A thymidine kinase-deleted vaccinia virus expressing human GM-CSF (hGM-CSF) with oncolytic activity. Upon intratumoral or intravenous administration, pexastimogene-devacirepvec may selectively infect and lyse tumor cells. While vaccinia displays a natural tumor cell tropism, deletion of the thymidine kinase gene increases the tumor selectivity of vaccinia by limiting viral replication to transformed cells. hGM-CSF expression by this agent may help recruit antigen processing cells (APCs), such as denritic cells and macrophages, to virally infected tumor cells, initiating a systemic antitumoral immune response.