Combination Treatment of Topical Imiquimod Plus Anti-PD-1 Antibody Exerts Significantly Potent Antitumor Effect

SIMPLE SUMMARY: Imiquimod (IMQ), a synthetic ligand of Toll-like receptor 7, is known to exert antitumor effects. However, the exact mechanisms of the IMQ-induced antitumor effect have not been fully understood. Although both topical IMQ treatment and anti-PD-1 antibody may be used for primary skin lesions or skin metastases of various cancers, the efficacy of each monotherapy for these lesions is insufficient. In our study using a murine tumor model, we show that IFN-γ produced by CD8(+) T cells may play a crucial role in the IMQ-induced antitumor effect. In addition, IMQ upregulated PD-1 expression in T cells as well as PD-L1/PD-L2 expression in myeloid cells. Furthermore, we also found that combination therapy of topical IMQ with anti-PD-1 antibody exerted a significantly potent antitumor effect when compared with each single therapy. Therefore, the combination therapy of topical IMQ plus anti-PD-1 antibody is promising therapy for skin cancer. ABSTRACT: The exact mechanisms of the imiquimod (IMQ)-induced antitumor effect have not been fully understood. Although both topical IMQ treatment and anti-PD-1 antibody may be used for primary skin lesions or skin metastases of various cancers, the efficacy of each monotherapy for these lesions is insufficient. Using a murine tumor model and human samples, we aimed to elucidate the detailed mechanisms of the IMQ-induced antitumor effect and analyzed the antitumor effect of combination therapy of topical IMQ plus anti-PD-1 antibody. Topical IMQ significantly suppressed the tumor growth of MC38 in wildtype mice. IMQ upregulated interferon γ (IFN-γ) expression in CD8(+) T cells in both the lymph nodes and the tumor, and the antitumor effect was abolished in both Rag1-deficient mice and IFN-γ-deficient mice, indicating that IFN-γ produced by CD8(+) T cells play a crucial role in the IMQ-induced antitumor effect. IMQ also upregulated PD-1 expression in T cells as well as PD-L1/PD-L2 expression in myeloid cells, suggesting that IMQ induces not only T-cell activation but also T-cell exhaustion by enhanced PD-1 inhibitory signaling. Combination therapy of topical IMQ plus anti-PD-1 antibody exerted a significantly potent antitumor effect when compared with each single therapy, indicating that the combination therapy is a promising therapy for the skin lesions of various cancers.