Abstract 5573: Mechanism of resistance against T cell bispecific antibodies depends on targeted antigen
Authors: Martínez-Sabadell Alex , Arenas Enrique J. , Rius Irene , Román Macarena , Morancho Beatriz , Bernado Cristina , Arribas Joaquin 
Published: September 2020

Abstract
Immunotherapy has revolutionized the way several types of cancers are treated, including breast and gastric cancer. One of the most successful therapies against HER2 positive tumors, trastuzumab, is a monoclonal antibody originally designed to inhibit HER2, which has been shown to be effective because not only impairs cell cycle but also promotes an immune reaction. Despite that, lots of 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, within a single entity, 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. Indeed, as all TCBs behave with the same mechanism of action, the same mechanism of resistance, independent of the target, is expected. In this piece of work, we tried to address for the first time this question, by using as a tool a gastric HER2+/CEA+ cell line MKN45, and TCBs targeting these antigens. We generated resistant cells against both HER2-TCB and CEA-TCB in vitro. To generate these immunoresistant models we co-cultured peripheral blood mononuclear cells (PBMCs) with the TCBs for several months in order to obtain resistance. We corroborated the resistance in a organotypic 3D model and also in an in vivo humanized PBMC model. In addition, we assessed the impact on T-cell effector function, and resistant cells impair T-cell activity. We next evaluated the cause of resistance, and one plausible mechanism that also happen in hematological malignancies treated with Chimeric Antigen Receptor T-cells (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 CEA-TCB resistant cells, in contrast to HER2-TCB resistant cells, which maintain HER2 levels. 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 HER2-TCB resistant MKN45 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, Irene Rius, Macarena Román, Beatriz Morancho, Cristina Bernado, Joaquin Arribas. Mechanism of resistance against T cell bispecific antibodies depends on targeted antigen [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5573.