activation Suppresses Orthotopic Oral Squamous Cell Carcinoma Growth

Although immune checkpoint inhibition has proven highly effective in patients with metastatic or advanced squamous cell carcinomas, such as oral cancer, the role of innate immune responses in regulating oral cancer progression remains poorly understood. In this study, we examined the impact of innate immune responses in an orthotopic oral squamous cell carcinoma model (NR-S1-Luc cells) using bioluminescence imaging. In severe combined immunodeficiency mice lacking adaptive immune cells, CD11b⁺ Ly6G^hi Ly6C^hi (Ly-6G⁺) myeloid cells accumulated more prominently than CD11b⁺ Ly6G^int Ly6C^hi (Ly-6C⁺) cells within NR-S1-Luc tumors. Although Ly-6G⁺ myeloid cells were predominant in orthotopic NR-S1 tumors, in vivo depletion of Ly-6G⁺ cells did not alter tumor growth. Treatment with a Toll-like receptor 3 (TLR3) agonist (poly I:C) significantly inhibited tumor growth, accompanied by enhanced inflammation and upregulation of Ly-6C expression on Ly-6G⁺ myeloid cells. Moreover, depletion of Ly-6G⁺ cells partially impaired this TLR3-mediated effect. Transcriptomic analysis of Ly-6G⁺ myeloid cells from NR-S1-Luc tumor-bearing mice revealed that poly I:C treatment induced functional reprogramming of these cells, accompanied by broad alterations in immune-related and metabolic pathways within the orthotopic oral tumor microenvironment. Collectively, these findings suggest that tumor-infiltrating Ly-6G⁺ myeloid cells are functionally plastic and can be reprogrammed by TLR3 stimulation, thereby contributing to the regulation of tumor progression.