(27) Oral Microbiome as a Therapeutic Modulator in OSCC: From Mechanism to Immunotherapy

Abstract: Oral squamous cell carcinoma (OSCC) represents approximately 4.5% of all malignancies worldwide and is characterized by a stagnant 5-year survival rate of 63%, underscoring the need for deeper insights into its pathogenesis and treatment. Emerging evidence suggests that the oral microbiome plays a pivotal role in shaping the tumor microenvironment (TME), modulating immune responses and influencing treatment efficacy. This study investigates the cellular and molecular interactions among the oral microbiome, immune system, and cancer cells in OSCC development and progression, and evaluates the impact of microbial composition and functionality on response to immunotherapy.

Using the 4-nitroquinoline-1-oxide (4-NQO)-induced OSCC model in both Specific Pathogen-Free (SPF) and Germ-Free (GF) mice, we demonstrate that the presence of a microbiome enhances tumor progression and fosters an immunosuppressive microenvironment. Flow cytometry and gene expression analyses revealed a marked increase in FoxP3+ regulatory T cells, neutrophils, and immunosuppressive cytokines (TGF-β, IL-10), alongside a reduction in cytotoxic CD8+ T cells in SPF mice compared to GF counterparts. Histopathological evaluation confirmed that GF mice developed significantly fewer and smaller lesions, with delayed progression.

To assess microbial contributions at the functional level, conditioned media derived from OSCC-associated bacterial communities were applied to murine OSCC cell lines, resulting in altered expression of PD-L1 and BCL2 — key regulators of immune evasion and cell survival — suggesting bacterial metabolites as modulators of tumor behavior. In parallel, anti-PD1 immune checkpoint inhibitor (ICI) treatment significantly mitigated tumor progression, restored cytotoxic T cell infiltration, and decreased immunosuppressive markers in SPF mice. Notably, microbial profiling post-ICI therapy revealed significant shifts in bacterial diversity and taxa composition, supporting a bidirectional interaction between therapy and microbiota.

Outcomes were corroborated using powered statistical analyses (t-tests, ANOVA), with all in vivo and in vitro experiments performed in triplicate. A temporal analysis further delineated dynamic shifts in immune cell populations and microbial communities during tumor progression and under ICI treatment. Patient-derived data from ongoing clinical sampling are being integrated to validate murine findings and explore microbial and immune signatures as potential biomarkers for prognosis and therapeutic responsiveness.

These findings highlight the oral microbiome as a key modulator of OSCC development and immunotherapy efficacy. Our results support a causal link between microbial presence and immunosuppression, and demonstrate that manipulating the microbiota alters tumor progression and therapy outcomes. This study lays the groundwork for microbiome-based diagnostic tools and adjunct therapeutic strategies in OSCC.

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