Functional pH-Responsive Nanoparticles for Immune Reprogramming in MSS Colorectal Cancer via ER Stress-Induced Proteostasis Disruption, PD-L1-Targeting miRNA, and TLR7 Activation

<b>Background:</b> Colorectal cancer (CRC), particularly the microsatellite-stable (MSS) subtype, remains largely unresponsive to immune checkpoint inhibitors (ICIs) due to immune escape, tumor-associated macrophage (TAM) enrichment, and cytokine-driven suppression that sustain a TAM-dominant tumor microenvironment (TME). To overcome these barriers, a pH-responsive solid lipid nanoparticle (SLN) system was engineered to co-deliver CB-5083 (a VCP/p97 inhibitor), miR-142 (a PD-L1-targeting microRNA), and imiquimod (R, a TLR7 agonist) for spatially confined induction of endoplasmic reticulum stress (ERS) and immune reprogramming in MSS CRC. <b>Methods:</b> The SLNs were coated with PEG-PGA for pH-triggered de-shielding and functionalized with PD-L1- and EGFR-binding peptides plus an ER-homing peptide, enabling tumor-selective and subcellular targeting. <b>Results:</b> The nanoplatform displayed acid-triggered PEG-PGA detachment, selective CRC/TAM uptake, and ER localization. CB-mediated VCP inhibition activated IRE1α/XBP1s/LC3II, PERK/eIF2α/ATF4/CHOP, and JNK/Beclin signaling, driving apoptosis and autophagy, while miR-142 suppressed PD-L1 expression and epithelial-mesenchymal transition markers. R facilitated dendritic cell maturation and M1 polarization. Combined CB + miR + R/SLN-CSW suppressed IL-17, G-CSF, and CXCL1, increased infiltration of CD4⁺ and CD8⁺ T cells, reduced Tregs and M2-TAMs, and inhibited tumor growth in CT-26 bearing mice. The treatment induced immunogenic cell death, reprogramming the TME into a T cell-permissive state and conferring resistance to tumor rechallenge. Biodistribution analysis confirmed tumor-preferential accumulation with minimal off-target exposure, and biosafety profiling demonstrated low systemic toxicity. <b>Conclusions:</b> This TME-responsive nanoplatform therefore integrates ERS induction, checkpoint modulation, and cytokine suppression to overcome immune exclusion in MSS CRC, representing a clinically translatable strategy for chemo-immunotherapy in immune-refractory tumors.