Abstract 4995: Mechanism of action of anti-PD-L1 antibody in a PD-L1-negative and immune desert-like tumor model

Purpose: Intratumoral PD-L1 plays an important role in preventing T cell-mediated cancer cell killing. Anti-PD-1/PD-L1 antibodies generally show higher clinical benefit specifically in patients with high intratumoral PD-L1 expression. However, anti-PD-L1 antibody, atezolizumab, provide clinical benefit even in the subgroup of cancer patients where PD-L1 is low or undetectable on tumor cells and tumor-infiltrating immune cells (TC0/IC0). In this study, we investigated the mechanism of action by which PD-L1-negative tumors responded to an anti-PD-L1 therapy by using a syngeneic mouse tumor model. Experimental Design: We tested the antitumor effect of an anti-mouse PD-L1 monoclonal antibody (mAb) in vivo in murine models. Flow cytometry, tumor-stimulated IFNγ release assay, T cell repertoire analysis, RNA sequencing, protein immunoassays, and immunohistochemistry were performed on isolated tumors or on tumor-draining lymph nodes (dLN). Results: The anti-PD-L1 mAb showed significant antitumor activity in 6 of the 17 tumor models tested. Model FM3A, one of these 6 sensitive models, showed little intratumoral PD-L1 expression and was classified as an immune desert-like tumor model, but PD-L1 was found to be highly expressed on CD103+CD11c+ cells in dLN in this model. Some of the CD44+CD62L−CD8α+ effector T cells in dLN expressed PD-L1 receptors, i.e. they were PD-1- and/or B7-1-positive cells. Using lymphocytes from dLN, we found that tumor cell-stimulated IFNγ production was significantly increased even in the control group, and it was further significantly increased in the anti-PD-L1 mAb group compared to that in the control group. We found that anti-PD-L1 mAb treatment firstly enhanced T cell priming and increased CXCR3+ activated T cells in dLN. FM3A tumors expressed CXCR3 ligands regardless of anti-PD-L1 mAb treatment. CXCR3 blockade significantly prevented activated CD8α+ T cells from increasing in tumors as a result of anti-PD-L1 mAb treatment but not in dLN and significantly attenuated tumor growth inhibition of anti-PD-L1 mAb. Conclusions: We showed that an anti-PD-L1 mAb exerted antitumor activity in a TC0/IC0-like and immune desert-like tumor model through the blocking of PD-L1 in dLN. Tumor antigen presentation and T cell priming had already occurred in dLN at baseline prior to anti-PD-L1 mAb treatment in the FM3A model but PD-L1 on antigen-presenting cells prevented further T cell priming. We suggested that PD-L1 blockade in dLN appeared to trigger re-activation of T cells leading to antitumor activity. The results also suggested that the antitumor activity of anti-PD-L1 Ab requires both the enhancement of T cell priming by PD-L1 blockade as well as the trafficking of T cells to the tumor in response to the tumor via CXCR3 and its ligands system. Citation Format: Toshiki Iwai, Masamichi Sugimoto, Osamu Kondo. Mechanism of action of anti-PD-L1 antibody in a PD-L1-negative and immune desert-like tumor model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4995.