Abstract B179: Enhancing intratumoural Treg depletion through antibody engineering and analysis of the checkpoint landscape of mouse and human cancers
Authors: Vargas Frederick Arce , Furness Andrew J. , Lesko Marta H. , Pule Martin , Ravetch Jeffrey V. , Peggs Karl S. , Quezada Sergio A. 
Published: May 2016

Abstract
Different components of the tumor microenvironment hamper the immune response against cancer cells and are a critical impediment for the success of cancer immunotherapy. Among these, the accumulation of regulatory T cells (Treg) and the reduction of the effector T cells (Teff)/Treg ratio favors tumor progression and is associated with worse prognosis in several human cancers. Treg elimination in the tumor microenvironment can be achieved through antibodies that target molecules expressed preferentially in Treg and that induce antibody-mediated cell cytotoxicity (ADCC). CTLA-4 is highly expressed on tumor-infiltrating Treg and therapy with anti-CTLA-4 has been effective for cancer treatment in murine models and in humans. In mice, we and others have shown that anti-CTLA-4 depletes Treg in the tumor but not in peripheral lymphoid organs. This site specificity in mice is given by the expression of the Fcγ-receptor IV (FcγRIV) in myeloid cells present in the tumor microenvironment but not in the periphery. However, the role of FcγRs in the effectiveness of anti-CTLA-4 therapy or therapies with other immunomodulatory antibodies in humans is not known. We therefore investigated the density of targets of clinically relevant immunomodulatory antibodies in different subpopulations of tumor-infiltrating immune cells as well as the expression of FcγR subtypes in murine tumor models. Because of the inter-species differences in the repertoire and the cellular distribution of FcγRs, we also studied this in humanized-FcγR (huFcγR) mice, in which there is no expression of murine FcγRs but of their human homologues. We compared this with samples of human melanoma and observed surprisingly similar expression profiles of both FcγRs and targets of immunomodulatory antibodies, including CTLA-4. We then asked whether anti-CTLA-4 would also lead to selective Treg depletion in the tumor microenvironment in huFcγR mice. Using an antibody targeting mCTLA-4 with a human IgG1 isotype, we observed that was the case. Furthermore, mutation of the Fc region of the antibody to enhance its affinity to activating huFcγRs resulted in more effective selective Treg depletion in the tumor. These findings are not only relevant for CTLA-4 but also for other targets expressed preferentially on Treg instead of Teff and must be taken into account when designing immunomodulatory antibodies for clinical use.
Citation Format: Frederick Arce Vargas, Andrew J. Furness, Marta H. Lesko, Martin Pule, Jeffrey V. Ravetch, Karl S. Peggs, Sergio A. Quezada. Enhancing intratumoural Treg depletion through antibody engineering and analysis of the checkpoint landscape of mouse and human cancers. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr B179.