Immunotherapy with immune‐checkpoint therapy has recently been used to treat oral squamous cell carcinomas (OSCCs). However, improvements in current immunotherapy are expected because response rates are limited. Transforming growth factor‐β (TGF‐β) creates an immunosuppressive tumor microenvironment (TME) by inducing the production of regulatory T‐cells (Tregs) and cancer‐associated fibroblasts and inhibiting the function of cytotoxic T‐lymphocytes (CTLs) and natural killer cells. TGF‐β may be an important target in the development of novel cancer immunotherapies. In this study, we investigated the suppressive effect of TGF‐β on CTL function in vitro using OSCC cell lines and their specific CTLs. Moreover, TGFB1 mRNA expression and T‐cell infiltration in 25 OSCC tissues were examined by in situ hybridization and multifluorescence immunohistochemistry. We found that TGF‐β suppressed the function of antigen‐specific CTLs in the priming and effector phases in vitro. Additionally, TGF‐β inhibitor effectively restored the CTL function, and TGFB1 mRNA was primarily expressed in the tumor invasive front. Interestingly, we found a significant negative correlation between TGFB1 mRNA expression and the CD8⁺ T‐cell/Treg ratio and between TGFB1 mRNA expression and the Ki‐67 expression in CD8⁺ T‐cells, indicating that TGF‐β also suppressed the function of CTLs in situ. Our findings suggest that the regulation of TGF‐β function restores the immunosuppressive TME to active status and is important for developing new immunotherapeutic strategies, such as a combination of immune‐checkpoint inhibitors and TGF‐β inhibitors, for OSCCs.

The successful use of immune checkpoint inhibitors has been big breakthrough in the development of cancer immunotherapy. Anti-CTLA-4 monoclonal antibody, ipilimumab, is the first-approved immune checkpoint inhibitor and has shown durable objective responses for advanced melanoma beyond the effect of dacarbazine. Anti-PD-1 monoclonal antibodies, nivolumab and pembrolizumab, are other immune checkpoint inhibitors that have demonstrated more effective results than conventional drugs in clinical trials for a variety of advanced solid tumors including melanoma, non-small cell lung carcinoma and renal carcinoma. These studies have indicated that the enhancement of anti-cancer immunity by controlling the immune suppressive environment in cancer tissues is an important issue for the development of cancer immune-therapy. Accordingly, in recent years, the enthusiasm for research of cancer immunology has shifted to studies regarding the formation of the immune suppressive environment, immune suppression mechanisms in cancer tissues and the molecules and cells involved in these pathways. Novel findings from these studies might lead to the development of cancer immunotherapy based on control of the immune suppressive environment.

Immune‐checkpoint inhibitors improve the survival of head and neck squamous cell carcinoma (HNSCC) patients. Although recent studies have demonstrated that the tumor immune microenvironment (TIME) has critical roles in immunotherapy, the precise mechanisms involved are unclear. Therefore, further investigations of TIME are required for the improvement of immunotherapy. The frequency of effector regulatory T‐cells (eTregs) and the expression of immune‐checkpoint molecules (ICM) on eTregs and conventional T‐cells (Tconvs) both in peripheral blood lymphocytes (PBL) and tumor‐infiltrating lymphocytes (TIL) from HNSCC patients were analyzed by flow cytometry and their distributions were evaluated by multi‐color immunofluorescence microscopy. High frequency eTreg infiltration into HNSCC tissues was observed and high expressions of CD25, FOXP3, stimulatory‐ICM (4‐1BB, ICOS, OX40 and GITR) and inhibitory‐ICM (programmed cell death‐1 [PD‐1] and cytotoxic T‐lymphocyte‐associated protein‐4 [CTLA‐4]) were found on invasive eTregs. In contrast, the expression of stimulatory‐ICM on Tconvs was low and the expression of inhibitory‐ICM was high. In addition, ICM‐ligands (programmed cell death‐1 [PD‐L1], galectin‐9 and CEACAM‐1) were frequently expressed on cancer cells. PD‐L1 and galectin‐9 were also expressed on macrophages. PD‐1⁺ T‐cells interacted with PD‐L1⁺ cancer cells or PD‐L1⁺ macrophages. This suggested that in TIL, eTregs are highly activated, but Tconvs are exhausted or inactivated by eTregs and immune‐checkpoint systems, and ICM and eTregs are strongly involved in the creation of an immunosuppressive environment in HNSCC tissues. These suggested eTreg targeting drugs are expected to be a combination partner with immune‐checkpoint inhibitors that will improve immunotherapy of HNSCC.