Characterization of the Immune Response to PD-1 Blockade during Chemoradiotherapy for Head and Neck Squamous Cell Carcinoma

SIMPLE SUMMARY: Blockade of the PD-1/L1 interaction represents a breakthrough in the treatment for recurrent/metastatic head and neck squamous cell carcinoma (HNSCC). Clinical and translational research suggests that this interaction may play a role in immune evasion during chemoradiotherapy. Using an immune-competent murine model of HNSCC, we demonstrate improved efficacy of PD-1 blockade with concurrent cisplatin-based chemoradiotherapy. Taking this approach into a clinical trial evaluating the anti-PD-1 agent, pembrolizumab, combined with chemoradiotherapy in HNSCC, we characterized the peripheral blood immune response to therapy. Our findings highlight that this combination is active in the murine model and circulating PD-1+ T-cell proportions were decreased during the clinical trial. However, additional findings from the clinical trial suggest a shift towards expression of other markers of immune exhaustion. As this treatment approach is being explored in large, randomized trials, these findings provide insight into potential pathways for treatment failure. ABSTRACT: Background: Chemoradiotherapy is a standard treatment for HNSCC. Blockade of the PD-1/L1-2 interaction may represent a target to overcome immune escape during this treatment. Methods: Utilizing a HNSCC mEERL C57BL/6 mouse model, we evaluated a PD-1 blockade alone or in combination with cisplatin-based chemoradiotherapy. Next, we evaluated peripheral blood mononuclear cells (PBMCs) with relative PD-1, TIM-3, and LAG-3 expression, and myeloid-derived suppressor-like (MDSC-like) populations from a clinical trial evaluating PD-1 blockade with chemoradiotherapy in HNSCC. Finally, we analyzed the effect of therapy on human T-cell clonality through T-cell Receptor (TCR) sequencing. Results: Anti-PD-1 monotherapy induced no response in the mEERL model; however, combination with chemoradiotherapy improved tumor clearance and survival. PBMCs from patients treated with this combination therapy demonstrate a decline in circulating T-cell populations with knockdown of PD-1 expressing CD3+CD4+ and CD3+CD8+ T cells during treatment. However, TIM-3, LAG-3 expressing T-cell and MDSC-like populations concordantly rose. During treatment, the TCR repertoire demonstrates overall clonal expansion, with both unique and previously reported T-cell clones. Conclusions: Our murine HNSCC model demonstrates efficacy of PD-1 blockade during chemoradiotherapy. However, while PD-1-expressing T cells decreased with this therapy, human PBMC findings also identified an increase in populations contributing to immune exhaustion. These findings further characterize PD-1 blockade during chemoradiotherapy for HNSCC and highlight potential competing mechanisms of immune evasion.