An In Vivo Screen Implicates PPP2R2D as an Inhibitor of T-Cell Function

Blockade of T-cell inhibitory receptors has emerged as a promising approach for cancer therapy, but our understanding of pathways that regulate T-cell function and create immunosuppressive tumor microenvironments remains incomplete. Hypothesizing that additional T-cell negative regulators exist that would be overlooked in vitro, Zhou and colleagues developed an in vivo pooled shRNA screening method in which T cells were lentivirally infected with shRNAs targeting kinases, phosphatases, and genes overexpressed in dysfunctional T cells and injected into mice bearing melanoma xenografts that express a recognizable tumor antigen. Genes targeted by shRNAs that were enriched in tumor-infiltrating T cells but not T cells within secondary lymphoid organs represented putative T-cell negative regulators and were chosen for secondary screening with additional shRNAs. In addition to shRNAs targeting known inhibitors of T-cell receptor signaling and T-cell function, the authors found that shRNAs targeting Ppp2r2d, which encodes a regulatory subunit of protein phosphatase 2A (PP2A), induced the most dramatic accumulation of CD4⁺ and CD8⁺ T cells in tumors compared with control shRNA-expressing tumors and with the spleen. Ppp2r2d knockdown significantly increased T-cell proliferation and survival in response to tumor cells and increased T-cell cytokine secretion. Silencing of Ppp2r2d in CD4⁺ and CD8⁺ T cells also enhanced the efficacy of adoptive T-cell therapy in mice with established tumors, leading to an increase in the number of effector T cells in tumors, an increase in apoptosis with tumors, reduced tumor burden, and prolonged survival. These findings show that it is feasible to identify regulators of T-cell function in the context of complex tissue microenvironments and establish an adaptable approach for analysis of immune regulation and discovery of cancer immunotherapy targets.

Zhou P, Shaffer DR, Alvarez Arias DA, Nakazaki Y, Pos W, Torres AJ, et al. In vivo discovery of immunotherapy targets in the tumour microenvironment. Nature 2014 Jan 29 [Epub ahead of print].