Background: Gastric cancer is a leading cause of cancer-related deaths worldwide. Many of those with advanced gastric cancer are responsive to immune checkpoint antibody therapy, although the median survival even with these new agents is less than 12 months. The gastrointestinal peptide, gastrin, has been shown to stimulate growth of gastric cancer through the cholecystokinin-B receptor (CCK-BR) and the inhibition of gastrin’s action results in decreased growth. In the current investigation we studied the effects of a cancer vaccine, Polyclonal Antibody Stimulator (PAS) that targets the gastrin peptide on growth of gastric cancer in mice. Methods: RNA was extracted from NCC-S1 and YTN-16 murine gastric cancers and analyzed by qRT-PCR for gastrin, CCK-BR, and PD-L1 expression. YTN-16 (5x10⁶) cells were injected subcutaneously in (N = 40) syngeneic female C57BL/6 mice. Mice were divided into 4 groups of equal tumor size and treated with PBS (control), PD-1 antibody (PD-1 Ab) 50 μg, PAS 250μg, or the combination of the PD-1 Ab and PAS given at one week after tumor inoculation (week 0) and at weeks 1, 3, and 6. A PAS booster was also given at week 9.Tumor growth rate was monitored weekly with calipers. At the end of treatment, tumors were excised, weighed and analyzed with Masson’s trichrome stain for fibrosis, and for Ki67, CD8 T-cells, and M2-polarized macrophages by immunohistochemistry. Metastases were counted and confirmed by histology. Results: Gastrin, CCK-BR, and PD-L1 expression were confirmed by qRT-PCR in the gastric cancer cells. PAS monotherapy (P = 0.023) or PAS in combination with PD-1 Ab (P = 0.0003) resulted in significantly slowed tumor growth and with no metastases observed. Tumor weights were 40-52% smaller in mice treated with PAS or the combination therapy compared to PBS or PD-1 Ab, respectively, but this difference did not reach significance (P = 0.09). PD-1 Ab monotherapy did not change tumor size compared to PBS-treated control mouse tumors. Ki67 staining was decreased by 62-67% and fibrosis staining was decreased by 51-61% in tumors of mice treated with PAS monotherapy or in combination with PD-1 Ab, respectively (P < 0.0001). PAS monotherapy resulted in a 4-fold increase of CD8+ T-cells and a 56% decrease in M2-polarized macrophages (P = 0.0001) compared to control tumors. PAS and PD-1 Ab combination therapy resulted in an additive effect with 29% more CD8+ T-cells (P < 0.05) and 54% fewer M2-polarized macrophages (P < 0.0001) compared to tumors of PAS monotherapy-treated mice. Conclusions: Treatment of mice with an anti-gastrin vaccine, PAS, slows growth of gastric cancer and prevents metastases. PAS vaccination improves the effectiveness of PD-1 Ab therapy in part by decreasing tumor fibrosis and altering the tumor immune cell signature. This study suggests that addition of PAS to immune checkpoint antibody therapy in gastric cancer may be beneficial.