Abstract 903: IFN-ã induced PD-L1 on tumor and host cells co-operatively prevents tumor immune elimination after cancer immunoediting

Tumor cells can escape immune destruction via engaging the PD-1/PD-L1 inhibitory pathway. However since PD-L1 is induced on many different cell types by IFN-ã, the importance of PD-L1 on tumor versus host cells in mediating immune escape remains unclear. In this study, we assessed the role of PD-L1 expression on two closely-related but distinct clones derived from the same d42m1 methylcholanthrene (MCA)-induced sarcoma line. Edited d42m1-T3 sarcoma cells grow progressively in wild type (WT) mice but are rejected in mice treated with anti-PD-L1. Upregulation of PD-L1 was observed on both immune and tumor cells in vivo. To assess the role of PD-L1 specifically on tumor cells, we generated d42m1-T3 cells lacking PD-L1 (T3ÄPDL1) using CRISPR-Cas9. T3ÄPDL1 cells were spontaneously rejected in naïve syngeneic WT mice but formed progressively growing tumors in Rag2-/- mice. To explore the role of host-cell expressed PD-L1, we challenged WT mice with 3-fold higher numbers of T3ÄPDL1 cells and found that the tumor cells formed progressively growing tumors in WT mice. However, when tumor bearing mice injected with the high dose of T3ÄPDL1 cells were treated with blocking PD-L1 mAb, they all rejected their tumors. Thus, PD-L1 expression on both tumor and host cells plays a critical role in preventing immune elimination of edited MCA sarcoma cells. Alternatively highly immunogenic, unedited d42m1-T9 sarcoma cells were spontaneously rejected in WT mice due to the presence of the strong rejection antigen, mutant Spectrin-â2. Enforced expression of PD-L1 on unedited d42m1-T9 cells to levels similar to those induced under physiologic in vivo conditions did not alter the regressor phenotype of these cells. In contrast, enforced overexpression of PD-L1 on d42m-T9 cells to levels that were 20 fold higher than physiologic led to formation of progressively growing tumors in WT mice. Thus, physiologic levels of PD-L1 expression on highly immunogenic unedited tumor cells are not sufficient to counteract immune effector mechanisms driven by “strong” tumor antigens. This result shows that adaptive immune resistance occurs only on tumor cells that have undergone immunoediting.

Citation Format: Takuro Noguchi, Jeffrey P. Ward, Matthew M. Gubin, Cora Arthur, Robert D. Schreiber. IFN-ã induced PD-L1 on tumor and host cells co-operatively prevents tumor immune elimination after cancer immunoediting. [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 903.