P66: CTLA-4 Blockade Increases Tumor-infiltrating CD8+ Cells in a Syngeneic Model of Gastric Cancer

Anti-CTLA-4 therapy has shown promising clinical efficacy in multiple cancer types including hepatocellular carcinoma, renal cell carcinoma, and non-small cell lung cancer. In particular, CTLA-4 blockade has been hypothesized to expand the tumor infiltrating T cell population and may be combined with anti-PD-1/-L1 therapy for improved efficacy. However, the role of CTLA-4 blockade in gastric cancer is not well-delineated. We assessed the effects of anti-CTLA-4 therapy on tumor growth and CD8+ tumor infiltrating lymphocytes in a syngeneic mouse model of gastric cancer.



Methods:

Primary cell lines were derived from gastric cancer formed in transgenic mice on a C57BL/6 background. These mice had parietal cell-specific, concomitant deletion of Trp53, Cdh1, and Arid1a. The tumor cells were injected into the flanks of wild-type C57BL/6 mice. Once the tumor volumes exceeded 200 mm³, the mice were randomized to receive intraperitoneal injections of vehicle, 100 mg, or 300 mg of anti-CTLA-4 monoclonal antibody every 3 days for 3 doses. 9 days after initiation of treatment, mice were euthanized, and tumor tissue was collected for histology. Immunohistochemical staining for CD8 was performed on tumor tissue, and CD8+ cells per high-power field (HPF) were counted.



Results:

Median tumor volume on day 0 of treatment was 210, 217, and 232 mm³ for the vehicle, 100 mg, and 300 mg group, respectively (P = 0.66). At day 9 after initiation of treatment, tumors in the 300 mg treatment group were significantly smaller than those in the vehicle and 100 mg treatment groups (Figure 1A; P = 0.009) with median tumor volumes of 618, 687, and 510 mm³ for the vehicle, 100 mg, and 300 mg group, respectively.

On immunohistochemical analysis, tumors in the 300 mg treatment group had significantly more CD8+ cells per HPF than both the vehicle and 100 mg treatment groups. Median value of cells per HPF were 8, 3, and 32 for the vehicle, 100 mg, and 300 mg group, respectively (Figure 1B; P < 0.0001).



Conclusions:

CTLA-4 blockade resulted in decreased tumor volume and increased CD8+ tumor infiltrating lymphocytes in a syngeneic mouse model of gastric cancer. CTLA-4 blockade shows promising antitumor activity and warrants further investigation in gastric cancer, which has poor clinical response to immune checkpoint inhibitors.