Circulating CD137⁺Treg cells and LOX-1⁺PMN-MDSCs as biomarkers of immunotherapy resistance in (R/M) HNSCC patients

Background: Recurrent/metastatic head and neck squamous cell carcinoma ((R/M) HNSCC) represents one of the most aggressive and immunosuppressive cancers. Despite the introduction of immune checkpoint inhibitors (ICIs), only a limited number of patients obtain long-term benefits. In (R/M) HNSCC patients, the antitumor immune response is defective, conferring resistance and promoting tumor progression. Therefore, the identification of novel biomarkers for superior clinical outcomes and easily accessible in standard clinical settings is still an unmet clinical need. Methods: Blood liquid biopsies obtained from (R/M) HNSCC patients undergoing pembrolizumab therapy (monotherapy or in combination with chemotherapy) were analyzed by flow cytometry to evaluate the levels of circulating immunosuppressive regulatory T cells (Tregs) and myeloid derived suppressor cells (MDSCs), at baseline and during therapy. Correlations between these immunosuppressive immune cell subsets and clinical parameters (clinical response rate, progression-free survival (PFS), overall survival (OS) and performance status (PS)) were performed. Results: Univariate analysis showed that before therapy, higher circulating levels of both CD137⁺Tregs and LOX-1⁺PMN-MDSCs, identified patients with significantly worse survival. Furthermore, CD137⁺Tregs resulted also positively correlated with worse PS, while high levels of LOX-1⁺PMN-MDSCs negatively affected response to pembrolizumab, with a significant increase in non-responsive patients during therapy. Interestingly, both CD137⁺Tregs as well as LOX-1⁺PMN-MDSCs exerted a higher immunosuppression on T cell proliferation than CD137-Tregs and LOX-1⁻PMN-MDSCs, respectively. Multivariate analysis revealed that the circulating LOX-1⁺PMN-MDSC subset resulted as an independent prognostic factor for survival by multivariate analysis, as confirmed in an independent validation cohort. Conclusions: The levels of blood circulating LOX-1⁺PMN-MDSCs may be proposed as non-invasive biomarkers to predict clinical outcomes of (R/M) HNSCC patients developing resistance to immunotherapy, improving patient selection and suggesting novel personalized therapies.