Abstract 3995: A novel chimeric Fas signal redirect receptor enhances the durability of anti-tumor activity and serial killing potential of CAR T cells
Authors: Ecker Christopher , Peralta Eigen , Chu Hui-Yi , Loter Lorraine , Tsuda Masanao , Avramis Earl , Denholtz Matthew , Witty Alec , Lee Tom , Valamehr Bahram 
Published: March 2024

Abstract
Chimeric antigen receptor (CAR) T-cell therapies have demonstrated remarkable success in hematological malignancies but only modest efficacy in solid tumors. Apoptosis induced by Fas receptor signaling can be a substantial barrier to CAR T-cell antitumor activity, as many solid tumors express high levels of Fas ligand (FasL). Here we describe a novel synthetic construct where the ectodomain of Fas receptor is fused to the signaling endodomain of a stimulatory receptor, creating a Fas signal redirect receptor (Fas-SRR). This unique design not only prevents Fas-dependent cell death, but also augments the long-term killing potential of induced pluripotent stem cell (iPSC)-derived CAR-T (CAR-iT) cells. Fas-SRR constructs with unique T-cell activation signaling endodomains were designed and compared to two control arms: a dominant negative receptor (Fas-DNR), which blocks Fas signal but does not promote activation; and a Fas-41BB fusion receptor, where the 41BB endodomain is fused to FasR ectodomain to direct activation upon FasL:Fas engagement. Fas-SRR designs were validated in Jurkat reporter cells treated with Fas agonist to measure signal redirection via Fas-SRR. To conduct a head-to-head comparison, CAR-iT cells were engineered to express a CAR under the regulation of the TRAC locus and to express either a Fas-DNR, a Fas-41BB, or FAS-SRR constructs at high purity (&gt;90% transgene expression). No major differences were seen in maturation phenotype (&gt;95% TRAC-CAR expression) of these cells, indicating compatibility of the FAS-SRR constructs with CAR-iT cell generation. Functional testing of the Fas-SRR+ CAR-iT cells was performed in a long-term restimulation assay where effector cells were repeatedly exposed to target cells over multiple rounds of co-culture. Complete loss of target cell lysis was observed by round six in the co-cultures with FAS-DNR cells, where loss of CAR-iT cell persistence led to accumulation of target cells. After eight rounds of co-culture, the Fas-41BB cells and the majority of Fas-SSR cell constructs also began to exhibit minimal activity toward target cells. However, the top Fas-SRR cell constructs continued to show durable antitumor activity (&gt;97% cytolysis) through the end of the 9-round rechallenge assay. Of note, GSEA analysis identified significant upregulation of NFkB and RELA target genes in cells having the top FAS-SRR construct versus the less durable designs. These results reinforce the advantage of attenuating Fas-FasL interactions to prevent apoptosis and, more importantly, demonstrate the unique therapeutic value in redirecting Fas signaling to improve T-cell potency over an extended duration. Additional studies are underway to further investigate the implications of Fas signal redirection in CAR T and CAR-iT cells and to further understand the module of genes involved in supporting serial stimulation performance.
Citation Format: Christopher Ecker, Eigen Peralta, Hui-Yi Chu, Lorraine Loter, Masanao Tsuda, Earl Avramis, Matthew Denholtz, Alec Witty, Tom Lee, Bahram Valamehr. A novel chimeric Fas signal redirect receptor enhances the durability of anti-tumor activity and serial killing potential of CAR T cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3995.