Tumor‐Targeting Salmonella typhimurium A1‐R Promotes Tumoricidal CD8+ T Cell Tumor Infiltration and Arrests Growth and Metastasis in a Syngeneic Pancreatic‐Cancer Orthotopic Mouse Model

The present study determined the effect of the tumor‐targeting strain Salmonella typhimurium A1‐R (S. typhimurium A1‐R) on CD8⁺ tumor‐infiltrating lymphocytes (TILs) in a syngeneic pancreatic‐cancer orthotopic mouse model. The effect of tumor‐targeting S. typhimurium A1‐R on CD8⁺ TILs was determined on the Pan02 murine pancreatic‐adenocarcinoma implanted orthotopically in the pancreatic tail of C57BL/6 immunocompromised mice. Three weeks after orthotopic implantation, mice were randomized as follows G1: untreated control group (n = 8); and G2: S. typhimurium A1‐R‐treatment group (n = 8, 1 × 10⁷ colony forming units CFU]/body, iv, weekly, 3 weeks). On the 22nd day from initial treatment, all mice were sacrificed and tumors were harvested. The tumor‐volume ratio was defined as ratio of tumor volume on the 22nd day relative to the 1st day. The tumor volume ratio was significantly lower in the S. typhimurium A1‐R‐treated group (G2) (3.0 ± 2.8) than the untreated control (G1) (39.9 ± 30.7, P < 0.01). Hematoxylin and easin (H&E) staining on tumor sections was performed to evaluate tumor destruction which was classified according to the Evans grading system and found to be much greater in the S. typhimurium A1‐R‐treated mice (G2). Six mice in G1 had peritoneal dissemination, whereas no mice showed peritoneal dissemination in G2 (P < 0.01). Immunohistochemical staining with anti‐mouse CD8⁺ antibody was performed in order to detect TILs determined by calculating the average number of CD8⁺ cells in three high power fields (200×) in the treated and untreated tumors. The TIL score was significantly higher in G2 (133.5 ± 32.2) than G1 (45.1 ± 19.4, P < 0.001). The present study demonstrates that S. typhimurium A1‐R promotes CD8⁺ T cell infiltration and inhibition of tumor growth and metastasis. J. Cell. Biochem. 119: 634–639, 2018. © 2017 Wiley Periodicals, Inc.