Abstract 3551: Signal 1 boosters that address the antigen-access problem for solid tumor cell therapy

Background: Access to tumor antigens in solid tumors is one of the major impediments to progress in oncology, exemplified by the stark difference between the success of cell therapies for blood cancers vs. solid tumors. To overcome this obstacle, cell therapy research and development has explored a wide range of methods to enhance potency without worsening toxicity. Special attention has been paid to onboard potency boosters expressed by engineered immune cells, because they more readily avoid systemic toxicity. Investigation has focused primarily on signals 2 (co-stimulatory factors) and 3 (cytokines), but other mechanisms have also been considered, including expression of transcription factors, anti-apoptotic proteins, and TGFβ pathway inhibitors. Despite the magnitude and range of these efforts to boost potency, little attention has been directed at enhancers of signal 1, beyond designs intended to improve acute sensitivity of chimeric antigen receptors (CARs) and t-cell receptors (TCRs). Methods: Three different small-molecule-regulated systems were explored as inducible modules to regulate tonic signaling by CARs and TCRs. Acute and long-term behavior was examined, with an emphasis on inducer-dependent activation of immune function. Fusions of known members of the TCR signaling pathway were compared to identify modules with different functional properties; for example, background signaling level and ligand-induced induction ratio. Results: A variety of constructs, including fusions to LAT, SLP76, CD3ε, and CD3ζ were shown to affect tonic signaling in a fashion that could be controlled by small-molecule inducers. Acute effects measured in Jurkat cells were also observed in primary T cells. Three clinically-tested classes of small molecule were able to act with their cognate ligand-binding fusion proteins as signal 1 boosters, demonstrating flexible small molecule control of the booster. This tunable-booster approach was extended to TmodTM, a dual-receptor NOT gate based on the LIR-1 inhibitory receptor (Hamburger AE, et al. Mol Immunol. 2020). Conclusions: We have developed an approach to mitigate a major challenge for solid tumor cell therapy—access to tumor antigens in the blood. The system utilizes signal 1 agonists and is tunable, flexible, and modular. These signal 1 booster constructs can be added to CARs, TCRs, and other receptor systems to mimic an antigen stimulus with a small molecule, thereby triggering tonic (antigen-independent) signaling that reversibly activates the cells in the absence of antigen. Citation Format: Diane Manry, Kelly Radecki, Alexander Partin, Charles Kirsh, Anagha Krishnagiri, Navneet Kumar, Yuta Ando, Jon Torres, Breanna DiAndreth, Timothy P. Riley, Alexander Kamb. Signal 1 boosters that address the antigen-access problem for solid tumor cell therapy [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 3551.