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Head and neck squamous cell carcinoma (HNSCC) has a low five-year survival rate because of its high rate of recurrence and metastasis. After surgical resection or radiation, the main treatments for HNSCC, patients sometimes experience functional or aesthetic disorders. Therefore, there is a great demand for the development of non-surgical treatment strategies to improve clinical outcomes and patients' quality of life. One such non-surgical treatment is mild hyperthermia (mHT). Many studies have investigated combination treatments with mHT and immune checkpoint inhibitors in preclinical settings. However, there have been no detailed reports on the effects of mHT on immune checkpoint molecules. Here, we investigated the effects of mHT on the tumor microenvironment (TME), particularly on programmed cell death receptor-1 (PD-1)/programmed cell death ligand-1 (PD-L1), in SCCVII cells and a squamous cell carcinoma mouse model. First, we found that PD-L1 mRNA levels and surface PD-L1 expression significantly increased after mHT. Second, a single tumor model was used to determine the effect of HT on the TME. mHT enhanced the accumulation of CD4+ and CD8+ T cells, elevated PD-L1 expression in the TME, and decreased the PD-1 positive rate of CD4+ T cells. Finally, using a bilateral tumor model, we found that anti-PD-L1 monotherapy and combination therapy resulted in longer survival than the isotype control or mHT monotherapy. Moreover, the combination therapy resulted in a significantly higher survival rate than anti-PD-L1 monotherapy. In conclusion, our findings elucidate changes in PD-L1 expression in the TME and strengthen the rationale for mHT and PD-L1 blockade combination therapy. [ABSTRACT FROM AUTHOR]