ABSTRACT Adenovirus-based vectors are a very attractive platform for development of novel drug candidates for treatment of numerous genetic diseases and cancer. Currently, the prevailing view of requisites for effective virus-based cancer therapeutics includes i) a potent cytotoxic capacity to trigger virus-mediated killing of as many tumor cells as possible in the shortest period of time, and ii) the capacity for stimulating adaptive anti-tumor immune response. While early efforts to develop potent oncolytic therapeutics were primarily focused on harnessing or even increasing virus-mediated tumor cell cytotoxicity, new data suggest that in vivo, therapeutic efficacy of viruses with low cytotoxic capacity is comparable or even better than that of viruses which are highly effective at killing cancer cells in vitro. The mechanistic factors underlying this mismatch between efficacy and potency in vivo remain poorly understood. While evaluating the therapeutic efficacy of systemically administered adenovirus-based vectors in a mouse model of disseminated lung cancer, we found that virus dose escalation leads to a reduction in median survival of tumor-bearing mice. In mice that developed a ‘non-responder’ phenotype, myeloid cells become highly activated in response to adenovirus administration. Based on these findings we propose the novel concept of a “reparative call”, whereby efficient tumor cell killing by the virus triggers activation of tumor-associated myeloid cells, which in turn secrete pro-tumorigenic growth factors, triggering accelerated tumor growth. This project is designed to comprehensively address mechanistic aspects of this concept by analyzing how cytotoxic potency may affect the efficacy of systemic virotherapy with Ad-based vectors. We will determine the mechanistic role of tumor-derived IL-33 in driving myeloid cell activation and will develop novel Ad vectors expressing soluble IL-33R to block IL-33-dependent tumor-cell activation. We will further analyze the therapeutic efficacy of soluble IL-33R-expressing Ad vectors in combination with clinically relevant drugs that target myeloid cells and the PD-1 pathway. The successful completion of this project will significantly advance our understanding of fundamental factors that are critical for effective clinical translation of Ad-based vector systems for therapy of neoplastic disease.