Abstract 2340: PD-1 blockade suppresses gastric cancer development by promoting antitumor immunity in mice

Abstract

Abstract Since advanced gastric cancer is difficult to cure with chemo and/or radiation therapy, new therapeutic approaches are clearly needed. Targeting immune checkpoints such as CTLA-4 and PD-1/PD-L1 has shown promising antitumor immunity in many malignancies by modulating T cell activity. PD-L1 and to a lesser extent of PD-L2 are expressed in human gastric cancer and expression of PD-L1 is associated with poor prognosis. Therefore, developing immunotherapies is an attractive strategy for the treatment of gastric cancer. We evaluated the efficacy of anti-PD-1 therapy in a mouse model of gastric cancer. Gastrin-deficient (GAS-KO) mice have been shown to develop large antral tumors with the combination of N-methyl-N-nitrosourea (MNU) and Helicobacter felis (H. felis) infection. Tumors developed in GAS-KO mice exhibited increased expression of PD-L1 and higher mutation rates than those in wild-type mice. To study the effect of anti-PD-1 antibody in tumor initiation or an early stage of progression, GAS-KO mice were induced with H.felis/MNU to develop tumors, one week later (10 weeks post-MNU) treated with anti-PD-1 antibody or isotype control (10 mg/kg IP qw) and sacrificed at 30 weeks post-MNU. We confirmed that GAS-KO mice treated with isotype control had moderate tumor burdens. Of interest, however, less and much smaller tumors were observed in mice treated with anti-PD-1. To compare immune cell populations in spleen tissue and peripheral blood from each group of mice, flow cytometric analysis was performed. Anti-PD-1-treated mice showed an increase in T cells including cytotoxic CD8+ T cells and a decrease in regulatory T cells and myeloid-derived suppressor cell (MDSC) populations compared with controls. In accordance with this, CD3-positive T lymphocytes were more frequently found in gastric tumors of anti-PD-1-treated mice than controls. Furthermore, qRT-PCR analysis revealed differential cytokine gene expression profiles including Il6, Il1b, and Il23 following anti-PD-1 treatment. Together, these data implicate that PD-1 blockade promotes immune infiltrates and therefore potentiates antitumor immune responses, suggesting targeting PD-1 as a potential new anticancer strategy in gastric cancer. Understanding anti-PD-1 therapy at the cellular and molecular level could provide new strategies for developing highly efficacious combination therapies. Citation Format: Woosook Kim, Yoku Hayakawa, James G. Fox, Timothy C. Wang. PD-1 blockade suppresses gastric cancer development by promoting antitumor immunity in mice. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2340.