CTLA-4 and PD-L1 checkpoint blockade enhances oncolytic measles virus therapy

We hypothesized that the combination of oncolytic virotherapy with immune checkpoint modulators would reduce tumor burden by direct cell lysis and stimulate antitumor immunity. In this study, we have generated attenuated Measles virus (MV) vectors encoding antibodies against CTLA-4 and PD-L1 (MV-aCTLA-4 and MVaPD- L1). We characterized the vectors in terms of growth kinetics, antibody expression, and cytotoxicity in vitro. Immunotherapeutic effects were assessed in a newly established, fully immunocompetent murine model of malignant melanoma, B16-CD20. Analyses of tumorinfiltrating lymphocytes and restimulation experiments indicated a favorable immune profile after MV-mediated checkpoint modulation. Therapeutic benefits in terms of delayed tumor progression and prolonged median overall survival were observed for animals treated with vectors encoding anti-CTLA-4 and anti-PD-L1, respectively. Combining systemic administration of antibodies with MV treatment also improved therapeutic outcome. In vivo oncolytic efficacy against human tumors was studied in melanoma xenografts. MV-aCTLA-4 and MV-aPDL1 were equally efficient as parental MV in this model, with high rates of complete tumor remission ( 80%). Furthermore, we could demonstrate lysis of tumor cells and transgene expression in primary tissue from melanoma patients. The current results suggest rapid translation of combining immune checkpoint modulation with oncolytic viruses into clinical application.