Exosomal miR-552-5p Regulates the Role of NK Cells in EMT of Gastric Cancer via the PD-1/PD-L1 Axis

<b>Background:</b> While previous studies have established the role of exosomal miR-552-5p in promoting gastric cancer (GC) progression, the exact mechanisms through which it modulates the PD-1/PD-L1 axis to affect NK cell function and subsequently influence GC epithelial-mesenchymal transition (EMT) remain to be elucidated. <b>Methods:</b> Western blot, transmission electron microscopy (TEM), and nanoparticle tracking analysis were used to characterize exosomes that were isolated from GC cell supernatants. Subcutaneous AGS cell injections expressing either Lv-miR-552-5p or Lv-NC were administered to nude BALB/C mice. Mice received intraperitoneal injections of anti-PD-L1 antibody (12.5 mg/kg) or isotype control IgG weekly for two weeks. Flow cytometry assessed NK cell proportions and activation receptor (NKG2D, NKp46) and PD-L1 expression. ELISA measured cytokine levels (IFN-γ, granzyme B, perforin). Immunohistochemistry evaluated EMT marker expression in tumor tissues. An <i>in vitro</i> co-culture of NK cells with Exo-Lv-miR-552-5p or Exo-Lv-NC and GC cells was established. EMT protein expression in GC cells was analyzed by Western blot and immunofluorescence. Transwell assays and a tail vein-lung metastasis model in nude mice tested GC cell migration and invasion. <b>Results:</b> Expression of NKG2D, NKp46, and PD-L1 was significantly reduced in Exo-Lv-miR-552-5p mice peripheral blood NK cell percentage. Increased treatment with PD-L1 inhibitors reversed the considerable reduction in IFN-γ, granzyme B, and perforin cytokine expression levels. Exosomal miR-552-5p overexpression in NK cells reduced E-cadherin expression while increasing N-cadherin and vimentin expression in GC cells, promoting migratory and invasive properties. <b>Conclusions:</b> GC-derived exosomal miR-552-5p promotes EMT in GC by inhibiting NK cell activity via the PD-1/PD-L1 axis, which provides new insights into the role of exosomal miR-552-5p in GC progression and immune escape.