Abstract 2308: The secondary immune selection is the dominant mechanism for acquired resistance against adoptive cell therapy
Authors: Kamada Takahiro , Togashi Yosuke , Ohue Yoshihiro , Nishikawa Hiroyoshi 
Published: August 2019

Abstract
Background: Adoptive cell therapy (ACT) has shown a promising result in the treatment of various types of cancers. However, as treated patients sometimes experience recurrences, understanding the mechanism(s) of the recurrences is important to maximize the clinical effect of ACT. Although tumor antigen loss has been reported as a major cause of acquired resistance in ACT, how and when these antigen loss variants are developed remains unclear. Objective:To clarify the mechanism(s) developing tumor antigen loss variants by ACT.
Results: BALB/c mice were inoculated subcutaneously with CMS5a cells (CMS5aP) harboring a neoantigen derived from mutated ERK2 (mERK2). Splenic CD8+ T cells from DUC18 mice (TCR specific for mERK2 transgenic mice) were transferred intravenously into CMS5a tumor-bearing mice. Although all tumors were totally eradicated by ACT, tumors were recurred in 4/12 treated mice. Cell lines were established from recurrent tumors (CMS5aR) and were subjected to further analyses. Whole exome sequence revealed a loss of mERK2 genes due to the deletion of chromosome 16 which contained mERK2 gene in resistant CMS5aR cells, but not in CMS5aP cells. The deletion of chromosome 16 was also confirmed with spectral karyotyping. We therefore addressed whether CMS5aR which lost chromosome 16 were newly developed by ACT or expanded from minor population which had exited at the initiation of ACT. Single cell clones (CMS5aS) were generated from the CMS5aP cells and confirmed to harbor mERK2. Mice were inoculated with these single clones and treated with ACT. Tumors were completely eradicated and no relapses were observed. When ACT was delayed, the frequency of tumor recurrence was increased, indicating that a resistant clone is newly developed in the course of tumor progression and selected during ACT. Then, the sensitive CMS5aS cells and resistant CMS5aR cells were labeled with DsRed-Express and GFP, respectively. Resistant CMS5aR cells were mixed in various proportions with sensitive CMS5aS cells, and the mixture was inoculated into BALB/c mice and treated with ACT. Tumor relapses were observed even in the mixture of resistant CMS5aR cells with the sensitive CMS5aS cells in 1/10000. These relapsed tumors were composed of only resistant CMS5aR tumors labeled with GFP. Based on the experimental data, a mathematical model for the prediction of recurrence was established using tumor-cell growth rates and tumor sizes, which completely followed the outcome of tumors in vivo model.
Conclusion: We propose that the acquired resistance clone against ACT is secondarily induced in the course of tumor progression and selected during the ACT.
Citation Format: Takahiro Kamada, Yosuke Togashi, Yoshihiro Ohue, Hiroyoshi Nishikawa. The secondary immune selection is the dominant mechanism for acquired resistance against adoptive cell therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2308.