Abstract 7305: Increasing TCR/antigen sensitivity using anti-CD3 Fabs enhances anti-tumor effects but not immunotoxicity of immunotherapies blocking T cell checkpoint receptors

Abstract Cancer patients treated with immune checkpoint inhibition (ICI), such as monoclonal antibodies (mAbs) that block the inhibitory functions of the T cell receptors, CTLA-4 and PD-1, are at risk of experiencing immune-related adverse effects (IRAEs). These effects may require interruption, neutralization, or discontinuation of treatments, potentially hindering control of cancer progression. The risk for IRAEs is elevated in individuals with underlying autoimmune diseases (AIDs), resulting in this subpopulation of cancer patients being undertreated with ICI therapies. Establishing novel approaches to enhance anti-tumor control by T cells using ICI without increasing concomitant immunotoxicity could benefit a broad range of cancer patients, including those with autoimmune pathologies. Targeting the CD3ε subunit of the T cell antigen receptor (TCR) with a Fab fragment that strictly controls monovalence (Mono-Fab) improves sensitivity to TCR/antigen stimulation and T cell effector function. We have demonstrated that anti-CD3ε Mono-Fabs are effective immunotherapies against murine melanoma, depending on the presence of functional T cells and the availability of tumor-associated antigens (TAAs) in melanoma cells. Here, the efficacy of the anti-mouse CD3ε Mono-7D6-Fab and its potential to induce IRAEs has been compared to ICI therapies using the mAbs anti-PD-1 and anti-CTLA-4 in mice implanted with subcutaneous melanoma. Results indicate that Mono-7D6-Fab enhances the performance of ICIs based on the level of tumor immunogenicity. However, Mono-7D6-Fab did not increase the immunotoxicity risk of either anti-CTLA-4 or anti-PD-1 monoclonal antibodies when combined with these treatments, nor in experiments involving the three types of immunoglobulins, as seen in mice with experimental autoimmune encephalomyelitis (EAE), which reported a worsening of T cell-mediated immune pathology. Additionally, T, B, and innate immune cell subpopulations found in peripheral lymphoid and CNS tissues, which could mediate EAE, were down-modulated in mice treated with Mono-7D6-Fab alone or combined with ICIs while elevated in mice treated solely with ICIs. Similarly, treatment with ICIs alone failed to control tumor growth in the context of underlying autoimmune disease; however, combining Mono-7D6-Fab with ICIs restores the anti-tumor benefit of ICI without further exacerbating autoimmune toxicities. These promising findings justify further experimentation for the eventual translation of anti-human CD3 Mono-Fabs, potentially enhancing and broadening the application of cancer immunotherapies that target T-cell anti-tumor responses. Citation Format: Onyekachi Juliet Okpasuo, Chandrashekara P. Mahadevappa, Hien Huynh, Christa Cheatam, Vijay Radhakrishnan, Adam Schrum, Diana Gil. Increasing TCR/antigen sensitivity using anti-CD3 Fabs enhances anti-tumor effects but not immunotoxicity of immunotherapies blocking T cell checkpoint receptors [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 7305.