Background: Treatment with poxvirus-based active immunotherapies shows evidence of robust immune responses against a variety of tumor-associated antigens in preclinical and clinical studies. These active immunotherapies are in clinical trials to treat tumors expressing PSA (PROSTVAC/Phase 3), CEA and MUC-1 (CV-301/Phase 2), HER-2 (MVA-BN-HER2), and Brachyury (MVA-BN-Brachyury). Because these immunotherapies drive IFNγ-producing T cells to the tumor, we hypothesized that the tumors would upregulate PD-L1 in an attempt to evade the immune response. Thus, combination treatment of poxvirus-based immunotherapy with PD-1 axis blockade has the potential to yield therapeutic synergy against tumors in preclinical and clinical regimens. Methods: In therapeutic MC38-CEA and CT26-HER2 tumor models, mice were treated with CV-301 or MVA-BN-HER2, respectively, in combination with anti-PD-1 antibody. PD-L1 expression was evaluated in the tumor microenvironment by immunofluorescence and FACS. Results: Poxvirus-based immunotherapies induced activation and trafficking of antigen specific T cells that produce high levels of IFNγ in the tumor microenvironment.TumorPD-L1 expression was upregulated following CV-301 and MVA-BN-HER2 immunotherapy in vivo as measured by immunofluorescence. The PD-L1 expression was significantly elevated on tumor cells (p < 0.01) and infiltrating immune cells (p < 0.05) as measured by FACS. Combining poxvirus-based immunotherapy with PD-1 blockade in therapeutic mouse tumor models demonstrated synergistic anti-tumor efficacy. Conclusions: In preclinical studies, CV-301 and MVA-BN-HER2 poxvirus-based immunotherapies drive a robust, tumor-infiltrating T cell response that provokes tumor PD-L1 expression. Synergistic anti-tumor efficacy in mice resulted from combining active immunotherapy with PD-1 immune checkpoint inhibition. The potential of active immunotherapy to drive productive antigen specific T cell immunity could provide patients with PD-L1^neg/low tumors an opportunity to benefit from PD-1/PD-L1 axis blockade when given in combination with poxvirus-based immunotherapies.