Fluorogenic Peptide-Modulated 2D ZnO/MnO ₂ Nanosheet Heterostructure Enables Monitoring of Tumor Early Response to Immunotherapy

Real-time monitoring of the adoptive transfer of natural killer (NK) cells is crucial for evaluating the efficacy of cancer immunotherapy. However, no effective strategy to verify the activity of NK cells in vivo. Herein, we developed an activatable nanosensor to visualize the in vivo activity of adoptive NK cells. The nanosensor consists of a 2D ZnO/MnO₂ nanosheet heterostructure (ZnO/MnO₂) modified with a fluorogenic peptide (Pep-ZnO/MnO₂), in which a granzyme B (Gzm B)-cleavable peptide sequence was conjugated with a fluorescent dye and a quencher pair. Upon cell endocytosis, the intracellular high concentration of glutathione (GSH) destroys the ZnO/MnO₂ and disassembles the fluorogenic peptide. The reduced Mn²⁺ by GSH can be used as a magnetic resonance imaging (MRI) contrast agent. Gzm B cleaves the peptide sequence, providing a fluorescence "turn-on" assay for the activity of Gzm B. The as-prepared nanosensor enables in situ imaging and monitoring of the Gzm B-mediated NK cell responses against cancer cells. Furthermore, the nanosensor can also be used to evaluate the efficacy of NK cell-based immunotherapies in vivo.