Add-On Effect of Hemagglutinating Virus of Japan Envelope Combined with Chemotherapy or Immune Checkpoint Inhibitor against Malignant Pleural Mesothelioma: An In Vivo Study

SIMPLE SUMMARY: In this study, we report that the combination of existing therapeutic agents and hemagglutinating virus of Japan envelope (HVJ-E), a virus-inactivating agent, may be a more effective treatment agent for pleural mesothelioma, a refractory cancer for which no effective treatment has been established. We have already reported that HVJ-E, which activates antitumor immunity and induces apoptosis in cancer, can provide a different mechanism for effective treatment. In this report, the antitumor effect of HVJ-E in combination with chemotherapy or immune checkpoint inhibitors (ICIs) activated antitumor immunity in a mouse model of malignant pleural mesothelioma-bearing tumor and significantly enhanced antitumor efficacy compared to single-agent therapy. We report that the use of HVJ-E in combination with chemotherapy or ICIs, which are already used in clinical practice, may be more effective than single-agent therapy. However, further clinical studies are needed before the combination therapy can be used in clinical practice. ABSTRACT: Malignant pleural mesothelioma (MPM) is a refractory tumor because most of the lesions are already disseminated at diagnosis. Previously, the main treatment for MPM was combination chemotherapy. However, recently, immune checkpoint inhibitors (ICIs) are also used. For better efficacy of MPM treatment, we focused on hemagglutinating virus of Japan envelope (HVJ-E), which activates antitumor immunity and induces tumor-specific cell death. In this paper, we aimed to determine whether HVJ-E as a single agent therapy or in combination with chemotherapy or ICIs is effective in MPM bearing mouse. We confirmed its antitumor efficacy in MPM-bearing mouse. HVJ-E significantly prolonged the survival of human MPM-bearing mouse compared to that of control mouse and when combined with CDDP. This efficacy was lost in NOD-SCID mouse, suggesting that activation of innate immunity by HVJ-E was related to the survival rate. HVJ-E also showed antitumor efficacy in murine MPM-bearing mouse. The combination of chemotherapy and HVJ-E caused a significant increase in cytotoxic T cells (CTLs) compared to chemotherapy alone, suggesting that not only innate immunity activated by HVJ-E but also the increase in CTLs contributed to improved survival. The combination of anti-PD-1 antibody and HVJ-E significantly prolonged the survival rate of murine MPM-bearing mouse. Further, HVJ-E might have exhibited antitumor effects by maintaining immunogenicity against tumors. We believe that HVJ-E may be a beneficial therapy to improve MPM treatment in the future.