Purpose: The purpose of the present study was to evaluate granulocyte macrophage colony-stimulating factor (GM-CSF)–secreting tumor cell immunotherapy, which is known to stimulate potent and long-lasting antigen-specific immune responses, in combination with PD-1 blockade, which has been shown to augment cellular immune responses.

Experimental Design: Survival studies were done in the B16 melanoma and CT26 colon carcinoma tumor models. Immune monitoring studies were done in the B16 model. GM-CSF–secreting tumor cell immunotherapy was administered s.c. and the anti–PD-1 antibody was administered i.p.

Results: The studies reported here show that combining PD-1 blockade with GM-CSF–secreting tumor cell immunotherapy prolonged the survival of tumor-bearing animals compared with animals treated with either therapy alone. Prolonged survival correlated with strong antigen-specific T-cell responses detected by tetramer staining and an in vivo CTL assay, higher secretion levels of proinflammatory cytokines by splenocytes, and the persistence of functional CD8⁺ T cells in the tumor microenvironment. Furthermore, in the biweekly multiple treatment setting, repeated antigen-specific T-cell expansion was only observed following administration of the cellular immunotherapy with the PD-1 blockade and not when the cellular immunotherapy or PD-1 blockade was used as monotherapy.

Conclusion: The combination of PD-1 blockade with GM-CSF–secreting tumor cell immunotherapy leads to significantly improved antitumor responses by augmenting the tumor-reactive T-cell responses induced by the cellular immunotherapy. Readministration of the cellular immunotherapy with the anti–PD-1 antibody in subsequent immunotherapy cycles was required to reactivate these T-cell responses.