Hyperthermia Enhances the Effects of ICI by Increasing PD-L1 Expression Through Induction of CD8 + Cell Infiltration

Background/Aim: Hyperthermia induces a multifactorial immune cascade reaction, potentially enhancing pharmacological effects of immune checkpoint inhibitors (ICIs) and overcoming limitations in their clinical application. This study aimed to clarify how hyperthermia affects tumor immunity and enhances the efficacy of ICIs. Materials and Methods: Expression of phospho-signal transducer and activator of transcription 1 (p-STAT1), interferon regulatory factor-1 (IRF1), and programmed cell death 1 ligand 1 (PD-L1) was assessed on PD-L1-expressing MC-38 mouse colorectal cancer cells under hyperthermia or interferon gamma (IFN-γ) treatment. Effects of hyperthermia on p-STAT1, IRF1, CD8, and PD-L1 were also assessed in vivo. Antitumor effects and infiltrating CD8+ cells were evaluated in mice treated with PD-L1 antibody, hyperthermia, or a combination of the two. Immunohistochemistry was performed to assess PD-L1 expression and CD8+ cell infiltration in samples from a patient after treatment with hyperthermia for breast cancer and a patient after hyperthermia and ICI treatment for Lynch syndrome-related colorectal cancer. Results: Expression of p-STAT1, IRF1, and PD-L1 was enhanced in vitro by IFN-γ treatment but not by hyperthermia. Hyperthermia enhanced the expression of p-STAT1, IRF1, CD8, and PD-L1 in vivo. The combination therapy of hyperthermia and PD-L1 antibody demonstrated a greater antitumor effect than either therapy alone. Infiltration of CD8+ T-cells was significantly increased by hyperthermia compared to the control and was even more strongly increased by treatment using hyperthermia combined with PD-L1 antibody. PD-L1 expression was mildly elevated in the sample from the patient with breast cancer and strongly elevated in that from the patient with colorectal cancer; CD8+ cell infiltration was also elevated significantly in both. Conclusion: Hyperthermia potentially enhances PD-L1 expression by increasing the efficiency of CD8+ cell infiltration into tumors and induces a strong antitumor effect in combination with an ICI.