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Background CAR-T cell therapies have shown significant success in treating hematological malignancies; however, their application to solid tumors remains challenging due to the hostile tumor microenvironment (TME) and T cell exhaustion. Among the many factors in the TME that impair T cell function, elevated sodium (Na +) levels, and the resulting hyperosmotic stress, contribute to tumor progression and resistance to therapy. In response to hyperosmotic stress, T cells activate the transcription factor NFAT5, which drives the expression of osmo-adaptive genes to restore cellular homeostasis. However, recent studies have shown that sustained NFAT5 activation in the TME can reinforce CD8+ T cell exhaustion. To address these challenges, we engineered hyperosmotic stress-responsive nucleocytoplasmic tags (NCtags) using distinct parts of the NFAT5 intrinsically disordered regions (IDRs), which can sense increases in cytoplasmic ionic strength and respond by translocating into the nucleus. We fused these NCtags to a synthetic transcription factor (synTF) to drive the expression of anti-exhaustion transgenes in CAR-T cells, with the goal of enhancing their efficacy in solid tumors.Methods We constructed various lentiviral vectors containing NFAT5-derived NCtags fused to a synTF under a constitutive promoter. To assess transcriptional activity, a mScarlet fluorescent reporter was placed under the control of an inducible promoter responsive to the synTF in the same vector. This system was used to evaluate reporter expression in HER2-CAR T cells exposed to hyperosmotic stress in 3D spheroid models of breast cancer cell lines.Results Our results demonstrate that NFAT5-derived NCtags function as effective intracellular sensors of hyperosmotic stress in the TME. In HER2-CAR T cells, exposure to elevated Na + levels in breast cancer spheroid models led to increased expression of the mScarlet reporter, indicating successful activation of the synthetic transcriptional program. These findings confirm that NCtags can drive transgene expression in response to osmotic cues within a tumor-like environment.Conclusions Together, these findings highlight a novel strategy to link environmental sensing with therapeutic gene expression in CAR-T cells using NFAT5-derived NCtags. By coupling synthetic transcriptional control to hyperosmotic stress cues, this approach provides a modular platform to enhance CAR-T cell function in solid tumors.