SUMMARY, Project 2 Radiation therapy (RT) is commonly used for locally recurrent head and neck squamous cell carcinoma (HNSCCs), yet no standard concomitant systemic therapy exists, and RT resistance rates are high. Antibodies against programmed death-1 (PD-1) are FDA approved for treating relapsed/recurrent HNSCCs, but the response rate is low. RT induces anti-tumor immunity by causing DNA damage and tumor cell killing that release neoantigens to trigger an “in situ tumor vaccine” and activation of STING (stimulator of IFN genes) for local and systemic immune activation. Conversely, RT also induces transforming growth factor-β1 (TGFβ1), an immune suppressor, and PD-L1, a ligand of PD-1. These RT effects make dual TGFβ/PD-L1 inhibition a rational combination being tested in this project. We have reported that TGFβ1 is elevated in >60% of tobacco-associated HNSCCs. TGFβ1-mediated DNA repair contributes to RT resistance. TGFβ1 also contributes to RT-induced toxicity, e.g., oral mucositis and fibrosis. Using our mouse HNSCC models, we found that TGFβ/PD-L1 dual inhibition eradicated SCCs better than anti-PD-L1 alone in tumors with high TGFβ1 levels and high numbers of PD-L1+/CD11b+ cells. We also found that TGFβ inhibition reduced metastases in athymic mice correlated with reduced CD11b+ myeloid cells. We hypothesize that in advanced HNSCCs, TGFβ/PD-L1 dual inhibition enhances RT-induced in situ vaccination, reverses immune suppression, and overcome RT resistance via T cell-dependent and -independent mechanisms. We will test this hypothesis with experimental therapeutics, mechanistic studies and analyses of HNSCC patient specimens. Aim 1 will determine if TGFβ/PD- L1 dual inhibition enhances RT-induced in situ vaccination and systemic immune activation in oral SCC mouse models. Experimental therapeutics of RT plus TGFβ/PD-L1 dual inhibition will be performed using mouse SCC lines transplanted orthotopically to syngeneic mice, and T-cell dependent anti-tumor mechanisms will be analyzed at the cellular and molecular levels. Aim 2 will determine how RT regimens in combination with TGFβ/PD-L1 inhibition target tumor epithelial death and myeloid cells in mouse and human HNSCC models. T cell-independent therapeutic benefit of RT in combination with TGFβ/PD-L1 inhibition will be analyzed. Aim 3 will conduct a Phase Ib trial for RT with M7824 (TGFβ/PD-L1 bidirectional inhibitor) in locally recurrent and oligometastatic HNSCC patients and identify cellular and molecular markers as therapeutic targets. By performing the proposed studies, we aim to bring a therapeutic intervention in real time to simultaneously enhance immunotherapy and reduce RT resistance in HNSCC patients with poor prognosis. Additionally, identifying predictive markers to the proposed treatment will lead to a true biomarker-driven Phase II trial with pre-selected patients.