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Abstract

Replicating oncolytic viruses (OVs) that are able to selectively destroy malignant cells are emerging as clinically relevant cancer therapeutics. Along with direct tumor cell lysis, activation of specific anti-tumor immune responses contributes to efficacy of virotherapy, allowing to consider it for a type of cancer immunotherapy. Combinations with different immunomodulation strategies have been shown to enhance the immunostimulatory effects of OVs and contribute to increased therapeutic efficacy. Based on the hypothesis that certain immunomodulation types might more efficiently contribute to efficacy of virotherapy in a given tumor type, this study compared efficacy of oncolytic measles vectors encoding immunomodulators from different classes. Furthermore, to identify immune effector mechanisms associated with successful therapeutic strategies, analysis of the tumor immune environment was performed following treatment with the most promising vectors. Measles Schwarz vaccine strain vectors (MeVac) encoding immunomodulators to target the main phases in establishment of an anti-tumor immune response were developed. Therapeutic efficacy of the novel vectors was compared in a fully immunocompetent murine colon adenocarcinoma model, MC38cea. MeVac vectors encoding an antibody against the negative T cell regulator PD-L1 (anti-PD-L1) and a fusion protein of murine interleukin-12 (FmIL-12), respectively, were identified as the most promising in terms of increased survival of animals. Importantly, MeVac encoding FmIL-12 was the most effective, ensuring complete tumor remissions in 90% of the treated animals. After MeVac therapy, long-term survivors rejected secondary tumor engraftments, indicating establishment of a systemic anti-tumor immune response. Profiling of the tumor environment four days after the last treatment with the anti-PD-L1 encoding vector revealed a slight benefit for cell mediated immune responses, as observed by a slight upregulation of the effector cytokines IFN-γ and TNF-α as well as an increase in the intratumoral T cell population. More pronounced modulation of the tumor immune environment was observed following treatment with the FmIL-12 encoding vector. One day after treatment with MeVac encoding FmIL-12 an increase of effector cytokines IFN-γ and TNF-α was observed, suggesting activation of a cell mediated immune response. Analysis of tumor infiltrating lymphocytes revealed an increase in the T cell population, a massive decrease in the natural killer (NK) cell population and upregulation of an activation marker on NK cells. These results indicated early activation of the immune effector cells following treatment with MeVac encoding FmIL-12, which in case of the NK cells could be associated with activation induced cell death. Furthermore, immune cell depletion experiments revealed that the CD4+ T cells and NK cells do not importantly contribute to the therapeutic efficacy of the MeVac encoding FmIL-12 in this model, but that the cytotoxic CD8+ T cells are essential. This study presents MeVac encoding FmIL-12 as an effective therapeutic for activation of cell mediated anti-tumor immune responses. Furthermore, the MeVac vector is established as a flexible platform for targeted local delivery of immunomodulators. The tumor immune profiling data provide a basis for further rational vector modifications to develop immunomodulation strategies tailored to the individual tumor immune environment.