Abstract LB070: In vitro modeling of antigen-driven exhaustion in human CAR T cells

Abstract A major hurdle for chimeric antigen receptor (CAR) T cells to function in solid tumors is chronic antigen exposure leading to terminal lymphocyte exhaustion and impaired anti-tumor activity. Therefore, it is necessary to establish improved in vitro models of antigen-driven CAR T cell exhaustion to evaluate new CAR design candidates prior to validation with costly in vivo studies. Using the human tumor antigen mesothelin (MSLN) and MSLN-targeting CAR T cells as a model, we developed two in vitro assays that drive an exhausted phenotype in human CAR T cells and can be used to predict anti-tumor responses in vivo: 1) a chronic stimulation assay with plate-bound recombinant antigen (MSLN), ICAM-1, and CD58 and 2) a high antigen-expressing 3D tumor spheroid co-culture assay. In the chronic antigen stimulation assay, repetitive exposure to plate-bound MSLN (alongside co-stimulation signals provided by recombinant ICAM-1 and CD58) reprogrammed the transcriptomic profile of MSLN CAR T cells leading to increased surface expression of inhibitory checkpoint molecules and reduced cytokine production. Moreover, chronically stimulated MSLN CAR T cells demonstrated a drastically impaired ability to kill MSLN+ tumor cells compared to their unstimulated or acutely stimulated counterparts. A similar exhausted phenotype was observed in the high-antigen tumor spheroid model: when MSLN CAR T cells were co-cultured with MSLN-high human NCI-H226 spheroids for 7 days in a 96-well plate, expression of inhibitory checkpoint molecules markedly increased on the surface of CAR T cells. This increased expression of exhaustion markers coincided with impairment of NCI-H226 spheroid clearance. By contrast, other tumor spheroid models expressing low to intermediate levels of MSLN (NCI-H1650, NCI-H2052, NCI-H292, and SKOV-3 tumor cells) did not substantially induce expression of inhibitory checkpoint molecules and the spheroids were successfully cleared by MSLN CAR T cells. Importantly, the ability (or inability) of a CAR construct to clear NCI-H226 high-antigen tumor spheroids in vitro accurately predicted CAR T cell performance in in vivo solid tumor studies: the CAR construct that showed effective killing in the in vitro spheroid model was the only condition to provide durable regression of NCI-H1650 tumors in a NSG xenograft mouse model. Additionally, the rank order of CAR efficacy for a series of MSLN CAR binders obtained using the tumor spheroid model mirrored in vivo efficacy. As the CAR T cell field develops novel strategies to combat antigen-driven exhaustion for use in solid tumors, these new antigen-dependent in vitro models of exhaustion can enable more cost-efficient and effective evaluation of those strategies than using in vivo models alone. Citation Format: Maria Steele, Tina Tan, Jason Yokoyama, Allan Wang, Ty Crowl, Jared Hammer, Jerry Chen, Kevin Haworth, Howell Moffett, Aaron Foster, Marc Lajoie, Scott Boyken, Rupesh Amin. In vitro modeling of antigen-driven exhaustion in human CAR T cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB070.