Immunotherapies through targeting of activating and inhibitory immune checkpoints (ICP) has revolutionized the treatment of cancers. To facilitate the translation of basic research into clinical application of these immunotherapies, we developed a pipeline of immunocompetent mouse models expressing humanized ICP instead of their mouse counterparts, thus compounds can be tested in the absence of endogenous cross-reacting mouse targets. The humanization strategy is target-dependent but ensures that the biology of the target, its physiological regulation and interacting partners are preserved. Herein, we describe a novel OX40 humanized mouse model (hOX40 model), designed to assess the efficacy and mechanism of action of compounds targeting human OX40 (hOX40). The expression of the fully human OX40 protein (human extracellular, transmembrane and intracellular domains) did not alter the distribution of immune cells in spleen, blood and bone marrow. hOX40 was expressed on CD4 and CD8 T cells upon activation and the kinetic of expression mirrored the expression of mouse OX40 in WT cells activated under the same conditions. Treg cells isolated from hOX40 mice expressed hOX40 and this expression was upregulated upon activation, also recapitulating the expression of mouse OX40 in WT cells, and suggesting that the regulation of OX40 expression was kept in the hOX40 mice. hOX40 was also found to be functional on CD4 T cells, as evidenced by the increased proliferation and cytokine production upon stimulation with human OX40 ligand. Importantly, hOX40 mice bearing MC38 tumors showed a tumor growth inhibition upon treatment with an anti-human OX40 therapy with agonist activity. This model should also enable the investigation of mechanism of action as well as tumor immunological memory, as a similar design of the hOX40 model was used for the assessment of a bispecific therapy targeting OX40 and CTLA-4 (Kvarnhammar, 2019). The hOX40 mouse model is currently being crossed with other models expressing humanized immune checkpoint to enable the assessment of bispecifics and combo therapies.

Citation Format: Fabiane Sônego, Gaëlle Martin, Audrey Beringer, Chloé Beuraud, Yacine Cherifi, Alexandre Fraichard, Patricia Isnard Petit, Kader Thiam. A novel OX40 humanized mouse model for efficacy assessment of OX40-targeting therapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1811.