924P Polyfunctional HPV16-Specific T Cell Responses in Subjects Receiving PDS0101 and Pembrolizumab Combination Treatment for Recurrent/metastatic HPV16-positive Head and Neck Squamous Cell Carcinoma (HNSCC)

Generation of multifunctional, anti-tumor T cell responses and immune memory are critical to effective long term control of tumor growth and clinical outcomes. The investigational T cell activating HPV-targeted immunotherapy PDS0101 is being studied in combination with pembrolizumab (NCT04260126) in patients with HPV16-positive head and neck cancer. Biomarkers of response are undetermined. The Isoplexis IsoCode single-cell proteomic assay was utilized to assess the functional state of tumor-specific CD4 and CD8 T cells. Cryopreserved PBMCs were thawed and rested overnight in the presence of IL-2, with magnetic bead column enrichment and surface staining for CD8/CD4 subsets following in vitro stimulation for 16 hrs with an overlapping HPV16 E6/E7 peptide pool. Single-cell production of 32 immune analytes was measured and polyfunctionality was assessed at 3 timepoints: pre-treatment, and 12 and 36 weeks following 4 and 5 cycles of combination therapy, respectively. Eighteen subjects with confirmed HPV16-positive tumors and CPS ≥1 (72% with CPS ≥20), median age 63 yrs (range 46-83), 94% male, 94% immune checkpoint inhibitor (ICI)-naïve, had specimens for this analysis and had received a minimum of 4 cycles of combination therapy. Confirmed objective response (OR; 1 CR and 5PR) was seen in 6 subjects (33%); 10 subjects (56%) had SD and 2 subjects had PD (11%). Polyfunctional T cells expressing 2-5+ proteins were observed in subjects with CPS 1-19 and CPS ≥ 20 and in both CD4 and CD8 subsets. Across timepoints and subjects, the proteins secreted with the highest frequency were Granzyme B, IL-8, IL-9, MIP-1α, MIP-1β and TNFα. There was no difference in polyfunctionality scores in subjects with confirmed OR versus those with stable or progressive disease. The PDS0101/pembrolizumab combination generates polyfunctional CD4 and CD8 T cells that secrete multiple cytokines and chemokines associated with a predominant effector, stimulatory functional profile. Further investigation of T cell polyfunctionality and correlation with clinical outcomes is warranted.