Abstract 6548: Tumor microenvironment of previously irradiated recurrent head and neck squamous cell carcinoma in humans

Abstract With increasing annual incidence, head and neck squamous cell carcinoma (HNSCC) is the most common head and neck cancer histology. Approximately half of HNSCC patients will develop recurrent disease. It is, therefore, important to understand the mechanism(s) and factors that may affect patient outcomes including the tumor microenvironment (TME). Previous studies have shown that patients with more TCF1+ PD-1+ CD8+ cells in the tumor prior to anti-PD-1 have superior outcomes. It is unknown whether these cells are present in recurrent tumors following radiation and what prognostic role they serve when patients receive concurrent radiation and anti-PD-1 (nivolumab). In our prior work, we evaluated PBMCs from recurrent HNSCC patients receiving reirradiation and nivolumab and correlated findings with progression free survival (PFS). Results indicated a shorter PFS for patients with a 1.5-fold increase in peripheral blood PD1+Ki67+CD4+ T cells from baseline to week 2 or 4 of treatment, suggesting the further need to assess potential biomarkers in the tumor. Thus, in this study, we provide a phenotypic analysis of cells present in recurrent previously irradiated tumors at baseline and plan to correlate with patient outcomes. Retrospective analysis of patient TME prior to receiving reirradiation and nivolumab was assessed utilizing multiplex immunofluorescence (mIF). T cells within the TME of previously irradiated disease revealed that 48.64% of CD8+ cells were PD-1+ at the time of resection (N=20, range=8.050%-62.16%). Of the PD-1+ CD8+ T cells, a higher percentage of effector TCF1-PD1+CD8+ cells (median= 58.94%, N=20, range= 28.43%-85.90%) were present compared to stem-like TCF1+PD1+CD8+ cells (median= 41.06%, N=20, range= 14.10%-71.57%). Likewise, cell density per mm2 for effector TCF1-PD1+CD8+ cells (median=46.01, N=20, range= 1.006-265.9) was higher compared to stem-like TCF1+PD1+CD8+ cells (median= 30.81, N=20, range=0.7669-142.7). In parallel, a prospective analysis of PBMCs of standard of care HNSCC patients with no prior history of treatment was assessed longitudinally during radiation by flow cytometry. We found the percentage of CD8+ T cells remained relatively stable throughout radiation treatment though the percentage of CD8 TEMRA cells increased 1.2-fold from baseline to 3 months post treatment. Naïve CD8 T cells, in contrast, experienced a 2-fold decrease from baseline to 3 months post treatment after an initial increase during treatment. Activated CD8 T cells (CD38+ HLA-DR+) decreased gradually from baseline to week 3 of treatment before rebounding 3 months post treatment. These data suggest that previously irradiated tumors still possess the T cell population necessary to mount a robust anti-PD-1 and anti-tumor immune response. Additionally, H&N radiation has an impact on circulating T cell populations in the absence of immunotherapy. Citation Format: Tarralyn Y. Clark, Jeromy Dooyema, Nabil Saba, Shlomo Koyfman, Haydn Kissick, Nicole C. Schmitt, Christine Lee-Poturalski, Tim Chan, David Yu, Zachary S. Buchwald. Tumor microenvironment of previously irradiated recurrent head and neck squamous cell carcinoma in humans [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6548.