Head and neck squamous cell carcinoma (HNSCC) is a deadly and disfiguring disease that will accounted for more than 800000 cases worldwide in 2023. The sixth most common cancer worldwide, HNSCC continues to increase, in part due to human papillomavirus (HPV) associated oropharynx cancers. The current standard therapeutic regimens of surgery, chemotherapy, therapy and anti-PD-1 therapy are associated with significant morbidity and loss of quality of life, with only modest 5-year survival rates. Immune therapy has offered new options for HNSCC patients; treatment with T cell checkpoint inhibitors or therapeutic vaccines has led to gains in survival for a portion of treated patients. However, the majority of HNSCC patients are resistant to these therapies. Improved therapeutic approaches that target mechanisms of treatment escape hold promise for this disease. We have found that HNSCC tumors are abundantly infiltrated by immune suppressive myeloid cells, including monocytes, macrophages, and granulocytes, which inhibit T cell recruitment and activation, leading to immune suppression and resistance to checkpoint inhibitors in HNSCC. Inhibitory targeting of phosphatidylinositol-4,5-bisphosphate 3-kinase, PI3Kgamma (PI3Kg), a myeloid cell specific PI3K isoform, reduces myeloid cell accumulation and converts remaining myeloid cells into pro-inflammatory cells, leading to T cell activation and tumor inhibition in mouse models of HPV+ and HPV- HNSCC tumors. PI3Kg inhibition synergizes with checkpoint inhibitors, stimulating T cell recruitment, activation, and memory formation. Based on these findings, the PI3Kg inhibitor, IPI-549 (eganelisib), was developed as an immune oncology therapeutic and has exhibited anti-tumor responses in Phase 1 and 2 clinical trials for the treatment of newly diagnosed cancer. Spatial transcriptomics of HNSCC patient tissues from clinical trials showed us that PI3Kg antagonism enhances biomarkers of myeloid cell and T cell activation in HNSCC patients. These bench to bedside to bench studies identified novel myeloid cell, T cell and B cell biomarkers of response to PI3Kg inhibitors, as well as biomarkers of treatment resistance. However, they also revealed potential mechanisms of therapeutic resistance. To advance our understanding of PI3Kg inhibition in HNSCC, we propose to test the hypothesis that inhibitory targeting of PI3Kg in tumor associated macrophages promotes both humoral and cellular immune responses that suppress HNSCC tumor cell survival but also activates novel mechanisms of therapeutic resistance. The specific aims of this proposal are: 1) To determine critical molecular and cellular regulators of PI3Kg mediated tumor immune suppression in mouse models of HPV+ and HPV- HNSCC 2) To discover and target mechanisms of resistance to PI3Kg inhibition in models of head and neck cancer.3) To identify circulating biomarkers of response and resistance to PI3Kg inhibition that reflect intratumoral tumor biology in HNSCC pa...