Unlocking antigen flexibility with widened patient access: CD3FR in off-the-shelf CAR T cells

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Combining chimeric antigen receptor (CAR) engineering with CRISPR-Cas9 gene editing paves the way toward precise and patient-specific cancer therapies.¹ Treatment with autologous anti-CD19 CAR T cells has already become clinical routine, as it shows high remission rates in B cell malignancies.² Broader application of CAR T cell therapy, however, remains challenging. This is in large part due to low CAR T cell sensitivity toward antigens, an overall paucity of tumor-specific antigens qualifying as CAR targets, the frequently inferior fitness of patient-derived T cells for CAR expression, especially after chemotherapy, if CAR T cell therapy is not first line of treatment, and the complex and costly manufacturing process.³

Using highly functional allogeneic T cells isolated from healthy donors as a source of CAR T cells offers, in many regards, a compelling, scalable, and off-the-shelf alternative.⁴ Applying, in parallel, bispecific CD3-directed T cell engagers (TCEs), which redirect both CAR and endogenous T cells to attack tumor sites, can enhance clinical efficacy and durability by accounting for heterogeneity in antigen expression and by reducing immune escape.⁵ Importantly, removing endogenously expressed T cell receptors (TCRs) in allogeneic settings is essential to prevent manifestation of graft versus host disease. However, doing so also eliminates CD3 expression, rendering T cells unresponsive to antigen- and CD3-targeting TCEs. In this issue of Molecular Therapy, Lu et al. address this problem by engineering a CD3ε fusion receptor (CD3FR), which restores surface CD3 in TCR-deficient CAR T cells and, as a consequence, TCE compatibility.⁶ Moreover, as the authors advocate, generating CD3FR⁺ CAR T cells from induced pluripotent stem cells (iPSCs) could enable scalable manufacturing of high-quality T cell products to be administered to a much larger community of patients in need.⁷

The simplicity of the CD3FR concept and its game-changing potential

The study focuses primarily on the design of a novel CD3FR engineered to reconstitute CD3 surface expression on TCR-deficient T cells. By fusing the CD3ε ectodomain (ED) with a CD28 transmembrane domain (TM) followed by the cytoplasmic tail of CD3ε (CT), forming the construct CD3FR-1 (Figure 1), the authors achieved stable surface expression of CD3FR in TCR-deficient Jurkat T cells as well as iPSC-derived T cells (iT cells). Ectopic expression of the CD3FR construct supported T cell activation via TCEs in the absence of native TCR signaling.

Figure 1.

CD19 CAR iT cells derived from iPSCs and stably expressing CD3FR are activated through CAR-CD19 interactions as well as bispecific TCEs that simultaneously target CD3 and the tumor-associated antigens EpCAM/HER2

This multi-antigen targeting strategy results in enhanced production of pro-inflammatory cytokines, upregulation of activation markers, as well as increased cytolysis of tumor cells that ultimately reduce tumor burden in vivo. ED, CD3ε ectodomain; TM, CD28 transmembrane domain; CT, cytoplasmic tail of CD3ε. The figure was designed in BioRender.

Lu et al. demonstrated robust antigen-dependent tumor cell killing by CD3FR⁺ CAR iT cells in the presence of TCEs targeting EpCAM and HER2.⁶ Enhanced production of interferon γ, tumor necrosis factor alpha, and GM-CSF; upregulated expression of CD25; as well as increased cytolysis in vitro and in vivo were also observed in hematological (MM.1S and Nalm6) and solid (H226, SUIT2, and SKOV3) tumor settings. As shown in the study, CD3FR⁺ CD19 CAR iT cells effectively eliminated both CD19⁺ and CD19⁻ tumor cells in vitro and in vivo when combined with TcE_EpCAM/TcE_HER2, a finding that supports the synergy between CD3FR and CAR to counter immune escape by heterogeneous and highly adaptable tumors.

Notably, CD3FR⁺ CAR iT cells, which had been engineered to secrete TCEs, demonstrated enhanced antitumor activity (1) by means of autocrine activation and (2) through recruitment and activation of bystander primary T cells (pT cells). In fact, TCE secretion eliminated the need for external TCE administration and significantly boosted tumor cell killing, as evidenced by increased activation markers (CD25 and CD69) and elevated pro-inflammatory cytokines and cytolytic activity for both iT and pT cells.

Further expanding the platform’s applicability, Lu et al. demonstrated that CD3FR⁺ iPSC-derived natural killer (NK) cells can be effectively paired with TCEs, resulting in enhanced NK cell cytotoxicity and efficient antigen-dependent targeting of cancer cells. Altogether, these findings make a game-changing case for iPSC-derived CD3FR-expressing CAR T and NK cells to promote the efficacy, flexibility, and scalability of allogeneic adoptive immunotherapy in a clinical world of need.

New treatment possibilities and remaining questions

The sheer simplicity of the CD3FR design and of its clinical implementation appears as a major strength of this proposed therapeutic platform, promising precise tuning of intracellular signaling to enhance antitumor responses. By restoring CD3 functionality in TCR-deficient CAR T cells, the platform allows for integration of diverse TCEs without requiring additional genetic modifications to address tumor heterogeneity or antigen escape. Unlike fixed-target dual or tandem CARs, the use of CD3FR⁺ CAR T cells in combination with TCEs decouples antigen specificity from the cell product, enabling real-time adjustments to target various tumor antigens.

Another advantage constitutes the use of iPSCs as a source for CAR T cell production, supporting scalability, standardization, and cost efficiency. The CD3FR⁺ CAR iT cell platform also offers effective immune activation with potentially reduced on-target, off-tumor toxicity due to autocrine TCE production, including activation of conventional bystander T cells.⁸ Furthermore, compatibility with NK cells extends its utility across immune effector types.⁹ Beyond oncology, CD3FR⁺ CAR iT cells may provide durable responses in autoimmune diseases such as systemic lupus erythematosus, idiopathic inflammatory myopathies, and systemic sclerosis.¹⁰

Despite these promising applications, several critical issues will need to be addressed in future research. These include the signaling profile in the presence and absence of antigen, the immunogenicity of CD3FR, as well as long-term in vivo data on persistence, safety, and efficacy. Systemic toxicity and cytokine release must be evaluated to assess and possibly mitigate the risks associated with TCE activity. The increased complexity of genetic engineering with add-on features such as TCE expression also raises manufacturing and regulatory challenges, which need to be resolved for clinical translation. Key requirements for validation include defining the antigen sensitivity thresholds of CD3FR⁺ CAR T cells and assessing their function across diverse tumor types. In tumor microenvironments enriched with TCR-expressing regulatory T cells, TCEs produced locally by CD3FR⁺ CAR T cells may suppress rather than promote antitumor immunity. Finally, comprehensive analysis of manufacturing scalability and feasibility will be essential before this approach can be offered to patients as an off-the-shelf therapy.

Promises to be met by future implementation

Integrating CD3FR into iPSC-derived CAR iT cells could mark a major advancement in allogeneic cell therapy, offering a highly modular, scalable, and adaptable platform for targeted immunotherapy. Restoring CD3 function in TCR-deficient cells enables synergy with TCEs to address tumor heterogeneity, boost antitumor activity, and limit immune escape. Additional benefits—including autocrine TCE secretion, compatibility with NK cells, and applicability beyond cancer—broaden its therapeutic scope. With continued research into safety, efficacy, and production challenges, CD3FR-based therapies hold strong potential to become cost-effective, off-the-shelf solutions for next-generation immunotherapy.

Acknowledgments

N.N. and J.B.H. are supported by Emerging Fields Grant “Devising Advanced TCR-T cells to eradicate OsteoSarcoma (DART²OS)” EFP 45 of the Austrian Science Fund (FWF). J.B.H. receives further financial support from the German Cancer Consortium (DKTK) of the Helmholtz Association.

Declaration of interests

The authors declare no competing interests.