Abstract 3658: Dendritic cells dictate the responsiveness of PD-L1 blockade in cancer

Recent advances in cancer immunotherapies with PD-1/PD-L1 pathway blockade have transformed the way that cancer is being treated, leading to durable responses and prolonged overall survival. The general thinking is that PD-1/PD-L1 blockade reinvigorates tumor-infiltrating PD-1+ T cells with exhausted phenotypes. However, a mechanistic understanding on why only a subset of patients (10-30%) responds to checkpoint inhibition remains largely unknown, as does the exact immune mechanism of PD-1/PD-L1 blockade. Here, we discovered that PD-L1 blockade mediates anti-tumor immunity via dendritic cells. In human blood, in vitro and ex vivo dendritic cells (DCs) express both PD-1 and PD-L1, and an activation signal via TLR agonists triggers downregulation of PD-1 to empower the T cell stimulatory capability of DCs. In contrast, tolerogenic DCs remain PD-1 positive, correlating to T cell-unresponsiveness. Anti-PD-L1 Ab directly induces maturation of DCs and renders them capable of stimulating T cell proliferation. Similarly, anti-PD-L1 Ab treatment in tumor-bearing mice induces massive infiltration and activation of CD11c+ DCs in the spleen and draining lymph node, indicating that PD-1 is a negative regulator in DCs. Furthermore, ablation of DCs prior to anti-PD-L1 Ab treatment in an established MC38 tumor model in CD11c-DTR mice suggests a crucial role of DCs in mediating response to PD-L1 treatment. In support of the preclinical evidence that DCs are the primary target of anti-PD-L1 Ab, we analyzed RNA-seq data from tumor biopsies at baseline in patients with renal cell carcinoma prior to treatment with atezolizumab and found that the abundance of genes related to cross-presenting DC subsets (such as XCR1) correlates with a survival advantage in response to atezolizumab (HR=0.13, median OS is 16.2 months in patients with DC gene score 50%). In conclusion, we discovered abundance of tumor-infiltrating DCs as a novel biomarker that can predict the clinical response of PD-L1 blockade. We postulate that checkpoint inhibition directly on tumor-infiltrating DCs likely contributes to amplification of Ag-specific priming of tumor-specific T cells. Citation Format: Maud Mayoux, Marieke F. Fransen, Andreas Roller, Ines Matos, Vesna Pulko, Vaios Karanikas, Pablo Umana, Christian Klein, Ferry A. Ossendorp, Wei Xu. Dendritic cells dictate the responsiveness of PD-L1 blockade in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3658. doi:10.1158/1538-7445.AM2017-3658