IM-6 HVJ-E containing PD-L1 siRNA inhibits immunosuppressive activities and elicits antitumor immune responses in glioma

Inactivated Sendai virus particle, hemagglutinating virus of Japan-envelope (HVJ-E), is a non-replicating virus-derived vector, in which the genomic RNA of Sendai virus (HVJ) has been destroyed. HVJ-E is a promising vector that enables the highly efficient and safe introduction of enclosed molecules such as RNA into target cells. Moreover, HVJ-E provokes robust antitumoral immunity by activating natural killer (NK) cells and CD8+ T lymphocytes and their induction into the tumor periphery, and by suppressing regulatory T lymphocytes (Treg) locally in the tumor. In the present study, we investigated a novel combination of antitumor immunotherapy by the antitumor immune-activating effect of HVJ-E itself with the inhibition of tumor PD-L1 molecule expression. We confirmed that intratumoral injection of HVJ-E containing siRNA targeting PD-L1 (siPDL1/HVJ-E) inhibited tumor PD-L1 protein expression in a mouse subcutaneous tumor model using TS, a mouse glioma stem-like cell. We conducted treatment experiments in the mouse brain tumor model in three groups: control group (PBS), siNC/HVJ-E group (negative control siRNA + HVJ-E), and siPDL1/HVJ-E group. We obtained a significant prolongation of overall survival in the siPDL1/HVJ-E group. Flow cytometric analyses of brain tumor models showed that the proportions of brain-infiltrating CD8+ T lymphocytes and NK cells were significantly increased after giving siPDL1/HVJ-E; in contrast, the rate of Treg/CD4+ lymphocytes was significantly decreased in HVJ-E-treated tumors (siNC/HVJ-E and siPDL1/HVJ-E). No difference was observed in the proportions of macrophages or M2 macrophages. CD8 depletion abrogated the therapeutic effect of siPDL1/HVJ-E, indicating that CD8+ T lymphocytes mainly mediated this therapeutic effect. We believe that this non-replicating immunovirotherapy may be a novel therapeutic alternative to treat patients with glioblastoma. The full article has been published (Cancer Science. 2021 Jan;112(1):81–90).