Thymine-functionalized bPEI nanocomplexes for synergistic STING activation and PD-L1 silencing to potentiate cancer immunotherapy.

Cancer continues to pose a major global health burden. Although immunotherapy has reshaped treatment strategies, its therapeutic effect is frequently limited by a suppressive tumor microenvironment and systemic toxicity. The stimulator of interferon genes (STING) pathway has emerged as a key target for coordinating innate and adaptive immunity through type I interferon signaling, yet insufficient activation and tumor-intrinsic resistance often constrain its benefit. Herein, we present an innovative branched polyethyleneimine (bPEI)-based nanodelivery platform designed to address these challenges by integrating STING activation with PD-L1 checkpoint blockade. The core of this platform is a novel thymine-functionalized bPEI polymer (bPEI-Thy), which acts as a polyvalent STING agonist and concurrently encapsulates PD-L1 siRNA into well-defined nanocomplexes (named as bPEI-Thy/siPD-L1 NCs, 65.76 nm, PDI 0.193, zeta potential 17.4 mV). In vitro, the NCs were internalized, facilitated endosomal escape, activated STING signaling, and reduced PD-L1 expression. In vivo studies of both anti-PD-L1-responsive and resistant tumor models demonstrated that intratumoral delivery of bPEI-Thy/siPD-L1 NCs substantially reduced tumor growth and metastatic spread, induced apoptosis, alleviated immunosuppression, and extended survival. Mechanistic evaluation showed enhanced dendritic cell maturation, activation of CD4+ and CD8+ T cells together with natural killer cells, an increased M1/M2 macrophage ratio, and reduced Treg infiltration. Notably, unilateral injection also suppressed tumor growth at distant sites, indicating systemic immune activation. Overall, these dual-functional bPEI-Thy/siPD-L1 NCs provide an integrated platform that activates innate immune pathways and blocks adaptive immune checkpoint blockade, offering a safe and effective strategy for synergistic cancer immunotherapy.