Despite the significant clinical responses observed with checkpoint blockade, there is an urgent need to identify new therapeutic targets in immune cells for combination therapies. However, the use of functional genomic approaches in immune cells to discover immunotherapy targets is limited by inefficient vector delivery or perturbation of cell states. To circumvent these limitations, we developed CHIME: CHimeric IMmune Editing, a bone marrow chimeric CRISPR-Cas9 delivery system, to rapidly evaluate gene function in innate and adaptive immune cells in vivo without prior ex vivo manipulation. This approach enables efficient deletion of genes of interest in major immune lineages without altering immune development or function. To discover novel immunotherapy targets we performed an in vivo pooled genetic screen for negative regulators of CD8+ T-cell responses to LCMV Clone 13 viral infection. We found that deletion of the phosphatase Ptpn2 enhances CD8+ T-cell responses to chronic pathogens and cancer. In models of both chronic viral infection and transplantable tumors, Ptpn2 null CD8+ T cells expand more and show increased expression of effector genes compared to wild-type cells. Consistent with this, in the LCMV Clone 13 model, Ptpn2 deletion in CD8+ T cells affects the differentiation and expansion of exhausted subpopulations by increasing IFN-I signaling. Furthermore, deletion of Ptpn2 in the immune system induces a CD8+ T cell-dependent clearance of tumors and synergizes with PD-1 immune checkpoint blockade. Our results suggest that therapeutic inhibition of Ptpn2 in immune cells may enhance CD8+ T-cell effector function and mediate antitumor immunity to improve tumor control. More generally, these findings suggest that this genetic platform can enable rapid target discovery through pooled loss-of-function screening in immune cell lineages in vivo and presents a novel target for potential immune based therapies.

Citation Format: Martin LaFleur, Thao Nguyen, Matthew Coxe, Brian Miller, Kathleen Yates, Jacob Gillis, Debattama Sen, John Doench, Nicholas Haining, Arlene Sharpe. CHIME screening identifies PTPN2 as a novel regulator of antitumor immunity [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B47.