PURPOSE: PD-1 checkpoint blockade has revolutionized the field of cancer immunotherapy, yet the frequency of responding patients is limited by inadequate T-cell priming secondary to a paucity of activatory dendritic cells (DCs). DC signals can be bypassed by CD27 agonists and we therefore investigated if the effectiveness of anti-PD-1/L1 could be improved by combining with agonist anti-CD27 monoclonal antibodies (mAb). EXPERIMENTAL DESIGN: The efficacy of PD-1/L1 blockade or agonist anti-CD27 mAb was compared with a dual-therapy approach in multiple tumor models. Global transcriptional profiling and flow-cytometry analysis were used to delineate mechanisms underpinning the observed synergy. RESULTS: PD-1/PD-L1 blockade and agonist anti-CD27 mAb synergize for increased CD8+ T-cell expansion and effector function, exemplified by enhanced IFN-gamma, TNF-alpha, granzyme B and T-bet. Transcriptome analysis of CD8+ T cells revealed that combination therapy triggered a convergent program largely driven by IL-2 and Myc. However, division of labor was also apparent such that anti-PD-1/L1 activates a cytotoxicity-gene expression program whereas anti-CD27 preferentially augments proliferation. In tumor models, either dependent on endogenous CD8+ T cells or adoptive transfer of transgenic T cells, anti-CD27 mAb synergized with PD-1/L1 blockade for anti-tumor immunity. Finally, we show that a clinically-relevant anti-human CD27 mAb, varlilumab, similarly synergizes with PD-L1 blockade for protection against lymphoma in human-CD27 transgenic mice. CONCLUSIONS: Our findings suggest that suboptimal T-cell invigoration in cancer patients undergoing treatment with PD-1 checkpoint blockers will be improved by dual PD-1 blockade and CD27 agonism and provide mechanistic insight into how these approaches co-operate for CD8+ T-cell activation.