Although some head-and-neck squamous cell carcinomas (HNSCCs) are associated with human papillomavirus (HPV) infection, a large proportion of HNSCCs are HPV-negative and are characterized by mutational signatures associated with smoking, suggesting that HPV-negative HNSCCs are induced by carcinogen exposure. In many human cancers, including HNSCC, regulatory T (Treg) cells are found at elevated densities and are thought to be a major barrier to the generation of robust anti-tumor T-cell responses. However, recent studies indicate that Treg cells can serve diverse functions in non-lymphoid sites. Therefore, it is critical to elucidate the function of Treg cells infiltrating HNSCC lesions. To study the functional impact of Treg cells on the development and progression of HPV-negative HNSCC, we utilized an autochthonous mouse model of carcinogen-induced oral squamous cell carcinoma (SCC), in which mice are exposed long-term to the carcinogen 4-nitroquinoline-1-oxide (4-NQO) in the drinking water. Immunohistochemistry and flow cytometric analysis revealed elevated densities of CD3+ T-cells and Foxp3+ Treg cells in premalignant dysplastic lesions and SCC lesions within the tongue epithelium, suggesting that many lesions exhibit a T-cell-inflamed phenotype. Single-cell TCR sequencing demonstrated the presence of recurrent Treg cell clones within carcinogen-induced lesions, suggesting clonal expansion of Treg cells in the tumor microenvironment. Surprisingly, the systemic depletion of Treg cells at the later stages of carcinogen exposure did not induce tumor regression, but instead induced the opposite effect, driving increased incidence and burden of SCC. These findings suggest that Treg cells may play an unexpected role in restricting the progression of carcinogen-induced SCC in the oral cavity, prompting a re-examination of the common paradigm that Treg cells promote tumorigenesis by suppressing antitumor immunity. Clinically, our findings suggest that novel therapies could be developed to combat HNSCC by triggering the recruitment or activation of tumor-associated Treg cells, and suggest that checkpoint blockade therapies that induce intratumoral Treg cell depletion may have unintended adverse consequences.

Citation Format: Jaime L. Chao, Peter A. Savage. Impact of regulatory T-cells on carcinogenesis of oral squamous cell carcinoma [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A177.