PD-L1 intracellular domain-mimicked poly-metformin synergistically enhances macrophage antigen presentation for colorectal cancer oral immunotherapy

As the most prevalent subtype of colorectal cancer, microsatellite-stable colorectal cancer (MSS CRC) is resistant to T cell-focused immune checkpoint blockade due to its low mutational burden and immunosuppressive tumor microenvironment. Despite reprogramming tumor-associated macrophages toward a tumoricidal M1-like state being a promising alternative, its efficacy is limited by paradoxical upregulation of programmed cell death-ligand 1 (PD-L1) on M1-like macrophages and their inherently poor antigen cross-presentation capacity. Here, we report that poly-metformin (PMet) can mimic the intracellular domain of PD-L1, competitively inhibit the membrane anchoring of PD-L1, and thereby effectively downregulate cell-surface PD-L1. To evaluate the therapeutic efficacy of PMet in immunosuppressive MSS CRC, we further developed an oral probiotic outer membrane vesicle (OMV) gene delivery system, siYthdf2/PMet@Akk-OMV, and demonstrated its ability to reduce PD-L1 levels on both macrophages and tumor cells while synergistically enhancing macrophage cross-presentation of tumor antigens, leading to potent activation of specific antitumor immunity and significant tumor growth inhibition. This work thus provides both theoretical and experimental foundations for treating cancers resistant to traditional immune checkpoint blockade therapy.