Abstract 2251: Non-depleting anti-OX40 antibodies boosts tumor-infiltrating T cells and impairs Tregs without effector T cell depletion

Abstract While antagonistic antibodies targeting CTLA-4 and PD-1 have demonstrated significant success in cancer immunotherapy, agonistic antibodies such as anti-OX40 have shown limited efficacy. This disparity is potentially due to a lack of understanding of how antibody Fc regions interact with FcγRs on innate immune cells. Anti-OX40 antibodies currently in clinical trials are predominantly human IgG1, a strong FcγR engager. This design assumes differential surface OX40 expression will result in regulatory T cell (Treg) depletion and CD4+ effector T cell (Teff) activation, yet the therapeutic outcomes remain suboptimal. Using transplantable murine cancer models, we compared the efficacy of anti-OX40 isotypes with distinct FcγR-binding affinities. Weak FcγR-binding mouse IgG1 triggered robust pan-T cell activation and expansion within the tumor microenvironment while reducing Treg suppressive capacity and destabilizing their transcriptional program. Conversely, strong FcγR-binding mouse IgG2a resulted in the depletion of both Tregs and Teffs from tumors, compromising therapeutic efficacy. Notably, combining anti-OX40 mouse IgG1 with anti-PD-1 resulted in synergistic responses, achieving nearly 100% complete tumor rejection, while anti-OX40 mouse IgG2a failed to show such synergy. These findings indicate that anti-OX40 antibodies with strong FcγR-binding may hinder optimal anti-tumor activity. A more effective approach could involve using weak FcγR-binding isotypes, which preserve effector T cells while activating agonistic signaling and impairs Treg function, thereby enhancing therapeutic efficacy without off-target effects. Future studies will validate these results in patient-derived tumor models to inform the development of anti-OX40-based therapies. Citation Format: Cristobal Costoya, Mariela Navarrete, Aniruddh Tyagi, Dafne Franz Demane, Maria Vila de Mucha, Sergio A. Quezada. Non-depleting anti-OX40 antibodies boosts tumor-infiltrating T cells and impairs Tregs without effector T cell depletion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2251.