Abstract 2328: ICOS signaling promotes CD4+ effector T-cell function during antitumor responses

Inducible costimulator (ICOS) is a CD28-related cell surface receptor which provides an important costimulatory signal to T cells during activation. Prior work has found that anti-ICOS antibodies can promote anti-tumor immunity in vivo, but it is unclear whether this effect results from ICOS signaling blockade and/or agonism, or whether direct antibody-mediated depletion of ICOS+ Tregs is responsible for enhancing anti-tumor immunity. Here, we generated anti-ICOS-L blocking antibodies in mice and engineered such antibodies to contain mouse IgG1 Fc regions with low Fc receptor engagement and good pharmacokinetics. These reagents were used in vivo to specifically block ICOS signaling in syngeneic mouse tumor models without potential depletion of immune cells through ADCC. The role of ICOS signaling in anti-tumor immunity was addressed both in isolation, and together with combination immunotherapy treatments of mouse syngeneic tumor models, followed by flow cytometry-based characterization of peripheral and tumor-infiltrating T cell subsets. Finally, CT26 tumor cells were engineered to express mouse ICOS-L in a doxycycline-inducible fashion to promote additional ICOS signaling within the tumor microenvironment. Initial characterization of ICOS blockade in tumor models revealed that inhibition of ICOS signaling alone was sufficient to mediate a modest anti-tumor effect, and characterization of intratumoral T cell populations demonstrated a specific reduction of intratumoral Tregs with ICOS signal deprivation. A survey of previously identified immunomodulators found that, like anti-CTLA-4, anti-OX40 treatment of tumors was able to drive upregulation of ICOS on multiple T cell subsets, while anti-4-1BB therapy was not. Combining ICOS-L blockade with these additional immunotherapeutic approaches showed that inhibition of ICOS signaling had little impact on CD8+ T cell-mediated anti-tumor immunity induced by anti-4-1BB. However, the efficacy of anti-OX40 therapy, which depletes Tregs and relies to a large degree on CD4+ effector T cells responses, was significantly impaired with ICOS-L blockade. In contrast, the induction of additional intratumoral ICOS signaling in established tumors increased the frequency of tumor rejection and overall survival when combined with anti-OX40 therapy. Taken together, these results show that ICOS signaling during the anti-tumor immune response acts on both effector and regulatory T cell subsets, and thus manipulation of ICOS signaling alone may be an inefficient strategy for immunotherapy. However, the use of ICOS agonists in combination with Treg-depleting therapies such as anti-OX40 should limit the potential to promote Treg function or expansion and instead result only in enhanced CD4+ effector T cell responses. Citation Format: Todd C. Metzger, Hua Long, Shobha Potluri, John C. Lin, Reid M.R. Feldman. ICOS signaling promotes CD4+ effector T-cell function during antitumor responses. [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 2328.