Abstract 1859: The antigen target is critical in the mechanism of resistance to T-cell based therapies
Authors: Martínez-Sabadell Alex , Arenas Enrique J. , Morancho Beatriz , Rius Irene , Escorihuela Marta , Luque Antonio , Arribas Joaquín 
Published: July 2021

Abstract
Purpose: Immunotherapy has revolutionized the way several types of cancers are treated, including gastrointestinal tumors. Despite that, most solid tumor patients do not respond or relapse. T Cell Bispecific antibodies (TCBs) are a promising immunotherapeutic strategy designed to boost the immune response against tumors, as they redirect cytotoxic T cells against the tumor cells. TCBs are engineered molecules that include binding sites to the T cell receptor and to a tumor-specific or a tumor-associated antigen. Clinical evidence shows that TCBs are an effective immunotherapy to treat cancer. However, little is known about the mechanisms of resistance against this promising therapy. Therefore, there is a need to anticipate the mechanism of response and resistance in order to improve the clinical outcome of patients. All TCBs behave with the same mechanism of action, and the same mechanism of resistance, independent of the target, is expected. In this piece of work, we try to address for the first time this question.
Methods: We use as a tool the gastric HER2+/CEA+ cell line MKN45, and TCBs targeting these antigens. We generated resistant cells against both HER2-TCB (HER2R) and CEACAM5-TCB (CEAR) in vitro. To generate these immunoresistant models we co-cultured the tumor cells with peripheral blood mononuclear cells (PBMCs) and the TCBs for several months in order to obtain resistance. In addition, an in vivo model of resistance against CEACAM5-TCB was also generated with a CEA+ colorectal cancer patient derived xenograft (PDX) in a CD34+ humanized mice model.
Results: We successfully generated resistant models to both TCBs, both in vitro and in vivo. In vitro resistant models were corroborated in a organotypic 3D model as well as in an in vivo humanized PBMC model. We evaluated the cause of resistance, and one plausible mechanism that also happen in hematological malignancies treated with CAR Ts is the loss of the antigen, in this case HER2 and CEA. Contrary to our expectations, we observed a dramatic loss of antigen in the case of CEAR resistant cells, in contrast to HER2R resistant cells, which maintain HER2 levels. In vivo model of resistance against CEACAM5-TCB recapitulate the loss of antigen phenotype.
Conclusions: Our results show for the first time that the mechanism of resistance against TCBs can be totally different depending on the target, and future studies and therapeutic approaches should take this into consideration. In addition, the HER2R resistant cells can be used as a tool to identify unknown mechanisms of resistance against the redirection of T-cells.
Citation Format: Alex Martínez-Sabadell, Enrique J. Arenas, Beatriz Morancho, Irene Rius, Marta Escorihuela, Antonio Luque, Joaquín Arribas. The antigen target is critical in the mechanism of resistance to T-cell based 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 1859.