Targeting the crosstalk between cytokine-induced killer cells and myeloid-derived suppressor cells in hepatocellular carcinoma

[Display omitted] •CIK therapy recruits MDSCs in tumor tissues through inflammatory cytokines.•MDSCs inhibit CIK tumor lytic function in an ARG1 and iNOS dependent manner.•Tadalafil, an FDA approved PDE5 inhibitor, suppressed the number and function of MDSCs.•Tadalafil enhances antitumor efficacy of CIK cells in in vivo murine HCC models.•Human MDSCs inhibit human CIK cell cytotoxicity. Cytokine-induced killer (CIK) cell-based immunotherapy is effective as an adjuvant therapy in early stage hepatocellular carcinoma (HCC) but lacks efficacy in advanced HCC. We aimed to investigate immune suppressor mechanisms in HCC, focusing on the role of myeloid-derived suppressor cells (MDSCs) in response to CIK therapy. MDSCs were quantified by flow cytometry and quantitative real-time PCR. Cytokines were detected by cytokine array. A lactate dehydrogenase cytotoxicity assay was performed in the presence or absence of MDSCs to study CIK function against HCC cells in vitro. An FDA-approved PDE5 inhibitor, tadalafil, was used to target MDSCs in vitro and in vivo. Two different murine HCC cell lines were tested in subcutaneous and orthotopic tumor models in C57BL/6 and BALB/c mice. The antitumor effects of human CIKs and MDSCs were also tested in vitro. Adoptive cell transfer of CIKs into tumor-bearing mice induced inflammatory mediators (e.g., CX3CL1, IL-13) in the tumor microenvironment and an increase of tumor-infiltrating MDSCs, leading to impaired antitumor activity in 2 different HCC models. MDSCs efficiently suppressed the cytotoxic activity of CIKs in vitro. In contrast, treatment with a PDE5 inhibitor reversed the MDSC suppressor function via ARG1 and iNOS blockade and systemic treatment with a PDE5 inhibitor prevented MDSC accumulation in the tumor microenvironment upon CIK cell therapy and increased its antitumor efficacy. Similar results were observed when human CIKs were tested in vitro in the presence of CD14+HLA-DR−/low MDSCs. Treatment of MDSCs with a PDE5 inhibitor suppressed MDSC suppressor function and enhanced CIK activity against human HCC cell lines in vitro. Our results suggest that targeting MDSCs is an efficient strategy to enhance the antitumor efficacy of CIKs for the treatment of patients with HCC. Cytokine-induced killer cells are a mixture of immune cells given to eliminate cancer cells. However, not all patients respond to this treatment. Herein, we show in 2 different liver cancer models that myeloid-derived suppressor cells are increased i