Leveraging cell’s endogenous regulatory network: towards safer and more effective CAR T cell therapies for solid tumors

Despite immense success in multiple clinical trials for treatment of several hematological malignancies, chimeric antigen receptor (CAR) T cells therapy for solid tumors remains challenging by the limited availability of targetable tumor-specific antigens, the multifaceted immunosuppressive effects of the tumor microenvironment, and inherent tumor cell heterogeneity (1). One of the most promising strategies to overcome these hurdles has been armoring CAR T cells with therapeutic payloads to modulate the tumor microenvironment and enhance CAR T cell antitumor responses. Current “armored T cell” systems such as T cells redirected for universal cytokine-mediated killing (TRUCK) (2,3) with tethered inducible promoter [e.g., nuclear factor of activated T cells (NFAT)-based circuits] or the synthetic logic gate “synNotch” (4,5) offer elegant and customized cytokine program, but they do not completely constrain the delivery of immunostimulatory factors only to the tumor site to avoid undesirable side effects while maximizing efficacy.

In the recently published work on Nature journal, Chen et al. (6) introduced a strategy that uses dual-locus CRISPR knock-in approach to insert payload genes into specific genomic loci whose expressions are controlled by endogenous tumor-restricted promoters. This represents a novel approach that integrates cytokine payload secretion into activation-responsive loci to reprogram T cell signaling networks. By leveraging the cell’s endogenous regulatory mechanism, the transgene expression by the engineered CAR T cells can be restricted to the tumor site. To achieve this goal, the authors systemically screened endogenous T cell genes whose transcription is enriched in tumor-infiltrating cells, and identified NR4A2 and RGS16 as promising promoter candidates in controlling spatial transgene expression. In murine and xenograft models, the authors showed that these two promoters drive highly stringent tumor-localized expression of IL-12 and IL-2 cytokines, demonstrating sustained robust tumor control with a markedly improved safety window compared to the conventional armored CAR T systems.

A particularly smart insight of the work is that different payloads may require different levels of “tightness” in spatial control. Indeed, the authors showed that NR4A2 is ideal for expressing highly potent cytokines like IL-12, whereas RGS16 is more preferable for delivering less-toxic factors, such as IL-2. This flexibility in promoter selection allows a tailored balance of potency and safety. At translational perspective, the authors have gone beyond murine systems to show that human CAR T cells, including patient-derived cells, can be similarly engineered using the knock-in strategy. Moreover, the one-step protocol to simultaneously insert the CAR and payload into T cell receptor alpha constant (TRAC) and endogenous promoter loci increases the translational potential of this approach.

However, concern about antigen specificity remains unaddressed. If the targeted antigen is also expressed in healthy tissue, even at low level, could pose an on-target off-tumor toxicity risk due to the released cytokines. Perhaps, pairing this platform with dual CAR logic gates or AND-gate systems would further reduce the risk.

Nonetheless, this work generated a new class of “smart” CAR T (and other immune cell) therapies capable of making decision on where and when to exert their therapeutic activities by surveying the tumor microenvironment. The CRISPR knock-in CAR framework also provides a powerful tool for developing the next generation of precisely regulated cell therapies.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Translational Cancer Research. The article has undergone external peer review.

Peer Review File: Available at https://tcr.amegroups.com/article/view/10.21037/tcr-2025-aw-2291/prf

Funding: None.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://tcr.amegroups.com/article/view/10.21037/tcr-2025-aw-2291/coif). The authors have no conflicts of interest to declare.

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