Pembrolizumab is a humanized IgG4 kappa monoclonal antibody that blocks the interaction between PD-1 and its ligands, PD-L1 and PD-L2. Pembrolizumab is currently approved for the treatment of unresectable or metastatic melanoma. Analysis of tumor samples before and during treatment in an earlier study demonstrated that a clinical response was associated with an increase in the density of CD8+ T cells in the tumor parenchyma (center), while disease progression was associated with persistent low levels of those cells. In an autochthonous murine model of pancreatic adenocarcinoma, persistent tumor growth despite administration of anti-PD-L1 was similarly associated with failure of tumor-specific cytotoxic T cells to enter the TME despite their presence in the peripheral circulation. This immunosuppressed phenotype was associated with CXCL12 production by cancer-associated fibroblasts. Moreover, administration of a CXCR4 antagonist (AMD3100) induced rapid T-cell accumulation among the cancer cells and, in combination with anti-PD-L1, synergistically decreased tumor growth.