A Lymphomimetic Synthetic Immune Niche, Consisting of CCL21 and ICAM1, Accelerates the Expansion of Potent CAR T-cells

Background: Chimeric Antigen Receptor (CAR) T-cell therapy has transformed the treatment of hematologic malignancies, yet, its broader clinical application often faces major challenges, including slow expansion rates, variable transduction efficiency, exhaustion, and functional heterogeneity. Recent studies have demonstrated that a Synthetic Immune Niche (SIN) composed of immobilized CCL21 and ICAM1 promotes the proliferation of murine and human T-cells while preserving their cytotoxic potency. In this study, we explored the capacity of immobilized CCL21 and ICAM1 to enhance the production of highly potent CAR T-cells by facilitating both their expansion and cytotoxic capacity. Methods: CD19-directed CAR T-cells were generated from PBMCs of healthy donors using a clinical-grade protocol. Following retroviral transduction, the cells were expanded on CCL21 and ICAM1 coated plates, or on uncoated control plates. The effects of the SIN treatment on CAR T-cell expansion, morphology, physical properties, phenotypic markers, and potency were systematically evaluated. Results: SIN exposure significantly enhanced CAR T-cell expansion, achieving a 9.3-fold higher total cell yield by day 13, compared with control cultures. This proliferative advantage persisted even after withdrawal of the cells from the synthetic niche on day 10. SIN stimulation preferentially expanded the CAR-transduced population, increasing CAR T-cell frequencies from 45.4% to 77.6%, resulting in a 14.6-fold increase in the absolute number of CAR T-cells compared with untreated cultures. Morphological and phenotypic analyses revealed distinct activated cell morphology, manifested by increased cell size, polarity and granularity, and elevated expression of the activation markers CD137 and CD69. Importantly, CAR T-cells transiently exposed to the SIN retained cytokine secretion and cytotoxic activity comparable to that of continuously SIN-treated cells, indicating that the SIN effect is persistent. Overall, SIN-conditioned CAR T-cells displayed robust antigen-dependent IFN-{gamma} secretion and cytotoxicity against CD19-expressing target cells. Conclusion: Stimulation of CAR T-cells with a synthetic immune niche consisting of immobilized CCL21 and ICAM1 enhances overall expansion while selectively enriching the CAR-transduced population, thereby substantially increasing both the number and prominence of therapeutically relevant CAR T-cells. This effect is accompanied by a persistent activation phenotype and high functional potency. We propose that incorporating SIN stimulation into CAR T-cell manufacturing represents a simple and scalable strategy for improving CAR T-cell yield while maintaining high cytotoxic efficacy.