Abstract 4550: CD80 vIgD-Fc proteins combine checkpoint antagonism and costimulatory signaling for potent antitumor immunity

Introduction: PD-1 pathway antagonists have revealed the importance of checkpoint pathways in regulating antitumor immunity, but an existing immune response is generally required for clinical efficacy. Specific T cell costimulation through CD28is central to this process, but the CD28ligands CD80and CD86are often poorly expressed in the tumor microenvironment, accounting for a second important mechanism of immune evasion by tumors. In contrast, PD-L1expression has been found extensively in multiple tumor cell types. Therapeutics that combine PD-L1/PD-1 antagonism coupled with PD-L1dependent CD28 agonismmay therefore provide a more potent, yet safe immunotherapeutic approach. Experimental Procedures: The variant Ig Domain (vIgD) TM platform has generated a diversity of human CD80variants using yeast display affinity maturationand selections against all three CD80counterstructures CD28, CTLA-4, and PD-L1. CD80vIgDs were produced in a mammalian expression system as recombinant Fc fusion proteins ( CD80vIgD-Fc proteins) and their binding properties were quantified by flow cytometry. Functional activity was determined in vitro by assessing responses from human primary T cells or an IL-2-luciferase Jurkat T cell reporter line stimulated with PD-L1-expressing artificial antigen presenting cells(aAPCs). In vitro human T cell cytotoxicity assays with a human PD-L1-expressing tumor line were also performed. Antitumor activity was assessed in vivo with mice implanted with human PD-L1transduced MC38 tumors. Data Summary: The human CD80IgV fragment was found to be optimal for high affinity PD-L1and CD28binding. A large panel of CD80vIgD-Fc proteins demonstrated a range of binding towards CD28, PD-L1, and/or CTLA-4, and CD80vIgD-Fc proteins with high affinity for PD-L1antagonized the PD-L1/PD-1 interaction. Some CD80vIgD-Fc proteins agonized CD28in a PD-L1dependent fashion with increased luciferase activity in the Jurkat reporter assay as well as increased cytokine production by primary human T cells when stimulated with a PD-L1expressing aAPC in vitro. The same candidates also showed specific killing of human PD-L1expressing tumor cells in vitro compared to the parental tumor line lacking PD-L1expression. Importantly, selected CD80vIgD-Fc proteins caused significant tumor reduction in the MC38 in vivo tumor model. Conclusion: Engineered CD80vIgD-Fc proteins that deliver a localized CD28costimulatory signal to T cells while simultaneously antagonizing the inhibitory PD-L1/PD-1 pathway may provide a transformative mechanism of action to drive potent, tolerable antitumor immunity. Preclinical development of therapeutic candidates is under way. Citation Format: Ryan Swanson, Mark F. Maurer, Chris L. Navas, Chelsea J. Gudgeon, Joseph L. Kuijper, Martin Wolfson, Katherine E. Lewis, Stacey R. Dillon, Steve D. Levin, Michael G. Kornacker. CD80vIgD-Fc proteins combinecheckpoint antagonism and costimulatory signaling for potent antitumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4550.