Experiments with human anti-CD19 CAR-T and lymphoma cells in culture validated these murine results. Moreover, bystander effects also seemed to contribute to the efficacy of checkpoint inhibitors. In mice, PD-1 blockade enhanced T-cell killing of on-target cells while nearly doubling the elimination of bystander cells.
“It’s very interesting data suggesting that when you have to do any immune checkpoint modulation therapies or immunotherapies, Fas is a critical molecule that needs to be modulated ahead so that you have a better efficacy,” remarked Lalit Sehgal, PhD, an assistant professor in the division of hematology at The Ohio State University in Columbus who was not involved in the new research.
Bystander killing has been previously shown in cell culture, “but this is the first time that’s [been] physically shown [in vivo],” Sehgal added. In theory, “that means you can treat heterogenous antigens without having to worry about specific antigen enrichment of CAR-T cells.”
Importantly, the team examined tumoral RNA sequencing data (prior to patient receipt of a CD19-directed CAR construct) from a subset of patients with refractory DLBCL who were participating in the ZUMA-1 trial. Patients with high tumoral Fas expression tended to have significantly prolonged survival compared with those with lower expression.
Notably, a separate analysis of lymphoma treatment data signaled that the opposite might be the case for patients receiving regular chemotherapy, suggesting that the effect of Fas is context-dependent.5 “It’s not always just a death switch,” Brody said.
The human CAR-T data, Kenderian said, “is supportive [of the clinical reality] that Fas is an important mechanism . . . with CAR-T cell therapy.” Yet, regarding the extent to which the mechanism contributes to CAR-T failure and clearance of tumors, Kenderian does not “think we have a good answer to that yet.”
He added that the team largely relied on a single murine CAR-T construct in their animal experiments. Ideally, he’d like to see more validation that human CAR-T cells have the same effect in vivo, and that the phenomenon applies across different CAR constructs, he said.
To Brody, Fas has been a somewhat neglected molecule. Even some companies specializing in evaluating the efficacy of CAR-T cells often don’t include Fas in T-cell gene panels, he said. Yet, cell death receptors—a group of proteins to which Fas belongs—are steadily gaining more recognition. Another recent study reported that patients with ALL whose tumor cells have elevated death receptor signaling—including enrichment of Fas—had better responses to CAR-T cell therapy.6
The major question is how to translate such findings clinically. Tinkering with T cells to boost their Fas expression could “lead to uncontrolled proliferation of the T cells…more toxicity, and even potentially leukemic transformation of the T cells,” Kenderian said.
But, coaxing tumor cells into increasing their Fas expression, perhaps through epigenetic inhibitors of pathways that tumors use to reduce their Fas expression, might be a good strategy, Sehgal noted. Some of his research has identified an immune evasion mechanism whereby lymphoma cells evade cell death by secreting their Fas receptors into extracellular spaces, where they act as decoys for T cells.7
Brody’s lab is currently exploring the activity of some FDA-approved agents that appear to increase Fas signaling, including certain anti-BCL-2 agents, inhibitors of apoptosis, and histone deacetylase blockers. Such a strategy could have advantages over simply designing CARs that target new antigens.
“Going after a second target is good. [But] it has these big limitations: Sometimes the second target disappears along with the first target, and a lot of times, we don’t have a second target,” Brody said. “We think that targeting their geography [makes more sense]—not their CD19-ness…not their CD20-ness, but just their near-ness to an antigen-positive cell,” he concluded.
Disclosures: Some of the study authors disclosed financial relationships with the pharmaceutical industry and/or the medical device industry. For a full list of disclosures, please refer to the original study.
- Rosenthal J, Naqvi AS, Luo M et al. Heterogeneity of surface CD19 and CD22 expression in B lymphoblastic leukemia. Am J Hematol. 2018;93(11):E352-E355. doi:10.1002/ajh.25235
- Majzner RG, Mackall CL. Tumor antigen escape from CAR T-cell therapy. Cancer Discov. 2018;8(10):1219-1226. doi:10.1158/2159-8290.CD-18-0442
- Schuster SJ, Bishop MR, Tam CS, et al; JULIET Investigators. Tisagenleucel in adult relapsed or refractory diffuse large B-cell lymphoma. N Engl J Med. 2019;380(1)45-56. doi:10.1056/NEJMoa1804980
- Upadhyay R, Boiarsky JA, Pantsulaia G et al. A critical role for fas-mediated off-target tumor killing in T cell immunotherapy. Cancer Discov. Published online December 17, 2020. doi:10.1158/2159-8290.CD-20-0756
- National Cancer Institute. The Cancer Genome Atlas Program. Accessed January 18, 2020. https://www.cancer.gov/about-nci/organization/ccg/research/structural-genomics/tcga
- Singh N, Lee YG, Shestova O et al. Impaired death receptor signaling in leukemia causes antigen-independent resistance by inducing CAR T-cell dysfunction. Cancer Discov. 2020;10(4):552-567. doi:10.1158/2159-8290.CD-19-0813
- Sehgal L, Mathur R, Braun FK et al. FAS-antisense 1 IncRNA and production of soluble versus membrane Fas in B-cell lymphoma. Leukemia. 2014;28(12):2376-2387. doi:10.1038/leu.2014.126