Single-cell RNA and T cell receptor (TCR) sequencing in patients with HPV-positive head and neck squamous cell carcinoma (HNSCC) after induction CTLA4 and PD-1 immune checkpoint blockade (ICB).

6056 Background: Combination CTLA-4 and PD-1 ICB results in durable responses in patients with recurrent and metastatic HNSCC and therefore justifies further investigation in the induction or neoadjuvant setting. Methods: We investigated features of the tumor immune microenvironment (TIME) associated with MHR after induction CTLA-4 and PD-1 ICB in patients with newly diagnosed HPV-positive HNSCC. Tumor samples were collected at baseline and on treatment from 31 patients enrolled in a phase 2 trial of a 6-week cycle of CTLA-4 and PD-1 ICB induction followed by dose/volume-adapted IMRT (50-66Gy) concurrent with cycle 2. Tumors were evaluated for histological response and single-cell RNA sequencing (scRNA-seq) and scTCR-seq were performed on the 10X genomics platform. After stringent clustering and annotation of scRNA-seq data, 76,319 CD8 + and 78,622 CD4 + T cells with paired TCR sequence data were evaluable for transcriptomic, TCR repertoire and clonotype differentiation analysis. Results: Percent reduction in tumor viability on histology correlated significantly with reduction in tumor cell proportion in scRNA-seq data (R 2 = 0.7). Thirteen (48%) of 27 patients with paired samples (missing data due to COVID) had MHR, defined as ≤10% residual tumor viability on ICB treatment. Through TCR tracing, over 100 T cell clonotypes that expanded significantly (p < 0.05, Fisher’s exact) post-ICB were identified. Highly expanded clonotypes resided mainly in exhausted CD8 + T cell clusters and exhibited a tissue resident memory (TRM) phenotype. Examination of their functional states revealed a variety of T cell modulatory trajectories, including reverse transition from exhausted to less exhausted states and from memory to functional states. When compared to patients without MHR, those with MHR had significantly higher ICB-induced TCR clonotype expansion (p = 0.025, Wilcoxon), reductions of activated Tregs (p = 0.003, Wilcoxon) and terminally exhausted CD8 + T cells (p = 0.003, Wilcoxon), and increase of polyfunctional CD8+ T cells (p = 0.014, Wilcoxon). A lasso regression model associated T cell expansion, basal 7-gene TRM and 5-gene effector scores, and on-treatment proliferation score and effector cell proportion with percent reduction in tumor viability (R 2 = 0.66). Conclusions: In HPV-HNSCC, induction CTLA4 and PD1 ICB lead to extensive reinvigoration of CD8 + T cell clonotypes with both an exhausted and TRM phenotype, revealing a high degree of plasticity of distinct T cell clones. Paired RNA and TCR profiling facilitated identification of potentially tumor reactive and ICB-responsive T cells and clonotypes and identified several TIME features associated with tumor cell death. Clinical trial information: NCT03799445 .